ns The Manual (formerly ns Notes and Documentation)1
The VINT Project A Collaboration between researchers at UC Berkeley, LBL, USC/ISI, and Xerox PARC. Kevin Fall [e-mail] , Editor Kannan Varadhan [e-mail] , Editor
April 10, 2001
ns c is LBNLs Network Simulator [16]. The simulator is written in C++; it uses OTcl as a command and conguration interface. ns v2 has three substantial changes from ns v1: (1) the more complex objects in ns v1 have been decomposed into simpler components for greater exibility and composability; (2) the conguration interface is now OTcl, an object oriented version of Tcl; and (3) the interface code to the OTcl interpreter is separate from the main simulator. Ns documentation is available in html, Postscript, and PDF formats. See http://www.isi.edu/nsnam/ns/ns-documentation. html for pointers to these.
1 The VINT project is a joint effort by people from UC Berkeley, USC/ISI, LBL, and Xerox PARC. The project is supported by the Defense Advanced Research Projects Agency (DARPA) at LBL under DARPA grant DABT63-96-C-0105, at USC/ISI under DARPA grant ABT63-96-C-0054, at Xerox PARC under DARPA grant DABT63-96-C-0105. Any opinions, ndings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reect the views of the DARPA.
Contents
1 Introduction 2 Undocumented Facilities 10 14
I
Interface to the Interpreter
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17 17 18 18 19 19 19 20 20 21 21 22 23 25 26 28 29 31 32 33
3 OTcl Linkage 3.1 Concept Overview . . . . . . . . . . . . . . . . . . . . 3.2 Code Overview . . . . . . . . . . . . . . . . . . . . . . 3.3 Class Tcl . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.1 Obtain a Reference to the class Tcl instance . . . 3.3.2 Invoking OTcl Procedures . . . . . . . . . . . . 3.3.3 Passing Results to/fro the Interpreter . . . . . . . 3.3.4 Error Reporting and Exit . . . . . . . . . . . . . 3.3.5 Hash Functions within the Interpreter . . . . . . 3.3.6 Other Operations on the Interpreter . . . . . . . 3.4 Class TclObject . . . . . . . . . . . . . . . . . . . . . . 3.4.1 Creating and Destroying TclObjects . . . . . . . 3.4.2 Variable Bindings . . . . . . . . . . . . . . . . . 3.4.3 Variable Tracing . . . . . . . . . . . . . . . . . 3.4.4 commandMethods: Denition and Invocation . 3.5 Class TclClass . . . . . . . . . . . . . . . . . . . . . . . 3.5.1 How to Bind Static C++ Class Member Variables 3.6 Class TclCommand . . . . . . . . . . . . . . . . . . . . 3.7 Class EmbeddedTcl . . . . . . . . . . . . . . . . . . . . 3.8 Class InstVar . . . . . . . . . . . . . . . . . . . . . . .
II
Simulator Basics
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36 36 36 37 38 38 38 38 39
4 The Class Simulator 4.1 Simulator Initialization . . . . . . . . . 4.2 Schedulers and Events . . . . . . . . . 4.2.1 The List Scheduler . . . . . . . 4.2.2 the heap scheduler . . . . . . . 4.2.3 The Calendar Queue Scheduler 4.2.4 The Real-Time Scheduler . . . 4.3 Other Methods . . . . . . . . . . . . . 4.4 Commands at a glance . . . . . . . . .
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5 Nodes and Packet Forwarding 5.1 Simulator Methods: Creating the Topology 5.2 Node Methods: Conguring the Node . . . 5.3 Conguring Node Functionality . . . . . . 5.3.1 Node Conguration Interface . . . 5.4 The Classier . . . . . . . . . . . . . . . . 5.4.1 Address Classiers . . . . . . . . . 5.4.2 Multicast Classiers . . . . . . . . 5.4.3 MultiPath Classier . . . . . . . . 5.4.4 Hash Classier . . . . . . . . . . . 5.4.5 Replicator . . . . . . . . . . . . . . 5.5 Routing Module and Classier Organization 5.5.1 Routing Module . . . . . . . . . . 5.5.2 Node Interface . . . . . . . . . . . 5.6 Commands at a glance . . . . . . . . . . .
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42 42 44 46 46 48 50 50 52 52 53 54 55 57 57 60 61 63 63 64 67 67 68 69 70 71 75 76 76 77 79 79 79 80 81 81 81 82 82 82 83 84 85 86 86 86 87 87 89 89 89
6 Links: Simple Links 6.1 Instance Procedures for Links and SimpleLinks 6.2 Connectors . . . . . . . . . . . . . . . . . . . 6.3 Object hierarchy . . . . . . . . . . . . . . . . . 6.4 Commands at a glance . . . . . . . . . . . . . 7 Queue Management and Packet Scheduling 7.1 The C++ Queue Class . . . . . . . . . . 7.1.1 Queue blocking . . . . . . . . . 7.1.2 PacketQueue Class . . . . . . . 7.2 Example: Drop Tail . . . . . . . . . . . 7.3 Different types of Queue objects . . . . 7.4 Commands at a glance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8 Delays and Links 8.1 The LinkDelay Class . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.2 Commands at a glance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Agents 9.1 Agent state . . . . . . . . . . . . . . . . . . . . . . . . . . 9.2 Agent methods . . . . . . . . . . . . . . . . . . . . . . . . 9.3 Protocol Agents . . . . . . . . . . . . . . . . . . . . . . . . 9.4 OTcl Linkage . . . . . . . . . . . . . . . . . . . . . . . . . 9.4.1 Creating and Manipulating Agents . . . . . . . . . . 9.4.2 Default Values . . . . . . . . . . . . . . . . . . . . 9.4.3 OTcl Methods . . . . . . . . . . . . . . . . . . . . 9.5 Examples: Tcp, TCP Sink Agents . . . . . . . . . . . . . . 9.5.1 Creating the Agent . . . . . . . . . . . . . . . . . . 9.5.2 Starting the Agent . . . . . . . . . . . . . . . . . . 9.5.3 Processing Input at Receiver . . . . . . . . . . . . . 9.5.4 Processing Responses at the Sender . . . . . . . . . 9.5.5 Implementing Timers . . . . . . . . . . . . . . . . . 9.6 Creating a New Agent . . . . . . . . . . . . . . . . . . . . . 9.6.1 Example: A ping requestor (Inheritance Structure) 9.6.2 The recv() and timeout() Methods . . . . . . . . 9.6.3 Linking the ping Agent with OTcl . . . . . . . . . 9.6.4 Using the agent through OTcl . . . . . . . . . . . . 9.7 The Agent API . . . . . . . . . . . . . . . . . . . . . . . . 9.8 Different agent objects . . . . . . . . . . . . . . . . . . . . 2
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9.9
Commands at a glance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
92 94 94 95 95 98 98 99 99 101 101 102 102 105 105 106 107 109 109 110 111 112 113 113 114 115 115 115 115 116 117 117 117 117 118 119 119 119 120 120 120 122 123 123 124 124 124 124 124 125
10 Timers 10.1 C++ abstract base class TimerHandler . . . 10.1.1 Denition of a new timer . . . . . . 10.1.2 Example: Tcp retransmission timer 10.2 OTcl Timer class . . . . . . . . . . . . . . 10.3 Commands at a glance . . . . . . . . . . .
11 Packet Headers and Formats 11.1 A Protocol-Specic Packet Header . . . . . . . . . . . . . . . . 11.1.1 Adding a New Packet Header Type . . . . . . . . . . . 11.1.2 Selectively Including Packet Headers in Your Simulation 11.2 Packet Classes . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2.1 The Packet Class . . . . . . . . . . . . . . . . . . . . . 11.2.2 p_info Class . . . . . . . . . . . . . . . . . . . . . . . 11.2.3 The hdr_cmn Class . . . . . . . . . . . . . . . . . . . . 11.2.4 The PacketHeaderManager Class . . . . . . . . . . . . 11.3 Commands at a glance . . . . . . . . . . . . . . . . . . . . . . 12 Error Model 12.1 Implementation . . . . 12.2 Conguration . . . . . 12.3 Multi-state error model 12.4 Commands at a glance
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13 Local Area Networks 13.1 Tcl conguration . . . . . . . . . . . . . . . . . . . . . . . 13.2 Components of a LAN . . . . . . . . . . . . . . . . . . . . 13.3 Channel Class . . . . . . . . . . . . . . . . . . . . . . . . . 13.3.1 Channel State . . . . . . . . . . . . . . . . . . . . . 13.3.2 Example: Channel and classier of the physical layer 13.3.3 Channel Class in C++ . . . . . . . . . . . . . . . . 13.4 MacClassier Class . . . . . . . . . . . . . . . . . . . . . . 13.5 MAC Class . . . . . . . . . . . . . . . . . . . . . . . . . . 13.5.1 Mac State . . . . . . . . . . . . . . . . . . . . . . . 13.5.2 Mac Methods . . . . . . . . . . . . . . . . . . . . . 13.5.3 Mac Class in C++ . . . . . . . . . . . . . . . . . . 13.5.4 CSMA-based MAC . . . . . . . . . . . . . . . . . . 13.6 LL (link-layer) Class . . . . . . . . . . . . . . . . . . . . . 13.6.1 LL Class in C++ . . . . . . . . . . . . . . . . . . . 13.6.2 Example: Link Layer conguration . . . . . . . . . 13.7 LanRouterclass . . . . . . . . . . . . . . . . . . . . . . . 13.8 Other Components . . . . . . . . . . . . . . . . . . . . . . 13.9 LANs and ns routing . . . . . . . . . . . . . . . . . . . . . 13.10Commands at a glance . . . . . . . . . . . . . . . . . . . . 14 The (Revised) Addressing Structure in NS 14.1 The Default Address Format . . . . . . 14.2 The Hierarchical Address Format . . . . 14.2.1 Default Hierarchical Setting . . 14.2.2 Specic Hierarchical Setting . . 14.3 The Expanded Node-Address Format . 14.4 Expanding port-id eld . . . . . . . . . 14.5 Errors in setting address format . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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14.6 Commands at a glance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125 15 Mobile Networking in ns 15.1 The basic wireless model in ns . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15.1.1 Mobilenode: creating wireless topology . . . . . . . . . . . . . . . . . . . 15.1.2 Creating Node movements . . . . . . . . . . . . . . . . . . . . . . . . . . 15.1.3 Network Components in a mobilenode . . . . . . . . . . . . . . . . . . . . 15.1.4 Different types of Routing Agents in mobile networking . . . . . . . . . . 15.1.5 Trace Support . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15.1.6 Revised format for wireless traces . . . . . . . . . . . . . . . . . . . . . . 15.1.7 Generation of node-movement and trafc-connection for wireless scenarios 15.2 Extensions made to CMUs wireless model . . . . . . . . . . . . . . . . . . . . . 15.2.1 wired-cum-wireless scenarios . . . . . . . . . . . . . . . . . . . . . . . . 15.2.2 MobileIP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15.3 Commands at a glance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 Satellite Networking in ns 16.1 Overview of satellite models . . . . . 16.1.1 Geostationary satellites . . . . 16.1.2 Low-earth-orbiting satellites . 16.2 Using the satellite extensions . . . . . 16.2.1 Nodes and node positions . . 16.2.2 Satellite links . . . . . . . . . 16.2.3 Handoffs . . . . . . . . . . . 16.2.4 Routing . . . . . . . . . . . . 16.2.5 Trace support . . . . . . . . . 16.2.6 Error models . . . . . . . . . 16.2.7 Other conguration options . 16.2.8 nam support . . . . . . . . . 16.2.9 Integration with other modules 16.2.10 Example scripts . . . . . . . . 16.3 Implementation . . . . . . . . . . . . 16.3.1 Use of linked lists . . . . . . . 16.3.2 Node structure . . . . . . . . 16.3.3 Detailed look at satellite links 16.4 Commands at a glance . . . . . . . . 17 Radio Propagation Models 17.1 Free space model . . . . . . . . 17.2 Two-ray ground reection model 17.3 Shadowing model . . . . . . . . 17.3.1 Backgroud . . . . . . . 17.3.2 Using shadowing model 17.4 Communication range . . . . . . 17.5 Commands at a glance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126 126 126 127 130 133 135 138 141 142 142 144 145 148 148 148 149 151 151 152 154 155 156 157 158 158 158 158 159 159 160 161 162 164 164 165 165 165 167 167 168 169 169 169 170 171 171 172 172 172
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18 Debugging ns 18.1 Tcl-level Debugging . . . . . . . . . . . . 18.2 C++-Level Debugging . . . . . . . . . . . 18.3 Mixing Tcl and C debugging . . . . . . . . 18.4 Memory Debugging . . . . . . . . . . . . . 18.4.1 Using dmalloc . . . . . . . . . . . 18.4.2 Memory Conservation Tips . . . . . 18.4.3 Some statistics collected by dmalloc 18.5 Memory Leaks . . . . . . . . . . . . . . .
18.5.1 OTcl . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173 18.5.2 C/C++ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173 19 Energy Model in ns 174 19.1 The C++ EnergyModel Class . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 174 19.2 The OTcl interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 175
III Support
20 Mathematical Support 20.1 Random Number Generation . . . . . . . . 20.2 Random Variables . . . . . . . . . . . . . . 20.3 Integrals . . . . . . . . . . . . . . . . . . . 20.4 ns-random . . . . . . . . . . . . . . . . 20.5 Some mathematical-support related objects 20.6 Commands at a glance . . . . . . . . . . . 21 Trace and Monitoring Support 21.1 Trace Support . . . . . . . . . . . . 21.1.1 OTcl Helper Functions . . . 21.2 Library support and examples . . . 21.3 The C++ Trace Class . . . . . . . . 21.4 Trace File Format . . . . . . . . . . 21.5 Packet Types . . . . . . . . . . . . 21.6 Queue Monitoring . . . . . . . . . . 21.7 Per-Flow Monitoring . . . . . . . . 21.7.1 The Flow Monitor . . . . . 21.7.2 Flow Monitor Trace Format 21.7.3 The Flow Class . . . . . . . 21.8 Commands at a glance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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177 177 179 180 181 181 182 184 184 185 186 188 189 191 192 194 194 194 195 195 198 198 199 200 200 200 201 201 202 203 203 203 204 204
22 Nam Trace 22.1 Nam Trace format . . . . . . . . . . . . . . . . . . . . . . 22.1.1 Packet Traces . . . . . . . . . . . . . . . . . . . . 22.1.2 Node state . . . . . . . . . . . . . . . . . . . . . . 22.1.3 Node Marking . . . . . . . . . . . . . . . . . . . 22.1.4 Link/Queue State . . . . . . . . . . . . . . . . . . 22.1.5 Agent Tracing . . . . . . . . . . . . . . . . . . . 22.1.6 Variable Tracing . . . . . . . . . . . . . . . . . . 22.1.7 Miscellaneous Trace Events . . . . . . . . . . . . 22.2 Ns commands for creating and controlling nam animations 22.2.1 Node . . . . . . . . . . . . . . . . . . . . . . . . 22.2.2 Link/Queue . . . . . . . . . . . . . . . . . . . . . 22.2.3 Agent and Features . . . . . . . . . . . . . . . . . 22.2.4 Some Generic Commands . . . . . . . . . . . . .
IV
Routing
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206 206 207 208 209
23 Unicast Routing 23.1 The Interface to the Simulation Operator (The API) . . . 23.2 Other Conguration Mechanisms for Specialised Routing 23.3 Protocol Specic Conguration Parameters . . . . . . . 23.4 Internals and Architecture of Routing . . . . . . . . . .
5
23.4.1 The classes . . . . . . . . . . . . . . . . . . 23.4.2 Interface to Network Dynamics and Multicast 23.5 Protocol Internals . . . . . . . . . . . . . . . . . . . 23.6 Unicast routing objects . . . . . . . . . . . . . . . . 23.7 Commands at a glance . . . . . . . . . . . . . . . . 24 Multicast Routing 24.1 Multicast API . . . . . . . . . . . . . . . . . . . . 24.1.1 Multicast Behavior Monitor Conguration 24.1.2 Protocol Specic conguration . . . . . . . 24.2 Internals of Multicast Routing . . . . . . . . . . . 24.2.1 The classes . . . . . . . . . . . . . . . . . 24.2.2 Extensions to other classes in ns . . . . . . 24.2.3 Protocol Internals . . . . . . . . . . . . . . 24.2.4 The internal variables . . . . . . . . . . . . 24.3 Commands at a glance . . . . . . . . . . . . . . . 25 Network Dynamics 25.1 The user level API . . . . . . . . . . . . . . . . . 25.2 The Internal Architecture . . . . . . . . . . . . . . 25.2.1 The class rtModel . . . . . . . . . . . . . . 25.2.2 class rtQueue . . . . . . . . . . . . . 25.3 Interaction with Unicast Routing . . . . . . . . . . 25.3.1 Extensions to Other Classes . . . . . . . . 25.4 Decencies in the Current Network Dynamics API 25.5 Commands at a glance . . . . . . . . . . . . . . . 26 Hierarchical Routing 26.1 Overview of Hierarchical Routing . . 26.2 Usage of Hierarchical routing . . . . . 26.3 Creating large Hierarchical topologies 26.4 Hierarchical Routing with SessionSim 26.5 Commands at a glance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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209 213 213 214 215 217 217 218 219 220 220 222 225 227 227 230 230 232 232 233 234 234 235 235 237 237 237 240 240 240
V
Transport
242
27 UDP Agents 243 27.1 UDP Agents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 243 27.2 Commands at a glance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 244 28 TCP Agents 28.1 One-Way TCP Senders . . . . . . . . . . . 28.1.1 The Base TCP Sender (Tahoe TCP) 28.1.2 Conguration . . . . . . . . . . . . 28.1.3 Simple Conguration . . . . . . . . 28.1.4 Other Conguration Parameters . . 28.1.5 Other One-Way TCP Senders . . . 28.2 TCP Receivers (sinks) . . . . . . . . . . . 28.2.1 The Base TCP Sink . . . . . . . . . 28.2.2 Delayed-ACK TCP Sink . . . . . . 28.2.3 Sack TCP Sink . . . . . . . . . . . 28.3 Two-Way TCP Agents (FullTcp) . . . . . . 28.3.1 Simple Conguration . . . . . . . . 28.4 Architecture and Internals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 245 246 246 246 246 247 248 248 249 249 249 249 250 251
28.5 28.6 28.7 28.8
Tracing TCP Dynamics . . . . . . . . One-Way Trace TCP Trace Dynamics One-Way Trace TCP Trace Dynamics Commands at a glance . . . . . . . .
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252 252 253 253 254 254 254 256 257 258 260 260 262 262 264 265 265 265 266 267 269 269 271 272 272 272 273 274 274 274
29 Agent/SRM 29.1 Conguration . . . . . . . . . . . . . . . . . . 29.1.1 Trivial Conguration . . . . . . . . . . 29.1.2 Other Conguration Parameters . . . . 29.1.3 Statistics . . . . . . . . . . . . . . . . 29.1.4 Tracing . . . . . . . . . . . . . . . . . 29.2 Architecture and Internals . . . . . . . . . . . . 29.3 Packet Handling: Processing received messages 29.4 Loss DetectionThe Class SRMinfo . . . . . 29.5 Loss Recovery Objects . . . . . . . . . . . . . 29.6 Session Objects . . . . . . . . . . . . . . . . . 29.7 Extending the Base Class Agent . . . . . . . . 29.7.1 Fixed Timers . . . . . . . . . . . . . . 29.7.2 Adaptive Timers . . . . . . . . . . . . 29.8 SRM objects . . . . . . . . . . . . . . . . . . . 29.9 Commands at a glance . . . . . . . . . . . . . 30 PLM 30.1 Conguration . . . . . . . . . . . . . . 30.2 The Packet Pair Source Generator . . . 30.3 Architecture of the PLM Protocol . . . 30.3.1 Instantiation of a PLM Source . 30.3.2 Instantiation of a PLM Receiver 30.3.3 Reception of a Packet . . . . . . 30.3.4 Detection of a Loss . . . . . . . 30.3.5 Joining or Leaving a Layer . . . 30.4 Commands at a Glance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
VI
Application
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276
277 277 278 278 279 279 279 280 281 283 284 284 286
31 Applications and transport agent API 31.1 The class Application . . . . . . . . . . . . . . 31.2 The transport agent API . . . . . . . . . . . . . 31.2.1 Attaching transport agents to nodes . . 31.2.2 Attaching applications to agents . . . . 31.2.3 Using transport agents via system calls 31.2.4 Agent upcalls to applications . . . . . . 31.2.5 An example . . . . . . . . . . . . . . . 31.3 The class TrafcGenerator . . . . . . . . . . . 31.3.1 An example . . . . . . . . . . . . . . . 31.4 Simulated applications: Telnet and FTP . . . . 31.5 Applications objects . . . . . . . . . . . . . . . 31.6 Commands at a glance . . . . . . . . . . . . .
7
32 Web cache as an application 32.1 Using application-level data in ns . . . . . . . . . . . 32.1.1 ADU . . . . . . . . . . . . . . . . . . . . . 32.1.2 Passing data between applications . . . . . . 32.1.3 Transmitting user data over UDP . . . . . . . 32.1.4 Transmitting user data over TCP . . . . . . . 32.1.5 Class hierarchy related to user data handling 32.2 Overview of web cache classes . . . . . . . . . . . . 32.2.1 Managing HTTP connections . . . . . . . . 32.2.2 Managing web pages . . . . . . . . . . . . . 32.2.3 Debugging . . . . . . . . . . . . . . . . . . 32.3 Representing web pages . . . . . . . . . . . . . . . 32.4 Page pools . . . . . . . . . . . . . . . . . . . . . . . 32.4.1 PagePool/Math . . . . . . . . . . . . . . . . 32.4.2 PagePool/CompMath . . . . . . . . . . . . . 32.4.3 PagePool/ProxyTrace . . . . . . . . . . . . . 32.4.4 PagePool/Client . . . . . . . . . . . . . . . . 32.5 Web client . . . . . . . . . . . . . . . . . . . . . . . 32.6 Web server . . . . . . . . . . . . . . . . . . . . . . 32.7 Web cache . . . . . . . . . . . . . . . . . . . . . . . 32.7.1 Http/Cache . . . . . . . . . . . . . . . . . . 32.8 Putting together: a simple example . . . . . . . . . . 32.9 Http trace format . . . . . . . . . . . . . . . . . . . 32.10Commands at a glance . . . . . . . . . . . . . . . .
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287 287 287 288 289 290 291 291 291 292 293 293 294 294 295 295 296 297 298 298 298 299 301 302
VII
Scale
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304
305 305 305 307 307 308 308 309 310
33 Session-level Packet Distribution 33.1 Conguration . . . . . . . . . . 33.1.1 Basic Conguration . . 33.1.2 Inserting a Loss Module 33.2 Architecture . . . . . . . . . . . 33.3 Internals . . . . . . . . . . . . . 33.3.1 Object Linkage . . . . . 33.3.2 Packet Forwarding . . . 33.4 Commands at a glance . . . . .
VIII Emulation
34 Emulation 34.1 Introduction . . . . . . . . . . . . . 34.2 Real-Time Scheduler . . . . . . . . 34.3 Tap Agents . . . . . . . . . . . . . 34.4 Network Objects . . . . . . . . . . 34.4.1 Pcap/BPF Network Objects 34.4.2 IP Network Objects . . . . . 34.4.3 IP/UDP Network Objects . . 34.5 An Example . . . . . . . . . . . . . 34.6 Commands at a glance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
311
312 312 313 313 314 314 315 315 316 317
8
IX
Nam and Animation
318
35 Nam 319 35.1 Animations from nam . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 319
9
Chapter 1
Introduction
Lets start at the very beginning, a very nice place to start, when you sing, you begin with A, B, C, when you simulate, you begin with the topology, 1 ... This document (ns Notes and Documentation) provides reference documentation for ns. Although we begin with a simple simulation script, resources like Marc Greiss tutorial web pages (originally at his web site, now at http://www.isi. edu/nsnam/ns/tutorial/) or the slides from one of the ns tutorials are problably better places to begin for the ns novice. We rst begin by showing a simple simulation script. This script is also available in the sources in ~ns/tcl/ex/simple.tcl. This script denes a simple topology of four nodes, and two agents, a UDP agent with a CBR trafc generator, and a TCP agent. The simulation runs for . The output is two trace les, out.tr and out.nam. When the simulation completes at the end of , it will attempt to run a nam visualisation of the simulation on your screen. # The preamble set ns [new Simulator] # Predene tracing set f [open out.tr w] $ns trace-all $f set nf [open out.nam w] $ns namtrace-all $nf
;# initialise the simulation
1 with
apologies to Rodgers and Hammerstein
10
# so, we lied. now, we dene the topology # # n0 # # 5Mb # 2ms # # n2 --------- n3 # / 1.5Mb # 5Mb / 10ms # 2ms / # / # n1 # set n0 [$ns node] set n1 [$ns node] set n2 [$ns node] set n3 [$ns node] $ns duplex-link $n0 $n2 5Mb 2ms DropTail $ns duplex-link $n1 $n2 5Mb 2ms DropTail $ns duplex-link $n2 $n3 1.5Mb 10ms DropTail # Some agents. set udp0 [new Agent/UDP] $ns attach-agent $n0 $udp0 set cbr0 [new Application/Traffic/CBR] $cbr0 attach-agent $udp0 $udp0 set class_ 0 set null0 [new Agent/Null] $ns attach-agent $n3 $null0 $ns connect $udp0 $null0 $ns at 1.0 "$cbr0 start" puts [$cbr0 set packetSize_] puts [$cbr0 set interval_] # A FTP over TCP/Tahoe from $n1 to $n3, owid 2 set tcp [new Agent/TCP] $tcp set class_ 1 $ns attach-agent $n1 $tcp set sink [new Agent/TCPSink] $ns attach-agent $n3 $sink set ftp [new Application/FTP] $ftp attach-agent $tcp $ns at 1.2 "$ftp start" ;# TCP does not generate its own trafc
;# A UDP agent ;# on node $n0 ;# A CBR trafc generator agent ;# attached to the UDP agent ;# actually, the default, but. . . ;# Its sink ;# on node $n3
$ns connect $tcp $sink $ns at 1.35 "$ns detach-agent $n0 $tcp ; $ns detach-agent $n3 $sink"
11
12
# The simulation runs for . # The simulation comes to an end when the scheduler invokes the nish{} procedure below. # This procedure closes all trace les, and invokes nam visualization on one of the trace les. $ns at 3.0 "finish" proc finish {} { global ns f nf $ns flush-trace close $f close $nf puts "running nam..." exec nam out.nam & exit 0 } # Finally, start the simulation. $ns run
13
Chapter 2
Undocumented Facilities
Ns is often growing to include new protocols. Unfortunately the documention doesnt grow quite as often. This section lists what remains to be documented, or what needs to be improved. (The documentation is in the doc subdirectory of the ns source code if you want to add to it. :-)
Interface to the Interpreter Simulator Basics
nothing currently
LANs need to be updated for new wired/wireless support (Yuri updated this?)
wireless support needs to be added (done) should explicitly list queueing options in the queue mgt chapter?
Support
should pick a single list mgt package and document it
should document the trace-post-processing utilities in bin
The usage and design of link state and MPLS routing modules are not documented at all. (Note: link state and MPLS appeared only in daily snapshots and releases after 09/14/2000.)
Routing
need to document hierarchical routing/addressing (Padma has done) need a chapter on supported ad-hoc routing protocols
CBQ needs documentation (can maybe build off of ftp://ftp.ee.lbl.gov/papers/cbqsims. ps.Z?)
Queueing Transport
need to document MFTP
need to document RTP (session-rtp.cc, etc.) need to document multicast building blocks should repair and document snoop and tcp-int
Trafc and scenarios (new section)
should add a description of how to drive the simulator from traces should add discussion of the scenario generator should add discussion of http trafc sources
Application
is the non-Haobo http stuff documented? no.
14
Scale
should add disucssion of mixed mode (pending)
Emulation Other
nothing currently
should document admission control policies?
should add a validation chapter and snarf up the contents of ns-tests.html should snarf up Marc Greis tutorial rather than just referring to it?
15
Part I
Interface to the Interpreter
16
Chapter 3
OTcl Linkage
ns is an object oriented simulator, written in C++, with an OTcl interpreter as a frontend. The simulator supports a class hierarchy in C++ (also called the compiled hierarchy in this document), and a similar class hierarchy within the OTcl interpreter (also called the interpreted hierarchy in this document). The two hierarchies are closely related to each other; from the users perspective, there is a one-to-one correspondence between a class in the interpreted hierarchy and one in the compiled hierarchy. The root of this hierarchy is the class TclObject. Users create new simulator objects through the interpreter; these objects are instantiated within the interpreter, and are closely mirrored by a corresponding object in the compiled hierarchy. The interpreted class hierarchy is automatically established through methods dened in the class TclClass. user instantiated objects are mirrored through methods dened in the class TclObject. There are other hierarchies in the C++ code and OTcl scripts; these other hierarchies are not mirrored in the manner of TclObject.
3.1 Concept Overview
Why two languages? ns uses two languages because simulator has two different kinds of things it needs to do. On one hand, detailed simulations of protocols requires a systems programming language which can efciently manipulate bytes, packet headers, and implement algorithms that run over large data sets. For these tasks run-time speed is important and turn-around time (run simulation, nd bug, x bug, recompile, re-run) is less important. On the other hand, a large part of network research involves slightly varying parameters or congurations, or quickly exploring a number of scenarios. In these cases, iteration time (change the model and re-run) is more important. Since conguration runs once (at the beginning of the simulation), run-time of this part of the task is less important. ns meets both of these needs with two languages, C++ and OTcl. C++ is fast to run but slower to change, making it suitable for detailed protocol implementation. OTcl runs much slower but can be changed very quickly (and interactively), making it ideal for simulation conguration. ns (via tclcl) provides glue to make objects and variables appear on both langauges. For more information about the idea of scripting languages and split-language programming, see Ousterhouts article in IEEE Computer [18]. For more information about split level programming for network simulation, see the ns paper [2]. Which language for what? Having two languages raises the question of which language should be used for what purpose. Our basic advice is to use OTcl:
for conguration, setup, and one-time stuff 17
if you can do what you want by manipulating existing C++ objects
and use C++:
if you are doing anything that requires processing each packet of a ow if you have to change the behavior of an existing C++ class in ways that werent anticipated
For example, links are OTcl objects that assemble delay, queueing, and possibly loss modules. If your experiment can be done with those pieces, great. If instead you want do something fancier (a special queueing dicipline or model of loss), then youll need a new C++ object. There are certainly grey areas in this spectrum: most routing is done in OTcl (although the core Dijkstra algorithm is in C++). Weve had HTTP simulations where each ow was started in OTcl and per-packet processing was all in C++. This approache worked OK until we had 100s of ows starting per second of simulated time. In general, if youre ever having to invoke Tcl many times per second, you problably should move that code to C++.
3.2 Code Overview
In this document, we use the term interpreter to be synonymous with the OTcl interpreter. The code to interface with the interpreter resides in a separate directory, tclcl. The rest of the simulator code resides in the directory, ns-2. We will use the notation ~tclcl/ le to refer to a particular le in the Tcl directory. Similarly, we will use the notation, ~ns/ le to refer to a particular le in the ns-2 directory. There are a number of classes dened in ~tclcl/. We only focus on the six that are used in ns: The Class Tcl (Section 3.3) contains the methods that C++ code will use to access the interpreter. The class TclObject (Section 3.4) is the base class for all simulator objects that are also mirrored in the compiled hierarchy. The class TclClass (Section 3.5) denes the interpreted class hierarchy, and the methods to permit the user to instantiate TclObjects. The class TclCommand (Section 3.6) is used to dene simple global interpreter commands. The class EmbeddedTcl (Section 3.7) contains the methods to load higher level builtin commands that make conguring simulations easier. Finally, the class InstVar (Section 3.8) contains methods to access C++ member variables as OTcl instance variables. The procedures and functions described in this chapter can be found in ~tclcl/Tcl.{cc, h}, ~tclcl/Tcl2.cc, ~tclcl/tcl-object.tcl, and, ~tclcl/tracedvar.{cc, h}. The le ~tclcl/tcl2c++.c is used in building ns, and is mentioned briey in this chapter.
3.3 Class Tcl
The class Tcl encapsulates the actual instance of the OTcl interpreter, and provides the methods to access and communicate with that interpreter. The methods described in this section are relevant to the ns programmer who is writing C++ code. The class provides methods for the following operations:
obtain a reference to the Tcl instance; invoke OTcl procedures through the interpreter; retrieve, or pass back results to the interpreter; report error situations and exit in an uniform manner; and
18
store and lookup TclObjects. acquire direct access to the interpreter.
We describe each of the methods in the following subsections.
3.3.1 Obtain a Reference to the class Tcl instance
A single instance of the class is declared in ~tclcl/Tcl.cc as a static member variable; the programmer must obtain a reference to this instance to access other methods described in this section. The statement required to access this instance is: Tcl& tcl = Tcl::instance();
3.3.2 Invoking OTcl Procedures
There are four different methods to invoke an OTcl command through the instance, tcl. They differ essentially in their calling arguments. Each function passes a string to the interpreter, that then evaluates the string in a global context. These methods will return to the caller if the interpreter returns TCL_OK. On the other hand, if the interpreter returns TCL_ERROR, the methods will call tkerror{}. The user can overload this procedure to selectively disregard certain types of errors. Such intricacies of OTcl programming are outside the scope of this document. The next section (Section 3.3.3) describes methods to access the result returned by the interpreter.
tcl.eval(char* ) invokes Tcl_GlobalEval() to execute through the interpreter. tcl.evalc(const char* ) preserves the argument string . It copies the string into its internal buffer; it then invokes the previous eval(char* ) on the internal buffer. tcl.eval() assumes that the command is already stored in the class internal bp_; it directly invokes tcl.eval(char* bp_). A handle to the buffer itself is available through the method tcl.buffer(void). tcl.evalf(const char* , . . . ) is a Printf(3) like equivalent. It uses vsprintf(3) internally to create the input string.
As an example, here are some of the ways of using the above methods: Tcl& tcl = Tcl::instance(); char wrk[128]; strcpy(wrk, "Simulator set NumberInterfaces_ 1"); tcl.eval(wrk); sprintf(tcl.buffer(), "Agent/SRM set requestFunction_ %s", "Fixed"); tcl.eval(); tcl.evalc("puts stdout hello world"); tcl.evalf("%s request %d %d", name_, sender, msgid);
3.3.3 Passing Results to/fro the Interpreter
When the interpreter invokes a C++ method, it expects the result back in the private member variable, tcl_->result. Two methods are available to set this variable. 19
tcl.result(const char* ) Pass the result string back to the interpreter.
tcl.resultf(const char* fmt, . . . ) varargs(3) variant of above to format the result using vsprintf(3), pass the result string back to the interpreter.
if (strcmp(argv[1], "now") == 0) { tcl.resultf("%.17g", clock()); return TCL_OK; } tcl.result("Invalid operation specified"); return TCL_ERROR;
Likewise, when a C++ method invokes an OTcl command, the interpreter returns the result in tcl_->result.
tcl.result(void) must be used to retrieve the result. Note that the result is a string, that must be converted into an internal format appropriate to the type of result.
tcl.evalc("Simulator set NumberInterfaces_"); char* ni = tcl.result(); if (atoi(ni) != 1) tcl.evalc("Simulator set NumberInterfaces_ 1");
3.3.4 Error Reporting and Exit
This method provides a uniform way to report errors in the compiled code.
tcl.error(const char* ) performs the following functions: write to stdout; write tcl_->result to stdout; exit with error code 1.
tcl.resultf("cmd = %s", cmd); tcl.error("invalid command specified"); /*NOTREACHED*/
Note that there are minor differences between returning TCL_ERROR as we did in the previous subsection (Section 3.3.3), and calling Tcl::error(). The former generates an exception within the interpreter; the user can trap the exception and possibly recover from the error. If the user has not specied any traps, the interpreter will print a stack trace and exit. However, if the code invokes error(), then the simulation user cannot trap the error; in addition, ns will not print any stack trace.
3.3.5 Hash Functions within the Interpreter
ns stores a reference to every TclObject in the compiled hierarchy in a hash table; this permits quick access to the objects. The hash table is internal to the interpreter. ns uses the name of the TclObject as the key to enter, lookup, or delete the TclObject in the hash table.
20
tcl.enter(TclObject* ) will insert a pointer to the TclObject into the hash table. It is used by TclClass::create_shadow() to insert an object into the table, when that object is created. tcl.lookup(char* ) will retrieve the TclObject with the name . It is used by TclObject::lookup(). tcl.remove(TclObject* ) will delete references to the TclObject from the hash table. It is used by TclClass::delete_shadow() to remove an existing entry from the hash table, when that object is deleted.
These functions are used internally by the class TclObject and class TclClass.
3.3.6 Other Operations on the Interpreter
If the above methods are not sufcient, then we must acquire the handle to the interpreter, and write our own functions.
tcl.interp(void) returns the handle to the interpreter that is stored within the class Tcl.
3.4 Class TclObject
class TclObject is the base class for most of the other classes in the interpreted and compiled hierarchies. Every object in the class TclObject is created by the user from within the interpreter. An equivalent shadow object is created in the compiled hierarchy. The two objects are closely associated with each other. The class TclClass, described in the next section, contains the mechanisms that perform this shadowing. In the rest of this document, we often refer to an object as a TclObject 1 . By this, we refer to a particular object that is either in the class TclObject, or in a class that is derived from the class TclObject. If it is necessary, we will explicitly qualify whether that object is an object within the interpreter, or an object within the compiled code. In such cases, we will use the abbreviations interpreted object, and compiled object to distinguish the two. and within the compiled code respectively.
Differences from ns v1 Unlike ns v1, the class TclObject subsumes the earlier functions of the NsObject class. It therefore stores the interface variable bindings (Section 3.4.2) that tie OTcl instance variables in the interpreted object to corresponding C++ member variables in the compiled object. The binding is stronger than in ns v1 in that any changes to the OTcl variables are trapped, and the current C++ and OTcl values are made consistent after each access through the interpreter. The consistency is done through the class InstVar (Section 3.8). Also unlike ns v1, objects in the class TclObject are no longer stored as a global link list. Instead, they are stored in a hash table in the class Tcl (Section 3.3.5).
Example conguration of a TclObject The following example illustrates the conguration of an SRM agent (class Agent/SRM/Adaptive). set srm [new Agent/SRM/Adaptive] $srm set packetSize_ 1024 $srm traffic-source $s0
1 In the latest release of ns and ns/tclcl, this object has been renamed to SplitObjefct, which more accurately reects its nature of existence. However, for the moment, we will continue to use the term TclObject to refer to these objects and this class.
21
By convention in ns, the class Agent/SRM/Adaptive is a subclass of Agent/SRM, is a subclass of Agent, is a subclass of TclObject. The corresponding compiled class hierarchy is the ASRMAgent, derived from SRMAgent, derived from Agent, derived from TclObject respectively. The rst line of the above example shows how a TclObject is created (or destroyed) (Section 3.4.1); the next line congures a bound variable (Section 3.4.2); and nally, the last line illustrates the interpreted object invoking a C++ method as if they were an instance procedure (Section 3.4.4).
3.4.1 Creating and Destroying TclObjects
When the user creates a new TclObject, using the procedures new{} and delete{}; these procedures are dened in ~tclcl/tcl-object.tcl. They can be used to create and destroy objects in all classes, including TclObjects. 2 . In this section, we describe the internal actions executed when a TclObject is created.
Creating TclObjects By using new{}, the user creates an interpreted TclObject. the interpreter will execute the constructor for that object, init{}, passing it any arguments provided by the user. ns is responsible for automatically creating the compiled object. The shadow object gets created by the base class TclObjects constructor. Therefore, the constructor for the new TclObject must call the parent class constructor rst. new{} returns a handle to the object, that can then be used for further operations upon that object. The following example illustrates the Agent/SRM/Adaptive constructor: Agent/SRM/Adaptive instproc init args { eval $self next $args $self array set closest_ "requestor 0 repairor 0" $self set eps_ [$class set eps_] } The following sequence of actions are performed by the interpreter as part of instantiating a new TclObject. For ease of exposition, we describe the steps that are executed to create an Agent/SRM/Adaptive object. The steps are: 1. Obtain an unique handle for the new object from the TclObject name space. The handle is returned to the user. Most handles in ns have the form _o NNN , where NNN is an integer. This handle is created by getid{}. It can be retrieved from C++ with the name(){} method. 2. Execute the constructor for the new object. Any user-specied arguments are passed as arguments to the constructor. This constructor must invoke the constructor associated with its parent class. In our example above, the Agent/SRM/Adaptive calls its parent class in the very rst line. Note that each constructor, in turn invokes its parent class constructor ad nauseum. The last constructor in ns is the TclObject constructor. This constructor is responsible for setting up the shadow object, and performing other initializations and bindings, as we explain below. It is preferable to call the parent constructors rst before performing the initializations required in this class. This allows the shadow objects to be set up, and the variable bindings established. 3. The TclObject constructor invokes the instance procedure create-shadow{} for the class Agent/SRM/Adaptive. 4. When the shadow object is created, ns calls all of the constructors for the compiled object, each of which may establish variable bindings for objects in that class, and perform other necessary initializations. Hence our earlier injunction that it is preferable to invoke the parent constructors prior to performing the class initializations. 5. After the shadow object is successfully created, create_shadow(void)
2 As an example, the classes Simulator, Node, Link, or rtObject, are classes that are not derived from the class TclObject. Objects in these classes are not, therefore, TclObjects. However, a Simulator, Node, Link, or route Object is also instantiated using the new procedure in ns.
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(a) adds the new object to hash table of TclObjects described earlier (Section 3.3.5). (b) makes cmd{} an instance procedure of the newly created interpreted object. This instance procedure invokes the command() method of the compiled object. In a later subsection (Section 3.4.4), we describe how the command method is dened, and invoked. Note that all of the above shadowing mechanisms only work when the user creates a new TclObject through the interpreter. It will not work if the programmer creates a compiled TclObject unilaterally. Therefore, the programmer is enjoined not to use the C++ new method to create compiled objects directly.
Deletion of TclObjects The delete operation destroys the interpreted object, and the corresponding shadow object. For example, use-scheduler{ scheduler } uses the delete procedure to remove the default list scheduler, and instantiate an alternate scheduler in its place. Simulator instproc use-scheduler type { $self instvar scheduler_ delete scheduler_ set scheduler_ [new Scheduler/$type] } As with the constructor, the object destructor must call the destructor for the parent class explicitly as the very last statement of the destructor. The TclObject destructor will invoke the instance procedure delete-shadow, that in turn invokes the equivalent compiled method to destroy the shadow object. The interpreter itself will destroy the interpreted object. ;# rst delete the existing list scheduler
3.4.2 Variable Bindings
In most cases, access to compiled member variables is restricted to compiled code, and access to interpreted member variables is likewise conned to access via interpreted code; however, it is possible to establish bi-directional bindings such that both the interpreted member variable and the compiled member variable access the same data, and changing the value of either variable changes the value of the corresponding paired variable to same value. The binding is established by the compiled constructor when that object is instantiated; it is automatically accessible by the interpreted object as an instance variable. ns supports ve different data types: reals, bandwidth valued variables, time valued variables, integers, and booleans. The syntax of how these values can be specied in OTcl is different for each variable type.
Real and Integer valued variables are specied in the normal form. For example, $object set realvar 1.2e3 $object set intvar 12
Bandwidth is specied as a real value, optionally sufxed by a k or K to mean kilo-quantities, or m or M to mean mega-quantities. A nal optional sufx of B indicates that the quantity expressed is in Bytes per second. The default is bandwidth expressed in bits per second. For example, all of the following are equivalent: $object set bwvar 1.5m $object set bwvar 1.5mb $object set bwvar 1500k 23
$object $object $object $object
set set set set
bwvar bwvar bwvar bwvar
1500kb .1875MB 187.5kB 1.5e6
Time is specied as a real value, optionally sufxed by a m to express time in milli-seconds, n to express time in nano-seconds, or p to express time in pico-seconds. The default is time expressed in seconds. For example, all of the following are equivalent: $object $object $object $object set set set set timevar timevar timevar timevar 1500m 1.5 1.5e9n 1500e9p
Note that we can also safely add a to reect the time unit of seconds. ns will ignore anything other than a valid real number specication, or a trailing m, n, or p.
Booleans can be expressed either as an integer, or as T or t for true. Subsequent characters after the rst letter are ignored. If the value is neither an integer, nor a true value, then it is assumed to be false. For example, $object set boolvar t $object set boolvar true $object set boolvar 1 $object set boolvar false $object set boolvar junk $object set boolvar 0 ;# set to true ;# or any non-zero value ;# set to false
The following example shows the constructor for the ASRMAgent 3 . ASRMAgent::ASRMAgent() { bind("pdistance_", &pdistance_); bind("requestor_", &requestor_); bind_time("lastSent_", &lastSessSent_); bind_bw("ctrlLimit_", &ctrlBWLimit_); bind_bool("running_", &running_); }
/* real variable */ /* integer variable */ /* time variable */ /* bandwidth variable */ /* boolean variable */
Note that all of the functions above take two arguments, the name of an OTcl variable, and the address of the corresponding compiled member variable that is linked. While it is often the case that these bindings are established by the constructor of the object, it need not always be done in this manner. We will discuss such alternate methods when we describe the class InstVar (Section 3.8) in detail later. Each of the variables that is bound is automatically initialised with default values when the object is created. The default values are specied as interpreted class variables. This initialisation is done by the routing init-instvar{}, invoked by methods in the class Instvar, described later (Section 3.8). init-instvar{} checks the class of the interpreted object, and all of the parent class of that object, to nd the rst class in which the variable is dened. It uses the value of the variable in that class to initialise the object. Most of the bind initialisation values are dened in ~ns/tcl/lib/ns-default.tcl. For example, if the following class variables are dened for the ASRMAgent:
3 Note
that this constructor is embellished to illustrate the features of the variable binding mechanism.
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Agent/SRM/Adaptive set pdistance_ 15.0 Agent/SRM set pdistance_ 10.0 Agent/SRM set lastSent_ 8.345m Agent set ctrlLimit_ 1.44M Agent/SRM/Adaptive set running_ f Therefore, every new Agent/SRM/Adaptive object will have pdistance_ set to 15.0; lastSent_ is set to 8.345m from the setting of the class variable of the parent class; ctrlLimit_ is set to 1.44M using the class variable of the parent class twice removed; running is set to false; the instance variable pdistance_ is not initialised, because no class variable exists in any of the class hierarchy of the interpreted object. In such instance, init-instvar{} will invoke warn-instvar{}, to print out a warning about such a variable. The user can selectively override this procedure in their simulation scripts, to elide this warning. Note that the actual binding is done by instantiating objects in the class InstVar. Each object in the class InstVar binds one compiled member variable to one interpreted member variable. A TclObject stores a list of InstVar objects corresponding to each of its member variable that is bound in this fashion. The head of this list is stored in its member variable instvar_ of the TclObject. One last point to consider is that ns will guarantee that the actual values of the variable, both in the interpreted object and the compiled object, will be identical at all times. However, if there are methods and other variables of the compiled object that track the value of this variable, they must be explicitly invoked or changed whenever the value of this variable is changed. This usually requires additional primitives that the user should invoke. One way of providing such primitives in ns is through the command() method described in the next section.
3.4.3 Variable Tracing
In addition to variable bindings, TclObject also supports tracing of both C++ and Tcl instance variables. A traced variable can be created and congured either in C++ or Tcl. To establish variable tracing at the Tcl level, the variable must be visible in Tcl, which means that it must be a bounded C++/Tcl or a pure Tcl instance variable. In addition, the object that owns the traced variable is also required to establish tracing using the Tcl trace method of TclObject. The rst argument to the trace method must be the name of the variable. The optional second argument species the trace object that is responsible for tracing that variable. If the trace object is not specied, the object that own the variable is responsible for tracing it. For a TclObject to trace variables, it must extend the C++ trace method that is virtually dened in TclObject. The Trace class implements a simple trace method, thereby, it can act as a generic tracer for variables.
class Trace : public Connector { ... virtual void trace(TracedVar*); };
Below is a simple example for setting up variable tracing in Tcl:
# \$tcp tracing its own variable cwnd_ \$tcp trace cwnd_ # the variable ssthresh_ of \$tcp is traced by a generic \$tracer set tracer [new Trace/Var] \$tcp trace ssthresh_ \$tracer
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For a C++ variable to be traceable, it must belong to a class that derives from TracedVar. The virtual base class TracedVar keeps track of the variables name, owner, and tracer. Classes that derives from TracedVar must implement the virtual method value, that takes a character buffer as an argument and writes the value of the variable into that buffer.
class TracedVar { ... virtual char* value(char* buf) = 0; protected: TracedVar(const char* name); const char* name_; // name of the variable TclObject* owner_; // the object that owns this variable TclObject* tracer_; // callback when the variable is changed ... };
The TclCL library exports two classes of TracedVar: TracedInt and TracedDouble. These classes can be used in place of the basic type int and double respectively. Both TracedInt and TracedDouble overload all the operators that can change the value of the variable such as assignment, increment, and decrement. These overloaded operators use the assign method to assign the new value to the variable and call the tracer if the new value is different from the old one. TracedInt and TracedDouble also implement their value methods that output the value of the variable into string. The width and precision of the output can be pre-specied.
3.4.4 command Methods: Denition and Invocation
For every TclObject that is created, ns establishes the instance procedure, cmd{}, as a hook to executing methods through the compiled shadow object. The procedure cmd{} invokes the method command() of the shadow object automatically, passing the arguments to cmd{} as an argument vector to the command() method. The user can invoke the cmd{} method in one of two ways: by explicitly invoking the procedure, specifying the desired operation as the rst argument, or implicitly, as if there were an instance procedure of the same name as the desired operation. Most simulation scripts will use the latter form, hence, we will describe that mode of invocation rst. Consider the that the distance computation in SRM is done by the compiled object; however, it is often used by the interpreted object. It is usually invoked as: $srmObject distance? agentAddress If there is no instance procedure called distance?, the interpreter will invoke the instance procedure unknown{}, dened in the base class TclObject. The unknown procedure then invokes $srmObject cmd distance? agentAddress to execute the operation through the compiled objects command() procedure. Ofcourse, the user could explicitly invoke the operation directly. One reason for this might be to overload the operation by using an instance procedure of the same name. For example, Agent/SRM/Adaptive instproc distance? addr { 26
$self instvar distanceCache_ if ![info exists distanceCache_($addr)] { set distanceCache_($addr) [$self cmd distance? $addr] } set distanceCache_($addr) } We now illustrate how the command() method using ASRMAgent::command() as an example. int ASRMAgent::command(int argc, const char*const*argv) { Tcl& tcl = Tcl::instance(); if (argc == 3) { if (strcmp(argv[1], "distance?") == 0) { int sender = atoi(argv[2]); SRMinfo* sp = get_state(sender); tcl.tesultf("%f", sp->distance_); return TCL_OK; } } return (SRMAgent::command(argc, argv)); } We can make the following observations from this piece of code:
The function is called with two arguments: The rst argument (argc) indicates the number of arguments specied in the command line to the interpreter. The command line arguments vector (argv) consists of argv[0] contains the name of the method, cmd. argv[1] species the desired operation. If the user specied any arguments, then they are placed in argv[2...(argc - 1)]. The arguments are passed as strings; they must be converted to the appropriate data type.
If the operation is successfully matched, the match should return the result of the operation using methods described earlier (Section 3.3.3). command() itself must return either TCL_OK or TCL_ERROR to indicate success or failure as its return code. If the operation is not matched in this method, it must invoke its parents command method, and return the corresponding result. This permits the user to concieve of operations as having the same inheritance properties as instance procedures or compiled methods. In the event that this command method is dened for a class with multiple inheritance, the programmer has the liberty to choose one of two implementations: 1) Either they can invoke one of the parents command method, and return the result of that invocation, or 2) They can each of the parents command methods in some sequence, and return the result of the rst invocation that is successful. If none of them are successful, then they should return an error.
In our document, we call operations executed through the command() instproc-likes. This reects the usage of these operations as if they were OTcl instance procedures of an object, but can be very subtly different in their realisation and usage. 27
3.5 Class TclClass
This compiled class (class TclClass) is a pure virtual class. Classes derived from this base class provide two functions: construct the interpreted class hierarchy to mirror the compiled class hierarchy; and provide methods to instantiate new TclObjects. Each such derived class is associated with a particular compiled class in the compiled class hierarchy, and can instantiate new objects in the associated class. As an example, consider a class such as the class RenoTcpClass. It is derived from class TclClass, and is associated with the class RenoTcpAgent. It will instantiate new objects in the class RenoTcpAgent. The compiled class hierarchy for RenoTcpAgent is that it derives from TcpAgent, that in turn derives from Agent, that in turn derives (roughly) from TclObject. RenoTcpClass is dened as static class RenoTcpClass: public TclClass { public: RenoTcpClass() : TclClass("Agent/TCP/Reno") {} TclObject* create(int argc, const char*const* argv) { return (new RenoTcpAgent()); } } class_reno; We can make the following observations from this denition: 1. The class denes only the constructor, and one additional method, to create instances of the associated TclObject. 2. ns will execute the RenoTcpClass constructor for the static variable class_reno, when it is rst started. This sets up the appropriate methods and the interpreted class hierarchy. 3. The constructor species the interpreted class explicitly as Agent/TCP/Reno. This also species the interpreted class hierarchy implicitly. Recall that the convention in ns is to use the character slash (/) is a separator. For any given class A/B/C/D, the class A/B/C/D is a sub-class of A/B/C, that is itself a sub-class of A/B, that, in turn, is a sub-class of A. A itself is a sub-class of TclObject. In our case above, the TclClass constructor creates three classes, Agent/TCP/Reno sub-class of Agent/TCP subclass of Agent sub-class of TclObject. 4. This class is associated with the class RenoTcpAgent; it creats new objects in this associated class. 5. The RenoTcpClass::create method returns TclObjects in the class RenoTcpAgent. 6. When the user species new Agent/TCP/Reno, the routine RenoTcpClass::create is invoked. 7. The arguments vector (argv) consists of argv[0] contains the name of the object. argv[1...3] contain $self, $class, and $proc.Since create is called through the instance procedure create-shadow, argv[3] contains create-shadow. argv[4] contain any additional arguments (passed as a string) provided by the user. The class Trace illustrates argument handling by TclClass methods. class TraceClass : public TclClass { public: 28
TraceClass() : TclClass("Trace") {} TclObject* create(int args, const char*const* argv) { if (args >= 5) return (new Trace(*argv[4])); else return NULL; } } trace_class; A new Trace object is created as new Trace "X" Finally, the nitty-gritty details of how the interpreted class hierarchy is constructed: 1. The object constructor is executed when ns rst starts. 2. This constructor calls the TclClass constructor with the name of the interpreted class as its argument. 3. The TclClass constructor stores the name of the class, and inserts this object into a linked list of the TclClass objects. 4. During initialization of the simulator, Tcl_AppInit(void) invokes TclClass::bind(void) 5. For each object in the list of TclClass objects, bind() invokes register{}, specifying the name of the interpreted class as its argument. 6. register{} establishes the class hierarchy, creating the classes that are required, and not yet created. 7. Finally, bind() denes instance procedures create-shadow and delete-shadow for this new class.
