CCNA1 4 OSI Transport Layer.docx

CCNA1 4 OSI Transport Layer.docx

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CCNA14OSITransportLayer

CCNAExploration-NetworkFundamentals

4OSITransportLayer

4.0ChapterIntroduction

4.0.1ChapterIntroduction

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DatanetworksandtheInternetsupportthehumannetworkbysupplyingseamless,reliablecommunicationbetweenpeople-bothlocallyandaroundtheglobe.Onasingledevice,peoplecanusemultipleservicessuchase-mail,theweb,andinstantmessagingtosendmessagesorretrieveinformation.Applicationssuchase-mailclients,webbrowsers,andinstantmessagingclientsallowpeopletousecomputersandnetworkstosendmessagesandfindinformation.

Datafromeachoftheseapplicationsispackaged,transported,anddeliveredtotheappropriateserverdaemonorapplicationonthedestinationdevice.TheprocessesdescribedintheOSITransportlayeracceptdatafromtheApplicationlayerandprepareitforaddressingattheNetworklayer.TheTransportlayerisresponsiblefortheoverallend-to-endtransferofapplicationdata.

Inthischapter,weexaminetheroleoftheTransportlayerinencapsulatingapplicationdataforusebytheNetworklayer.TheTransportlayeralsoencompassesthesefunctions:

∙Enablesmultipleapplicationstocommunicateoverthenetworkatthesametimeonasingledevice

∙Ensuresthat,ifrequired,allthedataisreceivedreliablyandinorderbythecorrectapplication

∙Employserrorhandlingmechanisms

LearningObjectives

Uponcompletionofthischapter,youwillbeableto:

∙ExplaintheneedfortheTransportlayer.

∙IdentifytheroleoftheTransportlayerasitprovidestheend-to-endtransferofdatabetweenapplications.

∙DescribetheroleoftwoTCP/IPTransportlayerprotocols:

TCPandUDP.

∙ExplainthekeyfunctionsoftheTransportlayer,includingreliability,portaddressing,andsegmentation.

∙ExplainhowTCPandUDPeachhandlekeyfunctions.

∙IdentifywhenitisappropriatetouseTCPorUDPandprovideexamplesofapplicationsthatuseeachprotocol.

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4.1RolesoftheTransportLayer

4.1.1PurposeoftheTransportLayer

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TheTransportlayerprovidesforthesegmentationofdataandthecontrolnecessarytoreassemblethesepiecesintothevariouscommunicationstreams.Itsprimaryresponsibilitiestoaccomplishthisare:

∙Trackingtheindividualcommunicationbetweenapplicationsonthesourceanddestinationhosts

∙Segmentingdataandmanagingeachpiece

∙Reassemblingthesegmentsintostreamsofapplicationdata

∙Identifyingthedifferentapplications

TrackingIndividualConversations

Anyhostmayhavemultipleapplicationsthatarecommunicatingacrossthenetwork.Eachoftheseapplicationswillbecommunicatingwithoneormoreapplicationsonremotehosts.ItistheresponsibilityoftheTransportlayertomaintainthemultiplecommunicationstreamsbetweentheseapplications.

SegmentingData

Aseachapplicationcreatesastreamdatatobesenttoaremoteapplication,thisdatamustbepreparedtobesentacrossthemediainmanageablepieces.TheTransportlayerprotocolsdescribeservicesthatsegmentthisdatafromtheApplicationlayer.Thisincludestheencapsulationrequiredoneachpieceofdata.EachpieceofapplicationdatarequiresheaderstobeaddedattheTransportlayertoindicatetowhichcommunicationitisassociated.

ReassemblingSegments

Atthereceivinghost,eachpieceofdatamaybedirectedtotheappropriateapplication.Additionally,theseindividualpiecesofdatamustalsobereconstructedintoacompletedatastreamthatisusefultotheApplicationlayer.TheprotocolsattheTransportlayerdescribethehowtheTransportlayerheaderinformationisusedtoreassemblethedatapiecesintostreamstobepassedtotheApplicationlayer.

IdentifyingtheApplications

Inordertopassdatastreamstotheproperapplications,theTransportlayermustidentifythetargetapplication.Toaccomplishthis,theTransportlayerassignsanapplicationanidentifier.TheTCP/IPprotocolscallthisidentifieraportnumber.Eachsoftwareprocessthatneedstoaccessthenetworkisassignedaportnumberuniqueinthathost.ThisportnumberisusedintheTransportlayerheadertoindicatetowhichapplicationthatpieceofdataisassociated.

TheTransportlayeristhelinkbetweentheApplicationlayerandthelowerlayerthatareresponsiblefornetworktransmission.Thislayeracceptsdatafromdifferentconversationsandpassesitdowntothelowerlayersasmanageablepiecesthatcanbeeventuallymultiplexedoverthemedia.

