7ApplicationLayer ApplicationLayer Introduction DomainNameSystem DNS HierarchicalNamespace ResourceRecords Nameservers NameResolution FileTransferProtocol FTP EmailSystemandProtocols SystemComponents MessageFormat SMTP POP3 IMAP WorldWideWeb WWWArchitecture HypertextTransferProtocol HTTP Homework Introduction DutiesandFeaturesofApplicationLayer ProvidingvariousapplicationstoendusersMaximumnumberofprotocolsandmostcomplexEachprotocolisspecifiedtoatypeofapplication nogeneral purposeprotocols ConventionalInternetApplicationProtocols Client serverParadigm Thetasks capabilitiesandprotocolmachinesaredifferentateachside Client siderequestsservice Server sideprovidesservice Formsthebasisofcomputernetworks Peer to peerParadigm TheprotocolmachinesarethesameatbothsidesThesamestate flowdiagramThesamesetoftasksandresponsibilitiesThesamesetofcapabilitiesBitTorrent eMule DHT KAD DomainNameSystem DNS HierarchicalNamespace ResourceRecords DNS DomainNameSystem Out dated hosts txt Aclient serverapplicationthatidentifieseachhostontheInternetwithauniqueuser friendlyname domainname insteadof211 68 71 130ProvidingsupportforotherapplicationsTranslatingdomainname hostname toIPaddressCfunction gethostbyname WindowsXPcommands nslookup ipconfig displaydns ipconfig flushdnsFeaturesUDPdatagram DNSServerHierarchicalnamespace HosthaveacompositenamesthatareallhierarchicallyorganizedDistributeddatabaseNamingfollowsorganizationalboundaries notphysicalnetworks HierarchicalNamespace WithinanorganizationSubdivisionpossibleArbitrarylevelspossible 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 Azonecorrespondstoanadministrativeauthoritythatisresponsibleforthatportionofthehierarchye g buptcontrols ResourceRecords RR EachdomainintheDNShasoneormoreResourceRecords RRs EachRRhasthefollowinginformationOwner thedomainnameType specifiesthetypeoftheresourceinthisRRSOA StartOfAuthority identifiesthezoneA HostAddressMX MailExchangerCNAME CanonicalName aliasHINFO HostInformation Class specifiestheprotocolfamilytouseIN theInternetsystemTTL specifiestheTimeToLive inunitofsecond ofthecachedRRs SampleofDNSDatabase TypeValue Domainname Time to live Class Nameservers NameResolution DNSClient ServerInteraction Client resolversoftwarerunningonclient accessatleastonenameserverMultipleDNSserversusedArrangedinhierarchyEachserverhasauthorityoveraportionofthehierarchyEachservercontainsalltherecordsforthehostsinitszoneHowdoesaserverknowaboutotherserversthatareresponsiblefortheotherzones EveryserverknowstherootRootserverknowsaboutalltop leveldomainsEveryserverknowstheserversfurtherdownthehierarchy HierarchyofNameServers PrimaryandSecondaryServers DNSusesbackupserversPrimaryserver creates maintains andupdatesinformationaboutitszoneSecondaryserver getsitsinformationfromaprimaryserver asabackupserverincaseoffailureBothservershaveauthorityovertheirzoneISPsOfferingDNSservicetosubscribersSmallorganizationsandindividualsOnlyneeddomainnamesforcomputersrunningserversContractwithanISPfordomainservice DNSLookup ApplicationsBecomingDNSclientSendingrequestfirsttolocalnameserverLocalnameserverListeningatport53 normallyusingUDPIfanswerknown returnsresponseIfanswerunknownRequeststop levelnameserverFollowslinksUntilreachingAuthoritativeNameServer whichperformsnametranslationandreturnsresponse NameResolutionMethods RecursiveresolutionAqueryismadetoalocalnameserverIfthequeriedserverdoesnothavetheinformation itmustmakeaquerytoanotherIterativeresolutionAniterativequeryismadetoanameserver whichmaythenrespondwiththeaddressofanotherserverthelocalnameserver onbehalfofresolver thenqueriesthatserver whichmightrespondwiththeaddressofanotherserver andsoon CommonlyusedbynameserversonInternet ExampleofRecursiveResolution ExampleofIterativeResolution DNSCaching ServeralwayscachesanswersHostcancacheanswersWorkswellbecausehighlocalityofreferenceTTL FileTransferProtocol FTP FTP FileTransferProtocol RFC959 UsedtotransferfilesbetweenhostsUsedtomanipulateremotefilesAuthListdirectoriesDeletefilesRenamefilesUsingclient servermodelUsingTCPforreliabletransfers FTPModel 1 FileSystem ServerPI ServerDTP ClientPI ClientDTP UserInterface FileSystem User FTPserver FTPclient ControlConnection DataConnection FTPModel 2 PI ProtocolInterpreter ImplementsFTPprotocol interpretingFTPcommandsandreplies DTP DataTransferProcess Establishesandmanagesthedataconnection ControlConnection Thecommunicationpathbetweentheclient PIandserver PIfortheexchangeofFTPcommandsandrepliesDataConnection AfullduplexconnectionoverwhichdataistransferredThedatatransferredmaybeapartofafile anentirefile FTPModel 3 FTPClientUsersinteractwithFTPClientdirectlyActiveopenofcontrolconnectionControlconnectionusesASCIIplain textSendscommands overcontrolconnection Receivesreplies overcontrolconnection DataconnectionusedtotransferfiledataFTPServerSystempro