Unformatted text preview: Data and Computer Communications Communications
Chapter 19 – Internetwork Operation Eighth Edition by William Stallings Lecture slides by Lawrie Brown Internetwork Operation Internetwork
She occupied herself with studying a map on the opposite wall because she knew she would have to change trains at some point. Tottenham Court Road must be that point, an interchange from the black line to the red. This train would take her there, was bearing her there rapidly now, and at the station she would follow the signs, for signs there must be, to the Central Line going westward —King Solomon's Carpet, Barbara Vine (Ruth Rendell) Internetwork Operation Internetwork consider mechanisms for handling growth in consider network traffic network from low-volume text based terminal/email to high volume multi-media web/voice/video historically IP nets gave best-effort datagram historically delivery to all services delivery now want variety of QoS in IP networks explore some new network services / functions Multicasting Multicasting sending packet to addresses referring to sending group of hosts on one or more networks group multimedia “broadcast” teleconferencing database distributed computing real time workgroups have design issues in addressing / routing LAN Multicast LAN LAN multicast is easy send to IEEE 802 multicast MAC address since broadcast all stations will see packet those in multicast group will accept it only single copy of packet is needed but much harder in internetwork Example Config Config Broadcast / Multiple Unicast / Multicast Multicast could broadcast packet to each network if server does not know members of group requires 13 packets to each net with members in multicast group requires 11 packets which send single packets over any link duplicating as needed to reach dest nets requires 8 packets could send multiple unicast packets or use true multicast True Multicast True determine least cost path to each network determine that has host in group that results in a spanning tree of just those nets with members in group transmit single packet along spanning tree routers replicate packets at branch points routers of spanning tree of Multicast Example Multicast Requirements for Multicasting Multicasting router may have to forward more than one router copy of packet copy need convention to identify multicast need addresses (IPv4 Class D or IPv6 prefix) addresses nodes translate between IP multicast nodes addresses and list of networks containing group members group router must translate between IP multicast router address and network multicast address address Requirements for Multicasting Multicasting mechanism required for hosts to join and leave mechanism multicast group multicast routers must exchange info which networks include members of given group sufficient info to work out shortest path to each sufficient network network routing algorithm to work out shortest path routers must determine routing paths based on routers source and destination addresses source Spanning Tree from Router C to Multicast Group to Internet Group Management Protocol (IGMP) RFC 3376 to exchange multicast group info RFC 3376 between hosts & routers on a LAN between hosts send messages to routers to subscribe to hosts and unsubscribe from multicast group and routers check which multicast groups of interest routers to which hosts to IGMP currently version 3 Operation of IGMPv1 & v2 Operation IGMPv1 hosts could join group routers used timer to unsubscribe members IGMPv2 enabled hosts to unsubscribe operational model: receivers have to subscribe to groups sources do not have to subscribe to groups sources groups any host can send traffic to any multicast group spamming of multicast groups establishment of distribution trees is problematic finding globally unique multicast addresses difficult problems: IGMP v3 IGMP addresses weaknesses: allows hosts to specify list from which they want to receive traffic want traffic from other hosts blocked at routers allows hosts to block packets from sources that send unwanted traffic that IGMP Message Formats IGMP Membership Query sent by multicast router three types: general query, group-specific query, groupand-source specific query Membership Query Fields Membership Type Max Response Time Checksum Group Address S Flag QRV (querier's robustness variable) QQIC (querier's querier interval code) Number of Sources Source addresses IGMP Message Formats IGMP Membership Report IGMP Message Formats IGMP Group Record IGMP Operation - Joining IGMP IGMP host wants to make itself known as group IGMP wants member to other hosts and routers on LAN member IGMPv3 can signal group membership with IGMPv3 filtering capabilities with respect to sources filtering EXCLUDE mode – all members except those listed INCLUDE mode – only from group members listed address field multicast address of group sent in IP datagram sent current group members receive & learn new member routers listen to all IP multicast addresses to hear all reports reports to join send IGMP membership report message to IGMP Operation – Keeping Lists Valid Keeping routers periodically issue IGMP general query message message in datagram with all-hosts multicast address hosts must read such datagrams hosts respond with report