计算机网络概论 英文版.ppt

2009PearsonEducationInc UpperSaddleRiver NJ Allrightsreserved 1 ComputerNetworksandInternets 5eByDouglasE Comer LecturePowerPointsByLamiKaya LKaya ieee org 2009PearsonEducationInc UpperSaddleRiver NJ Allrightsreserved 2 Chapter3InternetApplicationsandNetworkProgramming 2009PearsonEducationInc UpperSaddleRiver NJ Allrightsreserved 3 TopicsCovered 3 1Introduction3 2TwoBasicInternetCommunicationParadigms3 3Connection OrientedCommunication3 4TheClient ServerModelofInteraction3 5CharacteristicsofClientsandServers3 6ServerProgramsandServer ClassComputers3 7Requests Responses andDirectionofDataFlow3 8MultipleClientsandMultipleServers3 9ServerIdentificationandDemultiplexing3 10ConcurrentServers3 11CircularDependenciesAmongServers3 12Peer to PeerInteractions3 13NetworkProgrammingandtheSocketAPI 2009PearsonEducationInc UpperSaddleRiver NJ Allrightsreserved 4 TopicsCovered 3 14Sockets Descriptors andNetworkI O3 15ParametersandtheSocketAPI3 16SocketCallsinaClientandServer3 17SocketFunctionsUsedbyBothClientandServer3 18TheConnectionFunctionUsedOnlybyaClient3 19SocketFunctionsUsedOnlybyaServer3 20SocketFunctionsUsedwiththeMessageParadigm3 21OtherSocketFunctions3 22Sockets Threads andInheritance 2009PearsonEducationInc UpperSaddleRiver NJ Allrightsreserved 5 3 1Introduction TheInternetoffersusersarichdiversityofservicesnoneoftheservicesispartoftheunderlyingcommunicationinfrastructureInternetprovidesageneralpurposemechanismonwhichallservicesarebuiltandindividualservicesaresuppliedbyapplicationprogramsthatrunoncomputersattachedtotheInternetItispossibletodevisenewserviceswithoutchangingtheInternetChaptercoverstwokeyconceptsofInternetapplications describestheconceptualparadigmthatapplicationsfollowwhentheycommunicateovertheInternetpresentsthedetailsofthesocketApplicationProgrammingInterface socketAPI thatInternetapplicationsuse 2009PearsonEducationInc UpperSaddleRiver NJ Allrightsreserved 6 3 1Introduction InternetapplicationprogrammerscangetstartedeasilyItispossibletocreateInternetapplicationswithoutknowinghownetworksoperateHowever understandingnetworkprotocolsandtechnologiesallowsthemtowriteefficientandreliablecodethatenablesapplicationstoscaleacrossmanysitesLaterpartsofthetextprovidethenecessaryinformationbyexplainingdatacommunicationsandprotocolsusedtoformtheInternet 2009PearsonEducationInc UpperSaddleRiver NJ Allrightsreserved 7 3 2TwoBasicInternetCommunicationParadigms TheInternetsupportstwobasiccommunicationparadigms 3 2 1StreamTransportintheInternet3 2 2MessageTransportintheInternetFigure3 1summarizesthedifferences 3 2 1StreamTransportintheInternet StreamdenotesaparadigminwhichasequenceofbytesflowsfromoneapplicationprogramtoanotherInternet smechanismarrangestwostreamsbetweenapairofcommunicatingapplications oneineachdirectionThenetworkacceptsinputfromeitherapplication anddeliversthedatatotheotherapplicationThestreammechanismtransfersasequenceofbyteswithoutattachingmeaningtothebytesandwithoutinsertingboundariesAsendingapplicationcanchoosetogenerateonebyteatatime orcangenerateblocksofbytesThenetworkchoosesthenumberofbytestodeliveratanytimethenetworkcanchoosetocombinesmallerblocksintoonelargeblockorcandividealargeblockintosmallerblocks 2009PearsonEducationInc UpperSaddleRiver NJ Allrightsreserved 8 3 2 2MessageTransportintheInternet Inamessageparadigm thenetworkacceptsanddeliversmessagesEachmessagedeliveredtoareceivercorrespondstoamessagethatwastransmittedbyasenderthenetworkneverdeliverspartofamessage nordoesitjoinmultiplemessagestogetherifasenderplacesexactlynbytesinanoutgoingmessage thereceiverwillfindexactlynbytesintheincomingmessageThemessageparadigmallowsdeliveryindifferentforms Unicastamessagecanbesentfromanapplicationononecomputerdirectlytoanapplicationonanother 1 to 1Multicastamessagecanbemulticasttosomeofthecomputersonanetwork 1 to manyBroadcastamessagecanbebroadcasttoallcomputersonagivennetwork 1 to all 2009PearsonEducationInc UpperSaddleRiver NJ Allrightsreserved 9 3 2 