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Transport layer



Introduction

ThetransportlayeristhefourthlayerintheOpenSystemsInterconnection(OSI)referencemodelproposedbytheInternationalOrganizationforStandardization.Thislayerofprotocolprovidesreliableandeffectivemessagetransferservicesbetweenprocessesonnetworkendpointhosts.Itsfunctioncloselydependsonthevirtualcircuitordatagramserviceofthenetworklayer.Thetransportlayerdefinestheend-to-endconnectivitybetweenhostapplications.Thetransportlayerisalsocalledthetransportlayer.Thetransportlayeronlyexistsintheopenendsystem.Itisalayerbetweenthelower3-layercommunicationsubnetsystemandthehigher3-layer,butitisaveryimportantlayerbecauseitisthesourceend.Thelastlayerthatcontrolsthedatatransmissionfromlowtohightothedestination.

Theserviceofthetransportlayergenerallyhastogothroughthreestages:thetransmissionconnectionestablishmentphase,thedatatransmissionphase,andthetransmissionconnectionreleasephasetocompleteacompleteserviceprocess.Inthedatatransmissionstage,itisdividedintotwoforms:generaldatatransmissionandaccelerateddatatransmission.ThetwomostcommonprotocolsinthetransportlayeraretheTransmissionControlProtocol(TCP)andtheUserDatagramProtocol(UDP).Thetransportlayerprovidesservicessuchaslogicalconnectionestablishment,transportlayeraddressing,datatransmission,transportconnectionrelease,flowcontrol,congestioncontrol,multiplexinganddemultiplexing,andcrashrecovery.

Portconcept

Thetaskofthetransportlayeristomakethebestuseofnetworkresourcesaccordingtothecharacteristicsofthecommunicationsubnettoprovideestablishmentandmaintenancebetweenthesessionlayersofthetwoendsystems.Andcancelthefunctionoftransmissionconnection,responsibleforend-to-endreliabledatatransmission.Atthislayer,theprotocoldataunitofinformationtransmissioniscalledsegmentormessage.

Thenetworklayeronlytransmitsthedatapacketssentbythesourcenodetothedestinationnodeaccordingtothenetworkaddress,whilethetransportlayerisresponsibleforreliablytransmittingthedatatothecorrespondingport.Theresourcesubnetinthecomputernetworkistheinitiatorandreceiverofcommunication,andeachdeviceinitiscalledanendpoint;thecommunicationsubnetprovidescommunicationservicesinthenetwork,andthedevicesinitarecallednodes.IntheOSIreferencemodel,thefollowingfourlayersareusedforcommunicationcontrol,buttheircontrolobjectsaredifferent.

Basicfunctions

Thetransportlayerprovidesend-to-endservicesbetweenhostapplicationprocesses.Thebasicfunctionsareasfollows:

(1)Splitandreorganizedata

(2)Addressingbyportnumber

(3)Connectionmanagement

(4)Errorcontrolandflowcontrol,errorcorrectionfunction

Thetransportlayershouldprovidethereliabilityofthecommunicationservicetothesessionlayertoavoiderrors,loss,delaytimedisorder,repetition,disorderandothererrorsofthemessage.

Typeofservice

ThetransportlayerisnotonlythehighestlayerresponsiblefordatacommunicationintheOSIlayermodel,butalsothemiddlelayerbetweenthelowerthreelayersfornetworkcommunicationandthehigherthreelayersforinformationprocessingFloor.Thislayerfillsupthegapbetweentheservicesrequiredbythehigh-levelandtheservicesprovidedbythenetworklayer,andshieldsthedetailsofthecommunicationsubnetfromthehigh-levelusers,sothatthehigh-levelusersonlyseeanend-to-end,end-to-endconnectionbetweenthetwotransmissionentities.Areliabledatapaththatcanbecontrolledandsetbytheuser.ServiceTypes

Theservicesprovidedbythetransportlayercanbedividedintotransmissionconnectionservicesanddatatransmissionservices.

