Even though the issues are primarily os related and

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Chapter 14 / Exercise 14-2
Traffic and Highway Engineering, Enhanced Edition
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Even though the issues are primarily OS related and quite useful by themselves, the study illustrates how the design of networking protocols impacts their implementation, and hence proliferation, in common operating systems. It gives the important message that architecture and system issues should be given due consideration when designing network protocols. 4.1 Motivation Several routing protocols have been designed and studied through simulations but fewer implementations exist for the testbeds. This chapter 1 focuses on system issues, the goal being to make implementation of ad hoc routing protocols easier. It explores several system issues regarding design and implementation of routing protocols for ad hoc wireless networks. It ex- amines the routing architecture in current operating systems and shows how it is insufficient on several counts, especially for supporting on-demand or reactive routing protocols. Exam- ples include lack of mechanisms for queuing outstanding packets awaiting route discovery, and mechanisms for communicating route usage information from kernel to user-space. This work proposes an architecture and a generic API for any operating system to augment the current routing architecture. Implementing this API, however, requires kernel modifications in general. 1 The work described here has been done jointly with Yongguang Zhang (HRL Laboratories, LLC) and Binita Gupta. 86
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Chapter 14 / Exercise 14-2
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For the Linux operating system, we provide an implementation of the aforesaid API using a small loadable kernel module and standard facilities like tun devices, Netfilter and Raw IP sockets, which are available in the Linux 2.4 kernel. We call this the Ad hoc Support Library (ASL). ASL is provided as a shared user-space library which uses a custom kernel module, but no kernel recompilation is needed. We provide a full-fledged implementation of the AODV protocol using ASL, to demonstrate the viability and usefulness of our framework. 4.2 Challenges in Mobile Ad Hoc Routing This section first outlines the routing architecture in current operating systems and then describes the various challenges in implementing on-demand routing protocols. 4.2.1 Current routing architecture The current Internetworking architecture segregates the routing functionality into two parts: packet forwarding and packet routing (see Section 4.2 of [85]). Packet forwarding refers to the process of taking a packet, consulting a table (the forwarding table), and sending the packet towards its destination as determined by that table. Packet routing, on the other hand, refers to the process of building the forwarding table. Forwarding is a well-defined process performed locally at each node, whereas routing involves a complex distributed decision making process commonly referred to as the routing algorithm or the routing protocol . The forwarding table contains enough information to accomplish the forwarding function, whereas the routing table contains information used by the routing algorithm to discover and maintain routes.

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