message needs to know at least one group member still active each host in group sets timer with random delay host hearing another report cancels own host cancels if timer expires, host sends report only one member of each group reports to router to router don’t know every host in a group router don’t IGMP Operation - Leaving IGMP host leaves group by sending leave group by message to all-routers static multicast address message sends a membership report message with EXCLUDE option and null list of source addresses option router determines if have any remaining group router members using group-specific query message members Group Membership with IPv6 Group IGMP defined for IPv4 uses 32-bit addresses IPv6 internets need functionality IGMP functions included in Internet IGMP Control Message Protocol v 6 (ICMPv6) Control ICMPv6 has functionality of ICMPv4 & IGMP ICMPv6 includes group-membership ICMPv6 query and group-membership report message message Routing Protocols Routing routers receive and forward packets make decisions based on knowledge of make topology and traffic/delay conditions topology use dynamic routing algorithm distinguish between: routing information - about topology & delays routing algorithm - that makes routing routing decisions based on information decisions Autonomous Systems (AS) Autonomous is a group of routers and networks is managed by single organization which exchange information via a common which routing protocol routing form a connected network at least one path between any pair of nodes except in times of failure Interior Router Protocol & Exterior Routing Protocol Exterior interior router protocol (IRP) passes routing information between routers within AS can be tailored to specific applications needs detailed model of network to function routing algorithms & tables may differ between them may have more than one AS in internet routers need info on networks outside own AS use an exterior router protocol (ERP) for this supports summary information on AS reachability Application of IRP and ERP Application Approaches to Routing – Distance-vector each node (router or host) exchange information each with neighboring nodes with first generation routing algorithm for ARPANET eg. used by Routing Information Protocol (RIP) each node maintains vector of link costs for each each directly attached network and distance and nextdirectly hop vectors for each destination requires transmission of much info by routers distance vector & estimated path costs changes take long time to propagate changes time Approaches to Routing – Link-state Link-state designed to overcome drawbacks of distance-vector each router determines link cost on each interface advertises set of link costs to all other routers in topology if link costs change, router advertises new values change new each router constructs topology of entire configuration can calculate shortest path to each dest use to construct routing table with first hop to each dest use routing do not use distributed routing algorithm, but any suitable but alg to determine shortest paths, eg. Dijkstra's algorithm determine eg. Open Shortest Path First (OSPF) is a link-state protocol What Exterior Routing Protocols are not Protocols link-state and distance-vector not effective for exterior router protocol exterior distance-vector assumes routers share common distance metric but different ASs may have different priorities & needs but have no info on AS’s visited along route different ASs may use different metrics and have different different restrictions different flooding of link state information to all routers flooding unmanageable link-state Exterior Router Protocols – Exterior Path-vector alternative path-vector routing protocol alternative path-vector provides info about which networks can be reached provides by a given router and ASs crossed to get there by does not include distance or cost estimate distance hence dispenses with concept of routing metrics have list of all ASs visited on a route enables router to perform policy routing eg. avoid path to avoid transiting particular AS eg. avoid eg. link speed, capacity, tendency to become congested, and overall quality of operation, security congested, eg. minimizing number of transit ASs eg. minimizing Border Gateway Protocol (BGP) (BGP) developed for use with TCP/IP internets is preferred EGP of the Internet uses messages sent over TCP connection current version is BGP-4 (RFC1771) functional procedures neighbor acquisition - when agree to exchange info neighbor reachability - to maintain relationship network reachability - to update database of routes BGP Messages Messages Open Update Keep alive Notification Message Types Message Open & KeepAlive router makes TCP connection to neighbor Open message sent by connection initiator includes proposed hold time receiver uses minimum of own/sent hold time receiver max time between Keepalive and/or Update To tell other routers that this router is still here Keep Alive message Message Types - Update Message Update message conveys two info types: Info about single routes through internet List of routes being withdrawn Network Layer Reachability Information (NLRI) Total Path Attributes Length Total Path Attributes info on a route uses 3 fields: withdraw route identified by dest IP address Message Types - Update Message Origin - IGP or EGP AS_Path - list of AS traversed Next_hop - IP address of border router Multi_Exit_Disc - info on routers internal to AS Local_pref - inform routers in AS of route pref Atomic_Aggregate, Aggregator - implement Atomic_Aggregate, route aggregation to reduce amount of info route AS_Path and Next_Hop Use AS_Path AS_Path used to implement routing policies