3.5.1 How to Bind Static C++ Class Member Variables
In Section 3.4, we have seen how to expose member variables of a C++ object into OTcl space. This, however, does not apply to static member variables of a C++ class. Of course, one may create an OTcl variable for the static member variable of every C++ object; obviously this defeats the whole meaning of static members. We cannot solve this binding problem using a similar solution as binding in TclObject, which is based on InstVar, because InstVars in TclCL require the presence of a TclObject. However, we can create a method of the corresponding TclClass and access static members of a C++ class through the methods of its corresponding TclClass. The procedure is as follows: 1. Create your own derived TclClass as described above; 2. Declare methods bind() and method() in your derived class; 3. Create your binding methods in the implementation of your bind() with add_method("your_method"), then implement the handler in method() in a similar way as you would do in TclObject::command(). Notice that the number of arguments passed to TclClass::method() are different from those passed to TclObject::command(). The former has two more arguments in the front. As an example, we show a simplied version of PacketHeaderClass in ~ns/packet.cc. Suppose we have the following class Packet which has a static variable hdrlen_ that we want to access from OTcl: 29
class Packet { ...... static int hdrlen_; }; Then we do the following to construct an accessor for this variable: class PacketHeaderClass : public TclClass { protected: PacketHeaderClass(const char* classname, int hdrsize); TclObject* create(int argc, const char*const* argv); /* These two implements OTcl class access methods */ virtual void bind(); virtual int method(int argc, const char*const* argv); }; void PacketHeaderClass::bind() { /* Call to base class bind() must precede add_method() */ TclClass::bind(); add_method("hdrlen"); } int PacketHeaderClass::method(int ac, const char*const* av) { Tcl& tcl = Tcl::instance(); /* Notice this argument translation; we can then handle them as if in TclObject::command() */ int argc = ac - 2; const char*const* argv = av + 2; if (argc == 2) { if (strcmp(argv[1], "hdrlen") == 0) { tcl.resultf("%d", Packet::hdrlen_); return (TCL_OK); } } else if (argc == 3) { if (strcmp(argv[1], "hdrlen") == 0) { Packet::hdrlen_ = atoi(argv[2]); return (TCL_OK); } } return TclClass::method(ac, av); } After this, we can then use the following OTcl command to access and change values of Packet::hdrlen_: PacketHeader hdrlen 120 set i [PacketHeader hdrlen]
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3.6 Class TclCommand
This class (class TclCommand) provides just the mechanism for ns to export simple commands to the interpreter, that can then be executed within a global context by the interpreter. There are two functions dened in ~ns/misc.cc: ns-random and ns-version. These two functions are initialized by the function init_misc(void), dened in ~ns/misc.cc; init_misc is invoked by Tcl_AppInit(void) during startup.
class VersionCommand denes the command ns-version. It takes no argument, and returns the current ns version string. % ns-version 2.0a12 ;# get the current version
class RandomCommand denes the command ns-random. With no argument, ns-random returns an integer, uniformly distributed in the interval . When specied an argument, it takes that argument as the seed. If this seed value is 0, the command uses a heuristic seed value; otherwise, it sets the seed for the random number generator to the specied value. % ns-random 2078917053 % ns-random 0 858190129 % ns-random 23786 23786 ;# return a random number ;#set the seed heuristically ;#set seed to specied value
Note that, it is generally not advisable to construct top-level commands that are available to the user. We now describe how to dene a new command using the example class say_hello. The example denes the command hi, to print the string hello world, followed by any command line arguments specied by the user. For example, % hi this is ns [ns-version] hello world, this is ns 2.0a12 1. The command must be dened within a class derived from the class TclCommand. The class denition is: class say_hello : public TclCommand { public: say_hello(); int command(int argc, const char*const* argv); };
2. The constructor for the class must invoke the TclCommand constructor with the command as argument; i.e., say_hello() : TclCommand("hi") {}
The TclCommand constructor sets up "hi" as a global procedure that invokes TclCommand::dispatch_cmd(). 3. The method command() must perform the desired action. The method is passed two arguments. The rst argument, argc, contains the number of actual arguments passed by the user. 31
The actual arguments passed by the user are passed as an argument vector (argv) and contains the following: argv[0] contains the name of the command (hi). argv[1...(argc - 1)] contains additional arguments specied on the command line by the user. command() is invoked by dispatch_cmd(). #include <streams.h> /* because we are using stream I/O */
int say_hello::command(int argc, const char*const* argv) { cout << "hello world:"; for (int i = 1; i < argc; i++) cout << << argv[i]; cout << n; return TCL_OK; } 4. Finally, we require an instance of this class. TclCommand instances are created in the routine init_misc(void). new say_hello;
Note that there used to be more functions such as ns-at and ns-now that were accessible in this manner. Most of these functions have been subsumed into existing classes. In particular, ns-at and ns-now are accessible through the scheduler TclObject. These functions are dened in ~ns/tcl/lib/ns-lib.tcl. % set ns [new Simulator] _o1 % $ns now 0 % $ns at ... ... ;# get new instance of simulator ;# query simulator for current time ;# specify at operations for simulator
3.7 Class EmbeddedTcl
ns permits the development of functionality in either compiled code, or through interpreter code, that is evaluated at initialization. For example, the scripts ~tclcl/tcl-object.tcl or the scripts in ~ns/tcl/lib. Such loading and evaluation of scripts is done through objects in the class EmbeddedTcl. The easiest way to extend ns is to add OTcl code to either ~tclcl/tcl-object.tcl or through scripts in the ~ns/tcl/lib directory. Note that, in the latter case, ns sources ~ns/tcl/lib/ns-lib.tcl automatically, and hence the programmer must add a couple of lines to this le so that their script will also get automatically sourced by ns at startup. As an example, the le ~ns/tcl/mcast/srm.tcl denes some of the instance procedures to run SRM. In ~ns/tcl/lib/ns-lib.tcl, we have the lines: source tcl/mcast/srm.tcl to automatically get srm.tcl sourced by ns at startup. Three points to note with EmbeddedTcl code are that rstly, if the code has an error that is caught during the eval, then ns will not run. Secondly, the user can explicitly override any of the code in the scripts. In particular, they can re-source the entire 32
script after making their own changes. Finally, after adding the scripts to ~ns/tcl/lib/ns-lib.tcl, and every time thereafter that they change their script, the user must recompile ns for their changes to take effect. Of course, in most cases 4 , the user can source their script to override the embedded code. The rest of this subsection illustrate how to integrate individual scripts directly into ns. The rst step is convert the script into an EmbeddedTcl object. The lines below expand ns-lib.tcl and create the EmbeddedTcl object instance called et_ns_lib: tclsh bin/tcl-expand.tcl tcl/lib/ns-lib.tcl | ../Tcl/tcl2c++ et_ns_lib > gen/ns_tcl.cc The script, ~ns/bin/tcl-expand.tcl expands ns-lib.tcl by replacing all source lines with the corresponding source les. The program, ~tclcl/tcl2cc.c, converts the OTcl code into an equivalent EmbeddedTcl object, et_ns_lib. During initialization, invoking the method EmbeddedTcl::load explicitly evaluates the array. ~tclcl/tcl-object.tcl is evaluated by the method Tcl::init(void); Tcl_AppInit() invokes Tcl::Init(). The exact command syntax for the load is: et_tclobject.load();
Similarly, ~ns/tcl/lib/ns-lib.tcl is evaluated directly by Tcl_AppInit in ~ns/ns_tclsh.cc. et_ns_lib.load();
3.8 Class InstVar
This section describes the internals of the class InstVar. This class denes the methods and mechanisms to bind a C++ member variable in the compiled shadow object to a specied OTcl instance variable in the equivalent interpreted object. The binding is set up such that the value of the variable can be set or accessed either from within the interpreter, or from within the compiled code at all times. There are ve instance variable classes: class InstVarReal, class InstVarTime, class InstVarBandwidth, class InstVarInt, and class InstVarBool, corresponding to bindings for real, time, bandwidth, integer, and boolean valued variables respectively. We now describe the mechanism by which instance variables are set up. We use the class InstVarReal to illustrate the concept. However, this mechanism is applicable to all ve types of instance variables. When setting up an interpreted variable to access a member variable, the member functions of the class InstVar assume that they are executing in the appropriate method execution context; therefore, they do not query the interpreter to determine the context in which this variable must exist. In order to guarantee the correct method execution context, a variable must only be bound if its class is already established within the interpreter, and the interpreter is currently operating on an object in that class. Note that the former requires that when a method in a given class is going to make its variables accessible via the interpreter, there must be an associated
4 The few places where this might not work are when certain variables might have to be dened or undened, or otherwise the script contains code other than procedure and variable denitions and executes actions directly that might not be reversible.
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class TclClass (Section 3.5) dened that identies the appropriate class hierarchy to the interpreter. The appropriate method execution context can therefore be created in one of two ways. An implicit solution occurs whenever a new TclObject is created within the interpreter. This sets up the method execution context within the interpreter. When the compiled shadow object of the interpreted TclObject is created, the constructor for that compiled object can bind its member variables of that object to interpreted instance variables in the context of the newly created interpreted object. An explicit solution is to dene a bind-variables operation within a command function, that can then be invoked via the cmd method. The correct method execution context is established in order to execute the cmd method. Likewise, the compiled code is now operating on the appropriate shadow object, and can therefore safely bind the required member variables. An instance variable is created by specifying the name of the interpreted variable, and the address of the member variable in the compiled object. The constructor for the base class InstVar creates an instance of the variable in the interpreter, and then sets up a trap routine to catch all accesses to the variable through the interpreter. Whenever the variable is read through the interpreter, the trap routine is invoked just prior to the occurrence of the read. The routine invokes the appropriate get function that returns the current value of the variable. This value is then used to set the value of the interpreted variable that is then read by the interpreter. Likewise, whenever the variable is set through the interpreter, the trap routine is invoked just after to the write is completed. The routine gets the current value set by the interpreter, and invokes the appropriate set function that sets the value of the compiled member to the current value set within the interpreter.
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Part II
Simulator Basics
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Chapter 4
The Class Simulator
The overall simulator is described by a Tcl class Simulator. It provides a set of interfaces for conguring a simulation and for choosing the type of event scheduler used to drive the simulation. A simulation script generally begins by creating an instance of this class and calling various methods to create nodes, topologies, and congure other aspects of the simulation. A subclass of Simulator called OldSim is used to support ns v1 backward compatibility. The procedures and functions described in this chapter can be found in ~ns/tcl/lib/ns-lib.tcl, ~ns/scheduler.{cc,h}, and, ~ns/heap.h.
4.1 Simulator Initialization
When a new simulation object is created in tcl, the initialization procedure performs the following operations:
initialize the packet format (calls create_packetformat) create a scheduler (defaults to a calendar scheduler) create a null agent (a discard sink used in various places)
The packet format initialization sets up eld offsets within packets used by the entire simulation. It is described in more detail in the following chapter on packets (Chapter 11). The scheduler runs the simulation in an event-driven manner and may be replaced by alternative schedulers which provide somewhat different semantics (see the following section for more detail). The null agent is created with the following call: set nullAgent_ [new Agent/Null] This agent is generally useful as a sink for dropped packets or as a destination for packets that are not counted or recorded.
4.2 Schedulers and Events
The simulator is an event-driven simulator. There are presently four schedulers available in the simulator, each of which is implemented using a different data structure: a simple linked-list, heap, calendar queue (default), and a special type 36
called real-time. Each of these are described below. The scheduler runs by selecting the next earliest event, executing it to completion, and returning to execute the next event. Presently, the simulator is single-threaded, and only one event in execution at any given time. If more than one event are scheduled to execute at the same time, their execution is performed on the rst scheduled rst dispatched manner. Simultaneous events are not re-ordered anymore by schedulers (as it was in earlier versions) and all schedulers should yeild the same order of dispatching given the same input. No partial execution of events or pre-emption is supported. An event generally comprises a ring time and a handler function. The actual denition of an event is found in ~ns/scheduler.h: class Event { public: Event* next_; /* event list */ Handler* handler_; /* handler to call when event ready */ double time_; /* time at which event is ready */ int uid_; /* unique ID */ Event() : time_(0), uid_(0) {} }; /* * The base class for all event handlers. When an events scheduled * time arrives, it is passed to handle which must consume it. * i.e., if it needs to be freed it, it must be freed by the handler. */ class Handler { public: virtual void handle(Event* event); }; Two types of objects are derived from the base class Event: packets and at-events. Packets are described in detail in the next chapter (Chapter 11.2.1). An at-event is a tcl procedure execution scheduled to occur at a particular time. This is frequently used in simulation scripts. A simple example of how it is used is as follows: ... set ns_ [new Simulator] $ns_ use-scheduler Heap $ns_ at 300.5 "$self complete_sim" ... This tcl code fragment rst creates a simulation object, then changes the default scheduler implementation to be heap-based (see below), and nally schedules the function $self complete_sim to be executed at time 300.5 (seconds)(Note that this particular code fragment expects to be encapsulated in an object instance procedure, where the appropriate reference to $self is correctly dened.). At-events are implemented as events where the handler is effectively an execution of the tcl interpreter.
4.2.1 The List Scheduler
The list scheduler (class Scheduler/List) implements the scheduler using a simple linked-list structure. The list is kept in time-order (earliest to latest), so event insertion and deletion require scanning the list to nd the appropriate entry. Choosing the next event for execution requires trimming the rst entry off the head of the list. This implementation preserves event execution in a FIFO manner for simultaneous events. 37
4.2.2 the heap scheduler
The heap scheduler (class Scheduler/Heap) implements the scheduler using a heap structure. This structure is superior to the list structure for a large number of events, as insertion and deletion times are in for events. This implementation in ns v2 is borrowed from the MaRS-2.0 simulator [1]; it is believed that MaRS itself borrowed the code from NetSim [11], although this lineage has not been completely veried.
4.2.3 The Calendar Queue Scheduler
The calendar queue scheduler (class Scheduler/Calendar) uses a data structure analogous to a one-year desk calendar, in which events on the same month/day of multiple years can be recorded in one day. It is formally described in [6], and informally described in Jain (p. 410) [12]. The implementation of Calendar queues in ns v2 was contributed by David Wetherall (presently at MIT/LCS).
4.2.4 The Real-Time Scheduler
The real-time scheduler (class Scheduler/RealTime) attempts to synchronize the execution of events with real-time. It is currently implemented as a subclass of the list scheduler. The real-time capability in ns is still under development, but is used to introduce an ns simulated network into a real-world topology to experiment with easily-congured network topologies, cross-trafc, etc. This only works for relatively slow network trafc data rates, as the simulator must be able to keep pace with the real-world packet arrival rate, and this synchronization is not presently enforced.
4.3 Other Methods
The Simulator class provides a number of methods used to set up the simulation. They generally fall into three categories: methods to create and manage the topology (which in turn consists of managing the nodes (Chapter 5) and managing t he links (Chapter 6)), methods to perform tracing (Chapter 21), and helper functions to deal with the scheduler. The following is a list of the non-topology related simulator methods: Simulator Simulator Simulator Simulator Simulator Simulator Simulator Simulator instproc instproc instproc instproc instproc instproc instproc instproc now ;# return schedulers notion of current time at args ;# schedule execution of code at specied time cancel args ;# cancel event run args ;# start scheduler halt ;# stop (pause) the scheduler flush-trace ;# ush all trace object write buffers create-trace type files src dst ;# create trace object create_packetformat ;# set up the simulators packet format
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4.4 Commands at a glance
Synopsis: ns <otclfile> <arg> <arg>.. Description: Basic command to run a simulation script in ns. The simulator (ns) is invoked via the ns interpreter, an extension of the vanilla otclsh command shell. A simulation is defined by a OTcl script (file). Several examples of OTcl scripts can be found under ns/tcl/ex directory.
The following is a list of simulator commands commonly used in simulation scripts: set ns_ [new Simulator] This command creates an instance of the simulator object.
set now [$ns_ now] The scheduler keeps track of time in a simulation. This returns schedulers notion of current time.
$ns_ halt This stops or pauses the scheduler.
$ns_ run This starts the scheduler.
$ns_ at <time> <event> This schedules an <event> (which is normally a piece of code) to be executed at the specified <time>. e.g $ns_ at $opt(stop) "puts S EXITING.. ; $ns_ halt" N or, $ns_ at 10.0 "$ftp start"
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$ns_ cancel <event> Cancels the event. In effect, event is removed from schedulers list of ready to run events.
$ns_ create-trace <type> <file> <src> <dst> <optional arg: op> This creates a trace-object of type <type> between <src> and <dst> objects and attaches trace-object to <file> for writing trace-outputs. If op is defined as "nam", this creates nam tracefiles; otherwise if op is not defined, ns tracefiles are created on default.
$ns_
flush-trace
Flushes all trace object write buffers.
$ns_ gen-map This dumps information like nodes, node components, links etc created for a given simulation. This may be broken for some scenarios (like wireless).
$ns_ at-now <args> This is in effect like command "$ns_ at $now $args". Note that this function may not work because of tcls string number resolution.
These are additional simulator (internal) helper functions (normally used for developing/changing the ns core code) : $ns_ use-scheduler <type> Used to specify the type of scheduler to be used for simulation. The different types of scheduler available are List, Calendar, Heap and RealTime. Currently Calendar is used as default.
$ns_ after <delay> <event> Scheduling an <event> to be executed after the lapse of time <delay>.
$ns_ clearMemTrace Used for memory debugging purposes.
$ns_ is-started This returns true if simulator has started to run and false if not.
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$ns_ dumpq Command for dumping events queued in scheduler while scheduler is halted.
$ns_ create_packetformat This sets up simulators packet format.
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Chapter 5
Nodes and Packet Forwarding
This chapter describes one aspect of creating a topology in ns, i.e., creating the nodes. In the next chapter (Chapter 6), we will describe second aspect of creating the topology, i.e., connecting the nodes to form links. This chapter does not describe the detailed internal organization of a node (although some schematics are given), nor the interaction between a node and its routing modules; please refer to Chapter ?? for detailed discussion. Recall that each simulation requires a single instance of the class Simulator to control and operate that simulation. The class provides instance procedures to create and manage the topology, and internally stores references to each element of the topology. We begin by describing the procedures in the class Simulator (Section 5.1). We then describe the instance procedures in the class Node (Section 5.2) to access and operate on individual nodes. We conclude with detailed descriptions of the Classier (Section 5.4) from which the more complex node objects are formed. The procedures and functions described in this chapter can be found in ~ns/tcl/lib/ns-lib.tcl, ~ns/tcl/lib/ns-nodes.tcl, ~ns/tcl/lib/ns-rtmodule.tcl, ~ns/rtmodule.{cc,h}, ~ns/classier.{cc, h}, ~ns/classier-addr.cc, ~ns/classier-mcast.cc, ~ns/classiermpath.cc, and, ~ns/replicator.cc.
5.1 Simulator Methods: Creating the Topology
The basic primitive for acquiring a node is set ns [new Simulator] $ns node The instance procedure node constructs a node out of more simple classier objects (Section 5.4). The Node itself is a standalone class in OTcl. However, most of the components of the node are themselves TclObjects. The typical structure of a node is as shown in Figure 5.1. Before ns release 2.1b6, the address of an agent in a node is 16 bits wide: the higher 8 bits dene the node id_, the lower 8 bits identify the individual agent at the node. This limits the number of nodes in a simulation to 256 nodes. If the user needs to create a topology larger than 256 nodes, then they should rst expand the address space before creating any nodes, as $ns set-address-format expanded
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NODE
Port Classifier
Agent
Agent
dmux_ Addr Classifier Node entry entry_ classifier_ agents_
Agent
Link
Link
Link
Figure 5.1: Structure of an Unicast Node. Notice that entry_ is simply a label variable instead of a real object, e.g., the classier_. This expands the address space to 30 bits, the higher 22 of which are used to assign node numbers. Note: Starting from release 2.1b6, the above is no longer necessary; ns uses 32-bit integers for both address and port. Therefore, the above restriction of 256 nodes is no longer applicable. By default, nodes in ns are constructed for unicast simulations. In order to enable multicast simulation, the simulation should be created with an option -multicast on, e.g.: set ns [new Simulator -multicast on] The internal structure of a typical multicast node is shown in Figure 5.2. When a simulation uses multicast routing, the highest bit of the address indicates whether the particular address is a multicast address or an unicast address. If the bit is 0, the address represents a unicast address, else the address represents a multicast address. This implies that, by default, a multicast simulation is restricted to 128 nodes.
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MULTICAST NODE
dmux_
Agent
classifier_ Agent
Agent agents_ Node entry entry_
switch_
<S1,G1>
Replicators
Multicast Classifier
multiclassifier_
<S2,G2>
Link
Figure 5.2: Internal Structure of a Multicast Node.
Link
Link
5.2 Node Methods: Conguring the Node
Procedures to congure an individual node can be classied into: Control functions Address and Port number management, unicast routing functions Agent management Adding neighbors We describe each of the functions in the following paragraphs.
Control functions 1. $node entry returns the entry point for a node. This is the rst element which will handle packets arriving at that node. 44
The Node instance variable, entry_, stores the reference this element. For unicast nodes, this is the address classier that looks at the higher bits of the destination address. The instance variable, classifier_ contains the reference to this classier. However, for multicast nodes, the entry point is the switch_ which looks at the rst bit to decide whether it should forward the packet to the unicast classier, or the multicast classier as appropriate. 2. $node reset will reset all agents at the node.
Address and Port number management The procedure $node id returns the node number of the node. This number is automatically incremented and assigned to each node at creation by the class Simulator method, $ns node.The class Simulator also stores an instance variable array 1 , Node_, indexed by the node id, and contains a reference to the node with that id. The procedure $node agent port returns the handle of the agent at the specied port. If no agent at the specied port number is available, the procedure returns the null string. The procedure alloc-port returns the next available port number. It uses an instance variable, np_, to track the next unallocated port number. The procedures, add-route and add-routes, are used by unicast routing (Chapter 23) to add routes to populate the classifier_ The usage syntax is $node add-route destination id TclObject . TclObject is the entry of dmux_, the port demultiplexer at the node, if the destination id is the same as this nodes id, it is often the head of a link to send packets for that destination to, but could also be the the entry for other classiers or types of classiers. $node add-routes destination id TclObjects is used to add multiple routes to the same destination that must be used simultaneously in round robin manner to spread the bandwidth used to reach that destination across all links equally. It is used only if the instance variable multiPath_ is set to 1, and detailed dynamic routing strategies are in effect, and requires the use of a multiPath classier. We describe the implementation of the multiPath classier later in this chapter (Section 5.4); however, we defer the discussion of multipath routing (Chapter 23) to the chapter on unicast routing. The dual of add-routes{} is delete-routes{}. It takes the id, a list of TclObjects, and a reference to the simulators nullagent. It removes the TclObjects in the list from the installed routes in the multipath classier. If the route entry in the classier does not point to a multipath classier, the routine simply clears the entry from classifier_, and installs the nullagent in its place. Detailed dynamic routing also uses two additional methods: the instance procedure init-routing{} sets the instance variable multiPath_ to be equal to the class variable of the same name. It also adds a reference to the route controller object at that node in the instance variable, rtObject_. The procedure rtObject?{} returns the handle for the route object at the node. Finally, the procedure intf-changed{} is invoked by the network dynamics code if a link incident on the node changes state. Additional details on how this procedure is used are discussed later in the chapter on network dynamics (Chapter 25).
Agent management Given an agent , the procedure attach{} will add the agent to its list of agents_, assign a port number the agent and set its source address, set the target of the agent to be its (i.e., the nodes) entry{}, and add a pointer to the port demultiplexer at the node (dmux_) to the agent at the corresponding slot in the dmux_ classier. Conversely, detach{}will remove the agent from agents_, and point the agents target, and the entry in the node dmux_ to nullagent.
1 i.e.,
an instance variable of a class that is also an array variable
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Tracking Neighbors Each node keeps a list of its adjacent neighbors in its instance variable, neighbor_. The procedure add-neighbor{} adds a neighbor to the list. The procedure neighbors{} returns this list.
5.3 Conguring Node Functionality
Simulator::node-config{} accomodates exible and modular construction of different node denitions within the same base Node class. For instance, to create a mobile node capable of wireless communication, one no longer needs a specialized node creation command, e.g., dsdv-create-mobile-node{}; instead, one simply say node-config -adhocRouting dsdv{}. Together with routing modules, this allows one to combine arbitrary routing functionalities within a single node without resorting to multiple inheritance and other fancy object gimmicks. We will describe this in more detail in Chapter ??. This section describes node conguration interfaces and compare them with the old ones. The functions and procedures relevant to the new node APIs may be found in ~ns/tcl/lib/ns-node.tcl. NOTE: This API, especially its internal implementation which is messy at this point, is still a moving target. It may undergo signicant changes in the near future. However, we will do our best to maintain the same interface as described in this chapter. In addition, this API currently does not cover all existing nodes in the old format, namely, nodes built using inheritance. In particular, satellite simulation and part of mobile IP are not covered. [Sep 15, 2000].
5.3.1 Node Conguration Interface
The node conguration interface consists of two parts. The rst part deals with node conguration, while the second part actually creates nodes of the specied type. We have already seen the latter in Section 5.1, in this section we will describe the conguration part. Node conguration essentially consists of dening the different node characteristics before creating them. They may consist of the type of addressing structure used in the simulation, dening the network components for mobilenodes, turning on or off the trace options at Agent/Router/MAC levels, selecting the type of adhoc routing protocol for wirelessnodes or dening their energy model. The node conguration API in its entirety looks as the following: OPTION_TYPE ------------$ns_ node-config -addressingType -adhocRouting -llType -macType -propType -ifqType -ifqLen -phyType -antType -channelType -topoInstance -wiredRouting -mobileIP -energyModel -initialEnergy AVAILABLE OPTION_VALUES -------------------------flat or hierarchical or expanded DSDV or DSR or TORA or AODV LL Mac/802_11 Propagation/TwoRayGround Queue/DropTail/PriQueue 50 Phy/WirelessPhy Antenna/OmniAntenna Channel/WirelessChannel $topo_instance ON or OFF ON or OFF EnergyModel (in Joules) 46
-rxPower -txPower -agentTrace -routerTrace -macTrace -movementTrace -reset
(in W) (in W) ON or OFF ON or OFF ON or OFF ON or OFF
The default values for all the above options are NULL except -addressingType whose default value is at. The option -reset can be used to reset all node-cong parameters to their default value. Thus node-conguration for a wireless, mobile node that runs AODV as its adhoc routing protocol in a hierarchical topology would be as shown below. We decide to turn tracing on at the agent and router level only. Also we assume a topology has been instantiated with "set topo [new Topography]". The node-cong command would look like the following: $ns_ node-config -addressingType hierarchical -adhocRouting AODV -llType LL -macType Mac/802_11 -ifqType Queue/DropTail/PriQueue -ifqLen 50 -antType Antenna/OmniAntenna -propType Propagation/TwoRayGround -phyType Phy/WirelessPhy -topoInstance $topo -channelType Channel/WirelessChannel -agentTrace ON -routerTrace ON -macTrace OFF -movementTrace OFF Note that the cong command can be broken down into separate lines like $ns_ node-config -addressingType hier $ns_ node-config -macTrace ON The options that need to be changed may only be called. For example after conguring for AODV mobilenodes as shown above (and after creating AODV mobilenodes), we may congure for AODV base-station nodes in the following way: $ns_ node-config -wiredRouting ON While all other features for base-station nodes and mobilenodes are same, the base-station nodes are capable of wired routing, while mobilenodes are not. In this way we can change node-conguration only when it is required. All node instances created after a given node-conguration command will have the same property unless a part or all of the node-cong command is executed with different parameter values. And all parameter values remain unchanged unless they are expicitly changed. So after creation of the AODV base-station and mobilenodes, if we want to create simple nodes, we will use the following node-conguration command: $ns_ node-config -reset 47
This will set all parameter values to their default setting which basically denes conguration of a simple node.
5.4 The Classier
The function of a node when it receives a packet is to examine the packets elds, usually its destination address, and on occasion, its source address. It should then map the values to an outgoing interface object that is the next downstream recipient of this packet. In ns, this task is performed by a simple classier object. Multiple classier objects, each looking at a specic portion of the packet forward the packet through the node. A node in ns uses many different types of classiers for different purposes. This section describes some of the more common, or simpler, classier objects in ns. We begin with a description of the base class in this section. The next subsections describe the address classier (Section 5.4.1), the multicast classier (Section 5.4.2), the multipath classier (Section 5.4.3), and nally, the replicator (Section 5.4.5). A classier provides a way to match a packet against some logical criteria and retrieve a reference to another simulation object based on the match results. Each classier contains a table of simulation objects indexed by slot number. The job of a classier is to determine the slot number associated with a received packet and forward that packet to the object referenced by that particular slot. The C++ class Classifier (dened in ~ns/classier.h) provides a base class from which other classiers are derived. class Classifier : public NsObject { public: ~Classifier(); void recv(Packet*, Handler* h = 0); protected: Classifier(); void install(int slot, NsObject*); void clear(int slot); virtual int command(int argc, const char*const* argv); virtual int classify(Packet *const) = 0; void alloc(int); NsObject** slot_; /* table that maps slot number to a NsObject */ int nslot_; int maxslot_; }; The classify() method is pure virtual, indicating the class Classifier is to be used only as a base class. The alloc() method dynamically allocates enough space in the table to hold the specied number of slots. The install() and clear() methods add or remove objects from the table. The recv() method and the OTcl interface are implemented as follows in ~ns/classier.cc: /* * objects only ever see "packet" events, which come either * from an incoming link or a local agent (i.e., packet source). */ void Classifier::recv(Packet* p, Handler*) { NsObject* node; 48
int cl = classify(p); if (cl < 0 || cl >= nslot_ || (node = slot_[cl]) == 0) { Tcl::instance().evalf("%s no-slot %d", name(), cl); Packet::free(p); return; } node->recv(p); } int Classifier::command(int argc, const char*const* argv) { Tcl& tcl = Tcl::instance(); if (argc == 3) { /* * $classifier clear $slot */ if (strcmp(argv[1], "clear") == 0) { int slot = atoi(argv[2]); clear(slot); return (TCL_OK); } /* * $classifier installNext $node */ if (strcmp(argv[1], "installNext") == 0) { int slot = maxslot_ + 1; NsObject* node = (NsObject*)TclObject::lookup(argv[2]); install(slot, node); tcl.resultf("%u", slot); return TCL_OK; } if (strcmp(argv[1], "slot") == 0) { int slot = atoi(argv[2]); if ((slot >= 0) || (slot < nslot_)) { tcl.resultf("%s", slot_[slot]->name()); return TCL_OK; } tcl.resultf("Classifier: no object at slot %d", slot); return (TCL_ERROR); } } else if (argc == 4) { /* * $classifier install $slot $node */ if (strcmp(argv[1], "install") == 0) { int slot = atoi(argv[2]); NsObject* node = (NsObject*)TclObject::lookup(argv[3]); install(slot, node); return (TCL_OK); } } return (NsObject::command(argc, argv)); }
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When a classier recv()s a packet, it hands it to the classify() method. This is dened differently in each type of classier derived from the base class. The usual format is for the classify() method to determine and return a slot index into the table of slots. If the index is valid, and points to a valid TclObject, the classier will hand the packet to that object using that objects recv() method. If the index is not valid, the classier will invoke the instance procedure no-slot{} to attempt to populate the table correctly. However, in the base class Classifier::no-slot{} prints and error message and terminates execution. The command() method provides the following instproc-likes to the interpreter:
clear{ slot } clears the entry in a particular slot. installNext{ object } installs the object in the next available slot, and returns the slot number. Note that this instproc-like is overloaded by an instance procedure of the same name that stores a reference to the object stored. This then helps quick query of the objects installed in the classier from OTcl.
slot{ index } returns the object stored in the specied slot. install{ index , object } installs the specied object at the slot index . Note that this instproc-like too is overloaded by an instance procedure of the same name that stores a reference to the object stored. This is also to quickly query of the objects installed in the classier from OTcl.
5.4.1 Address Classiers
An address classier is used in supporting unicast packet forwarding. It applies a bitwise shift and mask operation to a packets destination address to produce a slot number. The slot number is returned from the classify() method. The class AddressClassifier (dened in ~ns/classier-addr.cc) ide dened as follows: class AddressClassifier : public Classifier { public: AddressClassifier() : mask_(~0), shift_(0) { bind("mask_", (int*)&mask_); bind("shift_", &shift_); } protected: int classify(Packet *const p) { IPHeader *h = IPHeader::access(p->bits()); return ((h->dst() >> shift_) & mask_); } nsaddr_t mask_; int shift_; }; The class imposes no direct semantic meaning on a packets destination address eld. Rather, it returns some number of bits from the packets dst_ eld as the slot number used in the Classifier::recv() method. The mask_ and shift_ values are set through OTcl.
5.4.2 Multicast Classiers
The multicast classier classies packets according to both source and destination (group) addresses. It maintains a (chained hash) table mapping source/group pairs to slot numbers. When a packet arrives containing a source/group unknown to the 50
classier, it invokes an Otcl procedure Node::new-group{} to add an entry to its table. This OTcl procedure may use the method set-hash to add new (source, group, slot) 3-tuples to the classiers table. The multicast classier is dened in ~ns/classier-mcast.cc as follows: static class MCastClassifierClass : public TclClass { public: MCastClassifierClass() : TclClass("Classifier/Multicast") {} TclObject* create(int argc, const char*const* argv) { return (new MCastClassifier()); } } class_mcast_classifier; class MCastClassifier : public Classifier { public: MCastClassifier(); ~MCastClassifier(); protected: int command(int argc, const char*const* argv); int classify(Packet *const p); int findslot(); void set_hash(nsaddr_t src, nsaddr_t dst, int slot); int hash(nsaddr_t src, nsaddr_t dst) const { u_int32_t s = src ^ dst; s ^= s >> 16; s ^= s >> 8; return (s & 0xff); } struct hashnode { int slot; nsaddr_t src; nsaddr_t dst; hashnode* next; }; hashnode* ht_[256]; const hashnode* lookup(nsaddr_t src, nsaddr_t dst) const; }; int MCastClassifier::classify(Packet *const pkt) { IPHeader *h = IPHeader::access(pkt->bits()); nsaddr_t src = h->src() >> 8; /*XXX*/ nsaddr_t dst = h->dst(); const hashnode* p = lookup(src, dst); if (p == 0) { /* * Didnt find an entry. * Call tcl exactly once to install one. * If tcl doesnt come through then fail. */ Tcl::instance().evalf("%s new-group %u %u", name(), src, dst); p = lookup(src, dst); if (p == 0) return (-1); } 51
return (p->slot); } The class MCastClassifiermplements a chained hash table and applies a hash function on both the packet source and destination addresses. The hash function returns the slot number to index the slot_ table in the underlying object. A hash miss implies packet delivery to a previously-unknown group; OTcl is called to handle the situation. The OTcl code is expected to insert an appropriate entry into the hash table.
5.4.3 MultiPath Classier
This object is devised to support equal cost multipath forwarding, where the node has multiple equal cost routes to the same destination, and would like to use all of them simultaneously. This object does not look at any eld in the packet. With every succeeding packet, it simply returns the next lled slot in round robin fashion. The denitions for this classier are in ~ns/classier-mpath.cc, and are shown below: class MultiPathForwarder : public Classifier { public: MultiPathForwarder() : ns_(0), Classifier() {} virtual int classify(Packet* const) { int cl; int fail = ns_; do { cl = ns_++; ns_ %= (maxslot_ + 1); } while (slot_[cl] == 0 && ns_ != fail); return cl; } private: int ns_; /* next slot to be used. Probably a misnomer? */ };
5.4.4 Hash Classier
This object is used to classify a packet as a member of a particular ow. As their name indicates, hash classiers use a hash table internally to assign packets to ows. These objects are used where ow-level information is required (e.g. in ow-specic queuing disciplines and statistics collection). Several ow granularities are available. In particular, packets may be assigned to ows based on ow ID, destination address, source/destination addresses, or the combination of source/destination addresses plus ow ID. The elds accessed by the hash classier are limited to the ip header: src(), dst(), flowid() (see ip.h). The hash classier is created with an integer argument specifying the initial size of its hash table. The current hash table size may be subsequently altered with the resize method (see below). When created, the instance variables shift_ and mask_ are initialized with the simulators current NodeShift and NodeMask values, respectively. These values are retrieved from the AddrParams object when the hash classier is instantiated. The hash classier will fail to operate properly if the AddrParams structure is not initialized. The following constructors are used for the various hash classiers: Classifier/Hash/SrcDest Classifier/Hash/Dest
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Classifier/Hash/Fid Classifier/Hash/SrcDestFid The hash classier receives packets, classies them according to their ow criteria, and retrieves the classier slot indicating the next node that should receive the packet. In several circumstances with hash classiers, most packets should be associated with a single slot, while only a few ows should be directed elsewhere. The hash classier includes a default_ instance variable indicating which slot is to be used for packets that do not match any of the per-ow criteria. The default_ may be set optionally. The methods for a hash classier are as follows: $hashcl $hashcl $hashcl $hashcl set-hash buck src dst fid slot lookup buck src dst fid del-hash src dst fid resize nbuck
The set-hash() method inserts a new entry into the hash table within the hash classier. The buck argument species the hash table bucket number to use for the insertion of this entry. When the bucket number is not known, buck may be specied as auto. The src, dst and fid arguments specify the IP source, destination, and ow IDs to be matched for ow classication. Fields not used by a particular classier (e.g. specifying src for a ow-id classier) is ignored. The slot argument indicates the index into the underlying slot table in the base Classifier object from which the hash classier is derived. The lookup function returns the name of the object associated with the given buck/src/dst/fid tuple. The buck argument may be auto, as for set-hash. The del-hash function removes the specied entry from the hash table. Currently, this is done by simply marking the entry as inactive, so it is possible to populate the hash table with unused entries. The resize function resizes the hash table to include the number of buckets specied by the argument nbuck. Provided no default is dened, a hash classier will perform a call into OTcl when it receives a packet which matches no ow criteria. The call takes the following form: $obj unknown-flow src dst flowid buck Thus, when a packet matching no ow criteria is received, the method unknown-flow of the instantiated hash classier object is invoked with the source, destination, and ow id elds from the packet. In addition, the buck eld indicates the hash bucket which should contain this ow if it were inserted using set-hash. This arrangement avoids another hash lookup when performing insertions into the classier when the bucket is already known.
5.4.5 Replicator
The replicator is different from the other classiers we have described earlier, in that it does not use the classify function. Rather, it simply uses the classier as a table of slots; it overloads the recv() method to produce copies of a packet, that are delivered to all objects referenced in the table. To support multicast packet forwarding, a classier receiving a multicast packet from source destined for group computes a hash function giving a slot number in the classiers object table. In multicast delivery, the packet must be copied once for each link leading to nodes subscribed to minus one. Production of additional copies of the packet is performed by a Replicator class, dened in replicator.cc: /* * A replicator is not really a packet classier but 53
* we simply nd convenience in leveraging its slot table. * (this object used to implement fan-out on a multicast * router as well as broadcast LANs) */ class Replicator : public Classifier { public: Replicator(); void recv(Packet*, Handler* h = 0); virtual int classify(Packet* const) {}; protected: int ignore_; }; void Replicator::recv(Packet* p, Handler*) { IPHeader *iph = IPHeader::access(p->bits()); if (maxslot_ < 0) { if (!ignore_) Tcl::instance().evalf("%s drop %u %u", name(), iph->src(), iph->dst()); Packet::free(p); return; } for (int i = 0; i < maxslot_; ++i) { NsObject* o = slot_[i]; if (o != 0) o->recv(p->copy()); } /* we know that maxslot is non-null */ slot_[maxslot_]->recv(p); } As we can see from the code, this class does not really classify packets. Rather, it replicates a packet, one for each entry in its table, and delivers the copies to each of the nodes listed in the table. The last entry in the table gets the original packet. Since the classify() method is pure virtual in the base class, the replicator denes an empty classify() method.
5.5 Routing Module and Classier Organization
As we have seen, a ns node is essentially a collection of classiers. The simplest node (unicast) contains only one address classier and one port classier, as shown in Figure 5.1. When one extends the functionality of the node, more classiers are added into the base node, for instance, the multicast node shown in Figure 5.2. As more function blocks is added, and each of these blocks requires its own classier(s), it becomes important for the node to provide a uniform interface to organize these classiers and to bridge these classiers to the route computation blocks. The classical method to handle this case is through class inheritance. For instance, if one wants a node that supports hierarchical routing, one simply derive a Node/Hier from the base node and override the classier setup methods to insert hierarchical classiers. This method works well when the new function blocks are independent and cannot be arbitrarily mixed. For instance, both hierarchical routing and ad hoc routing use their own set of classiers. Inheritance would require that we have Node/Hier that supports the former, and Node/Mobile for the latter. This becomes slightly problematic when one wants an ad hoc routing node that supports hierarchical routing. In this simple case one may use multiple inheritance to solve the problem, but this quickly becomes infeasible as the number of such function blocks increases. 54
Routing Modules RtModule/Base routing
add-route delete-route
Base Hier Mcast MPLS
Node routing
add-route delete-route
Route Computation User Simulation
transport
attach detach
transport
attach detach
Management
register unregister
Classifier
insert-entry install-entry install-demux
Figure 5.3: Interaction among node, routing module, and routing. The dashed line shows the details of one routing module.
The only method to solve this problem is object composition. The base node needs to dene a set of interfaces for classier access and organization. These interfaces should
allow individual routing modules that implement their own classiers to insert their classiers into the node; allow route computation blocks to populate routes to classiers in all routing modules that need this information, provide a single point of management for existing routing modules.
In addition, we should also dene a uniform interface for routing modules to connect to the node interfaces, so as to provide a systematic approach to extending node functionality. In this section we will describe the design of routing modules as well as that of the corresponding node interfaces.
......
5.5.1 Routing Module
In general, every routing implementation in ns consists of three function blocks:
Routing agent exchanges routing packet with neighbors, Route logic uses the information gathered by routing agents (or the global topology database in the case of static routing) to perform the actual route computation, Classiers sit inside a Node. They use the computed routing table to perform packet forwarding.
Notice that when implementing a new routing protocol, one does not necessarily implement all of these three blocks. For instance, when one implements a link state routing protocol, one simply implement a routing agent that exchanges information in the link state manner, and a route logic that does Dijkstra on the resulting topology database. It can then use the same classiers as other unicast routing protocols. When a new routing protocol implementation includes more than one function blocks, especially when it contains its own classier, it is desirable to have another object, which we call a routing module, that manages all these function blocks and to interface with node to organize its classiers. Figure 5.3 shows functional relation among these objects. Notice that routing 55
modules may have direct relationship with route computation blocks, i.e., route logic and/or routing agents. However, route computation MAY not install their routes directly through a routing module, because there may exists other modules that are interested in learning about the new routes. This is not a requirement, however, because it is possible that some route computation is specic to one particular routing module, for instance, label installation in the MPLS module. A routing module contains three major functionalities: 1. A routing module initializes its connection to a node through register{}, and tears the connection down via unregister{}. Usually, in register{} a routing module (1) tells the node whether it interests in knowing route updates and transport agent attachments, and (2) creates its classiers and install them in the node (details described in the next subsection). In unregister{} a routing module does the exact opposite: it deletes its classiers and removes its hooks on routing update in the node. 2. If a routing module is interested in knowing routing updates, the node will inform the module via RtModule::add-route{dst, target} and RtModule::delete-route{dst, nullagent}. 3. If a routing module is interested in learning about transport agent attachment and detachment in a node, the node will inform the module via RtModule::attach{agent, port} and RtModule::detach{agent, nullagent}. There are two steps to write your own routing module: 1. You need to declare the C++ part of your routing module (see ~ns/rtmodule.{cc,h}). For many modules this only means to declare a virtual method name() which returns a string descriptor of the module. However, you are free to implement as much functionality as you like in C++; if necessary you may later move functionality from OTcl into C++ for better performance. 2. You need to look at the above interfaces implemented in the base routing module (see ~ns/tcl/lib/ns-rtmodule.tcl) and decide which one youll inherit, which one youll override, and put them in OTcl interfaces of your own module. There are several derived routing module examples in ~ns/tcl/lib/ns-rtmodule.tcl, which may serve as templates for your modules. Currently, there are six routing modules implemented in ns: Module Name RtModule/Base RtModule/Mcast Functionality Interface to unicast routing protocols. Provide basic functionality to add/delete route and attach/detach agents. Interface to multicast routing protocols. Its only purpose is establishes multicast classiers. All other multicast functionalities are implemented as instprocs of Node. This should be converted in the future. Hierarchical routing. Its a wrapper for managing hierarchical classiers and route installation. Can be combined with other routing protocols, e.g., ad hoc routing. Manual routing. Uses virtual classier instead of vanilla classier. Implements MPLS functionality. This is the only existing module that is completely selfcontained and does not pollute the Node namespace. Table 5.1: Available routing modules
RtModule/Hier RtModule/Manual RtModule/VC RtModule/MPLS
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5.5.2 Node Interface
To connect to the above interfaces of routing module, a node provides a similar set of interfaces:
In order to know which module to register during creation, the Node class keeps a list of modules as a class variable. The default value of this list contains only the base routing module. The Node class provides the following two procs to manipulate this module list: Node::enable-module{[name]} Node::disable-module{[name]} If module RtModule/[name] exists, this proc puts [name] into the module list. If [name] is in the module list, remove it from the list.
When a node is created, it goes through the module list of the Node class, creates all modules included in the list, and register these modules at the node. After a node is created, one may use the following instprocs to list modules registered at the node, or to get a handle of a module with a particular name: Node::list-modules{} Node::get-module{[name]}
Return a list of the handles (shadow objects) of all registered modules. Return a handle of the registered module whose name matches the given one. Notice that any routing module can only have a single instance registered at any node.
To allow routing modules register their interests of routing updates, a node object provide the following instprocs: Node::route-notify{module} Node::unreg-route-notify{module} Add module into route update notication list. Remove module from route update notication list. Add module into agent attachment notication list. Remove module from agent attachment notication list.
Similarly, the following instprocs provide hooks on the attachment of transport agents: Node::port-notify{module} Node::unreg-port-notify{module}
Notice that in all of these instprocs, parameter module should be a module handle instead of a module name.
Node provides the following instprocs to manipulate its address and port classiers: Node::insert-entry{module, clsfr, hook} inserts classier clsfr into the entry point of the node. It also associates the new classier with module so that if this classier is removed later, module will be unregistered. If hook is specied as a number, the existing classier will be inserted into slot hook of the new classier. In this way, one may establish a chain of classiers; see Figure 5.2 for an example. NOTE: clsfr needs NOT to be a classier. In some cases one may want to put an agent, or any class derived from Connector, at the entry point of a node. In such cases, one simply supplies target to parameter hook. Node::install-entry{module, clsfr, hook} differs from Node::insert-entry in that it deletes the existing classier at the node entry point, unregisters any associated routing module, and installs the new classier at that point. If hook is given, and the old classier is connected into a classier chain, it will connect the chain into slot hook of the new classier. As above, if hook equals to target, clsfr will be treated as an object derived from Connector instead of a classier. Node::install-demux{demux, port} places the given classier demux as the default demultiplexer. If port is given, it plugs the existing demultiplexer into slot port of the new one. Notice that in either case it does not delete the existing demultiplexer.
5.6 Commands at a glance
Following is a list of common node commands used in simulation scripts:
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$ns_ node [<hier_addr>] Command to create and return a node instance. If <hier_addr> is given, assign the node address to be <hier_addr>. Note that the latter MUST only be used when hierarchical addressing is enabled via either set-address-format hierarchical{} or node-config -addressType hierarchical{}. $ns_ node-config -<config-parameter> <optional-val> This command is used to congure nodes. The different cong-parameters are addressingType, different type of the network stack components, whether tracing will be turned on or not, mobileIP ag is truned or not, energy model is being used or not etc. An option -reset maybe used to set the node conguration to its default state. The default setting of node-cong, i.e if no values are specied, creates a simple node (base class Node) with at addressing/routing. For the syntax details see Section 5.3. $node id Returns the id number of the node. $node node-addr Returns the address of the node. In case of at addressing, the node address is same as its node-id. In case of hierarchical addressing, the node address in the form of a string (viz. "1.4.3") is returned. $node reset Resets all agent attached to this node. $node agent <port_num> Returns the handle of the agent at the specied port. If no agent is found at the given port, a null string is returned. $node entry Returns the entry point for the node. This is rst object that handles packet receiving at this node. $node attach <agent> <optional:port_num> Attaches the <agent> to this node. Incase no specic port number is passed, the node allocates a port number and binds the agent to this port. Thus once the agent is attached, it receives packets destined for this host (node) and port. $node detach <agent> <null_agent> This is the dual of "attach" described above. It detaches the agent from this node and installs a null-agent to the port this agent was attached. This is done to handle transit packets that may be destined to the detached agent. These on-the-y packets are then sinked at the null-agent. $node neighbors This returns the list of neighbors for the node. $node add-neighbor <neighbor_node> This is a command to add <neighbor_node> to the list of neighbors maintained by the node.
Following is a list of internal node methods: $node add-route <destination_id> <target> This is used in unicast routing to populate the classier. The target is a Tcl object, which may be the entry of dmux_ (port demultiplexer in the node) incase the <destination_id> is same as this node-id. Otherwise it is usually the head of the link for that destination. It could also be the entry for other classiers. $node alloc-port <null_agent> This returns the next available port number.
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$node incr-rtgtable-size The instance variable rtsize_ is used to keep track of size of routing-table in each node. This command is used to increase the routing-table size every time an routing-entry is added to the classiers. There are other node commands that supports hierarchical routing, detailed dynamic routing, equal cost multipath routing, manual routing, and energy model for mobile nodes. These and other methods described earlier can be found in ~ns/tcl/lib/ns-node.tcl and ~ns/tcl/lib/ns-mobilenode.tcl.
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Chapter 6
Links: Simple Links
This is the second aspect of dening the topology. In the previous chapter (Chapter 5), we had described how to create the nodes in the topology in ns. We now describe how to create the links to connect the nodes and complete the topology. In this chapter, we restrict ourselves to describing the simple point to point links. ns supports a variety of other media, including an emulation of a multi-access LAN using a mesh of simple links, and other true simulation of wireless and broadcast media. They will be described in a separate chapter. The CBQlink is derived from simple links and is a considerably more complex form of link that is also not described in this chapter. We begin by describing the commands to create a link in this section. As with the node being composed of classiers, a simple link is built up from a sequence of connectors. We also briey describe some of the connectors in a simple link. We then describe the instance procedures that operate on the various components of dened by some of these connectors (Section 6.1). We conclude the chapter with a description the connector object (Section 6.2), including brief descriptions of the common link connectors. The class Link is a standalone class in OTcl, that provides a few simple primitives. The class SimpleLink provides the ability to connect two nodes with a point to point link. ns provides the instance procedure simplex-link{} to form a unidirectional link from one node to another. The link is in the class SimpleLink. The following describes the syntax of the simplex link: set ns [new Simulator] $ns simplex-link node0
node1
bandwidth
delay
queue_type
The command creates a link from node0 to node1 , with specied bandwidth and delay characteristics. The link uses a queue of type queue_type . The procedure also adds a TTL checker to the link. Five instance variables dene the link: head_ Entry point to the link, it points to the rst object in the link. queue_ Reference to the main queue element of the link. Simple links usually have one queue per link. Other more complex types of links may have multiple queue elements in the link. link_ A reference to the element that actually models the link, in terms of the delay and bandwidth characteristics of the link. ttl_ Reference to the element that manipulates the ttl in every packet. drophead_ Reference to an object that is the head of a queue of elements that process link drops. In addition, if the simulator instance variable, $traceAllFile_, is dened, the procedure will add trace elements that 60
Link
head_ enqT_ queue_ deqT_ link_ ttl_ rcvT_
drophead_
drpT_
Figure 6.1: Composite Construction of a Unidirectional Link track when a packet is enqueued and dequeued from queue_. Furthermore, tracing interposes a drop trace element after the drophead_. The following instance variables track the trace elements: enqT_ Reference to the element that traces packets entering queue_. deqT_ Reference to the element that traces packets leaving queue_. drpT_ Reference to the element that traces packets dropped from queue_. rcvT_ Reference to the element that traces packets received by the next node. Note however, that if the user enable tracing multiple times on the link, these instance variables will only store a reference to the last elements inserted. Other conguration mechanisms that add components to a simple link are network interfaces (used in multicast routing), link dynamics models, and tracing and monitors. We give a brief overview of the related objects at the end of this chapter (Section 6.2), and discuss their functionality/implementation in other chapters. The instance procedure duplex-link{} constructs a bi-directional link from two simplex links.
6.1 Instance Procedures for Links and SimpleLinks
Link procedures The class Link is implemented entirely in Otcl. The OTcl SimpleLink class uses the C++ LinkDelay class to simulate packet delivery delays. The instance procedures in the class Link are:
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head{} queue{} link{} up{} down{} up?{} all-connectors{} cost{} cost?{}
returns the handle for head_. returns the handle for queue_. returns the handle for the delay element, link_. set link status to up in the dynamics_ element. Also, writes out a trace line to each le specied through the procedure trace-dynamics{}. As with up{}, set link status to down in the dynamics_ element. Also, writes out a trace line to each le specied through the procedure trace-dynamics{}. returns status of the link. Status is up or down; status is up if link dynamics is not enabled. Apply specied operation to all connectors on the link.p An example of such usage is link all-connectors reset. set link cost to value specied. returns the cost of the link. Default cost of link is 1, if no cost has been specied earlier.
SimpleLink Procedures The Otcl class SimpleLink implements a simple point-to-point link with an associated queue and delay 1 . It is derived from the base Otcl class Link as follows: Class SimpleLink -superclass Link SimpleLink instproc init { src dst bw delay q { lltype "DelayLink" } } { $self next $src $dst $self instvar link_ queue_ head_ toNode_ ttl_ ... set queue_ $q set link_ [new Delay/Link] $link_ set bandwidth_ $bw $link_ set delay_ $delay $queue_ target $link_ $link_ target [$toNode_ entry] ... # XXX # put the ttl checker after the delay # so we dont have to worry about accounting # for ttl-drops within the trace and/or monitor # fabric # set ttl_ [new TTLChecker] $ttl_ target [$link_ target] $link_ target $ttl_ } Notice that when a SimpleLink object is created, new Delay/Link and TTLChecker objects are also created. Note also that, the Queue object must have already been created. There are two additional methods implemented (in OTcl) as part of the SimpleLink class: trace and init-monitor. These functions are described in further detail in the section on tracing (Chapter 21).
1 The
current version also includes an object to examine the network layer ttl eld and discard packets if the eld reaches zero.
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6.2 Connectors
Connectors, unlink classiers, only generate data for one recipient; either the packet is delivered to the target_ neighbor, or it is sent to he drop-target_. A connector will receive a packet, perform some function, and deliver the packet to its neighbor, or drop the packet. There are a number of different types of connectors in ns. Each connector performs a different function. networkinterface labels packets with incoming interface identierit is used by some multicast routing protocols. The class variable Simulator NumberInterfaces_ 1 tells ns to add these interfaces, and then, it is added to either end of the simplex link. Multicast routing protocols are discussed in a separate chapter (Chapter 24). Object that gates trafc depending on whether the link is up or down. It expects to be at the head of the link, and is inserted on the link just prior to simulation start. Its status_ variable control whether the link is up or down. The description of how the DynaLink object is used is in a separate chapter (Chapter 25). Object that models the links delay and bandwidth characteristics. If the link is not dynamic, then this object simply schedules receive events for the downstream object for each packet it receives at the appropriate time for that packet. However, if the link is dynamic, then it queues the packets internally, and schedules one receives event for itself for the next packet that must be delivered. Thus, if the link goes down at some point, this objects reset() method is invoked, and the object will drop all packets in transit at the instant of link failure. We discuss the specics of this class in another chapter (Chapter 8). model the output buffers attached to a link in a real router in a network. In ns, they are attached to, and are considered as part of the link. We discuss the details of queues and different types of queues in nsin another chapter (Chapter 7). will decrement the ttl in each packet that it receives. If that ttl then has a positive value, the packet is forwarded to the next element on the link. In the simple links, TTLCheckers are automatically added, and are placed as the last element on the link, between the delay element, and the entry for the next node.
DynaLink
DelayLink
Queues
TTLChecker
6.3 Object hierarchy
The base class used to represent links is called Link. Methods for this class are listed in the next section. Other link objects derived from the base class are given as follows:
SimpleLink Object A SimpleLink object is used to represent a simple unidirectional link. There are no state variables or conguration parameters associated with this object. Methods for this class are: $simplelink enable-mcast <src> <dst> This turns on multicast for the link by creating an incoming network interface for the destination and adds an outgoing interface for the source. $simplelink trace <ns> <file> <optional:op> Build trace objects for this link and update object linkage. If op is specied as "nam" create nam trace les. $simplelink nam-trace <ns> <file> Sets up nam tracing in the link. $simplelink trace-dynamics <ns> <file> <optional:op> This sets up tracing specially for dynamic links. <op> allows setting up of nam tracing as well.
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$simplelink init-monitor <ns> <qtrace> <sampleInterval> Insert objects that allow us to monitor the queue size of this link. Return the name of the object that can be queried to determine the average queue size. $simplelink attach-monitors <insnoop> <outsnoop> <dropsnoop> <qmon> This is similar to init-monitor, but allows for specication of more of the items. $simplelink dynamic Sets up the dynamic ag for this link. $simplelink errormodule <args> Inserts an error module before the queue. $simpleilnk insert-linkloss <args> Inserts the error module after the queue. //Other link objects derived from class SimpleLink are FQLink, CBQLink and IntServLink. Conguration parameters for FQLink are: queueManagement_ The type of queue management used in the link. Default value is DropTail. No conguration parameters are specied for CBQLink and IntServLink objects.
DelayLink Object The DelayLink Objects determine the amount of time required for a packet to traverse a link. This is dened to be size/bw + delay where size is the packet size, bw is the link bandwidth and delay is the link propagation delay. There are no methods or state variables associated with this object. Conguration Parameters are: bandwidth_ Link bandwidth in bits per second. delay_ Link propagation delay in seconds.
6.4 Commands at a glance
Following is a list of common link commands used in simulation scripts: $ns_ simplex-link <node1> <node2> <bw> <delay> <qtype> <args> This command creates an unidirectional link between node1 and node2 with specied bandwidth (BW) and delay characteristics. The link uses a queue type of <qtype> and depending on the queue type different arguments are passed through <args>. $ns_ duplex-link <node1> <node2> <bw> <delay> <qtype> <args> This creates a bi-directional link between node1 and node2. This procedure essentially creates a duplex-link from two simplex links, one from node1 to node2 and the other from node2 to node1. The syntax for duplex-link is same as that of simplex-link described above. $ns_ duplex-intserv-link <n1> <n2> <bw> <dly> <sched> <signal> <adc> <args> This creates a duplex-link between n1 and n2 with queue type of intserv, with specied BW and delay. This type of queue implements a scheduler with two level services priority. The type of intserv queue is given by <sched>, with admission control unit type of <adc> and signal module of type <signal>. $ns_ simplex-link-op <n1> <n2> <op> <args> This is used to set attributes for a simplex link. The attributes may be the orientation, color or queue-position. $ns_ duplex-link-op <n1> <n2> <op> <args> This command is used to set link attributes (like orientation of the links, color or queue-position) for duplex links. 64
$ns_ link-lossmodel <lossobj> <from> <to> This function generates losses (using the loss model <lossobj> inserted in the link between <from> node and <to> node) in the link that can be visualized by nam. $ns_ lossmodel <lossobj> <from> <to> This is used to insert a loss module in regular links. Following is a list of internal link-related procedures: $ns_ register-nam-linkconfig <link> This is an internal procedure used by "$link orient" to register/update the order in which links should be created in nam. $ns_ remove-nam-linkconfig <id1> <id2> This procedure is used to remove any duplicate links (duplicate links may be created by GT-ITM topology generator). $link head Returns the instance variable head_ for the link. The head_ is the entry pont to the link and it points to the rst object in the link. $link add-to-head <connector> This allows the <connector> object to be now pointed by the head_ element in the link, i.e, <connector> now becomes the rst object in the link. $link link Returns the instance variable link_. The link_ is the element in the link that actually models the link in terms of delay and bandwidth characteristics of the link. $link queue Returns the instance variable queue_. queue_ is queue element in the link. There may be one or more queue elements in a particular link. $link cost <c> This sets a link cost of <c>. $link cost? Returns the cost value for the link. Default cost of link is set to 1. $link if-label? Returns the network interfaces associated with the link (for multicast routing). $link up This sets the link status to "up". This command is a part of network dynamics support in ns. $link down Similar to up, this command marks the link status as "down". $link up? Returns the link status. The status is always "up" as default, if link dynamics is not enabled. $link all-connectors op This command applies the specied operation <op> to all connectors in the link. Like, $link all-connectors reset or $link all-connectors isDynamic.
