2.-第二章課后習題及答案

第二章

1.(Q2)Foracommunicationsessionbetweenapairofprocesses,whichprocessistheclientandwhichistheserver?

Answer:Theprocesswhichinitiatesthecommunicationistheclient;theprocessthatwaitstobecontactedistheserver.

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2.(Q3)Whatisthedifferencebetweennetworkarchitectureandapplicationarchitecture?

Answer:Networkarchitecturereferstotheorganizationofthecommunicationprocessintolayers(e.g.,thefive-layerInternetarchitecture).Applicationarchitecture,ontheotherhand,isdesignedbyanapplicationdeveloperanddictatesthebroadstructureoftheapplication(e.g.,client-serverorP2P)

3.(Q4)Whatinformationisusedbyaprocessrunningononehosttoidentifyaprocessrunningonanotherhost?

Answer:TheIPaddressofthedestinationhostandtheportnumberofthedestinationsocket.

4.(Q6)ReferringtoFigure2.4,weseethatnoneoftheapplicationlistedinFigure2.4requiresbothnodatalossandtiming.Canyouconceiveofanapplicationthatrequiresnodatalossandthatisalsohighlytime-sensitive?

Answer:Therearenogoodexampleofanapplicationthatrequiresnodatalossandtiming.Ifyouknowofone,sendane-mailtotheauthors

5.(Q9)WhydoHTTP,FTP,SMTP,andPOP3runontopofTCPratherthanonUDP?

Answer:Theapplicationsassociatedwiththoseprotocolsrequirethatallapplicationdatabereceivedinthecorrectorderandwithoutgaps.TCPprovidesthisservicewhereasUDPdoesnot.

6.(Q11)Whatismeantbyahandshakingprotocol?

Answer:Aprotocoluseshandshakingifthetwocommunicatingentitiesfirstexchangecontrolpacketsbeforesendingdatatoeachother.SMTPuseshandshakingattheapplicationlayerwhereasHTTPdoesnot.

7.(Q13)TelnetintoaWebserverandsendamultilinerequestmessage.IncludeintherequestmessagetheIf-modified-since:headerlinetoforcearesponsemessagewiththe304NotModifiedstatuscode.

Answer:Issuedthefollowingcommand(inWindowscommandprompt)followedbytheHTTPGETmessagetothe“”webserver:

>telnet80

Sincetheindex.htmlpageinthiswebserverwasnotmodifiedsinceFri,18May200709:23:34GMT,thefollowingoutputwasdisplayedwhentheabovecommandswereissuedonSat,19May2007.Notethatthefirst4linesaretheGETmessageandheaderlinesinputbytheuserandthenext4lines(startingfromHTTP/1.1304NotModified)istheresponsefromthewebserver.

8.(Q14)Considerane-commercesitethatwantstokeepapurchaserecordforeachofitscustomers.Describehowthiscanbedonewithcookies.

Answer:Whentheuserfirstvisitsthesite,thesitereturnsacookienumber.Thiscookienumberisstoredontheuser’shostandismanagedbythebrowser.Duringeachsubsequentvisit(andpurchase),thebrowsersendsthecookienumberbacktothesite.Thusthesiteknowswhenthisuser(moreprecisely,thisbrowser)isvisitingthesite.

9.(Q15)SupposeAlice,withaWeb-basede-mailaccount(suchasHotmailorgmail),sendsamessagetoBob,whoaccesseshismailfromhismailserverusingPOP3.DiscusshowthemessagegetsfromAlice’shosttoBob’shost.Besuretolisttheseriesofapplication-layerprotocolsthatareusedtomovethemessagebetweenthetwohosts.

Answer:MessageissentfromAlice’shosttohermailserveroverHTTP.Alice’smailserverthensendsthemessagetoBob’smailserveroverSMTP.BobthentransfersthemessagefromhismailservertohishostoverPOP3.

10.(Q10)RecallthatTCPcanbeenhancedwithSSLtoprovideprocess-to-processsecurityservices,includingencryption.DoesSSLoperateatthetransportlayerortheapplicationlayer?IftheapplicationdeveloperwantsTCPtobeenhancedwithSSL,whatdoesthedeveloperhavetodo?

Answer:SSLoperatesattheapplicationlayer.TheSSLsockettakesunencrypteddatafromtheapplicationlayer,encryptsitandthenpassesittotheTCPsocket.IftheapplicationdeveloperwantsTCPtobeenhancedwithSSL,shehastoincludetheSSLcodeintheapplication.

