计网-第二章作业

Chapter2注：括弧之中红色标注为与课本习题对应的编号2.1，2.2节后作业：1.(R2)Foracommunicationsessionbetweenapairofprocesses,whichprocessistheclientandwhichistheserver?答：Theprocessisalways-onhostcalledtheserver.Theprocesswhichinitiatesthecommunicationistheclient.2.(R3)Whatisthedifferencebetweennetworkarchitectureandapplicationarchitecture?答：Networkarchitecturereferstotheorganizationofthecommunicationprocessintolayers.Applicationarchitecture,ontheotherhand,isdesignedbyanapplicationdeveloperanddicatesthebroadstructureoftheapplication.3.(R4)Whatinformationisusedbyaprocessrunningononehosttoidentifyaprocessrunningonanotherhost?答：TheIPaddressofthedestinationhostandtheportnumberofthedestinationsocket.4.(R7)Supposeyouwantedtodoatransactionfromaremoteclienttoaserverasfastaspossible.WouldyouuseUDPorTCP?Why?答：WewoulduseUDP.WithUDP,thetransactioncanbecompletedinoneroundtriptime–theclientsendsthetransactionrequestintoaUDPsocket,andtheserversendsthereplybacktotheclient’sUDPsocket.WithTCP,aminimumoftwoRTTsareneeded–onetoset-uptheTCPconnection,andanotherfortheclienttosendtherequest,andfortheservertosendbackthereply.5.(R8)Listthefourbroadclassesofservicesthatatransportprotocolcanprovide.Foreachoftheserviceclasses,indicateifeitherUDPorTCP(orboth)providessuchaservice.答：(1)ReliableDataTransfer:TCPprovidesareliablebyte-streambetweenclientandserverbutUDPdoesnot.(2)Throughput:Aguaranteethatacertainvalueforthroughputwillbemaintained;Neither.(3)Timing:Aguaranteethatdatawillbedeliveredwithinaspecifiedamountoftime;Neither.(4)Security:Neither.6.(R9)WhydoHTTP,FTP,SMTP,andPOP3runontopofTCPratherthanonUDP?答：Theapplicationsassociatedwiththoseprotocolsrequirethatallapplicationdatabereceivedinthecorrectorderandwithoutgaps.TCPprovidesthisservicewhereasUDPdoesnot.7.(R11)Whatismeantbyahandshakingprotocol?答：Aprotocoluseshandshakingifthetwocommunicatingentitiesfirstexchangecontrolpacketsbeforesendingdatatoeachother.SMTPuseshandshakingattheapplicationlayerwhereasHTTPdoesnot.2.3，2.4节后作业8.(R15)SupposeAlice,withaWeb-basede-mailaccount(suchasHotmailorgmail),sendsamessagetoBob,whoaccesseshismailfromhismailserverusingPOP3.DiscusshowthemessagegetsfromAlice'shosttoBob'shost.Besuretolisttheseriesofapplication-layerprotocolsthatareusedtomovethemessagebetweenthetwohosts.答：MessageissentfromAlice’shosttohermailserveroverHTTP.Alice’smailserverthensendsthemessagetoBob’smailserveroverSMTP.BobthentransfersthemessagefromhismailservertohishostoverPOP3.9.(R17)Fromauser'sperspective,whatisthedifferencebetweenthedownload-and-deletemodeandthedownload-and-keepmodeinPOP3?答：Withdownloadanddelete,afterauserretrievesitsmessagesfromaPOPserver,themessagesaredeleted.Thisposesaproblemforthenomadicuser,whomaywanttoaccessthemessagesfrommanydifferentmachines.Inthedownloadandkeepconfiguration,messagesarenotdeletedaftertheuserretrievesthemessages.Thiscanalsobeinconvenient,aseachtimetheuserretrievesthestoredmessagesfromanewmachine,allofnon-deletedmessageswillbetransferredtothenewmachine.10.(R18)Isitpossibleforanorganization'sWebserverandmailservertohaveexactlythesamealiasforahostname(forexample,foo.com)?WhatwouldbethetypefortheRRthatcontainsthehostnameofthemailserver?答：Yes,anorganization’smailserverandWebservercanhavethesamealiasforahostname.TheMXrecordisusedtomapthemailserver’shostnametoitsIPaddress.11.(R19)WhyisitsaidthatFTPsendscontrolinformationout-of-band?答：FTPusestwoparallelTCPconnections,oneconnectionforsendingcontrolinformationandanotherconnectionforactuallytransferringthefile.Becausethecontrolinformationisnotsentoverthesameconnectionthatthefileissentover,FTPsendscontrolinformationoutofband.2.5，2.6节后作业12.(P1)Trueorfalse?a.AuserrequestsaWebpagethatconsistsofsometextandthreeimages.Forthispage,theclientwillsendonerequestmessageandreceivefourresponsemessages.b.TwodistinctWebpages(forexample,)canbesentoverthesamepersistentconnection.c.Withnonpersistentconnectionsbetweenbrowserandoriginserver,itispossibleforasingleTCPsegmenttocarrytwodistinctHTTPrequestmessages.d.TheDate:headerintheHTTPresponsemessageindicateswhentheobjectintheresponsewaslastmodified.答：a.Fb.Tc.Fd.F13.(P7)SupposewithinyourWebbrowseryouclickonalinktoobtainaWebpage.TheIPaddressfortheassociatedURLisnotcachedinyourlocalhost,soaDNSlookupisnecessarytoobtaintheIPaddress.SupposethatnDNSserversarevisitedbeforeyourhostreceivestheIPaddressfromDNS;thesuccessivevisitsincuranRTTofnRTTRTT,...,1.FurthersupposethattheWebpageassociatedwiththelinkcontainsexactlyoneobject,consistingofasmallamountofHTMLtext.Let0RTTdenotetheRTTbetweenthelocalhostandtheservercontainingtheobject.Assumingzerotransmissiontimeoftheobject,howmuchtimeelapsesfromwhentheclientclicksonthelinkuntiltheclientreceivestheobject?答：ThetimetogetIPaddressisRTT1+RTT2+……+RTTn,oncetheIPaddressisknown,theresponsetimeis2RTT0+RTT1+……+RTTn.14.(P8)ReferringtoProblemP7,supposetheHTMLfilereferencesthreeverysmallobjectsonthesameserver.Neglectingtransmissiontimes,howmuchtimeelapseswitha.Non-persistentHTTPwithnoparallelTCPconnections?b.Non-persistentHTTPwithparal