课件第04章网络层简介

1、 1Chapter 4 Network LayerIntroductionSwitchingPacket switching at network layerNetwork layer servicesOther network layer issuesSummary 24.1 IntroductionInternet不是一种新的网络建立在底层网络上的网际网底层网络“物理网”网际网“互联网”Black boxCombination of LANs and WANs connected togetherConnecting devicesSwitching，交换 34.2 Switching（交

2、换）Circuit Switching（电路交换）Example: the early telephone systemsPacket Switching （分组交换）Example: the network layer in the InternetApproachesDatagram （数据报）Virtual Circuit （ VC，虚电路） 4Circuit SwitchingConnection setupData transferConnection release 54.3 Packet Switching At N. L.Connectionless network - dat

3、agramConnection-oriented network - virtual circuitABYXIIIIIIIV12321ABYXIIIIIIIV2313-phasesetup, data transfer, teardown 6Connection-oriented vs. Connectionless ServicesService（服务）Network application：业务OSI/RM：下层为上层提供的一组操作TCP/IP：通信协议 + 层间接口操作面向连接无连接通信过程建立连接交付分组终止连接交付分组路由选择仅在建立连接时选路每个分组独立选路传输路径相同可以不同Fo

4、rwarding ProcessConnectionless network - datagramDecision Routing table Connection-oriented network - virtual circuitDecisionRouting table 7Destination addressOutput interfaceIn PortOut PortIn LabelOut LabelSADataDASADataDALabelDelay（时延）Connectionless network - datagramConnection-oriented network -

5、virtual circuit 8传输时间等待时间等待时间总时延传输时间建链总时延拆链 94.4 Network Layer ServicesConnectionless service dominant service Services Provided at the Source ComputerPacketizingFinding the logical address of the next hopFinding the physical (MAC) address of the next hopFragmenting the datagram if necessaryServices

6、 Provided at Each RouterValidating each datagram or each fragmentSame as the last 3 processes mentioned for a sourceServices Provided at the Destination ComputerValidating each datagram or each fragmentDepacketizingReassembling the datagram if necessary 10Source Computer次序？ Each Router11次序？ Destinat

7、ion Computer124.5 Other Network Layer IssuesError control，差错控制Flow control，流量控制Congestion control，拥塞控制Quality of Service (QoS)，服务质量To support multimedia communicationBe mostly implemented in the upper layerRouting，路由选择To make, maintain, and update the routing tableSecurity，安全性IPsec 13Error Control，差

8、错控制Detecting and Correcting errorscorrupted, lost, or duplicate datagramsNetwork layer in the InternetOmitting the error checkingTo make this layer operate simply and fastFragmentation would make error checking at this layer very inefficientChecksum：control any corruption in the headerICMP：report so

9、me errors 14Flow Control，流量控制Regulating the amount of data a source can send without overwhelming the receiverNetwork layer in the InternetNo flow controlThe receiver is so simple that it may rarely be overwhelmedThe upper layers can implement buffers to receive dataMost of the upper layer protocols

10、 provide flow control 15Congestion Control，拥塞控制Congestion（拥塞）Too many datagrams are present in an area of the InternetCC in a connectionless networkTo inform the sender to slow down the sending of packetsSignaling：backward vs. forwardChoke packet（扼流分组）：ICMPRoutersDrop the less important packetsCC in a connection-oriented network：easierAdvanced negotiat