數(shù)據(jù)與計(jì)算機(jī)通信課件22

WilliamStallings

DataandComputerCommunications

7thEditionChapter22DistributedApplicationsElectronicMailMostheavilyusedapplicationonanynetworkSimpleMailTransferProtocol(SMTP)TCP/IPDeliveryofsimpletextmessagesMulti-purposeInternetMailExtension(MIME)DeliveryofothertypesofdataVoice,images,videoclipsSMTPRFC821NotconcernedwithformatofmessagesordataCoveredinRFC822(seelater)SMTPusesinfowrittenonenvelopeofmailMessageheaderDoesnotlookatcontentsMessagebodyExcept:Standardizemessagecharactersetto7bitASCIIAddloginfotostartofmessageShowspathtakenBasicOperationMailcreatedbyuseragentprogram(mailclient)Messageconsistsof:Headercontainingrecipient’saddressandotherinfoBodycontaininguserdataMessagesqueuedandsentasinputtoSMTPsenderprogramTypicallyaserverprocess(daemononUNIX)MailMessageContentsEachqueuedmessagehas:MessagetextRFC822headerwithmessageenvelopeandlistofrecipientsMessagebody,composedbyuserAlistofmaildestinationsDerivedbyuseragentfromheaderMaybelistedinheaderMayrequireexpansionofmailinglistsMayneedreplacementofmnemonicnameswithmailboxnamesIfBCCsindicated,useragentneedstopreparecorrectmessageformatSMTPSenderTakesmessagefromqueueTransmitstoproperdestinationhostViaSMTPtransactionOveroneormoreTCPconnectionstoport25HostmayhavemultiplesendersactiveHostshouldbeabletocreatereceiversondemandWhendeliverycomplete,senderdeletesdestinationfromlistforthatmessageWhenalldestinationsprocessed,messageisdeletedOptimizationIfmessagedestinedformultipleusersonagivenhost,itissentonlyonceDeliverytousershandledatdestinationhostIfmultiplemessagesreadyforgivenhost,asingleTCPconnectioncanbeusedSavesoverheadofsettingupanddroppingconnectionPossibleErrorsHostunreachableHostoutofoperationTCPconnectionfailduringtransferSendercanre-queuemailGiveupafteraperiodFaultydestinationaddressUsererrorTargetuserchangedaddressRedirectifpossibleInformuserifnotSMTPProtocol-ReliabilityUsedtotransfermessagesfromsendertoreceiveroverTCPconnectionAttemptstoprovidereliableserviceNoguaranteetorecoverlostmessagesNoendtoendacknowledgementtooriginatorErrorindicationdeliverynotguaranteedGenerallyconsideredreliableSMTPReceiverAcceptsarrivingmessagePlacesinusermailboxorcopiestooutgoingqueueforforwardingReceivermust:VerifylocalmaildestinationsDealwitherrorsTransmissionLackofdiskspaceSenderresponsibleformessageuntilreceiverconfirmcompletetransferIndicatesmailhasarrivedathost,notuserSMTPForwardingMostlydirecttransferfromsenderhosttoreceiverhostMaygothroughintermediatemachineviaforwardingcapabilitySendercanspecifyrouteTargetusermayhavemovedConversationSMTPlimitedtoconversationbetweensenderandreceiverMainfunctionistotransfermessagesRestofmailhandlingbeyondscopeofSMTPMaydifferbetweensystemsSMTPMailFlowSMTPSystemOverviewCommandsandresponsesbetweensenderandreceiverInitiativewithsenderEstablishesTCPconnectionSendersendscommandstoreceivere.g.HELO<SP><domain><CRLF>Eachcommandgeneratesexactlyonereplye.g.250requestedmailactionok;completedSMTPRepliesLeadingdigitindicatescategoryPositivecompletionreply(2xx)Positiveintermediatereply(3xx)Transientnegativecompletionreply(4xx)Permanentnegativecompletionreply(5xx)OperationPhasesConnectionsetupExchangeofcommand-responsepairsConnectionterminationConnectionSetupSenderopensTCPconnectionwithreceiverOnceconnected,receiveridentifiesitself220<domain>servicereadySenderidentifiesitselfHELOReceiveracceptssender’sidentification250OKIfmailservicenotavailable,step2abovees:421servicenotavailableMailTransferSendermaysendoneormoremessagestoreceiverMAILcommandidentifiesoriginatorGivesreversepathtousedforerrorreportingReceiverreturns250OKorappropriatefail/errormessageOneormoreRCPTcommandsidentifiesrecipientsforthemessageSeparatereplyforeachrecipientDATAcommandtransfersmessagetextEndofmessageindicatedbylinecontainingjustperiod(.)ClosingConnectionTwostepsSendersendsQUITandwaitsforreplyTheninitiateTCPcloseoperationReceiverinitiatesTCPcloseaftersendingreplytoQUITFormatforTextMessages

