1Chapter5NetworkLayerCollegeofComputerScienceChongqingUniversity2CollegeofComputerScienceNetworkLayerChapter5–DesignIssues–RoutingAlgorithms–CongestionControl–QualityofService–Internetworking–NetworkLayeroftheInternet3CollegeofComputerScienceTheNetworkLayer•Responsiblefordeliveringpacketsbetweenendpointsovermultiplelinks•Thenetworklayeristhelowestlayerthatdealswithend-to-endtransmission.Itmust:-Knowaboutthetopologyofthecommunicationsubnet;-Takecaretochooseroutes,and-Dealwithdataexchangebetweendifferentnetworks.PhysicalLinkNetworkTransportApplication4CollegeofComputerScienceDesignIssues–Store-and-forwardpacketswitching»–Connectionlessservice–datagrams»–Connection-orientedservice–virtualcircuits»–Comparisonofvirtual-circuitsanddatagrams»5CollegeofComputerScienceStore-and-ForwardPacketSwitching•Hostssendpacketsintothenetwork;packetsareforwardedbyroutersISP’sequipmentTheenvironmentofthenetworklayerprotocols.6CollegeofComputerScienceServicesProvidedtotheTransportLayer•Thenetworkservicestothetransportlayergoals:–Theservicesshouldbeindependentoftheroutertechnology.–Thetransportlayershouldbeshieldedfromtherouterspresent.–TheuniformaddressesplanacrossLANsandWANs.•Connection-orientedorconnectionless?•-therouters'jobismovingpacketsaround.Thesubnetisinherentlyunreliable.Therefore,thehostsshoulddoerrorcontrolandflowcontrolthemselves.Theconclusionisconnectionless,withprimitivesSENDPACKETandRECEIVEPACKETandlittleelse.•-thesubnetserviceshouldbereliable,connection-oriented.•ThesetwocampsareexemplifiedbytheInternetandATM.7CollegeofComputerScienceConnectionlessService—Datagrams•Packetisforwardedusingdestinationaddressinsideit–DifferentpacketsmaytakedifferentpathsISP’sequipmentA’stable(initially)A’stable(later)C’sTableE’sTableDest.LineRoutingwithinadiagramsubnet8CollegeofComputerScienceConnection-Oriented—VirtualCircuits•Packetisforwardedalongavirtualcircuitusingtaginsideit–Virtualcircuit(VC)issetupaheadoftimeISP’sequipmentA’stableC’sTableE...