☆Transmissionconnectionservice:Generally,foreachtransmissionconnectionrequiredbythesessionlayer,thetransmissionlayermustestablishacorrespondingconnectiononthenetworklayer.

☆Datatransmissionservice:Emphasizestheprovisionofconnection-orientedreliableservices(OSIonlybegantodeveloprelevantstandardsforconnectionlessservicesverylate),andprovidesflowcontrol,errorcontrolandsequencecontroltoachievetwoterminalsThereisnoerror,noloss,norepetition,andnodisorderinthemessagestransmittedbetweensystems.

Protocollevelandtransportlayerprotocol

Servicequality

Thetransportlayerserviceisreflectedbytheagreement,sothetransportlayerprotocolleveliscloselyrelatedtothenetworkservicequality.Accordingtothenatureoferrors,networkservicescanbedividedintothefollowingthreetypesaccordingtotheirquality:

☆Aservice:lowerrorrateconnection,thatis,withacceptableresidualerrorrateandfailurenotificationrate;

☆TypeBservice:higherrorrateconnection,thatis,withunacceptableresidualerrorrateandfailurenotificationrate;

☆TypeCservice:betweenTypeAserviceandTypeBservice.

Agreementlevel

Theacceptanceorunacceptabilityoftheerrorratedependsontheuser.Therefore,theclassificationofnetworkservicequalityisbasedonuserrequirements.Accordingtothefunctionalcharacteristicsofthetransportlayer,OSIdefinesthefollowingfiveprotocollevels:

☆Level0:Simpleconnection.Onlyasimpleend-to-endtransmissionconnectionisestablished,andlongmessagescanbetransmittedinsegments.

☆Level1:Basicerrorrecoverylevel.Ithasarecoveryfunctionwhenthenetworkconnectionisdisconnected,thenetworkconnectionfails,oranunrecognizedtransmissionconnectiondataunitisreceived.

☆Level2:Multiplexing.Allowmultipletransmissionstosharethesamenetworkconnectionandhavecorrespondingflowcontrolfunctions.

☆Level3:Errorrecoveryandmultiplexing.Itisasynthesisoflevel1andlevel2protocols.

☆Level4:Errordetection,recoveryandmultiplexing.Anerrordetectionfunctionisaddedonthebasisofthelevel3protocol.

Transportlayerprotocol

Connection-orientedtransmissionprotocol(TCP):Theconnectionmustbeestablishedbeforedatatransmission,andtheconnectionmustbereleasedafterthedatatransmissioniscompleted.Onlysupportunicasttransmission:eachtransmissionconnectioncanonlyhavetwoendpoints,andonlypoint-to-pointconnectionscanbemade.Multicastandbroadcasttransmissionmethodsarenotsupported.UDPissupported.

Providereliabledeliveryservices:thetransmitteddataiserror-free.Noloss,norepetition,andthesequenceisconsistentwiththesourcedata.

Thetransmissionunitisthedatasegment:thedatasegmentsenteachtimeisnotfixed,andisaffectedbythesizeoftheapplicationlayertransmissionmessageandtheMTU(MaximumTransmissionUnit)valueinthenetwork.Thesmallestdatasegmentmayonlyhave21bytes(ofwhich20bytesbelongtotheTCPheader,andthedatapartisonly1byte).

Supportfull-duplextransmission:bothpartiescansendandreceivedataatthesametime.

TCPconnectionisbasedonbytestream:UDPisbasedonpacketstream.

TCPisatransportlayerprotocolthatcanguaranteereliabledatatransmission.Itmainlyadoptsthefollowingfourmechanismstoachievereliabledatatransmission.

Bytenumberingmechanism:TCPdatasegmentnumbersthe"data"partofthedatasegmentonebyoneinbytestoensurethateachbyteofdatacanbetransmittedandreceivedinanorderlymanner.

Datasegmentconfirmationmechanism:everytimeadatasegmentisreceived,thereceivingendmustreturnaconfirmationdatasegmenttothesendingend,andtheconfirmationnumberindicatesthesequencenumberofthedatasegmentthathasbeencorrectlyreceived.

