现代计算机网络讲义7(英语)+应用层课件

TheApplicationLayerChapter71TheApplicationLayerChapter77.1DNS–TheDomainNameSystemMapASCIIstringsontonetworkaddressesThereasonsforusingDNSThesizeofthefilewouldbecometoolarge.HostnameconflictswouldoccurconstantlyTheloadandlatency.AhierarchicalDomain-basednamingschemeAdistributeddatabasesystemTheDNSNameSpaceResourceRecordsNameServers27.1DNS–TheDomainNameSyst7.1.1TheDNSNameSpaceAportionoftheInternetdomainnamespace.37.1.1TheDNSNameSpaceAport7.1.2ResourceRecords(1)EverydomaincanhaveasetofresourcerecordsDNSistomapdomainnamesontoresourcerecordsAresourcerecordisafive-tuple:Domain_nameTime_to_liveClassTypeValueTheprincipalDNSresourcerecordstypes.47.1.2ResourceRecords(1)Ever7.1.2ResourceRecords(2)AportionofapossibleDNSdatabaseforcs.vu.nl.57.1.2ResourceRecords(2)Apo7.1.3NameServers(1)PartoftheDNSnamespaceshowingthedivisionintozones.Eachzonecontainssomepartofthetreeandalsocontainsnameserversholdingtheinformationaboutthatzone.67.1.3NameServers(1)Partoft7.1.3NameServers(2)Recursivequeryaresolverlooksuparemotenameineightsteps.Orreturnthenameofthenextservertotry77.1.3NameServers(2)Recursiv7.2ElectronicMailArchitectureandServicesTheUserAgentMessageFormatsMessageTransferFinalDelivery87.2ElectronicMailArchitectur7.2.1ArchitectureandServicesBasicfunctionsCompositionTransferReportingDisplayingDispositionE-mailMessages:aprimitiveenvelopesomenumberofheaderfields,ablankline,themessagebody.97.2.1ArchitectureandService7.2.2TheUserAgentSendingE-mailUsermustprovidethemessage,thedestinationaddress,andpossiblysomeotherparameters.ReadingE-mailFromtheuser'smailboxforincominge-mail107.2.2TheUserAgentSendingE-7.2.3MessageFormats-RFC822(1)RFC822headerfieldsrelatedtomessagetransport.117.2.3MessageFormats-RFC822(7.2.3MessageFormats–RFC822(2)SomefieldsusedintheRFC822messageheader.127.2.3MessageFormats–RFC82Problemswithinternationallanguages:Languageswithaccents(French,German).Languagesinnon-Latinalphabets

(Hebrew,Russian).Languageswithoutalphabets(Chinese,Japanese).Messagesnotcontainingtextatall(audioorimages).MIME–MultipurposeInternetMailExtensions7.2.3MessageFormats–RFC822(3)13Problemswithinternationalla7.2.3MessageFormats–RFC822(4)RFC822headersaddedbyMIME.147.2.3MessageFormats–RFC827.2.3MessageFormats–RFC822(5)TheMIMEtypesandsubtypesdefinedinRFC2045.157.2.3MessageFormats–RFC82Amultipartmessagecontainingenrichedandaudioalternatives.16Amultipartmessagecontaining7.2.4MessageTransfer(1)SMTP—TheSimpleMailTransferProtocolasimpleASCIIprotocolEstablishingtheTCPconnectionatport25Sendingmachine,operatingastheclientReceivingmachine,operatingastheserver,talkfirst.Theserverstartsbysendingitsidentityandtellingwhetheritispreparedtoreceivemail.Ifitisnot,theclientreleasestheconnectionandtriesagainlater.Ifitis,theclientannounceswhomthee-mailiscomingfromandwhomitisgoingto.Ifsucharecipientexistsatthedestination,theservergivestheclientthego-aheadtosendthemessage.Thentheclientsendsthemessageandtheserveracknowledgesit.177.2.4MessageTransfer(1)SMTP7.2.4MessageTransfer(2)Transferringamessagefromelinor@tocarolyn@.187.2.4MessageTransfer(2)TranSomeoneisnotOn-linePOP3(PostOfficeProtocolVersion3)Theuseragentstocopye-mailfromthetransferagentThreestatusAuthorization.Transactions.Update7.2.5FinalDelivery(1)19SomeoneisnotOn-line7.2.5F7.2.5FinalDelivery(2)theuseragentandthemessagetransferagentrunsonthesamemachine.thereceiverhasadial-upconnectiontoanISP.207.2.5FinalDelivery(2)theu7.2.5FinalDelivery(3)UsingPOP3tofetchthreemessages.POP3serverlistensatport110217.2.5FinalDelivery(3)Using7.3TheWorldWideWebArchitecturalOverviewStaticWebDocumentsDynamicWebDocumentsHTTP–TheHyperTextTransferProtocolPerformanceEnhancementsTheWirelessWeb227.3TheWorldWideWebArchitec7.3.1ArchitecturalOverview(1)ThepartsoftheWebmodel.237.3.1ArchitecturalOverview(TheClientSide(1)ThebrowserdeterminestheURL(byseeingwhatwasselected).ThebrowserasksDNSfortheIPaddress.DNSreplieswithaddressThebrowsermakesaTCPconnectiontoport80ItthensendsarequestaskingforfileTheserversendsthefileTheTCPconnectionisreleased.Thebrowserdisplaysallthetextinthefile.Thebrowserfetchesanddisplaysallimagesinthisfile.24TheClientSide(1)ThebrowserTheClientSide(2)Todisplaythefile,thebrowserhastounderstanditsformat.

