Chap4.1 Network Layer

Chap4.1 Network Layer - 3/16/10 Chapter 4: Network...

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Unformatted text preview: 3/16/10 Chapter 4: Network Layer Chapter goals: •  understand principles behind network layer services: –  network layer service models –  forwarding versus rouEng –  how a router works –  rouEng (path selecEon) –  dealing with scale –  advanced topics: IPv6, mobility Network Layer Part 1 Many slides in this document adapted from materials for Kurose and Ross, Computer Networking, 5th edition. copyright 1996-2009 J.F Kurose and K.W. Ross, All Rights Reserved •  instanEaEon, implementaEon in the Internet Network Layer 4 ­2 Network layer •  transport segment from sending to receiving host •  on sending side encapsulates segments into datagrams •  on rcving side, delivers segments to transport layer •  network layer protocols in every host, router •  router examines header fields in all IP datagrams passing through it applicaEon transport network data link physical network data link physical network data link physical network data link physical network data link physical network data link physical network data link physical network data link physical network data link physical network data link physical Interplay between rouEng and forwarding routing algorithm local forwarding table header value output link 0100 0101 0111 1001 3 2 2 1 network data link physical network data link physical applicaEon transport network data link physical value in arriving packet’s header 0111 1 32 Network Layer 4 ­3 Network Layer 4 ­4 ConnecEon setup •  3rd important funcEon in some network architectures: Network service model Q: What service model for “channel” transporEng datagrams from sender to receiver? Example services for individual datagrams: •  guaranteed delivery •  guaranteed delivery with less than 40 msec delay Example services for a flow of datagrams: •  in ­order datagram delivery •  guaranteed minimum bandwidth to flow •  restricEons on changes in inter ­packet spacing –  ATM, frame relay, X.25 •  before datagrams flow, two end hosts and intervening routers establish virtual connecEon –  routers get involved •  network vs transport layer connecEon service: –  network: between two hosts (may also involve intervening routers in case of VCs) –  transport: between two processes Network Layer 4 ­5 Network Layer 4 ­6 3/16/10 Network layer service models: Network Architecture Internet ATM ATM ATM ATM Service Model Guarantees ? Congestion Bandwidth Loss Order Timing feedback no yes yes no no no yes yes yes yes no yes yes no no no (inferred via loss) no congestion no congestion yes no best effort none CBR VBR ABR UBR constant rate guaranteed rate guaranteed minimum none Network layer connecEon and connecEon ­less service •  datagram network provides network ­layer connecEonless service •  VC network provides network ­layer connecEon service •  analogous to the transport ­layer services, but: –  service: host ­to ­host –  no choice: network provides one or the other –  implementaEon: in network core Network Layer 4 ­7 Network Layer 4 ­8 Virtual circuits “source ­to ­dest path behaves much like telephone circuit” –  performance ­wise –  network acEons along source ­to ­dest path •  call setup, teardown for each call before data can flow •  each packet carries VC idenEfier (not desEnaEon host address) •  every router on source ­dest path maintains “state” for each passing connecEon •  link, router resources (bandwidth, buffers) may be allocated to VC (dedicated resources = predictable service) VC implementaEon a VC consists of: 1.  path from source to desEnaEon 2.  VC numbers, one number for each link along path 3.  entries in forwarding tables in routers along path •  packet belonging to VC carries VC number (rather than dest address) •  VC number can be changed on each link. –  New VC number comes from forwarding table Network Layer 4 ­9 Network Layer 4 ­10 Forwarding table VC number 12 22 32 Virtual circuits: signaling protocols 3 1 2 Forwarding table in northwest router: interface number •  used to setup, maintain teardown VC •  used in ATM, frame ­relay, X.25 •  not used in today’s Internet Incoming interface Incoming VC # Outgoing interface Outgoing VC # 1 12 3 22 2 63 1 18 3 7 2 17 1 97 3 87 … … … … applicaEon transport 5. Data flow begins network 4. Call connected data link 1. IniEate call physical 6. Receive data 3. Accept call 2. incoming call applicaEon transport network data link physical Routers maintain connecEon state informaEon! Network