• eg. to avoid a particular AS, security, performance, eg. quality, number of AS crossed quality, Next_Hop only a few routers implement BGP responsible for informing outside routers of responsible routes to other networks in AS routes Notification Message Notification sent when some error condition detected: Message header error Open message error Update message error Hold time expired Finite state machine error Cease BGP Routing Information Exchange Exchange within AS a router builds topology picture within using IGP using router issues Update message to other router routers outside AS using BGP routers these routers exchange info with other these routers in other AS routers AS_Path field used to prevent loops routers must then decide best routes Open Shortest Path First (RFC2328) (RFC2328) IGP of Internet replaced Routing Information Protocol (RIP) uses Link State Routing Algorithm each router keeps list of state of local links to network transmits update state info little traffic as messages are small and not sent often uses least cost based on user cost metric topology stored as directed graph vertices or nodes (router, transit or stub network) edges (between routers or router to network) Example OSPF AS OSPF Directed Graph of AS AS SPF Tree SPF for for Router 6 Router Integrates Services Architecture Architecture changes in traffic demands require variety changes of quality of service eg. internet phone, multimedia, multicast new functionality required in routers new means of requesting QoS IETF developing a suite of Integrated IETF Services Architecture (ISA) standards Services RFC 1633 defines overall view of ISA Internet Traffic Categories Internet elastic traffic can cope with wide changes in delay and/or can throughput throughput traditional TCP/IP traffic eg. FTP, email, telnet, SNMP, HTTP different sensitivity to throughput, delay, different congestion congestion does not easily adapt to variations inelastic traffic Inelastic Traffic Requirements Inelastic throughput delay jitter packet loss need preferential treatment for some traffic need types types require elastic traffic to be supported ISA Approach ISA IP nets control congestion by routing algorithms packet discard ISA provides enhancements to traditional IP in ISA associate each packet with a flow ISA functions: admission control routing algorithm queuing discipline discard policy ISA in Router in ISA Services ISA Guaranteed assured data rate upper bound on queuing delay no queuing loss approximates best effort behavior on unloaded net no specific upper bound on queuing delay very high delivery success traditional IP service Controlled load Best Effort Token Bucket Scheme Token Queuing Discipline Queuing traditionally FIFO no special treatment for high priority flow packets large packet can hold up smaller packets greedy connection can crowd out less greedy greedy connection connection multiple queues used on each output port packet is placed in queue for its flow round robin servicing of queues can have weighted fair queuing need some form of fair queuing FIFO and Fair Queue FIFO Resource Reservation: RSVP Resource RFC 2205 unicast applications can reserve resources in unicast routers to meet QoS routers if router can not meet request, application informed some members of group may not require delivery some from particular source over given time from some group members may only be able to handle a some portion of the transmission portion reservation means routers can decide in advance if reservation can meet requirements can multicast more demanding, but may be reduced Soft State Soft have different resource reservation needs have to traditional connection-oriented networks to must dynamically change set of state info in router that expires unless set refreshed refreshed use concept of Soft State applications must periodically renew applications requests during transmission requests RSVP Characteristics RSVP unicast and multicast simplex receiver initiated reservation maintain soft state in the internet provide different reservation styles transparent operation through non-RSVP transparent routers routers support for IPv4 and IPv6 Differentiated Services Differentiated simple, easily implemented, low overhead tool to simple, support a range of differentiated network services support IP Packets labeled for differing QoS using existing IP IPv4 Type of Service or IPv6 DS field IPv4 have service level agreement established have between provider and customer prior to use of DS between have built in aggregation iimplemented by queuing and forwarding based on mplemented DS octet DS most widely used QoS mechanism today DS Domains DS DS Services DS is defined within a DS domain a contiguous portion of internet over which consistent contiguous set of DS policies are administered set typically under control of one organization