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.
Основни функции
Транспортният слой предоставя услуги от край до край между процесите на приложения на хоста. Областите на основните функции са както следва:
(1) Splitandreorganizedata
(2) Адресиране по номер на порт
(3) Управление на връзката
(4)Контрол на грешки и контрол на потока,функция за коригиране на грешки
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:
☆Услуга: ниска честота на грешка при свързване, която е с приемлива остатъчна честота на грешка и честота на уведомяване за отказ;
☆Тип Bуслуга: връзка с висок процент на грешки, т.е., с неприемлив процент на остатъчна грешка и процент на уведомяване за отказ;
☆Услуга TypeC:между услугатаTypeA и услугатаTypeB.
Agreementlevel
Theacceptanceorunacceptabilityoftheerrorratedependsontheuser.Therefore,theclassificationofnetworkservicequalityisbasedonuserrequirements.Accordingtothefunctionalcharacteristicsofthetransportlayer,OSIdefinesthefollowingfiveprotocollevels:
☆Ниво 0: Проста връзка. Установява се само проста връзка за предаване от край до край и дълги съобщения могат да се предават на сегменти.
☆Level1:Basicerrorrecoverylevel.Ithasarecoveryfunctionwhenthenetworkconnectionisdisconnected,thenetworkconnectionfails,oranunrecognizedtransmissionconnectiondataunitisreceived.
☆Level2:Multiplexing.Allowmultipletransmissionstosharethesamenetworkconnectionandhavecorrespondingflowcontrolfunctions.
☆Ниво 3: Възстановяване на грешки и мултиплексиране. Това е синтез на протоколи от ниво 1 и ниво 2.
☆Ниво 4: Откриване на грешки, възстановяване и мултиплексиране. Функцията за откриване на грешки е добавена към базата на протокола ниво 3.
Transportlayerprotocol
Connection-orientedtransmissionprotocol(TCP):Theconnectionmustbeestablishedbeforedatatransmission,andtheconnectionmustbereleasedafterthedatatransmissioniscompleted.Onlysupportunicasttransmission:eachtransmissionconnectioncanonlyhavetwoendpoints,andonlypoint-to-pointconnectionscanbemade.Multicastandbroadcasttransmissionmethodsarenotsupported.UDPissupported.
Доставчик на надеждни услуги за доставка: предадените данни са без грешки. Без загуба, без повторение и последователността е в съответствие с изходните данни.
Thetransmissionunitisthedatasegment:thedatasegmentsenteachtimeisnotfixed,andisaffectedbythesizeoftheapplicationlayertransmissionmessageandtheMTU(MaximumTransmissionUnit)valueinthenetwork.Thesmallestdatasegmentmayonlyhave21bytes(ofwhich20bytesbelongtotheTCPheader,andthedatapartisonly1byte).
Поддържа пълно дуплексно предаване: и двете страни могат да изпращат и получават данни по едно и също време.
TCP връзката е базирана на байтов поток: UDP е базиран на пакетен поток.
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-битов номер на порт на източника: номерът на порта, използван от приложната програма на хоста източник.
2)16-битов номер на порт на местоназначение: номерът на порт, използван от приложната програма на хоста на местоназначение.
3)16-битова UDPдължина: отнася се до дължината на UDP заглавието и UDP данните. Тъй като дължината на UDP заглавието е 8 байта, минималната стойност на полето е 8.
4)16-bitUDP контролна сума: това поле предоставя същата функция като TCP полето за проверка; това поле е по избор.
Importance
Thetransportlayeristhecoreoftheentireprotocolhierarchyandistheonlylayerresponsibleforoveralldatatransmissionandcontrol.IntheOSIseven-layermodel,thetransportlayeristhehighestlayerresponsiblefordatacommunication,anditisthemiddlelayerbetweenthelowerthreelayersfornetworkcommunicationandtheupperthreelayersforinformationprocessing.Becausethenetworklayerdoesnotnecessarilyguaranteethereliabilityoftheservice,anduserscannotdirectlycontrolthecommunicationsubnet,atransmissionlayerisaddedabovethenetworklayertoimprovetransmissionquality.
Thetransportlayerusestheservicesprovidedbythenetworklayerandprovidescommunicationportsforhigh-leveluserstotransmitdatathroughthetransportlayeraddress,sothatthesharingofhigh-levelresourcesbetweensystemsdoesnothavetoconsiderdatacommunicationandunreliabledatatransmissionTheproblem.Itsmainfunctionsare:toprovidereliabletransmissionservicesforanongoingconversationorconnection,torealizethemultiplexingoftheconnectiononasinglephysicalconnectiontothenetwork,andtoprovideend-to-endserialnumberandflowcontrolanderrorcontrolonasingleconnectionAndrecoveryservices.