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$link install-error <errmodel> This installs an error module after the link_ element. In addition to the Link and link-related commands listed above, there are other procedures to support the specic requirements of different types of links derived from the base class "Link" like simple-link (SimpleLink), integrated service (IntServLink), class-based queue (CBQLink), fair queue (FQLink) and procedures to support multicast routing, sessionsim, nam etc. These and the above procedures may be found in ns/tcl/lib(ns-lib.tcl, ns-link.tcl, ns-intserv.tcl, ns-namsupp.tcl, ns-queue.tcl), ns/tcl/mcast/(McastMonitor.tcl, ns-mcast.tcl), ns/tcl/session/session.tcl.
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Chapter 7
Queue Management and Packet Scheduling
Queues represent locations where packets may be held (or dropped). Packet scheduling refers to the decision process used to choose which packets should be serviced or dropped. Buffer management refers to any particular discipline used to regulate the occupancy of a particular queue. At present, support is included for drop-tail (FIFO) queueing, RED buffer management, CBQ (including a priority and round-robin scheduler), and variants of Fair Queueing including, Fair Queueing (FQ), Stochastic Fair Queueing (SFQ), and Decit Round-Robin (DRR). In the common case where a delay element is downstream from a queue, the queue may be blocked until it is re-enabled by its downstream neighbor. This is the mechanism by which transmission delay is simulated. In addition, queues may be forcibly blocked or unblocked at arbitrary times by their neighbors (which is used to implement multi-queue aggregate queues with inter-queue ow control). Packet drops are implemented in such a way that queues contain a drop destination; that is, an object that receives all packets dropped by a queue. This can be useful to (for example) keep statistics on dropped packets.
7.1 The C++ Queue Class
The Queue class is derived from a Connector base class. It provides a base class used by particular types of (derived) queue classes, as well as a call-back function to implement blocking (see next section). The following denitions are provided in queue.h: class Queue : public Connector { public: virtual void enque(Packet*) = 0; virtual Packet* deque() = 0; void recv(Packet*, Handler*); void resume(); int blocked(); void unblock(); void block(); protected: Queue(); int command(int argc, const char*const* argv); int qlim_; /* maximum allowed pkts in queue */ int blocked_; int unblock_on_resume_; /* unblock q on idle? */ QueueHandler qh_; 67
}; The enque and deque functions are pure virtual, indicating the Queue class is to be used as a base class; particular queues are derived from Queue and implement these two functions as necessary. Particular queues do not, in general, override the recv function because it invokes the the particular enque and deque. The Queue class does not contain much internal state. Often these are special monitoring objects (Chapter 21). The qlim_ member is constructed to dictate a bound on the maximum queue occupancy, but this is not enforced by the Queue class itself; it must be used by the particular queue subclasses if they need this value. The blocked_ member is a boolean indicating whether the queue is able to send a packet immediately to its downstream neighbor. When a queue is blocked, it is able to enqueue packets but not send them.
7.1.1 Queue blocking
A queue may be either blocked or unblocked at any given time. Generally, a queue is blocked when a packet is in transit between it and its downstream neighbor (most of the time if the queue is occupied). A blocked queue will remain blocked as long as it downstream link is busy and the queue has at least one packet to send. A queue becomes unblocked only when its resume function is invoked (by means of a downstream neighbor scheduling it via a callback), usually when no packets are queued. The callback is implemented by using the following class and methods: class QueueHandler : public Handler { public: inline QueueHandler(Queue& q) : queue_(q) {} void handle(Event*); /* calls queue_.resume() */ private: Queue& queue_; }; void QueueHandler::handle(Event*) { queue_.resume(); } Queue::Queue() : drop_(0), blocked_(0), qh_(*this) { Tcl& tcl = Tcl::instance(); bind("limit_", &qlim_); } void Queue::recv(Packet* p, Handler*) { enque(p); if (!blocked_) { /* * Were not block. Get a packet and send it on. * We perform an extra check because the queue * might drop the packet even if it was * previously empty! (e.g., RED can do this.) */ p = deque(); if (p != 0) { blocked_ = 1; target_->recv(p, &qh_); 68
} } } void Queue::resume() { Packet* p = deque(); if (p != 0) target_->recv(p, &qh_); else { if (unblock_on_resume_) blocked_ = 0; else blocked_ = 1; } } The handler management here is somewhat subtle. When a new Queue object is created, it includes a QueueHandler object (qh_) which is initialized to contain a reference to the new Queue object (Queue& QueueHandler::queue_). This is performed by the Queue constructor using the expression qh_(*this). When a Queue receives a packet it calls the subclass (i.e. queueing discipline-specic) version of the enque function with the packet. If the queue is not blocked, it is allowed to send a packet and calls the specic deque function which determines which packet to send, blocks the queue (because a packet is now in transit), and sends the packet to the queues downstream neighbor. Note that any future packets received from upstream neighbors will arrive to a blocked queue. When a downstream neighbor wishes to cause the queue to become unblocked it schedules the QueueHandlers handle function by passing &qh_ to the simulator scheduler. The handle function invokes resume, which will send the next-scheduled packet downstream (and leave the queue blocked), or unblock the queue when no packet is ready to be sent. This process is made more clear by also referring to the LinkDelay::recv() method (Section 8.1).
7.1.2 PacketQueue Class
The Queue class may implement buffer management and scheduling but do not implement the low-level operations on a particular queue. The PacketQueue class is used for this purpose, and is dened as follows (see queue.h): class PacketQueue { public: PacketQueue(); int length(); /* queue length in packets */ void enque(Packet* p); Packet* deque(); Packet* lookup(int n); /* remove a specific packet, which must be in the queue */ void remove(Packet*); protected: Packet* head_; Packet** tail_; int len_; // packet count }; This class maintains a linked-list of packets, and is commonly used by particular scheduling and buffer management disciplines to hold an ordered set of packets. Particular scheduling or buffer management schemes may make use of several
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PacketQueue objects. The PacketQueue class maintains current counts of the number of packets held in the queue which is returned by the length() method. The enque function places the specied packet at the end of the queue and updates the len_ member variable. The deque function returns the packet at the head of the queue and removes it from the queue (and updates the counters), or returns NULL if the queue is empty. The lookup function returns the th packet from the head of the queue, or NULL otherwise. The remove function deletes the packet stored in the given address from the queue (and updates the counters). It causes an abnormal program termination if the packet does not exist.
7.2 Example: Drop Tail
The following example illustrates the implementation of the Queue/DropTail object, which implements FIFO scheduling and drop-on-overow buffer management typical of most present-day Internet routers. The following denitions declare the class and its OTcl linkage: /* * A bounded, drop-tail queue */ class DropTail : public Queue { protected: void enque(Packet*); Packet* deque(); PacketQueue q_; }; The base class Queue, from which DropTail is derived, provides most of the needed functionality. The drop-tail queue maintains exactly one FIFO queue, implemented by including an object of the PacketQueue class. Drop-tail implements its own versions of enque and deque as follows: /* * drop-tail */ void DropTail::enque(Packet* p) { q_.enque(p); if (q_.length() >= qlim_) { q_.remove(p); drop(p); } } Packet* DropTail::deque() { return (q_.deque()); } Here, the enque function rst stores the packet in the internal packet queue (which has no size restrictions), and then checks the size of the packet queue versus qlim_. Drop-on-overow is implemented by dropping the packet most recently added to the packet queue if the limit is reached or exceeded. Simple FIFO scheduling is implemented in the deque function by always returning the rst packet in the packet queue.
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7.3 Different types of Queue objects
A queue object is a general class of object capable of holding and possibly marking or discarding packets as they travel through the simulated topology. Conguration Parameters used for queue objects are: limit_ The queue size in packets. blocked_ Set to false by default, this is true if the queue is blocked (unable to send a packet to its downstream neighbor). unblock_on_resume_ Set to true by default, indicates a queue should unblock itself at the time the last packet packet sent has been transmitted (but not necessarily received). Other queue objects derived from the base class Queue are drop-tail, FQ, SFQ, DRR, RED and CBQ queue objects. Each are described as follows:
Drop-tail objects: Drop-tail objects are a subclass of Queue objects that implement simple FIFO queue. There are no methods, conguration parameter, or state variables that are specic to drop-tail objects. FQ objects: FQ objects are a subclass of Queue objects that implement Fair queuing. There are no methods that are specic to FQ objects. Conguration Parameters are: secsPerByte_ There are no state variables associated with this object.
SFQ objects: SFQ objects are a subclass of Queue objects that implement Stochastic Fair queuing. There are no methods that are specic to SFQ objects. Conguration Parameters are: maxqueue_ buckets_ There are no state variables associated with this object.
DRR objects: DRR objects are a subclass of Queue objects that implement decit round robin scheduling. These objects implement decit round robin scheduling amongst different ows ( A particular ow is one which has packets with the same node and port id OR packets which have the same node id alone). Also unlike other multi-queue objects, this queue object implements a single shared buffer space for its different ows. Conguration Parameters are: buckets_ Indicates the total number of buckets to be used for hashing each of the ows. blimit_ Indicates the shared buffer size in bytes. quantum_ Indicates (in bytes) how much each ow can send during its turn. mask_ mask_, when set to 1, means that a particular ow consists of packets having the same node id (and possibly different port ids), otherwise a ow consists of packets having the same node and port ids.
RED objects: RED objects are a subclass of Queue objects that implement random early-detection gateways. The object can be congured to either drop or mark packets. There are no methods that are specic to RED objects. Conguration Parameters are: bytes_ Set to "true" to enable byte-mode RED, where the size of arriving packets affect the likelihood of marking (dropping) packets. queue-in-bytes_ Set to "true" to measure the average queue size in bytes rather than packets. Enabling this option also causes thresh_ and maxthresh_ to be automatically scaled by mean_pktsize_ (see below). thresh_ The minimum threshold for the average queue size in packets.
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maxthresh_ The maximum threshold for the average queue size in packets. mean_pktsize_ A rough estimate of the average packet size in bytes. Used in updating the calculated average queue size after an idle period. q_weight_ The queue weight, used in the exponential-weighted moving average for calculating the average queue size. wait_ Set to true to maintain an interval between dropped packets. linterm_ As the average queue size varies between "thresh_" and "maxthresh_", the packet dropping probability varies between 0 and "1/linterm". setbit_ Set to "true" to mark packets by setting the congestion indication bit in packet headers rather than drop packets. drop-tail_ Set to true to use drop-tail rather than randomdrop when the queue overows or the average queue size exceeds "maxthresh_". For a further explanation of these variables, see [2]. None of the state variables of the RED implementation are accessible.
CBQ objects: CBQ objects are a subclass of Queue objects that implement class-based queueing. $cbq insert <class> Insert trafc class class into the link-sharing structure associated with link object cbq. $cbq bind <cbqclass> <id1> [$id2] Cause packets containing ow id id1 (or those in the range id1 to id2 inclusive) to be associated with the trafc class cbqclass. $cbq algorithm <alg> Select the CBQ internal algorithm. <alg> may be set to one of: "ancestor-only", "top-level", or "formal".
CBQ/WRR objects: CBQ/WRR objects are a subclass of CBQ objects that implement weighted round-robin scheduling among classes of the same priority level. In contrast, CBQ objects implement packet-by-packet round-robin scheduling among classes of the same priority level. Conguration Parameters are: maxpkt_ The maximum size of a packet in bytes. This is used only by CBQ/WRR objects in computing maximum bandwidth allocations for the weighted round-robin scheduler.
CBQCLASS OBJECTS CBQClass objects implement the trafc classes associated with CBQ objects. $cbqclass setparams <parent> <okborrow> <allot> <maxidle> <prio> <level> Sets several of the conguration parameters for the CBQ trafc class (see below). $cbqclass parent <cbqcl|none> specify the parent of this class in the link-sharing tree. The parent may be specied as none to indicate this class is a root. $cbqclass newallot <a> Change the link allocation of this class to the specied amount (in range 0.0 to 1.0). Note that only the specied class is affected. $cbqclass install-queue <q> Install a Queue object into the compound CBQ or CBQ/WRR link structure. When a CBQ object is initially created, it includes no internal queue (only a packet classier and scheduler). Conguration Parameters are: okborrow_ is a boolean indicating the class is permitted to borrow bandwidth from its parent. allot_ is the maximum fraction of link bandwidth allocated to the class expressed as a real number between 0.0 and 1.0.
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maxidle_ is the maximum amount of time a class may be required to have its packets queued before they are permitted to be forwarded priority_ is the class priority level with respect to other classes. This value may range from 0 to 10, and more than one class may exist at the same priority. Priority 0 is the highest priority. level_ is the level of this class in the link-sharing tree. Leaf nodes in the tree are considered to be at level 1; their parents are at level 2, etc. extradelay_ increase the delay experienced by a delayed class by the specied time QUEUE-MONITOR OBJECTS QueueMonitor Objects are used to monitor a set of packet and byte arrival, departure and drop counters. It also includes support for aggregate statistics such as average queue size, etc. $queuemonitor reset all the cumulative counters described below (arrivals, departures, and drops) to zero. Also, reset the integrators and delay sampler, if dened. $queuemonitor set-delay-samples <delaySamp_> Set up the Samples object delaySamp_ to record statistics about queue delays. delaySamp_ is a handle to a Samples object i.e the Samples object should have already been created. $queuemonitor get-bytes-integrator Returns an Integrator object that can be used to nd the integral of the queue size in bytes. $queuemonitor get-pkts-integrator Returns an Integrator object that can be used to nd the integral of the queue size in packets. $queuemonitor get-delay-samples Returns a Samples object delaySamp_ to record statistics about queue delays. There are no conguration parameters specic to this object. State Variables are: size_ Instantaneous queue size in bytes. pkts_ Instantaneous queue size in packets. parrivals_ Running total of packets that have arrived. barrivals_ Running total of bytes contained in packets that have arrived. pdepartures_ Running total of packets that have departed (not dropped). bdepartures_ Running total of bytes contained in packets that have departed (not dropped). pdrops_ Total number of packets dropped. bdrops_ Total number of bytes dropped. bytesInt_ Integrator object that computes the integral of the queue size in bytes. The sum_ variable of this object has the running sum (integral) of the queue size in bytes. pktsInt_ Integrator object that computes the integral of the queue size in packets. The sum_ variable of this object has the running sum (integral) of the queue size in packets.
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QUEUEMONITOR/ED OBJECTS This derived object is capable of differentiating regular packet drops from early drops. Some queues distinguish regular drops (e.g. drops due to buffer exhaustion) from other drops (e.g. random drops in RED queues). Under some circumstances, it is useful to distinguish these two types of drops. State Variables are: epdrops_ The number of packets that have been dropped early. ebdrops_ The number of bytes comprising packets that have been dropped early. Note: because this class is a subclass of QueueMonitor, objects of this type also have elds such as pdrops_ and bdrops_. These elds describe the total number of dropped packets and bytes, including both early and non-early drops. QUEUEMONITOR/ED/FLOWMON OBJECTS These objects may be used in the place of a conventional QueueMonitor object when wishing to collect per-ow counts and statistics in addition to the aggregate counts and statistics provided by the basic QueueMonitor. $fmon classifier <cl> This inserts (read) the specied classier into (from) the ow monitor object. This is used to map incoming packets to which ows they are associated with. $fmon dump Dump the current per-ow counters and statistics to the I/O channel specied in a previous attach operation. $fmon flows Return a character string containing the names of all ow objects known by this ow monitor. Each of these objects are of type QueueMonitor/ED/Flow. $fmon attach <chan> Attach a tcl I/O channel to the ow monitor. Flow statistics are written to the channel when the dump operation is executed. Conguration Parameters are: enable_in_ Set to true by default, indicates that per-ow arrival state should be kept by the ow monitor. If set to false, only the aggregate arrival information is kept. enable_out_ Set to true by default, indicates that per-ow departure state should be kept by the ow monitor. If set to false, only the aggregate departure information is kept. enable_drop_ Set to true by default, indicates that per-ow drop state should be kept by the ow monitor. If set to false, only the aggregate drop information is kept. enable_edrop_ Set to true by default, indicates that per-ow early drop state should be kept by the ow monitor. If set to false, only the aggregate early drop information is kept. QUEUEMONITOR/ED/FLOW OBJECTS These objects contain per-ow counts and statistics managed by a QueueMonitor/ED/Flowmon object. They are generally created in an OTcl callback procedure when a ow monitor is given a packet it cannot map on to a known ow. Note that the ow monitors classier is responsible for mapping packets to ows in some arbitrary way. Thus, depending on the type of classier used, not all of the state variables may be relevant (e.g. one may classify packets based only on ow id, in which case the source and destination addresses may not be signicant). State Variables are: src_ The source address of packets belonging to this ow. 74
dst_ The destination address of packets belonging to this ow. owid_ The ow id of packets belonging to this ow.
7.4 Commands at a glance
Following is a list of queue commands used in simulation scripts: $ns_ queue-limit <n1> <n2> <limit> This sets a limit on the maximum buffer size of the queue in the link between nodes <n1> and <n2>. $ns_ trace-queue <n1> <n2> <optional:file> This sets up trace objects to log events in the queue. If tracele is not passed, it uses traceAllFile_ to write the events. $ns_ namtrace-queue <n1> <n2> <optional:file> Similar to trace-queue above, this sets up nam-tracing in the queue. $ns_ monitor-queue <n1> <n2> <optional:qtrace> <optional:sampleinterval> This command inserts objects that allows us to monitor the queue size. This returns a handle to the object that may be queried to determine the average queue size. The default value for sampleinterval is 0.1.
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Chapter 8
Delays and Links
Delays represent the time required for a packet to traverse a link. A special form of this object (dynamic link) also captures the possibility of a link failure. The amount of time required for a packet to traverse a link is dened to be where is the packet size (as recorded in its IP header), is the speed of the link in bits/sec, and is the link delay in seconds. The implementation of link delays is closely associated with the blocking procedures described for Queues in Section7.1.1.
8.1 The LinkDelay Class
The class LinkDelay is derived from the base class Connector. Its denition is in ~ns/delay.cc, and is briey excerpted below: class LinkDelay : public Connector { public: LinkDelay(); void recv(Packet* p, Handler*); void send(Packet* p, Handler*); void handle(Event* e); double delay(); /* line latency on this link */ double bandwidth(); /* bandwidth on this link */ inline double txtime(Packet* p) { /* time to send pkt p on this link */ hdr_cmn* hdr = (hdr_cmn*) p->access(off_cmn_); return (hdr->size() * 8. / bandwidth_); } protected: double bandwidth_; double delay_; int dynamic_; Event inTransit_; PacketQueue* itq_; Packet* nextPacket_; Event intr_; };
/* bandwidth of underlying link (bits/sec) */ /* line latency */ /* indicates whether or not link is ~ */ /* internal packet queue for dynamic links */ /* to be delivered for a dynamic link. */
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The recv() method overrides the base class Connector version. It is dened as follows: void LinkDelay::recv(Packet* p, Handler* h) { double txt = txtime(p); Scheduler& s = Scheduler::instance(); if (dynamic_) { Event* e = (Event*)p; e->time_ = s.clock() + txt + delay_; itq_->enque(p); schedule_next(); } else { s.schedule(target_, p, txt + delay_); } /* XXX only need one intr_ since upstream object should * block until its handler is called * * This only holds if the link is not dynamic. If it is, then * the link itself will hold the packet, and call the upstream * object at the appropriate time. This second interrupt is * called inTransit_, and is invoked through schedule_next() */ s.schedule(h, &intr_, txt); } This object supports one instproc-like, $object dynamic, to set its variable, dynamic_. This variable determines whether the link is dynamic or not (i.e., prone to fail/recover at appropriate times). The internal behavior of the link in each case is different. For non-dynamic links, this method operates by receiving a packet, , and scheduling two events. Assume these two events are called and , and that event is scheduled to occur before . is scheduled to occur when the upstream node attached to this delay element has completed sending the current packet (which takes time equal to the packet size divided by the link bandwidth). is usually associated with a Queue object, and will cause it to (possibly) become unblocked (see section 7.1.1). represents the packet arrival event at the downstream neighbor of the delay element. Event occurs a number of seconds later than equal to the link delay. Alternately, when the link is dynamic, and receives , then it will schedule to possibly unblock the queue at the appropriate time. However, is scheduled only if is the only packet currently in transit. Otherwise, there is at least one packet in transit on the link that must be delivered before at . Therefore, packet is held in the objects inTransit queue, itq_. When the packet just before in transit on the link is delivered at the neighbor node, the DelayLink object will schedule an event for itself to re at . At that appropriate time then, its handle() method will directly send to its target. The objects internal schedule_next() method will schedule these events for packet sin transit at the appropriate time.
8.2 Commands at a glance
The LinkDelay object represents the time required by a packet to transverse the link and is used internally within a Link. Hence we donot list any linkdelay related commands suitable for simulation scripts here.
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Chapter 9
Agents
Agents represent endpoints where network-layer packets are constructed or consumed, and are used in the implementation of protocols at various layers. The class Agent has an implementation partly in OTcl and partly in C++. The C++ implementation is contained in ~ns/agent.cc and ~ns/agent.h, and the OTcl support is in ~ns/tcl/lib/ns-agent.tcl.
9.1 Agent state
The C++ class Agent includes enough internal state to assign various elds to a simulated packet before it is sent. This state includes the following: addr_ dst_ size_ type_ fid_ prio_ flags_ defttl_ node address of myself (source address in packets) where I am sending packets to packet size in bytes (placed into the common packet header) type of packet (in the common header, see packet.h) the IP ow identier (formerly class in ns-1) the IP priority eld packet ags (similar to ns-1) default IP ttl value
These variables may be modied by any class derived from Agent, although not all of them may be needed by any particular agent.
9.2 Agent methods
The class Agent supports packet generation and reception. The following member functions are implemented by the C++ Agent class, and are generally not over-ridden by derived classes: Packet* allocpkt() Packet* allocpkt(int) allocate new packet and assign its elds allocate new packet with a data payload of n bytes and assign its elds
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The following member functions are also dened by the class Agent, but are intended to be over-ridden by classes deriving from Agent: void timeout(timeout number) void recv(Packet*, Handler*) subclass-specic time out method receiving agent main receive path
The allocpkt() method is used by derived classes to create packets to send. The function lls in the following elds in the common packet header (Section 11): uid, ptype, size, and the following elds in the IP header: src, dst, flowid, prio, ttl. It also zero-lls in the following elds of the Flags header: ecn, pri, usr1, usr2. Any packet header information not included in these lists must be must be handled in the classes derived from Agent. The recv() method is the main entry point for an Agent which receives packets, and is invoked by upstream nodes when sending a packet. In most cases, Agents make no use of the second argument (the handler dened by upstream nodes).
9.3 Protocol Agents
There are several agents supported in the simulator. These are their names in OTcl: TCP TCP/Reno TCP/NewReno TCP/Sack1 TCP/Fack TCP/FullTcp TCP/Vegas TCP/Vegas/RBP TCP/Vegas/RBP TCP/Asym TCP/Reno/Asym TCP/Newreno/Asym TCPSink TCPSink/DelAck TCPSink/Asym TCPSink/Sack1 TCPSink/Sack1/DelAck UDP RTP RTCP LossMonitor IVS/Source IVS/Receiver a Tahoe TCP sender (cwnd = 1 on any loss) a Reno TCP sender (with fast recovery) a modied Reno TCP sender (changes fast recovery) a SACK TCP sender a forward SACK sender TCP a more full-functioned TCP with 2-way trafc a Vegas TCP sender a Vegas TCP with rate based pacing a Reno TCP with rate based pacing an experimental Tahoe TCP for asymmetric links an experimental Reno TCP for asymmetric links an experimental NewReno TCP for asymmetric links a Reno or Tahoe TCP receiver (not used for FullTcp) a TCP delayed-ACK receiver an experimental TCP sink for asymmetric links a SACK TCP receiver a delayed-ACK SACK TCP receiver a basic UDP agent an RTP sender and receiver an RTCP sender and receiver a packet sink which checks for losses an IVS source an IVS receiver
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CtrMcast/Encap CtrMcast/Decap Message Message/Prune SRM SRM/Adaptive Tap Null rtProto/DV
a centralised multicast encapsulator a centralised multicast de-encapsulator a protocol to carry textual messages processes multicast routing prune messages an SRM agent with non-adaptive timers an SRM agent with adaptive timers interfaces the simulator to a live network a degenerate agent which discards packets distance-vector routing protocol agent
Agents are used in the implementation of protocols at various layers. Thus, for some transport protocols (e.g. UDP) the distribution of packet sizes and/or inter-departure times may be dictated by some separate object representing the demands of an application. To this end, agents expose an application programming interface (API) to the application. For agents used in the implementation of lower-layer protocols (e.g. routing agents), size and departure timing is generally dictated by the agents own processing of protocol messages.
9.4 OTcl Linkage
Agents may be created within OTcl and an agents internal state can be modied by use of Tcls set function and any Tcl functions an Agent (or its base classes) implements. Note that some of an Agents internal state may exist only within OTcl, and is thus is not directly accessible from C++.
9.4.1 Creating and Manipulating Agents
The following example illustrates the creation and modication of an Agent in OTcl: set newtcp [new Agent/TCP] $newtcp set window_ 20 $newtcp target $dest $newtcp set portID_ 1 ;# create new object (and C++ shadow object) ;# sets the tcp agents window to 20 ;# target is implemented in Connector class ;# exists only in OTcl, not in C++
9.4.2 Default Values
Default values for member variables, those visible in OTcl only and those linked between OTcl and C++ with bind are initialized in the ~ns/tcl/lib/ns-default.tcl le. For example, Agent is initialized as follows: Agent set fid_ 0 Agent set prio_ 0 Agent set addr_ 0 81
Agent set dst_ 0 Agent set flags_ 0 Generally these initializations are placed in the OTcl namespace before any objects of these types are created. Thus, when an Agent object is created, the calls to bind in the objects constructors will causes the corresponding member variables to be set to these specied defaults.
9.4.3 OTcl Methods
The instance procedures dened for the OTcl Agent class are currently found in ~ns/tcl/lib/ns-agent.tcl. They are as follows: port the agents port identier dst-port the destinations port identier create and attach a Source object to an agent attach-source stype
9.5 Examples: Tcp, TCP Sink Agents
The class TCP represents a simplied TCP sender. It sends data to a TCPSink agent and processes its acknowledgments. It has a separate object associated with it which represents an applications demand. By looking at the class TCPAgent and class TCPSinkAgent, we may see how relatively complex agents are constructed. An example from the Tahoe TCP agent TCPAgent is also given to illustrate the use of timers.
9.5.1 Creating the Agent
The following OTcl code fragment creates a TCP agent and sets it up: set tcp [new Agent/TCP] $tcp set fid_ 2 set sink [new Agent/TCPSink] $ns attach-agent $n0 $tcp $ns attach-agent $n3 $sink $ns connect $tcp $sink set ftp [new Application/FTP] $ftp attach-agent $tcp $ns at 1.2 "$ftp start" ;# create sender agent ;# set IP-layer ow ID ;# create receiver agent ;# put sender on node $n0 ;# put receiver on node $n3 ;# establish TCP connection ;# create an FTP source "application" ;# associate FTP with the TCP sender ;#arrange for FTP to start at time 1.2 sec
The OTcl instruction new Agent/TCP results in the creation of a C++ TcpAgent class object. Its constructor performs rst invokes the constructor of the Agent base class and then performs its own bindings. These two constructors appear as follows: The TcpSimpleAgent constructor (~ns/tcp.cc): TcpAgent::TcpAgent() : Agent(PT_TCP), rtt_active_(0), rtt_seq_(-1), rtx_timer_(this), delsnd_timer_(this) { bind("window_", &wnd_); 82
bind("windowInit_", &wnd_init_); bind("windowOption_", &wnd_option_); bind("windowConstant_", &wnd_const_); ... bind("off_ip_", &off_ip_); bind("off_tcp_", &off_tcp_); ... } The Agent constructor (~ns/agent.cc): Agent::Agent(int pkttype) : addr_(-1), dst_(-1), size_(0), type_(pkttype), fid_(-1), prio_(-1), flags_(0) { memset(pending_, 0, sizeof(pending_)); // this is really an IP agent, so set up // for generating the appropriate IP elds. . . bind("addr_", (int*)&addr_); bind("dst_", (int*)&dst_); bind("fid_", (int*)&fid_); bind("prio_", (int*)&prio_); bind("flags_", (int*)&flags_); ... }
/* timers */
These code fragments illustrate the common case where an agents constructor passes a packet type identier to the Agent constructor. The values for the various packet types are used by the packet tracing facility (Section 21.5) and are dened in ~ns/trace.h. The variables which are bound in the TcpAgent constructor are ordinary instance/member variables for the class with the exception of the special integer values off_tcp_ and off_ip_. These are needed in order to access a TCP header and IP header, respectively. Additional details are in the section on packet headers (Section 11.1). Note that the TcpAgent constructor contains initializations for two timers, rtx_timer_ and delsnd_timer_. TimerHandler objects are initialized by providing a pointer (the this pointer) to the relevant agent.
9.5.2 Starting the Agent
The TcpAgent agent is started in the example when its FTP source receives the start directive at time 1.2. The start operation is an instance procedure dened on the class Application/FTP (Section 31.4). It is dened in ~ns/tcl/lib/ns-source.tcl as follows: Application/FTP instproc start {} { [$self agent] send -1 } In this case, agent refers to our simple TCP agent and send -1 is analogous to sending an arbitrarily large le. The call to send eventually results in the simple TCP sender generating packets. The following function output performs this:
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void TcpAgent::output(int seqno, int reason) { Packet* p = allocpkt(); hdr_tcp *tcph = (hdr_tcp*)p->access(off_tcp_); double now = Scheduler::instance().clock(); tcph->seqno() = seqno; tcph->ts() = now; tcph->reason() = reason; Connector::send(p, 0); ... if (!(rtx_timer_.status() == TIMER_PENDING)) /* No timer pending. Schedule one. */ set_rtx_timer(); } Here we see an illustration of the use of the Agent::allocpkt() method. This output routine rst allocates a new packet (with its common and IP headers already lled in), but then must ll in the appropriate TCP-layer header elds. To nd the TCP header in a packet (assuming it has been enabled (Section 11.2.4)) the off_tcp_ must be properly initialized, as illustrated in the constructor. The packet access() method returns a pointer to the TCP header, its sequence number and time stamp elds are lled in, and the send() method of the class Connector is called to send the packet downstream one hop. Note that the C++ :: scoping operator is used here to avoid calling TcpSimpleAgent::send() (which is also dened). The check for a pending timer uses the timer method status() which is dened in the base class TimerHandler. It is used here to set a retransmission timer if one is not already set (a TCP sender only sets one timer per window of packets on each connection).
9.5.3 Processing Input at Receiver
Many of the TCP agents can be used with the class TCPSink as the peer. This class denes the recv() and ack() methods as follows: void TcpSink::recv(Packet* pkt, Handler*) { hdr_tcp *th = (hdr_tcp*)pkt->access(off_tcp_); acker_->update(th->seqno()); ack(pkt); Packet::free(pkt); } void TcpSink::ack(Packet* opkt) { Packet* npkt = allocpkt(); hdr_tcp *otcp = (hdr_tcp*)opkt->access(off_tcp_); hdr_tcp *ntcp = (hdr_tcp*)npkt->access(off_tcp_); ntcp->seqno() = acker_->Seqno(); ntcp->ts() = otcp->ts(); hdr_ip* oip = (hdr_ip*)opkt->access(off_ip_); hdr_ip* nip = (hdr_ip*)npkt->access(off_ip_); nip->flowid() = oip->flowid();
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hdr_flags* of = (hdr_flags*)opkt->access(off_flags_); hdr_flags* nf = (hdr_flags*)npkt->access(off_flags_); nf->ecn_ = of->ecn_; acker_->append_ack((hdr_cmn*)npkt->access(off_cmn_), ntcp, otcp->seqno()); send(npkt, 0); } The recv() method overrides the Agent::recv() method (which merely discards the received packet). It updates some internal state with the sequence number of the received packet (and therefore requires the off_tcp_ variable to be properly initialized. It then generates an acknowledgment for the received packet. The ack() method makes liberal use of access to packet header elds including separate accesses to the TCP header, IP header, Flags header, and common header. The call to send() invokes the Connector::send() method.
9.5.4 Processing Responses at the Sender
Once the simple TCPs peer receives data and generates an ACK, the sender must (usually) process the ACK. In the TcpAgent agent, this is done as follows: /* * main reception path - should only see acks, otherwise the * network connections are miscongured */ void TcpAgent::recv(Packet *pkt, Handler*) { hdr_tcp *tcph = (hdr_tcp*)pkt->access(off_tcp_); hdr_ip* iph = (hdr_ip*)pkt->access(off_ip_); ... if (((hdr_flags*)pkt->access(off_flags_))->ecn_) quench(1); if (tcph->seqno() > last_ack_) { newack(pkt); opencwnd(); } else if (tcph->seqno() == last_ack_) { if (++dupacks_ == NUMDUPACKS) { ... } } Packet::free(pkt); send(0, 0, maxburst_); } This routine is invoked when an ACK arrives at the sender. In this case, once the information in the ACK is processed (by newack) the packet is no longer needed and is returned to the packet memory allocator. In addition, the receipt of the ACK indicates the possibility of sending additional data, so the TcpSimpleAgent::send() method is invoked which attempts to send more data if the TCP window allows.
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9.5.5 Implementing Timers
As described in the following chapter (Chapter 10), specic timer classes must be derived from an abstract base class TimerHandler dened in ~ns/timer-handler.h. Instances of these subclasses can then be used as various agent timers. An agent may wish to override the Agent::timeout() method (which does nothing). In the case of the Tahoe TCP agent, two timers are used: a delayed send timer delsnd_timer_ and a retransmission timer rtx_timer_. We describe the retransmission timer in TCP (Section 10.1.2) as an example of timer usage.
9.6 Creating a New Agent
To create a new agent, one has to do the following: 1. decide its inheritance structure (Section 9.6.1), and create the appropriate class denitions, 2. dene the recv() and timeout() methods (Section 9.6.2), 3. dene any necessary timer classes, 4. dene OTcl linkage functions (Section 9.6.3), 5. write the necessary OTcl code to access the agent (Section 9.6.4). The action required to create and agent can be illustrated by means of a very simple example. Suppose we wish to construct an agent which performs the ICMP ECHO REQUEST/REPLY (or ping) operations.
9.6.1 Example: A ping requestor (Inheritance Structure)
Deciding on the inheritance structure is a matter of personal choice, but is likely to be related to the layer at which the agent will operate and its assumptions on lower layer functionality. The simplest type of Agent, connectionless datagram-oriented transport, is the Agent/UDP base class. Trafc generators can easily be connected to UDP Agents. For protocols wishing to use a connection-oriented stream transport (like TCP), the various TCP Agents could be used. Finally, if a new transport or sub-transport protocol is to be developed, using Agent as the base class would likely be the best choice. In our example, well use Agent as the base class, given that we are constructing an agent logically belonging to the IP layer (or just above it). We may use the following class denitions: class ECHO_Timer; class ECHO_Agent : public Agent { public: ECHO_Agent(); int command(int argc, const char*const* argv); protected: void timeout(int); void sendit(); double interval_; ECHO_Timer echo_timer_; }; class ECHO_Timer : public TimerHandler { 86
public: ECHO_Timer(ECHO_Agent *a) : TimerHandler() { a_ = a; } protected: virtual void expire(Event *e); ECHO_Agent *a_; };
9.6.2 The recv() and timeout() Methods
The recv() method is not dened here, as this agent represents a request function and will generally not be receiving events or packets1 . By not dening the recv() method, the base class version of recv() (i.e., Connector::recv()) is used. The timeout() method is used to periodically send request packets. The following timeout() method is used, along with a helper method, sendit(): void ECHO_Agent::timeout(int) { sendit(); echo_timer_.resched(interval_); } void ECHO_Agent::sendit() { Packet* p = allocpkt(); ECHOHeader *eh = ECHOHeader::access(p->bits()); eh->timestamp() = Scheduler::instance().clock(); send(p, 0); // Connector::send() } void ECHO_Timer::expire(Event *e) { a_->timeout(0); } The timeout() method simply arranges for sendit() to be executed every interval_ seconds. The sendit() method creates a new packet with most of its header elds already set up by allocpkt(). The packet is only lacks the current time stamp. The call to access() provides for a structured interface to the packet header elds, and is used to set the timestamp eld. Note that this agent uses its own special header (ECHOHeader). The creation and use of packet headers is described in later chapter (Chapter 11); to send the packet to the next downstream node, Connector::send() is invoked without a handler.
9.6.3 Linking the ping Agent with OTcl
We have the methods and mechanisms for establishing OTcl Linkage earlier (Chapter 3). This section is a brief review of the essential features of that earlier chapter, and describes the minimum functionality required to create the ping agent. There are three items we must handle to properly link our agent with Otcl. First we need to establish a mapping between the OTcl name for our class and the actual object created when an instantiation of the class is requested in OTcl. This is done as follows:
1 This
is perhaps unrealistically simple. An ICMP ECHO REQUEST agent would likely wish to process ECHO REPLY messages.
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static class ECHOClass : public TclClass { public: ECHOClass() : TclClass("Agent/ECHO") {} TclObject* create(int argc, const char*const* argv) { return (new ECHO_Agent()); } } class_echo; Here, a static object class_echo is created. Its constructor (executed immediately when the simulator is executed) places the class name Agent/ECHO into the OTcl name space. The mixing of case is by convention; recall from Section 3.5 in the earlier chapters that the / character is a hierarchy delimiter for the interpreted hierarchy. The denition of the create() method species how a C++ shadow object should be created when the OTcl interpreter is instructed to create an object of class Agent/ECHO. In this case, a dynamically-allocated object is returned. This is the normal way new C++ shadow objects are created. Once we have the object creation set up, we will want to link C++ member variables with corresponding variables in the OTcl nname space, so that accesses to OTcl variables are actually backed by member variables in C++. Assume we would like OTcl to be able to adjust the sending interval and the packet size. This is accomplished in the classs constructor: ECHO_Agent::ECHO_Agent() : Agent(PT_ECHO) { bind_time("interval_", &interval_); bind("packetSize_", &size_); } Here, the C++ variables interval_ and size_ are linked to the OTcl instance variables interval_ and packetSize_, respectively. Any read or modify operation to the Otcl variables will result in a corresponding access to the underlying C++ variables. The details of the bind() methods are described elsewhere (Section 3.4.2). The dened constant PT_ECHO is passed to the Agent() constuctor so that the Agent::allocpkt() method may set the packet type eld used by the trace support (Section 21.5). In this case, PT_ECHO represents a new packet type and must be dened in ~ns/trace.h (Section 21.4). Once object creation and variable binding is set up, we may want to create methods implemented in C++ but which can be invoked from OTcl (Section 3.4.4). These are often control functions that initiate, terminate or modify behavior. In our present example, we may wish to be able to start the ping query agent from OTcl using a start directive. This may be implemented as follows: int ECHO_Agent::command(int argc, const char*const* argv) { if (argc == 2) { if (strcmp(argv[1], "start") == 0) { timeout(0); return (TCL_OK); } } return (Agent::command(argc, argv)); } Here, the start() method available to OTcl simply calls the C++ member function timeout() which initiates the rst packet generation and schedules the next. Note this class is so simple it does not even include a way to be stopped.
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9.6.4 Using the agent through OTcl
The agent we have created will have to be instantiated and attached to a node. Note that a node and simulator object is assumed to have already been created. The following OTcl code performs these functions: set echoagent [new Agent/ECHO] $simulator attach-agent $node $echoagent To set the interval and packet size, and start packet generation, the following OTcl code is executed: $echoagent $echoagent $echoagent $echoagent $echoagent $echoagent $echoagent set dst_ $dest set fid_ 0 set prio_ 0 set flags_ 0 set interval_ 1.5 set packetSize_ 1024 start
This will cause our agent to generate one 1024-byte packet destined for node $dest every 1.5 seconds.
9.7 The Agent API
Simulated applications may be implemented on top of protocol agents. Chapter 31 describes the API used by applications to access the services provided by the protocol agent.
9.8 Different agent objects
Class Agent forms the base class from which different types of objects like Nullobject, TCP etc are derived. The methods for Agent class are described in the next section. Conguration parameters for: d_ Flowid. prio_ Priority. agent_addr_ Address of this agent. agent_port_ Port adress of this agent. dst_addr_ Destination address for the agent. dst_port_ Destination port address for the agent. ags_ ttl_ TTL defaults to 32. There are no state variables specic to the generic agent class. Other objects derived from Agent are given below: 89
Null Objects Null objects are a subclass of agent objects that implement a trafc sink. They inherit all of the generic agent object functionality. There are no methods specic to this object. The state variables are:
sport_ dport_
LossMonitor Objects LossMonitor objects are a subclass of agent objects that implement a trafc sink which also maintains some statistics about the received data e.g., number of bytes received, number of packets lost etc. They inherit all of the generic agent object functionality. $lossmonitor clear Resets the expected sequence number to -1. State Variables are: nlost_ Number of packets lost. npkts_ Number of packets received. bytes_ Number of bytes received. lastPktTime_ Time at which the last packet was received. expected_ The expected sequence number of the next packet. TCP objects TCP objects are a subclass of agent objects that implement the BSD Tahoe TCP transport protocol as described in paper: "Fall, K., and Floyd, S. Comparisons of Tahoe, Reno, and Sack TCP. December 1995." URL ftp:// ftp.ee.lbl.gov/papers/sacks.ps.Z. They inherit all of the generic agent functionality. Conguration Parameters are: window_ The upper bound on the advertised window for the TCP connection. maxcwnd_ The upper bound on the congestion window for the TCP connection. Set to zero to ignore. (This is the default.) windowInit_ The initial size of the congestion window on slow-start. windowOption_ The algorithm to use for managing the congestion window. windowThresh_ Gain constant to exponential averaging lter used to compute awnd (see below). For investigations of different window-increase algorithms. overhead_ The range of a uniform random variable used to delay each output packet. The idea is to insert random delays at the source in order to avoid phase effects, when desired [see Floyd, S., and Jacobson, V. On Trafc Phase Effects in Packet-Switched Gateways. Internetworking: Research and Experience, V.3 N.3, September 1992. pp. 115-156 ]. This has only been implemented for the Tahoe ("tcp") version of tcp, not for tcp-reno. This is not intended to be a realistic model of CPU processing overhead. ecn_ Set to true to use explicit congestion notication in addition to packet drops to signal congestion. This allows a Fast Retransmit after a quench() due to an ECN (explicit congestion notication) bit. packetSize_ The size in bytes to use for all packets from this source. tcpTick_ The TCP clock granularity for measuring roundtrip times. Note that it is set by default to the non-standard value of 100ms. bugFix_ Set to true to remove a bug when multiple fast retransmits are allowed for packets dropped in a single window of data. maxburst_ Set to zero to ignore. Otherwise, the maximum number of packets that the source can send in response to a single incoming ACK. slow_start_restart_ Set to 1 to slow-start after the connection goes idle. On by default. Dened Constants are: MWS The Maximum Window Size in packets for a TCP connection. MWS determines the size of an array in tcpsink.cc. The default for MWS is 1024 packets. For Tahoe TCP, the "window" parameter, representing the receivers advertised window, should be less than MWS-1. For Reno TCP, the "window" parameter should be less than (MWS-1)/2. 90
State Variables are: dupacks_ Number of duplicate acks seen since any new data was acknowledged. seqno_ Highest sequence number for data from data source to TCP. t_seqno_ Current transmit sequence number. ack_ Highest acknowledgment seen from receiver. cwnd_ Current value of the congestion window. awnd_ Current value of a low-pass ltered version of the congestion window. For investigations of different windowincrease algorithms. ssthresh_ Current value of the slow-start threshold. rtt_ Round-trip time estimate. srtt_ Smoothed round-trip time estimate. rttvar_ Round-trip time mean deviation estimate. backoff_ Round-trip time exponential backoff constant. TCP/Reno Objects TCP/Reno objects are a subclass of TCP objects that implement the Reno TCP transport protocol described in paper: "Fall, K., and Floyd, S. Comparisons of Tahoe, Reno, and Sack TCP. December 1995." URL ftp:// ftp.ee.lbl.gov/papers/sacks.ps.Z. There are no methods, conguration parameters or state variables specic to this object. TCP/Newreno Objects TCP/Newreno objects are a subclass of TCP objects that implement a modied version of the BSD Reno TCP transport protocol. There are no methods or state variables specic to this object. Conguration Parameters are: newreno_changes_ Set to zero for the default NewReno described in "Fall, K., and Floyd, S. Comparisons of Tahoe, Reno, and Sack TCP. December 1995". Set to 1 for additional NewReno algorithms [see Hoe, J., Improving the Start-up Behavior of a Congestion Control Scheme for TCP. in SIGCOMM 96, August 1996, pp. 270-280. URL http://www.acm.org/sigcomm/sigcomm96/papers/hoe.html.]; this includes the estimation of the ssthresh parameter during slow-start. TCP/Vegas Objects There are no methods or conguration parameters specic to this object. State variables are:
v_alpha_ v_beta_ v_gamma_ v_rtt_
TCP/Sack1 Objects TCP/Sack1 objects are a subclass of TCP objects that implement the BSD Reno TCP transport protocol with Selective Acknowledgement Extensions described in "Fall, K., and Floyd, S. Comparisons of Tahoe, Reno, and Sack TCP. December 1995". URL ftp:// ftp.ee.lbl.gov/papers/sacks.ps.Z. They inherit all of the TCP object functionality. There are no methods, conguration parameters or state variables specic to this object. TCP/FACK Objects TCP/Fack objects are a subclass of TCP objects that implement the BSD Reno TCP transport protocol with Forward Acknowledgement congestion control. They inherit all of the TCP object functionality. There are no methods or state variables specic to this object. Conguration Parameters are: ss-div4 Overdamping algorithm. Divides ssthresh by 4 (instead of 2) if congestion is detected within 1/2 RTT of slow-start. (1=Enable, 0=Disable) rampdown Rampdown data smoothing algorithm. Slowly reduces congestion window rather than instantly halving it. (1=Enable, 0=Disable) TCP/FULLTCP Objects This section has not yet been added here. The implementation and the conguration parameters are described in paper: "Fall, K., Floyd, S., and Henderson, T., Ns Simulator Tests for Reno FullTCP. July, 1997." URL ftp://ftp.ee.lbl.gov/papers/fulltcp.ps. 91
TCPSINK Objects TCPSink objects are a subclass of agent objects that implement a receiver for TCP packets. The simulator only implements "one-way" TCP connections, where the TCP source sends data packets and the TCP sink sends ACK packets. TCPSink objects inherit all of the generic agent functionality. There are no methods or state variables specic to the TCPSink object. Conguration Parameters are packetSize_ The size in bytes to use for all acknowledgment packets. maxSackBlocks_ The maximum number of blocks of data that can be acknowledged in a SACK option. For a receiver that is also using the time stamp option [RFC 1323], the SACK option specied in RFC 2018 has room to include three SACK blocks. This is only used by the TCPSink/Sack1 subclass. This value may not be increased within any particular TCPSink object after that object has been allocated. (Once a TCPSink object has been allocated, the value of this parameter may be decreased but not increased). TCPSINK/DELACK Objects DelAck objects are a subclass of TCPSink that implement a delayed-ACK receiver for TCP packets. They inherit all of the TCPSink object functionality. There are no methods or state variables specic to the DelAck object. Conguration Parameters are: interval_ The amount of time to delay before generating an acknowledgment for a single packet. If another packet arrives before this time expires, generate an acknowledgment immediately. TCPSINK/SACK1 Objects TCPSink/Sack1 objects are a subclass of TCPSink that implement a SACK receiver for TCP packets. They inherit all of the TCPSink object functionality. There are no methods, conguration parameters or state variables specic to this object. TCPSINK/SACK1/DELACK Objects TCPSink/Sack1/DelAck objects are a subclass of TCPSink/Sack1 that implement a delayed-SACK receiver for TCP packets. They inherit all of the TCPSink/Sack1 object functionality. There are no methods or state variables specic to this object. Conguration Parameters are: interval_ The amount of time to delay before generating an acknowledgment for a single packet. If another packet arrives before this time expires, generate an acknowledgment immediately.
9.9 Commands at a glance
Following are the agent related commands used in simulation scripts: ns_ attach-agent <node> <agent> This command attaches the <agent> to the <node>. We assume here that the <agent> has already been created. An agent is typically created by set agent [new Agent/AgentType] where Agent/AgentType denes the class deniton of the specied agent type. $agent port This returns the port number to which the agent is attached. $agent dst-port This returns the port number of the destination. When any connection is setup between 2 nodes, each agent stores the destination port in its instance variable called dst_port_. $agent attach-app <s_type> This commands attaches an application of type <s_type> to the agent. A handle to the application object is returned. Also note that the application type must be dened as a packet type in packet.h. $agent attach-source <s_type> This used to be the procedure to attach source of type <s_type> to the agent. But this is obsolete now. Use attach-app (described above) instead. 92
$agent attach-tbf <tbf> Attaches a token bucket lter (tbf) to the agent. $ns_ connect <src> <dst> Sets up a connection between the src and dst agents. $ns_ create-connection <srctype> <src> <dsttype> <dst> <pktclass> This sets up a complete connection between two agents. First creates a source of type <srctype> and binds it to <src>. Then creates a destination of type <dsttype> and binds it to <dst>. Finally connects the src and dst agents and returns a handle to the source agent. $ns_ create-connection-list <srctype> <src> <dsttype> <dst> <pktclass> This command is exactly similar to create-connection described above. But instead of returning only the source-agent, this returns a list of source and destination agents. Internal procedures: $ns_ simplex-connect <src> <dst> This is an internal method that actually sets up an unidirectional connection between the <src> agent and <dst> agent. It simply sets the destination address and destination port of the <src> as <dst>s agent-address and agent-port. The "connect" described above calls this method twice to set up a bi-directional connection between the src and dst. $agent set <args> This is an internal procedure used to inform users of the backward compatibility issues resulting from the upgrade to 32-bit addressing space currently used in ns. $agent attach-trace <file> This attaches the <le> to the agent to allow nam-tracing of the agent events. In addition to the agent related procedures described here, there are additional methods that support different type of agents like Agent/Null, Agent/TCP, Agent/CBR, Agent/TORA, Agent/mcast etc. These additional methods along with the procedures described here can be found in ns/tcl/lib/(ns-agent.tcl, ns-lib.tcl, ns-mip.tcl, ns-mobilenode.tcl, ns-namsupp.tcl, ns-queue.tcl, ns-route.tcl, ns-sat.tcl, ns-source.tcl). They are also described in the previous section.
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Chapter 10
Timers
Timers may be implemented in C++ or OTcl. In C++, timers are based on an abstract base class dened in ~ns/timer-handler.h. They are most often used in agents, but the framework is general enough to be used by other objects. The discussion below is oriented towards the use of timers in agents. The procedures and functions described in this chapter can be found in ~ns/tcl/ex/timer.tcl, and ~ns/timer-handler.{cc, h}. In OTcl, a simple timer class is dened in ~ns/tcl/ex/timer.tcl. Subclasses can be derived to provide a simple mechanism for scheduling events at the OTcl level.
10.1 C++ abstract base class TimerHandler
The abstract base class TimerHandler contains the following public member functions: void sched(double delay) void resched(double delay) void cancel() int status() schedule a timer to expire delay seconds in the future reschedule a timer (similar to sched(), but timer may be pending) cancel a pending timer returns timer status (either TIMER_IDLE, TIMER_PENDING, TIMER_HANDLING)
or
The abstract base class TimerHandler contains the following protected members: virtual void expire(Event* e) virtual void handle(Event* e) int status_ Event event_ =0 =0 this method must be lled in by the timer client consumes an event keeps track of the current timer status event to be consumed upon timer expiration
The pure virtual functions must be dened by the timer classes deriving from this abstract base class. Finally, two private inline functions are dened: inline void _sched(double delay) { (void)Scheduler::instance().schedule(this, &event_, delay); 94
} inline void _cancel() { (void)Scheduler::instance().cancel(&event_); } From this code we can see that timers make use of methods of the Scheduler class.
10.1.1 Denition of a new timer
To dene a new timer, subclass this function and dene handle() if needed (handle() is not always required): class MyTimer : public TimerHandler { public: MyTimer(MyAgentClass *a) : TimerHandler() { a_ = a; } virtual double expire(Event *e); protected: MyAgentClass *a_; }; Then dene expire: double MyTimer::expire(Event *e) { // do the work // return TIMER_HANDLED; // return delay; }
// => do not reschedule timer // => reschedule timer after delay
Note that expire() can return either the ag TIMER_HANDLED or a delay value, depending on the requirements for this timer. Often MyTimer will be a friend of MyAgentClass, or expire() will only call a public function of MyAgentClass. Timers are not directly accessible from the OTcl level, although users are free to establish method bindings if they so desire.
10.1.2 Example: Tcp retransmission timer
TCP is an example of an agent which requires timers. There are three timers dened in the basic Tahoe TCP agent dened in tcp.cc: rtx_timer_; delsnd_timer_; burstsnd_timer_; /* Retransmission timer */ /* Delays sending of packets by a small random amount of time, */ /* to avoid phase effects */ /* Helps TCP to stagger the transmission of a large window */ /* into several smaller bursts */
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In ~ns/tcp.h, three classes are derived from the base class class TimerHandler: class RtxTimer : public TimerHandler { public: RtxTimer(TcpAgent *a) : TimerHandler() { a_ = a; } protected: virtual void expire(Event *e); TcpAgent *a_; }; class DelSndTimer : public TimerHandler { public: DelSndTimer(TcpAgent *a) : TimerHandler() { a_ = a; } protected: virtual void expire(Event *e); TcpAgent *a_; }; class BurstSndTimer : public TimerHandler { public: BurstSndTimer(TcpAgent *a) : TimerHandler() { a_ = a; } protected: virtual void expire(Event *e); TcpAgent *a_; }; In the constructor for TcpAgent in tcp.cc, each of these timers is initialized with the this pointer, which is assigned to the pointer a_. TcpAgent::TcpAgent() : Agent(PT_TCP), rtt_active_(0), rtt_seq_(-1), ... rtx_timer_(this), delsnd_timer_(this), burstsnd_timer_(this) { ... } In the following, we will focus only on the retransmission timer. Various helper methods may be dened to schedule timer events; e.g., /* * Set retransmit timer using current rtt estimate. By calling resched() * it does not matter whether the timer was already running. */ void TcpAgent::set_rtx_timer() { rtx_timer_.resched(rtt_timeout()); } /* * Set new retransmission timer if not all outstanding * data has been acked. Otherwise, if a timer is still 96
* outstanding, cancel it. */ void TcpAgent::newtimer(Packet* pkt) { hdr_tcp *tcph = (hdr_tcp*)pkt->access(off_tcp_); if (t_seqno_ > tcph->seqno()) set_rtx_timer(); else if (rtx_timer_.status() == TIMER_PENDING) rtx_timer_.cancel(); } In the above code, the set_rtx_timer() method reschedules the retransmission timer by calling rtx_timer_.resched(). Note that if it is unclear whether or not the timer is already running, calling resched() eliminates the need to explicitly cancel the timer. In the second function, examples are given of the use of the status() and cancel(void) methods. Finally, the expire(void) method for class RtxTimer must be dened. In this case, expire(void) calls the timeout(void) method for TcpAgent. This is possible because timeout() is a public member function; if it were not, then RtxTimer would have had to have been declared a friend class of TcpAgent. void TcpAgent::timeout(int tno) { /* retransmit timer */ if (tno == TCP_TIMER_RTX) { if (highest_ack_ == maxseq_ && !slow_start_restart_) { /* * TCP option: * If no outstanding data, then dont do anything. */ return; }; recover_ = maxseq_; recover_cause_ = 2; closecwnd(0); reset_rtx_timer(0,1); send_much(0, TCP_REASON_TIMEOUT, maxburst_); } else { /* * delayed-send timer, with random overhead * to avoid phase effects */ send_much(1, TCP_REASON_TIMEOUT, maxburst_); } } void RtxTimer::expire(Event *e) { a_->timeout(TCP_TIMER_RTX); } The various TCP agents contain additional examples of timers.
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10.2 OTcl Timer class
A simple timer class is dened in ~ns/tcl/mcast/timer.tcl. Subclasses of Timer can be dened as needed. Unlike the C++ timer API, where a sched() aborts if the timer is already set, sched() and resched() are the same; i.e., no state is kept for the OTcl timers. The following methods are dened in the Timer base class: $self $self $self $self $self sched $delay resched $delay cancel destroy expire ;# causes "$self timeout" to be called $delay seconds in the future ;# same as "$self sched $delay" ;# cancels any pending scheduled callback ;# same as "$self cancel" ;# calls "$self timeout" immediately
10.3 Commands at a glance
Following is a list of methods for the class Timer. Note that many different types of timers have been derived from this base class (viz. LogTimer, Timer/Iface, Timer/Iface/Prune, CacheTimer, Timer/Scuba etc). $timer sched <delay> This command cancels any other event that may have been scheduled and re-schedules another event after time <delay>. $timer resched <delay> Similar to "sched" described above. Added to have similar APIs as that of the C++ timers. $timer cancel This cancels any scheduled event. $timer destroy This is similar to cancel. Cancels any scheduled event. $timer expire This command calls for a time-out. However the time-out procedure needs to be dened in the sub-classes. All these procedures can be found in ns/tcl/mcast/timer.tcl.
