《计算机网络与通讯》课程教学资源（PPT课件讲稿，英文版）Chapter 4 Network Layer

Chapter 4: Network Layer Chapter goals: Overview: o understand principles D network layer services behind network layer services: 口 hierarchical routing o routing(path selection) IP o dealing with scale O what's inside a router? o how a router works o advanced topics IPv6 O IPv6 multicast g instantiation and implementation in the Internet 4: Network Layer 4a-1

4: Network Layer 4a-1 Chapter 4: Network Layer Chapter goals:  understand principles behind network layer services:  routing (path selection)  dealing with scale  how a router works  advanced topics: IPv6, multicast  instantiation and implementation in the Internet Overview:  network layer services  hierarchical routing  IP  what’s inside a router?  IPv6

Network layer functions o transport packet from pplication sending to receiving hosts g network layer protocols in network eve ery host, router data link physical ata link three important functions ●●● data link o path determination: route physical taken by packets from source network to dest. routing algorithms data link physical o switching: move packets from routers input to appropriate router output data link o call setup: some network architectures require router call setup along path before data flows 4: Network Layer 4a-2

4: Network Layer 4a-2 Network layer functions  transport packet from sending to receiving hosts  network layer protocols in every host, router three important functions:  path determination: route taken by packets from source to dest. Routing algorithms  switching: move packets from router’s input to appropriate router output  call setup: some network architectures require router call setup along path before data flows 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 application transport network data link physical application transport network data link physical

Network service model Q: What service model for channel transporting packets The most important abstraction provided from sender to by network layer receiver? s o guaranteed bandwidth? virtual circuit t o preservation of inter-packet or timing(no jitter)? datagram? go loss-free delivery o in-order delivery? 5 o congestion feedback to sender? 4: Network Layer 4a-3

4: Network Layer 4a-3 Network service model Q: What service model for “channel” transporting packets from sender to receiver?  guaranteed bandwidth?  preservation of inter-packet timing (no jitter)?  loss-free delivery?  in-order delivery?  congestion feedback to sender? ? ? ? virtual circuit or datagram? The most important abstraction provided by network layer:

Virtual circuits source-to-dest path behaves much like telephone circuit o performance-WIse o network actions along source-to-dest path o call setup, teardown for each call before data can flow D each packet carries VC identifier(not destination host OD) o every router on source-dest path s maintain state"for each passing connection o transport-layer connection only involved two end systems o link, router resources(bandwidth, buffers)may be allocated to vc o to get circuit-like perf 4: Network layer 4a-4

4: Network Layer 4a-4 Virtual circuits  call setup, teardown for each call before data can flow  each packet carries VC identifier (not destination host OD)  every router on source-dest path s maintain “state” for each passing connection  transport-layer connection only involved two end systems  link, router resources (bandwidth, buffers) may be allocated to VC  to get circuit-like perf. “source-to-dest path behaves much like telephone circuit”  performance-wise  network actions along source-to-dest path

Virtual circuits: signaling protocols D used to setup maintain teardown Vc O used in ATM, frame-relay, X25 g not used in today' s Internet applICaTion transport 5, Data flow begins 6. Receive data ap application network 4.Call connected transpon↑ 3. Accept ce data link 1 Initiate call network 2. incoming c data link physIca physical 4: Network Layer 4a-5

4: Network Layer 4a-5 Virtual circuits: signaling protocols  used to setup, maintain teardown VC  used in ATM, frame-relay, X.25  not used in today’s Internet application transport network data link physical application transport network data link physical 1. Initiate call 2. incoming call 4. Call connected 3. Accept call 5. Data flow begins 6. Receive data

Datagram networks: the Internet model o no call setup at network layer routers: no state about end-to-end connections o no network-level concept of"connection D packets typically routed using destination host ID o packets between same source-dest pair may take different paths applIcaTion fransport application network fransport data link 1.Send data neTwor k 2. Receive data data link physical t physical I 4: Network Layer 4a-6

4: Network Layer 4a-6 Datagram networks: the Internet model  no call setup at network layer  routers: no state about end-to-end connections  no network-level concept of “connection”  packets typically routed using destination host ID  packets between same source-dest pair may take different paths application transport network data link physical application transport network data link physical 1. Send data 2. Receive data

Network layer service models: Network Service Guarantees Congestion Architecture Model Bandwidth Loss Order Timing feedback Internet best effort none no no no(inferred via loss) ATM CBR constant yes yes yes no rate congestion ATM VBR guaranteed yesyes yes no rate congestion ATM ABR guaranteed no yes no yes minimum ATM UBR none yes no no o Internet model being extented: Intserv diffserv o Chapter 6 4: Network Layer 4a-7

4: Network Layer 4a-7 Network layer service models: Network Architecture Internet ATM ATM ATM ATM Service Model best effort CBR VBR ABR UBR Bandwidth none constant rate guaranteed rate guaranteed minimum none Loss no yes yes no no Order no yes yes yes yes Timing no yes yes no no Congestion feedback no (inferred via loss) no congestion no congestion yes no Guarantees ?  Internet model being extented: Intserv, Diffserv  Chapter 6

Datagram or Vc network why? Internet ATM o data exchange among D evolved from telephony computers o human conversation: o elastic"service. no strict timing req o strict timing, reliability requirements 口"smar+" end systems (computers) o need for guaranteed service o can adapt, perform 口"dumb" end systems control, error recovery o telephones o simple inside network complexity at edge o complexity inside o many link types network o different characteristics o uniform service difficult 4: Network Layer 4a-8

4: Network Layer 4a-8 Datagram or VC network: why? Internet  data exchange among computers  “elastic” service, no strict timing req.  “smart” end systems (computers)  can adapt, perform control, error recovery  simple inside network, complexity at “edge”  many link types  different characteristics  uniform service difficult ATM  evolved from telephony  human conversation:  strict timing, reliability requirements  need for guaranteed service  “dumb” end systems  telephones  complexity inside network

Routing Routing protocol Goal: determine"good"path 5 (sequence of routers)thru network from source to dest B3-3 2 5 CA 2 Graph abstraction for routing algorithms O graph nodes are routers good"path o graph edges are o typically means minimum hysical links phy cost path o link cost: delay, $s cost o other defs possible or congestion level 4: Network Layer 4a-9

4: Network Layer 4a-9 Routing Graph abstraction for routing algorithms:  graph nodes are routers  graph edges are physical links  link cost: delay, $ cost, or congestion level Goal: determine “good” path (sequence of routers) thru network from source to dest. Routing protocol A D E B C F 2 2 1 3 1 1 2 5 3 5  “good” path:  typically means minimum cost path  other def’s possible

Routing Algorithm classification Global or decentralized Static or dynamic? information? Static Global: o routes change slowly over o all routers have complete time topology link cost info Dynamic 0 link state"algorithms o routes change more quickly Decentralized o periodIc updaTe o router knows physically o in response to link cost connected neigh g bors link changes costs to neighbors g iterative process of computation exchange of info with neighbors o distance vector"algorithms 4: Network Layer 4a-10

4: Network Layer 4a-10 Routing Algorithm classification Global or decentralized information? Global:  all routers have complete topology, link cost info  “link state” algorithms Decentralized:  router knows physically￾connected neighbors, link costs to neighbors  iterative process of computation, exchange of info with neighbors  “distance vector” algorithms Static or dynamic? Static:  routes change slowly over time Dynamic:  routes change more quickly  periodic update  in response to link cost changes