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.
Perustoiminnot
Kuljetuskerros tarjoaa lähetyspalveluita isäntäsovellusprosessien välillä.Perustoiminnot ovat seuraavat:
(1)Jakaminen ja uudelleenorganisoitu data
(2)Osoittaminen porttinumeron mukaan
(3) Yhteydenhallinta
(4)Virheenhallinta ja virranhallinta, virheenkorjaustoiminto
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:
☆Palvelu: alempi virheyhteys, eli hyväksytyllä jäännösvirhe- ja epäonnistumisilmoitusprosentilla;
☆TyyppiBpalvelu:korkeampi yhteys, eli ei-hyväksyttävillä jäännösvirheillä ja epäonnistumisilmoitusten määrällä;
☆TypeCservice:TypeAservice-jaTypeBservice-välillä.
Agreementlevel
Theacceptanceorunacceptabilityoftheerrorratedependsontheuser.Therefore,theclassificationofnetworkservicequalityisbasedonuserrequirements.Accordingtothefunctionalcharacteristicsofthetransportlayer,OSIdefinesthefollowingfiveprotocollevels:
☆Taso 0: Yksinkertainen yhteys.Vain yksinkertainen lähetysyhteys on luotu ja pitkiä viestejä voidaan lähettää osissa.
☆Level1:Basicerrorrecoverylevel.Ithasarecoveryfunctionwhenthenetworkconnectionisdisconnected,thenetworkconnectionfails,oranunrecognizedtransmissionconnectiondataunitisreceived.
☆Level2:Multiplexing.Allowmultipletransmissionstosharethesamenetworkconnectionandhavecorrespondingflowcontrolfunctions.
☆Taso 3: Virheenpalautus ja multipleksointi. Se on taso1- ja taso2-protokollien synteesi.
☆Taso 4: Virheiden havaitseminen, palautus ja multipleksointi. Virheiden havaitsemistoiminto on lisättylevel3-protokollan perusteella.
Transportlayerprotocol
Connection-orientedtransmissionprotocol(TCP):Theconnectionmustbeestablishedbeforedatatransmission,andtheconnectionmustbereleasedafterthedatatransmissioniscompleted.Onlysupportunicasttransmission:eachtransmissionconnectioncanonlyhavetwoendpoints,andonlypoint-to-pointconnectionscanbemade.Multicastandbroadcasttransmissionmethodsarenotsupported.UDPissupported.
Tarjoa luotettavia toimituspalveluita: lähetetyt tiedot virheettömästi. Häviö, ei toistoa, ja nämä sekvenssit ovat yhdenmukaisia lähdetietojen kanssa.
Thetransmissionunitisthedatasegment:thedatasegmentsenteachtimeisnotfixed,andisaffectedbythesizeoftheapplicationlayertransmissionmessageandtheMTU(MaximumTransmissionUnit)valueinthenetwork.Thesmallestdatasegmentmayonlyhave21bytes(ofwhich20bytesbelongtotheTCPheader,andthedatapartisonly1byte).
Tuettu kaksipuolinen lähetys: molemmat osapuolet voivat lähettää ja vastaanottaa tietoja samaan aikaan.
TCP-yhteys perustuu tavuvirtaan:UDP perustuu pakettivirtaan.
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-bittinen lähdeportin numero: lähdeisännän sovellusohjelman käyttämä portin numero.
2)16-bittinen kohdeportin numero: kohdeisännän sovellusohjelman käyttämä portin numero.
3)16 bitin UDPlength: viittaaUDP-otsikon ja UDP-tietojen pituuden mukaan.KoskaUDP-otsikon pituus on 8 tavua, kentän vähimmäisarvo on8.
4) 16-bittinen UDP-tarkistussumma: Tämä kenttä tarjoaa saman toiminnon kuin TCP-tarkistuskenttä; tämä kenttä on valinnainen.
Importance
Thetransportlayeristhecoreoftheentireprotocolhierarchyandistheonlylayerresponsibleforoveralldatatransmissionandcontrol.IntheOSIseven-layermodel,thetransportlayeristhehighestlayerresponsiblefordatacommunication,anditisthemiddlelayerbetweenthelowerthreelayersfornetworkcommunicationandtheupperthreelayersforinformationprocessing.Becausethenetworklayerdoesnotnecessarilyguaranteethereliabilityoftheservice,anduserscannotdirectlycontrolthecommunicationsubnet,atransmissionlayerisaddedabovethenetworklayertoimprovetransmissionquality.
Thetransportlayerusestheservicesprovidedbythenetworklayerandprovidescommunicationportsforhigh-leveluserstotransmitdatathroughthetransportlayeraddress,sothatthesharingofhigh-levelresourcesbetweensystemsdoesnothavetoconsiderdatacommunicationandunreliabledatatransmissionTheproblem.Itsmainfunctionsare:toprovidereliabletransmissionservicesforanongoingconversationorconnection,torealizethemultiplexingoftheconnectiononasinglephysicalconnectiontothenetwork,andtoprovideend-to-endserialnumberandflowcontrolanderrorcontrolonasingleconnectionAndrecoveryservices.