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Chapter 11
Packet Headers and Formats
The procedures and functions described in this chapter can be found in ~ns/tcl/lib/ns-lib.tcl, ~ns/tcl/lib/ns-packet.tcl, and ~ns/packet.{cc, h}. Objects in the class Packet are the fundamental unit of exchange between objects in the simulation. The class Packet provides enough information to link a packet on to a list (i.e., in a PacketQueue or on a free list of packets), refer to a buffer containing packet headers that are dened on a per-protocol basis, and to refer to a buffer of packet data. New protocols may dene their own packet headers or may extend existing headers with additional elds. New packet headers are introduced into the simulator by dening a C++ structure with the needed elds, dening a static class to provide OTcl linkage, and then modifying some of the simulator initialization code to assign a byte offset in each packet where the new header is to be located relative to others. When the simulator is initialized through OTcl, a user may choose to enable only a subset of the compiled-in packet formats, resulting in a modest savings of memory during the execution of the simulation. Presently, most congured-in packet formats are enabled. The management of which packet formats are currently enabled in a simulation is handled by a special packet header manager object described below. This object supports an OTcl method used to specify which packet headers will be used in a simulation. If an object in the simulator makes use of a eld in a header which has not been enabled, a run-time fatal program abort occurs.
11.1 A Protocol-Specic Packet Header
Protocol developers will often wish to provide a specic header type to be used in packets. Doing so allows a new protocol implementation to avoid overloading already-existing header elds. We consider a simplied version of RTP as an example. The RTP header will require a sequence number elds and a source identier eld. The following classes create the needed header (see ~ns/rtp.h and ~ns/rtp.cc): From rtp.h: /* rtp packet. For now, just have srcid + seqno. */ struct hdr_rtp { u_int32_t srcid_; int seqno_; /* per-eld member functions */ u_int32_t& srcid() { return (srcid_); } 99
int& seqno() { return (seqno_); } /* Packet header access functions */ static int offset_; inline static int& offset() { return offset_; } inline static hdr_rtp* access(const Packet* p) { return (hdr_rtp*) p->access(offset_); } }; From rtp.cc: class RTPHeaderClass : public PacketHeaderClass { public: RTPHeaderClass() : PacketHeaderClass("PacketHeader/RTP", sizeof(hdr_rtp)) { bind_offset(&hdr_rtp::offset_); } } class_rtphdr; void RTPAgent::sendpkt() { Packet* p = allocpkt(); hdr_rtp *rh = hdr_rtp::access(p); lastpkttime_ = Scheduler::instance().clock(); /* Fill in srcid_ and seqno */ rh->seqno() = seqno_++; rh->srcid() = session_->srcid(); target_->recv(p, 0); } RTPAgent::RTPAgent() : session_(0), lastpkttime_(-1e6) { type_ = PT_RTP; bind("seqno_", &seqno_); } The rst structure, hdr_rtp, denes the layout of the RTP packet header (in terms of words and their placement): which elds are needed and how big they are. This structure denition is only used by the compiler to compute byte offsets of elds; no objects of this structure type are ever directly allocated. The structure also provides member functions which in turn provide a layer of data hiding for objects wishing to read or modify header elds of packets. Note that the static class variable offset_ is used to nd the byte offset at which the rtp header is located in an arbitrary nspacket. Two methods are provided to utilize this variable to access this header in any packet: offset() and access(). The latter is what most users should choose to access this particular header in a packet; the former is used by the packet header management class and should seldom be used. For example, to access the RTP packet header in a packet pointed by p, one simply says hdr_rtp::access(p). The actual binding of offset_ to the position of this header in a packet is done by routines inside ~ns/tcl/lib/ns-packet.tcl and ~ns/packet.cc. The const in access()s argument provides (presumably) read-only access to a const Packet, lthough read-only is enforced since the return pointer is not const. One correct way to do this is to provide two methods, one for write access, the other for read-only access. However, this is not currently implemented. IMPORTANT: Notice that this is completely different from the original (and obsolete) method to access a packet header, which requires that an integer variable, off_ hdrname _, be dened for any packet header that one needs to access. This 100
method is now obsolete; its usage is tricky and its misuse can be very difcult to detect. The static object class_rtphdr of class RTPHeaderClass is used to provide linkage to OTcl when the RTP header is enabled at conguration time. When the simulator executes, this static object calls the PacketHeaderClass constructor with arguments "PacketHeader/RTP" and sizeof(hdr_rtp). This causes the size of the RTP header to be stored and made available to the packet header manager at conguration time (see below, Section 11.2.4). Notice that bind_offset() MUST be called in the constructor of this class, so that the packet header manager knows where to store the offset for this particular packet header. The sample member function sendpkt() method of RTPAgent creates a new packet to send by calling allocpkt(), which handles assignment of all the network-layer packet header elds (in this case, IP). Headers other than IP are handled separately. In this case, the agent uses the RTPHeader dened above. The Packet::access(void) member function returns the address of the rst byte in a buffer used to hold header information (see below). Its return value is cast as a pointer to the header of interest, after which member functions of the RTPHeader object are used to access individual elds.
11.1.1 Adding a New Packet Header Type
Assuming we wish to create a new header called newhdr the following steps are performed: 1. create a new structure dening the raw elds (called hdr_newhdr), dene offset_ and access methods. 2. dene member functions for needed elds. 3. create a static class to perform OTcl linkage (denes PacketHeader/Newhdr), do bind_offset() in its constructor. 4. edit ~ns/tcl/lib/ns-packet.tcl to enable new packet format (see 11.2.2, 11.2.4). This is the recommended way to add your packet headers. If you do not follow this method, your simulation may still work, but it may behave in a unpredictable way when more protocols are added into your simulation. The reason is that the BOB (Bag of Bits, Section 11.2.1) in nspacket is a large sparse space, assigning one wrong packet header offset may not trigger failure immediately.
11.1.2 Selectively Including Packet Headers in Your Simulation
By default, ns includes ALL packet headers of ALL protocols in ns in EVERY packet in your simulation. This is a LOT of overhead, and will increase as more protocols are added into ns. For packet-intensive simulations, this could be a huge overhead. For instance, as of now (Aug 30, 2000), the size of packet headers of all protocols in ns is about 1.9KB; however, if you turn on only the common header, the IP header and the TCP header, they add up to about 100 bytes. If you are doing large-scale web trafc simulation with many big fat pipes, reducing unused packet headers can lead to major memory saving. To include only the packet headers that are of interest to you in your specic simulation, follow this pattern (e.g., you want to remove AODV and ARP headers from your simulation): remove-packet-header AODV ARP ...... set ns [new Simulator] Notice that remove-packet-header MUST go before the simulator is created. All packet header names are in the forms of PacketHeader/[hdr]. You only need to supply the [hdr] part, not the prex. To nd the names of packet headers, you may either look them up in ~ns/tcl/lib/ns-packet.tcl, or run the following simple commands in ns: 101
foreach cl [PacketHeader info subclass] { puts $cl } To include only a specic set of headers in your simulation, e.g., IP and TCP, follow this pattern: remove-all-packet-headers add-packet-header IP TCP ...... set ns [new Simulator] IMPORTANT: You MUST never remove common header from your simulation. As you can see in ~ns/tcl/lib/ns-packet.tcl, this is enforced by these header manipulation procs. Notice that by default, all packet headers are included.
11.2 Packet Classes
There are four C++ classes relevant to the handling of packets and packet headers in general: Packet, p_info PacketHeader, and PacketHeaderManager. The class Packet denes the type for all packets in the simulation; it is a subclass of Event so that packets may be scheduled (e.g. for later arrival at some queue). The class packet_info holds all text representations for packet names. The class PacketHeader provides a base class for any packet header congured into the simulation. It essentially provides enough internal state to locate any particular packet header in the collection of packet headers present in any given packet. The class PacketHeaderManager denes a class used to collect and manage currently-congured headers. It is invoked by a method available to OTcl at simulation conguration time to enable some subset of the compiled-in packet headers.
11.2.1 The Packet Class
The class Packet denes the structure of a packet and provides member functions to handle a free list for objects of this type. It is illustrated in Figure 11.1 and dened as follows in packet.h: class Packet : public Event { private: friend class PacketQueue; u_char* bits_; u_char* data_; /* variable size buffer for data */ u_int datalen_; /* length of variable size buffer */ protected: static Packet* free_; public: Packet* next_; /* for queues and the free list */ static int hdrlen_; Packet() : bits_(0), datalen_(0), next_(0) {} u_char* const bits() { return (bits_); } Packet* copy() const; static Packet* alloc(); 102
Packet
next_ hdrsize_ bits()
points to next packet in either free list or in a PacketQueue accessdata() packet data
size determined at compile time Size Determined at Simulator Config Time, stored in hdrsize_
ip header body
size determined at compile time size determined at compile time size determined at compile time
Figure 11.1: A Packet Object
tcp header body
rtp header body trace header body
static inline static inline
Packet* alloc(int); void allocdata(int); void free(Packet*); u_char* access(int off) { if (off < 0) abort(); return (&bits_[off]);
} inline u_char* accessdata() { return data_; } }; This class holds a pointer to a generic array of unsigned characters (commonly called the bag of bits or BOB for short) where packet header elds are stored. It also holds a pointer to packet data (which is often not used in simulations). The bits_ variable contains the address of the rst byte of the BOB. Effectively BOB is (currently implemented as) a concatenation of all the structures dened for each packet header (by convention, the structures with names beginning hdr_ something ) that have been congured in. BOB generally remains a xed size throughout a simulation, and the size is recorded in the Packet::hdrlen_ member variable. This size is updated during simulator conguration by OTcl 1 . The other methods of the class Packet are for creating new packets and storing old (unused) ones on a private free list. Such allocation and deallocation is performed by the following code (in ~ns/packet.h): inline Packet* Packet::alloc() { Packet* p = free_; if (p != 0) free_ = p->next_;
1 It
is not intended to be updated after conguration time. Doing so should be possible, but is currently untested.
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else { p = new Packet; p->bits_ = new u_char[hdrsize_]; if (p == 0 || p->bits_ == 0) abort(); } return (p); } /* allocate a packet with an n byte data buffer */ inline Packet* Packet::alloc(int n) { Packet* p = alloc(); if (n > 0) p->allocdata(n); return (p); } /* allocate an n byte data buffer to an existing packet */ inline void Packet::allocdata(int n) { datalen_ = n; data_ = new u_char[n]; if (data_ == 0) abort(); } inline void Packet::free(Packet* p) { p->next_ = free_; free_ = p; if (p->datalen_) { delete p->data_; p->datalen_ = 0; } } inline Packet* Packet::copy() const { Packet* p = alloc(); memcpy(p->bits(), bits_, hdrlen_); if (datalen_) { p->datalen_ = datalen_; p->data_ = new u_char[datalen_]; memcpy(p->data_, data_, datalen_); } return (p); } The alloc() method is a support function commonly used to create new packets. It is called by Agent::allocpkt() method on behalf of agents and is thus not normally invoked directly by most objects. It rst attempts to locate an old packet on the free list and if this fails allocates a new one using the C++ new operator. Note that Packet class objects and BOBs are allocated separately. The free() method frees a packet by returning it to the free list. Note that packets are never returned to 104
the systems memory allocator. Instead, they are stored on a free list when Packet::free() is called. The copy() member creates a new, identical copy of a packet with the exception of the uid_ eld, which is unique. This function is used by Replicator objects to support multicast distribution and LANs.
11.2.2 p_info Class
This class is used as a glue to bind numeric packet type values with their symbolic names. When a new packet type is dened, its numeric code should be added to the enumeration packet_t (see ~ns/packet.h) 2 and its symbolic name should be added to the constructor of p_info: enum packet_t { PT_TCP, ... PT_NTYPE // This MUST be the LAST one }; class p_info { public: p_info() { name_[PT_TCP]= "tcp"; ... } }
11.2.3 The hdr_cmn Class
Every packet in the simulator has a common header which is dened in ~ns/packet.h as follows: struct hdr_cmn { double packet_t int int int
ts_; ptype_; uid_; size_; iface_;
/* timestamp: for q-delay measurement */ /* packet type (see above) */ /* unique id */ /* simulated packet size */ /* receiving interface (label) */
/* Packet header access functions */ static int offset_; inline static int& offset() { return offset_; } inline static hdr_cmn* access(Packet* p) { return (hdr_cmn*) p->access(offset_); } /* per-eld member functions */ int& ptype() { return (ptype_); } int& uid() { return (uid_); } int& size() { return (size_); } int& iface() { return (iface_); }
2 Note:
PT_NTYPE should remain the last element of this enumeration.
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double& timestamp() { return (ts_); } }; This structure primarily denes elds used for tracing the ow of packets or measuring other quantities. The time stamp eld is used to measure queuing delay at switch nodes. The ptype_ eld is used to identify the type of packets, which makes reading traces simpler. The uid_ eld is used by the scheduler in scheduling packet arrivals. The size_ eld is of general use and gives the simulated packets size in bytes. Note that the actual number of bytes consumed in the simulation may not relate to the value of this eld (i.e., size_ has no relationship to sizeof(struct hdr_cmn) or other ns structures). Rather, it is used most often in computing the time required for a packet to be delivered along a network link. As such it should be set to the sum of the application data size and IP-, transport-, and application-level headers for the simulated packet. The iface_ eld is used by the simulator when performing multicast distribution tree computations. It is a label indicating (typically) on which link a packet was received.
11.2.4 The PacketHeaderManager Class
An object of the class PacketHeaderManager is used to manage the set of currently-active packet header types and assign each of them unique offsets in the BOB. It is dened in both the C++ and OTcl code: From tcl/lib/ns-packet.tcl: PacketHeaderManager set hdrlen_ 0 ...... foreach prot { AODV ARP aSRM Common CtrMcast Diffusion ...... TORA UMP }{ add-packet-header $prot } Simulator instproc create_packetformat {} { PacketHeaderManager instvar tab_ set pm [new PacketHeaderManager] foreach cl [PacketHeader info subclass] { if [info exists tab_($cl)] { set off [$pm allochdr $cl] $cl offset $off } } $self set packetManager_ $pm } PacketHeaderManager instproc allochdr cl { set size [$cl set hdrlen_] $self instvar hdrlen_ set NS_ALIGN 8 ;# round up to nearest NS_ALIGN bytes, (needed on sparc/solaris) set incr [expr ($size + ($NS_ALIGN-1)) & ~($NS_ALIGN-1)] set base $hdrlen_ 106
incr hdrlen_ $incr return $base } From packet.cc: /* manages active packet header types */ class PacketHeaderManager : public TclObject { public: PacketHeaderManager() { bind("hdrlen_", &Packet::hdrlen_); } }; The code in ~ns/tcl/lib/ns-packet.tcl is executed when the simulator initializes. Thus, the foreach statement is executed before the simulation begins, and initializes the OTcl class array tab_ to contain the mapping between class the name and the names of the currently active packet header classes. As discussed above (11.1), packet headers should be accessed using hdr_ hdrname ::access(). The create_packetformat{} instance procedure is part of the basic Simulator class and is called one time during simulator conguration. It rst creates a single PacketHeaderManager object. The C++ constructor links the OTcl instance variable hdrlen_ (of class PacketHeaderManager) to the C++ variable Packet::hdrlen_ (a static member of the Packet class). This has the effect of setting Packet::hdrlen_ to zero. Note that binding across class types in this fashion is unusual. After creating the packet manager, the foreach loop enables each of the packet headers of interest. This loop iterates through the list of dened packet headers of the form where is the name of the th header and is the name of the variable containing the location of the header in BOB. The placement of headers is performed by the allochdr instproc of the PacketHeaderManager OTcl class. The procedure keeps a running variable hdrlen_ with the current length of BOB as new packet headers are enabled. It also arranges for 8-byte alignment for any newly-enabled packet header. This is needed to ensure that when double-world length quantities are used in packet headers on machines where double-word alignment is required, access faults are not produced. 3.
11.3 Commands at a glance
Following is a list of packet-header related procedures: Simulator::create_packetformat This is an internal simulator procedure and is called once during the simulator conguration to setup a packetHeaderManager object. PacketHeaderManager::allochdr This is another internal procedure of Class PacketHeaderManager that keeps track of a variable called hdrlen_ as new packet-headers are enabled. It also allows 8-byte allignment for any newly-enabled pkt header. add-packet-header takes a list of arguments, each of which is a packet header name (without PacketHeader/ prex). This global proc will tell simulator to include the specied packet header(s) in your simulation.
3 In some processer architectures, including the Sparc and HP-PA, double-word access must be performed on a double-word boundary (i.e. addresses ending in 0 mod 8). Attempting to perform unaligned accesses result in an abnormal program termination.
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remove-packet-header operates in the same syntax, but it removes the specied headers from your simulation; notice that it does not remove the common header even it is instructed to do so. remove-all-packet-headers is a global Tcl proc. It takes no argument and removes all packet headers, except the common header, from your simulation. add-all-packet-headers is its counterpart.
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Chapter 12
Error Model
This chapter describes the implementation and conguration of error models, which introduces packet losses into a simulation.
12.1 Implementation
The procedures and functions described in this section can be found in ~ns/errmodel.{cc, h}. Error model simulates link-level errors or loss by either marking the packets error ag or dumping the packet to a drop target. In simulations, errors can be generated from a simple model such as the packet error rate, or from more complicated statistical and empirical models. To support a wide variety of models, the unit of error can be specied in term of packet, bits, or time-based. The ErrorModel class is derived from the Connector base class. As the result, it inherits some methods for hooking up objects such as target and drop-target. If the drop target exists, it will received corrupted packets from ErrorModel. Otherwise, ErrorModel just marks the error_ ag of the packets common header, thereby, allowing agents to handle the loss. The ErrorModel also denes additional Tcl method unit to specify the unit of error and ranvar to specify the random variable for generating errors. If not specied, the unit of error will be in packets, and the random variable will be uniform distributed from 0 to 1. Below is a simple example of creating an error model with the packet error rate of 1 percent (0.01): # create a loss_module and set its packet error rate to 1 percent set loss_module [new ErrorModel] $loss_module set rate_ 0.01 # optional: set the unit and random variable $loss_module unit pkt $loss_module ranvar [new RandomVariable/Uniform] # set target for dropped packets $loss_module drop-target [new Agent/Null] In C++, the ErrorModel contains both the mechanism and policy for dropping packets. The packet dropping mechanism is handled by the recv method, and packet corrupting policy is handled by the corrupt method.
;# error unit: packets (the default)
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enum ErrorUnit { EU_PKT=0, EU_BIT, EU_TIME }; class ErrorModel : public Connector { public: ErrorModel(); void recv(Packet*, Handler*); virtual int corrupt(Packet*); inline double rate() { return rate_; } protected: int command(int argc, const char*const* argv); ErrorUnit eu_; /* error unit in pkt, bit, or time */ RandomVariable* ranvar_; double rate_; }; The ErrorModel only implements a simple policy based on a single error rate, either in packets of bits. More sophisticated dropping policy can be implemented in C++ by deriving from ErrorModel and redening its corrupt method.
12.2 Conguration
The previous section talked about error model, in this section we discuss how to use error models in ns. To use an error model, it has to be inserted into a SimpleLink object. Because a SimpleLink is a composite object (Chapter 6), an error model can be inserted to many places. Currently we provide the following methods to insert an error module into three different places.
Insert an error module in a SimpleLink BEFORE the queue module. This is provided by the following two OTcl methods: SimpleLink::errormodule args When an error model is given as a parameter, it inserts the error module into the simple link, right after the queue module, and set the drop-target of the error model to be the drop trace object of the simple link. Note that this requires the following conguration order: ns namtrace-all followed by link congurations, followed by error model insertion. When no argument is given, it returns the current error model in the link, if theres any. This method is dened in ns/tcl/lib/ns-link.tcl Call SimpleLink::errormodule to insert the given error module into the simple link (src, dst). Its simply a wrapper for the above method. This method is dened in ns/tcl/lib/ns-lib.tcl.
Simulator::lossmodel em
src
dst
Insert an error module in a SimpleLink AFTER the queue but BEFORE the delay link. This is provided by the following two methods: SimpleLink::insert-linkloss args This methods behavior is identical to that of SimpleLink::errormodule, except that it inserts an error module immediately after the queue object. Its dened in ns/tcl/lib/ns-link.tcl This is a wrapper for SimpleLink::insert-linkloss. Its dened in ns/tcl/lib/ns-lib.tcl
Simulator::link-lossmodel em
src
dst
The nam traces generated by error models inserted using these two methods do not require special treatment and can be visualized using an older version of nam.
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Insert an error module in a Link AFTER the delay link module. This can be done by Link::install-error. Currently this API doesnt produce any trace. It only serves as a placeholder for possible future extensions.
12.3 Multi-state error model
Contributed by Jianping Pan ([e-mail]). The multi-state error model implements time-based error state transitions. Transitions to the next error state occur at the end of the duration of the current state. The next error state is then selected using the transition state matrix. To create a multi-state error model, the following parameters should be supplied (as dened in ns/tcl/lib/ns-errmodel.tcl):
states: an array of states (error models). periods: an array of state durations. trans: the transition state model matrix. transunit: one of [pkt|byte|time]. sttype: type of state transitions to use: either time or pkt. nstates: number of states. start: the start state.
Here is a simple example script to create a multi-state error model: set tmp [new ErrorModel/Uniform 0 pkt] set tmp1 [new ErrorModel/Uniform .9 pkt] set tmp2 [new ErrorModel/Uniform .5 pkt] # Array of states (error models) set m_states [list $tmp $tmp1 $tmp2] # Durations for each of the states, tmp, tmp1 and tmp2, respectively set m_periods [list 0 .0075 .00375] # Transition state model matrix set m_transmx { {0.95 0.05 0} {0 0 1} {1 0 0} } set m_trunit pkt # Use time-based transition set m_sttype time set m_nstates 3 set m_nstart [lindex $m_states 0] set em [new ErrorModel/MultiState $m_states $m_periods $m_transmx $m_trunit $m_sttype $m_nstates $m_nstart]
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12.4 Commands at a glance
The following is a list of error-model related commands commonly used in simulation scripts:
set $em $em $em $em
em [new ErrorModel] unit pkt set rate_ 0.02 ranvar [new RandomVariable/Uniform] drop-target [new Agent/Null]
This is a simple example of how to create and congure an error model. The commands to place the error-model in a simple link will be shown next.
$simplelink errormodule <args> This commands inserts the error-model before the queue object in simple link. However in this case the error-models drop-target points to the links drophead_ element. $ns_ lossmodel <em> <src> <dst> This command places the error-model before the queue in a simplelink dened by the <src> and <dst> nodes. This is basically a wrapper for the above method. $simplelink insert-linkloss <args> This inserts a loss-module after the queue, but right before the delay link_ element in the simple link. This is because nam can visualize a packet drop only if the packet is on the link or in the queue. The error-modules drop-target points to the links drophead_ element. $ns_ link-lossmodel <em> <src> <dst> This too is a wrapper method for insert-linkloss method described above. That is this inserts the error-module right after the queue element in a simple link (src-dst). In addition to the methods described above for the class ErrorModel, there are several other types of error modules derived from this base class like SRMErrorModel, ErrorModel/Trace, ErrorModel/Periodic etc. These denitions can be found in ns/tcl/lib/(ns-errmodel.tcl and ns-default.tcl).
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Chapter 13
Local Area Networks
The characteristics of the wireless and local area networks (LAN) are inherently different from those of point-to-point links. A network consisting of multiple point-to-point links cannot capture the sharing and contention properties of a LAN. To simulate these properties, we created a new type of a Node, called LanNode. The OTcl congurations and interfaces for LanNode reside in the following two les in the main ns directory:
tcl/lan/vlan.tcl tcl/lan/ns-ll.tcl tcl/lan/ns-mac.tcl
13.1 Tcl conguration
The interface for creating and conguring a LAN slightly differs from those of point-to-point link. At the top level, the OTcl class Simulator exports a new method called make-lan. The parameters to this method are similar to the method duplex-link, except that make-lan only accepts a list of nodes as a single parameter instead of 2 parameters as in duplex-link:
Simulator instproc make-lan {nodes bw delay lltype ifqtype mactype chantype}
The optional parameters to make-lan specify the type of objects to be created for the link layer (LL), the interface queue, the MAC layer (Mac), and the physical layer (Channel). Below is an example of how a new CSMA/CD (Ethernet) LAN is created. Example: $ns make-lan "$n1 $n2" $bw $delay LL Queue/DropTail Mac/Csma/Cd creates a LAN with basic link-layer, drop-tail queue, and CSMA/CD MAC.
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Higher Layers
Node 1
Node 2
...
Node N
Link Layer
Queue
Queue
Queue
...
LL LL LL
Mac Layer
Mac
Mac
...
Mac
Physical Layer
Channel
Classifier/Mac
Figure 13.1: Connectivity within a LAN
13.2 Components of a LAN
LanLink captures the functionality of the three lowest layers in the network stack: 1. Link Layer (LL) 2. Medium Access Control (MAC) Layer 3. Physical (PHY) Layer Figure 13.1 illustrates the extended network stack that makes simulations of local area network possible in ns. A packet sent down the stack ows through the link layer (Queue and LL), the MAC layer (Mac), and the physical layer (Channel to Classifier/Mac). The packet then makes its way up the stack through the Mac, and the LL. At the bottom of the stack, the physical layer is composed of two simulation objects: the Channel and Classifier/Mac. The Channel object simulates the shared medium and supports the medium access mechanisms of the MAC objects on the sending side of the transmission. On the receiving side, the Classifier/Mac is responsible for delivering and optionally replicating packets to the receiving MAC objects. Depending on the type of physical layer, the MAC layer must contain a certain set of functionalities such as: carrier sense, collision detection, collision avoidance, etc. Since these functionalities affect both the sending and receiving sides, they are 114
implemented in a single Mac object. For sending, the Mac object must follow a certain medium access protocol before transmitting the packet on the channel. For receiving, the MAC layer is responsible for delivering the packet to the link layer. Above the MAC layer, the link layer can potentially have many functionalities such as queuing and link-level retransmission. The need of having a wide variety of link-level schemes leads to the division of functionality into two components: Queue and LL (link-layer). The Queue object, simulating the interface queue, belongs to the same Queue class that is described in Chapter 7. The LL object implements a particular data link protocol, such as ARQ. By combining both the sending and receiving functionalities into one module, the LL object can also support other mechanisms such as piggybacking.
13.3 Channel Class
The Channel class simulates the actual transmission of the packet at the physical layer. The basic Channel implements a shared medium with support for contention mechanisms. It allows the MAC to carry out carrier sense, contention, and collision detection. If more than one transmissions overlaps in time, a channel raises the collision ag. By checking this ag, the MAC object can implement collision detection and handling. Since the transmission time is a function of the number of bits in the packet and the modulation speed of each individual interface (MAC), the Channel object only sets its busy signal for the duration requested by the MAC object. It also schedules the packets to be delivered to the destination MAC objects after the transmission time plus the propagation delay.
13.3.1 Channel State
The C++ class Channel includes enough internal state to schedule packet delivery and detect collisions. It exports the following OTcl conguration parameter: delay_ propagation delay on the channel
13.3.2 Example: Channel and classier of the physical layer
set channel_ [new Channel] $channel_ set delay_ 4us
# propagation delay
set mcl_ [new Classifier/Mac] $channel_ target $mcl_ $mcl_ install $mac_DA $recv_iface ...
13.3.3 Channel Class in C++
In C++, the class Channel extends the Connector object with several new methods to support a variety of MAC protocols. The class is dened as follow in ~ns/channel.h:
class Channel : public Connector { public: Channel(); 115
void recv(Packet* p, Handler*); virtual int send(Packet* p, double txtime); virtual void contention(Packet*, Handler*); int hold(double txtime); virtual int collision() { return numtx_ > 1; } virtual double txstop() { return txstop_; } ... }; The important methods of the class Channel are:
txstop() method returns the time when the channel will become idle, which can be used by the MAC to implement carrier sense. contention() method allows the MAC to contend for the channel before sending a packet. The channel then use this packet to signal the corresponding Mac object at the end of each contention period. collision() method indicates whether a collision occurs during the contention period. When the Channel signal the end of the contention period, the MAC can use the collision() method to detect collision. send() method allows the MAC object to transmit a packet on the channel for a specied duration of time. hold() method allows the MAC object to hold the channel for a specied duration of time without actually transmitting any packets. This is useful in simulating the jamming mechanism of some MAC protocols.
13.4 MacClassier Class
The MacClassifier class extends the Classifier class to implement a simple broadcasting mechanism. It modies the recv() method in the following way: since the replication of a packet is expensive, normally a unicast packet will be classied by the MAC destination address macDA_ and delivered directly to the MAC object with such an address. However, if the destination object cannot be found or if the MAC destination address is explicitly set to the broadcast address BCAST_ADDR, the packet will be replicated and sent to all MACs on the lan excluding the one that is the source of the packet. Finally, by setting the bound variable MacClassifier::bcast_ to a nonzero value, will cause MacClassifier always to replicate packets. class MacClassifier : public Classifier { public: void recv(Packet*, Handler*); }; void MacClassifier::recv(Packet* p, Handler*) { Mac* mac; hdr_mac* mh = hdr_mac::access(p); if (bcast_ || mh->macDA() == BCAST_ADDR || (mac = (Mac *)find(p)) == 0) { // Replicate packets to all slots (broadcast) ... return; } mac->recv(p); } 116
13.5 MAC Class
The Mac object simulates the medium access protocols that are necessary in the shared medium environment such as the wireless and local area networks. Since the sending and receiving mechanisms are tightly coupled in most types of MAC layers, it is essential for the Mac object to be duplex. On the sending side, the Mac object is responsible for adding the MAC header and transmitting the packet onto the channel. On the receiving side, the Mac object asynchronously receives packets from the classier of the physical layer. After MAC protocol processing, it passes the data packet to the link layer.
13.5.1 Mac State
The C++ class Mac class contains enough internal state to simulate the particular MAC protocol. It also exports the following OTcl conguration parameter: bandwidth_ hlen_ label_ modulation rate of the MAC additional bytes added to packet for MAC header MAC address
13.5.2 Mac Methods
The class Mac class added several Tcl methods for conguration, in particular, linking with other simulation objects: channel classifier maclist specify the channel for transmission the classier that deliver packets to receiving MAC a link list of MAC interfaces on the same node
13.5.3 Mac Class in C++
In C++, the Mac class derives from Connector. When the recv() method gets a packet, it identies the direction of the packet based on the presence of a callback handler. If there is a callback handler, the packet is outgoing, otherwise, it is incoming.
class Mac : public Connector { public: Mac(); virtual void recv(Packet* p, Handler* h); virtual void send(Packet* p); virtual void resume(Packet* p = 0); ... };
When a Mac object receives a packet via its recv() method, it checks whether the packet is outgoing or incoming. For an outgoing packet, it assumes that the link-layer of the sender has obtained the destination MAC address and lled in the macDA_ eld of the MAC header, hdr_mac. The Mac object lls in the rest of the MAC header with the source MAC address 117
and the frame type. It then passes the packet to its send() method, which carries out the medium access protocol. For the basic Mac object, the send method calls txtime() to compute the transmission time, then invokes Channel::send to transmit the packet. Finally, it schedules itself to resume after the transmission time has elapsed. For an incoming packet, the MAC object does its protocol processing and passes the packet to the link-layer.
13.5.4 CSMA-based MAC
The class CsmaMac extends the Mac class with new methods that implements carrier sense and backoff mechanisms. The CsmaMac::send() method detects when the channel becomes idle using Channel::txtime(). If the channel is busy, the MAC schedules the next carrier sense at the moment the channel turns idle. Once the channel is idle, the CsmaMac object initiates the contention period with Channel::contention(). At the end of the contention period, the endofContention() method is invoked. At this time, the basic CsmaMac just transmits the packet using Channel::send.
class CsmaMac : public Mac { public: CsmaMac(); void send(Packet* p); void resume(Packet* p = 0); virtual void endofContention(Packet* p); virtual void backoff(Handler* h, Packet* p, double delay=0); ... }; class CsmaCdMac : public CsmaMac { public: CsmaCdMac(); void endofContention(Packet*); }; class CsmaCaMac : public CsmaMac { public: CsmaCaMac(); virtual void send(Packet*); };
The CsmaCdMac extends CsmaMac to carry out collision detection procedure of the CSMA/CD (Ethernet) protocol. When the channel signals the end of contention period, the endofContention method checks for collision using the Channel::collisi method. If there is a collision, the MAC invokes its backoff method to schedule the next carrier sense to retransmit the packet. The CsmaCaMac extends the send method of CsmaMac to carry out the collision avoidance (CSMA/CA) procedure. Instead of transmitting immediately when the channel is idle, the CsmaCaMac object backs off a random number of slots, then transmits if the channel remains idle until the end of the backoff period.
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13.6 LL (link-layer) Class
The link-layer object is responsible for simulating the data link protocols. Many protocols can be implemented within this layer such as packet fragmentation and reassembly, and reliable link protocol. Another important function of the link layer is setting the MAC destination address in the MAC header of the packet. In the current implementation this task involves two separate issues: nding the nexthopnodes IP address (routing) and resolving this IP address into the correct MAC address (ARP). For simplicity, the default mapping between MAC and IP addresses is onetoone, which means that IP addresses are reused at the MAC layer.
13.6.1 LL Class in C++
The C++ class LL derives from the LinkDelay class. Since it is a duplex object, it keeps a separate pointer for the send target, sendtarget, and the receive target, recvtarget. It also denes the methods recvfrom() and sendto() to handle the incoming and outgoing packets respectively. class LL : public LinkDelay { public: LL(); virtual void recv(Packet* p, Handler* h); virtual Packet* sendto(Packet* p, Handler* h = 0); virtual Packet* recvfrom(Packet* p); inline inline inline inline inline inline int seqno() return seqno_; int ackno() return ackno_; int macDA() return macDA_; Queue *ifq() return ifq_; NsObject* sendtarget() return sendtarget_; NsObject* recvtarget() return recvtarget_;
protected: int command(int argc, const char*const* argv); void handle(Event* e) recv((Packet*)e, 0); inline virtual int arp (int ip_addr) return ip_addr; int seqno_; // link-layer sequence number int ackno_; // ACK received so far int macDA_; // destination MAC address Queue* ifq_; // interface queue NsObject* sendtarget_; // for outgoing packet NsObject* recvtarget_; // for incoming packet LanRouter* lanrouter_; // for lookups of the next hop };
13.6.2 Example: Link Layer conguration
set ll_ [new LL] set ifq_ [new Queue/DropTail] $ll_ lanrouter [new LanRouter $ns $lan] # LanRouter is one object 119
$ll_ $ll_ $ll_ $ll_
set delay_ $delay set bandwidth_ $bw sendtarget $mac recvtarget $iif ...
# # # #
# per LAN link-level overhead bandwidth interface queue at the sender side input interface of the receiver
13.7 LanRouter class
By default, there is just one LanRouter object per LAN, which is created when a new LanNode is initialized. For every node on the LAN, the link layer object (LL) has a pointer to the LanRouter, so it is able to nd the next hop for the packet that is sent on the LAN: Packet* LL::sendto(Packet* p, Handler* h) { int nh = (lanrouter_) ? lanrouter_->next_hop(p) : -1; ... } LanRouter is able to nd the next hop by querying the current RouteLogic: int LanRouter::next_hop(Packet *p) { int next_hopIP; if (enableHrouting_) { routelogic_->lookup_hier(lanaddr_, adst, next_hopIP); } else { routelogic_->lookup_flat(lanaddr_, adst, next_hopIP); } One limitation of this is that RouteLogic may not be aware of dynamic changes to the routing. But it is always possible to derive a new class from LanRouter so that to redene its next_hop method to handle dynamic changes appopriately.
13.8 Other Components
In addition to the C++ components described above, simulating local area networks also requires a number of existing components in ns such as Classifier, Queue, and Trace, networkinterface, etc. Conguring these objects requires knowledge of what the user wants to simulate. The default conguration is implemented in the two Tcl les mentioned at the beginning of this chapter. To obtain more realistic simulations of wireless networks, one can use the ErrorModel described in Chapter 12.
13.9 LANs and ns routing
When a LAN is created using either make-lan or newLan, a virtual LAN node LanNode is created. LanNode keeps together all shared objects on the LAN: Channel, Classifier/Mac, and LanRouter. Then for each node on the LAN, 120
a LanIface object is created. LanIface contains all other objects that are needed on the pernode basis: a Queue, a link layer (LL), Mac, etc. It should be emphasized that LanNode is a node only for routing algorithms: Node and LanNode have very little in common. One of few things that they share is an identier taken from the Node IDspace. If hierarchical routing is used, LanNode has to be assigned a hierarchical address just like any other node. From the point of view of ns (static) routing, LanNode is just another node connected to every node on the LAN. Links connecting the LanNode with
n1 n2 n1 n2
LAN
LAN
n3
n4
n3
n4
Figure 13.2: Actual LAN conguration (left) and as seen by ns routing (right) the nodes on the LAN are also virtual (Vlink). The default routing cost of such a link is , so the cost of traversing two n2) is counted as just one hop. Vlinks (e.g. n1 LAN Most important method of Vlink is the one that gives the head of the link: Vlink instproc head {} { $self instvar lan_ dst_ src_ if {$src_ == [$lan_ set id_]} { # if this is a link FROM the lan vnode, # it doesnt matter what we return, because # its only used by $lan add-route (empty) return "" } else { # if this is a link TO the lan vnode, # return the entry to the lanIface object set src_lif [$lan_ set lanIface_($src_)] return [$src_lif entry] } } This method is used by static (default) routing to install correct routes at a node (see Simulator methods compute-flat-routes and compute-hier-routes in tcl/lib/ns-route.tcl, as well as Node methods add-route and add-hroute in tcl/lib/ns-node.tcl). From the code fragment above it can be seen that it returns LAN interface of the node as a head of the link to be installed in the appropriate classier. Thus, Vlink does not impose any delay on the packet and serves the only purpose to install LAN interfaces instead of normal links at nodes classiers. Note, that this design allows to have nodes connected by parallel LANs, while in the current implementation it is impossible to have nodes connected by parallel simple links and use them both (the array Simulator instvar link_ holds the link object for each connected pair of source and destination, and it can be only one object per source/destination pair). 121
13.10 Commands at a glance
The following is a list of lan related commands commonly used in simulation scripts: $ns_ make-lan <nodelist> <bw> <delay> <LL> <ifq> <MAC> <channel> <phy> Creates a lan from a set of nodes given by <nodelist>. Bandwidth, delay characteristics along with the link-layer, Interface queue, Mac layer and channel type for the lan also needs to be dened. Default values used are as follows: <LL> .. LL <ifq>.. Queue/DropTail <MAC>.. Mac <channel>.. Channel and <phy>.. Phy/WiredPhy $ns_ newLan <nodelist> <BW> <delay> <args> This command creates a lan similar to make-lan described above. But this command can be used for ner control whereas make-lan is a more convinient and easier command. For example newLan maybe used to create a lan with hierarchical addresses. See ns/tcl/ex/vlantest-hier.tcl, vlantest-mcst.tcl, lantest.tcl, mac-test.tcl for usage of newLan. The possible argument types that can be passed are LL, ifq, MAC, channel, phy and address. $lannode cost <c> This assigns a cost of c/2 to each of the (uni-directional) links in the lan. $lannode cost? Returns the cost of (bi-directional) links in the lan, i.e c. Internal procedures : $lannode addNode <nodes> <bw> <delay> <LL> <ifq> <MAC> <phy> Lan is implemented as a virtual node. The LanNode mimics a real node and uses an address (id) from nodes address space. This command adds a list of <nodes> to the lan represented by lannode. The bandwidth, delay and network characteristics of nodes are given by the above arguments. This is an internal command used by make-lan and newLan. $lannode id Returns the virtual nodes id. $lannode node-addr Returns virtual nodess address. $lannode dump-namconfig This command creates a given lan layout in nam. This function may be changed to redene the lan layout in a different way. $lannode is-lan? This command always returns 1, since the node here is a virtual node representing a lan. The corresponding command for base class Node $node is-lan? always returns a 0.
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Chapter 14
The (Revised) Addressing Structure in NS
This chapter describes the internals of the revised addressing format implemented in ns. The chapter consists of ve sections. We describe the APIs that can be used for allocating bits to the ns addressing structure. The address space as described in chapter 3, can be thought of a contiguous eld of n bits, where n may vary as per the address requirement of the simulation. The default value of n is 16 (as dened by MAXADDRSIZE_). The maximum value of n is set to 32 (dened as MAXADDRSIZE_). These default and maximum address sizes are dened in ~ns//tcl/lib/ns-default.tcl. The address space consists of 2 parts, the node-id and the port-id. The higher bits are assigned as the nodes address or id_ and remaining lower bits are assigned to form port-id or the identication of the agent attached to the node. Of the higher bits, 1 bit is assigned for multicast. The default settings allow 7 higher bits for node-id, the MSB for multicast and the lower remaining 8 bits for port-id. Naturally this limits the simulation to 128 nodes. This address space may be expanded to accomodate larger number of nodes in the simulation. The port-id may also be expanded to suppprt higher number of agents. Additionally, the address space may also be set in hierarchical format, consisting of multiple levels of addressing hierarchy. We shall be describing the APIs for setting address structure in different formats as described above as well as expanding the address space. The procedures and functions described in this chapter can be found in ~ns/tcl/lib/ns-address.tcl, address.cc and address.h.
14.1 The Default Address Format
The default settings allocates 8 lower bits for port-id, 1 higher bit for mcast and the rest 7 higher bits for node-id. The procedure to set the address format in default mode is called during initialisation of the simulator as: # The preamble set ns [new Simulator]
;# initialise the simulation
It can also be called explicitly set as: $ns set-address-format def
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14.2 The Hierarchical Address Format
There are two options for setting an address to hierarchical format, the default and the specied.
14.2.1 Default Hierarchical Setting
The default hierarchical node-id consists of 3 levels with 8 bits assigned to each level. The hierarchical conguration may be invoked as follows: $ns set-address-format hierarchical This sets : * 8 bits for port-id, * 24 bits for node-id assigned in - 3 levels of hierarchy - 8 bits for each level - looks like 8 8 8 - or 7 8 8, if multicast is enabled.
14.2.2 Specic Hierarchical Setting
The second option allows a hierarchical address to be set with specied number of levels with number of bits assigned for each level. The API would be as the following: $ns set-address-format hierarchical <#n hierarchy levels> <#bits for level1> <#bits for level 2> ....<#bits for nth level> An example conguration would be: $ns set-address-format hierarchical 2 8 15 where 2 levels of hierarchy is specied, assigning 8 bits for the 1st level and 15 bits for the second.
14.3 The Expanded Node-Address Format
On the event of requirement of more bits to the address space, the expanded address API may be used as: $ns set-address-format expanded This expands the address space to 30 bits, allocating 22 higher bits to node-id and lower 8 bits to port-id.
14.4 Expanding port-id eld
This primitive may be used in case of need to expand portid in the event of requirement to attach a large number of agents to the nodes. This may be used in conjunction with set-addres-format command (with different options) explained above. Synopsis for this command shall be: expand-port-eld-bits <#bits for portid> 124
expand-port-eld-bits checks and raises error in the following if the requested portsize cannot be accomodated (i.e if sufcient num.of free bits are not available) or if requested portsize is less than or equal to the existing portsize.
14.5 Errors in setting address format
Errors are returned for both set-address-format and expand-port-eld-bits primitives in the following cases: * if number of bits specied is less than 0. * if bit positions clash (contiguous number of requested free bits not * found). * if total number of bits exceed MAXADDRSIZE_. * if expand-port-eld-bits is attempted with portbits less than or * equal to the existing portsize. * if number of hierarchy levels donot match with number of bits * specied (for each level).
14.6 Commands at a glance
The following is a list of address-format related commands used in simulation scripts: $ns_ set-address-format def This command is used internally to set the address format to its default value of 8 lower bits for port-id, 1 higher bit for mcast and the rest 7 higher bits for port-id. However this API has been replaced by the new node API $ns_ node-config -addressType flat. $ns_ set-address-format hierarchical This command is used to set the address format to the hierarchical conguration that consists of 3 levels with 8bits assigned to each level and 8 lower bits for port-id. However this API has been replaced by the new node API $ns_ node-config -addressType hierarchical. $ns_ set-address-format hierarchical <levels> <args> This command is used to set the address format to a specic hierarchical setting. The <levels> indicate the number of levels of hierarchy in the addressing structure, while the args dene number of bits for each level. An example would be $ns_ set-address-format hierachical 3 4 4 16 , where 4, 4 and 16 denes the number of bits to be used for the address space in level 1 , 2 and 3 respectively. $ns_ set-address-format expanded This command was used to expand the address space to 30 bits, allocating 22 higher bits for node-id and lower 8 bits for port-id. However this command is obsoleted now by 32 bit addressing, i.e node-id eld is 32 bit wide. expand-port-field-bits <bits-for-portid> Similar to the command above, this was used to expand the address space for the port-id eld to <bits-for-portid> number of bits. However this command is obsolete now that the ports are 32 bit wide.
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Chapter 15
Mobile Networking in ns
This chapter describes the wireless model that was originally ported as CMUs Monarch groups mobility extension to ns. This chapter consists of two sections and several subsections. The rst section covers the original mobility model ported from CMU/Monarch group. In this section, we cover the internals of a mobilenode, routing mechanisms and network components that are used to construct the network stack for a mobilenode. The components that are covered briey are Channel, Networkinterface, Radio propagation model, MAC protocols, Interface Queue, Link layer and Address resolution protocol model (ARP). CMU trace support and Generation of node movement and trafc scenario les are also covered in this section. The original CMU model allows simulation of pure wireless LANs or multihop ad-hoc networks. Further extensions were made to this model to allow combined simulation of wired and wireless networks. MobileIP was also extended to the wireless model. These are discussed in the second section of this chapter.
15.1 The basic wireless model in ns
The wireless model essentially consists of the MobileNode at the core,with additional supporting features that allows simulations of multi-hop ad-hoc networks, wireless LANs etc. The MobileNode object is a split object. The C++ class MobileNode is derived from parent class Node. Refer to Chapter 5 for details on Node. A MobileNode thus is the basic Node object with added functionalities of a wireless and mobile node like ability to move within a given topology, ability to receive and transmit signals to and from a wireless channel etc. A major difference between them, though, is that a MobileNode is not connected by means of Links to other nodes or mobilenodes. In this section we shall describe the internals of MobileNode, its routing mechanisms, the routing protocols dsdv, aodv, tora and dsr, creation of network stack allowing channel access in MobileNode, brief description of each stack component, trace support and movement/trafc scenario generation for wireless simulations.
15.1.1 Mobilenode: creating wireless topology
MobileNode is the basic nsNode object with added functionalities like movement, ability to transmit and receive on a channel that allows it to be used to create mobile, wireless simulation environments. The class MobileNode is derived from the base class Node. MobileNode is a split object. The mobility features including node movement, periodic position updates, maintaining topology boundary etc are implemented in C++ while plumbing of network components within MobileNode itself (like classiers, dmux , LL, Mac, Channel etc) have been implemented in Otcl. The functions and procedures described in this subsection can be found in ~ns/mobilenode.{cc,h}, ~ns/tcl/lib/ns-mobilenode.tcl, ~ns/tcl/mobility/dsdv.tcl, ~ns/tcl/mobility/dsr.tcl, ~ns/tcl/mobility/tora.tcl. Example scripts can be found in ~ns/tcl/ex/wireless-test.tcl and ~ns/tcl/ex/wireless.tcl.
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While the rst example uses a small topology of 3 nodes, the second example runs over a topology of 50 nodes. These scripts can be run simply by typing
$ns tcl/ex/wireless.tcl (or /wireless-test.tcl)
The four ad-hoc routing protocols that are currently supported are Destination Sequence Distance Vector (DSDV), Dynamic Source Routing (DSR), Temporally ordered Routing Algorithm (TORA) and Adhoc On-demand Distance Vector (AODV). The APIs for creating a mobilenode depends on which routing protocol it would be using. Hence the primitive to create a mobilenode is
set mnode [$opt(rp)-create-mobile-node $id]
where $opt(rp) denes "dsdv", "aodv", "tora" or "dsr" and id is the index for the mobilenode. The above procedure creates a mobilenode (split)object, creates a routing agent as specied, creates the network stack consisting of a link layer, interface queue, mac layer, and a network interface with an antenna, interconnects these components and con nects the stack to the channel. The mobilenode now looks like the schematic in Figure 15.1. The mobilenode structure used for DSR routing is slightly different from the mobilenode described above. The class SRNode is derived from class MobileNode. SRNode doesnot use address demux or classiers and all packets received by the node are handed dow n to the DSR routing agent by default. The DSR routing agent either receives pkts for itself by handing it over to the port dmux or forwards pkts as per source routes in the pkt hdr or sends out route requests and route replies for fresh packets. Details on DSR routing agent may be found in section 15.1.4. The schematic model for a SRNode is shown in Figure 15.2.
15.1.2 Creating Node movements
The mobilenode is designed to move in a three dimensional topology. However the third dimension (Z) is not used. That is the mobilenode is assumed to move always on a at terrain with Z always equal to 0. Thus the mobilenode has X, Y, Z(=0) co-ordinates that is continually adjusted as the node moves. There are two mechanisms to induce movement in mobilenodes. In the rst method, starting position of the node and its future destinations may be set explicitly. These directives are normally included in a separate movement scenario le. The start-position and future destinations for a mobilenode may be set by using the following APIs: $node set X_ <x1> $node set Y_ <y1> $node set Z_ <z1> $ns at $time $node setdest <x2> <y2> <speed> At $time sec, the node would start moving from its initial position of (x1,y1) towards a destination (x2,y2) at the dened speed. In this method the node-movement-updates are triggered whenever the position of the node at a given time is required to be known. This may be triggered by a query from a neighbouring node seeking to know the distance between them, or the setdest directive described above that changes the direction and speed of the node.
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port demux addr demux entry_ defaulttarget_ IP address 255
Src/Sink
RTagent (DSDV) target_ arptable_ uptarget_ LL ARP
downtarget_
IFq
downtarget_
mac_
MAC
uptarget_
downtarget_ Radio Propagation Model propagation_
uptarget_
NetIF channel_ uptarget_ Channel
Figure 15.1: Schematic of a mobilenode under the CMU monarchs wireless extensions to ns An example of a movement scenario le using the above APIs, can be found in ~ns/tcl/mobility/scene/scen-670x670-50-60020-0. Here 670x670 denes the length and width of the topology with 50 nodes moving at a maximum speed of 20m/s with average pause time of 600s. These node movement les may be generated using CMUs scenario generator to be found under ~ns/indep-utils/cmu-scen-gen/setdest. See subsection 15.1.7 for details on generation of node movement scenarios.
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port demux
Src/Sink
entry_
DSR
target_ ll_ uptarget_ ll_(0) arptable_ LL ARP
downtarget_
IFq
downtarget_
MAC
uptarget_
downtarget_ Radio Propagation Model propagation_
uptarget_
NetIF channel_ uptarget_ Channel
Figure 15.2: Schematic of a SRNode under the CMU monarchs wireless extensions to ns The second method employs random movement of the node. The primitive to be used is: $mobilenode start
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which starts the mobilenode with a random position and have routined updates to change the direction and speed of the node. The destination and speed values are generated in a random fashion. We have not used the second method and leave it to the user to explore the details. The mobilenode movement is implemented in C++. See methods in ~ns/mobilenode.{cc.h} for the implementational details. Irrespective of the methods used to generate node movement, the topography for mobilenodes needs to be dened. It should be dened before creating mobilenodes. Normally at topology is created by specifying the length and width of the topography using the following primitive: set topo [new Topography] $topo load_flatgrid $opt(x) $opt(y) where opt(x) and opt(y) are the boundaries used in simulation. The movement of mobilenodes may be logged by using a procedure like the following: proc log-movement {} { global logtimer ns_ ns set ns $ns_ source ../mobility/timer.tcl Class LogTimer -superclass Timer LogTimer instproc timeout {} { global opt node_; for {set i 0} {$i < $opt(nn)} {incr i} { $node_($i) log-movement } $self sched 0.1 } set logtimer [new LogTimer] $logtimer sched 0.1 } In this case, mobilenode positions would be logged every 0.1 sec.
15.1.3 Network Components in a mobilenode
The network stack for a mobilenode consists of a link layer(LL), an ARP module connected to LL, an interface priority queue(IFq), a mac layer(MAC), a network interface(netIF), all connected to the channel. These network components are created and plumbed together in OTcl. The relevant MobileNode method add-interface() in ~ns/tcl/lib/ns-mobilenode.tcl is shown below: # # The following setups up link layer, mac layer, network interface # and physical layer structures for the mobile node. # Node/MobileNode instproc add-interface { channel pmodel lltype mactype qtype qlen iftype anttype } { 130
$self instvar arptable_ nifs_ $self instvar netif_ mac_ ifq_ ll_ global ns_ MacTrace opt set t $nifs_ incr nifs_ set set set set set netif_($t) mac_($t) ifq_($t) ll_($t) ant_($t) [new [new [new [new [new $iftype] $mactype] $qtype] $lltype] $anttype] ;# ;# ;# ;# net-interface mac layer interface queue link layer
# # Local Variables # set nullAgent_ [$ns_ set nullAgent_] set netif $netif_($t) set mac $mac_($t) set ifq $ifq_($t) set ll $ll_($t) # # Initialize ARP table only once. # if { $arptable_ == "" } { set arptable_ [new ARPTable $self $mac] set drpT [cmu-trace Drop "IFQ" $self] $arptable_ drop-target $drpT } # # Link Layer # $ll arptable $arptable_ $ll mac $mac $ll up-target [$self entry] $ll down-target $ifq # # Interface Queue # $ifq target $mac $ifq set qlim_ $qlen set drpT [cmu-trace Drop "IFQ" $self] $ifq drop-target $drpT # # Mac Layer # $mac netif $netif $mac up-target $ll
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$mac down-target $netif $mac nodes $opt(nn) # # Network Interface # $netif channel $channel $netif up-target $mac $netif propagation $pmodel $netif node $self $netif antenna $ant_($t) # # Physical Channel # $channel addif $netif
;# Propagation Model ;# Bind node <---> interface ;# attach antenna
;# add to list of interfaces
# ============================================================ # Setting up trace objects if { $MacTrace == "ON" } { # # Trace RTS/CTS/ACK Packets # set rcvT [cmu-trace Recv "MAC" $self] $mac log-target $rcvT
# # Trace Sent Packets # set sndT [cmu-trace Send "MAC" $self] $sndT target [$mac sendtarget] $mac sendtarget $sndT # # Trace Received Packets # set rcvT [cmu-trace Recv "MAC" $self] $rcvT target [$mac recvtarget] $mac recvtarget $rcvT # # Trace Dropped Packets # set drpT [cmu-trace Drop "MAC" $self] $mac drop-target $drpT } else { $mac log-target [$ns_ set nullAgent_] $mac drop-target [$ns_ set nullAgent_] } # ============================================================
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$self addif $netif } The plumbing in the above method creates the network stack we see in Figure 15.1. Each component is briey described here. Hopefully more detailed docuentation from CMU shall be available in the future. Link Layer The LL used by mobilenode is same as described in Chapter 13. The only difference being the link layer for mobilenode, has an ARP module connected to it which resolves all IP to hardware (Mac) address conversions. Normally for all outgoing (into the channel) packets, the packets are handed down to the LL by the Routing Agent. The LL hands down packets to the interface queue. For all incoming packets (out of the channel), the mac layer hands up packets to the LL which is then handed off at the node_entry_ point. The class LL is implemented in ~ns/ll.{cc,h} and ~ns/tcl/lan/ns-ll.tcl. ARP The Address Resolution Protocol (implemented in BSD style) module receives queries from Link layer. If ARP has the hardware address for destination, it writes it into the mac header of the packet. Otherwise it broadcasts an ARP query, and caches the packet temporarily. For each unknown destination hardware address, there is a buffer for a single packet. Incase additional packets to the same destination is sent to ARP, the earlier buffered packet is dropped. Once the hardware address of a packet"s next hop is known, the packet is inserted into the interface queue. The class ARPTable is implemented in ~ns/arp.{cc,h} and ~ns/tcl/lib/ns-mobilenode.tcl. Interface Queue The class PriQueue is implemented as a priority queue which gives priority to routing rotocol packets, inserting them at the head of the queue. It supports running a lter over all packets in the queue and removes those with a specied destination address. See ~ns/priqueue.{cc,h} for interface queue implementation. Mac Layer The IEEE 802.11 distributed coordination function (DCF) Mac protocol has been implemented by CMU. It uses a RTS/CTS/DATA/ACK pattern for all unicast packets and simply sends out DATA for all broadcast packets. The implementation uses both physical and virtual carrier sense. The class Mac802_11 is implemented in ~ns/mac802_11.{cc,h}. Tap Agents Agents that subclass themselves as class Tap dened in mac.h can register themselves with the mac object using method installTap(). If the particular Mac protocol permits it, the tap will promiscuously be given all packets received by the mac layer, before address ltering is done. See ~ns/mac.{cc,h} for class Tapmplementation. Network Interfaces The Network Interphase layer serves as a hardware interface which is used by mobilenode to access the channel. The wireless shared media interface is implemented as class Phy/WirelessPhy. This interface subject to collisions and the radio propagation model receives packets transmitted by other node interfaces to the channel. The interface stamps each transmitted packet with the meta-data related to the transmitting interface like the transmission power, wavelength etc. This meta-data in pkt header is used by the propagation model in receiving network interface to determine if the packet has minimum power to be received and/or captured and/or detected (carrier sense) by the receiving node. The model approximates the DSSS radio interface (Lucent WaveLan direct-sequence spread-spectrum). See ~ns/phy.{cc.h} and ~ns/wireless-phy.{cc,h} for network interface implementations. Radio Propagation Model It uses Friss-space attenuation ( ) at near distances and an approximation to Two ray Ground ( ) at far distances. The approximation assumes specular reection off a at ground plane. See ~ns/tworayground.{cc,h} for implementation. Antenna An omni-directional antenna having unity gain is used by mobilenodes. See ~ns/antenna.{cc,h} for implementation details.
