第5章osi層次傳輸層

OSILayer4:

TransportLayerOSILayer4:TransportLayerAnoverviewoflayer4

TCP(TransmissionControlProtocol)UDP(UserDatagramProtocol)Anapplication:NATandPATLayer4performsmultiplefunctions:segmentingupper-layerapplicationdataestablishingend-to-endoperationssendingsegmentsfromoneendhosttoanotherFlowcontrolandreliabilitycanbecomparedtotalkingtoaforeigner.Oftenyouwouldasktheforeignertorepeathis/herwords(reliability)andtospeakslowly(flowcontrol)OSILayer4:TransportLayer

TwoparticularlyimportantLayer4protocols:

TransmissionControlProtocol(TCP)UserDatagramProtocol(UDP)Layer4:TheTransportLayerDivideoutgoingmessagesintosegmentsReassemblemessagesatthedestinationstationTCP:reliableConnection-orientedSoftwarecheckingforsegmentRe-sendanythinglostorerrorUsesacknowledgmentsProvidesflowcontrolUDP:

unreliable

connectionless

providesnosoftwarecheckingforsegmentusesnoacknowledgmentsprovidesnoflowcontrolLayer4:TheTransportLayerBothTCPandUDPuseport

tokeeptrackofdifferentconversationsthatcrossthenetworkatthesametimeApplicationsoftwaredevelopershaveagreedtousethewell-knownportnumbersthataredefinedinRFC1700

Portnumbersbelow255arereservedforTCPandUDPpublicapplications.ServiceModelSocketispresentedas(IP_address,port)Everyconnectionisexpressedas(socketsource,socketdestination),whichisapoint-to-pointfull-duplexchannelDoesnotsupportmulticastandbroadcastSocketLayer4:TheTransportLayerAnoverviewoflayer4TCP(TransmissionControlProtocol)UDP(UserDatagramProtocol)Anapplication:NATandPATTCPServiceModelProblemsmustbesolvedinTCP:ReliabletransferFlowcontrolSlidingwindowcongestionavoidance…ConnectionmanagementEstablishconnection：threehandshakesReleaseconnection：fourhandshakesTCP首部20字節(jié)固定首部目的端口數(shù)據(jù)偏移檢驗和選項（長度可變）源端口序號緊急指針窗口確認(rèn)號保留FINSYNRSTPSHACKURG位08162431填充源端口和目的端口字段——各占2字節(jié)端口是運(yùn)輸層與應(yīng)用層的服務(wù)接口運(yùn)輸層的復(fù)用和分用功能都要通過端口才能實現(xiàn)TCP首部20字節(jié)固定首部目的端口數(shù)據(jù)偏移檢驗和選項（長度可變）源端口序號緊急指針窗口確認(rèn)號保留FINSYNRSTPSHACKURG位08162431填充序號字段——占4字節(jié)TCP傳送的數(shù)據(jù)流中的每一個字節(jié)都編上一個序號序號字段的值指本報文段所發(fā)送的數(shù)據(jù)的第一個字節(jié)的序號TCP首部20字節(jié)固定首部目的端口數(shù)據(jù)偏移檢驗和選項（長度可變）源端口序號緊急指針窗口確認(rèn)號保留FINSYNRSTPSHACKURG位08162431填充確認(rèn)號字段——占4字節(jié)，是期望收到對方的下一個報文段的數(shù)據(jù)的第一個字節(jié)的序號TCP首部20字節(jié)固定首部目的端口數(shù)據(jù)偏移檢驗和選項（長度可變）源端口序號緊急指針窗口確認(rèn)號保留FINSYNRSTPSHACKURG位08162431填充數(shù)據(jù)偏移（即首部長度）——占4位指出TCP報文段的數(shù)據(jù)起始處距TCP報文段的起始處的長度單位是32位字（以4字節(jié)為計算單位）TCP首部20字節(jié)固定首部目的端口數(shù)據(jù)偏移檢驗和選項（長度可變）源端口序號緊急指針窗口確認(rèn)號保留FINSYNRSTPSHACKURG位08162431填充保留字段——占6位，保留為今后使用，目前置0TCP首部20字節(jié)固定首部目的端口數(shù)據(jù)偏移檢驗和選項（長度可變）源端口序號緊急指針窗口確認(rèn)號保留FINSYNRSTPSHACKURG位08162431填充緊急URG1時，表明緊急指針字段有效告訴系統(tǒng)此報文段中有緊急數(shù)據(jù)，應(yīng)盡快傳送(相當(dāng)于高優(yōu)先級的數(shù)據(jù))