Applicationsdonotneedtoknowtheoperationaldetailsofthenetworkinuse.Theapplicationsgeneratedatathatissentfromoneapplicationtoanother,withoutregardtothedestinationhosttype,thetypeofmediaoverwhichthedatamusttravel,thepathtakenbythedata,thecongestiononalink,orthesizeofthenetwork.

Additionally,thelowerlayersarenotawarethattherearemultipleapplicationssendingdataonthenetwork.Theirresponsibilityistodeliverdatatotheappropriatedevice.TheTransportlayerthensortsthesepiecesbeforedeliveringthemtotheappropriateapplication.

DataRequirementsVary

Becausedifferentapplicationshavedifferentrequirements,therearemultipleTransportlayerprotocols.Forsomeapplications,segmentsmustarriveinaveryspecificsequenceinordertobeprocessedsuccessfully.Insomecases,allofthedatamustbereceivedforanyofittobeofuse.Inothercases,anapplicationcantoleratesomelossofdataduringtransmissionoverthenetwork.

Intoday'sconvergednetworks,applicationswithverydifferenttransportneedsmaybecommunicatingonthesamenetwork.ThedifferentTransportlayerprotocolshavedifferentrulesallowingdevicestohandlethesediversedatarequirements.

Someprotocolsprovidejustthebasicfunctionsforefficientlydeliveringthedatapiecesbetweentheappropriateapplications.Thesetypesofprotocolsareusefulforapplicationswhosedataissensitivetodelays.

OtherTransportlayerprotocolsdescribeprocessesthatprovideadditionalfeatures,suchasensuringreliabledeliverybetweentheapplications.WhiletheseadditionalfunctionsprovidemorerobustcommunicationattheTransportlayerbetweenapplications,theyhaveadditionaloverheadandmakelargerdemandsonthenetwork.

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SeparatingMultipleCommunications

Consideracomputerconnectedtoanetworkthatissimultaneouslyreceivingandsendinge-mailandinstantmessages,viewingwebsites,andconductingaVoIPphonecall.Eachoftheseapplicationsissendingandreceivingdataoverthenetworkatthesametime.However,datafromthephonecallisnotdirectedtothewebbrowser,andtextfromaninstantmessagedoesnotappearinane-mail.

Further,usersrequirethatane-mailorwebpagebecompletelyreceivedandpresentedfortheinformationtobeconsidereduseful.Slightdelaysareconsideredacceptabletoensurethatthecompleteinformationisreceivedandpresented.

Incontrast,occasionallymissingsmallpartsofatelephoneconversationmightbeconsideredacceptable.Onecaneitherinferthemissingaudiofromthecontextoftheconversationorasktheotherpersontorepeatwhattheysaid.Thisisconsideredpreferabletothedelaysthatwouldresultfromaskingthenetworktomanageandresendmissingsegments.Inthisexample,theuser-notthenetwork-managestheresendingorreplacementofmissinginformation.

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Asexplainedinapreviouschapter,sendingsometypesofdata-avideoforexample-acrossanetworkasonecompletecommunicationstreamcouldpreventothercommunicationsfromoccurringatthesametime.Italsomakeserrorrecoveryandretransmissionofdamageddatadifficult.

Dividingdataintosmallparts,andsendingthesepartsfromthesourcetothedestination,enablesmanydifferentcommunicationstobeinterleaved（multiplexed）onthesamenetwork.

Segmentationofthedata,inaccordancewithTransportlayerprotocols,providesthemeanstobothsendandreceivedatawhenrunningmultipleapplicationsconcurrentlyonacomputer.Withoutsegmentation,onlyoneapplication,thestreamingvideoforexample,wouldbeabletoreceivedata.Youcouldnotreceivee-mails,chatoninstantmessenger,orviewwebpageswhilealsoviewingthevideo.

AttheTransportlayer,eachparticularsetofpiecesflowingbetweenasourceapplicationandadestinationapplicationisknownasaconversation.

Toidentifyeachsegmentofdata,theTransportlayeraddstothepieceaheadercontainingbinarydata.Thisheadercontainsfieldsofbits.ItisthevaluesinthesefieldsthatenabledifferentTransportlayerprotocolstoperformdifferentfunctions.

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4.1.2ControllingtheConversations

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TheprimaryfunctionsspecifiedbyallTransportlayerprotocolsinclude:

SegmentationandReassembly-MostnetworkshavealimitationontheamountofdatathatcanbeincludedinasinglePDU.TheTransportlayerdividesapplicationdataintoblocksofdatathatareanappropriatesize.Atthedestination,theTransportlayerreassemblesthedatabeforesendingittothedestinationapplicationorservice.

ConversationMultiplexing-Theremaybemanyapplicationsorservicesrunningoneachhostinthenetwork.EachoftheseapplicationsorservicesisassignedanaddressknownasaportsothattheTransportlayercandeterminewithwhichapplicationorservicethedataisidentified.

Inadditiontousingtheinformationcontainedintheheaders,forthebasicfunctionsofdatasegmentationandreassembly,someprotocolsattheTransportlayer