《计算机网络》课程教学资源（PPT课件讲稿，第三版）Chapter 04 网络层 Network Layer

Chapter 4: network layer chapter goals so understand principles behind network layer services. network layer service models forwarding versus routing how a router works routing(path selection) broadcast multicast ,s instantiation implementation in the internet Network Layer 4-2

Network Layer 4-2 Chapter 4: network layer chapter goals: ❖ understand principles behind network layer services: ▪ network layer service models ▪ forwarding versus routing ▪ how a router works ▪ routing (path selection) ▪ broadcast, multicast ❖ instantiation, implementation in the Internet

Chapter 4: outline 4.1 introduction 4.5 routing algorithms 4.2 virtual circuit and link state datagram networks distance vector 4.3 what's inside a router hierarchical routing 4.4 IP: Internet protocol 4.6 routing in the Internet datagram format RIP IPv4 addressing OSPF ICMP BGP IPv6 4.7 broadcast and multicast routing Network Layer 4-3

Network Layer 4-3 4.1 introduction 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 4.5 routing algorithms ▪ link state ▪ distance vector ▪ hierarchical routing 4.6 routing in the Internet ▪ RIP ▪ OSPF ▪ BGP 4.7 broadcast and multicast routing Chapter 4: outline

Network layer s transport segment from sending to receiving host data link 今 on sending side network data link data link data link physica physical encapsulates segments sica into datagrams physical physical go on receiving side to network data link data link delivers segments sical data link transport layer transport network layer protocols data link physical data link data link n every host router data link physical ohysical physical router examines header fields in all ip datagrams passing through it Network Layer 4-4

Network Layer 4-4 Network layer ❖ transport segment from sending to receiving host ❖ on sending side encapsulates segments into datagrams ❖ on receiving side, delivers segments to transport layer ❖ network layer protocols in every host, router ❖ router examines header fields in all IP datagrams passing through it application transport network data link physical application 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 network data link physical network data link physical

Two key network-layer functions ,o forwarding: move analogy: packets from routers input to appropriate s routing: process of router output planning trip from source to dest .o routing: determine route taken by packets s forwarding:, process from source to dest of getting throug single interchange routing algorithms Network Layer 4-5

Network Layer 4-5 Two key network-layer functions ❖ forwarding: move packets from router’s input to appropriate router output ❖ routing: determine route taken by packets from source to dest. ▪ routing algorithms analogy: ❖ routing: process of planning trip from source to dest ❖ forwarding: process of getting through single interchange

Interplay between routing and forwarding routing algorithm routing algorithm determines end-end-path through network local forwarding table forwarding table determines header value output link local forwarding at this router 01003 01012 01112 10011 value in arriving packets header 011 Network Layer 4-6

Network Layer 4-6 1 2 3 0111 value in arriving packet’s header routing algorithm local forwarding table header value output link 0100 0101 0111 1001 3 2 2 1 Interplay between routing and forwarding routing algorithm determines end-end-path through network forwarding table determines local forwarding at this router

Connection setup ,s 3rd important function in some network architectures: ATM, frame relay, X25 before datagrams flow, two end hosts and intervening routers establish virtual connection routers get involved network vs transport layer connection service: network: between two hosts(may also involve intervening routers in case of Vcs) transport: between two processes Network Layer 4-7

Network Layer 4-7 Connection setup ❖ 3rd important function in some network architectures: ▪ ATM, frame relay, X.25 ❖ before datagrams flow, two end hosts and intervening routers establish virtual connection ▪ routers get involved ❖ network vs transport layer connection service: ▪ network: between two hosts (may also involve intervening routers in case of VCs) ▪ transport: between two processes

Network service model Q: What service mode/ for " channel transporting datagrams from sender to receiver example services for example services for individual a flow of datagrams. datagrams: quaranteed deliver . s in-order datagram .s guaranteed delivery deliver with less than 40 msec quaranteed minimum d bandwidth to flow .. restrictions on changes in inter- packet spacing Network Layer 4-8

Network Layer 4-8 Network service model Q: What service model for “channel” transporting 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 ❖ restrictions on changes in inter￾packet spacing

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 Network Layer 4-9

Network Layer 4-9 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 ?

Chapter 4: outline 4.1 introduction 4.5 routing algorithms 4.2 virtual circuit and link state datagram networks distance vector 4.3 what's inside a router hierarchical routing 4.4 IP: Internet protocol 4.6 routing in the datagram format Internet IPv4 addressing RIP ICMP OSPF IPv6 BGP 4.7 broadcast and multicast routing Network Layer 4-10

Network Layer 4-10 4.1 introduction 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 4.5 routing algorithms ▪ link state ▪ distance vector ▪ hierarchical routing 4.6 routing in the Internet ▪ RIP ▪ OSPF ▪ BGP 4.7 broadcast and multicast routing Chapter 4: outline

onnection connection-ess service datagram network provides network-layer connection/ess service virtual-circuit network provides network- layer connection service ,o analogous to TCP/ UDP connecton-oriented 7 connectionless transport-layer services but a service: host-to-host no choice: network provides one or the other implementation: in network core Network Layer 4-11

Network Layer 4-11 Connection, connection-less service ❖ datagram network provides network-layer connectionless service ❖ virtual-circuit network provides network￾layer connection service ❖ analogous to TCP/UDP connecton-oriented / connectionless transport-layer services, but: ▪ service: host-to-host ▪ no choice: network provides one or the other ▪ implementation: in network core