TCP首部20字節(jié)固定首部目的端口數(shù)據(jù)偏移檢驗和選項（長度可變）源端口序號緊急指針窗口確認(rèn)號保留FINSYNRSTPSHACKURG位08162431填充ACK1時確認(rèn)號字段有效ACK0時確認(rèn)號字段無效TCP首部20字節(jié)固定首部目的端口數(shù)據(jù)偏移檢驗和選項（長度可變）源端口序號緊急指針窗口確認(rèn)號保留FINSYNRSTPSHACKURG位08162431填充推送PSH(PuSH)——接收TCP收到PSH=1的報文段，就盡快地交付接收應(yīng)用進(jìn)程，而不再等到整個緩存都填滿了后再向上交付TCP首部20字節(jié)固定首部目的端口數(shù)據(jù)偏移檢驗和選項（長度可變）源端口序號緊急指針窗口確認(rèn)號保留FINSYNRSTPSHACKURG位08162431填充復(fù)位RST(ReSeT1時，表明TCP連接中出現(xiàn)嚴(yán)重差錯（如由于主機(jī)崩潰或其他原因），必須釋放連接，然后再重新建立運(yùn)輸連接TCP首部20字節(jié)固定首部目的端口數(shù)據(jù)偏移檢驗和選項（長度可變）源端口序號緊急指針窗口確認(rèn)號保留FINSYNRSTPSHACKURG位08162431填充同步SYN=1表示這是一個連接請求或連接接受報文TCP首部20字節(jié)固定首部目的端口數(shù)據(jù)偏移檢驗和選項（長度可變）源端口序號緊急指針窗口確認(rèn)號保留FINSYNRSTPSHACKURG位08162431填充終止FIN(FINis)——用來釋放一個連接。FIN1表明此報文段的發(fā)送端的數(shù)據(jù)已發(fā)送完畢，并要求釋放運(yùn)輸連接。TCP首部20字節(jié)固定首部目的端口數(shù)據(jù)偏移檢驗和選項（長度可變）源端口序號緊急指針窗口確認(rèn)號保留FINSYNRSTPSHACKURG位08162431填充窗口字段——占2字節(jié)，用來讓對方設(shè)置發(fā)送窗口的依據(jù)，單位為字節(jié)。TCP首部20字節(jié)固定首部目的端口數(shù)據(jù)偏移檢驗和選項（長度可變）源端口序號緊急指針窗口確認(rèn)號保留FINSYNRSTPSHACKURG位08162431填充檢驗和——占2字節(jié)。檢驗和字段檢驗的范圍包括首部和數(shù)據(jù)這兩部分TCP首部20字節(jié)固定首部目的端口數(shù)據(jù)偏移檢驗和選項（長度可變）源端口序號緊急指針窗口確認(rèn)號保留FINSYNRSTPSHACKURG位08162431填充緊急指針字段——占16位，指出在本報文段中緊急數(shù)據(jù)共有多少個字節(jié)（緊急數(shù)據(jù)放在本報文段數(shù)據(jù)的最前面）

TCP首部20字節(jié)固定首部目的端口數(shù)據(jù)偏移檢驗和選項（長度可變）源端口序號緊急指針窗口確認(rèn)號保留FINSYNRSTPSHACKURG比特08162431填充TCP最初只有一種選項，即最大報文段長度MSS(MaximumSegmentSize)MSS告訴對方緩存所能接收的報文段的數(shù)據(jù)字段的最大長度是MSS個字節(jié)數(shù)據(jù)字段加上TCP首部才等于整個的TCP報文段。TCP首部20字節(jié)固定首部目的端口數(shù)據(jù)偏移檢驗和選項（長度可變）源端口序號緊急指針窗口確認(rèn)號保留FINSYNRSTPSHACKURG位08162431填充填充字段——這是為了使整個首部長度是4字節(jié)的整數(shù)倍。TCPProtocolHostsexchangedatabyusingsegment(TPDU)Eachsegmenthas:aheaderof20bytes(exceptoptionalparts)

0ormoredatabytesThesizeofthesegmentmustbematchedwithIPpackets,andalsomustsatisfythedemandofbottomlayersForexample,theMTU(MaximalTransferUnit)ofEthernetis1500bytesEachbytehasa32bitssequencenumberReliableConnection?