HTMLinterpreterAbrowserplug-in,fromaspecialdirectoryonthediskAhelperapplication,programsindependentlyofthebrowser25TheClientSide(2)TodisplayTheServerSide(1)AcceptaTCPconnectionfromaclient(abrowser).Getthenameofthefilerequested.Getthefile(fromdisk).Returnthefiletotheclient.ReleasetheTCPconnection26TheServerSide(1)AcceptaTCTheServerSide(2)GetthefileCache,savingsintimeshouldworththeexpenseMultithreadMultipledisk27TheServerSide(2)GetthefilTheServerSide(3)domorethanjustreturnfilesResolvethenameoftheWebpagerequested.Authenticatetheclient.Accesscontrolontheclient.AccesscontrolontheWebpage.Checkthecache.Fetchtherequestedpagefromdisk.DeterminetheMIMEtypetoincludeintheresponse.Takecareofmiscellaneousoddsandends.Returnthereplytotheclient.Makeanentryintheserverlog.28TheServerSide(3)domorethaTheServerSide(4)Aserverfarm.MultipleCPUFrontendkeeptrackofwhereitsendseachrequestandsendsubsequentrequestsforthesamepagetothesamenode.29TheServerSide(4)AserverfaTheServerSide(5)(a)Normalrequest-replymessagesequence.(b)SequencewhenTCPhandoffisused.30TheServerSide(5)(a)Normal7.3.4HTTP—TheHyperTextTransferProtocolConnectionHTTP1.0,oneconnectiononlyforonerequestHTTP1.1,persistentconnection,formultiplerequestThebuilt-inHTTPrequestmethodsGETHEADPUTPOSTDELETETRACECONNECTOPTIONSMessageHeaders317.3.4HTTP—TheHyperTextTrans7.3.5PerformanceEnhancement*

WorldWideWaitCachingServerreplicationContentdeliverynetworks.327.3.5PerformanceEnhancement*Caching(1)Whoshoulddothecatching?Hierarchicalcachingwiththreeproxies.33Caching(1)33Caching(2)Howlongshouldpagesbecached?thecacheabilityofapagemayvarywildlyovertimeBasetheholdingtimeontheLast-ModifiedtimeIf-Modified-Sinceproactivecaching34Caching(2)HowlongshouldpagServerReplication-MirroringStaticmirroredsitesAutomaticallycloneitselftohandleamassiveincreaseintrafficDynamicreplicasonaper35ServerReplication-MirroringStContentDeliveryNetworks(1)PagepreprocessOriginalWebpageSamepageaftertransformation.36ContentDeliveryNetworks(1)PaStepsinlookingupaURLwhenaCDNisused.ContentDeliveryNetworks(2)37StepsinlookingupaURLwhen7.3.6TheWirelessWeb*WAP–TheWirelessApplicationProtocolUsetheexistingdigitalwirelessinfrastructure.UserscanliterallycallupaWAPgatewayoverthewirelesslinkandsendpagerequeststoit.Thegatewaythenchecksitscacheforthepagerequested.Ifpresent,sendsit;ifabsent,itfetchesitoverthewiredInternet.Problem:AccessingtheInternetonatinyscreenPayingbytheminute387.3.6TheWirelessWeb*WAP–TWAP1.0–TheWirelessApplicationProtocol(1)TheWAPprotocolstack.39WAP1.0–TheWirelessApplicatWAP1.0(2)WAElayerusesaWML(WirelessMarkupLanguage),anapplicationofXML.TheWAParchitecture.40WAP1.0(2)WAElayerusesaWMWAP2.0

Second-GenerationWirelessWeb(1)NewfeaturesofWAP2.0.Pushmodelaswellaspullmodel.Supportforintegratingtelephonyintoapps.Multimediamessaging.Voiceanddataarestartingtomerge.Inclusionof264pictograms.Interfacetoastoragedevice.Supportforplug-insinthebrowser.41WAP2.0

Second-GenerationWirelWAP2.0

Second-GenerationWirelessWeb(2)TwoprotocolstacksXHTMLBasic,forsmallwirelessdevices..42WAP2.0