15.1.4 Different types of Routing Agents in mobile networking
The four different ad-hoc routing protocols currently implemented for mobile networking in nsare dsdv, dsr, aodv and tora. In this section we shall briey discuss each of them. 133
DSDV In this routing protocol routing messages are exchanged between neighbouring mobilenodes (i.e mobilenodes that are within range of one another). Routing updates may be triggered or routine. Updates are triggered in case a routing information from one of t he neighbours forces a change in the routing table. A packet for which the route to its destination is not known is cached while routing queries are sent out. The pkts are cached until route-replies are received from the destination. There is a maximum buffer size for caching the pkts waiting for routing information beyond which pkts are dropped. All packets destined for the mobilenode are routed directly by the address dmux to its port dmux. The port dmux hands the packets to the respective destination agents. A port number of 255 is used to attach routing agent in mobilenodes. The mobilenodes al so use a default-target in their classier (or address demux). In the event a target is not found for the destination in the classier (which happens when the destination of the packet is not the mobilenode itself), the pkts are handed to the default-ta rget which is the routing agent. The routing agent assigns the next hop for the packet and sends it down to the link layer. The routing protocol is mainly implemented in C++. See ~ns/dsdv directory and ~ns/tcl/mobility/dsdv.tcl for all procedures related to DSDV protocol implementation.
DSR This section briey describes the functionality of the dynamic source routing protocol. As mentioned earlier the SRNode is different from the MobileNode. The SRNodes entry_ points to the DSR routing agent, thus forcing all packets received by the node to be handed down to the routing agent. This model is required for future implementation of piggy-backed routing information on data packets which otherwise would not ow through the routing agent. The DSR agent checks every data packet for source-route information. It forwards the packet as per the routing information. Incase it doesnot nd routing information in the packet, it provides the source route, if route is known, or caches the packet and sends out route queries if route to destination is not known. Routing queries, always triggered by a data packet with no route to its destination, are initially broadcast to all neighbours. Route-replies are send back either by intermediate nodes or the destination node, to the source, if it can nd routing info for the destination in the route-query. It hands over all packets destined to itself to the port dmux. In SRNode the port number 255 points to a null agent since the packet has already been processed by the routing agent. See ~ns/dsr directory and ~ns/tcl/mobility/dsr.tcl for implementation of DSR protocol.
TORA Tora is a distributed routing protocol based on "link reversal" algorithm. At every node a separate copy of TORA is run for every destination. When a node needs a route to a given destination it broadcasts a QUERY message containing the address of the destination for which it requires a route. This packet travels through the network until it reaches the destination or an intermediate node that has a route to the destination node. This recepient node node then broadcasts an UPDATE packet listing its height wrt the destination. As this node propagates through the network each node updates its height to a value greater than the height of the neighbour from which it receives the UPDATE. This results in a series of directed links from the node that originated the QUERY to the destination node. If a node discovers a particular destination to be unreachable it sets a local maximum value of height for that destination. Incase the node cannot nd any neighbour having nite height wrt this destination it attempts to nd a new route. In case of network partition, the node broadcasts a CLEAR message that resets all routing states and removes invalid routes from the network. TORA operates on top of IMEP (Internet MANET Encapsulation Protocol) that provides reliable delivery of route-messages and informs the routing protocol of any changes of the links to its neighbours. IMEP tries to aggregate IMEP and TORA 134
messages into a single packet (called block) in order to reduce overhead. For link-status sensing and maintaining a list of neighbour nodes, IMEP sends out periodic BEACON messages which is answered by each node that hears it by a HELLO reply message. See ns/tora directory and ns/tcl/mobility/tora.tcl for implementation of tora in ns.
AODV AODV is a combination of both DSR and DSDV protocols. It has the basic route-discovery and route-maintenance of DSR and uses the hop-by-hop routing, sequence numbers and beacons of DSDV. The node that wants to know a route to a given destination generates a ROUTE REQUEST. The route request is forwarded by intermediate nodes that also creates a reverse route for itself from the destination. When the request reaches a node with route to destination it generates a ROUTE REPLY containing the number of hops requires to reach destination. All nodes that participates in forwarding this reply to the source node creates a forward route to destination. This state created from each node from source to destination is a hop-by-hop state and not the entire route as is done in source routing. See ns/aodv and ns/tcl/lib/ns-lib.tcl for implementational details of aodv.
15.1.5 Trace Support
The trace support for wireless simulations currently use cmu-trace objects. In the future this shall be extended to merge with trace and monitoring support available in ns, which would also include nam support for wireless modules. For now we will explain briey with cmu-trace objects and how they may be used to trace packets for wireless scenarios. The cmu-trace objects are of three types - CMUTrace/Drop, CMUTrace/Recv and CMUTrace/Send. These are used for tracing packets that are dropped, received and sent by agents, routers, mac layers or interface queues in ns. The methods and procedures used for implementing wireless trace support can be found under ~ns/trace.{cc,h} and ~ns/tcl/lib/ns-cmutrace.tcl. A cmu-trace object may be created by the following API: set sndT [cmu-trace Send "RTR" $self] which creates a trace object, sndT, of the type CMUTrace/Send for tracing all packets that are sent out in a router. The trace objects may be used to trace packets in MAC, agents (routing or others), routers or any other NsObject. The cmu-trace object CMUTrace is derived from the base class Trace. See Chapter 21 for details on class Trace. The class CMUTrace is dened as the following:
class CMUTrace : public Trace { public: CMUTrace(const char *s, char t); void recv(Packet *p, Handler *h); void recv(Packet *p, const char* why); private: int off_arp_; int off_mac_; int off_sr_; char tracename[MAX_ID_LEN + 1]; int tracetype; MobileNode *node_;
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int initialized() { return node_ && 1; } int void void void void void void void void }; The type eld (described in Trace class denition) is used to differentiate among different types of traces. For cmu-trace this can be s for sending, r for receiving or D for dropping a packet. A fourth type f is used to denote forwarding of a packet (When the node is not the originator of the packet). Similar to the method Trace::format(), the CMUTrace::format() denes and dictates the trace le format. The method is shown below: void CMUTrace::format(Packet* p, const char *why) { hdr_cmn *ch = HDR_CMN(p); int offset = 0; /* * Log the MAC Header */ format_mac(p, why, offset); offset = strlen(wrk_); switch(ch->ptype()) { case PT_MAC: break; case PT_ARP: format_arp(p, offset); break; default: format_ip(p, offset); offset = strlen(wrk_); switch(ch->ptype()) { case PT_DSR: format_dsr(p, offset); break; case PT_MESSAGE: case PT_UDP: 136 command(int argc, const char*const* argv); format(Packet *p, const char *why); format_mac(Packet *p, const char *why, int offset); format_ip(Packet *p, int offset); format_arp(Packet format_dsr(Packet format_msg(Packet format_tcp(Packet format_rtp(Packet *p, *p, *p, *p, *p, int int int int int offset); offset); offset); offset); offset);
format_msg(p, offset); break; case PT_TCP: case PT_ACK: format_tcp(p, offset); break; case PT_CBR: format_rtp(p, offset); break; .......... } } } The above function calls different format functions depending on the type of the packet being traced. All traces are written to the buffer wrk_. A count of the offset for the buffer is kept and is passed along the different trace functions. The most basic format is dened by format_mac() and is used to trace all pkt types. The other format functions print additional information as dened by the packet types. The mac format prints the following: #ifdef LOG_POSITION double x = 0.0, y = 0.0, z = 0.0; node_->getLoc(&x, &y, &z); #endif sprintf(wrk_ + offset, #ifdef LOG_POSITION "%c %.9f %d (%6.2f %6.2f) %3s %4s %d %s %d [%x %x %x %x] ", #else "%c %.9f _%d_ %3s %4s %d %s %d [%x %x %x %x] ", #endif op, // s, r, D or f Scheduler::instance().clock(), // time stamp src_, // the nodeid for this node #ifdef LOG_POSITION x, // x co-ord y, // y co-ord #endif tracename, // name of object type tracing why, // reason, if any ch->uid(), // identifier for this event packet_info.name(ch->ptype()), // packet type ch->size(), // size of cmn header mh->dh_duration, // expected time to send data ETHER_ADDR(mh->dh_da), // mac_destination address ETHER_ADDR(mh->dh_sa), // mac_sender address GET_ETHER_TYPE(mh->dh_body)); // type - arp or IP If the LOG_POSITION is dened the x and y co-ordinates for the mobilenode is also printed. The descriptions for different elds in the mac trace are given in the comments above. For all IP packets additional IP header elds are also added to the 137
above trace. The IP trace is described below: sprintf(wrk_ + offset, "------- [%d:%d %d:%d %d %d] ", src, // IP src address ih->sport_, // src port number dst, // IP dest address ih->dport_, // dest port number ih->ttl_, // TTL value (ch->next_hop_ < 0) ? 0 : ch->next_hop_); // next hopaddress, if any. An example of a trace for a tcp packet is as follows: r 160.093884945 _6_ RTR --- 5 tcp 1492 [a2 4 6 800] ------- [655 36:0 16777984:0 31 16777984] [1 0] 2 0 Here we see a TCP data packet being received by a node with id of 6. UID of this pkt is 5 with a cmn hdr size of 1492. The mac details shows an IP pkt (ETHERTYPE_IP is dened as 0x0800, ETHERTYPE_IP is 0x0806 ), mac-id of this receiving node is 6. That of the sending node is 4 and expected time to send this data pkt over the wireless channel is a2 (hex2dec conversion: 160+2 sec). Additionally, IP traces information about IP src and destination addresses. The src translates (using a 3 level hier-address of 8/8/8) to a address string of 0.1.0 with port of 0. The dest address is 1.0.3 with port address of 0. The TTL value is 31 and the destination was a hop away from the src. Additionally TCP format prints information about tcp seqno of 1, ackno of 0. See other formats described in ~ns//cmu-trace.cc for DSR, UDP/MESSAGE, TCP/ACK and CBR packet types. Other trace formats are also used by the routing agents (TORA and DSR) to log certain special routing events like "originating" (adding a SR header to a packet) or "ran off the end of a source route" indicating some sort of routing problem with the source route etc. These special event traces begin with "S" for DSR and "T" for Tora and may be found in ~ns/tora/tora.cc for TORA and ~ns/dsr/dsrgent.cc for DSR routing agent.
15.1.6 Revised format for wireless traces
In an effort to merge wireless trace, using cmu-trace objects, with ns tracing, a new, inproved trace format has been introduced. This revised trace support is backwards compatible with the old trace formatting and can be enabled by the following command: $ns use-newtrace This command should be called before the universal trace command $ns trace-all <trace-fd>. Primitive use-newtrace sets up new format for wireless tracing by setting a simulator variable called newTraceFormat. Currently this new trace support is available for wireless simulations only and shall be extended to rest of nsin the near future. An example of the new trace format is shown below: s -t 0.267662078 -Hs 0 -Hd -1 -Ni 0 -1.000000 -Nl RTR -Nw --- -Ma 0 -Md message -Il 32 -If 0 -Ii 0 -Iv 32 s -t 1.511681090 -Hs 1 -Hd -1 -Ni 1 -1.000000 -Nl RTR -Nw --- -Ma 0 -Md -Nx 5.00 -Ny 2.00 -Nz 0.00 -Ne 0 -Ms 0 -Mt 0 -Is 0.255 -Id -1.255 -It -Nx 390.00 -Ny 385.00 -Nz 0.00 -Ne 0 -Ms 0 -Mt 0 -Is 1.255 -Id -1.255 -It 138
message -Il 32 -If 0 -Ii 1 -Iv 32 s -t 10.000000000 -Hs 0 -Hd -2 -Ni 0 -Nx 5.00 -Ny 2.00 -Nz 0.00 -Ne -1.000000 -Nl AGT -Nw --- -Ma 0 -Md 0 -Ms 0 -Mt 0 -Is 0.0 -Id 1.0 -It tcp -Il 1000 -If 2 -Ii 2 -Iv 32 -Pn tcp -Ps 0 -Pa 0 -Pf 0 -Po 0 r -t 10.000000000 -Hs 0 -Hd -2 -Ni 0 -Nx 5.00 -Ny 2.00 -Nz 0.00 -Ne -1.000000 -Nl RTR -Nw --- -Ma 0 -Md 0 -Ms 0 -Mt 0 -Is 0.0 -Id 1.0 -It tcp -Il 1000 -If 2 -Ii 2 -Iv 32 -Pn tcp -Ps 0 -Pa 0 -Pf 0 -Po 0 r -t 100.004776054 -Hs 1 -Hd 1 -Ni 1 -Nx 25.05 -Ny 20.05 -Nz 0.00 -Ne -1.000000 -Nl AGT -Nw --- -Ma a2 -Md 1 -Ms 0 -Mt 800 -Is 0.0 -Id 1.0 -It tcp -Il 1020 -If 2 -Ii 21 -Iv 32 -Pn tcp -Ps 0 -Pa 0 -Pf 1 -Po 0 s -t 100.004776054 -Hs 1 -Hd -2 -Ni 1 -Nx 25.05 -Ny 20.05 -Nz 0.00 -Ne -1.000000 -Nl AGT -Nw --- -Ma 0 -Md 0 -Ms 0 -Mt 0 -Is 1.0 -Id 0.0 -It ack -Il 40 -If 2 -Ii 22 -Iv 32 -Pn tcp -Ps 0 -Pa 0 -Pf 0 -Po 0
Explanation of new trace format The new trace format as seen above can be can be divided into the following elds : Event type In the traces above, the rst eld (as in the older trace format) describes the type of event taking place at the node and can be one of the four types: s send r receive ]item[d] drop f forward General tag The second eld starting with "-t" may stand for time or global setting -t time -t * (global setting) Node property tags This eld denotes the node properties like node-id, the level at which tracing is being done like agent, router or MAC. The tags start with a leading "-N" and are listed as below: -Ni: node id -Nx: nodes x-coordinate -Ny: nodes y-coordinate -Nz: nodes z-coordinate -Ne: node energy level -Nl: trace level, such as AGT, RTR, MAC -Nw: reason for the event. The different reasons for dropping a packet are given below: "END" DROP_END_OF_SIMULATION "COL" DROP_MAC_COLLISION "DUP" DROP_MAC_DUPLICATE "ERR" DROP_MAC_PACKET_ERROR "RET" DROP_MAC_RETRY_COUNT_EXCEEDED "STA" DROP_MAC_INVALID_STATE "BSY" DROP_MAC_BUSY "NRTE" DROP_RTR_NO_ROUTE i.e no route is available. "LOOP" DROP_RTR_ROUTE_LOOP i.e there is a routing loop 139
"TTL" DROP_RTR_TTL i.e TTL has reached zero. "TOUT" DROP_RTR_QTIMEOUT i.e packet has expired. "CBK" DROP_RTR_MAC_CALLBACK "IFQ" DROP_IFQ_QFULL i.e no buffer space in IFQ. "ARP" DROP_IFQ_ARP_FULL i.e dropped by ARP "OUT" DROP_OUTSIDE_SUBNET i.e dropped by base stations on receiving routing updates from nodes outside its domain. Packet information at IP level The tags for this eld start with a leading "-I" and are listed along with their explanations as following: -Is: source address.source port number -Id: dest address.dest port number -It: packet type -Il: packet size -If: ow id -Ii: unique id -Iv: ttl value Next hop info This eld provides next hop info and the tag starts with a leading "-H". -Hs: id for this node -Hd: id for next hop towards the destination. Packet info at MAC level This eld gives MAC layer information and starts with a leading "-M" as shown below: -Ma: duration -Md: dsts ethernet address -Ms: srcs ethernet address -Mt: ethernet type Packet info at "Application level" The packet information at application level consists of the type of application like ARP, TCP, the type of adhoc routing protocol like DSDV, DSR, AODV etc being traced. This eld consists of a leading "-P" and list of tags for different application is listed as below: -P arp Address Resolution Protocol. Details for ARP is given by the following tags: -Po: ARP Request/Reply -Pm: src mac address -Ps: src address -Pa: dst mac address -Pd: dst address -P dsr This denotes the adhoc routing protocol called Dynamic source routing. Information on DSR is represented by the following tags: -Pn: how many nodes traversed -Pq: routing request ag -Pi: route request sequence number -Pp: routing reply ag -Pl: reply length -Pe: src of srcrouting->dst of the source routing -Pw: error report ag ? 140
-Pm: number of errors -Pc: report to whom -Pb: link error from linka->linkb -P cbr Constant bit rate. Information about the CBR application is represented by the following tags: -Pi: sequence number -Pf: how many times this pkt was forwarded -Po: optimal number of forwards -P tcp Information about TCP ow is given by the following subtags: -Ps: -Pa: -Pf: -Po: seq number ack number how many times this pkt was forwarded optimal number of forwards
This eld is still under development and new tags shall be added for other applications as they get included along the way.
15.1.7 Generation of node-movement and trafc-connection for wireless scenarios
Normally for large topologies, the node movement and trafc connection patterns are dened in separate les for convinience. These movement and trafc les may be generated using CMUs movement- and connection-generators. In this section we shall describe both separately.
MobileNode Movement Some examples of node movement les may be found in ~ns/tcl/mobility/scene/scen-670x670-50-600-20-*. These les dene a topology of 670 by 670m where 50 nodes move with a speed of 20m/s with pause time of 600s. each node is assigned a starting position. The information regarding number of hops between the nodes is fed to the central object "GOD" (XXX but why/where is this information used??-answer awaited from CMU.) Next each node is a speed and a direction to move to. The generator for creating node movement les are to be found under ~ns/indep-utils/cmu-scen-gen/setdest/ directory. Compile the les under setdest to create an executable. run setdest with arguments in the following way: ./setdest -n <num_of_nodes> -p <pausetime> -s <maxspeed> -t <simtime> -x <maxx> -y <maxy> > <outdir>/<scenario-file> Note that the index used for nodes now start from 0 instead of 1 as was in the original CMU version, to match with nss tradition of assigning node indices from 0.
Generating trafc pattern les The examples for trafc patterns may be found in ~ns/tcl/mobility/scene/cbr-50-{10-4-512, 20-4-512}. The trafc generator is located under ~ns/indep-utils/cmu-scen-gen/ and are called cbrgen.tcl and tcpgen.tcl. They may be used for generating CBR and TCP connections respectively. To create CBR connecions, run 141
ns cbrgen.tcl [-type cbr|tcp] [-nn nodes] [-seed seed] [-mc connections] [-rate rate] To create TCP connections, run ns tcpgen.tcl [-nn nodes] [-seed seed] You will need to pipe the outputs from above to a cbr-* or a tcp-* le.
15.2 Extensions made to CMUs wireless model
As mentioned earlier, the original CMU wireless model allows simulation of wireless LANs and ad-hoc networks. However in order to use the wireless model for simulations using both wired and wireless nodes we had to add certain extensions to cmu model. We call this wired-cum-wireless feature. Also SUNs MobileIP (implemented for wired nodes) was integrated into the wireless model allowing mobileIP to run over wireless mobilenodes. The following two subsections describe these two extensions to the wireless model in ns.
15.2.1 wired-cum-wireless scenarios
The mobilenodes described so far mainly supports simulation of multi-hop ad-hoc networks or wireless LANs. But what if we need to simulate a topology of multiple wireless LANs connected through wired nodes, or may need to run mobileIP on top of these wireless nodes? The extensions made to the CMU wireless model allows us to do that. The main problem facing the wired-cum-wireless scenario was the issue of routing. In ns, routing information is generated based on the connectivity of the topology, i.e how nodes are connected to one another through Links. Mobilenodes on the other hand have no concept of links. They route packets among themselves, within the wireless topology, using their routing protocol. so how would packets be exchanged between these two types of nodes? So a node called BaseStationNode is created which plays the role of a gateway for the wired and wireless domains. The BaseStationNode is essentially a hybrid between a Hierarchical node 1 (HierNode) and a MobileNode. The basestation node is responsible for delivering packets into and out of the wireless domain. In order to achieve this we need Hierarchical routing. Each wireless domain along with its base-station would have an unique domain address assigned to them. All packets destined to a wireless node would reach the base-station attached to the domain of that wireless node, who would eventually hand the packet over to the destination (mobilenode). And mobilenodes route packets, destined to outside their (wireless) domain, to their base-station node. The base-station knows how to forward these packets towards the (wired) destination. The schematic of a BaseStationNode is shown in Figure 15.3. The mobilenodes in wired-cum-wireless scenario are required to support hierarchical addressing/routing. Thus the MobileNode looks exactly like the BaseStationNode. The SRNode, however, simply needs to have its own hier-address since it does not require any address demuxes and thus is not required to support hier routing 2. The DSDV agent on having to forward a packet checks to see if the destination is outside its (wireless) subnet. If so, it tries to forward the packet to its base-station node. In case no route to base-station is found the packet is dropped. Otherwise
to Chapter 26 for details on hierarchical routing and internals of HierNode. order to do away with all these different variations of the denition of a node, we are planning to revise the node architecture that would allow a more exible and modularised construction of a node without the necessity of having to dene and be limited to certain Class denitions only.
2 In 1 Refer
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Hierarchical classifiers node entry level n level 2 level 1 IP address
port demux 255
Src/Sink
RTagent (DSDV) defaulttarget_ target_ arptable_ uptarget_ LL ARP
downtarget_
IFq
downtarget_
mac_
MAC
uptarget_
downtarget_ Radio Propagation Model propagation_
uptarget_
NetIF channel_ uptarget_ Channel
Figure 15.3: Schematic of a baseStationNode
the packet is forwarded to the next_hop towards the base-station. Which is then routed towards the wired network by basestations classiers. The DSR agent, on receiving a pkt destined outside its subnet, sends out a route-query for its base-station in case the route to base-station is not known. The data pkt is temporarily cached while it waits to hear route replies from base-station. On getting
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a reply the packet is provided with routing information in its header and send away towards the base-station. The base-station address demuxes routes it correctly toward the wired network. The example script for a wired-cum-wireless simulation can be found at ~ns/tcl/ex/wired-cum-wireless-sim.tcl. The methods for wired-cum-wireless implementations are dened in ~ns/tcl/lib/ns-bsnode.tcl, ~ns/tcl/mobility/{com.tcl,dsr.tcl, dsdv.tcl}, ~ns/dsdv/dsdv.{cc,h} and ~ns/dsr/dsragent.{cc,h}.
15.2.2 MobileIP
The wired-cum-wireless extensions for the wireless model paved the path for supporting wireless MobileIP in ns. Sun Microsystems (Charlie Perkins et al) MobileIP model was based on nss wired model (consisting of Nodes and Links) and thus didnot use CMUs mobility model. Here we briey describe the wireless MobileIP implementation. We hope that Sun would provide the detailed version of the documentation in the future. The mobileIP scenario consists of Home-Agents(HA) and Foreign-Agents(FA) and have Mobile-Hosts(MH) moving between their HA and FAs. The HA and FA are essentially base-station nodes we have described earlier. While MHs are basically the mobileNodes described in section 15.1.1. The methods and procedures for MobileIP extensions are described in ~ns/mip.{cc,h}, ~ns/mip-reg.cc, ~ns/tcl/lib/ns-mip.tcl and ~ns/tcl/lib/ns-wireless-mip.tcl. The HA and FA nodes are dened as MobileNode/MIPBS having a registering agent (regagent_) that sends beacon out to the mobilenodes, sets up encapsulator and decapsulator, as required and replies to solicitations from MHs. The MH nodes are dened as MobileNode/MIPMH which too have a regagent_ that receives and responds to beacons and sends out solicitations to HA or FAs. Figure 15.4 illustrates the schematic of a MobileNode/MIPBS node. The MobileNode/MIPMH node is very similar to this except for the fact that it doesnot have any encapsulator or decapsulator. As for the SRNode version of a MH, it doesnot have the hierarchical classiers and the RA agent forms the entry point of the node. See Figure 15.2 for model of a SRNode. The MobileNode/MIPBS node routinely broadcasts beacon or advertisement messages out to MHs. A solicitation from a mobilenode generates an ad that is send directly to the requesting MH. The address of the base-station sending out beacon is heard by MH and is used as the COA (care-of-address) of the MH. Thus as the MH moves from its native to foreign domains, its COA changes. Upon receiving reg_request (as reply to ads) from a mobilehost the base-station checks to see if it is the HA for the MH. If not, it sets up its decapsulator and forwards the reg_request towards the HA of the MH. In case the base-station is the HA for the requesting MH but the COA doesnot match its own, it sets up an encapsulator and sends reg-request-reply back to the COA (address of the FA) who has forwarded the reg_request to it. so now all packets destined to the MH reaching the HA would be tunneled through the encapsulator which encapsulates the IP pkthdr with a IPinIP hdr, now destined to the COA instead of MH. The FAs decapsulator recives this packet, removes the encapsulation and sends it to the MH. If the COA matches that of the HA, it just removes the encapsulator it might have set up (when its mobilehost was roaming into foreign networks) and sends the reply directly back to the MH, as the MH have now returned to its native domain. The mobilehost sends out solicitations if it doesnot hear any ads from the base-stations. Upon receiving ads, it changes its COA to the address of the HA/FA it has heard the ad from, and replies back to the COA with a request for registration (reg-request). Initially the MH maybe in the range of the HA and receives all pkts directly from its COA which is HA in this case. Eventually as the MH moves out of range of its HA and into the a foreign domain of a FA, the MHs COA changes from its HA to that of the FA. The HA now sets up an encapsulator and tunnels all pkts destined for MH towards the FA. The FA decapsulates the pkts and hands them over to the MH. The data from MH destined for the wired world is always routed towards its current COA. An example script for wireless mobileIP can be found at ~ns/tcl/ex/wireless-mip-test.tcl. The simulation consists of a MH moving between its HA and a FA. The HA and FA are each connected to a wired domain on one
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target_ target_ encapsulator reg_agent_
MH IP address
0 1
decapsulator_
255 Hierarchical classifiers level n entry_ level 2 Rtg Agent level 1 defaulttarget_ target_ own IP address default target_
uptarget_
LL
downtarget_ Channel
Figure 15.4: Schematic of a Wireless MobileIP BaseStation Node side and to their wireless domains on the other. TCP ows are set up between the MH and a wired node.
15.3 Commands at a glance
Following is a list of commands used in wireless simulations: $ns_ node-config -addressingType <usually flat or hierarchical used for wireless topologies> -adhocRouting <adhoc rotuing protocol like DSDV, DSR, 145
-llType -macType -propType -ifqType -ifqLen -phyType -antType -channelType -topoInstance -wiredRouting -mobileIP -energyModel -initialEnergy -rxPower -txPower -agentTrace -routerTrace -macTrace -movementTrace
TORA, AODV etc> <LinkLayer> <MAC type like Mac/802_11> <Propagation model like Propagation/TwoRayGround> <interface queue type like Queue/DropTail/PriQueue> <interface queue length like 50> <network inteface type like Phy/WirelessPhy> <antenna type like Antenna/OmniAntenna> <Channel type like Channel/WirelessChannel> <the topography instance> <turning wired routing ON or OFF> <setting the flag for mobileIP ON or OFF> <EnergyModel type> <specified in Joules> <specified in W> <specified in W> <tracing at agent level turned ON or OFF> <tracing at router level turned ON or OFF> <tracing at mac level turned ON or OFF> <mobilenode movement logging turned ON or OFF>
This command is used typically to congure for a mobilenode. For more info about this command (part of new node APIs) see chapter titled "Restructuring ns node and new Node APIs" in ns Notes and Documentation. $ns_ node <optional:hier address> This command is used to create a mobilenode after node conguration is done as shown in the node-cong command. Incase hierarchical addressing is being used, the hier address of the node needs to be passed as well. $node log-movement This command previously used to enable logging of mobilenodes movement has now been replaced by $ns_ node-config -movementTrace <ON or OFF>. create-god <num_nodes> This command is used to create a God instance. The number of mobilenodes is passed as argument which is used by God to create a matrix to store connectivity information of the topology. $topo load_flatgrid <X> <Y> <optional:res> This initializes the grid for the topography object. <X> and <Y> are the x-y co-ordinates for the topology and are used for sizing the grid. The grid resolution may be passed as <res>. A default value of 1 is normally used. $topo load_demfile <file-descrptor> For loading DEMFile objects into topography. See ns/dem.cc,.h for details on DEMFiles. $ns_ namtrace-all-wireless <namtrace> <X> <Y> This command is used to initialize a namtrace le for logging node movements to be viewed in nam. The namtrace le descriptor, the X and Y co-ordinates of the wireless topology is passed as parameters with this command. $ns_ nam-end-wireless <stop-time> This command is used to tell nam the simulation stop time given by <stop-time>. 146
$ns_ initial_node_pos <node> <size> This command denes the node initial position in nam. <size> denotes the size of node in nam. This function must be called after mobility model has been dened. $mobilenode random-motion <0 or 1> Random-motion is used to turn on random movements for the mobilenode, in which case random destinations are assigned to the node. 0 disables and 1 enables random-motion. $mobilenode setdest <X> <Y> <s> This command is used to setup a destination for the mobilenode. The mobile node starts moving towards destination given by <X> and <Y> at a speed of <s> m/s. $mobilenode reset This command is used to reset all the objects in the nodes (network components like LL, MAC, phy etc). Internal procedures Following is a list of internal procedures used in wireless networking: $mobilenode base-station <BSnode-hier-addr> This is used for wired-cum-wireless scenarios. Here the mobilenode is provided with the base-stationnode info for its domain. The address is hierarchical since wired-cum-wireless scenarios typically use hierarchical addressing. $mobilenode log-target <target-object> The <target-object>, which is normally a trace object, is used to log mobilenode movements and their energy usage, if energy model is provided. $mobilenode topography <topoinstance> This command is used to provide the node with a handle to the topography object. $mobilenode addif A mobilenode may have more than one network interface. This command is used to pass handle for a network interface to the node. $mobilenode namattach <namtracefd> This command is used to attach the namtrace le descriptor <namtracefd> to the mobilenode. All nam traces for the node are then written into this namtrace le. $mobilenode radius <r> The radius <r> denotes the nodes range. All mobilenodes that fall within the circle of radius <r> with the node at its center are considered as neighbours. This info is typically used by the gridkeeper. $mobilenode start This command is used to start off the movement of the mobilenode.
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Chapter 16
Satellite Networking in ns
This chapter describes extensions that enable the simulation of satellite networks in ns. In particular, these extensions enable ns to model the following: i) traditional geostationary bent-pipe satellites with multiple users per uplink/downlink and asymmetric links, ii) geostationary satellites with processing payloads (either regenerative payloads or full packet switching), and iii) polar orbiting LEO constellations such as Iridium and Teledesic. These satellite models are principally aimed at using ns to study networking aspects of satellite systems; in particular, MAC, link layer, routing, and transport protocols.
16.1 Overview of satellite models
Exact simulation of satellite networks requires a detailed modelling of radio frequency characteristics (interference, fading), protocol interactions (e.g., interactions of residual burst errors on the link with error checking codes), and second-order orbital effects (precession, gravitational anomalies, etc.). However, in order to study fundamental characteristics of satellite networks from a networking perspective, certain features may be abstracted out. For example, the performance of TCP over satellite links is impacted little by using an approximate rather than detailed channel model performance can be characterized to rst order by the overall packet loss probability. This is the approach taken in this simulation model to create a framework for studying transport, routing, and MAC protocols in a satellite environment consisting of geostationary satellites or constellations of polar-orbiting low-earth-orbit (LEO) satellites. Of course, users may extend these models to provide more detail at a given layer.
16.1.1 Geostationary satellites
Geostationary satellites orbit the Earth at an altitude of 22,300 miles above the equator. The position of the satellites is specied in terms of the longitude of the nadir point (subsatellite point on the Earths surface). In practice, geostationary satellites can drift from their designated location due to gravitational perturbations these effects are not modelled in ns. Two kinds of geostationary satellites can be modelled. Traditional bent-pipe geostationary satellites are merely repeaters in orbit all packets received by such satellites on an uplink channel are piped through at RF frequencies to a corresponding downlink, and the satellite node is not visible to routing protocols. Newer satellites will increasingly use baseband processing, both to regenerate the digital signal and to perform fast packet switching on-board the spacecraft. In the simulations, these satellites can be modelled more like traditional ns nodes with classiers and routing agents. Previously, users could simulate geostationary satellite links by simply simulating a long delay link using traditional ns links and nodes. The key enhancement of these satellite extensions with respect to geostationary satellites is the capability to 148
Counter-rotating planes cause rapid crossseam ISL handoffs
Overlap of coverage at the poles Interplane intersatellite links (ISLs) are turned off
An intraplane ISL An interplane ISL
Figure 16.1: Example of a polar-orbiting LEO constellation. This gure was generated using the SaVi software package from the geometry center at the University of Minnesota. simulate MAC protocols. Users can now dene many terminals at different locations on the Earths surface and connect them to the same satellite uplink and downlink channels, and the propagation delays in the system (which are slightly different for each user) are accurately modelled. In addition, the uplink and downlink channels can be dened differently (perhaps with different bandwidths or error models).
16.1.2 Low-earth-orbiting satellites
Polar orbiting satellite systems, such as Iridium and the proposed Teledesic system, can be modelled in ns. In particular, the simulator supports the specication of satellites that orbit in purely circular planes, for which the neighboring planes are corotating. There are other non-geostationary constellation congurations possible (e.g., Walker constellations) the interested user may develop new constellation classes to simulate these other constellation types. In particular, this would mainly require dening new intersatellite link handoff procedures. The following are the parameters of satellite constellations that can currently be simulated:
Basic constellation denition Includes satellite altitude, number of satellites, number of planes, number of satellites per plane. Orbits Orbit inclination can range continuously from 0 to 180 degrees (inclination greater than 90 degrees corresponds to retrograde orbits). Orbit eccentricity is not modeled. Nodal precession is not modeled. Intersatellite spacing within a given plane is xed. Relative phasing between planes is xed (although some systems may not control phasing between planes). Intersatellite (ISL) links For polar orbiting constellations, intraplane, interplane, and crossseam ISLs can be dened. Intraplane ISLs exist between satellites in the same plane and are never deactivated or handed off. Interplane ISLs exist between satellites of neighboring co-rotating planes. These links are deactivated near the poles (above the ISL latitude threshold in the table) because the antenna pointing mechanism cannot track these links in the polar regions. Like intraplane ISLs, interplane ISLs are never handed off. Crossseam ISLs may exist in a constellation between satellites
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in counter-rotating planes (where the planes form a so-called seam in the topology). GEO ISLs can also be dened for constellations of geostationary satellites.
Ground to satellite (GSL) links Multiple terminals can be connected to a single GSL satellite channel. GSL links for GEO satellites are static, while GSL links for LEO channels are periodically handed off as described below. Elevation mask The elevation angle above which a GSL link can be operational. Currently, if the (LEO) satellite serving a terminal drops below the elevation mask, the terminal searches for a new satellite above the elevation mask. Satellite terminals check for handoff opportunities according to a timeout interval specied by the user. Each terminal initiates handoffs asynchronously; it would be possible also to dene a system in which each handoff occurs synchronously in the system.
The following table lists parameters used for example simulation scripts of the Iridium 1 and Teledesic2 systems. Iridium Altitude Planes Satellites per plane Inclination (deg) Interplane separation (deg) Seam separation (deg) Elevation mask (deg) Intraplane phasing Interplane phasing ISLs per satellite ISL bandwidth Up/downlink bandwidth Cross-seam ISLs ISL latitude threshold (deg) 780 km 6 11 86.4 31.6 22 8.2 yes yes 4 25 Mb/s 1.5 Mb/s no 60 Teledesic 1375 km 12 24 84.7 15 15 40 yes no 8 155 Mb/s 1.5 Mb/s yes 60
Table 16.1: Simulation parameters used for modeling a broadband version of the Iridium system and the proposed 288-satellite Teledesic system. Both systems are examples of polar orbiting constellations.
1 Aside from the link bandwidths (Iridium is a narrowband system only), these parameters are very close to what a broadband version of the Iridium system might look like. 2 These Teledesic constellation parameters are subject to change; thanks to Marie-Jose Montpetit of Teledesic for providing tentative parameters as of January 1999. The link bandwidths are not necessarily accurate.
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Z
R
X
o 0 longitude at equator
Y
Figure 16.2: Spherical coordinate system used by satellite nodes
16.2 Using the satellite extensions
16.2.1 Nodes and node positions
There are two basic kinds of satellite nodes: geostationary and non-geostationary satellite nodes. In addition, terminal nodes can be placed on the Earths surface. As is explained later in Section 16.3, each of these three different types of nodes is actually implemented with the same class SatNode object, but with different position, handoff manager, and link objects attached. The position object keeps track of the satellite nodes location in the coordinate system as a function of the elapsed simulation time. This position information is used to determine link propagation delays and appropriate times for link handoffs. Figure 16.2 illustrates the spherical coordinate system, and the corresponding Cartesian coordinate system. The coordi nate system is centered at the Earths center, and the axis coincides with the Earths axis of rotation. corresponds to longitude (prime meridian) on the equator. Specically, there is one class of satellite node Class Node/SatNode, to which one of three types of Position objects may be attached. Each SatNode and Position object is a split OTcl/C++ object, but most of the code resides in C++. The following types of position objects exist:
Position/Sat/Term A terminal is specied by its latitude and longitude. Latitude ranges from and longitude ranges from , with negative values corresponding to south and west, respectively. As simulation time evolves, the terminals move along with the Earths surface. The Simulator instproc satnode can be used to create a terminal with an attached position object as follows: $ns satnode terminal $lat $lon
Position/Sat/Geo A geostationary satellite is specied by its longitude above the equator. As simulation time evolves, the geostationary satellite moves through the coordinate system with the same orbital period as that of the
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Earths rotation. The longitude ranges from degrees. The Simulator instproc satnode can be used to create a geostationary satellite with an attached position object as follows: $ns satnode geo $lon
Position/Sat/Polar A polar orbiting satellite has a purely circular orbit along a xed plane in the coordinate system; the Earth rotates underneath this orbital plane, so there is both an east-west and a north-south component to the track of a polar satellites footprint on the Earths surface. Strictly speaking, the polar position object can be used to model the movement of any circular orbit in a xed plane; we use the term polar here because we later use such satellites to model polar-orbiting constellations. Satellite orbits are usually specied by six parameters: altitude, semi-major axis, eccentricity, right ascension of ascending node, inclination, and time of perigee passage. The polar orbiting satellites in ns have purely circular orbits, so we simplify the specication of the orbits to include only three parameters: altitude, inclination, and longitude, with a fourth parameter alpha specifying initial position of the satellite in the orbit, as described below. Altitude is specied in kilometers above the Earths surface, and inclination can range from degrees, with corresponding to pure polar orbits and angles greater than degrees corresponding to retrograde orbits. The ascending node refers to the point where the footprint of the satellite orbital track crosses the equator moving from south to north. In this simulation model, the parameter longitude of ascending node species the earth-centric longitude at which the satellites nadir point crosses the equator moving south to north. 3 Longitude of ascending node can range from degrees. The fourth parameter, alpha, species the initial position of the satellite along this orbit, starting from the ascending node. For example, an alpha of degrees indicates that the satellite is initially above the equator moving from north to south. Alpha can range from degrees. Finally, a fth parameter, plane, is specied when creating polar satellite nodes all satellites in the same plane are given the same plane index. The Simulator instproc satnode can be used to create a polar satellite with an attached position object as follows: $ns satnode polar $alt $inc $lon $alpha $plane Note that the above methods use the atomic satnode instproc. In Section 16.2.2, we introduce wrapper methods that may be more convenient for generating various types of satellite nodes: $ns $ns $ns $ns satnode-terminal $latitude $longitude satnode-polar $alt $inc $lon $alpha $plane $linkargs $chan satnode-geo $longitude $linkargs $chan satnode-geo-repeater $longitude $chan
16.2.2 Satellite links
Satellite links resemble wireless links, which are described in Chapter 15. Each satellite node has one or more satellite network interface stacks, to which channels are connected to the physical layer object in the stack. Figure 16.3 illustrates the major components. Satellite links differ from ns wireless links in two major respects: i) the transmit and receive interfaces must be connected to different channels, and ii) there is no ARP implementation. Currently, the Radio Propagation Model is a placeholder for users to add more detailed error models if so desired; the current code does not use a propagation model. Network interfaces can be added with the following instproc of Class Node/SatNode: $node add-interface $type $ll $qtype $qlim $mac $mac_bw $phy The add-interface instproc returns an index value that can be used to access the network interface stack later in the simulation. By convention, the rst interface created on a node is attached to the uplink and downlink channels of a satellite or terminal. The following parameters must be provided:
3 Traditionally, the right ascension of the ascending node is specied for satellite orbits the right ascension corresponds to the celestial longitude. In our case, we do not care about the orientation in a celestial coordinate system, so we specify the earth-centric longitude instead.
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LL
IFq
MAC
Phy_tx
Phy_rx
Radio Propagation Model
Channel
Channel
Figure 16.3: Main components of a satellite network interface
type: The following link types can be indicated: geo or polar for links from a terminal to a geo or polar satellite, respectively, gsl and gsl-repeater for links from a satellite to a terminal, and intraplane, interplane, and crossseam ISLs. The type eld is used internally in the simulator to identify the different types of links, but structurally they are all very similar. ll: The link layer type (class LL/Sat is currently the only one dened). qtype: The queue type (e.g., class Queue/DropTail). Any queue type may be used however, if additional parameters beyond the length of the queue are needed, then this instproc may need to be modied to include more arguments. qlim: The length of the interface queue, in packets. mac: The MAC type. Currently, two types are dened: class Mac/Sat a basic MAC for links with only one receiver (i.e., it does not do collision detection), and Class Mac/Sat/UnslottedAloha an implementation of unslotted Aloha. mac_bw: The bandwidth of the link is set by this parameter, which controls the transmission time how fast the MAC sends. The packet size used to calculate the transmission time is the sum of the values size() in the common packet header and LINK_HDRSIZE, which is the size of any link layer headers. The default value for LINK_HDRSIZE is 16 bytes (settable in satlink.h). The transmission time is encoded in the packet header for use at the receive MAC (to simulate waiting for a whole packet to arrive). phy: The physical layer currently two Phys (Class Phy/Sat and Class Phy/Repeater) are dened. The class Phy/Sat just pass the information up and down the stack as in the wireless code described in Chapter 15, a radio propagation model could be attached at this point. The class Phy/Repeater pipes any packets received on a receive interface straight through to a transmit interface.
An ISL can be added between two nodes using the following instproc: $ns add-isl $ltype $node1 $node2 $bw $qtype $qlim
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This creates two channels (of type Channel/Sat), and appropriate network interfaces on both nodes, and attaches the channels to the network interfaces. The bandwidth of the link is set to bw. The linktype (ltype) must be specied as either intraplane, interplane, or crossseam. A GSL involves adding network interfaces and a channel on board the satellite (this is typically done using the wrapper methods described in the next paragraph), and then dening the correct interfaces on the terrestrial node and attaching them to the satellite link, as follows: $node add-gsl $type $ll $qtype $qlim $mac $bw_up $phy [$node_satellite set downlink_] [$node_satellite set uplink_] Here, the type must be either geo or polar, and we make use of the downlink_ and uplink_ instvars of the satellite; therefore, the satellites uplink and downlink must be created before this instproc is called. Finally, the following wrapper methods can be used to create nodes of a given type and, in the case of satellite nodes, give them an uplink and downlink interface as well as create and attach an uplink and downlink channel: $ns $ns $ns $ns satnode-terminal $latitude $longitude satnode-polar $alt $inc $lon $alpha $plane $linkargs $chan satnode-geo $longitude $linkargs $chan satnode-geo-repeater $longitude $chan
where linkargs is a list of link argument options for the network interfaces (i.e., $ll $qtype $qlim $mac $mac_bw $phy).
16.2.3 Handoffs
Satellite handoff modelling is a key component of LEO satellite network simulations. It is difcult to predict exactly how handoffs will occur in future LEO systems because the subject is not well treated in the literature. In these satellite extensions, we establish certain criteria for handoffs, and allow nodes to independently monitor for situations that require a handoff. An alternative would be to have all handoff events synchronized across the entire simulation it would not be difcult to change the simulator to work in such a manner. There are no link handoffs involving geostationary satellites, but there are two types of links to polar orbiting satellites that must be handed off: GSLs to polar satellites, and crossseam ISLs. A third type of link, interplane ISLs, are not handed off but are deactivated at high latitudes as we describe below. Each terminal connected to a polar orbiting satellite runs a timer that, upon expiry, causes the HandoffManager to check whether the current satellite has fallen below the elevation mask of the terminal. If so, the handoff manager detaches the terminal from that satellites up and down links, and searches through the linked list of satellite nodes for another possible satellite. First, the next satellite in the current orbital plane is checked a pointer to this satellite is stored in the Position object of each polar satellite node and is set during simulation conguration using the Node/SatNode instproc $node set_next $next_node. If the next satellite is not suitable, the handoff manager searches through the remaining satellites. If it nds a suitable polar satelite, it connects its network interfaces to that satellites uplink and downlink channels, and restarts the handoff timer. If it does not nd a suitable satellite, it restarts the timer and tries again later. If any link changes occur, the routing agent is notied. The elevation mask and handoff timer interval are settable via OTcl: HandoffManager/Term set elevation_mask_ 10; # degrees 154
HandoffManager/Term set term_handoff_int_ 10; # seconds In addition, handoffs may be randomized to avoid phase effects by setting the following variable: HandoffManager set handoff_randomization_ 0; # 0 is false, 1 is true If handoff_randomization_ is true, then the next handoff interval is a random variate picked from a uniform distribution across _ _ _ _ _ _. Crossseam ISLs are the only type of ISLs that are handed off. The criteria for handing off a crossseam ISL is whether or not there exists a satellite in the neighboring plane that is closer to the given satellite than the one to which it is currently connected. Again, a handoff timer running within the handoff manager on the polar satellite determines when the constellation is checked for handoff opportunities. Crossseam ISL handoffs are initiated by satellites in the lower-numbered plane of the two. It is therefore possible for a transient condition to arise in which a polar satellite has two crossseam ISLs (to different satellites). The satellite handoff interval is again settable from OTcl and may also be randomized: HandoffManager/Sat set sat_handoff_int_ 10; # seconds Interplane and crossseam ISLs are deactivated near the poles, because the pointing requirements for the links are too severe as the satellite draw close to one another. Shutdown of these links is governed by a parameter: HandoffManager/Sat set latitude_threshold_ 70; # degrees The values for this parameter in the example scripts are speculative; the exact value is dependent upon the satellite hardware. The handoff manager checks the latitude of itself and its peer satellite upon a handoff timeout; if either or both of the satellites is above latitude_threshold_ degrees latitude (north or south), the link is deactivated until both satellites drop below this threshold. Finally, if crossseam ISLs exist, there are certain situations in which the satellites draw too close to one another in the midlatitudes (if the orbits are not close to being pure polar orbits). We check for the occurence of this orbital overlap with the following parameter: HandoffManager/Sat set longitude_threshold_ 10; # degrees Again, the values for this parameter in the example scripts are speculative. If the two satellites are closer together in longitude than longitude_threshold_ degrees, the link between them is deactivated. This parameter is disabled (set to ) by default all defaults for satellite-related bound variables can be found in ~ns/tcl/lib/ns-sat.tcl.
16.2.4 Routing
The current status of routing is that it is incomplete. Ideally, one should be able to run all existing ns routing protocols over satellite links. However, many of the existing routing protocols implemented in OTcl require that the conventional ns links be used. The ns developers are working to remedy this situation so that existing routing implementations are independent of the underlying link objects. With that being said, the current routing implementation is similar to Session routing described in Chapter 23, except that it is implemented entirely in C++. Upon each topology change, a centralized routing genie determines the global network 155
topology, computes new routes for all nodes, and uses the routes to build a forwarding table on each node. Currently, the slot table is kept by a routing agent on each node, and packets not destined for agents on the node are sent by default to this routing agent. For each destination for which the node has a route, the forwarding table contains a pointer to the head of the corresponding outgoing link. As noted in Chapter 23, the user is cautioned that this type of centralized routing can lead to minor causality violations. The routing genie is a class SatRouteObject and is created and invoked with the following OTcl commands: set satrouteobject_ [new SatRouteObject] $satrouteobject_ compute_routes where the call to compute_routes is performed after all of the links and nodes in the simulator have been instantiated. Like the Scheduler, there is one instance of a SatRouteObject in the simulation, and it is accessed by means of an instance variable in C++. For example, the call to recompute routes after a topology change is: SatRouteObject::instance().recompute(); Despite the current use of centralized routing, the design of having a routing agent on each node was mainly done with distributed routing in mind. Routing packets can be sent to port 255 of each node. The key to distributed routing working correctly is for the routing agent to be able to determine from which link a packet arrived. This is accomplished by the inclusion of a class NetworkInterface object in each link, which uniquely labels the link on which the packet arrived. A helper function NsObject* intf_to_target(int label) can be used to return the head of the link corresponding to a given label. The use of routing agents parallels that of the mobility extensions, and the interested reader can turn to those examples to see how to implement distributed routing protocols in this framework. The shortest-path route computations use the current propagation delay of a link as the cost metric. It is possible to compute routes using only the hop count and not the propagation delays; in order to do so, set the following default variable to "false": SatRouteObject set metric_delay_ "true" Finally, for very large topologies (such as the Teledesic example), the centralized routing code will produce a very slow runtime because it executes an all-pairs shortest path algorithm upon each topology change even if there is no data currently being sent. To speed up simulations in which there is not much data transfer but there are lots of satellites and ISLs, one can disable handoff-driven and enable data-driven route computations. With data-driven computations, routes are computed only when there is a packet to send, and furthermore, a single-source shortest-path algorithm (only for the node with a packet to send) is executed instead of an all-pairs shortest path algorithm. The following OTcl variable can congure this option (which is set to "false" by default): SatRouteObject set data_driven_computation_ "false"
16.2.5 Trace support
Traceles using satellite nodes and links are very similar to conventional ns tracing described in Chapter 21. Special SatTrace objects (class SatTrace derives from class Trace) are used to log the geographic latitude and longitude of the node logging the trace (in the case of a satellite node, the latitude and longitude correspond to the nadir point of the satellite). For example, a packet on a link from node 66 to node 26 might normally be logged as:
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+ 1.0000 66 26 cbr 210 ------- 0 66.0 67.0 0 0 but in the satellite simulation, the position information is appended: + 1.0000 66 26 cbr 210 ------- 0 66.0 67.0 0 0 37.90 -122.30 48.90 -120.94 In this case, node 66 is at latitude 37.90 degrees, longitude -122.30 degrees, while node 26 is a LEO satellite whose subsatellite point is at 48.90 degrees latitude, -120.94 degrees longitude (negative latitude corresponds to south, while negative longitude corresponds to west). One addition is the Class Trace/Sat/Error, which traces any packets that are errored by an error model. The error trace logs packets dropped due to errors as follows, for example: e 1.2404 12 13 cbr 210 ------- 0 12.0 13.0 0 0 -0.00 10.20 -0.00 -10.00 To enable tracing of all satellite links in the simulator, use the following commands before instantiating nodes and links: set f [open out.tr w] $ns trace-all $f Then use the following line after all node and link creation (and all error model insertion, if any) to enable tracing of all satellite links: $ns trace-all-satlinks $f Specically, this will put tracing around the link layer queues in all satellite links, and will put a receive trace between the mac and the link layer for received packets. To enable tracing only on a specic link on a specic node, one may use the command: $node trace-inlink-queue $f $i $node trace-outlink-queue $f $i where is the index of the interface to be traced. The implementations of the satellite trace objects can be found in ~ns/tcl/lib/ns-sat.tcl and ~ns/sattrace.{cc,h}.
16.2.6 Error models
ns error models are described in Chapter 12. These error models can be set to cause packets to be errored according to various probability distributions. These error models are simple and dont necessarily correspond to what would be experienced on an actual satellite channel (particularly a LEO channel). Users are free to dene more sophisticated error models that more closely match a particular satellite environment. The following code provides an example of how to add an error model to a link:
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set em_ [new ErrorModel] $em_ unit pkt $em_ set rate_ 0.02 $em_ ranvar [new RandomVariable/Uniform] $node interface-errormodel $em_ This will add an error model to the receive path of the rst interface created on node $node (specically, between the MAC and link layer) this rst interface generally corresponds to the uplink and downlink interface for a satellite or a terminal (if only one uplink and/or downlink exists). To add the error model to a different stack (indexed by ), use the following code: $node interface-errormodel $em_ $i
16.2.7 Other conguration options
Given an initial conguration of satellites specied for time , it is possible to start the satellite conguration from any arbitrary point in time through the use of the time_advance_ parameter (this is really only useful for LEO simulations). During the simulation run, this will set the position of the object to the position at time Scheduler::instance().clock + time_advance_ seconds. Position/Sat set time_advance_ 0; # seconds
16.2.8 nam support
nam is not currently supported. Addition of nam for GEO satellite topologies is an active work item. nam support for LEO constellations is not planned (interested users are encouraged to develop this component).
16.2.9 Integration with other modules
Currenty, these satellite extensions do not interoperate with the wireless code, mobile-IP code, and OTcl routing protocols (in particular, multicast). This is an active work item for the ns developers and more support along these lines will be added later.
16.2.10 Example scripts
Example scripts can be found in the ~ns/tcl/ex directory, including:
sat-mixed.tcl A simulation with a mixture of polar and geostationary satellites. sat-repeater.tcl Demonstrates the use of a simple bent-pipe geostationary satellite, and also error models. sat-aloha.tcl Simulates one hundred terminals in a mesh-VSAT conguration using an unslotted Aloha MAC protocol with a bent-pipe geostationary satellite. Terminals listen to their own transmissions (after a delay), and if they do not successfully receive their own packet within a timeout interval, they perform exponential backoff and then retransmit the packet.
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name lh_first obj
name is the name of the structure containing the list head lh_first is a pointer of type *obj, initialized to NULL lh_next is a pointer of type *obj lh_prev (of type **obj) is a pointer to the previous lh_next obj obj NULL
lh_next lh_prev
lh_next lh_prev
lh_next lh_prev
Figure 16.4: Linked list implementation in ns.
sat-iridium.tcl Simulates a broadband LEO constellation with parameters similar to that of the Iridium constellation (with supporting scripts sat-iridium-links.tcl, sat-iridium-linkswithcross.tcl, and sat-iridium-nodes.tcl).
sat-teledesic.tcl Simulates a broadband LEO constellation with parameters similar to those proposed for the 288 satellite Teledesic constellation (with supporting scripts sat-teledesic-links.tcl and sat-teledesic-nodes.t
In addition, there is a test suite script that tries to exercise a lot of features simultaneously, it can be found at ~ns/tcl/test/testsuite-sat.tcl.
16.3 Implementation
The code for the implementation of satellite extensions can be found in ~ns/{sat.h, sathandoff.{cc,h}, satlink.{cc,h}, satnode.{cc,h}, satposition.{cc,h}, satroute.{cc,h}, sattrace.{cc,h}}, and ~ns/tcl/lib/ns-sat.tcl. Almost all of the mechanism is implemented in C++. In this section, we focus on some of the key of components the implementation; namely, the use of linked lists, the node structure, and a detailed look at the satellite link structure.
16.3.1 Use of linked lists
There are a number of linked lists used heavily in the implementation:
class Node maintains a (static) list of all objects of class Node in the simulator. The variable Node::nodehead_ stores the head of the list. The linked list of nodes is used for centralized routing, for nding satellites to hand off to, and for tracing. class Node maintains a list of all (satellite) links on the node. Specically, the list is a list of objects of class LinkHead. The variable linklisthead_ stores the head of the list. The linked list of LinkHeads is used for checking whether or not to handoff links, and to discover topology adjacencies. class Channel maintains a list of all objects of class Phy on the channel. The head of the list is stored in the variable if_head_. This list is used to determine the set of interfaces on a channel that should receive a copy of a packet.
Figure 16.4 provides a schematic of how the linked list is organized. Each object in the list is linked through a LINK_ENTRY that is a protected member of the class. This entry contains a pointer to the next item in the list and also a pointer to the address 159
class SatNode : public Node port demux addr demux entry_ IP address 255 RTagent LinkHandoffMgr Src/Sink SatPosition
defaulttarget_
SatTrace List of pointers: node_head nodehead_ linklist_head linklisthead_ Channel* uplink_ Channel* downlink_ Other link objects Other link objects
SatLinkHead
...
SatLinkHead
Figure 16.5: Structure of class SatNode. of the previous next pointer in the preceding object. Various macros found in ~ns/list.h can be used to manipulate the list; the implementation of linked-lists in ns is similar to the queue implementation found in some variants of BSD UNIX.
16.3.2 Node structure
Figure 16.5 is a schematic of the main components of a SatNode. The structure bears resemblance to the MobileNode in the wireless extensions, but there are several differences. Like all ns nodes, the SatNode has an entry point to a series of classiers. The address classier contains a slot table for forwarding packets to foreign nodes, but since OTcl routing is not used, all packets not destined for this node (and hence forwarded to the port classier), are sent to the default target, which points to a routing agent. Packets destined on the node for port 255 are classied as routing packets and are also forwarded to the routing agent. Each node contains one or more network stacks that include a generic SatLinkHead at the entry point of the link. The SatLinkHead is intended to serve as an API to get at other objects in the link structure, so it contains a number of pointers (although the API here has not been nalized). Packets leaving the network stack are sent to the nodes entry. An important feature is that each packet leaving a network stack has its iface_ eld in the common packet header coded with the unique NetworkInterface index corresponding to the link. This value can be used to support distributed routing as described below. The base class routing agent is class SatRouteAgent; it can be used in conjunction with centralized routing. SatRouteAgents contain a forwarding table that resolves a packets address to a particular LinkHead target it is the job of the SatRouteObject to populate this table correctly. The SatRouteAgent populates certain elds in the header and then sends the packet down to the approprate link. To implement a distributed routing protocol, a new SatRouteAgent could be dened this would learn about topology by noting the interface index marked in each packet as it came up the stack a helper function in the node intf_to_target() allows it to resolve an index value to a particular LinkHead. There are pointers to three additional objects in a SatNode. First, each SatNode contains a position object, discussed in the previous section. Second, each SatNode contains a LinkHandoffMgr that monitors for opportunities to hand links off and coordinates the handoffs. Satellite nodes and terminal nodes each have their specialized version of a LinkHandoffMgr.