Two-armyProblem

TCP:EstablishConnectionSYN=1,seq=xClosedClosedActiveopenPassiveopenABClientServerTheFirstHandshakeServer:executesLISTEN

andACCEPTprimitive,andmonitorspassivelyClient:executesCONNECTprimitive,generateaTCPsegmentwithSYN=1andACK=0,whichstandsforconnectionrequestSYN=1,seq=xSYN=1,ACK=1,seq=y,ack=x1TheSecondHandshakeServerchecksifexistsserviceprocessmonitoringtheportIfnoneprocess,answeraTCPsegmentwithRST=1Ifexistsprocess,decidestorejectortoaccepttherequestIfaccepttheconnectionrequest,sendasegmentwithSYN=1andACK=1ClosedClosedActiveopenPassiveopenABClientServer

TCP:EstablishConnectionSYN=1,seq=xACK=1,seq=x+1,ack=y1SYN=1,ACK=1,seq=y,ack=x1TheThirdHandshakeTheclientsendsasegmentwithSYN=0andACK=1

toacknowledgetheconnectionClosedClosedActiveopenPassiveopenABClientServer

TCP:EstablishConnectionSYN=1,seq=xACK=1,seq=x+1,ack=y1DataTransferSYN=1,ACK=1,seq=y,ack=x1Whentheserverreceivestheacknowledgement,itinformstheupperlayerapplications

TCP:EstablishConnectionClosedClosedActiveopenPassiveopenABClientServerSYN-SENTESTAB-LISHEDSYN-RCVDLISTENESTAB-LISHEDSYN=1,seq=xACK=1,seq=x+1,ack=y1DataTransferSYN=1,ACK=1,seq=y,ack=x1

TCP:EstablishConnectionClosedClosedActiveopenPassiveopenABClientServerExample:EstablishConnection

TCPATCPB1.CLOSEDLISTEN2.SYN-SENT--><SEQ=100><CTL=SYN>-->SYN-RECEIVED3.ESTABLISHED<--<SEQ=300><ACK=101><CTL=SYN,ACK><--SYN-RECEIVED4.ESTABLISHED--><SEQ=101><ACK=301><CTL=ACK>-->ESTABLISHED5.ESTABLISHED--><SEQ=101><ACK=301><CTL=ACK><DATA>-->ESTABLISHEDBasic3-WayHandshakeforConnectionSynchronizationNotethattheACKdoesnotoccupysequencenumberspace(ifitdid,wewouldwindupACKingACK's!)Datatransfer

——stop-and-waitprotocol(a)NoerrorASend

M1ACK

M1BSend

M2Send

M3ACKM2ACK

M3ASend

M1BResend

M1Send

M2ACK

M1Disposethesegment(b)OutoftimettttDatatransfer

——stop-and-waitprotocolAftersendingasegment,preserveabackuptemporarilyEachsegmentandACKmusthaveIDTheresend-timemustbemorethanaverage-travel-time*2stop-and-waitprotocolisasimpleprotocol,buthaspoorefficiencyDatatransfer

——LostACKandLateACKASend

M1BOutoftimeResendM1SendM2DisposetherepeatedM1andresendACKM2(a)LostACKACK

M1ASend

M1BOutoftimeResendM1Send

M2DisposetherepeatedM1andresendACKM1(b)LateACKACKM1ReceivethelateACKanddonothingttttReliableCommunicationARQ(AutomaticRepeatreQuest)。Itmeansthe‘resendrequest’isautomaticallysentandthereceiverneednotrequestthesendertoresendtheerrorsegmentContiguousARQProtocol123456789101112(a)發(fā)送方維持發(fā)送窗口（發(fā)送窗口是5）發(fā)送窗口(b)收到一個確認(rèn)后發(fā)送窗口向前滑動向前123456789101112發(fā)送窗口SegmentttACKFIN=1,seq=uESTAB-LISHEDCLOSEDDataTransferESTAB-LISHEDTCP:ReleaseConnectionActiveClosePassiveCloseABClientServerFIN=1,seq=uACK=1,seq=v,ack=u1ESTAB-LISHEDDataTransferESTAB-LISHEDActiveCloseInformUpperLayerABClientServerTCP:ReleaseConnectionFIN=1,seq=uACK=1,seq=v,ack=u1FIN=1,ACK=1,seq=w,ack=u1PassiveCloseDataTransferESTAB-LISHEDDataTransferESTAB-LISHEDActiveCloseInformUpperLayerABClientServerTCP:ReleaseConnectionFIN=1,seq=uACK=1,seq=v,ack=u1FIN=1,ACK=1,seq=w,ack=u1ACK=1,seq=u+1,ack=w1TCP:ReleaseConnectionPassiveCloseDataTransferESTAB-LISHEDDataTransferESTAB-LISHEDActiveCloseInformUpperLayerABClientServerCLOSEDACK=1,seq=u+1,ack=w1FIN=1,seq=uACK=1,seq=v,ack=u1FIN=1,ACK=1,seq=w,ack=u1FIN-WAIT-1CLOSE-WAITFIN-WAIT-2LAST-ACKWait