Timeoutretransmissionmechanism:Thereisaretransmissiontimer(RTT)inTCP.Whensendingadatasegment,thistimerisalsoturnedon.Ifthetimerexpiresandnoconfirmationisreturned,thetimerStopandretransmitthedata.

Selectiveacknowledgmentmechanism:(SelectiveACK,SACK)/Onlythemissingpartofthedataisretransmitted,andthedatathathasbeenreceivedcorrectlywillnotberetransmitted.

UDPistheabbreviationofUserDatagramProtocol.TheChinesenameisUserDatagramProtocol.ItisaconnectionlesstransportlayerprotocolintheOSI(OpenSystemInterconnection)referencemodel.Itprovidestransaction-orientedIETFRFC768istheofficialspecificationofUDP.TheprotocolnumberofUDPintheIPmessageis17.

ThefullnameoftheUDPprotocolistheUserDatagramProtocol.ItisusedtoprocessdatapacketsinthesamenetworkastheTCPprotocol,andisaconnectionlessprotocol.IntheOSImodel,thefourthlayer-thetransportlayer,isintheupperlayeroftheIPprotocol.UDPhasthedisadvantagesofnotprovidingdatapacketgrouping,assemblyandinabilitytosortdatapackets.Thatistosay,afterthemessageissent,itisimpossibletoknowwhetherithasarrivedsafelyandcompletely.UDPisusedtosupportnetworkapplicationsthatneedtotransferdatabetweencomputers.Numerousclient/servernetworkapplicationsincludingnetworkvideoconferencingsystemsneedtousetheUDPprotocol.UDPmessageisdividedintotwoparts:UDPmessageheaderandUDPdataarea.Theheaderiscomposedofsourceport,destinationport,messagelength,andchecksum.UDPissuitableforreal-timedatatransmission,suchasvoiceandvideocommunications.ComparedwithTCP,UDPhashighertransmissionefficiencyandloweroverhead,butitcannotguaranteethereliabilityofdatatransmission.TheUDPheaderisidentifiedasfollows:

1)16-bitsourceportnumber:theportnumberusedbythesourcehost'sapplicationprogram.

2)16-bitdestinationportnumber:theportnumberusedbytheapplicationprogramofthedestinationhost.

3)16-bitUDPlength:referstothebytelengthoftheUDPheaderandUDPdata.BecausethelengthoftheUDPheaderis8bytes,theminimumvalueofthefieldis8.

4)16-bitUDPchecksum:ThisfieldprovidesthesamefunctionastheTCPcheckfield;thisfieldisoptional.

Importance

Thetransportlayeristhecoreoftheentireprotocolhierarchyandistheonlylayerresponsibleforoveralldatatransmissionandcontrol.IntheOSIseven-layermodel,thetransportlayeristhehighestlayerresponsiblefordatacommunication,anditisthemiddlelayerbetweenthelowerthreelayersfornetworkcommunicationandtheupperthreelayersforinformationprocessing.Becausethenetworklayerdoesnotnecessarilyguaranteethereliabilityoftheservice,anduserscannotdirectlycontrolthecommunicationsubnet,atransmissionlayerisaddedabovethenetworklayertoimprovetransmissionquality.

Thetransportlayerusestheservicesprovidedbythenetworklayerandprovidescommunicationportsforhigh-leveluserstotransmitdatathroughthetransportlayeraddress,sothatthesharingofhigh-levelresourcesbetweensystemsdoesnothavetoconsiderdatacommunicationandunreliabledatatransmissionTheproblem.Itsmainfunctionsare:toprovidereliabletransmissionservicesforanongoingconversationorconnection,torealizethemultiplexingoftheconnectiononasinglephysicalconnectiontothenetwork,andtoprovideend-to-endserialnumberandflowcontrolanderrorcontrolonasingleconnectionAndrecoveryservices.

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