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from routing agent
to Node->entry
SatLinkHead
Network Interface
LL Sat/Recv Sat/Enque Queue Sat/Drop Sat/Deque Sat/Error Mac Error model
Phy_tx
Phy_rx
to SatChannel
from SatChannel
Figure 16.6: Detailed look at network interface stack. Finally, a number of pointers to objects are contained in a SatNode. We discussed linklisthead_ and nodehead_ in the previous subsection. The uplink_ and downlink_ pointers are used for convenience under the assumption that, in most simulations, a satellite or a terminal has only one uplink and downlink channel.
16.3.3 Detailed look at satellite links
Figure 16.6 provides a more detailed look at how satellite links are composed. In this section, we describe how packets move up and down the stack, and the key things to note at each layer. The le ~ns/tcl/lib/ns-sat.tcl contains the various OTcl instprocs that assemble links according to Figure 16.6. We describe the composite structure herein as a network stack. Most of the code for the various link components is in ~ns/satlink.{cc,h}. The entry point to a network stack is the SatLinkHead object. The SatLinkHead object derives from Class LinkHead; the aim of link head objects is to provide a uniform API for all network stacks. 4 The SatLinkHead object contains pointers to the LL, Queue, MAC, Error model, and both Phy objects. The SatLinkHead object can also be queried as to what type of network stack it is e.g., GSL, interplane ISL, crossseam ISL, etc.. Valid codes for the type_ eld are currently found in ~ns/sat.h. Finally, the SatLinkHead stores a boolean variable linkup_ that indicates whether the link to at least one other node on the channel is up. The C++ implementation of SatLinkHead is found in ~ns/satlink.{cc,h}. Packets leaving a node pass through the SatLinkHead transparently to the class SatLL object. The SatLL class derives from LL (link layer). Link layer protocols (like ARQ protocols) can be dened here. The current SatLL assigns a MAC address to the packet. Note that in the satellite case, we do not use an Address Resolution Protocol (ARP); instead, we simply use the MAC index_ variable as its address, and we use a helper function to nd the MAC address of the corresponding interface of the next-hop node. Since class LL derives from class LinkDelay, the delay_ parameter of LinkDelay can be used to model any processing delay in the link layer; by default this delay is zero.
4 In
the authors opinion, all network stacks in ns should eventually have a LinkHead object at the front the class SatLinkHead would then disappear.
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The next object an outgoing packet encounters is the interface queue. However, if tracing is enabled, tracing elements may surround the queue, as shown in Figure 16.6. This part of a satellite link functions like a conventional ns link. The next layer down is the MAC layer. The MAC layer draws packets from the queue (or deque trace) object a handshaking between the MAC and the queue allows the MAC to draw packets out of the queue as it needs them. The transmission time of a packet is modelled in the MAC also the MAC computes the transmission delay of the packet (based on the sum of the LINK_HDRSIZE eld dened in satlink.h and the size eld in the common packet header), and does not call up for another packet until the current one has been sent to the next layer down. Therefore, it is important to set the bandwidth of the link correctly at this layer. For convenience, the transmit time is encoded in the mac header; this information can be used at the receiving MAC to calculate how long it must wait to detect a collision on a packet, for example. Next, the packet is sent to a transmitting interface (Phy_tx) of class SatPhy. This object just sends the packet to the attached channel. We noted earlier in this chapter that all interfaces attached to a channel are part of the linked list for that channel. This is not true for transmit interfaces, however. Only receive interfaces attached to a channel comprise this linked list, since only receive interfaces should get a copy of transmitted packets. The use of separate transmit and receive interfaces mirrors the real world where full-duplex satellite links are made up of RF channels at different frequencies. The outgoing packet is next sent to a SatChannel, which copies the packet to every receiving interface (of class SatPhy) on the channel. The Phy_rx sends the packet to the MAC layer. At the MAC layer, the packet is held for the duration of its transmission time (and any appropriate collision detection is performed if the MAC, such as the Aloha MAC, supports it). If the packet is determined to have arrived safely at the MAC, it next passes to an ErrorModel object, if it exists. If not, the packet moves through any receive tracing objects to the SatLL object. The SatLL object passes the packet up after a processing delay (again, by default, the value for delay_ is zero). The nal object that a received packet passes through is an object of class NetworkInterface. This object stamps the iface_ eld in the common header with the network stacks unique index value. This is used to keep track of which network stack a packet arrived on. The packet then goes to the entry of the SatNode (usually, an address classier). Finally, geo-repeater satellites exist, as described earlier in this chapter. Geo-repeater network stacks are very simple they only contain a Phy_tx and a Phy_rx of class RepeaterPhy, and a SatLinkHead. Packets received by a Phy_rx are sent to the Phy_tx without delay. The geo-repeater satellite is a degenerate satellite node, in that it does not contain things like tracing elements, handoff managers, routing agents, or any other link interfaces other than repeater interfaces.
16.4 Commands at a glance
Following is a list of commands related to satellite networking: $ns_ satnode-polar <alt> <inc> <lon> <alpha> <plane> <linkargs> <chan> This a simulator wrapper method for creating a polar satellite node. Two links, uplink and downlink, are created along with two channels, uplink channel and downlink channel. <alt> is the polar satellite altitude, <inc> is orbit inclination w.r.t equator, <lon> is the longitude of ascending node, <alpha> gives the initial position of the satellite along this orbit, <plane> denes the plane of the polar satellite. <linkargs> is a list of link argument options that denes the network interface (like LL, Qtype, Qlim, PHY, MAC etc). $ns_ satnode-geo <lon> <linkargs> <chan> This is a wrapper method for creating a geo satellite node that rst creates a satnode plus two link interfaces (uplink and downlink) plus two satellite channels (uplink and downlink). <chan> denes the type of channel. $ns_ satnode-geo-repeater <lon> <chan> This is a wrapper method for making a geo satellite repeater node that rst creates a satnode plus two link interfaces (uplink and downlink) plus two satellite channels (uplink and downlink). 162
$ns_ satnode-terminal <lat> <lon> This is a wrapper method that simply creates a terminal node. The <lat> and <lon> denes the latitude and longitude respectively of the terminal. $ns_ satnode <type> <args> This is a more primitive method for creating satnodes of type <type> which can be polar, geo or terminal. $satnode add-interface <type> <ll> <qtype> <qlim> <mac_bw> <phy> This is an internal method of Node/SatNode that sets up link layer, mac layer, interface queue and physical layer structures for the satellite nodes. $satnode add-isl <ltype> <node1> <node2> <bw> <qtype> <qlim> This method creates an ISL (inter-satellite link) between the two nodes. The link type (inter, intra or cross-seam), BW of the link, the queue-type and queue-limit are all specied. $satnode add-gsl <ltype> <opt_ll> <opt_ifq> <opt_qlim> <opt_mac> <opt_bw> <opt_phy> <opt_inlink> <opt_outlink> This method creates a GSL (ground to satellite link). First a network stack is created that is dened by LL, IfQ, Qlim, MAC, BW and PHY layers. Next the node is attached to the channel inlink and outlink.
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Chapter 17
Radio Propagation Models
This chapter describes the radio propagation models implemented in ns. These models are used to predict the received signal power of each packet. At the physical layer of each wireless node, there is a receiving threshold. When a packet is received, if its signal power is below the receiving threshold, it is marked as error and dropped by the MAC layer. Up to now there are three propagation models in ns, which are the free space model 1 , two-ray ground reection model 2 and the shadowing model 3 . Their implementation can be found in ~ns/propagation.{cc,h}, ~ns/tworayground.{cc,h} and ~ns/shadowing.{cc,h}. This documentation reects the APIs in ns-2.1b7.
17.1 Free space model
The free space propagation model assumes the ideal propagation condition that there is only one clear line-of-sight path between the transmitter and receiver. H. T. Friis presented the following equation to calculate the received signal power in free space at distance from the transmitter [10].
(17.1)
where is the transmitted signal power. and are the antenna gains of the transmitter and the receiver respectively. is the system loss, and is the wavelength. It is common to select and in ns simulations.
The free space model basically represents the communication range as a circle around the transmitter. If a receiver is within the circle, it receives all packets. Otherwise, it loses all packets The OTcl interface for utilizing a propagation model is the node-config command. One way to use it here is
$ns_ node-config -propType Propagation/FreeSpace
Another way is
1 Based
on the code contributed to ns from the CMU Monarch project. to ns from the CMU Monarch project. 3 Implemented in ns by Wei Ye at USC/ISI
2 Contributed
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set prop [new Propagation/FreeSpace] $ns_ node-config -propInstance $prop
17.2 Two-ray ground reection model
A single line-of-sight path between two mobile nodes is seldom the only means of propation. The two-ray ground reection model considers both the direct path and a ground reection path. It is shown [20] that this model gives more accurate prediction at a long distance than the free space model. The received power at distance is predicted by
(17.2)
where and are the heights of the transmit and receive antennas respectively. Note that the original equation in [20] assumes . To be consistent with the free space model, is added here. The above equation shows a faster power loss than Eqn. (17.1) as distance increases. However, The two-ray model does not give a good result for a short distance due to the oscillation caused by the constructive and destructive combination of the two rays. Instead, the free space model is still used when is small. , Eqn. (17.1) is used. When Therefore, a cross-over distance is calculated in this model. When is used. At the cross-over distance, Eqns. (17.1) and (17.2) give the same result. So can be calculated as , Eqn. (17.2)
(17.3)
Similarly, the OTcl interface for utilizing the two-ray ground reection model is as follows.
$ns_ node-config -propType Propagation/TwoRayGround
Alternatively, the user can use
set prop [new Propagation/TwoRayGround] $ns_ node-config -propInstance $prop
17.3 Shadowing model
17.3.1 Backgroud
The free space model and the two-ray model predict the received power as a deterministic function of distance. They both represent the communication range as an ideal circle. In reality, the received power at certain distance is a random variable due to multipath propagation effects, which is also known as fading effects. In fact, the above two models predicts the mean received power at distance . A more general and widely-used model is called the shadowing model [20].
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Environment Outdoor In building Free space Shadowed urban area Line-of-sight Obstructed
2 2.7 to 5 1.6 to 1.8 4 to 6
Table 17.1: Some typical values of path loss exponent
Environment Outdoor Ofce, hard partition Ofce, soft partition Factory, line-of-sight Factory, obstructed (dB) 4 to 12 7 9.6 3 to 6 6.8
Table 17.2: Some typical values of shadowing deviation
The shadowing model consists of two parts. The rst one is known as path loss model, which also predicts the mean received power at distance , denoted by . It uses a close-in distance as a reference. is computed relative to as follows.
(17.4)
is called the path loss exponent, and is usually empirically determined by eld measurement. From Eqn. (17.1) we know that for free space propagation. Table 17.1 gives some typical values of . Larger values correspond to more obstructions and hence faster decrease in average received power as distance becomes larger. can be computed from Eqn. (17.1).
The path loss is usually measured in dB. So from Eqn. (17.4) we have
(17.5)
The second part of the shadowing model reects the variation of the received power at certain distance. It is a log-normal random variable, that is, it is of Gaussian distribution if measured in dB. The overall shadowing model is represented by
(17.6)
where is a Gaussian random variable with zero mean and standard deviation . is called the shadowing deviation, . Eqn. (17.6) is also known as a log-normal and is also obtained by measurement. Table 17.2 shows some typical values of shadowing model. The shadowing model extends the ideal circle model to a richer statistic model: nodes can only probabilistically communicate when near the edge of the communication range.
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17.3.2 Using shadowing model
Before using the model, the user should select the values of the path loss exponent and the shadowing deviation according to the simulated environment. The OTcl interface is still the node-config command. One way to use it is as follows, and the values for these parameters are just examples. # first set values of Propagation/Shadowing Propagation/Shadowing Propagation/Shadowing Propagation/Shadowing shadowing model set pathlossExp_ 2.0 set std_db_ 4.0 set dist0_ 1.0 set seed_ 0
;# ;# ;# ;#
path loss exponent shadowing deviation (dB) reference distance (m) seed for RNG
$ns_ node-config -propType Propagation/Shadowing The shadowing model creates a random number generator (RNG) object. The RNG has three types of seeds: raw seed, pre-dened seed (a set of known good seeds) and the huristic seed (details in Section 20.1). The above API only uses the pre-dened seed. If a user want different seeding method, the following API can be used. set prop [new Propagation/Shadowing] $prop set pathlossExp_ 2.0 $prop set std_db_ 4.0 $prop set dist0_ 1.0 $prop seed <seed-type> 0 $ns_ node-config -propInstance $prop The <seed-type> above can be raw, predef or heuristic.
;# user can specify seeding method
17.4 Communication range
In some applications, a user may want to specify the communication range of wireless nodes. This can be done by set an appropriate value of the receiving threshold in the network interface, i.e., Phy/WirelessPhy set RXThresh_ <value> A separate C program is provided at ~ns/indep-utils/propagation/threshold.cc to compute the receiving threshold. It can be used for all the propagation models discussed in this chapter. Assume you have compiled it and get the excutable named as threshold. You can use it to compute the threshold as follows threshold -m <propagation-model> [other-options] distance where <propagation-model> is either FreeSpace, TwoRayGround or Shadowing, and the distance is the communication range in meter. 167
[other-options] are used to specify parameters other than their default values. For the shadowing model there is a necessary parameter, -r <receive-rate>, which species the rate of correct reception at the distance. Because the communication range in the shadowing model is not an ideal circle, an inverse Q-function [20] is used to calculate the receiving threshold. For example, if you want 95% of packets can be correctly received at the distance of 50m, you can compute the threshold by
threshold -m Shadowing -r 0.95 50
Other available values of [other-options] are shown below
-pl <path-loss-exponent> -std <shadowing-deviation> -Pt <transmit-power> -fr <frequency> -Gt <transmit-antenna-gain> -Gr <receive-antenna-gain> -L <system-loss> -ht <transmit-antenna-height> -hr <receive-antenna-height> -d0 <reference-distance>
17.5 Commands at a glance
Following is a list of commands for propagation models.
$ns_ node-config -propType <propagation-model> This command selects <propagation-model> in the simulation. the <propagation model> can be Propagation/FreeSpace, Propagation/TwoRayGround or Propagation/Shadowing $ns_ node-config -propInstance $prop This command is another way to utilize a propagation model. $prop is an instance of the <propagation-model>. $sprop_ seed <seed-type> <value> This command seeds the RNG. $sprop_ is an instance of the shadowing model. threshold -m <propagation-model> [other-options] distance This is a separate program at ~ns/indep-utils/propagation/threshold.cc, which is used to compute the receiving threshold for a specied communication range.
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Chapter 18
Debugging ns
nsis a simulator engine built in C++ and has an OTcl (Object-oriented Tcl) interface that is used for conguration and commands. Thus in order to debug nswe will have to deal with debugging issues involving both OTcl and C++. This chapter gives debugging tips at Tcl and C++ level and shows how to move to-fro the Tcl and C++ boundaries. It also briey covers memory debugging and memory conservation in ns.
18.1 Tcl-level Debugging
Ns supports Don Libs Tcl debugger ( see its Postscript documentation at http://expect.nist.gov/tcl-debug/tcl-debug.ps.Z and its source code at http://expect.nist.gov/tcl-debug/tcl-debug.tar.gz ). Install the program or leave the source code in a directory parallel to ns-2 and it will be built. Unlike expect, described in the tcl-debug documentation, we do not support the -D ag. To enter the debugger, add the lines "debug 1" to your script at the appropriate location. In order to build ns with the debugging ag turned on, congure ns with conguration option "enable-debug" and incase the Tcl debugger has been installed in a directory not parallel to ns-2, provide the path with conguration option "with-tcldebug=<give/your/path/to/tcldebug/library>". An useful debugging command is $ns_ gen-map which may be used to list all OTcl objects in a raw form. This is useful to correlate the position and function of an object given its name. The name of the object is the OTcl handle, usually of the form _o###. For TclObjects, this is also available in a C++ debugger, such as gdb, as this->name_.
18.2 C++-Level Debugging
Debugging at C++ level can be done using any standard debugger. The following macro for gdb makes it easier to see what happens in Tcl-based subroutines: ## for Tcl code define pargvc set $i=0 while $i < argc p argv[$i] set $i=$i+1
169
end end document pargvc Print out argc argv[i]s common in Tcl code. (presumes that argc and argv are defined) end
18.3 Mixing Tcl and C debugging
It is a painful reality that when looking at the Tcl code and debugging Tcl level stuff, one wants to get at the C-level classes, and vice versa. This is a smallish hint on how one can make that task easier. If you are running ns through gdb, then the following incantation (shown below) gets you access to the Tcl debugger. Notes on how you can then use this debugger and what you can do with it are documented earlier in this chapter and at this URL (http://expect.nist.gov/tcl-debug/tcl-debug.ps.Z). (gdb) run Starting program: /nfs/prot/kannan/PhD/simulators/ns/ns-2/ns ... Breakpoint 1, AddressClassifier::AddressClassifier (this=0x12fbd8) at classifier-addr.cc:47 (gdb) p this->name_ $1 = 0x2711e8 "_o73" (gdb) call Tcl::instance().eval("debug 1") 15: lappend auto_path $dbg_library dbg15.3> w *0: application 15: lappend auto_path /usr/local/lib/dbg dbg15.4> Simulator info instances _o1 dbg15.5> _o1 now 0 dbg15.6> # and other fun stuff dbg15.7> _o73 info class Classifier/Addr dbg15.8> _o73 info vars slots_ shift_ off_ip_ offset_ off_flags_ mask_ off_cmn_ dbg15.9> c (gdb) w Ambiguous command "w": while, whatis, where, watch. (gdb) where #0 AddressClassifier::AddressClassifier (this=0x12fbd8) at classifier-addr.cc:47 #1 0x5c68 in AddressClassifierClass::create (this=0x10d6c8, argc=4, argv=0xefffcdc0) at classifier-addr.cc:63 ... (gdb) In a like manner, if you have started ns through gdb, then you can always get gdbs attention by sending an interrupt, usually a <Ctrl-c> on berkeloidrones. However, note that these do tamper with the stack frame, and on occasion, may (sometimes can (and rarely, does)) screw up the stack so that, you may not be in a position to resume execution. To its credit, gdb appears to be smart enough to warn you about such instances when you should tread softly, and carry a big stick. 170
18.4 Memory Debugging
The rst thing to do if you run out of memory is to make sure you can use all the memory on your system. Some systems by default limit the memory available for individual programs to something less than all available memory. To relax this, use the limit or ulimit command. These are shell functionssee the manual page for your shell for details. Limit is for csh, ulimit is for sh/bash. Simulations of large networks can consume a lot of memory. Ns-2.0b17 supports Gray Watsons dmalloc library (see its web documentation at http://www.letters.com/dmalloc/ and get the source code from ftp://ftp.letters.com/src/dmalloc/dmalloc.tar.gz ). To add it, install it on your system or leave its source in a directory parallel to ns-2 and specify with-dmalloc when conguring ns. Then build all components of ns for which you want memory information with debugging symbols (this should include at least ns-2, possibly tclcl and otcl and maybe also tcl).
18.4.1 Using dmalloc
In order to use dmalloc do the following:
Dene an alias for csh: alias dmalloc eval \dmalloc -C \!*, for bash: function dmalloc { eval command dmalloc -b $* }%$
Next turn debugging on by typing dmalloc -l logfile low Run your program (which was congured and built with dmalloc as described above). Interpret logle by running dmalloc_summarize ns <logfile. (You need to download dmalloc_summarize separately.)
On some platforms you may need to link things statically to get dmalloc to work. On Solaris this is done by linking with these options: "-Xlinker -B -Xlinker -static libraries -Xlinker -B -Xlinker -dynamic -ldl -lX11 -lXext". (Youll need to change Makele. Thanks to Haobo Yu and Doug Smith for working this out.) We can interpret a sample summary produced from this process on ns-2/tcl/ex/newmcast/cmcast-100.tcl with an exit statement after the 200th duplex-link-of-interefaces statement: Ns allocates 6MB of memory. 1MB is due to TclObject::bind 900KB is StringCreate, all in 32-byte chunks 700KB is NewVar, mostly in 24-byte chunks Dmalloc_summarize must map function names to and from their addresses. It often cant resolve addresses for shared libraries, so if you see lots of memory allocated by things beginning with ra=, thats what it is. The best way to avoid this problem is to build ns statically (if not all, then as much as possible). Dmallocs memory allocation scheme is somewhat expensive, plus theres bookkeeping costs. Programs linked against dmalloc will consume more memory than against most standard mallocs. Dmalloc can also diagnose other memory errors (duplicate frees, buffer overruns, etc.). See its documentation for details.
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18.4.2 Memory Conservation Tips
Some tips to saving memory (some of these use examples from the cmcast-100.tcl script): If you have many links or nodes: Avoid trace-all : $ns trace-all $f causes trace objects to be pushed on all links. If you only want to trace one link, theres no need for this overhead. Saving is about 14 KB/link. Use arrays for sequences of variables : Each variable, say n$i in set n$i [$ns node], has a certain overhead. If a sequence of nodes are created as an array, i.e. n($i), then only one variable is created, consuming much less memory. Saving is about 40+ Byte/variable. Avoid unnecessary variables : If an object will not be referred to later on, avoid naming the object. E.g. set cmcast(1) [new CtrMcast $ns $n(1) $ctrmcastcomp [list 1 1]] would be better if replaced by new CtrMcast $ns $n(1) $ctrmcastcomp [list 1 1]. Saving is about 80 Byte/variable. Run on top of FreeBSD : malloc() overhead on FreeBSD is less than on some other systems. We will eventually port that allocator to other platofrms. Dynamic binding : Using bind() in C++ consumes memory for each object you create. This approach can be very expensive if you create many identical objects. Changing bind() to delay_bind() changes this memory requirement to per-class. See ns/object.cc for an example of how to do binding, either way. Disabling packet headers : For packet-intensive simulations, disabling all packet headers that you will not use in your simulation may signicantly reduce memory usage. See Section 11.1 for detail.
18.4.3 Some statistics collected by dmalloc
A memory consumption problem occured in recent simulations (cmcast-[150,200,250].tcl), so we decided to take a closer look at scaling issue. See page http://www-mash.cs.berkeley.edu/ns/ns-scaling.html which demostrates the efforts in nding the bottlneck. The following table summarises the results of investigating the bottleneck:
KBytes trace-all turn off trace-all use array remove unnecessay variables on SunOS
cmcast-50.tcl(217 Links) 8,084 5,095 5,091 5,087 5,105
cmcast-100.tcl(950 Links) 28,541 15,465 15,459 15,451 15,484
18.5 Memory Leaks
This section deals with memory leak problems in ns, both in Tcl as well as C/C++.
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18.5.1 OTcl
OTcl, especially TclCL, provides a way to allocate new objects. However, it does not accordingly provide a garbage collection mechanism for these allocated objects. This can easily lead to unintentional memory leaks. Important: tools such as dmalloc and purify are unable to detect this kind of memory leaks. For example, consider this simple piece of OTcl script: set ns [new Simulator] for set i 0 $i < 500 incr i set a [new RandomVariable/Constant]
One would expect that the memory usage should stay the same after the rst RandomVariable is allocated. However, because OTcl does not have garbage collection, when the second RandomVariable is allocated, the previous one is not freed and hence results in memory leak. Unfortunately, there is no easy x for this, because garbage collection of allocated objects is essentially incompatible with the spirit of Tcl. The only way to x this now is to always explicitly free every allocated OTcl object in your script, in the same way that you take care of malloc-ed object in C/C++.
18.5.2 C/C++
Another source of memory leak is in C/C++. This is much easier to track given tools that are specically designed for this task, e.g., dmalloc and purify. nshas a special target ns-pure to build puried ns executable. First make sure that the macro PURIFY in the ns Makele contains the right -collector for your linker (check purify man page if you dont know what this is). Then simply type make ns-pure. See earlier sections in this chapter on how to use ns with libdmalloc.
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Chapter 19
Energy Model in ns
Energy Model, as implemented in ns, is a node attribute. The energy model represents level of energy in a mobile host. The energy model in a node has a initial value which is the level of energy the node has at the beginning of the simulation. This is known as initialEnergy_. It also has a given energy usage for every packet it transmits and receives. These are called txPower_ and rxPower_. The les where the energy model is dened are ns/energymodel[.cc and.h]. Other functions/methods described in this chapter may be found in ns/wireless-phy.cc, ns/cmu-trace.cc, ns/tcl/lib[ns-lib.tcl, nsnode.tcl, ns-mobilenode.tcl].
19.1 The C++ EnergyModel Class
The basic energy model is very simple and is dened by class EnergyModel as shown below:
class EnergyModel : public TclObject public: EnergyModel(double energy) energy_ = energy; inline double energy() return energy_; inline void setenergy(double e) energy_ = e; virtual void DecrTxEnergy(double txtime, double P_tx) energy_ -= (P_tx * txtime); virtual void DecrRcvEnergy(double rcvtime, double P_rcv) energy_ -= (P_rcv * rcvtime); protected: double energy_; ;
As seen from the EnergyModel Class denition above, there is only a single class variable energy_ which represents the level of energy in the node at any given time. The constructor EnergyModel(energy) requires the initial-energy to be passed along as a parameter. The other class methods are used to decrease the energy level of the node for every packet transmitted ( DecrTxEnergy(txtime, P_tx)) and every packet received ( DecrRcvEnergy (rcvtime, P_rcv)) by the node. P_tx and P_rcv are the transmitting and receiving power (respectively) required by the nodes interface or PHY. At the beginning of simulation, energy_ is set to initialEnergy_ which is then decremented for every transmission and 174
reception of packets at the node. When the energy level at the node goes down to zero, no more packets can be received or transmitted by the node. If tracing is turned on, line DEBUG: node <node-id> dropping pkts due to energy = 0 is printed in the tracele.
19.2 The OTcl interface
Since the energy model is a node attribute, it may be dened by the following node conguration APIs: $ns_ node-config -energyModel $energymodel \ -rxPower $p_rx \ -txPower $p_tx \ -initialEnergy $initialenergy Optional values for above conguration parameters of the energy model are given in the following table: Attribute -energyModel -rxPower -txPower -initialEnergy optional values "EnergyModel" receiving power in watts (e.g 0.3) transmitting power in watts (e.g 0.4) energy in joules (e.g 0.1) default values none 281.8mW 281.8mW 0.0
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Part III
Support
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Chapter 20
Mathematical Support
The simulator includes a small collection of mathematical functions used to implement random variate generation and integration. This area of the simulator is currently undergoing some changes. The procedures and functions described in this chapter can be found in ~ns/rng.{cc, h}, ~ns/random.{cc, h}, ~ns/ranvar.{cc, h}, ~ns/pareto.{cc, h}, ~ns/expoo.{cc, h}, ~ns/tcl/lib/ns-random.cc, and ~ns/integrator.{cc, h}.
20.1 Random Number Generation
The RNG class contains an implementation of the minimal standard multiplicative linear congruential generator of Park and Miller [19]. Multiple instances of the RNG class can be created to allow a simulation to draw random numbers from independent random number streams. For instance, a user who wants to generate the same trafc (based on some random process) in 2 different simulation experiments that compare different dropping algorithms that are themselves based on random processes may choose to base the trafc generation on one random number stream and the dropping algorithms on another stream. However, when using multiple RNG objects in a simulation care should be taken to insure that they are seeded in such a way as to guarantee that they produce independent, high-quality streams of random numbers. We describe approaches to seed the RNG below. Most users will be satised with a single instance of the RNG. Hence, a default RNG, created at simulator initialization time, is provided.
C++ Support This random number generator is implemented by the RNG class, dened in ~ns/rng.h: class RNG : public TclObject { enum RNGSources { RAW_SEED_SOURCE, PREDEF_SEED_SOURCE, HEURISTIC_SEED_SOURCE }; ... // These are primitive but maybe useful. inline int uniform_positive_int() { // range [0, MAXINT] return (int)(stream_.next()); } inline double uniform_double() { // range [0.0, 1.0) 177
return stream_.next_double(); } inline int uniform(int k) { return (uniform_positive_int() % (unsigned)k); } inline double uniform(double r) { return (r * uniform_double());} inline double uniform(double a, double b) { return (a + uniform(b - a)); } inline double exponential() { return (-log(uniform_double())); } inline double exponential(double r) { return (r * exponential());} inline double pareto(double scale, double shape) { return (scale * (1.0/pow(uniform_double(), 1.0/shape)));} ... }; The uniform_positive_int method generates random integers in the range member functions provide the following random variate generation: uniform(double r) uniform(double a, double b) exponential() integer(int k)
. In particular, Additional
generate a oating-point number uniformly distributed on generate a oating-point number uniformly distributed on generate a oating-point number exponentially distributed (with parameter 1) on generate an integer uniformly distributed on
The Random class (in random.h) is an older interface to the standard random number stream. Heres a sample use of RNG modeled on RED. rng_ is an instance of class RNG: ... // drop probability is computed, pick random number and act double u = rng_->uniform_double(); if (u <= edv_.v_prob) { edv_.count = 0; if (edp_.setbit) iph->flags() |= IP_ECN; else return (1); } ...
/* ip ecn bit */
Seeding the random number generator When doing simulations often you will either want to get absolutely repeatable (deterministic) results or different results each time. Each approach requires a different seeding method. To get deterministic behavior, invoke the set_seed() method with the rst parameter either RAW_SEED_SOURCE or PREDEF_SEED_SOURCE. With a raw seed, the second parameter species the numeric seed that will be used (any integer). The alternative is predened seeds, where the second selects seeds from a table of 64 known good seeds (which happen to be about 33 million steps apart in the default random number generation stream). Predened seeds are know to provide a good random number stream, but there are limited numbers of them. Raw seeds may not provide a statistically good random number stream but are easy to generate. 178
To get non-deterministic behavior, pick a seed with either RAW_SEED_SOURCE or HEURISTIC_SEED_SOURCE. For raw streams the second argument species the stream. For heuristic streams the second argument is ignored and a seed is generated based on the current time of day and a counter. Both have the caveat that they may not provide a statistically good random number stream. It is very unlikely that any two heuristic seeds will be identical.
OTcl support The RNG class can be accessed from OTcl. For example, a new RNG is created and seeded with: set rng [new RNG] $rng seed 0 $rng seed n $rng next-random $rng uniform a b $rng integer k $rng exponential
;# seeds the RNG heuristically ;# seeds the RNG with value n ;# return the next random number ;# return a number uniformly distributed on [a, b] ;# return an integer uniformly distributed on [0, (k-1)] ;# return a number from an exponential distribution with average 1.
Currently there is no way to select predened seeds from OTcl.
20.2 Random Variables
The class RandomVariable provides a thin layer of functionality on top of the base random number generator and the default random number stream. It is dened in ~ns/ranvar.h: class RandomVariable : public TclObject { public: virtual double value() = 0; int command(int argc, const char*const* argv); RandomVariable(); protected: RNG* rng_; }; Classes derived from this abstract class implement specic distributions. Each distribution is parameterized with the values of appropriate parameters. The value method is used to return a value from the distribution. The currently dened distributions, and their associated parameters are: class UniformRandomVariable class ExponentialRandomVariable class ParetoRandomVariable class ConstantRandomVariable val_ class HyperExponentialRandomVariable min_, max_ avg_ avg_, shape_ avg_, cov_
The RandomVariable class is available in OTcl. For instance, to create a random variable that generates number uniformly on [10, 20]: set u [new RandomVariable/Uniform] 179
$u set min_ 10 $u set max_ 20 $u value By default, RandomVariable objects use the default random number generator described in the previous section. The use-rng method can be used to associate a RandomVariable with a non-default RNG: set rng [new RNG] $rng seed 0 set e [new RandomVariable/Exponential] $e use-rng $rng
20.3 Integrals
The class Integrator supports the approximation of (continuous) integration by (discrete) sums; it is dened in ~ns/integrator.h as From integrator.h: class Integrator : public TclObject { public: Integrator(); void set(double x, double y); void newPoint(double x, double y); int command(int argc, const char*const* argv); protected: double lastx_; double lasty_; double sum_; }; From integrator.cc: Integrator::Integrator() : lastx_(0.), lasty_(0.), sum_(0.) { bind("lastx_", &lastx_); bind("lasty_", &lasty_); bind("sum_", &sum_); } void Integrator::set(double x, double y) { lastx_ = x; lasty_ = y; sum_ = 0.; } void Integrator::newPoint(double x, double y) { sum_ += (x - lastx_) * lasty_; lastx_ = x; 180
lasty_ = y; } int Integrator::command(int argc, const char*const* argv) { if (argc == 4) { if (strcmp(argv[1], "newpoint") == 0) { double x = atof(argv[2]); double y = atof(argv[3]); newPoint(x, y); return (TCL_OK); } } return (TclObject::command(argc, argv)); } This class provides a base class used by other classes such as QueueMonitor that keep running sums. Each new element of the running sum is added by the newPoint(x, y) function. After the th execution of newPoint, the running sum is equal to unless lastx_, lasty_, or sum_ are reset via OTcl. Note that a new where point in the sum can be added either by the C++ member newPoint or the OTcl member newpoint. The use of integrals to compute certain types of averages (e.g. mean queue lengths) is given in (pp. 429430, [12]).
20.4 ns-random
ns-random is an obsolete way to generate random numbers. This information is provided only for backward compatibility. ns-random is implemented in ~ns/misc.{cc,h}. When called with no argument, it generates a random number with uniform distribution between 0 and MAXINT. When an integer argument is provided, it seeds the random generater with the given number. A special case is when ns-random 0 is called, it randomly seeds the generator based on current time. This feature is useful to produce non-deterministic results across runs.
20.5 Some mathematical-support related objects
INTEGRATOR OBJECTS Integrator Objects support the approximate computation of continuous integrals using discrete sums. The running sum(integral) is computed as: sum_ += [lasty_ * (x lastx_)] where (x, y) is the last element entered and (lastx_, lasty_) was the element previous to that added to the sum. lastx_ and lasty_ are updated as new elements are added. The rst sample point defaults to (0,0) that can be changed by changing the values of (lastx_,lasty_). $integrator newpoint <x> <y> Add the point (x,y) to the sum. Note that it does not make sense for x to be less than lastx_. There are no conguration parameters specic to this object.
State Variables are: lastx_ x-coordinate of the last sample point.
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lasty_ y-coordinate of the last sample point. sum_ Running sum (i.e. the integral) of the sample points. SAMPLES OBJECT Samples Objects support the computation of mean and variance statistics for a given sample. $samples mean Returns mean of the sample. $samples variance Returns variance of the sample. $samples cnt Returns a count of the sample points considered. $samples reset Reset the Samples object to monitor a fresh set of samples. There are no conguration parameters or state variables specic to this object.
20.6 Commands at a glance
Following is a list of mathematical support related commands commonly used in simulation scripts: set rng [new RNG] This creates a new random number generator. $rng seed <0 or n> This command seeds the RNG. If 0 is specied, the RNG is seeded heuristically. Otherwise the RNG is seeded with the value <n>. $rng next-random This returns the next random number from RNG. $rng uniform <a> <b> This returns a number uniformly distributed on <a> and <b>. $rng integer <k> This returns an integer uniformly distributed on 0 and k-1. $rng exponential This returns a number that has exponential distribution with average 1. set rv [new Randomvariable/<type of random-variable>] This creates an instance of a random variable object that generates random variables with specic distribution. The different types of random variables derived from the base class are: RandomVariable/Uniform, RandomVariable/Exponential, RandomVariable/Pareto, RandomVariable/Constant, RandomVariable/HyperExponential. Each of these distribution types are parameterized with values of appropriate parameters. For details see section 20.2 of this chapter. $rv use-rng <rng> 182
This method is used to associated a random variable object with a non-default RNG. Otherwise by default, the random variable object is associated with the default random number generator.
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Chapter 21
Trace and Monitoring Support
The procedures and functions described in this chapter can be found in ~ns/trace.{cc, h}, ~ns/tcl/lib/ns-trace.tcl, ~ns/queuemonitor.{cc, h}, ~ns/tcl/lib/ns-link.tcl, ~ns/packet.h, ~ns/owmon.cc, and ~ns/classier-hash.cc. There are a number of ways of collecting output or trace data on a simulation. Generally, trace data is either displayed directly during execution of the simulation, or (more commonly) stored in a le to be post-processed and analyzed. There are two primary but distinct types of monitoring capabilities currently supported by the simulator. The rst, called traces, record each individual packet as it arrives, departs, or is dropped at a link or queue. Trace objects are congured into a simulation as nodes in the network topology, usually with a Tcl Channel object hooked to them, representing the destination of collected data (typically a trace le in the current directory). The other types of objects, called monitors, record counts of various interesting quantities such as packet and byte arrivals, departures, etc. Monitors can monitor counts associated with all packets, or on a per-ow basis using a ow monitor below (Section 21.7). To support traces, there is a special common header included in each packet (this format is dened in ~ns/packet.h as hdr_cmn). It presently includes a unique identier on each packet, a packet type eld (set by agents when they generate packets), a packet size eld (in bytes, used to determine the transmission time for packets), and an interface label (used for computing multicast distribution trees). Monitors are supported by a separate set of objects that are created and inserted into the network topology around queues. They provide a place where arrival statistics and times are gathered and make use of the class Integrator (Section 20.3) to compute statistics over time intervals.
21.1 Trace Support
The trace support in OTcl consists of a number of specialized classes visible in OTcl but implemented in C++, combined with a set of Tcl helper procedures and classes dened in the ns library. All following OTcl classes are supported by underlying C++ classes dened in ~ns/trace.cc. Objects of the following types are inserted directly in-line in the network topology:
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Trace/Hop Trace/Enque Trace/Deque Trace/Drop Trace/Recv SnoopQueue/In SnoopQueue/Out SnoopQueue/Drop SnoopQueue/EDrop
trace a hop (XXX what does this mean exactly; it is not really used XXX) a packet arrival (usually at a queue) a packet departure (usually at a queue) packet drop (packet delivered to drop-target) packet receive event at the destination node of a link on input, collect a time/size sample (pass packet on) on output, collect a time/size sample (pass packet on) on drop, collect a time/size sample (pass packet on) on an "early" drop, collect a time/size sample (pass packet on)
Objects of the following types are added in the simulation and a referenced by the objects listed above. They are used to aggregate statistics collected by the SnoopQueue objects: QueueMonitor QueueMonitor/ED QueueMonitor/ED/Flowmon QueueMonitor/ED/Flow QueueMonitor/Compat receive and aggregate collected samples from snoopers queue-monitor capable of distinguishing between early and standard packet drops per-ow statistics monitor (manager) per-ow statistics container a replacement for a standard QueueMonitor when ns v1 compatibility is in use
21.1.1 OTcl Helper Functions
The following helper functions may be used within simulation scripts to help in attaching trace elements (see ~ns/tcl/lib/nslib.tcl); they are instance procedures of the class Simulator: flush-trace {} create-trace { type file src dst } ush buffers for all trace objects in simulation create a trace object of type type between the given src and dest nodes. If le is non-null, it is interpreted as a Tcl channel and is attached to the newly-created trace object. The procedure returns the handle to the newly created trace object. arrange for tracing on the link between nodes n1 and n2. This function calls create-trace, so the same rules apply with respect to the le argument. arranges to call command when a line is to be traced. The procedure treats command as a string and evaluates it for every line traced. See ~ns/tcl/ex/callback_demo.tcl for additional details on usage. this function calls the init-monitor function on the link between nodes n1 and n2. the given trace object is made the drop-target of the queue associated with the link between nodes n1 and n2.
trace-queue { n1 n2 file }
trace-callback{ ns command }
monitor-queue { n1 n2 } drop-trace { n1 n2 trace }
The create-trace{} procedure is used to create a new Trace object of the appropriate kind and attach an Tcl I/O channel to it (typically a le handle). The src_ and dst_ elds are are used by the underlying C++ object for producing the trace output le so that trace output can include the node addresses dening the endpoints of the link which is being traced. Note that they are not used for matching. Specically, these values in no way relate to the packet header src and dst elds, which are also displayed when tracing. See the description of the Trace class below (Section 21.3). The trace-queue function enables Enque, Deque, and Drop tracing on the link between nodes n1 and n2. The Link trace procedure is described below (Section 21.2).
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The monitor-queue function is constructed similarly to trace-queue. By calling the links init-monitor procedure, it arranges for the creation of objects (SnoopQueue and QueueMonitor objects) which can, in turn, be used to ascertain time-aggregated queue statistics. The drop-trace function provides a way to specify a Queues drop target without having a direct handle of the queue.
21.2 Library support and examples
The Simulator procedures described above require the trace and init-monitor methods associated with the OTcl Link class. Several subclasses of link are dened, the most common of which is called SimpleLink. Thus, the trace and init-monitor methods are actually part of the SimpleLink class rather than the Link base class. The trace function is dened as follows (in ns-link.tcl): # # Build trace objects for this link and # update the object linkage # SimpleLink instproc trace { ns f } { $self instvar enqT_ deqT_ drpT_ queue_ link_ head_ fromNode_ toNode_ $self instvar drophead_ set enqT_ [$ns create-trace Enque $f $fromNode_ $toNode_] set deqT_ [$ns create-trace Deque $f $fromNode_ $toNode_] set drpT_ [$ns create-trace Drop $f $fromNode_ $toNode_] $drpT_ target [$drophead_ target] $drophead_ target $drpT_ $queue_ drop-target $drpT_ $deqT_ target [$queue_ target] $queue_ target $deqT_ if { [$head_ info class] == "networkinterface" } { $enqT_ target [$head_ target] $head_ target $enqT_ # puts "head is i/f" } else { $enqT_ target $head_ set head_ $enqT_ # puts "head is not i/f" } } This function establishes Enque, Deque, and Drop traces in the simulator $ns and directs their output to I/O handle $f. The function assumes a queue has been associated with the link. It operates by rst creating three new trace objects and inserting the Enque object before the queue, the Deque object after the queue, and the Drop object between the queue and its previous drop target. Note that all trace output is directed to the same I/O handle. This function performs one other additional tasks. It checks to see if a link contains a network interface, and if so, leaves it as the rst object in the chain of objects in the link, but otherwise inserts the Enque object as the rst one.
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The following functions, init-monitor and attach-monitor, are used to create a set of objects used to monitor queue sizes of a queue associated with a link. They are dened as follows: SimpleLink instproc attach-monitors { insnoop outsnoop dropsnoop qmon } { $self instvar queue_ head_ snoopIn_ snoopOut_ snoopDrop_ $self instvar drophead_ qMonitor_ set snoopIn_ $insnoop set snoopOut_ $outsnoop set snoopDrop_ $dropsnoop $snoopIn_ target $head_ set head_ $snoopIn_ $snoopOut_ target [$queue_ target] $queue_ target $snoopOut_ $snoopDrop_ target [$drophead_ target] $drophead_ target $snoopDrop_ $snoopIn_ set-monitor $qmon $snoopOut_ set-monitor $qmon $snoopDrop_ set-monitor $qmon set qMonitor_ $qmon } # # Insert objects that allow us to monitor the queue size # of this link. Return the name of the object that # can be queried to determine the average queue size. # SimpleLink instproc init-monitor { ns qtrace sampleInterval} { $self instvar qMonitor_ ns_ qtrace_ sampleInterval_ set set set set ns_ $ns qtrace_ $qtrace sampleInterval_ $sampleInterval qMonitor_ [new QueueMonitor]
$self attach-monitors [new SnoopQueue/In] [new SnoopQueue/Out] [new SnoopQueue/Drop] $qMonitor_ set bytesInt_ [new Integrator] $qMonitor_ set-bytes-integrator $bytesInt_ set pktsInt_ [new Integrator] $qMonitor_ set-pkts-integrator $pktsInt_ return $qMonitor_ } These functions establish queue monitoring on the SimpleLink object in the simulator ns. Queue monitoring is implemented by constructing three SnoopQueue objects and one QueueMonitor object. The SnoopQueue objects are linked in around a Queue in a way similar to Trace objects. The SnoopQueue/In(Out) object monitors packet arrivals(departures) and reports them to an associated QueueMonitor agent. In addition, a SnoopQueue/Out object is 187
also used to accumulate packet drop statistics to an associated QueueMonitor object. For init-monitor the same QueueMonitor object is used in all cases. The C++ denitions of the SnoopQueue and QueueMonitor classes are described below.
21.3 The C++ Trace Class
Underlying C++ objects are created in support of the interface specied in Section 21.3 and are linked into the network topology as network elements. The single C++ Trace class is used to implement the OTcl classes Trace/Hop, Trace/Enque, Trace/Deque, and Trace/Drop. The type_ eld is used to differentiate among the various types of traces any particular Trace object might implement. Currently, this eld may contain one of the following symbolic characters: + for enque, - for deque, h for hop, and d for drop. The overall class is dened as follows in ~ns/trace.cc: class Trace : public Connector { protected: int type_; nsaddr_t src_; nsaddr_t dst_; Tcl_Channel channel_; int callback_; char wrk_[256]; void format(int tt, int s, int d, Packet* p); void annotate(const char* s); int show_tcphdr_; // bool ags; backward compat public: Trace(int type); ~Trace(); int command(int argc, const char*const* argv); void recv(Packet* p, Handler*); void dump(); inline char* buffer() { return (wrk_); } }; The src_, and dst_ internal state is used to label trace output and is independent of the corresponding eld names in packet headers. The main recv() method is dened as follows: void Trace::recv(Packet* p, Handler* h) { format(type_, src_, dst_, p); dump(); /* hack: if trace object not attached to anything free packet */ if (target_ == 0) Packet::free(p); else send(p, h); /* Connector::send() */ } The function merely formats a trace entry using the source, destination, and particular trace type character. The dump function writes the formatted entry out to the I/O handle associated with channel_. The format function, in effect, dictates the trace le format. 188
21.4 Trace File Format
The Trace::format() method denes the trace le format used in trace les produced by the Trace class. It is constructed to maintain backward compatibility with output les in earlier versions of the simulator (i.e., ns v1) so that ns v1 post-processing scripts continue to operate. The important pieces of its implementation are as follows: // this function should retain some backward-compatibility, so that // scripts dont break. void Trace::format(int tt, int s, int d, Packet* p) { hdr_cmn *th = (hdr_cmn*)p->access(off_cmn_); hdr_ip *iph = (hdr_ip*)p->access(off_ip_); hdr_tcp *tcph = (hdr_tcp*)p->access(off_tcp_); hdr_rtp *rh = (hdr_rtp*)p->access(off_rtp_); packet_t t = th->ptype(); const char* name = packet_info.name(t); if (name == 0) abort(); int seqno; /* XXX */ /* CBRs now have seqnos too */ if (t == PT_RTP || t == PT_CBR) seqno = rh->seqno(); else if (t == PT_TCP || t == PT_ACK) seqno = tcph->seqno(); else seqno = -1; ... if (!show_tcphdr_) { sprintf(wrk_, "%c %g %d %d %s %d %s %d %d.%d %d.%d tt, Scheduler::instance().clock(), s, d, name, th->size(), flags, iph->flowid() /* was p->class_ */, iph->src() >> 8, iph->src() & 0xff, // iph->dst() >> 8, iph->dst() & 0xff, // seqno, th->uid() /* was p->uid_ */); } else { sprintf(wrk_, "%c %g %d %d %s %d %s %d %d.%d %d.%d %d %d %d 0x%x tt, Scheduler::instance().clock(), s, d, 189
%d %d",
XXX XXX
%d",
name, th->size(), flags, iph->flowid() /* was p->class_ */, iph->src() >> 8, iph->src() & 0xff, iph->dst() >> 8, iph->dst() & 0xff, seqno, th->uid(), /* was p->uid_ */ tcph->ackno(), tcph->flags(), tcph->hlen()); }
// XXX // XXX
This function is somewhat unelegant, primarily due to the desire to maintain backward compatibility. It formats the source, destination, and type elds dened in the trace object (not in the packet headers), the current time, along with various packet header elds including, type of packet (as a name), size, ags (symbolically), ow identier, source and destination packet header elds, sequence number (if present), and unique identier. The show_tcphdr_ variable indicates whether the trace output should append tcp header information (ack number, ags, header length) at the end of each output line. This is especially useful for simulations using FullTCP agents (Section 28.3). An example of a trace le (without the tcp header elds) migh appear as follows:
+ r r + r + d r + + 1.84375 0 2 cbr 210 ------- 0 0.0 3.1 225 610 1.84375 0 2 cbr 210 ------- 0 0.0 3.1 225 610 1.84471 2 1 cbr 210 ------- 1 3.0 1.0 195 600 1.84566 2 0 ack 40 ------- 2 3.2 0.1 82 602 1.84566 0 2 tcp 1000 ------- 2 0.1 3.2 102 611 1.84566 0 2 tcp 1000 ------- 2 0.1 3.2 102 611 1.84609 0 2 cbr 210 ------- 0 0.0 3.1 225 610 1.84609 2 3 cbr 210 ------- 0 0.0 3.1 225 610 1.84609 2 3 cbr 210 ------- 0 0.0 3.1 225 610 1.8461 2 3 cbr 210 ------- 0 0.0 3.1 192 511 1.84612 3 2 cbr 210 ------- 1 3.0 1.0 196 603 1.84612 2 1 cbr 210 ------- 1 3.0 1.0 196 603 1.84612 2 1 cbr 210 ------- 1 3.0 1.0 196 603 1.84625 3 2 cbr 210 ------- 1 3.0 1.0 199 612
Here we see 14 trace entries, ve enque operations (indicated by + in the rst column), four deque operations (indicated by -), four receive events (indicated by r), and one drop event. (this had better be a trace fragment, or some packets would have just vanished!). The simulated time (in seconds) at which each event occurred is listed in the second column. The next two elds indicate between which two nodes tracing is happening. The next eld is a descriptive name for the the type of packet seen (Section 21.5). The next eld is the packets size, as encoded in its IP header. The next eld contains the ags, which not used in this example. The ags are dened in the ags[] array in trace.cc. Four of the ags are used for ECN: C for Ecn and N for Ecn-Capable in the IP header, and A for Congestion Action, and E for Congestion Experienced in the TCP header. For the other ags, P is for priority, and F is for TCP Fast Start. The next eld gives the IP ow identier eld as dened for IP version 6. 1 . The subsequent two elds indicate the packets source and destination node addresses, respectively. The following eld indicates the sequence number. 2 The last eld is a unique packet identier. Each new packet created in the simulation is assigned a new, unique identier.
1 In ns v1, each packet included a class eld, which was used by CBQ to classify packets. It then found additional use to differentiate between ows at one trace point. In ns v2, the ow ID eld is available for this purpose, but any additional information (which was commonly overloaded into the class eld in ns v1) should be placed in its own separate eld, possibly in some other header 2 In ns v1, all packets contained a sequence number, whereas in ns v2 only those Agents interested in providing sequencing will generate sequence numbers. Thus, this eld may not be useful in ns v2 for packets generated by agents that have not lled in a sequence number. It is used here to remain backward compatible with ns v1.
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21.5 Packet Types
Each packet contains a packet type eld used by Trace::format to print out the type of packet encountered. The type eld is dened in the TraceHeader class, and is considered to be part of the trace support; it is not interpreted elsewhere in the simulator. Initialization of the type eld in packets is performed by the Agent::allocpkt(void) method. The type eld is set to integer values associated with the denition passed to the Agent constructor (Section 9.6.3). The currently-supported denitions, their values, and their associated symblic names are as follows (dened in ~ns/packet.h): enum packet_t { PT_TCP, PT_UDP, PT_CBR, PT_AUDIO, PT_VIDEO, PT_ACK, PT_START, PT_STOP, PT_PRUNE, PT_GRAFT, PT_GRAFTACK, PT_JOIN, PT_ASSERT, PT_MESSAGE, PT_RTCP, PT_RTP, PT_RTPROTO_DV, PT_CtrMcast_Encap, PT_CtrMcast_Decap, PT_SRM, /* simple signalling messages */ PT_REQUEST, PT_ACCEPT, PT_CONFIRM, PT_TEARDOWN, PT_LIVE,// packet from live network PT_REJECT, PT_TELNET,// not needed: telnet use TCP PT_FTP, PT_PARETO, PT_EXP, PT_INVAL, PT_HTTP, /* new encapsulator */ PT_ENCAPSULATED, PT_MFTP, /* CMU/Monarchs extnsions */ PT_ARP, PT_MAC, PT_TORA, PT_DSR, PT_AODV,
191
// insert new packet types here PT_NTYPE // This MUST be the LAST one }; The constructor of class p_info glues these constants with their string values: p_info() { name_[PT_TCP]= "tcp"; name_[PT_UDP]= "udp"; name_[PT_CBR]= "cbr"; name_[PT_AUDIO]= "audio"; ... name_[PT_NTYPE]= "undefined"; } See also section 11.2.2 for more details.
21.6 Queue Monitoring
Queue monitoring refers to the capability of tracking the dynamics of packets at a queue (or other object). A queue monitor tracks packet arrival/departure/drop statistics, and may optionally compute averages of these values. Monitoring may be applied all packets (aggregate statistics), or per-ow statistics (using a Flow Monitor). Several classes are used in supporting queue monitoring. When a packet arrives at a link where queue monitoring is enabled, it generally passes through a SnoopQueue object when it arrives and leaves (or is dropped). These objects contain a reference to a QueueMonitor object. A QueueMonitor is dened as follows (~ns/queue-monitor.cc): class QueueMonitor : public TclObject { public: QueueMonitor() : bytesInt_(NULL), pktsInt_(NULL), delaySamp_(NULL), size_(0), pkts_(0), parrivals_(0), barrivals_(0), pdepartures_(0), bdepartures_(0), pdrops_(0), bdrops_(0), srcId_(0), dstId_(0), channel_(0) { bind("size_", &size_); bind("pkts_", &pkts_); bind("parrivals_", &parrivals_); bind("barrivals_", &barrivals_); bind("pdepartures_", &pdepartures_); bind("bdepartures_", &bdepartures_); bind("pdrops_", &pdrops_); bind("bdrops_", &bdrops_); bind("off_cmn_", &off_cmn_); }; 192
int size() const { return (size_); } int pkts() const { return (pkts_); } int parrivals() const { return (parrivals_); } int barrivals() const { return (barrivals_); } int pdepartures() const { return (pdepartures_); } int bdepartures() const { return (bdepartures_); } int pdrops() const { return (pdrops_); } int bdrops() const { return (bdrops_); } void printStats(); virtual void in(Packet*); virtual void out(Packet*); virtual void drop(Packet*); virtual void edrop(Packet*) { abort(); }; // not here virtual int command(int argc, const char*const* argv); ... // packet arrival to a queue void QueueMonitor::in(Packet* p) { hdr_cmn* hdr = (hdr_cmn*)p->access(off_cmn_); double now = Scheduler::instance().clock(); int pktsz = hdr->size(); barrivals_ += pktsz; parrivals_++; size_ += pktsz; pkts_++; if (bytesInt_) bytesInt_->newPoint(now, double(size_)); if (pktsInt_) pktsInt_->newPoint(now, double(pkts_)); if (delaySamp_) hdr->timestamp() = now; if (channel_) printStats(); } ... in(), out(), drop() are all defined similarly ... It addition to the packet and byte counters, a queue monitor may optionally refer to objects that keep an integral of the queue size over time using Integrator objects, which are dened in Section 20.3. The Integrator class provides a simple implementation of integral approximation by discrete sums. All bound variables beginning with p refer to packet counts, and all variables beginning with b refer to byte counts. The variable size_ records the instantaneous queue size in bytes, and the variable pkts_ records the same value in packets. When a QueueMonitor is congured to include the integral functions (on bytes or packets or both), it computes the approximate integral of the queue size (in bytes) with respect to time over the interval , where is either the start of the simulation or the last time the sum_ eld of the underlying Integrator class was reset. The QueueMonitor class is not derived from Connector, and is not linked directly into the network topology. Rather, objects of the SnoopQueue class (or its derived classes) are inserted into the network topology, and these objects contain references to an associated queue monitor. Ordinarily, multiple SnoopQueue objects will refer to the same queue monitor. Objects constructed out of these classes are linked in the simulation topology as described above and call QueueMonitor 193
out, in, or drop procedures, depending on the particular type of snoopy queue.
21.7 Per-Flow Monitoring
A collection of specialized classes are used to to implement per-ow statistics gathering. These classes include: QueueMonitor/ED/Flowmon, QueueMonitor/ED/Flow, and Classifier/Hash. Typically, an arriving packet is inspected to determine to which ow it belongs. This inspection and ow mapping is performed by a classier object (described in section 21.7.1). Once the correct ow is determined, the packet is passed to a ow monitor, which is responsible for collecting per-ow state. Per-ow state is contained in ow objects in a one-to-one relationship to the ows known by the ow monitor. Typically, a ow monitor will create ow objects on-demand when packets arrive that cannot be mapped to an already-known ow.
21.7.1 The Flow Monitor
The QueueMonitor/ED/Flowmon class is responsible for managing the creation of new ow objects when packets arrive on previously unknown ows and for updating existing ow objects. Because it is a subclass of QueueMonitor, each ow monitor contains an aggregate count of packet and byte arrivals, departures, and drops. Thus, it is not necessary to create a separate queue monitor to record aggregate statistics. It provides the following OTcl interface: classier attach dump ows get(set) classier to map packets to ows attach a Tcl I/O channel to this monitor dump contents of ow monitor to Tcl channel return string of ow object names known to this monitor
The classifier function sets or gets the name of the previously-allocated object which will perform packet-to-ow mapping for the ow monitor. Typically, the type of classier used will have to do with the notion of ow held by the user. One of the hash based classiers that inspect various IP-level header elds is typically used here (e.g. d, src/dst, src/dst/d). Note that while classiers usually receive packets and forward them on to downstream objects, the ow monitor uses the classier only for its packet mapping capability, so the ow monitor acts as a passive monitor only and does not actively forward packets. The attach and dump functions are used to associate a Tcl I/O stream with the ow monitor, and dump its contents on-demand. The le format used by the dump command is described below. The flows function returns a list of the names of ows known by the ow monitor in a way understandable to Tcl. This allows tcl code to interrogate a ow monitor in order to obtain handles to the individual ows it maintains.