2MSLTIME-WAITCLOSEDBeforereleasingconnection,theclientmustwaitfor2MSLTCP:ReleaseConnectionPassiveCloseDataTransferESTAB-LISHEDDataTransferESTAB-LISHEDActiveCloseInformUpperLayerABClientServerWhymustwaitfor2MSL?ToensurethelastACKsentbyAcanreachBTopreventanyinvalidconnectionrequestsegmentfromemergingAfterwaitingfor2MSL,wecanmakesurethatallsegmentsontheconnectionhavedisappearedT

C

P

的

有

限

狀

態(tài)

機(jī)CLOSEDESTABLISHEDLISTENCLOSE_WAITFIN_WAIT_1SYN_RCVDFIN_WAIT_2CLOSINGTIME_WAITSYN_SENTLAST_ACK主動打開被動打開被動關(guān)閉主動關(guān)閉起點被動打開主動打開發(fā)送SYN同時打開收到SYN，發(fā)送SYN,ACK收到ACK數(shù)據(jù)傳送階段

關(guān)閉發(fā)送FIN

關(guān)閉發(fā)送FIN

關(guān)閉發(fā)送FIN收到RST

收到SYN發(fā)送SYN,ACK

關(guān)閉或超時收到ACK

收到SYN,ACK發(fā)送ACK收到ACK收到ACK收到FIN發(fā)送ACK收到FIN,ACK

發(fā)送ACK收到FIN發(fā)送ACK同時關(guān)閉收到FIN發(fā)送ACK發(fā)送SYN定時經(jīng)過兩倍報文段壽命后關(guān)閉Layer4:TheTransportLayerAnOverviewofLayer4TCP(TransmissionControlProtocol)UDP(UserDatagramProtocol)Anapplication:NATandPATUDP(UserDatagramProtocol)WhydoweneedUDP?Noconnectionestablishment(whichcanadddelay)Simple:noconnectionstateatsender,receiverSmallsegmentheaderNocongestioncontrol:UDPcanblastawayasfastasdesiredUDP(UserDatagramProtocol)Connectionless:nohandshakingbetweenUDPsender,receivereachUDPsegmenthandledindependentlyofothersOftenusedforstreamingmultimediaapplicationslosstolerantratesensitiveUDPareusedin:RIP:TosendtherouteinformationperiodicallyDNS:AvoidthedelaytosetuptheTCPconnectionSNMP:Whencongestion,SNMPmuststillrunable.Withoutthecongestionandreliabilitycontrolmechanism,UDPhasbetterperformancethanTCPunderthecircumstances.OtherprotocolsincludeTFTP,DHCPAddreliabilityatapplicationlayerifnecessaryUDP(UserDatagramProtocol)Layer4:TheTransportLayerAnOverviewofLayer4TCP(TransmissionControlProtocol)UDP(UserDatagramProtocol)Anapplication:NATandPATNAT,istheprocessofswappingoneaddressforanotherintheIPpacketheaderInpractice,NATisusedtoallowhoststhatareprivatelyaddressedtoaccesstheInternetOneofsolutionstoIPaddressdepletionConservesregistered(legal)addressesIncreasesFlexibilitywhenconnectingtoInternetRFC1631-

NetworkAddressTranslator(NAT)WhatisNAT?NATasimpleconceptPrivateaddresses

Registeredaddresses

NATtypesStaticNAT: FixedmappingofaninternaladdresstoanregisteredaddressDynamicNAT: Mappingisdonedynamicallyonafirstcomefirst

servedbasisPAT(Overload):

Portaddresstranslationisusedtoallowmanyinternaluserstoshareasingle‘insideglobal’