計算機網(wǎng)絡英文版課件：2-1 網(wǎng)絡應用

網(wǎng)絡應用ApplicationarchitecturesClient-server(C/S)Peer-to-peer(P2P)HybridofC/SandP2PClient-serverarchitectureserver:

always-onhostpermanentIPaddressserverfarmsforscalingclients:communicatewithservermaybeintermittentlyconnectedmayhavedynamicIPaddressesdonotcommunicatedirectlywitheachotherclient/serverPureP2Parchitecturenoalways-onserverarbitraryendsystemsdirectlycommunicatepeersareintermittentlyconnectedandchangeIPaddressespeer-peerHighlyscalablebutdifficulttomanageHybridofclient-serverandP2PSkypevoice-over-IPP2Papplicationcentralizedserver:findingaddressofremoteparty:client-clientconnection:direct(notthroughserver)InstantmessagingchattingbetweentwousersisP2Pcentralizedservice:clientpresencedetection/locationuserregistersitsIPaddresswithcentralserverwhenitcomesonlineusercontactscentralservertofindIPaddressesofbuddiesProcessescommunicatingProcess:programrunningwithinahost.withinsamehost,twoprocessescommunicateusinginter-processcommunication(definedbyOS).processesindifferenthostscommunicatebyexchangingmessagesNote:applicationswithP2Parchitectureshaveclientprocesses&serverprocessesClientprocess:processthatinitiatescommunicationServerprocess:processthatwaitstobecontactedSocketsprocesssends/receivesmessagesto/fromitssocketsocketanalogoustodoorsendingprocessshovesmessageoutdoorsendingprocessreliesontransportinfrastructureonothersideofdoorwhichbringsmessagetosocketatreceivingprocessprocessTCPwithbuffers,variablessockethostorserverprocessTCPwithbuffers,variablessockethostorserverInternetcontrolledbyOScontrolledbyappdeveloperAPI:(1)choiceoftransportprotocol;(2)abilitytofixafewparameters(lotsmoreonthislater)Addressingprocessestoreceivemessages,processmusthaveidentifierhostdevicehasunique32-bitIPaddressQ:doesIPaddressofhostsufficeforidentifyingtheprocess?Addressingprocessestoreceivemessages,processmusthaveidentifierhostdevicehasunique32-bitIPaddressQ:doesIPaddressofhostonwhichprocessrunssufficeforidentifyingtheprocess?A:

No,manyprocessescanberunningonsamehostidentifier

includesbothIPaddressandportnumbersassociatedwithprocessonhost.Exampleportnumbers:HTTPserver:80Mailserver:25tosendHTTPmessagetowebserver:IPaddress:

2Portnumber:

80moreshortly…App-layerprotocoldefinesPublic-domainprotocols:definedinRFCsallowsforinteroperabilitye.g.,HTTP,SMTPProprietaryprotocols:e.g.,SkypeTypesofmessagesexchanged,e.g.,request,responseMessagesyntax:whatfieldsinmessages&howfieldsaredelineatedMessagesemanticsmeaningofinformationinfieldsRulesforwhenandhowprocessessend&respondtomessagesWhattransportservicedoesanappneed?Datalosssomeapps(e.g.,audio)cantoleratesomelossotherapps(e.g.,filetransfer,telnet)require100%reliabledatatransferTimingsomeapps(e.g.,Internettelephony,interactivegames)requirelowdelaytobe“effective”Throughputsomeapps(e.g.,multimedia)requireminimumamountofthroughputtobe“effective”otherapps(“elasticapps”)makeuseofwhateverthroughputtheygetSecurityEncryption,dataintegrity,…TransportservicerequirementsofcommonappsApplicationfiletransfere-mailWebdocumentsreal-timeaudio/videostoredaudio/videointeractivegamesinstantmessagingDatalossnolossnolossnolossloss-tolerantloss-tolerantloss-tolerantnolossThroughputelasticelasticelasticaudio:5kbps-1Mbpsvideo:10kbps-5MbpssameasabovefewkbpsupelasticTimeSensitivenononoyes,100’smsecyes,fewsecsyes,100’smsecyesandnoInternettransportprotocolsservicesTCPservice:connection-oriented:

setuprequiredbetweenclientandserverprocessesreliabletransportbetweensendingandreceivingprocessflowcontrol:

senderwon’toverwhelmreceivercongestioncontrol:

throttlesenderwhennetworkoverloadeddoesnotprovide:

timing,minimumthroughputguarantees,securityUDPservice:unreliabledatatransferbetweensendingandreceivingprocessdoesnotprovide:connectionsetup,reliability,flowcontrol,congestioncontrol,timing,throughputguarantee,orsecurityQ:whybother?WhyisthereaUDP?Internetapps:application,transportprotocolsApplicatione-mailremoteterminalaccessWebfiletransferstreamingmultimediaInternettelephonyApplicationlayerprotocolSMTP