defined by service level agreements (SLA) specify service received for classes of packets once established customer submits packets with once DS marked indicating class DS service provider ensures agreed QoS within domain if transit other domains, provider chooses closest QoS SLA Parameters SLA detailed service performance such as: expected throughput drop probability latency constraints on ingress and egress points traffic profiles disposition of traffic in excess of profile Example Services Example level A - low latency level B - low loss level C - 90% of traffic < 50ms latency level D - 95% in profile traffic delivered level E - allotted twice bandwidth of level F level F - with drop precedence X has level higher probability of delivery than that of Y higher DS Field DS DS Field - DS Codepoint DS 6 bit field in IPv4 & IPv6 header 3 pools of code points xxxxx0 - assignment as standards
• 000000 - default best effort • xxx000 - IPv4 precedence compatibility xxxx11 - experimental or local use xxxx01 - experimental or local but may be xxxx01 allocated for standards in future allocated IPv4 Precedence Service IPv4 IPv4 TOS field included subfields IPv4 precedence (3 bit) - datagram urgency/priority TOS(4 bit) - guidance on selecting next hop route selection - smaller queue, has priority network service - supports precedence queuing discipline - support precedence queuing ordered queueing & discard lower precedence ordered can respond with DS Configuration and Operation Operation within domain, interpretation of DS code within points is uniform points interior nodes implement simple mechanisms per-hop behavior (PHB) on all routers have PHB & more sophisticated mechanisms hence most of complexity boundary nodes DS Traffic Conditioner DS Per Hop Behavior – Expedited Forwarding specific PHBs defined expedited forwarding (EF) PHB (RFC 3246) llow-loss, low-delay, low-jitter, assured bandwidth, ow-loss, end-to-end service through DS domains end-to-end simulates a point-to-point connection or leased line queues on node/router result in loss, delays, and jitter queues on unless internet grossly oversized, care needed in handling premium service traffic difficult in internet or packet-switching network EF PHB intent is to use empty/short queues to EF minimise delay, jitter & packet loss. minimise Expedited Forwarding Expedited Requirements EF PHB designed to configure nodes so traffic EF aggregate has minimum departure rate aggregate border routers condition traffic aggregate (via border policing / shaping) so arrival rate is less than minimum departure rate for nodes minimum iinterior nodes treat traffic so no queuing effects nterior so no specific queuing policy set for interior nodes note a simple priority scheme can achieve this note this EF traffic given absolute priority EF traffic must not overwhelm interior node EF must but packet flows for other PHB traffic disrupted Assured Forwarding PHB Assured provide service superior to best-effort without needing reservation of resources or without reservation detailed flow discrimination detailed based on explicit allocation users offered choice of classes of service traffic monitored at boundary node, marked in/out iinside network, no separation of traffic from different nside users or classes users when congested, drop out packets before in packets when congested, different users will see different levels of service advantage is simplicity AF PHB RFC 2597 AF four AF classes / traffic profiles are defined within each class, packets marked with three within with hree drop precedence values drop in congestion determines relative importance simpler, more flexible than resource reservation simpler, flexible within interior DS node, traffic from different within classes is treated separately treated different resources (buffer space, data rate) hence forwarding assurance depends on hence resources, current load & drop precedence resources, Service Level Agreements Service is a contract between network provider is and customer for aspects of service and typically includes: description of nature of service expected performance level of service process for monitoring & reporting service process level level similar to frame relay / ATM SLA’s but more difficult to realize Service Level Agreements Service IP Performance Metrics IP IP Performance Metrics working group is IP developing a standard set of metrics developing on quality, performance, reliability to provide common understanding singleton metric - elementary / atomic quantity sample metric - taken over time period statistical metric - derived from sample 3 stages of metrics active or passive measurement IP Performance Metrics IP
Metric Name One-Way Delay Round-Trip Delay One-Way Loss One-Way Loss Pattern (distance / period) Packet Delay Variation Connectivity Bulk Transfer Capacity Summary Summary reviewed various internetwork services & reviewed functions to support varying services functions multicasting routing protocols integrated services architecture differentiated services service level agreements IP performance metrics ...
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This note was uploaded on 04/06/2011 for the course EE 5363 taught by Professor Kang during the Spring '09 term at NYU Poly.
- Spring '09