To:Thisfieldindicatestheemailaddressofthereceiverofthemail.Intheexample,thereceiveris

prithula@

Subject:Thisgivesthesubjectofthemail(ifanyspecifiedbythesender).Intheexample,thesubjectspecifiedbythesenderis“Testmail”

Date:Thedateandtimewhenthemailwassentbythesender.Intheexample,thesendersentthemailon19thMay2007,attime23:52:36GMT.

Mime-version:MIMEversionusedforthemail.Intheexample,itis1.0.

Content-type:Thetypeofcontentinthebodyofthemailmessage.Intheexample,itis“text/html”.

Return-Path:Thisspecifiestheemailaddresstowhichthemailwillbesentifthe

receiverofthismailwantstoreplytothesender.Thisisalsousedbythesender’s

mailserverforbouncingbackundeliverablemailmessagesofmailer-daemon

errormessages.Intheexample,thereturnpathis

“prithuladhungel@”.

12.(Q18)Isitpossibleforanorganization’sWebserverandmailservertohaveexactlythesamealiasforahostname(forexample,)?WhatwouldbethetypefortheRRthatcontainsthehostnameofthemailserver?

Answer:Yesanorganization’smailserverandWebservercanhavethesamealiasforahostname.TheMXrecordisusedtomapthemailserver’shostnametoitsIPaddress.

13.(Q19)WhyisitsaidthatFTPsendscontrolinformation“out-of-band”?

Answer:FTPusestwoparallelTCPconnections,oneconnectionforsendingcontrolinformation(suchasarequesttotransferafile)andanotherconnectionforactuallytransferringthefile.Becausethecontrolinformationisnotsentoverthesameconnectionthatthefileissentover,FTPsendscontrolinformationoutofband.

14.(P6)ConsideranHTTPclientthatwantstoretrieveaWebdocumentatagivenURL.TheIPaddressoftheHTTPserverisinitiallyunknown.Whattransportandapplication-layerprotocolsbesidesHTTPareneededinthisscenario?

Answer:

Applicationlayerprotocols:DNSandHTTP

Transportlayerprotocols:UDPforDNS;TCPforHTTP

15.(P9)ConsiderFigure2.12,forwhichthereisaninstitutionalnetworkconnectedtotheInternet.Supposethattheaverageobjectsizeis900,000bitsandthattheaveragerequestratefromtheinstitution’sbrowserstotheoriginserversis10requestspersecond.AlsosupposethattheamountoftimeittakesfromwhentherouterontheInternetsideoftheaccesslinkforwardsanHTTPrequestuntilitreceivestheresponseistwosecondsonaverage(seeSection2.2.5).Modelthetotalaverageresponsetimesasthesumoftheaverageaccessdelay(thatis,thedelayfromInternetroutertoinstitutionrouter)andtheaverageInternetdelay.Fortheaverageaccessdelay,use△/(1-△β),where△istheaveragetimerequiredtosendanobjectovertheaccesslinkandβisthearrivalrateofobjectstotheaccesslink.

a.Findthetotalaverageresponsetime.

b.NowsupposeacacheisinstalledintheinstitutionalLAN.Supposethehitrateis0.6.Findthetotalresponsetime.

Answer:

ThetimetotransmitanobjectofsizeLoveralinkorrateRisL/R.TheaveragetimeistheaveragesizeoftheobjectdividedbyR:

Δ=(900,000bits)/(1,500,000bits/sec)=0.6sec

Thetrafficintensityonthelinkis(1.5requests/sec)(0.6sec/request)=0.9.Thus,theaverageaccessdelayis(0.6sec)/(1-0.9)=6seconds.Thetotalaverageresponsetimeistherefore6sec+2sec=8sec.

Thetrafficintensityontheaccesslinkisreducedby40%sincethe40%oftherequestsaresatisfiedwithintheinstitutionalnetwork.Thustheaverageaccessdelayis (0.6sec)/[1–(0.6)(0.9)]=1.2seconds.Theresponsetimeisapproximatelyzeroiftherequestissatisfiedbythecache(whichhappenswithprobability0.4);theaverageresponsetimeis1.2sec+2sec=3.2secforcachemisses(whichhappens60%ofthetime).Sotheaverageresponsetimeis(0.4)(0sec)+(0.6)(3.2sec)=1.92seconds.Thustheaverageresponsetimeisreducedfrom8secto1.92sec.

16.(P12)WhatisthedifferencebetweenMAILFROM:inSMTPandFrom:inthemailmessageitself?

Answer:TheMAILFROM:inSMTPisamessagefromtheSMTPclientthatidentifiesthesenderofthemailmessagetotheSMTPserver.TheFrom:onthemailmessageitselfisNOTanSMTPmessage,butratherisjustalineinthebodyofthemailmessage.

17.(P16)ConsiderdistributingafileofF=5GbitstoNpeers.Theserverh