RFC882MessageviewedashavingenvelopeandcontentsEnvelopecontainsinformationrequiredtotransmitanddelivermessageMessageissequenceoflinesoftextUsesgeneralmemoframeworkHeaderusuallykeywordfollowedbycolonfollowedbyargumentsExampleMessageDate:Tue,16Jan199610:37:17(EST)From:“WilliamStallings”<>Subject:ThesyntaxofRFC822To:Cc:Thisisthemaintext,delimitedfromtheheaderbyablankline.MultipurposeInternetMailExtension(MIME)ExtensiontoRFC822SMTPcannottransmitexecutablesUuencodeandotherschemesareavailableNotstandardizedCannottransmittextincludinginternationalcharacters(e.g.a,?,?,è,é,ê,?)Need8bitASCIIServersmayrejectmailovercertainsizeTranslationbetweenASCIIandEBCDICnotstandardSMTPgatewaystoX.400cannothandlenonetextdatainX.400messagesSomeSMTPimplementationsdonotadheretostandardCRLF,truncateorwraplonglines,removalofwhitespace,etc.OverviewofMIMEFivenewmessageheaderfieldsMIMEversionContenttypeContenttransferencodingContentIdContentDescriptionNumberofcontentformatsdefinesTransferencodingdefinedContentTypesTextbodyMultipartMixed,Parallel,Alternative,DigestMessageRFC822,Partial,External-bodyImagejpeg,gifVideompegAudioBasicApplicationPostscriptoctetstreamMIMETransferEncodingsReliabledeliveryacrosswidelargestrangeofenvironmentsContenttransferencodingfieldSixvaluesThree(7bit,8bit,binary)noencodingdoneProvideinfoaboutnatureofdataQuoted-printableDatalargelyprintableASCIIcharactersNon-printingcharactersrepresentedbyhexcodeBase64MapsarbitrarybinaryinputontoprintableoutputX-tokenNamednonstandardencodingRadix-64EncodingHypertextTransferProtocol