21.7.2 Flow Monitor Trace Format
The ow monitor denes a trace format which may be used by post-processing scripts to determine various counts on a per-ow basis. The format is dened by the folling code in ~ns/owmon.cc: void FlowMon::fformat(Flow* f) { 194
double now = Scheduler::instance().clock(); sprintf(wrk_, "%8.3f %d %d %d %d %d %d %d %d %d %d %d %d %d %d %d %d %d %d", now, f->flowid(), 0, f->ptype(), f->flowid(), f->src(), f->dst(), f->parrivals(), f->barrivals(), f->epdrops(), f->ebdrops(), parrivals(), barrivals(), epdrops(), ebdrops(), pdrops(), bdrops(), f->pdrops(), f->bdrops() ); }; Most of the elds are explained in the code comments. The category is historical, but is used to maintain loose backwardcompatibility with the ow manager format in ns version 1. // // // // flowid category type (from common header) flowid (formerly class)
// // // // // // // // // // // //
arrivals this flow (pkts) arrivals this flow (bytes) early drops this flow (pkts) early drops this flow (bytes) all arrivals (pkts) all arrivals (bytes) total early drops (pkts) total early drops (bytes) total drops (pkts) total drops (bytes) drops this flow (pkts) [includes edrops] drops this flow (bytes) [includes edrops]
21.7.3 The Flow Class
The class QueueMonitor/ED/Flow is used by the ow monitor for containing per-ow counters. As a subclass of QueueMonitor, it inherits the standard counters for arrivals, departures, and drops, both in packets and bytes. In addition, because each ow is typically identied by some combination of the packet source, destination, and ow identier elds, these objects contain such elds. Its OTcl interface contains only bound variables: src_ dst_ flowid_ source address on packets for this ow destination address on packets for this ow ow id on packets for this ow
Note that packets may be mapped to ows (by classiers) using criteria other than a src/dst/owid triple. In such circumstances, only those elds actually used by the classier in performing the packet-ow mapping should be considered reliable.
21.8 Commands at a glance
Following is a list of trace related commands commonly used in simulation scripts: $ns_ trace-all <tracefile> 195
This is the command used to setup tracing in ns. All traces are written in the <tracele>. $ns_ namtrace-all <namtracefile> This command sets up nam tracing in ns. All nam traces are written in to the <namtracele>. $ns_ namtrace-all-wireless <namtracefile> <X> <Y> This command sets up wireless nam tracing. <X> and <Y> are the x-y co-ordinates for the wireless topology and all wireless nam traces are written into the <namtracele>. $ns_ nam-end-wireless <stoptime> This tells nam the simulation stop time given in <stoptime>. $ns_ trace-all-satlinks <tracefile> This is a method to trace satellite links and write traces into <tracele>. $ns_ flush-trace This command ushes the trace buffer and is typically called before the simulation run ends. $ns_ get-nam-traceall Returns the namtrace le descriptor stored as the Simulator instance variable called namtraceAllFile_. $ns_ get-ns-traceall Similar to get-nam-traceall. This returns the le descriptor for ns tracele which is stored as the Simulator instance called traceAllFile_. $ns_ create-trace <type> <file> <src> <dst> <optional:op> This command creates a trace object of type <type> between the <src> and <dst> nodes. The traces are written into the <le>. <op> is the argument that may be used to specify the type of trace, like nam. if <op> is not dened, the default trace object created is for nstraces. $ns_ trace-queue <n1> <n2> <optional:file> This is a wrapper method for create-trace. This command creates a trace object for tracing events on the link represented by the nodes <n1> and <n2>. $ns_ namtrace-queue <n1> <n2> <optional:file> This is used to create a trace object for namtracing on the link between nodes <n1> and <n2>. This method is very similar to and is the namtrace counterpart of method trace-queue. $ns_ drop-trace <n1> <n2> <trace> This command makes the given <trace> object a drop-target for the queue associated with the link between nodes <n1> and <n2>. $ns_ monitor-queue <n1> <n2> <qtrace> <optional:sampleinterval> This sets up a monitor that keeps track of average queue length of the queue on the link between nodes <n1> and <n2>. The default value of sampleinterval is 0.1. $link trace-dynamics <ns> <fileID> Trace the dynamics of this link and write the output to leID lehandle. ns is an instance of the Simulator or MultiSim object that was created to invoke the simulation. The tracele format is backward compatible with the output les in the ns version 1 simulator so that ns-1 postprocessing scripts can still be used. Trace records of trafc for link objects with Enque, Deque, receive or Drop Tracing have the following form: <code> <time> <hsrc> <hdst> <packet> where
196
<code> := [hd+-] h=hop d=drop +=enque -=deque r=receive <time> := simulation time in seconds <hsrc> := first node address of hop/queuing link <hdst> := second node address of hop/queuing link <packet> := <type> <size> <flags> <flowID> <src.sport> <dst.dport> <seq> <pktID> <type> := tcp|telnet|cbr|ack etc. <size> := packet size in bytes <flags> := [CP] C=congestion, P=priority <flowID> := flow identifier field as defined for IPv6 <src.sport> := transport address (src=node,sport=agent) <dst.sport> := transport address (dst=node,dport=agent) <seq> := packet sequence number <pktID> := unique identifer for every new packet Only those agents interested in providing sequencing will generate sequence numbers and hence this eld may not be useful for packets generated by some agents. For links that use RED gateways, there are additional trace records as follows: <code> <time> <value> where
<code> := [Qap] Q=queue size, a=average queue size, p=packet dropping probability <time> := simulation time in seconds <value> := value
Trace records for link dynamics are of the form: <code> <time> <state> <src> <dst> where
<code> := [v] <time> := simulation time in seconds <state> := [link-up | link-down] <src> := first node address of link <dst> := second node address of link
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Chapter 22
Nam Trace
Nam is a Tcl/Tk based animation tool that is used to visualize the ns simulations and real world packet trace data. The rst step to use nam is to produce a nam trace le. The nam trace le should contain topology information like nodes, links, queues, node connectivity etc as well as packet trace information. In this chapter we shall describe the nam trace format and simple ns commands/APIs that can be used to produce topology congurations and control animation in nam.
22.1 Nam Trace format
The C++ class Trace used for ns tracing is used for nam tracing as well. Description of this class may be found under section 21.3. The method Trace::format() denes nam format used in nam trace les which are used by nam for visualization of ns simulations. Trace class method Trace::format() is described in section 21.4 of chapter 21. If the macro NAM_TRACE has been dened (by default it is dened in trace.h), then the following code is executed as part of the Trace::format() function:
if (namChan_ != 0) sprintf(nwrk_, "%c -t "TIME_FORMAT" -s %d -d %d -p %s -e %d -c %d -i %d -a %d -x %s.%s %s.%s %d %s %s", tt, Scheduler::instance().clock(), s, d, name, th->size(), iph->flowid(), th->uid(), iph->flowid(), src_nodeaddr, src_portaddr, dst_nodeaddr, dst_portaddr, seqno,flags,sname);
Every line in a nam trace le follows this format: 198
<event-type> -t <time> ... Depending on the event type, there are different ags following the time ag. In the following we describe nam trace event format in 7 classes: packet, node, node mark, link/queue, agent, feature, and miscellaneous.
22.1.1 Packet Traces
When a trace line describes a packet, the event type may be + (enqueue), - (dequeue), r (receive), d (drop), or h (hop). h Hop: The packet started to be transmitted on the link from src_addr to dst_addr and is forwarded to the next_hop towards its dst_addr. r Receive: The packet nished transmission and started to be received at the destination. d Drop: The packet was dropped from queue or link from src_addr to dst_addr. Drop here doesnt distinguish between dropping from queue or link. This is decided by the drop time. + Enter queue: The packet entered the queue from src_addr to dst_addr. - Leave queue: The packet left the queue from src_addr to dst_addr. The other ags have the following meaning: -t <time> is the time the event occurred. -s <src> is the originating node. -d <dst> is the destination node. -p <pkt-type> is the descriptive name of the type of packet seen. See section 21.5 for the different types of packets supported in ns. -e <extent> is the size (in bytes) of the packet. -c <conv> is the conversation id or ow-id of that session. -i <id> is the packet id in the conversation. -a <attr> is the packet attribute, which is currently used as color id. -x <src-na.pa> <dst-sa.na> <seq> <ags> <sname> is taken from ns-traces and it gives the source and destination node and port addresses, sequence number, ags (if any) and the type of message. For example -x 0.1 -2147483648.0 -1 ----- SRM_SESS denotes an SRM message being sent from node 0 (port 1). In addition to the above nam format for packet events there are nam traces that provide information about nam version, hierarchical addressing structure, node/link/queue states, node-marks, protocol states, color and annotations. These nam trace outputs typically have the following letters (or tags) as their rst eld and they represent the following trace types: n (node state), m (node marking), l (link state), q (queue), a (protocol state), f (protocol state variable), V (nam version), A (hierarchy information), c (nam color) and v (annotations).
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22.1.2 Node state
The nam trace format dening node state is: n -t <time> -a <src-addr> -s <src-id> -S <state> -v <shape> -c <color> -i <l-color> -o <color> "n" denotes the node state. Flags "-t" indicates time and "-a" and "-s" denotes the node address and id. "-S" gives the node state transition. The possible values:
UP, DOWN indicates node recovery and failure. COLOR indicates node color change. If COLOR is given, a following -c <color> is expected which gives the new color value. Also, ag -o is expected so that backtracing can restore the old color of a node. DLABEL indicates addition of label to node. If DLABEL is given, a following -l <old-label> -L <new-label> is expected that gives the old-label, if any (for backtracing) and current label. Shape gives the node shape. The color of a node label can be specied via the -i ag.
As an example, the line n -t * -a 4 -s 4 -S UP -v circle -c tan -i tan denes a node with address and id of 4 that has the shape of a circle, and color of tan and label-color (-i) of tan.
22.1.3 Node Marking
Node marks are colored concentric circles around nodes. They are created by: m -t <time> -n <mark name> -s <node> -c <color> -h <shape> -o <color> and can be deleted by: m -t <time> -n <mark name> -s <node> -X Note that once created, a node mark cannot change its shape. The possible choices for shapes are, circle, square, and hexagon. They are dened as lower-case strings exactly as above. A nam trace showing node mark is: m -t 4 -s 0 -n m1 -c blue -h circle indicating node 0 is marked with a blue circle at time 4.0. The name of the mark is m1.
22.1.4 Link/Queue State
The nam trace for link and queue states are given by (respectively): l -t <time> -s <src> -d <dst> -S <state> -c <color> -o orientation -r <bw> -D <delay> q -t <time> -s <src> -d <dst> -a <attr>
200
where <state> and <color> indicate the same attributes (and the same format) as described above in the node state traces. Flag -o gives the link orientation (angle between link and horizontal). Flags -r and -D give the bandwidth (in Mb) and delay (in ms), respectively. An example of a link trace is: l -t * -s 0 -d 1 -S UP -r 1500000 -D 0.01 -c black -o right Queues are visualized in nam as a straight line along which packets (small squares) are packed. In queue trace events, ag -a species the orientation of the line of the queue (angle between the queue line and the horizontal line, counter-clockwise). For example, the following line species a queue that grows vertically upwards with respect to the screen (here 0.5 means the angle of the queue line is ): q -t * -s 0 -d 1 -a 0.5
22.1.5 Agent Tracing
Agent trace events are used to visualize protocol state. They are always associated with nodes. An agent event has a name, which is a unique identier of the agent. An agent is shown as a square with its name inside, and a line link the square to its associated node Agent events are constructed using the following format: a -t <time> -n <agent name> -s <src> Because in ns, agents may be detached from nodes, an agent may be destructed in nam with: a -t <time> -n <agent name> -s <src> -X For example, the following nam trace line creates an agent named srm(5) associated with node 5 at time 0: a -t 0.00000000000000000 -s 5 -n srm(5)
22.1.6 Variable Tracing
To visualize state variables associated with a protocol agent, we use the feature trace events. Currently we allow a feature to display a simple variable, i.e., a variable with a single value. Notice that the value is simple treated as a string (without white space). Every feature is required to be associated with an agent. Then, it can be added or modied at any time after its agent is created. The trace line to create a feature is: f -t <time> -s <src> -a <agentname> -T <type> -n <varname> -v <value> -o <prev value> Flag <type> is l for a list, v for a simple variable, s for a stopped timer, u for an up-counting timer, d for a downcounting timer. However, only v is implemented in ns. Flag -v <value> gives the new value of the variable. Variable values are simple ASCII strings obeying the TCL string quoting conventions. List values obey the TCL list conventions. Timer values are ASCII numeric. Flag -o <prev value> gives the previous value of the variable. This is used in backward animation. Here is an example of a simple feature event: 201
\code{f -t 0.00000000000000000 -s 1 -n C1_ -a srm(1) -v 2.25000 -T v}\\ Features may be deleted using: f -t <time> -a <agent name> -n <var name> -o <prev value> -X
22.1.7 Miscellaneous Trace Events
There are other trace events in addition to the formats described above: Annotation This event is represented by event type v. It is used for generic annotation: v -t <time> <TCL script string> Notice that this event is very generic, in that it may include an arbitrary tcl script to be executed at a given time, as long as it is in one line (no more than 256 characters). There may be white spaces in the string. The order of ag and the string is important. Here is an example of this event: v -t 4 sim_annotation 4 3 node 0 added one mark This line calls a special tcl function sim_annotation in nam, which inserts the given string node 0 added one mark into nams annotation pane. Color Nam allows one to associate color names with integers. This is very useful in coloring packets, where ow id of a packet is used to color the packet using the corresponding color: c -t <time> -i <color id> -n <color name> Notice the color name should be one of the names listed in color database in X11 (/usr/X11/lib/rgb.txt). Version The following line dene the nam version as required to visualize the given trace: V -t <time> -v <version> -a <attr> Normally there is only one version string in a given tracele, and it is usually the rst line of the le. Hierarchy Hierarchical address information is dened by: A -t <time> -n <levels> -o <address-space size> -c <mcastshift> -a <mcastmask> -h <nth level> -m <mask in nth level> -s <shift in nth level> This trace gives the details of hierarchy, if hierarchical addressing is being used for simulation. Flag -n <levels> indicate the total number of hierarchical tiers, which is 1 for at addressing, 2 for a 2-level hierarchy etc. Flag -o <address space size> denotes the total number of bits used for addressing. Flag -h <nth level> species the level of the address hierarchy. Flag -m <mask> and -s <shift> describes the address mask and the bit shift of a given level in the address hierarchy, respectively. Here is an example of a trace for topology with 3 level hierachy: A A A A -t -t -t -t * * * * -n -h -h -h 3 1 2 3 -p -m -m -m 0 -o 1023 2047 2047 0xffffffff -c 31 -a 1 -s 22 -s 11 -s 0
The functions that implement the different nam trace formats described above may be found in the following les: ns/trace.cc, ns/trace.h, ns/tcl/lib/ns-namsupp.tcl. 202
22.2 Ns commands for creating and controlling nam animations
This section describes different APIs in nsthat may be used to manipulate nam animations for objects like nodes, links, queues and agents. The implementation of most of these APIs is contained in ns/tcl/lib/ns-namsupp.tcl. Demonstration of nam APIs may be found in ns/tcl/ex/nam-example.tcl.
22.2.1 Node
Nodes are created from the n trace event in trace le. Each node represents a host or a router. Nam terminates if there are duplicate denitions of the same node. Attributes specic to node are color, shape, label, label-color, position of label and adding/deleting mark on the node. Each node can have 3 shapes: circle (default), square, or hexagon. But once created, the shape of a node cannot be changed during the simulation. Different node may have different colors, and its color may be changed during animation. The following OTcl procedures are used to set node attributes, they are methods of the class Node:
$node $node $node $node $node $node $node
color [color] ;# sets color of node shape [shape] ;# sets shape of node label [label] ;# sets label on node label-color [lcolor] ;# sets color of label label-at [ldirection] ;# sets position of label add-mark [name] [color] [shape] ;# adds a mark to node delete-mark [name] ;# deletes mark from node
22.2.2 Link/Queue
Links are created between nodes to form a network topology. namlinks are internally simplex, but it is invisible to the users. The trace event l creates two simplex links and other necessary setups, hence it looks to users identical to a duplex link. Link may have many colors and it can change its color during animation. Queues are constructed in nam between two nodes. Unlike link, nam queue is associated to a simplex link. The trace event q only creates a queue for a simplex link. In nam, queues are visualized as stacked packets. Packets are stacked along a line, and the angle between the line and the horizontal line can be specied in the trace event q. Commands to setup different animation attributes of a link are as follows: $ns duplex-link-op <attribute> <value> The <attribute> may be one of the following: orient, color, queuePos. Orient or the link orientation denes the angle between the link and horizontal. The optional orientation values may be dined in degrees or by text like right (0), right-up (45), rightdown (-45), left (180), left-up (135), left-down (-135), up (90), down (-90). The queuePos or position of queue is dened as the angle of the queue line with horizontal. Examples for each attribute are given as following : $ns duplex-link-op orient right ;# orientation is set as right. The order ;# in which links are created in nam ;# depends on calling order of this function.
$ns duplex-link-op color "green" $ns duplex-link-op queuePos 0.5
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22.2.3 Agent and Features
Agents are used to separate protocol states from nodes. They are always associated with nodes. An agent has a name, which is a unique identier of the agent. It is shown as a square with its name inside, and a line link the square to its associated node. The following are commands that support agent tracing: $ns add-agent-trace <agent> <name> <optional:tracefile> $ns delete-agent-trace <agent> $ns monitor-agent-trace <agent> Once the above command is used to create an agent in nam trace, the tracevar method of the nsagent can be used to create feature traces of a given variable in the agent. For example, the following code segment creates traces of the variable C1_ in an SRM agent $srm(0):
$ns attach-agent $n($i) $srm(0) $ns add-agent-trace $srm($i) srm(0) $ns monitor-agent-trace $srm(0) ;# turn nam monitor on from the start $srm(0) tracevar C1_
22.2.4 Some Generic Commands
$ns color <color-id> denes color index for nam. Once specied, color-id can be used in place of the color name in nam traces. $ns trace-annotate <annotation> inserts an annotation in nam. Notice that if <annotation> contains white spaces, it must be quoted using the double quote. An example of this would be $ns at $time "$ns trace-annotate vent A happened This annotation appears in the nam window and is used to control playing of nam by events. "
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Part IV
Routing
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Chapter 23
Unicast Routing
This section describes the structure of unicast routing in ns. We begin by describing the interface to the user (Section 23.1), through methods in the class Simulator and the class RouteLogic. We then describe conguration mechanisms for specialized routing (Section 23.2) such as asymmetric routing, or equal cost multipath routing The next section describes the the conguration mechanisms for individual routing strategies and protocols (Section 23.3). We conclude with a comprehensive look at the internal architecture (Section 23.4) of routing in ns. The procedures and functions described in this chapter can be found in ~ns/tcl/lib/ns-route.tcl, ~ns/tcl/rtglib/route-proto.tcl, ~ns/tcl/mcast/McastProto.tcl, and ~ns/rtProtoDV.{cc, h}.
23.1 The Interface to the Simulation Operator (The API)
The user level simulation script requires one command: to specify the unicast routing strategy or protocols for the simulation. A routing strategy is a general mechanism by which ns will compute routes for the simulation. There are three routing strategies in ns: Static, Session, and Dynamic. Conversely, a routing protocol is a realization of a specic algorithm. Currently, Static and Session routing use the Dijkstras all-pairs SPF algorithm []; one type of dynamic routing strategy is currently implemented: the Distributed Bellman-Ford algorithm []. In ns, we blur the distinction between strategy and protocol for static and session routing, considering them simply as protocols 1. rtproto{} is the instance procedure in the class Simulator that species the unicast routing protocol to be used in the simulation. It takes multiple arguments, the rst of which is mandatory; this rst argument identies the routing protocol to be used. Subsequent arguments specify the nodes that will run the instance of this protocol. The default is to run the same routing protocol on all the nodes in the topology. As an example, the following commands illustrate the use of the rtproto{} command. $ns $ns $ns $ns rtproto rtproto rtproto rtproto Static Session DV $n1 $n2 $n3 LS $n1 $n2 ;# Enable static route strategy for the simulation ;# Enable session routing for this simulation ;# Run DV agents on nodes $n1, $n2, and $n3 ;# Run link state routing on specied nodes
If a simulation script does not specify any rtproto{} command, then ns will run Static routing on all the nodes in the topology.
1 The consideration is that static and session routing strategies/protocols are implemented as agents derived from the class Agent/rtProto similar to how the different dynamic routing protocols are implemented; hence the blurred distinctions.
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Multiple rtproto{} lines for the same or different routing protocols can occur in a simulation script. However, a simulation cannot use both centralized routing mechanisms such as static or session routing and detailed dynamic routing protocols such as DV. In dynamic routing, each node can be running more than one routing protocol. In such situations, more than one routing protocol can have a route to the same destination. Therefore, each protocol afxes a preference value to each of its routes. These values are non-negative integers in the range 0. . . 255. The lower the value, the more preferred the route. When multiple routing protocol agents have a route to the same destination, the most preferred route is chosen and installed in the nodes forwarding tables. If more than one agent has the most preferred routes, the ones with the lowest metric is chosen. We call the least cost route from the most preferred protocol the candidate route. If there are multiple candidate routes from the same or different protocols, then, currently, one of the agents routes is randomly chosen 2.
Preference Assignment and Control Each protocol agent stores an array of route preferences, rtpref_. There is one element per destination, indexed by the node handle. The default preference values used by each protocol are derived from a class variable, preference_, for that protocol. The current defaults are: Agent/rtProto set preference_ 200 Agent/rtProto/Direct3 set preference_ 100 Agent/rtProto/DV set preference_ 120 ;# global default preference
A simulation script can control routing by altering the preference for routes in one of three ways: alter the preference for a specic route learned via a particular protocol agent, alter the preference for all routes learned by the agent, or alter the class variables for the agent before the agent is created.
Link Cost Assignment and Control In the currently implemented route protocols, the metric of a route to a destination, at a node, is the cost to reach the destination from that node. It is possible to change the link costs at each of the links. The instance procedure cost{} is invoked as $ns cost node1 node2 cost ,and sets the cost of the link from node1 to node2 to cost . $ns cost $n1 $n2 10 $ns cost $n2 $n1 5 [$ns link $n1 $n2] cost? [$ns link $n2 $n1] cost? ;# set cost of link from $n1 to $n2 to 10 ;# set cost of link in reverse direction to 5 ;# query cost of link from $n1 to $n2 ;# query cost of link in reverse direction
Notice that the procedure sets the cost along one direction only. Similarly, the procedure cost?{} returns the cost of traversing the specied unidirectional link. The default cost of a link is 1.
23.2 Other Conguration Mechanisms for Specialised Routing
It is possible to adjust preference and cost mechanisms to get two special types of route congurations: asymmetric routing, and multipath routing.
2 This really is undesirable, and may be xed at some point. The x will probably be to favor the agents in class preference order. A user level simulation relying on this behavior, or getting into this situation in specic topologies is not recommended. 3 Direct is a special routing strategy that is used in conjunction with Dynamic routing. We will describe this in greater detail as part of the route architecture description.
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Asymmetric Routing Asymmetric routing occurs when the path from node to node is different from the path from to . The following shows a simple topology, and cost conguration that can achieve such a result: Nodes and use different paths to reach each other. All other pairs of nodes use symmetric paths to reach each other.
r1
n1
n2
$ns cost $n1 $r1 2 $ns cost $n2 $r2 2 $ns cost $r1 $n2 3
r2
Any routing protocol that uses link costs as the metric can observe such asymmetric routing if the link costs are appropriately congured 4.
MultiPath Routing Each node can be individually congured to use multiple separate paths to a particular destination. The instance variable multiPath_ determines whether or not that node will use multiple paths to any destination. Each node initialises its instance variable from a class variable of the same name. If multiple candidate routes to a destination are available, all of which are learned through the same protocol, then that node can use all of the different routes to the destination simultaneously. A typical conguration is as shown below: Node set multiPath_ 1 or alternately set n1 [$ns Node] $n1 set multiPath_ 1 ;# All new nodes in the simulation use multiPaths where applicable ;# only enable $n1 to use multiPaths where applicable
Currently, only DV routing can generate multipath routes.
23.3 Protocol Specic Conguration Parameters
Static Routing The static route computation strategy is the default route computation mechanism in ns. This strategy uses the Dijkstras all-pairs SPF algorithm []. The route computation algorithm is run exactly once prior to the start of the simulation. The routes are computed using an adjacency matrix and link costs of all the links in the topology.
Session Routing The static routing strategy described earlier only computes routes for the topology once in the course of a simulation. If the above static routing is used and the topology changes while the simulation is in progress, some sources and destinations may become temporarily unreachable from each other for a short time. Session routing strategy is almost identical to static routing, in that it runs the Dijkstra all-pairs SPF algorithm prior to the start of the simulation, using the adjacency matrix and link costs of the links in the topology. However, it will also run the same algorithm to recompute routes in the event that the topology changes during the course of a simulation. In other words, route recomputation and recovery is done instantaneously and there will not be transient routing outage as in static routing. Session routing provides complete and instantaneous routing changes in the presence of topology dynamics. If the topology is always connected, there is end-to-end connectivity at all times during the course of the simulation. However, the user should note that the instantaneous route recomputation of session routing does not prevent temporary violations of causality, such as packet reordering, around the instant that the topology changes.
4 Link
costs can also be used to favour or disregard specic links in order to achieve particular topology congurations.
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DV Routing DV routing is the implementation of Distributed Bellman-Ford (or Distance Vector) routing in ns. The implementation sends periodic route updates every advertInterval. This variable is a class variable in the class Agent/rtProto/DV. Its default value is 2 seconds. In addition to periodic updates, each agent also sends triggered updates; it does this whenever the forwarding tables in the node change. This occurs either due to changes in the topology, or because an agent at the node received a route update, and recomputed and installed new routes. Each agent employs the split horizon with poisoned reverse mechanisms to advertise its routes to adjacent peers. Split horizon is the mechanism by which an agent will not advertise the route to a destination out of the interface that it is using to reach that destination. In a Split horizon with poisoned reverse mechanism, the agent will advertise that route out of that interface with a metric of innity. Each DV agent uses a default preference_ of 120. The value is determined by the class variable of the same name. Each agent uses the class variable INFINITY (set at 32) to determine the validity of a route.
23.4 Internals and Architecture of Routing
We start with a discussion of the classes associated with unicast routing, and the code path used to congure and execute each of the different routing protocols. We conclude with a description of the interface between unicast routing and network dynamics, and that between unicast and multicast routing.
23.4.1 The classes
There are four main classes, the class RouteLogic, the class rtObject, the class rtPeer, and the base class Agent/rtProto for all protocols. In addition, the routing architecture extends the classes Simulator, Link, Node and Classier.
class RouteLogic This class denes two methods to congure unicast routing, and one method to query it for route information. It also denes an instance procedure that is applicable when the topology is dynamic. We discuss this last procedure in conjunction with the interface to network dynamics.
The instance procedure register{} is invoked by Simulator::rtproto{}. It takes the protocol and a list of nodes as arguments, and constructs an instance variable, rtprotos_, as an array; the array index is the name of the protocol, and the value is the list of nodes that will run this protocol. The configure{} reads the rtprotos_ instance variable, and for each element in the array, invokes route protocol methods to perform the appropriate initializations. It is invoked by the simulator run procedure. For each protocol rt-proto indexed in the rtprotos_ array, this routine invokes Agent/rtProto/ rt-proto init-all rtprotos_( rt-proto ). If there are no elements in rtprotos_, the routine invokes Static routing, as Agent/rtProto/Static init-all.
The instance procedure lookup{} takes two node numbers, the neighbor node that uses to reach .
and
,
as argument; it returns the id of
The procedure is used by the static route computation procedure to query the computed routes and populate the routes at each of the nodes. It is also used by the multicast routing protocols to perform the appropriate RPF check.
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Note that this procedure overloads an instproc-like of the same name. The procedure queries the appropriate rtObject entities if they exist (which they will if dynamic routing strategies are used in the simulation); otherwise, the procedure invokes the instproc-like to obtain the relevant information.
class rtObject is used in simulations that use dynamic routing. Each node has a rtObject associated with it, that acts as a co-ordinator for the different routing protocols that operate at a node. At any node, the rtObject at that node tracks each of the protocols operating at that node; it computes and installs the nest route to each destination available via each of the protocols. In the event that the routing tables change, or the topology changes, the rtObject will alert the protocols to take the appropriate action. The class denes the procedure init-all{}; this procedure takes a list of nodes as arguments, and creates a rtObject at each of the nodes in its argument list. It subsequently invokes its compute-routes. The assumption is that the constructor for each of the new objects will instantiate the Direct route protocol at each of these nodes. This route protocol is responsible for computing the routes to immediately adjacent neighbors. When compute-routes{} is run by the init-all{} procedure, these direct routes are installed in the node by the appropriate route object. The other instance procedures in this class are:
init{} The constructor sets up pointers from itself to the node, in its instance variable node_, and from the node to itself, through the Node instance procedure init-routing{} and the Node instance variable rtObject_. It then initializes an array of nextHop_, rtpref_, metric_, rtVia_. The index of each of these arrays is the handle of the destination node. The nextHop_ contains the link that will be used to reach the particular destination; rtpref_ and metric_ are the preference and metric for the route installed in the node; rtVia_ is the name of the agent whose route is installed in the node. The constructor also creates the instance of the Direct route protocol, and invokes compute-routes{} for that protocol.
add-proto{} creates an instance of the protocol, stores a reference to it in its array of protocols, rtProtos_. The index of the array is the name of the protocol. It also attaches the protocol object to the node, and returns the handle of the protocol object. lookup{} takes a destination node handle, and returns the id of the neighbor node that is used to reach the destination. If multiple paths are in use, then it returns a list of the neighbor nodes that will be used. If the node does not have a route to the destination, the procedure will return -1.
compute-routes{} is the core procedure in this class. It rst checks to see if any of the routing protocols at the node have computed any new routes. If they have, it will determine the best route to each destination from among all the protocols. If any routes have changed, the procedure will notify each of the protocols of the number of such changes, in case any of these protocols wants to send a fresh update. Finally, it will also notify any multicast protocol that new unicast route tables have been computed. The routine checks the protocol agents instance variable, rtsChanged_ to see if any of the routes in that protocol have changed since the protocol was last examined. It then uses the protocols instance variable arrays, nextHop_, rtpref_, and metric_ to compute its own arrays. The rtObject will install or modify any of the routes as the changes are found. If any of the routes at the node have changed, the rtObject will invoke the protocol agents instance procedures, send-updates{} with the number of changes as argument. It will then invoke the multicast route object, if it exists.
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The next set of routines are used to query the rtObject for various state information.
dump-routes{} takes a output le descriptor as argument, and writes out the routing table at that node on that le descriptor. A typical dump output is:
rtProto?{} takes a route protocol as argument, and returns a handle to the instance of the protocol running at the node. nextHop?{} takes a destination node handle, and returns the link that is used to reach that destination. Similarly, rtpref?{} and metric?{} take a destination node handle as argument, and return the preference and metric of the route to the destination installed at the node.
The class rtPeer is a container class used by the protocol agents. Each object stores the address of the peer agent, and the metric and preference for each route advertised by that peer. A protocol agent will store one object per peer. The class maintains the instance variable addr_, and the instance variable arrays, metric_ and rtpref_; the array indices are the destination node handles. The class instance procedures, metric{} and preference{}, take one destination and value, and set the respective array variable. The procedures, metric?{} and preference?{}, take a destination and return the current value for that destination. The instance procedure addr?{} returns the address of the peer agent.
class Agent/rtProto This class is the base class from which all routing protocol agents are derived. Each protocol agent must dene the procedureinit-all{} to initialize the complete protocol, and possibly instance procedures init{}, compute-routes{}, and send-updates{}. In addition, if the topology is dynamic, and the protocol supports route computation to react to changes in the topology, then the protocol should dene the procedure compute-all{}, and possibly the instance procedure intf-changed{}. In this section, we will briey describe the interface for the basic procedures. We will defer the description of compute-all{} and intf-changed{} to the section on network dynamics. We also defer the description of the details of each of the protocols to their separate section at the end of the chapter. The procedure init-all{} is a global initialization procedure for the class. It may be given a list of the nodes as an argument. This the list of nodes that should run this routing protocol. However, centralized routing protocols such as static and session routing will ignore this argument; detailed dynamic routing protocols such as DV will use this argument list to instantiate protocols agents at each of the nodes specied. Note that derived classes in OTcl do not inherit the procedures dened in the base class. Therefore, every derived routing protocol class must dene its own procedures explicitly. The instance procedure init{} is the constructor for protocol agents that are created. The base class constructor initializes the default preference for objects in this class, identies the interfaces incident on the node and their current status. The interfaces are indexed by the neighbor handle and stored in the instance variable array, ifs_; the corresponding status instance variable array is ifstat_. Centralized routing protocols such as static and session routing do not create separate agents per node, and therefore do not access any of these instance procedures. The instance procedure compute-routes{} computes the actual routes for the protocol. The computation is based on the routes learned by the protocol, and varies from protocol to protocol. This routine is invoked by the rtObject whenever the topology changes. It is also invoked when the node receives an update for the protocol. 211
If the routine computes new routes, rtObject::compute-routes{} needs to be invoked to recompute and possibly install new routes at the node. The actual invoking of the rtObject is done by the procedure that invoked this routine in the rst place. The instance procedure send-updates{} is invoked by the rtObject whenever the node routing tables have changed, and fresh updates have to be sent to all peers. The rtObject passes as argument the number of changes that were done. This procedure may also be invoked when there are no changes to the routes, but the topology incident on the node changes state. The number of changes is used to determine the list of peers to which a route update must be sent. Other procedures relate to responding to topology changes and are described later (Section 23.4.2).
Other Extensions to the Simulator, Node, Link, and Classier We have discussed the methods rtproto{} and cost{} in the class Simulator earlier (Section 23.1). The one other method used internally is get-routelogic{}; this procedure returns the instance of routelogic in the simulation. The method is used by the class Simulator, and unicast and multicast routing. The class Node contains these additional instance procedures to support dynamic unicast routing: init-routing{}, add-routes{}, delete-routes{}, and rtObject?{}. The instance procedure init-routing{} is invoked by the rtObject at the node. It stores a pointer to the rtObject, in its instance variable rtObject_, for later manipulation or retrieval. It also checks its class variable to see if it should use multiPath routing, and sets up an instance variable to that effect. If multiPath routing could be used, the instance variable array routes_ stores a count of the number of paths installed for each destination. This is the only array in unicast routing that is indexed by the node id, rather than the node handle. The instance procedure rtObject?{} returns the rtObject handle for that node. The instance procedure add-routes{} takes a node id, and a list of links. It will add the list of links as the routes to reach the destination identied by the node id. The realization of multiPath routing is done by using a separate Classier/multiPath. For any given destination id , if this node has multiple paths to , then the main classier points to this multipath classier instead of the link to reach the destination. Each of the multiple paths identied by the interfaces being used is installed in the multipath classier. The multipath classier will use each of the links installed in it for succeeding packets forwarded to it. The instance procedure delete-routes{} takes a node id, a list of interfaces, and a nullAgent. It removes each of the interfaces in the list from the installed list of interfaces. If the entry did not previously use a multipath classier, then it must have had only one route, and the route entry is set to point to the nullAgent specied. Q: WHY DOES IT NOT POINT TO NULLAGENT IF THE ENTRIES IN THE MPATHCLASSIFIER GOES TO ZERO? The main extension to the class Link for unicast routing is to support the notion of link costs. The instance variable cost_ contains the cost of the unidirectional link. The instance procedures cost{} and cost?{} set and get the cost on the link. Note that cost{} takes the cost as argument. It is preferable to use the simulator method to set the cost variable, similar to the simulator instance procedures to set the queue or delay on a link. The class Classifier contains three new procedures, two of which overloads an existing instproc-like, and the other two provide new functionality. The instance procedure install{} overloads the existing instproc-like of the same name. The procedure stores the entry being installed in the instance variable array, elements_, and then invokes the instproc-like. The instance procedure installNext{} also overloads the existing instproc-like of the same name. This instproclike simply installs the entry into the next available slot. The instance procedure adjacents{} returns a list of key, value pairs of all elements installed in the classier. 212
23.4.2 Interface to Network Dynamics and Multicast
This section describes the methods applied in unicast routing to respond to changes in the topology. The complete sequence of actions that cause the changes in the topology, and re the appropriate actions is described in a different section. The response to topology changes falls into two categories: actions taken by individual agents at each of the nodes, and actions to be taken globally for the entire protocol. Detailed routing protocols such as the DV implementation require actions to be performed by individual protocol agents at the affected nodes. Centralized routing protocols such as static and session routing fall into the latter category exclusively. Detailed routing protocols could use such techniques to gather statistics related to the operation of the routing protocol; however, no such code is currently implemented in ns.
Actions at the individual nodes Following any change in the topology, the network dynamics models will rst invoke rtObject::intf-changed{} at each of the affected nodes. For each of the unicast routing protocols operating at that node, rtObject::intf-changed{} will invoke each individual protocols instance procedure, intf-changed{}, followed by that protocols compute-routes{}. After each protocol has computed its individual routes rtObject::intf-changed{} invokes compute-routes{} to possibly install new routes. If new routes were installed in the node, rtObject::compute-routes{} will invoke send-updates{} for each of the protocols operating at the node. The procedure will also ag the multicast route object of the route changes at the node, indicating the number of changes that have been executed. rtObject::flag-multicast{} will, in turn, notify the multicast route object to take appropriate action. The one exception to the interface between unicast and multicast routing is the interaction between dynamic dense mode multicast and detailed unicast routing. This dynamicDM implementation in ns assumes neighbor nodes will send an implicit update whenever their routes change, without actually sending the update. It then uses this implicit information to compute appropriate parent-child relationships for the multicast spanning trees. Therefore, detailed unicast routing will invoke rtObject_ flag-multicast 1 whenever it receives a route update as well, even if that update does not result in any change in its own routing tables.
Global Actions Once the detailed actions at each of the affected nodes is completed, the network dynamics models will notify the RouteLogic instance (RouteLogic::notify{}) of changes to topology. This procedure invokes the procedure compute-all{} for each of the protocols that were ever installed at any of the nodes. Centralized routing protocols such as session routing use this signal to recompute the routes to the topology. Finally, the RouteLogic::notify{} procedure noties any instances of centralized multicast that are operating at the node.
23.5 Protocol Internals
In this section, we describe any leftover details of each of the routing protocol agents. Note that this is the only place where we describe the internal route protocol agent, Direct routing.
Direct Routing This protocol tracks the state of the incident links, and maintains routes to immediately adjacent neighbors only. As with the other protocols, it maintains instance variable arrays of nextHop_, rtpref_, and metric_, indexed by the handle of each of the possible destinations in the topology. The instance procedure compute-routes{} computes routes based on the current state of the link, and the previously known state of the incident links.
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No other procedures or instance procedures are dened for this protocol.
Static Routing The procedure compute-routes{} in the class RouteLogic rst creates the adjacency matrix, and then invokes the C++ method, compute_routes() of the shadow object. Finally, the procedure retrieves the result of the route computation, and inserts the appropriate routes at each of the nodes in the topology. The class only denes the procedure init-all{} that invokes compute-routes{}.
Session Routing The class denes the procedure init-all{} to compute the routes at the start of the simulation. It also denes the procedure compute-all{} to compute the routes when the topology changes. Each of these procedures directly invokes compute-routes{}.
DV Routing In a dynamic routing strategy, nodes send and receive messages, and compute the routes in the topology based on the messages exchanged. The procedure init-all{} takes a list of nodes as the argument; the default is the list of nodes in the topology. At each of the nodes in the argument, the procedure starts the class rtObject and a class Agent/rtProto/DV agents. It then determines the DV peers for each of the newly created DV agents, and creates the relevant rtPeer objects. The constructor for the DV agent initializes a number of instance variables; each agent stores an array, indexed by the destination node handle, of the preference and metric, the interface (or link) to the next hop, and the remote peer incident on the interface, for the best route to each destination computed by the agent. The agent creates these instance variables, and then schedules sending its rst update within the rst 0.5 seconds of simulation start. Each agent stores the list of its peers indexed by the handle of the peer node. Each peer is a separate peer structure that holds the address of the peer agent, the metric and preference of the route to each destination advertised by that peer. We discuss the rtPeer structure later when discuss the route architecture. The peer structures are initialized by the procedure add-peer{} invoked by init-all{}. The routine send-periodic-update{} invokes send-updates{} to send the actual updates. It then reschedules sending the next periodic update after advertInterval jittered slightly to avoid possible synchronization effects. send-updates{} will send updates to a select set of peers. If any of the routes at that node have changed, or for periodic updates, the procedure will send updates to all peers. Otherwise, if some incident links have just recovered, the procedure will send updates to the adjacent peers on those incident links only. send-updates{} uses the procedure send-to-peer{} to send the actual updates. This procedure packages the update, taking the split-horizon and poison reverse mechanisms into account. It invokes the instproc-like, send-update{} (Note the singular case) to send the actual update. The actual route update is stored in the class variable msg_ indexed by a nondecreasing integer as index. The instproc-like only sends the index to msg_ to the remote peer. This eliminates the need to convert from OTcl strings to alternate formats and back. When a peer receives a route update it rst checks to determine if the update from differs from the previous ones. The agent will compute new routes if the update contains new information.
23.6 Unicast routing objects
Routelogic and rtObject are two objects that are signicant to unicast routing in ns. Routelogic, essentially, represents the routing table that is created and maintained centrally for every unicast simulation. rtObject is the object that every node 214
taking part in dynamic unicast routing, has an instance of. Note that nodes will not have an instance of this object if Session routing is done as a detailed routing protocol is not being simulated in this case. The methods for RouteLogic and rtObject are described in the next section.
23.7 Commands at a glance
Following is a list of unicast routing related commands used in simulation scripts: $ns_ rtproto <routing-proto> <args> where <routing-proto> denes the type of routing protocol to be used, like Static, Manual, Session , DV etc. args may dene the list of nodes on which the protocol is to be run. The node list defaults to all nodes in the topology. Internal methods: $ns_ compute-routes This command computes next_hop information for all nodes in the topology using the topology connectivity. This next_hop info is then used to populate the node classiers or the routing tables. compute-routes calls compute-at-routes or compute-hier-routes depending on the type of addressing being used for the simulation. $ns_ get-routelogic This returns a handle to the RouteLogic object (the routing table), if one has been created. Otherwise a new routing table object is created. $ns_ dump-routelogic-nh This dumps next hop information in the routing table. $ns_ dump-routelogic-distance This dumps the distance information in the routing table. $node add-route <dst> <Target> This is a method used to add routing entries (nexthop information) in the nodes routing table. The nexthop to <dst> from this node is the <target> object and this info is added to the nodes classier. $routelogic lookup <srcid> <destid> Returns the id of the node that is the next hop from the node with id srcid to the node with id destid. $routelogic dump <nodeid> Dump the routing tables of all nodes whose id is less than nodeid. Node ids are typically assigned to nodes in ascending fashion starting from 0 by their order of creation. rtobject dump-routes <fileID> Dump the routing table to the output channel specied by leID. leID must be a le handle returned by the Tcl open 215
command and it must have been opened for writing. $rtobject rtProto? <proto>
Returns a handle to the routing protocol agent specied by proto if it exists at that node. Returns an empty string otherwise. $rtobject nextHop? <destID>
Returns the id of the node that is the next hop to the destination specied by the node id, <destID>. $rtobject rtpref? destID
Returns the preference for the route to destination node given by destid. $rtobject metric? destID
Returns metric for the route to destid.
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Chapter 24
Multicast Routing
This section describes the usage and the internals of multicast routing implementation in ns. We rst describe the user interface to enable multicast routing (Section 24.1), specify the multicast routing protocol to be used and the various methods and conguration parameters specic to the protocols currently supported in ns. We then describe in detail the internals and the architecture of the multicast routing implementation in ns (Section 24.2). The procedures and functions described in this chapter can be found in various les in the directories ~ns/tcl/mcast, ~ns/tcl/ctrmcast; additional support routines are in ~ns/mcast_ctrl.{cc,h}, ~ns/tcl/lib/ns-lib.tcl, and ~ns/tcl/lib/ns-node.tcl.
24.1 Multicast API
Multicast forwarding requires enhancements to the nodes and links in the topology. Therefore, the user must specify multicast requirements to the Simulator class before creating the topology. This is done in one of two ways: set ns [new Simulator -multicast on] or set ns [new Simulator] $ns multicast When multicast extensions are thus enabled, nodes will be created with additional classiers and replicators for multicast forwarding, and links will contain elements to assign incoming interface labels to all packets entering a node. A multicast routing strategy is the mechanism by which the multicast distribution tree is computed in the simulation. ns supports three multiast route computation strategies: centralised, dense mode(DM) or shared tree mode(ST). The method mrtproto{} in the Class Simulator species either the route computation strategy, for centralised multicast routing, or the specic detailed multicast routing protocol that should be used. The following are examples of valid invocations of multicast routing in ns: set cmc [$ns mrtproto CtrMcast] $ns mrtproto DM 217 ;# specify centralized multicast for all nodes ;# cmc is the handle for multicast protocol object ;# specify dense mode multicast for all nodes
$ns mrtproto ST
;# specify shared tree mode to run on all nodes
Notice in the above examples that CtrMcast returns a handle that can be used for additional conguration of centralised multicast routing. The other routing protocols will return a null string. All the nodes in the topology will run instances of the same protocol. Multiple multicast routing protocols can be run at a node, but in this case the user must specify which protocol owns which incoming interface. For this ner control mrtproto-iifs{} is used. New/unused multicast address are allocated using the procedure allocaddr{}. allocaddr{} is a class procedure in the class Node. The agents use the instance procedures join-group{} and leave-group{}, in the class Node to join and leave multicast groups. These procedures take two mandatory arguments. The rst argument identies the corresponding agent and second argument species the group address. An example of a relatively simple multicast conguration is: set set set set $ns ns [new Simulator -multicast on] group [Node allocaddr] node0 [$ns node] node1 [$ns node] duplex-link $node0 $node1 1.5Mb 10ms DropTail ;# enable multicast routing ;# allocate a multicast address ;# create multicast capable nodes
set mproto DM set mrthandle [$ns mrtproto $mproto] set udp [new Agent/UDP] $ns attach-agent $node0 $udp set src [new Application/Traffic/CBR] $src attach-agent $udp $udp set dst_addr_ $group $udp set dst_port_ 0
;# congure multicast protocol ;# all nodes will contain multicast protocol agents ;# create a source agent at node 0
set rcvr [new Agent/LossMonitor] $ns attach-agent $node1 $rcvr $ns at 0.3 "$node1 join-group $rcvr $group"
;# create a receiver agent at node 1 ;# join the group at simulation time 0.3 (sec)
24.1.1 Multicast Behavior Monitor Conguration
ns supports a multicast monitor module that can trace user-dened packet activity. The module counts the number of packets in transit periodically and prints the results to specied les. attach{} enables a monitor module to print output to a le. trace-topo{} insets monitor modules into all links. filter{} allows accounting on specied packet header, eld in the header), and value for the eld). Calling filter{} repeatedly will result in an AND effect on the ltering condition. print-trace{} noties the monitor module to begin dumping data. ptype() is a global arrary that takes a packet type name (as seen in trace-all{} output) and maps it into the corresponding value. A simple conguration to lter cbr packets on a particular group is:
set mcastmonitor [$ns McastMonitor] set chan [open cbr.tr w] $mmonitor attach $chan1 218
;# open trace le ;# attach trace le to McastMoniotor object
$mcastmonitor set period_ 0.02 ;# default 0.03 (sec) $mmonitor trace-topo ;# trace entire topology $mmonitor filter Common ptype_ $ptype(cbr) ;# lter on ptype_ in Common header $mmonitor filter IP dst_ $group ;# AND lter on dst_ address in IP header $mmonitor print-trace ;# begin dumping periodic traces to specied les
The following sample output illustrates the output le format (time, count):
0.16 0 0.17999999999999999 0.19999999999999998 0.21999999999999997 0.23999999999999996 0.25999999999999995 0.27999999999999997
0 0 6 11 12 12
24.1.2 Protocol Specic conguration
In this section, we briey illustrate the protocol specic conguration mechanisms for all the protocols implemented in ns.
Centralized Multicast The centralized multicast is a sparse mode implementation of multicast similar to PIM-SM [8]. A Rendezvous Point (RP) rooted shared tree is built for a multicast group. The actual sending of prune, join messages etc. to set up state at the nodes is not simulated. A centralized computation agent is used to compute the forwarding trees and set up multicast forwarding state, S, G at the relevant nodes as new receivers join a group. Data packets from the senders to a group are unicast to the RP. Note that data packets from the senders are unicast to the RP even if there are no receivers for the group. The method of enabling centralised multicast routing in a simulation is: set mproto CtrMcast set mrthandle [$ns mrtproto $mproto] ;# set multicast protocol
The command procedure mrtproto{} returns a handle to the multicast protocol object. This handle can be used to control the RP and the boot-strap-router (BSR), switch tree-types for a particular group, from shared trees to source specic trees, and recompute multicast routes. $mrthandle set_c_rp $node0 $node1 $mrthandle set_c_bsr $node0:0 $node1:1 $mrthandle get_c_rp $node0 $group $mrthandle get_c_bsr $node0 $mrthandle switch-treetype $group $mrthandle compute-mroutes ;# set the RPs ;# set the BSR, specied as list of node:priority ;# get the current RP ??? ;# get the current BSR ;# to source specic or shared tree ;# recompute routes. usually invoked automatically as needed
Note that whenever network dynamics occur or unicast routing changes, compute-mroutes{} could be invoked to recompute the multicast routes. The instantaneous re-computation feature of centralised algorithms may result in causality 219
violations during the transient periods.
Dense Mode The Dense Mode protocol (DM.tcl) is an implementation of a densemodelike protocol. Depending on the value of DM class variable CacheMissMode it can run in one of two modes. If CacheMissMode is set to pimdm (default), PIM-DM-like forwarding rules will be used. Alternatively, CacheMissMode can be set to dvmrp (loosely based on DVMRP [21]). The main difference between these two modes is that DVMRP maintains parentchild relationships among nodes to reduce the number of links over which data packets are broadcast. The implementation works on point-to-point links as well as LANs and adapts to the network dynamics (links going up and down). Any node that receives data for a particular group for which it has no downstream receivers, send a prune upstream. A prune message causes the upstream node to initiate prune state at that node. The prune state prevents that node from sending data for that group downstream to the node that sent the original prune message while the state is active. The time duration for which a prune state is active is congured through the DM class variable, PruneTimeout. A typical DM conguration is shown below: DM set PruneTimeout 0.3 DM set CacheMissMode dvmrp $ns mrtproto DM ;# default 0.5 (sec) ;# default pimdm
Shared Tree Mode Simplied sparse mode ST.tcl is a version of a sharedtree multicast protocol. Class variable array RP_ indexed by group determines which node is the RP for a particular group. For example: ST set RP_($group) $node0 $ns mrtproto ST At the time the multicast simulation is started, the protocol will create and install encapsulator objects at nodes that have multicast senders, decapsulator objects at RPs and connect them. To join a group, a node sends a graft message towards the RP of the group. To leave a group, it sends a prune message. The protocol currently does not support dynamic changes and LANs.
Bi-directional Shared Tree Mode BST.tcl is an experimental version of a bidirectional shared tree protocol. As in shared tree mode, RPs must be congured manually by using the class array RP_. The protocol currently does not support dynamic changes and LANs.
24.2 Internals of Multicast Routing
We describe the internals in three parts: rst the classes to implement and support multicast routing; second, the specic protocol implementation details; and nally, provide a list of the variables that are used in the implementations.
24.2.1 The classes
The main classes in the implementation are the class mrtObject and the class McastProtocol. There are also extensions to the base classes: Simulator, Node, Classier, etc. We describe these classes and extensions in this subsection. The specic protocol implementations also use adjunct data structures for specic tasks, such as timer mechanisms by detailed 220
dense mode, encapsulation/decapsulation agents for centralised multicast etc.; we defer the description of these objects to the section on the description of the particular protocol itself.
mrtObject class There is one mrtObject (aka Arbiter) object per multicast capable node. This object supports the ability for a node to run multiple multicast routing protocols by maintaining an array of multicast protocols indexed by the incoming interface. Thus, if there are several multicast protocols at a node, each interface is owned by just one protocol. Therefore, this object supports the ability for a node to run multiple multicast routing protocols. The node uses the arbiter to perform protocol actions, either to a specic protocol instance active at that node, or to all protocol instances at that node. addproto{instance, [iiist]} adds the handle for a protocol instance to its array of protocols. The second optional argument is the incoming interface list controlled by the protocol. If this argument is an empty list or not specied, the protocol is assumed to run on all interfaces (just one protocol). returns the handle to the protocol instance active at that node that matches the specied type (rst and only argument). This function is often used to locate a protocols peer at another node. An empty string is returned if the protocol of the given type could not be found. internal routine to execute op with args on all protocol instances active at that node. start/stop execution of all protocols. is called when a topology change occurs. The dummy argument is currently not used. dump multicast routes to specied le-handle. signals all protocol instances to join S, G . signals all protocol instances to leave S, G . signalled by node on forwarding errors in classier; this routine in turn signals the protocol instance that owns the incoming interface (iif) by invoking the appropriate handle function for that particular code. Called on packet drop, possibly to prune an interface.
getType{protocol}
all-mprotos{op, args} start{} stop{} notify{dummy} dump-mroutes{le-handle, [grp], [src]} join-group{G, S} leave-group{G, S} upcall{code, s, g, iif}
drop{rep, s, g, iif}
In addition, the mrtObject class supports the concept of well known groups, i.e., those groups that do not require explicit protocol support. Two well known groups, ALL_ROUTERS and ALL_PIM_ROUTERS are predened in ns. The class mrtObject denes two class procedures to set and get information about these well known groups. registerWellKnownGroups{name} getWellKnownGroup{name} assigns name a well known group address. returns the address allocated to well known group, name. If name is not registered as a well known group, then it returns the address for ALL_ROUTERS.
McastProtocol class This is the base class for the implementation of all the multicast protocols. It contains basic multicast functions: start{}, stop{} getStatus{} getType{} upcall{code args} Set the status_ of execution of this protocol instance. return the status of execution of this protocol instance. return the type of protocol executed by this instance. invoked when the node classier signals an error, either due to a cache-miss or a wrong-iif for incoming packet. This routine invokes the protocol specic handle, handle- code {} with specied args to handle the signal. 221
A few words about interfaces. Multicast implementation in ns assumes duplex links i.e. if there is a simplex link from node 1 to node 2, there must be a reverse simplex link from node 2 to node 1. To be able to tell from which link a packet was received, multicast simulator congures links with an interface labeller at the end of each link, which labels packets with a particular and unique label (id). Thus, incoming interface is referred to this label and is a number greater or equal to zero. Incoming interface value can be negative (-1) for a special case when the packet was sent by a local to the given node agent. In contrast, an outgoing interface refers to an object handler, usually a head of a link which can be installed at a replicator. This destinction is important: incoming interface is a numeric label to a packet, while outgoing interface is a handler to an object that is able to receive packets, e.g. head of a link.
24.2.2 Extensions to other classes in ns
In the earlier chapter describing nodes in ns (Chapter 5), we described the internal structure of the node in ns. To briey recap that description, the node entry for a multicast node is the switch_. It looks at the highest bit to decide if the destination is a multicast or unicast packet. Multicast packets are forwarded to a multicast classier which maintains a list of replicators; there is one replicator per source, group tuple. Replicators copy the incoming packet and forward to all outgoing interfaces.
Class Node Node support for multicast is realized in two primary ways: it serves as a focal point for access to the multicast protocols, in the areas of address allocation, control and management, and group membership dynamics; and secondly, it provides primitives to access and control interfaces on links incident on that node.
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expandaddr{}, allocaddr{}
Class procedures for address management. expandaddr{} increases the address space from 128 multicast capable nodes to . allocaddr{} allocates the next available multicast address.
start-mcast{}, stop-mcast{} notify-mcast{}
To start and stop multicast routing at that node. notify-mcast{} signals the mrtObject at that node to recompute multicastroutes following a topology change or unicast route update from a neighbour. returns a handle to mrtObject operating at that node. to dump the multicast forwarding tables at that node. When a multicast data packet is received, and the multicast classier cannot nd the slot corresponding to that data packet, it invokes Node nstproc new-group{} to establish the appropriate entry. The code indicates the reason for not nding the slot. Currently there are two possibilities, cache-miss and wrong-iif. This procedure noties the arbiter instance to establish the new group. An agent at a node that joins a particular group invokes node join-group <agent> <group>. The node signals the mrtObject to join the particular group, and adds agent to its list of agents at that group. It then adds agent to all replicators associated with group. Node instproc leave-group reverses the process described earlier. It disables the outgoing interfaces to the receiver agents for all the replicators of the group, deletes the receiver agents from the local Agents_ list; it then invokes the arbiter instances leave-group{}. Node instproc add-mfc adds a multicast forwarding cache entry for a particular source, group, iif . The mechanism is:
getArbiter{} dump-routes{le-handle} new-group{s g iif code}
join-group{a g}
leave-group{a g}
add-mfc{s g iif oiist}
create a new replicator (if one does not already exist), update the replicator_ instance variable array at the node, add all outgoing interfaces and local agents to the appropriate replicator, invoke the multicast classiers add-rep{} to create a slot for the replicator in the multicast classier.
del-mfc{s g oiist}
disables each oif in oiflist from the replicator for s, g .
The list of primitives accessible at the node to control its interfaces are listed below.
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add-iif{id link}, add-oif{link if} get-all-oifs{} get-all-iifs{} iif2link{id} link2iif{link} oif2link{oif} link2oif{link} rpf-nbr{src} getReps{s g} getReps-raw{s g} clearReps{s g}
Invoked during link creation to prep the node about its incoming interface label and outgoing interface object. Returns all oifs for this node. Returns all iifs for this node. Returns the link object labelled with given interface label. Returns the incoming interface label for the given link. Returns the link object corresponding to the given outgoing interface. Returns the outgoing interface for the link (ns object that is incident to the node). Returns a handle to the neighbour node that is its next hop to the specied src. Returns a handle to the replicator that matches s, g . Either argument can be a wildcard (*). As above, but returns a list of key, handle pairs. Removes all replicators associated with s, g .