2MessageTransportintheInternet MessageservicedoesnotmakeanyguaranteesSomessagesmaybeLost i e neverdelivered Duplicated morethanonecopyarrives Deliveredout of orderAprogrammerwhousesthemessageparadigmmustinsurethattheapplicationoperatescorrectlyevenifpacketsarelostorreorderedMostapplicationsrequiredeliveryguaranteesProgrammerstendtousethestreamserviceexceptinspecialsituationssuchasvideo wheremulticastisneededandtheapplicationprovidessupporttohandlepacketreorderingandlossThus wewillfocusonthestreamparadigm 2009PearsonEducationInc UpperSaddleRiver NJ Allrightsreserved 10 2009PearsonEducationInc UpperSaddleRiver NJ Allrightsreserved 11 3 3Connection OrientedCommunication TheInternetstreamserviceisconnection orientedItoperatesanalogoustoatelephonecall twoapplicationsmustrequestthataconnectionbecreatedonceithasbeenestablished theconnectionallowstheapplicationstosenddataineitherdirectionfinally whentheyfinishcommunicating theapplicationsrequestthattheconnectionbeterminatedAlgorithm3 1summarizestheinteraction 2009PearsonEducationInc UpperSaddleRiver NJ Allrightsreserved 12 3 4TheClient ServerModelofInteraction ThefirststepinAlgorithm3 1raisesaquestion howcanapairofapplicationsthatrunontwoindependentcomputerscoordinatetoguaranteethattheyrequestaconnectionatthesametime Theanswerliesinaformofinteractionknownastheclient servermodelAserverstartsfirstandawaitscontactAclientstartssecondandinitiatestheconnectionFigure3 2summarizestheinteractionSubsequentsectionsdescribehowspecificservicesusetheclient servermodelApplicationprogramsknownasclientsandservershandleallservicesintheInternet 2009PearsonEducationInc UpperSaddleRiver NJ Allrightsreserved 13 3 4TheClient ServerModelofInteraction 2009PearsonEducationInc UpperSaddleRiver NJ Allrightsreserved 14 3 5CharacteristicsofClientsandServers Mostinstancesofclient serverinteractionhavethesamegeneralcharacteristicsAclientsoftware Isanarbitraryapplicationprogramthatbecomesaclienttemporarilywhenremoteaccessisneeded butalsoperformsothercomputationIsinvokeddirectlybyauser andexecutesonlyforonesessionRunslocallyonauser spersonalcomputerActivelyinitiatescontactwithaserverCanaccessmultipleservicesasneeded butusuallycontactsoneremoteserveratatimeDoesnotrequireespeciallypowerfulcomputerhardware 2009PearsonEducationInc UpperSaddleRiver NJ Allrightsreserved 15 3 5CharacteristicsofClientsandServers Aserversoftware Isaspecial purpose privilegedprogramIsdedicatedtoprovidingoneservicethatcanhandlemultipleremoteclientsatthesametimeIsinvokedautomaticallywhenasystemboots andcontinuestoexecutethroughmanysessionsRunsonalarge powerfulcomputerWaitspassivelyforcontactfromarbitraryremoteclientsAcceptscontactfromarbitraryclients butoffersasingleserviceRequirespowerfulhardwareandasophisticatedoperatingsystem OS 2009PearsonEducationInc UpperSaddleRiver NJ Allrightsreserved 16 3 6ServerProgramsandServer ClassComputers TermserverreferstoaprogramthatwaitspassivelyforcommunicationNottothecomputeronwhichitexecutesHowever whenacomputerisdedicatedtorunningoneormoreserverprograms thecomputeritselfissometimescalledaserverHardwarevendorscontributetotheconfusionbecausetheyclassifycomputersthathavefastCPUs largememories andpowerfuloperatingsystemsasservermachinesFigure3 3illustratesthedefinitions 2009PearsonEducationInc UpperSaddleRiver NJ Allrightsreserved 17 3 6ServerProgramsandServer ClassComputers 2009PearsonEducationInc UpperSaddleRiver NJ Allrightsreserved 18 3 7Requests Responses andDirectionofDataFlow Clientandserver Whichsideinitiatescontact Oncecontacthasbeenestablished two waycommunicationispossible i e datacanflowfromaclienttoaserverorfromaservertoaclient Insomecases aclientsendsaseriesofrequestsandtheserverissuesaseriesofresponses e g adatabaseclientmightallowausertolookupmorethanoneitematatime 2009PearsonEducationInc UpperSaddleRiver NJ Allrightsreserved 19 3 8MultipleClientsandMultipleServers AllowingagivencomputertooperatemultipleserversisusefulbecausethehardwarecanbesharedasinglecomputerhaslowersystemadministrationoverheadthanmultiplecomputersystemsexperiencehasshownthatthedemandforaserverisoftensporadicaservercanremainidleforlongperiodsoftimeanidleserverdoesnotusetheCPUwhilewaitingforarequesttoarriveIfdemandforservicesislow consolidatingserversonasinglecomputercandramaticallyreducecostwithoutsignificantlyreducingperformance 2009PearsonEducationInc UpperSaddleRiver NJ Allrightsreserved 20 3 8MultipleClientsandMultipleServers Acomputercanrun AsingleclientAsingleserverMultiplecopiesofaclientthatcontactagivenserverMultipleclientsthateachcontactaparticularserverMultipleservers eachforaparticularserviceAllowingacomputertooperatemultipleclientsisusefulbecauseservicescanbeaccessedsimultaneouslyForexample ausercanhavethree 3 windowsopensimultaneouslyrunningthree 3 applications onethatretrievesanddisplaysemailanotherthatconnectstoachatserviceandathirdrunningawebbrowser 2009PearsonEducationInc UpperSaddleRiver NJ Allrightsreserved 21 3 9ServerIdentificationandDemultiplexing Howdoesaclientidentifyaserver TheInternetprotocolsdivideidentificationintotwopieces AnidentifierforthecomputeronwhichaserverrunsAnidentifierforaserviceonthecomputerIdentifyingacomputer EachcomputerintheInternetisassignedaunique32 bitidentifierknownasanInternetProtocoladdress IPaddress Aclientmustspecifytheserver sIPaddressTomakeserveridentificationeasyforhumans eachcomputerisalsoassignedaname andtheDomainNameSystem DNS describedinChapter4isusedtotranslateanameintoanaddressThus ratherthananintegeraddress 2009PearsonEducationInc UpperSaddleRiver NJ Allrightsreserved 22 3 9ServerIdentificationandDemultiplexing Identifyingaservice EachserviceavailableintheInternetisassignedaunique16 bitidentifierknownasaprotocolportnumber orportnumber Examples email portnumber25 andtheweb portnumber80WhenaserverbeginsexecutionitregisterswithitslocalOSbyspecifyingtheportnumberforitsserviceWhenaclientcontactsaremoteservertorequestservicetherequestcontainsaportnumberWhenarequestarrivesataserversoftwareontheserverusestheportnumberintherequesttodeterminewhichapplicationontheservercomputershouldhandletherequestFigure3 4summarizesthebasicsteps 2009PearsonEducationInc UpperSaddleRiver NJ Allrightsreserved 23 3 9ServerIdentificationandDemultiplexing 2009PearsonEducationInc UpperSaddleRiver NJ Allrightsreserved 24 3 10ConcurrentServers ThestepsinFigure3 4implythataserverhandlesoneclientatatimeAlthoughaserialapproachworksinafewtrivialcases mostserversareconcurrentThatis aserverusesmorethanonethreadofcontrolConcurrentexecutiondependsontheOSbeingusedConcurrentservercodeisdividedintotwopiecesamainprogram thread ahandlerThemainthreadacceptscontactfromaclientandcreatesathreadofcontrolfortheclientEachthreadofcontrolinteractswithasingleclientandrunsthehandlercode 2009PearsonEducationInc UpperSaddleRiver NJ Allrightsreserved 25 3 10ConcurrentServers AfterhandlingoneclientthethreadterminatesThemainthreadkeepstheserveraliveaftercreatingathreadtohandlearequestthemainthreadwaitsforanotherrequesttoarriveIfNclientsaresimultaneouslyusingaconcurrentserver N 1threadswillberunning themainthread 1 iswaitingforadditionalrequestsandNthreadsareeachinteractingwithasingleclient 2009PearsonEducationInc UpperSaddleRiver NJ Allrightsreserved 26 3 11CircularDependenciesAmongServers Inpractice thedistinctionblursbecauseaserverforoneservicecanactasaclientforanotherForexample beforeitcanfillinawebpage awebservermayneedtobecomeaclientofadatabaseAservermayalsobecometheclientofasecurityservice e g toverifythataclientisallowedtoaccesstheservice ProgrammersmustbecarefultoavoidcirculardependenciesamongserversForexample considerwhatcanhappenifaserverforserviceXbecomesaclientofserviceX whichbecomesaclientofserviceX whichbecomesaclientofXThechainofrequestscancontinueindefinitelyuntilallthreeserversexhaustresourcesThepotentialforcircularityisespeciallyhighwhenservicesaredesignedindependentlybecausenosingleprogrammercontrolsallservers 2009PearsonEducationInc UpperSaddleRiver NJ Allrightsreserved 27 3 