《计算机网络——自顶向下方法与Internet特色》幻灯片Lecture31

Chapter3 TransportLayer Ourgoals understandprinciplesbehindtransportlayerservices multiplexing demultiplexingreliabledatatransferflowcontrolcongestioncontrol learnabouttransportlayerprotocolsintheInternet UDP connectionlesstransportTCP connection orientedtransportTCPcongestioncontrol Chapter3outline 3 1Transport layerservices3 2Multiplexinganddemultiplexing3 3Connectionlesstransport UDP3 4Principlesofreliabledatatransfer 3 5Connection orientedtransport TCPsegmentstructurereliabledatatransferflowcontrolconnectionmanagement3 6Principlesofcongestioncontrol3 7TCPcongestioncontrol 3 1Transportservicesandprotocols providelogicalcommunicationbetweenappprocessesrunningondifferenthoststransportprotocolsruninendsystemssendside breaksappmessagesintosegments passestonetworklayerrcvside reassemblessegmentsintomessages passestoapplayermorethanonetransportprotocolavailabletoappsInternet TCPandUDP 3 1 1Tworklayer networklayer logicalcommunicationbetweenhoststransportlayer logicalcommunicationbetweenprocessesrelieson enhances networklayerservices Householdanalogy 12kidssendinglettersto12kidsprocesses kidsappmessages lettersinenvelopeshosts housestransportprotocol AnnandBillnetwork layerprotocol postalservice 3 1 2Internettransport layerprotocols TCP TransmissionControlProtocol andUDP UserDatagramProtocol SegmentandDatagram alsousedinNetworklayer Internet sNetworkLayerprotocol IP InternetProtocol IPprovideslogicalcommunicationbetweenhosts TheIPservicemodelisabest effortdeliveryservice 尽力而为交互服务 ThemostfundamentalresponsibilityofUDPandTCPistoextendIP sdeliveryservicebetweentwoendsystemtoadeliveryservicebetweentwoprocessesrunningontheendsystems Extendinghost to hostdeliverytoprocess to processdeliveryiscalledtransportlayermultiplexing 多路复用 anddemultiplexing 多路分解 3 1 2Internettransport layerprotocols reliable in orderdelivery TCP congestioncontrolflowcontrolconnectionsetupunreliable unordereddelivery UDPno frillsextensionof best effort IPservicesnotavailable delayguaranteesbandwidthguarantees Chapter3outline 3 1Transport layerservices3 2Multiplexinganddemultiplexing3 3Connectionlesstransport UDP3 4Principlesofreliabledatatransfer 3 5Connection orientedtransport TCPsegmentstructurereliabledatatransferflowcontrolconnectionmanagement3 6Principlesofcongestioncontrol3 7TCPcongestioncontrol 3 2Multiplexing demultiplexing application transport network link physical P1 application transport network link physical application transport network link physical P2 P3 P4 P1 host1 host2 host3 process socket deliveringreceivedsegmentstocorrectsocket gatheringdatafrommultiplesockets envelopingdatawithheader laterusedfordemultiplexing 1 Howdemultiplexingworks hostreceivesIPdatagramseachdatagramhassourceIPaddress destinationIPaddresseachdatagramcarries1transport layersegmenteachsegmenthassource destinationportnumberhostusesIPaddresses portnumberstodirectsegmenttoappropriatesocket sourceport destport 32bits applicationdata message otherheaderfields TCP UDPsegmentformat 2 Connectionlessdemultiplexing Createsocketswithportnumbers DatagramSocketmySocket1 newDatagramSocket 99111 DatagramSocketmySocket2 newDatagramSocket 99222 UDPsocketidentifiedbytwo tuple destIPaddress destportnumber WhenhostreceivesUDPsegment checksdestinationportnumberinsegmentdirectsUDPsegmenttosocketwiththatportnumberIPdatagramswithdifferentsourceIPaddressesand orsourceportnumbersdirectedtosamesocket 2 Connectionlessdemux cont DatagramSocketserverSocket newDatagramSocket 6428 SPprovides returnaddress 3 Connection orienteddemux TCPsocketidentifiedby4 tuple sourceIPaddresssourceportnumberdestIPaddressdestportnumberrecvhostusesallfourvaluestodirectsegmenttoappropriatesocket ServerhostmaysupportmanysimultaneousTCPsockets eachsocketidentifiedbyitsown4 tupleWebservershavedifferentsocketsforeachconnectingclientnon persistentHTTPwillhavedifferentsocketforeachrequest 3 Connection orienteddemux cont ClientIP B serverIP C SP 9157 DP 80 D IP C S IP A D IP C S IP B D IP C S IP B 4 Connection orienteddemux ThreadedWebServer ClientIP B serverIP C SP 9157 DP 80 P4 D IP C S IP A D IP C S IP B D IP C S IP B Chapter3outline 3 1Transport layerservices3 2Multiplexinganddemultiplexing3 3Connectionlesstransport UDP3 4Principlesofreliabledatatransfer 3 5Connection orientedtransport TCPsegmentstructurereliabledatatransferflowcontrolconnectionmanagement3 6Principlesofcongestioncontrol3 7TCPcongestioncontrol 3 3UDP UserDatagramProtocol RFC768 nofrills barebones Internettransportprotocol besteffort service UDPsegmentsmaybe lostdeliveredoutofordertoappconnectionless nohandshakingbetweenUDPsender receivereachUDPsegmenthandledindependentlyofothers WhyisthereaUDP Finerapp levelcontroloverwhatdataaresent andwhen noconnectionestablishment whichcanadddelay simple noconnectionstateatsender receiversmallsegmentheadernocongestioncontrol UDPcanblastawayasfastasdesired 3 3 1UDPSegmentstructure oftenusedforstreamingmultimediaappslosstolerantratesensitiveotherUDPusesDNSSNMPreliabletransferoverUDP addreliabilityatapplicationlayerapplication specificerrorrecovery sourceport destport 32bits Applicationdata message UDPsegmentformat length checksum Length inbytesofUDPsegment includingheader 3 3 2UDPchecksum Sender treatsegmentcontentsassequenceof16 bitintegerschecksum addition 1 scomplementsum ofsegmentcontentssenderputschecksumvalueintoUDPchecksumfield Receiver computechecksumofreceivedsegmentcheckifcomputedchecksumequalschecksumfieldvalue NO errordetectedYES noerrordetected Butmaybeerrorsnonetheless Morelater Goal detec