HTTPUnderlyingprotocoloftheWorldWideWebNotaprotocolfortransferringhypertextFortransmittinginformationwithefficiencynecessaryforhypertextjumpsCantransferplaintext,hypertext,audio,images,andInternetaccessibleinformationHTTPOverviewTransactionorientedclient/serverprotocolUsuallybetweenWebbrowser(clinet)andWebserverUsesTCPconnectionsStatelessEachtransactiontreatedindependentlyEachnewTCPconnectionforeachtransactionTerminateconnectionwhentransactioncompleteKeyTermsCacheClientConnectionEntityGatewayMessageOriginserverProxyResourceServerTunnelUseragentExamplesofHTTPOperationIntermediateHTTPSystemsHTTPMessagesRequestsClienttoserverResponsesServertoclientRequestlineResponselineGeneralheaderRequestheaderResponseheaderEntityheaderEntitybodyHTTPMessageStructureGeneralHeaderFieldsCachecontrolConnectionDataForwardedKeepaliveMIMEversionPragmaUpgradeRequestMethodsRequest-Line=Method<SP>Request_URL<SP>HTTP-Version<CRLF>Methods:OptionsGetHeadPostPutPatchCopyMoveDeleteLinkUnlinkTraceWrappedExtension-methodRequestHeaderFieldAcceptAcceptcharsetAcceptencodingAcceptlanguageAuthorizationFromHostIfmodifiedsinceProxyauthenticationRangeReferrerUnlessUseragentResponseMessagesStatuslinefollowedbyoneormoregeneral,responseandentityheaders,followedbyoptionalentitybodyStatus-Line=HTTP-Version<SP>Status-Code<SP>Reason-Phrase<CRLF>StatusCodesInformationalSuccessfulRedirectionClienterrorServererrorResponseHeaderFieldsLocationProxyauthenticationPublicRetryafterServerWWW-AuthenticateEntityHeaderFieldsAllowContentencodingContentlanguageContentlengthContentMD5ContentrangeContenttypeContentversionDerivedfromExpiresLastmodifiedLinkTitleTransferencodingURLheaderExtensionheaderEntityBodyArbitrarysequenceofoctetsHTTPtransfersanytypeofdataincluding:textbinarydataaudioimagesvideoInterpretationofdatadeterminedbyheaderfieldsContentencoding,contenttype,transferencodingNetworkManagement-SNMPSimpleNetworkManagementProtocolNetworksareingindispensableMorecomplexitymakesfailuremorelikelyRequireautomaticnetworkmanagementtoolsStandardsrequiredtoallowmulti-vendornetworksCovering:ServicesProtocolsManagementinformationbase(MIB)NetworkManagementSystemsCollectionoftoolsfornetworkmanagementSingleoperatorinterfacePowerful,userfriendlycommandsetPerformingmostorallmanagementtasksMinimalamountofseparateequipmenti.e.useexistingequipmentViewentirenetworkasunifiedarchitectureActiveelementsprovideregularfeedbackKeyElementsManagementstationormanagerAgentManagementinformationbaseNetworkmanagementprotocolManagementStationStandalonesystemorpartofsharedsystemInterfaceforhumannetworkmanagerSetofmanagementapplicationsDataanalysisFaultrecoveryInterfacetomonitorandcontrolnetworkTranslatemanager’srequirementsintomonitoringandcontrolofremoteelementsDatabaseofnetworkmanagementinformationextractedfrommanagedentitiesManagementAgentHosts,bridges,hubs,routersequippedwithagentsoftwareAllowthemtobemanagedfrommanagementstationRespondtorequestsforinformationRespondtorequestsforactionAsynchronouslysupplyunsolicitedinformationManagementInformationBaseMIBRepresentationofnetworkresourcesasobjectsEachobjectavariablerepresentingoneaspectofmanagedobjectMIBiscollectionofaccesspointsatagentformanagementofstationObjectsstandardizedacrossclassofsystemBridge,routeretc.NetworkManagementProtocolLinkbetweenmanagementstationandagentTCP/IPusesSNMPOSIusesCommonManagementInformationProtocol(CMIP)SNMPv2(enhancedSNMP)forOSIandTCP/IPProtocolCapabilitiesGetSetNotifyManagementLayoutMaybecentralizedinsimplenetworkMaybedistributedinlarge,complexnetworkMultiplemanagementserversEachmanagespoolofagentsManagementmaybedelegatedtointermediatemanagerExampleofDistributedNetworkManagementConfigurationNetworkManagementProtocolArchitectureApplication-levelprotocolPartofTCP/IPprotocolsuiteRunsoverUDPFrommanagementstation,threetypesofSNMPmessagesissuedGetRequest,GetNextRequest,andSetRequestPort161AgentreplieswithGetResponseAgentmayissuetrapmessageinresponsetoeventthataffectsMIBandunderlyingmanagedPort162SNMPv1ConfigurationRoleofSNMPv1SNMPv1August1988SNMPspecificationissuedStandalonemanagementstationsandbridges,routersworkstationsetcsuppliedwithagentsDefineslimited,easilyimplementedMIBofscalarvariablesandtwodimensionaltablesStreamlinedprotocolLimitedfunctionalityLackofsecuritySNMPv21993,revised1996RFC1901-1908SNMPv2(1)Frameworkonwhichnetworkmanagementapplicationscanbebuilte.gfaultmanagement,performancemonitoring,accountingProtocolusedtoexchangemanagementinformationEachplayermaintainslocalMIBStructuredefinedinstandardAtleastonesystemresponsibleformanagementHous