12Peer to PeerInteractions IfasingleserverprovidesagivenservicethenetworkconnectionbetweentheserverandtheInternetcanbecomeabottleneckFigure3 5illustratesthearchitecture 2009PearsonEducationInc UpperSaddleRiver NJ Allrightsreserved 28 3 12Peer to PeerInteractions CanInternetservicesbeprovidedwithoutcreatingacentralbottleneck Onewaytoavoidabottleneckformsthebasisoffilesharingknownasapeer to peer P2P architectureTheschemeavoidsplacingdataonacentralserverdataisdistributedequallyamongasetofNserversandeachclientrequestissenttotheappropriateserveragivenserveronlyprovides1 NofthedatatheamountoftrafficbetweenaserverandtheInternetis1 Nasmuchasinthesingle serverarchitectureServersoftwarecanrunonthesamecomputersasclientsFigure3 6illustratesthearchitecture 2009PearsonEducationInc UpperSaddleRiver NJ Allrightsreserved 29 3 12Peer to PeerInteractions 2009PearsonEducationInc UpperSaddleRiver NJ Allrightsreserved 30 3 13NetworkProgrammingandtheSocketAPI TheinterfaceanapplicationusestospecifycommunicationisknownasanApplicationProgramInterface API Appendix1containsasimplifiedAPI withonlysevenfunctions examplecodethatdemonstrateshowsuchanAPIcanbeusedtocreateInternetapplications includingaworkingwebserverDetailsofanAPIdependontheOSOneparticularAPIhasemergedasthedefactostandardforsoftwarethatcommunicatesovertheInternetknownasthesocketAPI andcommonlyabbreviatedsocketsThesocketAPIisavailableformanyOSsuchasMicrosoft sWindowssystemsaswellasvariousUNIXsystems includingLinux 2009PearsonEducationInc UpperSaddleRiver NJ Allrightsreserved 31 3 14Sockets Descriptors andNetworkI O BecauseitwasoriginallydevelopedaspartoftheUNIXOS thesocketAPIisintegratedwithI OWhenanapplicationcreatesasockettouseforInternetOSreturnsasmallintegerdescriptorthatidentifiesthesocketTheapplicationthenpassesthedescriptorasanargumentwhenitcallsfunctionstoperformanoperationonthesocket e g totransferdataacrossthenetworkortoreceivedata InmanyOS socketdescriptorsareintegratedwithotherI Odescriptorsasaresult anapplicationcanusethereadandwriteoperationsforsocketI OorI Otoafile 2009PearsonEducationInc UpperSaddleRiver NJ Allrightsreserved 32 3 15ParametersandtheSocketAPI SocketprogrammingdiffersfromconventionalI OAnapplicationmustspecifymanydetails suchastheaddressofaremotecomputertheprotocolportnumberandwhethertheapplicationwillactasaclientorasaserverToavoidhavingasinglesocketfunctionwithmanyparameters designersofthesocketAPIchosetodefinemanyfunctionsAnapplicationcreatesasocket andtheninvokesfunctionsfordetailsTheadvantageofthesocketapproachisthatmostfunctionshavethreeorfewerparametersThedisadvantageisthataprogrammermustremembertocallmultiplefunctionswhenusingsocketsFigure3 7summarizeskeyfunctionsinthesocketAPI 2009PearsonEducationInc UpperSaddleRiver NJ Allrightsreserved 33 2009PearsonEducationInc UpperSaddleRiver NJ Allrightsreserved 34 3 16SocketCallsinaClientandServer Figure3 8illustratesthesequenceofsocketcallsmadebyatypicalclientandserverthatuseastreamconnectionTheclientsendsdatafirstandtheserverwaitstoreceivedataInpractice someapplicationsarrangefortheservertosendfirst i e sendandrecvarecalledinthereverseorder 2009PearsonEducationInc UpperSaddleRiver NJ Allrightsreserved 35 2009PearsonEducationInc UpperSaddleRiver NJ Allrightsreserved 36 3 17SocketFunctionsUsedbyBothClientandServer Readersareencouragedtocheckthebookforthedetailsofthesefunctions3 17 1TheSocketFunction3 17 2TheSendFunction3 17 3TheRecvFunction3 17 4ReadandWritewithSockets3 17 5TheCloseFunction 2009PearsonEducationInc UpperSaddleRiver NJ Allrightsreserved 37 3 18TheConnectionFunctionUsedOnlybyaClient Clientscallconnecttoestablishaconnectionwithaspecificserver Theformis connect socket saddress saddresslen ArgumentsocketisthedescriptorofasockettousefortheconnectionReadersareencouragedtocheckthebookforthedetails 2009PearsonEducationInc UpperSaddleRiver NJ Allrightsreserved 38 3 19SocketFunctionsUsedOnlybyaServer Readersareencouragedtocheckthebookforthedetailsofthesefunctions3 19 1TheBindFuncti