Second-GenerationWirel7.4MultimediaIntroductiontoAudioAudioCompressionStreamingAudioInternetRadioVoiceoverIPIntroductiontoVideo*VideoCompression*VideoonDemand*TheMBone–TheMulticastBackbone*437.4MultimediaIntroductionto7.4.1IntroductiontoAudioAsinewave.Samplingthesinewave.Quantizingthesamplesto4bits.447.4.1IntroductiontoAudioAs7.4.2AudioCompression(a)Thethresholdofaudibilityasafunctionoffrequency.(b)Themaskingeffect.457.4.2AudioCompression(a)The7.4.3StreamingAudio(1)AstraightforwardwaytoimplementclickablemusiconaWebpage.Delay467.4.3StreamingAudio(1)Astra7.4.3StreamingAudio(2)MediaPlayerManagetheuserinterface.Handletransmissionerrors.Decompressthemusic.Eliminatejitter.BuffersinputfromthemediaserverPlaysfromthebuffer477.4.3StreamingAudio(2)Media7.4.3StreamingAudio(3)Pullserveraslongasthereisroominthebuffer,themediaplayerkeepssendingrequeststotheserver.Unnecessarydatarequests.Theserverknowsithassentthewholefile,sowhyhavetheplayerkeepasking?Pushserver,themediaplayersendsaPLAYrequest,theserverkeepspushingdata.Themediaserverrunsatnormalplaybackspeedperfectly,nocontrolmessagesarerequiredTheserverrunsfaster.Low-watermark&High-watermarkBufferisfilledtothehigh-watermark.themediaplayertellsittopauseWhenhitsthelow-watermark,themediaplayertellstostartagain.487.4.3StreamingAudio(3)Pull7.4.3StreamingAudio(4)RTSP(RealTimeStreamingProtocol

),tooperateapushserverCommandsfromtheplayertotheserver.RTPforthedatastream497.4.3StreamingAudio(4)RTSP(TheReal-TimeTransportProtocol(RTP)Tomultiplexseveralreal-timedatastreamsontoasinglestreamofUDPpackets

ThepositionofRTPintheprotocolstack.Packetnesting7.4.3StreamingAudio(5)50TheReal-TimeTransportProtoc7.4.3StreamingAudio(6)*RTPhead.517.4.3StreamingAudio(6)*RTPRTPHeadPbit,thepackethasbeenpaddedtoamultipleof4bytes.Xbit,anextensionheaderispresent.CCfield,tellshowmanycontributingsourcesarepresent.Mbit,anapplication-specificmarkerbit.Payloadtypefield,tellswhichencodingalgorithmhasbeenusedSequencenumber,incrementedoneachRTPpacket.RetransmissionisnotapracticaloptionRTPhasnoflowcontrol,noerrorcontrol,noacknowledgements,andnomechanismtorequestretransmissions.Timestamp,tonotewhenthefirstsampleinthepacketwasmade,allowsthedestinationplayeachsampleatrighttime,independentlyofwhenthepacketarrived,allowsmultiplestreamstobesynchronizedwitheachother.Synchronizationsourceidentifiertellswhichstreamthepacketbelongsto,usedtomultiplexanddemultiplexmultipledatastreamsontoasinglestreamofUDPpackets.Contributingsourceidentifiers,usedwhenmixersarepresentinthestudio.7.4.3StreamingAudio(7)*52RTPHead7.4.3StreamingAudioRTCP(RealtimeTransportControlProtocol).Toprovidefeedbackondelay,jitter,bandwidth,congestion,andothernetworkpropertiestothesources.Interstreamsynchronization.Differentstreamsmayusedifferentclocks,withdifferentgranularitiesanddifferentdriftrates.Namingthevarioussources,todisplayonthereceiver'sscreen.7.4.3StreamingAudio(8)53RTCP(RealtimeTransportContrLiveradioisalwaysbroadcastatexactlytherateitisgeneratedandplayedbackInternetradioshouldusemulticastingwiththeRTP/RTSPprotocols.Inpractice,thesoundissentovertheTCPconnectionfromInternetradiostation7.4.4InternetRadio(2-1)54Liveradioisalwaysbroadcast7.4.4InternetRadio(2-2)Astudentradiostation.557.4.4InternetRadio(2-2)Ast7.4.5VoiceoverIP(1)TheH323architecturalmodelforInternettelephony.GatewayGatekeeper,controlstheendpointsunderitszone567.4.5VoiceoverIP(1)TheH3237.4.5VoiceoverIP(2)TheH323protocolstack.G.711,Encoding&DecodingH.245,negotiatesotheraspectsoftheconnectionRTCP,controltheRTPchannels.Q.931,establishingandreleasingconnectionsofthestandardtelephonyH.225,RAS(Registration/Admission/Status),talkingtothegatekeeperRTP,fortheactualdatatransmission.577.4.5VoiceoverIP(2)TheH327.4.5VoiceoverIP(3)ConnectbetweenPCandphone587.4.5VoiceoverIP(3)Connect7.4.5VoiceoverIP(4)Logicalchannelsbetweenthecallerandcalleeduringacall.597.4.5VoiceoverIP