Class Link and SimpleLink This class provides methods to check the type of link, and the label it afxes on individual packets that traverse it. There is one additional method to actually place the interface objects on this link. These methods are: if-label?{} returns the interface label afxed by this link to packets that traverse it.
Class NetworkInterface This is a simple connector that is placed on each link. It afxes its label id to each packet that traverses it. The packet id is used by the destination node incident on that link to identify the link by which the packet reached it. The label id is congured by the Link constructor. This object is an internal object, and is not designed to be manipulated by user level simulation scripts. The object only supports two methods: label{id} label{} assigns ifid that this object will afx to each packet. returns the label that this object afxes to each packet.
The global class variable, ifacenum_, species the next available ifid number.
Class Multicast Classier Classifier/Multicast maintains a multicast forwarding cache. There is one multicast classier per node. The node stores a reference to this classier in its instance variable multiclassifier_. When this classier receives a packet, it looks at the source, group information in the packet headers, and the interface that the packet arrived from (the incoming interface or iif); does a lookup in the MFC and identies the slot that should be used to forward that packet. The slot will point to the appropriate replicator. However, if the classier does not have an entry for this source, group , or the iif for this entry is different, it will invoke an upcall new-group{} for the classier, with one of two codes to identify the problem:
cache-miss indicates that the classier did not nd any source, group entries; wrong-iif indicates that the classier found source, group entries, but none matching the interface that this packet arrived on.
These upcalls to TCL give it a chance to correct the situation: install an appropriate MFCentry (for cache-miss) or change the incoming interface for the existing MFCentry (for wrong-iif). The return value of the upcall determines what classier will do with the packet. If the return value is 1, it will assume that TCL upcall has appropriately modied MFC 224
will try to classify packet (lookup MFC) for the second time. If the return value is 0, no further lookups will be done, and the packet will be thus dropped. add-rep{} creates a slot in the classier and adds a replicator for source, group, iif to that slot.
Class Replicator When a replicator receives a packet, it copies the packet to all of its slots. Each slot points to an outgoing interface for a particular source, group . If no slot is found, the C++ replicator invokes the class instance procedure drop{} to trigger protocol specic actions. We will describe the protocol specic actions in the next section, when we describe the internal procedures of each of the multicast routing protocols. There are instance procedures to control the elements in each slot: insert{oif} disable{oif} enable{oif} is-active{} exists{oif} change-iface{source, group, oldiif, newiif} inserting a new outgoing interface to the next available slot. disable the slot pointing to the specied oif. enable the slot pointing to the specied oif. returns true if the replicator has at least one active slot. returns true if the slot pointing to the specied oif is active. modied the iif entry for the particular replicator.
24.2.3 Protocol Internals
We now describe the implementation of the different multicast routing protocol agents.
Centralized Multicast CtrMcast is inherits from McastProtocol. One CtrMcast agent needs to be created for each node. There is a central CtrMcastComp agent to compute and install multicast routes for the entire topology. Each CtrMcast agent processes membership dynamic commands, and redirects the CtrMcastComp agent to recompute the appropriate routes. join-group{} leave-group{} handle-cache-miss{} registers the new member with the CtrMCastComp agent, and invokes that agent to recompute routes for that member. is the inverse of join-group{}. called when no proper forwarding entry is found for a particular packet source. In case of centralized multicast, it means a new source has started sending data packets. Thus, the CtrMcast agent registers this new source with the CtrMcastComp agent. If there are any members in that group, compute the new multicast tree. If the group is in RPT (shared tree) mode, then 1. create an encapsulation agent at the source; 2. a corresponding decapsulation agent is created at the RP; 3. the two agents are connected by unicast; and 4. the S,G entry points its outgoing interface to the encapsulation agent.
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CtrMcastComp is the centralised multicast route computation agent. reset-mroutes{} compute-mroutes{} compute-tree{source, group} compute-branch{source, group, member} resets all multicast forwarding entries. (re)computes all multicast forwarding entries. computes a multicast tree for one source to reach all the receivers in a specic group. is executed when a receiver joins a multicast group. It could also be invoked by compute-tree{} when it itself is recomputing the multicast tree, and has to reparent all receivers. The algorithm starts at the receiver, recursively nding successive next hops, until it either reaches the source or RP, or it reaches a node that is already a part of the relevant multicast tree. During the process, several new replicators and an outgoing interface will be installed. is similar to compute-branch{} except the outgoing interface is disabled; if the outgoing interface list is empty at that node, it will walk up the multicast tree, pruning at each of the intermediate nodes, until it reaches a node that has a non-empty outgoing interface list for the particular multicast tree.
prune-branch{source, group, member}
Dense Mode join-group{group} sends graft messages upstream if S,G does not contain any active outgoing slots (i.e., no downstream receivers). If the next hop towards the source is a LAN, icrements a counter of receivers for a particular group for the LAN decrements LAN counters. depending on the value of CacheMissMode handle-cache-miss-pimdm or calls either handle-cache-miss-dvmrp. if the packet was received on the correct iif (from the node that is the next hop towards the source), fan out the packet on all oifs except the oif that leads back to the nexthopneighbor and oifs that are LANs for which this node is not a forwarder. Otherwise, if the interface was incorrect, send a prune back. fans out the packet only to nodes for which this node is a next hop towards the source (parent). sends a prune message back to the previous hop. resets the prune timer if the interface had been pruned previously; otherwise, it starts the prune timer and disables the interface; furthermore, if the outgoing interface list becomes empty, it propagates the prune message upstream. cancels any existing prune timer, andre-enables the pruned interface. If the outgoing interface list was previously empty, it forwards the graft upstream. This is invoked when the multicast classier drops a packet because it arrived on the wrong interface, and invoked new-group{}. This routine is invoked by mrtObject instproc new-group{}. When invoked, it sends a prune message back to the source.
leave-group{group} handle-cache-miss{srcID group iface}
handle-cache-miss-pimdm{srcID group iface}
handle-cache-miss-dvmrp{srcID group iface} drop{replicator source group iface} recv-prune{from source group iface}
recv-graft{from source group iface}
handle-wrong-iif{srcID group iface}
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24.2.4 The internal variables
Class mrtObject protocols_ mask-wkgroups wkgroups
An array of handles of protocol instances active at the node at which this protocol operates indexed by incoming interface. Class variabledenes the mask used to identify well known groups. Class array variablearray of allocated well known groups addresses, indexed by the group name. wkgroups(Allocd) is a special variable indicating the highest currently allocated well known group.
McastProtocol status_ type_ Simulator multiSim_ MrtHandle_ Node switch_ multiclassifier_ replicator_ Agents_ outLink_ inLink_ Link and SimpleLink iif_ head_ NetworkInterface ifacenum_
takes values up or down, to indicate the status of execution of the protocol instance. contains the type (class name) of protocol executed by this instance, e.g., DM, or ST. 1 if multicast simulation is enabled, 0 otherwise. handle to the centralised multicast simulation object.
handle for classier that looks at the high bit of the destination address in each packet to determine whether it is a multicast packet (bit = 1) or a unicast packet (bit = 0). handle to classier that performs the s, g, iif match. array indexed by s, g of handles that replicate a multicast packet on to the required links. array indexed by multicast group of the list of agents at the local node that listen to the specic group. Cached list of outgoing interfaces at this node. Cached list of incoming interfaces at this node. handle for the NetworkInterface object placed on this link. rst object on the link, a no-op connector. However, this object contains the instance variable, link_, that points to the container Link object. Class variableholds the next available interface number.
24.3 Commands at a glance
Following is a list of commands used for multicast simulations: set ns [new Simulator -mcast on] This turns the multicast ag on for the the given simulation, at the time of creation of the simulator object. ns_ multicast This like the command above turns the multicast ag on. ns_ multicast? This returns true if multicast ag has been turned on for the simulation and returns false if multicast is not turned on.
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$ns_ mrtproto <mproto> <optional:nodelist> This command species the type of multicast protocol <mproto> to be used like DM, CtrMcast etc. As an additional argument, the list of nodes <nodelist> that will run an instance of detailed routing protocol (other than centralised mcast) can also be passed. $ns_ mrtproto-iifs <mproto> <node> <iifs> This command allows a ner control than mrtproto. Since multiple mcast protocols can be run at a node, this command species which mcast protocol <mproto> to run at which of the incoming interfaces given by <iifs> in the <node>. Node allocaddr This returns a new/unused multicast address that may be used to assign a multicast address to a group. Node expandaddr This command expands the address space from 16 bits to 30 bits. However this command has been replaced by "ns_ set-address-format-expanded". $node_ join-group <agent> <grp> This command is used when the <agent> at the node joins a particular group <grp>. $node_ leave-group <agent> <grp> This is used when the <agent> at the nodes decides to leave the group <grp>. Internal methods: $ns_ run-mcast This command starts multicast routing at all nodes. $ns_ clear-mcast This stopd mcast routing at all nodes. $node_ enable-mcast <sim> This allows special mcast supporting mechanisms (like a mcast classier) to be added to the mcast-enabled node. <sim> is the a handle to the simulator object. In addition to the internal methods listed here there are other methods specic to protocols like centralized mcast (CtrMcast), dense mode (DM), shared tree mode (ST) or bi-directional shared tree mode (BST), Node and Link class methods and NetworkInterface and Multicast classier methods specic to multicast routing. All mcast related les may be found under ns/tcl/mcast directory. Centralised Multicast A handle to the CtrMcastComp object is returned when the protocol is specied as CtrMcast in mrtproto. Ctrmcast methods are: $ctrmcastcomp switch-treetype group-addr Switch from the Rendezvous Point rooted shared tree to source-specic trees for the group specied by group-addr. Note that this method cannot be used to switch from source-specic trees back to a shared tree for a multicast group. $ctrmcastcomp set_c_rp <node-list> This sets the RPs. $ctrmcastcomp set_c_bsr <node0:0> <node1:1> This sets the BSR, specied as list of node:priority. $ctrmcastcomp get_c_rp <node> <group> Returns the RP for the group as seen by the node node for the multicast group with address group-addr. Note that different nodes may see different RPs for the group if the network is partitioned as the nodes might be in different partitions. $ctrmcastcomp get_c_bsr <node>
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Returns the current BSR for the group. $ctrmcastcomp compute-mroutes This recomputes multicast routes in the event of network dynamics or a change in unicast routes. Dense Mode The dense mode (DM) protocol can be run as PIM-DM (default) or DVMRP depending on the class variable CacheMissMode. There are no methods specic to this mcast protocol object. Class variables are: PruneTimeout Timeout value for prune state at nodes. defaults to 0.5sec. CacheMissMode Used to set PIM-DM or DVMRP type forwarding rules. Shared Tree There are no methods for this class. Variables are: RP_ RP_ indexed by group determines which node is the RP for a particular group. Bidirectional Shared Tree There are no methods for this class. Variable is same as that of Shared Tree described above.
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Chapter 25
Network Dynamics
This chapter describes the capabilities in ns to make the simulation topologies dynamic. We start with the instance procedures to the class Simulator that are useful to a simulation script (Section 25.1). The next section describes the internal architecture (Section 25.2), including the different classes and instance variables and procedures; the following section describes the interaction with unicast routing (Section 25.3). This aspect of network dynamics is still somewhat experimental in ns. The last section of this chapter outlines some of the deciencies in the current realization (Section 25.4) of network dynamics, some one or which may be xed in the future. The procedures and functions described in this chapter can be found in ~ns/tcl/rtglib/dynamics.tcl and ~ns/tcl/lib/routeproto.tcl.
25.1 The user level API
The user level interface to network dynamics is a collection of instance procedures in the class Simulator, and one procedure to trace and log the dynamics activity. Reecting a rather poor choice of names, these procedures are rtmodel, rtmodel-delete, and rtmodel-at. There is one other procedure, rtmodel-configure, that is used internally by the class Simulator to congure the rtmodels just prior to simulation start. We describe this method later (Section 25.2). The instance procedure rtmodel{} denes a model to be applied to the nodes and links in the topology. Some examples of this command as it would be used in a simulation script are: $ns rtmodel Exponential 0.8 1.0 1.0 $n1 $ns rtmodel Trace dynamics.trc $n2 $n3 $ns rtmodel Deterministic 20.0 20.0 $node(1) $node(5) The procedure requires at least three arguments:
The rst two arguments dene the model that will be used, and the parameters to congure the model. The currently implemented models in ns are Exponential (On/Off), Deterministic (On/Off), Trace (driven), or Manual (one-shot) models. The number, format, and interpretation of the conguration parameters is specic to the particular model. 1. The exponential on/off model takes four parameters: [start time], up interval, down interval, [nish time] . start time defaults to from the start of the simulation, nish time defaults to the end of the simulation. up interval and down interval specify the mean of the exponential distribution dening the time that the 230
node or link will be up and down respectively. The default up and down interval values are and respectively. Any of these values can be specied as to default to the original value. The following are example specications of parameters to this model: 0.8 1.0 1.0 ;# start at , up/down = , nish is default 5.0 0.5 ;# start is default, up/down = , nish is default - 0.7 ;# start, up interval are default, down = , nish is default - - - 10 ;# start, up, down are default, nish at 2. The deterministic on/off model is similar to the exponential model above, and takes four parameters: [start time], up interval, down interval, [nish time] . start time defaults to the start of the simulation, nish time defaults to the end of the simulation. Only the interpretation of the up and down interval is different; up interval and down interval specify the exact duration that the node or link will be up and down respectively. The default values for these parameters are: start time is from start of simulation, up interval is , down interval is , and nish time is the duration of the simulation. 3. The trace driven model takes one parameter: the name of the trace le. The format of the input trace le is identical to that output by the dynamics trace modules, viz., v time link- operation node1 node2 . Lines that do not correspond to the node or link specied are ignored.
v 0.8123 link-up 3 5 v 3.5124 link-down 3 5
4. The manual one-shot model takes two parameters: the operation to be performed, and the time that it is to be performed.
The rest of the arguments to the rtmodel{} procedure dene the node or link that the model will be applied to. If only one node is specied, it is assumed that the node will fail. This is modeled by making the links incident on the node fail. If two nodes are specied, then the command assumes that the two are adjacent to each other, and the model is applied to the link incident on the two nodes. If more than two nodes are specied, only the rst is considered, the subsequent arguments are ignored. instance variable, traceAllFile_ is set.
The command returns the handle to the model that was created in this call. Internally, rtmodel{} stores the list of route models created in the class Simulator instance variable, rtModel_. The instance procedure rtmodel-delete{} takes the handle of a route model as argument, removes it from the rtModel_ list, and deletes the route model. The instance procedure rtmodel-at{} is a special interface to the Manual model of network dynamics. The command takes the time, operation, and node or link as arguments, and applies the operation to the node or link at the specied time. Example uses of this command are: $ns rtmodel-at 3.5 up $n0 $ns rtmodel-at 3.9 up $n(3) $n(5) $ns rtmodel-at 40 down $n4 Finally, the instance procedure trace-dynamics{} of the class rtModel enables tracing of the dynamics effected by this model. It is used as: set fh [open "dyn.tr" w] $rtmodel1 trace-dynamics $fh $rtmodel2 trace-dynamics $fh $rtmodel1 trace-dynamics stdout In this example, $rtmodel1 writes out trace entries to both dyn.tr and stdout; $rtmodel2 only writes out trace entries to dyn.tr. A typical sequence of trace entries written out by either model might be:
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v v v v
0.8123 0.8123 3.5124 3.5124
link-up 3 link-up 5 link-down link-down
5 3 35 53
These lines above indicate that Link 3, 5 failed at , and recovered at time
25.2 The Internal Architecture
Each model of network dynamics is implemented as a separate class, derived from the base class rtModel. We begin by describing the base class rtModel and the derived classes (Section 25.2.1). The network dynamics models use an internal queuing structure to ensure that simultaneous events are correctly handled, the class rtQueue. The next subsection (Section 25.2.2) describes the internals of this structure. Finally, we describe the extensions to the existing classes (Section 25.3.1): the Node, Link, and others.
25.2.1 The class rtModel
To use a new route model, the routine rtmodel{} creates an instance of the appropriate type, denes the node or link that the model will operate upon, congures the model, and possibly enables tracing; The individual instance procedures that accomplish this in pieces are: The constructor for the base class stores a reference to the Simulator in its instance variable, ns_. It also initializes the startTime_ and finishTime_ from the class variables of the same name. The instance procedure set-elements identies the node or link that the model will operate upon. The command stores two arrays: links_, of the links that the model will act upon; nodes_, of the incident nodes that will be affected by the link failure or recovery caused by the model. The default procedure in the base class to set the model conguration parameters is set-parms. It assumes a well dened start time, up interval, down interval, and a nish time, and sets up conguration parameters for some class of models. It stores these values in the instance variables: startTime_, upInterval_, downInterval_, finishTime_. The exponential and deterministic models use this default routine, the trace based and manual models dene their own procedures. The instance procedure trace{} enables trace-dynamics{} on each of the links that it affects. Additional details on trace-dynamics{} is discussed in the section on extensions to the class Link (Section 25.3.1). The next sequence of conguration steps are taken just prior to the start of the simulator. ns invokes rtmodel-configure{} just before starting the simulation. This instance procedure rst acquires an instance of the class rtQueue, and then invokes configure{} for each route model in its list, rtModel_. The instance procedure configure{} makes each link that is is applied to dynamic; this is the set of links stored in its instance variable array, links_. Then the procedure schedules its rst event. The default instance procedure set-first-event{} schedules the rst event to take all the links down at $startTime_ + upInterval_. Individual types of route models derived from this base class should redene tihs function.
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Two instance procedures in the base class , set-event{} and set-event-exact{}, can be used to schedule events in the route queue. set-event{interval, operation} schedules operation after interval seconds from the current time; it uses the procedure set-event-exact{} below. set-event-exact{reTime, operation} schedules operation to execute at fireTime. If the time for execution is greater than the finishTime_, then the only possible action is to take a failed link up. Finally, the base class provides the methods to take the links up{} or down{}. Each method invokes the appropriate procedure on each of the links in the instance variable, links_.
Exponential The model schedules its rst event to take the links down at startTime_ + E(upInterval_); It also denes the procedures, up{} and down{}; each procedure invokes the base class procedure to perform the actual operation. This routine then reschedules the next event at E(upInterval) or E(downInterval_) respectively.
Deterministic The model denes the procedures, up{} and down{}; each procedure invokes the base class procedure to perform the actual operation. This routine then reschedules the next event at upInterval or downInterval_ respectively.
Trace The model redenes the instance procedure set-parms{} to operan a trace le, and set events based on that input. The instance procedure get-next-event{} returns the next valid event from the trace le. A valid event is an event that is applicable to one of the links in this objects links_ variable. The instance procedure set-trace-events{} uses get-next-event{} to schedule the next valid event. The model redenes set-first-event{}, up{}, and down{} to use set-trace-events{}.
Manual The model is designed to re exactly once. The instance procedure set-parms{} takes an operation and the time to execute that operation as arguments. set-first-event{} will schedule the event at the appropriate moment. This routine also redenes notify{} to delete the object instance when the operation is completed. This notion of the object deleting itself is fragile code. Since the object only res once and does nto have to be rescheduled, it does not overload the procedures up{} or down{}.
25.2.2 class rtQueue
The simulator needs to co-ordinate multiple simultaneous network dynamics events, especially to ensure the right coherent behaviour. Hence, the network dynamics models use their own internal route queue to schedule dynamics events. There is one instance of this object in the simulator, in the class Simulator instance variable rtq_. The queue object stores an array of queued operations in its instance variable, rtq_. The index is the time at which the event will execute. Each element is the list of operations that will execute at that time. The instance procedures insq{} and insq-i{} can insert an element into the queue.
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The rst argument is the time at which this operation will execute. insq{} takes the exact time as argument; insq-i{} takes the interval as argument, and schedules the operation interval seconds after the current time. The following arguments specify the object, $obj, the instance procedure of that object, $iproc, and the arguments to that procedure, $args. These arguments are placed into the route queue for execution at the appropriate time. The instance procedure runq{} executes eval $obj $iproc $args at the appropriate instant. After all the events for that instance are executed, runq{} will notify{} each object about the execution. Finally, the instance procedure delq{} can remove a queued action with the time and the name of the object.
25.3 Interaction with Unicast Routing
In an earlier section, we had described how unicast routing reacts (Section 23.4.2) to changes to the topology. This section details the steps by which the network dynamics code will notify the nodes and routing about the changes to the topology. 1. rtQueue::runq{} will invoke the procedures specied by each of the route model instances. After all of the actions are completed, runq{} will notify each of the models. 2. notify{} will then invoke instance procedures at all of the nodes that were incident to the affected links. Each route model stores the list of nodes in its instance variable array, nodes_. It will then notify the RouteLogic instance of topology changes. 3. The rtModel object invokes the class Node instance procedure intf-changed{} for each of the affected nodes. 4. Node::intf-changed{} will notify any rtObject at the node of the possible changes to the topology. Recall that these route objects are created when the simulation uses detailed dynamic unicast routing.
25.3.1 Extensions to Other Classes
The existing classes assume that the topology is static by default. In this section, we document the necessary changes to these classes to support dynamic topologies. We have already described the instance procedures in the class Simulator to create or manipulate route models, i.e., rtmodel{}, rtmodel-at{}, rtmodel-delete{}, and rtmodel-configure{} in earlier sections (Section 25.2.1). Similarly, the class Node contains the instance procedure intf-changed{} that we described in the previous section (Section 25.3). The network dynamics code operates on individual links. Each model currently translates its specication into operations on the appropriate links. The following paragraphs describe the class Link and related classes.
class DynamicLink This class is the only TclObject in the network dynamics code. The shadow class is called class DynaLink. The class supports one bound variable, status_. status_ is 1 when the link is up, and 0 when the link is down. The shadow objects recv() method checks the status_ variable, to decide whether or not a packet should be forwarded.
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class Link This class supports the primitives: up and down, and up? to set and query status_. These primitives are instance procedures of the class. The instance procedures up{} and down{} set status_ to 1 and 0 respectively. In addition, when the link fails, down{} will reset all connectors that make up the link. Each connector, including all queues and the delay object will ush and drop any packets that it currently stores. This emulates the packet drop due to link failure. Both procedures then write trace entries to each le handle in the list, dynT_. The instance procedure up?{} returns the current value of status_. In addition, the class contains the instance procedure all-connectors{}. This procedure takes an operation as argument, and applies the operation uniformly to all of the class instance variables that are handles for TclObjects.
class SimpleLink The class supports two instance procedures dynamic{} and trace-dynamics{}. We have already described the latter procedure when describing the trace{} procedure in the class rtModel. The instance procedure dynamic{} inserts a DynamicLink object (Section 6.2) at the head of the queue. It points the downtarget of the object to the drop target of the link, drpT_, if the object is dened, or to the nullAgent_ in the simulator. It also signals each connector in the link that the link is now dynamic. Most connectors ignore this signal to be become dynamic; the exception is DelayLink object. This object will normally schedule each packet it receives for reception by the destination node at the appropriate time. When the link is dynamic, the object will queue each packet internally; it schedules only one event for the next packet that will be delivered, instead of one event per packet normally. If the link fails, the route model will signal a reset, at which point, the shadow object will execute its reset instproc-like, and ush all packets in its internal queue. Additional details about the DelayLink can be found in another chapter (Chapter 8).
25.4 Decencies in the Current Network Dynamics API
There are a number of decencies in the current API that should be changed in the next iteration: 1. There is no way to specify a cluster of nodes or links that behave in lock-step dynamic synchrony. 2. Node failure should be dealt with as its own mechanism, rather than a second grade citizen of link failure. This shows up in a number of situations, such as: (a) The method of emulating node failure as the failure of the incident links is broken. Ideally, node failure should cause all agents incident on the node to be reset. (b) There is no tracing associated with node failure. 3. If two distinct route models are applied to two separate links incident on a common node, and the two links experience a topology change at the same instant, then the node will be notied more than once.
25.5 Commands at a glance
Following is a list of commands used to simulate dynamic scenarios in ns: 235
$ns_ rtmodel <model> <model-params> <args> This command denes the dynamic model (currently implemented models are: Deterministic, Exponential, Manual or Trace) to be applied to nodes and links in the topology. The rst two arguments consists of the rtmodel and the parameter to congure the model. <args> stands for different type of arguments expected with different dynamic model types. This returns a handle to a model object corresponding to the specied model.
In the Deterministic model <model-params> is <start-time>, <up-interval>, <down-interval>, <nish-time>. Starting from start-time the link is made up for up-interval and down for down-interval till nish-time is reached. The default values for start-time, up-interval, downinterval are 0.5s, 2.0s, 1.0s respectively. nishtime defaults to the end of the simulation. The start-time defaults to 0.5s in order to let the routing protocol computation quiesce. If the Exponential model is used model-params is of the form <up-interval>, <down-interval> where the link up-time is an exponential distribution around the mean upinterval and the link down-time is an exponential distribution around the mean down-interval. Default values for up-interval and down-interval are 10s and 1s respectively. If the Manual distribution is used model-params is <at> <op> where at species the time at which the operation op should occur. op is one of up, down. The Manual distribution could be specied alternately using the rtmodel-at method described later in the section. If Trace is specied as the model the link/node dynamics is read from a Tracele. The model-params argument would in this case be the le-handle of the Tracele that has the dynamics information. The tracele format is identical to the trace output generated by the trace-dynamics link method (see TRACE AND MONITORING METHODS SECTION).
$ns_ rtmodel-delete <model> This command takes the handle of the routemodel <model> as an argument, removes it from the list of rtmodels maintained by simulator and deletes the model. $ns_ rtmodel-at <at> <op> <args> This command is a special interface to the Manual model of network dynamics. It takes the time <at>, type of operation <op> and node or link on which to apply the operation <args> as the arguments. At time <at>, the operation <op> which maybe up or down is applied to a node or link. $rtmodel trace <ns> <f> <optional:op> This enables tracing of dynamics effected by this model in the links. <ns> is an instance of the simulator, <f> the output le to write the traces to and <op> is an optional argument that may be used to dene a type of operation (like nam). This is a wrapper for the class Link procedure trace-dynamics. $link trace-dynamics <ns> <f> <optional:op> This is a class link instance procedure that is used to setup tracing of dynamics in that particular link. The arguments are same as that of class rtModels procedure trace described above. $link dynamic This command inserts a DynamicLink object at the head of the queue and signals to all connectors in the link that the link is now dynamic. Internal procedures: $ns_ rtmodel-configure This is an internal procedure that congures all dynamic models that are present in the list of models maintained by the simulator.
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Chapter 26
Hierarchical Routing
This chapter describes the internals of hierarchical routing implemented in ns. This chapter consists of two sections. In the rst section we give an overview of hierarchical routing. In the second section we walk through the APIs used for setting hierarchical routing and describe the architecture, internals and code path for hier rtg in the process. The functions and procedures described in this chapter can be found in ~ns/tcl/lib/ns-hiernode.tcl, tcl/lib/ns-address.tcl, tcl/lib/ns-route.tcl and route.cc.
26.1 Overview of Hierarchical Routing
Hierarchical routing was mainly devised, among other things, to reduce memory requirements of simulations over very large topologies. A topology is broken down into several layers of hierarchy, thus downsizing the routing table. The table size is reduced from , for at routing, to about log n for hierarchical routing. However some overhead costs results as number of hierarchy levels are increased. Optimum results were found for 3 levels of hierarchy and the current ns implementation supports upto a maximum of 3 levels of hierarchical routing. To be able to use hierarchical routing for the simulations, we need to dene the hierarchy of the topology as well as provide the nodes with hierarchical addressing. In at routing, every node knows about every other node in the topology, thus resulting in routing table size to the order of . For hierarchical routing, each node knows only about those nodes in its level. For all other destinations outside its level it forwards the packets to the border router of its level. Thus the routing table size gets downsized to the order of about log n.
26.2 Usage of Hierarchical routing
Hierarchical routing requires some additional features and mechanisms for the simualtion. For example, a new node object called HierNode is been dened for hier rtg. Therefore the user must specify hierarchical routing requirements before creating topology. This is done as shown below: First, the address format ( ) or the address space used for node and port address, needs to be set in the hierarchical mode. It may be done in one of the two ways: set ns [new Simulator] 237
$ns set-address-format hierarchical This sets the node address space to a 3 level hierarchy assigning 8 bits in each level. or, $ns set-address-format hierarchical <n hierarchy levels> <# bits in level 1> ...<# bits in nth level> which creates a node address space for n levels of hierarchy assigning bits as specied for every level. This other than creating a hierarchical address space also sets a ag called EnableHierRt_ and sets the Simulator class variable node_factory_ to HierNode. Therefore when nodes are created by calling Simulator method node as in : $ns node 0.0.1, a HierNode is created with an address of 0.0.1; Class AddrParams is used to store the topology hierarchy like number of levels of hierarchy, number of areas in each level like number of domains, number of clusters and number of nodes in each cluster. The API for supplying these information to AddrParams is shown below: AddrParams set domain_num_ 2 lappend cluster_num 2 2 AddrParams set cluster_num_ $cluster_num lappend eilastlevel 2 3 2 3 AddrParams set nodes_num_ $eilastlevel This denes a topology with 2 domains, say D1 and D2 with 2 clusters each (C11 & C12 in D1 and C21 & C22 in D2). Then number of nodes in each of these 4 clusters is specied as 2,3,2 and 3 respectively. The default values used by AddrParams provide a topology with a single domain with 4 clusters, with each cluster consisting of 5 nodes. Appropriate mask and shift values are generated by AddrParams for the hierarchical node address space. Each HierNode at the time of its creation calls the method mk-default-classier to setup n numbers of address classiers for n levels of hierarchy dened in the topology. HierNode instproc mk-default-classifier $self instvar np_ id_ classifiers_ agents_ dmux_ neighbor_ address_ # puts "id=$id_" set levels [AddrParams set hlevel_] for set n 1 $n <= $levels incr n set classifiers_($n) [new Classifier/Addr] $classifiers_($n) set mask_ [AddrParams set NodeMask_($n)] $classifiers_($n) set shift_ [AddrParams set NodeShift_($n)]
At the time of route computation, a call is made to add-hroute instead of the conventional add-route used for at-routing. Add-hroute populates classiers as shown in the otcl method below: 238
0 1 0 1 HierNode Entry 0 1 2 3 4 Level 2 2 3 4 Level 3 2 3 4
To HierNode Port Demux
Level 1
3-Level classifiers for HierNode (hier-addr:0.2.1)
Figure 26.1: Hierarchical classiers HierNode instproc add-hroute dst target $self instvar classifiers_ rtsize_ set al [$self split-addrstr $dst] set l [llength $al] for set i 1 $i <= $l incr i set d [lindex $al [expr $i-1]] if $i == $l $classifiers_($i) install $d $target else $classifiers_($i) install $d $classifiers_([expr $i + 1])
# increase the routing table size counter - keeps track of rtg # table size for each node set rtsize_ [expr $rtsize_ + 1]
For an example of 3 level of hierarchy, the level 1 classier demuxes for domains, level 2 for all clusters inside the nodes domain and nally classier 3 demuxes for all nodes in the particular cluster that the node itself resides. For such a topology, a HierNode with address of 0.1.2 looks like the gure below: Thus the size of the routing tables are considerably reduced from as seen for at routing where each node had to store the next_hop info of all other nodes in the topology. Instead, for hierarchical routing, a given node needs to know about its neighbours in its own cluster, about the all clusters in its domain and about all the domains. This saves on memory consumption as well as run-time for the simulations using several thousands of nodes in their topology.
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26.3 Creating large Hierarchical topologies
The previous section describes methods to create hierarchical topologies by hand. However, there is a script available in ns that converts Georgia-techs SGB-graphs into ns compatible hierarchical topologies. Please refer to http://wwwmash.CS.Berkeley.EDU/ns/ns-topogen.html for downloading as well as instructions on using the hierarchical converter package. See hier-rtg-10.tcl and hier-rtg-100.tcl in ~ns/tcl/ex for example scripts of hier routing on small and large topologies respectively.
26.4 Hierarchical Routing with SessionSim
Hierarchical routing may be used in conjunction with Session simulations (see Chapter 33). Session-level simulations which are used for running multicast simulations over very large topologies, gains additionally in terms of memory savings if used with hierarchical routing. See simulation script ~ns/tcl/ex/newmcast/session-hier.tcl for an example of sessionsim over hier rtg.
26.5 Commands at a glance
Following is a list of hierarchical routing/addressing related commands used in simulation scripts: $ns_ set-address-format hierarchical This command was used to setup hierarchical addressing in ns. However with the recent changes in node APIs, this command has been replaced by ns_ node-config -addressType hierarchical This creates a default topology of 3 levels of hierarchy, assigning 8 bits to each level. $ns_ set-address-format hierarchical <nlevels> <#bits in level1>....<#bits in level n> This command creates a hierarchy of <nlevels> and assigns the bits in each level as specied in the arguments. AddrParams set domain_num_ <n_domains> AddrParams set cluster_num_ <n_clusters> AddrParams set nodes_num_ <n_nodes> The above APIs are used to specify the hierarchical topology, i.e the number of domains, clusters and nodes present in the topology. Default values used by AddrParams (i.e if nothing is specied) provide a topology with a single domain with 4 clusters, with each cluster consisting of 5 nodes. Internal procedures: $Hiernode_ add-hroute <dst> <target> This procedure is used to add next-hop entries of a destination <dst> for a given <target>. Since hier-nodes have multiple classiers, one for each level of hierarchy, add-hroute populates hier-classiers correctly and should be used in place of add-route used for at-routing. $hiernode_ split-addrstr <str> 240
This splits up a hierarchical adrress string (say a.b.c) into a list of the addresses at each level (i.e, a,b and c).
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Part V
Transport
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Chapter 27
UDP Agents
27.1 UDP Agents
UDP agents are implemented in udp.{cc, h}. A UDP agent accepts data in variable size chunks from an application, and segments the data if needed. UDP packets also contain a monotonically increasing sequence number and an RTP timestamp. Although real UDP packets do not contain sequence numbers or timestamps, this sequence number does not incur any simulated overhead, and can be useful for tracele analysis or for simulating UDP-based applications. The default maximum segment size (MSS) for UDP agents is 1000 byte: Agent/UDP set packetSize_ 1000 ;# max segment size
This OTcl instvar is bound to the C++ agent variable size_. Applications can access UDP agents via the sendmsg() function in C++, or via the send or sendmsg methods in OTcl, as described in section 31.2.3. The following is a simple example of how a UDP agent may be used in a program. In the example, the CBR trafc generator is started at time 1.0, at which time the generator begins to periodically call the UDP agent sendmsg() function. set set set $ns ns [new Simulator] n0 [$ns node] n1 [$ns node] duplex-link $n0 $n1 5Mb 2ms DropTail
set udp0 [new Agent/UDP] $ns attach-agent $n0 $udp0 set cbr0 [new Application/Traffic/CBR] $cbr0 attach-agent $udp0 $udp0 set packetSize_ 536 set $ns $ns $ns null0 [new Agent/Null] attach-agent $n1 $null0 connect $udp0 $null0 at 1.0 "$cbr0 start" 243
;# set MSS to 536 bytes
27.2 Commands at a glance
The following commands are used to setup UDP agents in simulation scripts: set udp0 [new Agent/UDP] This creates an instance of the UDP agent. $ns_ attach-agent <node> <agent> This is a common command used to attach any <agent> to a given <node>. $traffic-gen attach-agent <agent> This a class Application/Trafc/<trafctype> method which connects the trafc generator to the given <agent>. For example, if we want to setup a CBR trafc ow for the udp agent, udp1, we given the following commands
set cbr1 [new Application/Traffic/CBR] $cbr1 attach-agent $udp1
$ns_ connect <src-agent> <dst-agent> This command sets up an end-to-end connection between two agents (at the transport layer).
$udp set packetSize_ <pktsize> $udp set dst_addr_ <address> $udp set dst_port_ <portnum> $udp set class_ <class-type> $udp set ttl_ <time-to-live> ..... etc
The above are different parameter values that may be set as shown above for udp agents. The default values can be found in ns/tcl/lib/ns-default.tcl. For a typical example of setting up an UDP agent used in a simulation, see the above section 27.1.
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Chapter 28
TCP Agents
This section describes the operation of the TCP agents in ns. There are two major types of TCP agents: one-way agents and a two-way agent. One-way agents are further subdivided into a set of TCP senders (which obey different congestion and error control techniques) and receivers (sinks). The two-way agent is symmetric in the sense that it represents both a sender and receiver. It is still under development. The les described in this section are too numerous to enumerate here. Basically it covers most les matching the regular expression ~ns/tcp*.{cc, h}. The one-way TCP sending agents currently supported are:
Agent/TCP - a tahoe TCP sender Agent/TCP/Reno - a Reno TCP sender Agent/TCP/NewReno - Reno with a modication Agent/TCP/Sack1 - TCP with selective repeat (follows RFC2018) Agent/TCP/Vegas - TCP Vegas Agent/TCP/Fack - Reno TCP with forward acknowledgment
The one-way TCP receiving agents currently supported are:
Agent/TCPSink - TCP sink with one ACK per packet Agent/TCPSink/DelAck - TCP sink with congurable delay per ACK Agent/TCPSink/Sack1 - selective ACK sink (follows RFC2018) Agent/TCPSink/Sack1/DelAck - Sack1 with DelAck
The two-way experimental sender currently supports only a Reno form of TCP:
Agent/TCP/FullTcp
The section comprises three parts: the rst part is a simple overview and example of conguring the base TCP send/sink agents (the sink requires no conguration). The second part describes the internals of the base send agent, and last part is a description of the extensions for the other types of agents that have been included in the simulator. 245
28.1 One-Way TCP Senders
The simulator supports several versions of an abstracted TCP sender. These objects attempt to capture the essence of the TCP congestion and error control behaviors, but are not intended to be faithful replicas of real-world TCP implementations. They do not contain a dynamic window advertisement, they do segment number and ACK number computations entirely in packet units, there is no SYN/FIN connection establishment/teardown, and no data is ever transferred (e.g. no checksums or urgent data).
28.1.1 The Base TCP Sender (Tahoe TCP)
The Tahoe TCP agent Agent/TCP performs congestion control and round-trip-time estimation in a way similar to the version of TCP released with the 4.3BSD Tahoe UNX system release from UC Berkeley. The congestion window is increased by one packet per new ACK received during slow-start (when _ _) and is increased by _ for each new ACK received during congestion avoidance (when _ _).
Responses to Congestion Tahoe TCP assumes a packet has been lost (due to congestion) when it observes NUMDUPACKS (dened in tcp.h, currently 3) duplicate ACKs, or when a retransmission timer expires. In either case, Tahoe TCP reacts by setting ssthresh_ to half of the current window size (the minimum of cwnd_ and window_) or 2, whichever is larger. It then initializes cwnd_ back to the value of windowInit_. This will typically cause the TCP to enter slow-start.
Round-Trip Time Estimation and RTO Timeout Selection Four variables are used to estimate the round-trip time and set the retransmission timer: rtt_, srtt_, rttvar_, tcpTick_, and backoff_. TCP initializes rttvar to _ and backoff to 1. When any future retransmission timer is set, its timeout is set to the current time plus seconds, where is the current backoff value, is the value of tcpTick, is the value of srtt, and is the value of rttvar. Round-trip time samples arrive with new ACKs. The RTT sample is computed as the difference between the current time and a time echo eld in the ACK packet. When the rst sample is taken, its value is used as the initial value for srtt_. Half the rst sample is used as the initial value for rttvar_. For subsequent samples, the values are updated as follows:
28.1.2 Conguration
Running an TCP simulation requires creating and conguring the agent, attaching an application-level data source (a trafc generator), and starting the agent and the trafc generator.
28.1.3 Simple Conguration
Creating the Agent
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set ns [new Simulator] set node1 [$ns node] set node2 [$ns node] set tcp1 [$ns create-connection TCP $node1 TCPSink $node2 42] $tcp set window_ 50 set ftp1 [new Application/FTP] $ftp1 attach-agent $tcp1 $ns at 0.0 "$ftp start"
;# preamble initialization ;# agent to reside on this node ;# agent to reside on this node
;# congure the TCP agent
This example illustrates the use of the simulator built-in function create-connection. The arguments to this function are: the source agent to create, the source node, the target agent to create, the target node, and the ow ID to be used on the connection. The function operates by creating the two agents, setting the ow ID elds in the agents, attaching the source and target agents to their respective nodes, and nally connecting the agents (i.e. setting appropriate source and destination addresses and ports). The return value of the function is the name of the source agent created.
TCP Data Source The TCP agent does not generate any application data on its own; instead, the simulation user can connect any trafc generation module to the TCP agent to generate data. Two applications are commonly used for TCP: FTP and Telnet. FTP represents a bulk data transfer of large size, and telnet chooses its transfer sizes randomly from tcplib (see the le tcplib-telnet.cc. Details on conguring these application source objects are in Section 31.4.
28.1.4 Other Conguration Parameters
In addition to the window_ parameter listed above, the TCP agent supports additional conguration variables. Each of the variables described in this subsection is both a class variable and an instance variable. Changing the class variable changes the default value for all agents that are created subsequently. Changing the instance variable of a particular agent only affects the values used by that agent. For example, Agent/TCP set window_ 100 $tcp set window_ 2.0 The default parameters for each TCP agent are: Agent/TCP Agent/TCP Agent/TCP Agent/TCP Agent/TCP Agent/TCP Agent/TCP Agent/TCP Agent/TCP Agent/TCP Agent/TCP Agent/TCP Agent/TCP Agent/TCP set set set set set set set set set set set set set set window_ 20 windowInit_ 1 windowOption_ 1 windowConstant_ 4 windowThresh_ 0.002 overhead_ 0 ecn_ 0 packetSize_ 1000 bugFix_ true slow_start_restart_ true tcpTick_ 0.1 maxrto_ 64 dupacks_ 0 ack_ 0 247 ;# max bound on window size ;# initial/reset value of cwnd ;# cong avoid algorithm (1: standard) ;# used only when windowOption != 1 ;# used in computing averaged window ;# !=0 adds random time between sends ;# TCP should react to ecn bit ;# packet size used by sender (bytes) ;# see explanation ;# see explanation ;# timer granulatiry in sec (.1 is NONSTANDARD) ;# bound on RTO (seconds) ;# duplicate ACK counter ;# highest ACK received ;# Changes the class variable ;# Changes window_ for the $tcp object only
Agent/TCP Agent/TCP Agent/TCP Agent/TCP Agent/TCP Agent/TCP Agent/TCP Agent/TCP
set set set set set set set set
cwnd_ 0 awnd_ 0 ssthresh_ 0 rtt_ 0 srtt_ 0 rttvar_ 0 backoff_ 0 maxseq_ 0
;# congestion window (packets) ;# averaged cwnd (experimental) ;# slow-stat threshold (packets) ;# rtt sample ;# smoothed (averaged) rtt ;# mean deviation of rtt samples ;# current RTO backoff factor ;# max (packet) seq number sent
For many simulations, few of the conguration parameters are likely to require modication. The more commonly modied parameters include: window_ and packetSize_. The rst of these bounds the window TCP uses, and is considered to play the role of the receivers advertised window in real-world TCP (although it remains constant). The packet size essentially functions like the MSS size in real-world TCP. Changes to these parameters can have a profound effect on the behavior of TCP. Generally, those TCPs with larger packet sizes, bigger windows, and smaller round trip times (a result of the topology and congestion) are more agressive in acquiring network bandwidth.
28.1.5 Other One-Way TCP Senders
Reno TCP The Reno TCP agent is very similar to the Tahoe TCP agent, except it also includes fast recovery, where the current congestion window is inated by the number of duplicate ACKs the TCP sender has received before receiving a new ACK. A new ACK refers to any ACK with a value higher than the higest seen so far. In addition, the Reno TCP agent does not return to slow-start during a fast retransmit. Rather, it reduces sets the congestion window to half the current window and resets ssthresh_ to match this value.
NewReno TCP This agent is based on the Reno TCP agent, but which modies the action taken when receiving new ACKS. In order to exit fast recovery, the sender must receive an ACK for the highest sequence number sent. Thus, new partial ACKs (those which represent new ACKs but do not represent an ACK for all outstanding data) do not deate the window (and possibly lead to a stall, characteristic of Reno).
Vegas TCP This agent implements Vegas TCP ([4, 5]). It was contributed by Ted Kuo.
Sack TCP This agent implements selective repeat, based on selective ACKs provided by the receiver. It follows the ACK scheme described in [15], and was developed with Matt Mathis and Jamshid Mahdavi.
Fack TCP This agent implements forward ACK TCP, a modication of Sack TCP described in [14].
28.2 TCP Receivers (sinks)
The TCP senders described above represent one-way data senders. They must peer with a TCP sink object.
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28.2.1 The Base TCP Sink
The base TCP sink object (Agent/TCPSink) is responsible for returning ACKs to a peer TCP source object. It generates one ACK per packet received. The size of the ACKs may be congured. The creation and conguration of the TCP sink object is generally performed automatically by a library call (see create-connection above).
conguration parameters Agent/TCPSink set packetSize_ 40
28.2.2 Delayed-ACK TCP Sink
A delayed-ACK sink object (Agent/Agent/TCPSink/DelAck) is available for simulating a TCP receiver that ACKs less than once per packet received. This object contains a bound variable interval_ which gives the number of seconds to wait between ACKs. The delayed ACK sink implements an agressive ACK policy whereby only ACKs for in-order packets are delayed. Out-of-order packets cause immediate ACK generation.
conguration parameters Agent/TCPSink/DelAck set interval_ 100ms
28.2.3 Sack TCP Sink
The selective-acknowledgment TCP sink (Agent/TCPSink/Sack1) implements SACK generation modeled after the description of SACK in RFC 2018. This object includes a bound variable maxSackBlocks_ which gives the maximum number of blocks of information in an ACK available for holding SACK information. The default value for this variable is 3, in accordance with the expected use of SACK with RTTM (see RFC 2018, section 3). Delayed and selective ACKs together are implemented by an object of type Agent/TCPSink/Sack1/DelAck.
conguration parameters Agent/TCPSink set maxSackBlocks_ 3
28.3 Two-Way TCP Agents (FullTcp)
The Agent/TCP/FullTcp object is a new addition to the suite of TCP agents supported in the simulator and is still under development. It is different from (and incompatible with) the other agents, but does use some of the same architecture. It differs from these agents in the following ways: following ways:
connections may be establised and town down (SYN/FIN packets are exchanged) bidirectional data transfer is supported 249
sequence numbers are in bytes rather than packets
The generation of SYN packets (and their ACKs) can be of critical importance in trying to model real-world behavior when using many very short data transfers. This version of TCP currently defaults to sending data on the 3rd segment of an initial 3way handshake, a behavior somewhat different than common real-world TCP implementations. A typical TCP connection proceeds with an active opener sending a SYN, the passive opener responding with a SYN+ACK, the active opener responding with an ACK, and then some time later sending the rst segment with data (corresponding to the rst application write). Thus, this version of TCP sends data at a time somewhat earlier than typical implementations. This TCP can also be congured to send data on the initial SYN segment. Future changes to FullTCP may include a modication to send the rst data segment later, and possibly to implement T/TCP functionality. Currently FullTCP is only implemented with Reno congestion control, but ultimately it should be available with the full range of congestion control algorithms (e.g., Tahoe, SACK, Vegas, etc.).
28.3.1 Simple Conguration
Running an Full TCP simulation requires creating and conguring the agent, attaching an application-level data source (a trafc generator), and starting the agent and the trafc generator.
Creating the Agent # set up connection (do not use "create-connection" method because # we need a handle on the sink object) set src [new Agent/TCP/FullTcp] set sink [new Agent/TCP/FullTcp] $ns_ attach-agent $node_(s1) $src $ns_ attach-agent $node_(k1) $sink $src set fid_ 0 $sink set fid_ 0 $ns_ connect $src $sink # set up TCP-level connections $sink listen $src set window_ 100;
;# create agent ;# create agent ;# bind src to node ;# bind sink to node ;# set ow ID eld ;# set ow ID eld ;# active connection src to sink
;# will gure out who its peer is
The creation of the FullTcp agent is similar to the other agents, but the sink is placed in a listening state by the listen method. Because a handle to the receiving side is required in order to make this call, the create-connection call used above cannot be used.
Conguration Parameters The following conguration parameters are available through Tcl for the FullTcp agent: Agent/TCP/FullTcp Agent/TCP/FullTcp Agent/TCP/FullTcp Agent/TCP/FullTcp Agent/TCP/FullTcp Agent/TCP/FullTcp set set set set set set segsperack_ 1 segsize_ 536 tcprexmtthresh_ 3 iss_ 0 nodelay_ false data_on_syn_ false ;# segs received before generating ACK ;# segment size (MSS size for bulk xfers) ;# dupACKs thresh to trigger fast rexmt ;# initial send sequence number ;# disable sender-side Nagle algorithm ;# send data on initial SYN? 250
Agent/TCP/FullTcp set dupseg_fix_ true ;# avoid fast rxt due to dup segs+acks Agent/TCP/FullTcp set dupack_reset_ false ;# reset dupACK ctr on !0 len data segs containing dup ACKs Agent/TCP/FullTcp set interval_ 0.1 ;# as in TCP above, (100ms is non-std)
28.4 Architecture and Internals
The base TCP agent (class Agent/TCP) is constructed as a collection of routines for sending packets, processing ACKs, managing the send window, and handling timeouts. Generally, each of these routines may be over-ridden by a function with the same name in a derived class (this is how many of the TCP sender variants are implemented).
The TCP header The TCP header is dened by the hdr_tcp structure in the le ~ns/tcp.h. The base agent only makes use of the following subset of the elds: ts_ ts_echo_ seqno_ reason_ /* current time packet was sent from source */ /* for ACKs: timestamp eld from packet associated with this ACK */ /* sequence number for this data segment or ACK (Note: overloading!) */ /* set by sender when (re)transmitting to trace reason for send */
Functions for Sending Data Note that generally the sending TCP never actually sends data (it only sets the packet size). send_much(force, reason, maxburst) - this function attempts to send as many packets as the current sent window allows. It also keeps track of how many packets it has sent, and limits to the total to maxburst. The function output(seqno, reason) sends one packet with the given sequence number and updates the maximum sent sequence number variable (maxseq_) to hold the given sequence number if it is the greatest sent so far. This function also assigns the various elds in the TCP header (sequence number, timestamp, reason for transmission). This function also sets a retransmission timer if one is not already pending.
Functions for Window Management The usable send window at any time is given by the function window(). It returns the minimum of the congestion window and the variable wnd_, which represents the receivers advertised window. opencwnd() - this function opens the congestion window. It is invoked when a new ACK arrives. When in slow-start, the function merely increments cwnd_ by each ACK received. When in congestion avoidance, the standard conguration increments cwnd_ by its reciprocal. Other window growth options are supported during congestion avoidance, but they are experimental (and not documented; contact Sally Floyd for details). closecwnd(int how) - this function reduces the congestion window. It may be invoked in several ways: when entering fast retransmit, due to a timer expiration, or due to a congestion notication (ECN bit set). Its argument how indicates how the congestion window should be reduced. The value 0 is used for retransmission timeouts and fast retransmit in Tahoe TCP. It typically causes the TCP to enter slow-start and reduce ssthresh_ to half the current window. The value 1 is used by Reno TCP for implementing fast recovery (which avoids returning to slow-start). The value 2 is used for reducing the window due to an ECN indication. It resets the congestion window to its initial value (usually causing slow-start), but does not alter ssthresh_.
Functions for Processing ACKs recv() - this function is the main reception path for ACKs. Note that because only one direction of data ow is in use, this function should only ever be invoked with a pure ACK packet (i.e. no data). The function stores the timestamp from the ACK in ts_peer_, and checks for the presence of the ECN bit (reducing the send window 251
if appropriate). If the ACK is a new ACK, it calls newack(), and otherwise checks to see if it is a duplicate of the last ACK seen. If so, it enters fast retransmit by closing the window, resetting the retransmission timer, and sending a packet by calling send_much. newack() - this function processes a new ACK (one that contains an ACK number higher than any seen so far). The function sets a new retransmission timer by calling newtimer(), updates the RTT estimation by calling rtt_update, and updates the highest and last ACK variables.
Functions for Managing the Retransmission Timer These functions serve two purposes: estimating the round-trip time and setting the actual retransmission timer. rtt_init - this function initializes srtt_ and rtt_ to zero, sets rttvar_ to _ _, and sets the backoff multiplier to one. rtt_timeout - this function gives the timeout value in seconds that should be used to schedule the next retransmission timer. It computes this based on the current estimates of the mean and deviation of the round-trip time. In addition, it implements Karns exponential timer backoff for multiple consecutive retransmission timeouts. rtt_update - this function takes as argument the measured RTT and averages it in to the running mean and deviation estimators according to the description above. Note that t_srtt_ and t_rttvar are both stored in xed-point (integers). They have 3 and 2 bits, respectively, to the right of the binary point. reset_rtx_timer - This function is invoked during fast retransmit or during a timeout. It sets a retransmission timer by calling set_rtx_timer and if invoked by a timeout also calls rtt_backoff. rtt_backoff - this function backs off the retransmission timer (by doubling it). newtimer - this function called only when a new ACK arrives. If the senders left window edge is beyond the ACK, then set_rtx_timer is called, otherwise if a retransmission timer is pending it is cancelled.
28.5 Tracing TCP Dynamics
The behavior of TCP is often observed by constructing a sequence number-vs-time plot. Typically, a trace is performed by enabling tracing on a link over which the TCP packets will pass. Two trace methods are supported: the default one (used for tracing TCP agents), and an extension used only for FullTcP.
28.6 One-Way Trace TCP Trace Dynamics
TCP packets generated by one of the one-way TCP agents and destined for a TCP sink agent passing over a traced link (see section 21) will generate a trace le lines of the form: + + d + + 0.94176 0.94276 0.94276 0.95072 0.95072 0.95176 0.95176 2 2 2 2 2 2 2 3 3 3 0 0 3 3 tcp tcp tcp ack ack tcp tcp 1000 -----1000 -----1000 -----40 ------ 0 40 ------ 0 1000 -----1000 -----0 0.0 3.0 25 40 0 0.0 3.0 26 41 0 0.0 3.0 26 41 3.0 0.0 14 29 3.0 0.0 14 29 0 0.0 3.0 21 36 0 0.0 3.0 27 42
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The exact format of this trace le is given in section 21.4. When tracing TCP, packets of type tcp or ack are relevant. Their type, size, sequence number (ack number for ack packets), and arrival/depart/drop time are given by eld positions 5, 6, 11, and 2, respectively. The + indicates a packet arrival, d a drop, and - a departure. A number of scripts process this le to produce graphical output or statistical summaries (see, for example, ~ns/test-suite.tcl, the finish procedure.
28.7 One-Way Trace TCP Trace Dynamics
TCP packets generated by FullTcp and passing over a traced link contain additional information not displayed by default using the regular trace object. By enabling the ag show_tcphdr_ on the trace object (see section refsec:traceformat), 3 additional header elds are written to the trace le: ack number, tcp-specic ags, and header length.
28.8 Commands at a glance
The following is a list of commands used to setup/manipulate TCP ows for simulations: set tcp0 [new Agent/TCP] This creates an instance of a TCP agent. There are several avors of TCP-sender and TCP-receiver (or sink) agent currently implemented in ns. TCP-senders currently available are: Agent/TCP, Agent/TCP/Reno, Agent/TCP/NewReno, Agent/TCP/Sack1, Agent/TCP/Vegas, Agent/TCP/Fack. TCP-receivers currently available are: Agent/TCPSink, Agent/TCPSink/DelAck, Agent/TCPSink/Sack1, Agent/TCPSink/Sack1/DelAck. There is also a two-way implementation of tcp called Agent/TCP/FullTcp. For details on the different TCP avors see earlier sections of this chapter. Conguration parameters for TCP ows maybe set as follows: $tcp set window_ <wnd-size> For all possible conguration parameters available for TCP see section 28.1.4. The default conguration values can also be found in ns/tcl/lib/ns-default.tcl. Following is an example of a simple TCP connection setup: set tcp [new Agent/TCP] $ns_ attach-agent $node_(s1) $tcp $tcp set fid_ 0 set ftp [new Application/FTP] $ftp attach-agent $tcp set sink [new Agent/TCPSink] $ns_ attach-agent $node_(k1) $sink $sink set fid_ 0 $ns_ connect $ftp $sink $ns_ at $start-time "$ftp start" For an example of setting up a full-tcp connection see section 28.3.1. ;# create tcp agent ;# bind src to node ;# set ow ID eld ;# create ftp trafc ;# bind ftp trafc to tcp agent ;# create tcpsink agent ;# bind sink to node ;# set ow ID eld ;# active connection src to sink ;# start ftp ow
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Chapter 29
Agent/SRM
This chapter describes the internals of the SRM implementation in ns. The chapter is in three parts: the rst part is an overview of a minimal SRM conguration, and a complete description of the conguration parameters of the base SRM agent. The second part describes the architecture, internals, and the code path of the base SRM agent. The last part of the chapter is a description of the extensions for other types of SRM agents that have been attempted to date. The procedures and functions described in this chapter can be found in ~ns/tcl/mcast/srm.tcl, ~ns/tcl/mcast/srm-adaptive.tcl, ~ns/tcl/mcast/srm-nam.tcl, ~ns/tcl/mcast/srm-debug.tcl, and ~ns/srm.{cc, h}.
29.1 Conguration
Running an SRM simulation requires creating and conguring the agent, attaching an application-level data source (a trafc generator), and starting the agent and the trafc generator.
29.1.1 Trivial Conguration
Creating the Agent set $ns set set ns [new Simulator] enableMcast node [$ns node] group [$ns allocaddr] ;# preamble initialization ;# agent to reside on this node ;# multicast group for this agent
set srm [new Agent/SRM] $srm set dst_ $group $ns attach-agent $node $srm $srm set fid_ 1 $srm log [open srmStats.tr w] $srm trace [open srmEvents.tr w]
;# congure the SRM agent
;# ...