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.
Základní funkce
Přenosová vrstva poskytuje služby typu "end-to-end" mezi hostitelskými aplikačními procesy. Základní funkce jsou následující:
(1)Splitandreorganizedata
(2)Addressingbyportnumber
(3)Správa připojení
(4)Ovládání chyb a řízení toku, funkce opravy chyb
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:
☆Služba: připojení s nízkou chybovostí, tedy s přijatelnou zbytkovou chybovostí a četností upozornění na selhání;
☆TypBservice:spojení s vysokou chybovostí, to znamená s nepřijatelnou zbytkovou chybou a četností oznámení o selhání;
☆TypCservice:meziTypAserviceaTypBservice.
Agreementlevel
Theacceptanceorunacceptabilityoftheerrorratedependsontheuser.Therefore,theclassificationofnetworkservicequalityisbasedonuserrequirements.Accordingtothefunctionalcharacteristicsofthetransportlayer,OSIdefinesthefollowingfiveprotocollevels:
☆Úroveň 0:Jednoduché připojení. Je vytvořeno pouze jednoduché přenosové spojení mezi koncovými body a v segmentech lze přenášet dlouhé zprávy.
☆Level1:Basicerrorrecoverylevel.Ithasarecoveryfunctionwhenthenetworkconnectionisdisconnected,thenetworkconnectionfails,oranunrecognizedtransmissionconnectiondataunitisreceived.
☆Level2:Multiplexing.Allowmultipletransmissionstosharethesamenetworkconnectionandhavecorrespondingflowcontrolfunctions.
☆Úroveň 3:Oprava chyb a multiplexování. Jedná se o syntézu protokolů úrovně 1 a úrovně 2.
☆Úroveň 4: Detekce chyb, obnova a multiplexování. Do základu protokolu úrovně 3 je přidána funkce detekce chyb.
Transportlayerprotocol
Connection-orientedtransmissionprotocol(TCP):Theconnectionmustbeestablishedbeforedatatransmission,andtheconnectionmustbereleasedafterthedatatransmissioniscompleted.Onlysupportunicasttransmission:eachtransmissionconnectioncanonlyhavetwoendpoints,andonlypoint-to-pointconnectionscanbemade.Multicastandbroadcasttransmissionmethodsarenotsupported.UDPissupported.
Poskytujte spolehlivé doručovací služby: přenášená data jsou bez chyb. Žádná ztráta, žádné opakování a sekvence jsou konzistentní se zdrojovými daty.
Thetransmissionunitisthedatasegment:thedatasegmentsenteachtimeisnotfixed,andisaffectedbythesizeoftheapplicationlayertransmissionmessageandtheMTU(MaximumTransmissionUnit)valueinthenetwork.Thesmallestdatasegmentmayonlyhave21bytes(ofwhich20bytesbelongtotheTCPheader,andthedatapartisonly1byte).
Podporovaný duplexní přenos: obě strany mohou odesílat a přijímat data současně.
Připojení TCP je založeno na toku bajtů: UDP je založeno na toku paketů.
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) 16bitové číslo portu zdroje: číslo portu používané aplikačním programem hostitele zdroje.
2)16bitové číslo cílového portu:číslo portu používané aplikačním programem cílového hostitele.
3)16bitová délka UDP: týká se délky bajtů hlavičky UDP a dat UDP. Protože délka hlavičky UDP je 8 bajtů, minimální hodnota pole je 8.
4) 16bitový kontrolní součet UDP: Toto pole poskytuje stejnou funkci jako pole kontroly TCP; toto pole je volitelné.
Importance
Thetransportlayeristhecoreoftheentireprotocolhierarchyandistheonlylayerresponsibleforoveralldatatransmissionandcontrol.IntheOSIseven-layermodel,thetransportlayeristhehighestlayerresponsiblefordatacommunication,anditisthemiddlelayerbetweenthelowerthreelayersfornetworkcommunicationandtheupperthreelayersforinformationprocessing.Becausethenetworklayerdoesnotnecessarilyguaranteethereliabilityoftheservice,anduserscannotdirectlycontrolthecommunicationsubnet,atransmissionlayerisaddedabovethenetworklayertoimprovetransmissionquality.
Thetransportlayerusestheservicesprovidedbythenetworklayerandprovidescommunicationportsforhigh-leveluserstotransmitdatathroughthetransportlayeraddress,sothatthesharingofhigh-levelresourcesbetweensystemsdoesnothavetoconsiderdatacommunicationandunreliabledatatransmissionTheproblem.Itsmainfunctionsare:toprovidereliabletransmissionservicesforanongoingconversationorconnection,torealizethemultiplexingoftheconnectiononasinglephysicalconnectiontothenetwork,andtoprovideend-to-endserialnumberandflowcontrolanderrorcontrolonasingleconnectionAndrecoveryservices.