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1、7ApplicationLayer ApplicationLayer Introduction DomainNameSystem DNS HierarchicalNamespace ResourceRecords Nameservers NameResolution FileTransferProtocol FTP EmailSystemandProtocols SystemComponents MessageFormat SMTP POP3 IMAP WorldWideWeb WWWArchitecture HypertextTransferProtocol HTTP Homework In
2、troduction DutiesandFeaturesofApplicationLayer ProvidingvariousapplicationstoendusersMaximumnumberofprotocolsandmostcomplexEachprotocolisspecifiedtoatypeofapplication nogeneral purposeprotocols ConventionalInternetApplicationProtocols Client serverParadigm Thetasks capabilitiesandprotocolmachinesare
3、differentateachside Client siderequestsservice Server sideprovidesservice Formsthebasisofcomputernetworks Peer to peerParadigm TheprotocolmachinesarethesameatbothsidesThesamestate flowdiagramThesamesetoftasksandresponsibilitiesThesamesetofcapabilitiesBitTorrent eMule DHT KAD DomainNameSystem DNS Hie
4、rarchicalNamespace ResourceRecords DNS DomainNameSystem Out dated hosts txt Aclient serverapplicationthatidentifieseachhostontheInternetwithauniqueuser friendlyname domainname insteadof211 68 71 130ProvidingsupportforotherapplicationsTranslatingdomainname hostname toIPaddressCfunction gethostbyname
5、WindowsXPcommands nslookup ipconfig displaydns ipconfig flushdnsFeaturesUDPdatagram DNSServerHierarchicalnamespace HosthaveacompositenamesthatareallhierarchicallyorganizedDistributeddatabaseNamingfollowsorganizationalboundaries notphysicalnetworks HierarchicalNamespace WithinanorganizationSubdivisio
6、npossibleArbitrarylevelspossible maximumdepth 128 Notstandardized controlledlocallybytheorganizationNamingpolicybythepathupwardfromleaftoroot separatedby com edu gov int mil net org ae cn zw mit cs xx cs edu bupt Genericdomains Countrydomains Unnamedroot Topleveldomains 2ndleveldomains Zones Azoneco
7、rrespondstoanadministrativeauthoritythatisresponsibleforthatportionofthehierarchye g buptcontrols ResourceRecords RR EachdomainintheDNShasoneormoreResourceRecords RRs EachRRhasthefollowinginformationOwner thedomainnameType specifiesthetypeoftheresourceinthisRRSOA StartOfAuthority identifiesthezoneA
8、HostAddressMX MailExchangerCNAME CanonicalName aliasHINFO HostInformation Class specifiestheprotocolfamilytouseIN theInternetsystemTTL specifiestheTimeToLive inunitofsecond ofthecachedRRs SampleofDNSDatabase TypeValue Domainname Time to live Class Nameservers NameResolution DNSClient ServerInteracti
9、on Client resolversoftwarerunningonclient accessatleastonenameserverMultipleDNSserversusedArrangedinhierarchyEachserverhasauthorityoveraportionofthehierarchyEachservercontainsalltherecordsforthehostsinitszoneHowdoesaserverknowaboutotherserversthatareresponsiblefortheotherzones Everyserverknowstheroo
10、tRootserverknowsaboutalltop leveldomainsEveryserverknowstheserversfurtherdownthehierarchy HierarchyofNameServers PrimaryandSecondaryServers DNSusesbackupserversPrimaryserver creates maintains andupdatesinformationaboutitszoneSecondaryserver getsitsinformationfromaprimaryserver asabackupserverincaseo
11、ffailureBothservershaveauthorityovertheirzoneISPsOfferingDNSservicetosubscribersSmallorganizationsandindividualsOnlyneeddomainnamesforcomputersrunningserversContractwithanISPfordomainservice DNSLookup ApplicationsBecomingDNSclientSendingrequestfirsttolocalnameserverLocalnameserverListeningatport53 n
12、ormallyusingUDPIfanswerknown returnsresponseIfanswerunknownRequeststop levelnameserverFollowslinksUntilreachingAuthoritativeNameServer whichperformsnametranslationandreturnsresponse NameResolutionMethods RecursiveresolutionAqueryismadetoalocalnameserverIfthequeriedserverdoesnothavetheinformation itm
13、ustmakeaquerytoanotherIterativeresolutionAniterativequeryismadetoanameserver whichmaythenrespondwiththeaddressofanotherserverthelocalnameserver onbehalfofresolver thenqueriesthatserver whichmightrespondwiththeaddressofanotherserver andsoon CommonlyusedbynameserversonInternet ExampleofRecursiveResolu
14、tion ExampleofIterativeResolution DNSCaching ServeralwayscachesanswersHostcancacheanswersWorkswellbecausehighlocalityofreferenceTTL FileTransferProtocol FTP FTP FileTransferProtocol RFC959 UsedtotransferfilesbetweenhostsUsedtomanipulateremotefilesAuthListdirectoriesDeletefilesRenamefilesUsingclient
15、servermodelUsingTCPforreliabletransfers FTPModel 1 FileSystem ServerPI ServerDTP ClientPI ClientDTP UserInterface FileSystem User FTPserver FTPclient ControlConnection DataConnection FTPModel 2 PI ProtocolInterpreter ImplementsFTPprotocol interpretingFTPcommandsandreplies DTP DataTransferProcess Est
16、ablishesandmanagesthedataconnection ControlConnection Thecommunicationpathbetweentheclient PIandserver PIfortheexchangeofFTPcommandsandrepliesDataConnection AfullduplexconnectionoverwhichdataistransferredThedatatransferredmaybeapartofafile anentirefile FTPModel 3 FTPClientUsersinteractwithFTPClientdirectlyActiveopenofcontrolconnectionControlconnectionusesASCIIplain textSendscommands overcontrolconnection Receivesreplies overcontrolconnection DataconnectionusedtotransferfiledataFTPServerSystempro
