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1、1Courtesy to Pearson Addison-Wesley because most of the slides are from Jim Kurose, Keith Ross, “Computer Networking: A Top Down Approach,” 5th edition Addison-Wesley. All material copyright 1996-2009. J.F Kurose and K.W. Ross, All Rights Reserved.Introduction to Computer Networks 計算機網概Chapter 4: Ne
2、twork Layer2Chapter 4: Network LayerChapter goals:?understand principles behind network layer services:?network layer service models?forwarding versus routing?how a router works?routing (path selection)?dealing with scale?advanced topics: IPv6, mobility?instantiation, implementation in the Internet3
3、Chapter 4: Network Layer?4. 1 Introduction?4.2 Virtual circuit and datagram networks?4.3 Whats 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 Broad
4、cast and multicast routing4Network 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 everyhost, router?router examines header fields in all IP datagrams passing
5、through itapplication transport network data link physicalapplication transport network data link physicalnetwork data link physicalnetwork data link physicalnetwork data link physicalnetwork data link physicalnetwork data link physicalnetwork data link physicalnetwork data link physicalnetwork data
6、 link physicalnetwork data link physicalnetwork data link physicalnetwork data link physical5Two Key Network-Layer Functions?forwarding:move packets from routers input to appropriate router output?routing:determine route taken by packets from source to dest. ?routing algorithmsanalogy:?routing: proc
7、ess of planning trip from source to dest?forwarding: process of getting through single interchange61230111value in arriving packets headerrouting algorithmlocal forwarding table header value output link0100 0101 0111 10013 2 2 1Interplay between routing and forwarding7Connection setup?3rdimportant f
8、unction in somenetwork architectures:?ATM, frame relay, X.25?before datagrams flow, two end hosts andintervening 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)?t
9、ransport: between two processes8Network service modelQ: What service modelfor “channel” transporting datagrams from sender to receiver?Example services for individual datagrams:?guaranteed delivery?guaranteed delivery with less than 40 msec delayExample services for a flow of datagrams:?in-order dat
10、agram delivery?guaranteed minimum bandwidth to flow?restrictions on changes in inter- packet spacing9Network layer service models:Network ArchitectureInternetATMATMATMATMService Modelbest effortCBRVBRABRUBRBandwidthnoneconstant rate guaranteed rate guaranteed minimum noneLossnoyesyesnonoOrdernoyesye
11、syesyesTimingnoyesyesnonoCongestion feedbackno (inferred via loss) no congestion no congestion yesnoGuarantees ?10Chapter 4: Network Layer?4. 1 Introduction?4.2 Virtual circuit and datagram networks?4.3 Whats inside a router?4.4 IP: Internet Protocol?Datagram format?IPv4 addressing?ICMP?IPv6?4.5 Rou
12、ting algorithms?Link state?Distance Vector?Hierarchical routing?4.6 Routing in the Internet?RIP?OSPF?BGP?4.7 Broadcast and multicast routing11Network layer connection and connection-less service?datagram network provides network-layer connectionless service?VC network provides network-layer connecti
13、on service?analogous to the transport-layer services, but:?service: host-to-host?no choice: network provides one or the other?implementation: in network core12Virtual circuits?call setup, teardown for each call beforedata can flow?each packet carries VC identifier (not destination host address)?ever
14、yrouter on source-dest path maintains “state” for each passing connection?link, router resources (bandwidth, buffers) may be allocated to VC (dedicated resources = predictable service)“source-to-dest path behaves much like telephone circuit”?performance-wise?network actions along source-to-dest path
15、13VC implementationa VC consists of:1.path from source to destination2.VC numbers, one number for each link along path3.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 table14Forwarding table122232123VC numberinterface numberIncoming interface Incoming VC # Outgoing interface Outgoing VC #1 12 3 22 2 63 1 18 3 7 2 17 1 97 3 87 Forwarding table in nor