Layer 4 ­11 Network Layer 4 ­12 3/16/10 Datagram networks •  no call setup at network layer •  routers: no state about end ­to ­end connecEons –  no network ­level concept of “connecEon” Forwarding table 4 billion possible entries Destination Address Range 11001000 00010111 00010000 00000000 through 11001000 00010111 00010111 11111111 11001000 00010111 00011000 00000000 through 11001000 00010111 00011000 11111111 11001000 00010111 00011001 00000000 through 11001000 00010111 00011111 11111111 otherwise Network Layer Link Interface 0 •  packets forwarded using desEnaEon host address –  packets between same source ­dest pair may take different paths 1 applicaEon transport network data link physical 1. Send data 2. Receive data applicaEon transport network data link physical 4 ­13 2 3 4 ­14 Network Layer Longest prefix matching Prefix Match 11001000 00010111 00010 11001000 00010111 00011000 11001000 00010111 00011 otherwise Examples DA: 11001000 00010111 00010110 10100001 DA: 11001000 00010111 00011000 10101010 Which interface? Which interface? Link Interface 0 1 2 3 Datagram or VC network: why? Internet (datagram) •  data exchange among computers •  evolved from telephony –  “elasEc” service, no strict •  human conversaEon: Eming req. –  strict Eming, reliability •  “smart” end systems (computers) requirements –  can adapt, perform control, –  need for guaranteed error recovery service –  simple inside network, •  “dumb” end systems complexity at “edge” –  telephones •  many link types –  complexity inside network –  different characterisEcs –  uniform service difficult ATM (VC) Network Layer 4 ­15 Network Layer 4 ­16 Chapter 4: Network Layer •  4. 1 IntroducEon •  4.2 Virtual circuit and datagram networks •  4.3 What’s inside a router •  4.4 IP: Internet Protocol –  –  –  –  Datagram format IPv4 addressing ICMP IPv6 Router Architecture Overview Two key router funcEons: •  •  run rouEng algorithms/protocol (RIP, OSPF, BGP) forwarding datagrams from incoming to outgoing link •  4.5 RouEng algorithms –  Link state –  Distance Vector –  Hierarchical rouEng •  4.6 RouEng in the Internet –  RIP –  OSPF –  BGP •  4.7 Broadcast and mulEcast rouEng Network Layer 4 ­17 Network Layer 4 ­18 3/16/10 Input Port FuncEons Content Addressable memory (CAM) Physical layer: bit ­level recepEon Data link layer: e.g., Ethernet see chapter 5 Decentralized switching: •  given datagram dest., lookup output port using forwarding table in input port memory •  goal: complete input port processing at ‘line speed’ •  queuing: if datagrams arrive faster than forwarding rate into switch fabric Network Layer 4 ­19 Three types of switching fabrics Switching Via Memory First generaEon routers: •  tradiEonal computers with switching under direct control of CPU • packet copied to system’s memory •  speed limited by memory bandwidth (2 bus crossings per datagram) Memory Input Port Output Port System Bus Network Layer 4 ­21 Network Layer 4 ­22 Switching Via a Bus •  datagram from input port memory to output port memory via a shared bus •  bus contenEon: switching speed limited by bus bandwidth •  32 Gbps bus, Cisco 5600: sufficient speed for access and enterprise routers Switching Via An InterconnecEon Network •  overcome bus bandwidth limitaEons •  Banyan networks, other interconnecEon nets iniEally developed to connect processors in mulEprocessor •  advanced design: fragmenEng datagram into fixed length cells, switch cells through the fabric. •  Cisco 12000: switches 60 Gbps through the interconnecEon network Network Layer 4 ­23 Network Layer 4 ­24 3/16/10 Output Ports Output port queueing •  Buffering required when datagrams arrive from fabric faster than the transmission rate •  Scheduling discipline chooses among queued datagrams for transmission •  buffering when arrival rate via switch exceeds output line speed •  queueing (delay) and loss due to output port buffer overflow! 4 ­25 Network Layer 4 ­26 Network Layer How much buffering? •  RFC 3439 rule of thumb: average buffering equal to “typical” RTT (say 250 msec) Emes link capacity C –  e.g., C = 10 Gps link: 2.5 Gbit buffer Input Port Queuing •  Fabric slower than input ports combined  ­> queueing may occur at input queues •  Head ­of ­the ­Line (HOL) blocking: queued datagram at front of queue prevents others in queue from moving forward •  queueing delay and loss due to input buffer overflow! •  Recent recommendaEon: with N flows, buffering equal to RTT C . N Network Layer 4 ­27 Network Layer 4 ­28 ...
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