计算机网络系统方法英文版课后习题及解答.docx
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计算机网络系统方法英文版课后习题及解答.docx
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计算机网络系统方法英文版课后习题及解答
第一章
(1.21.3节)
5.Calculatethetotaltimerequiredtotransfera1,000-KBfileinthefollowingcases,assuminganRTTof100ms,apacketsizeof1-KBdata,andaninitial2×RTTof“handshaking”beforedataissent.
(a)Thebandwidthis1.5Mbps,anddatapacketscanbesentcontinuously.
(b)Thebandwidthis1.5Mbps,butafterwefinishsendingeachdatapacket
wemustwaitoneRTTbeforesendingthenext.
(c)Thebandwidthis“infinite,”meaningthatwetaketransmittimetobe
zero,andupto20packetscanbesentperRTT.
(d)Thebandwidthisinfinite,andduringthefirstRTTwecansendone
packet(21−1),duringthesecondRTTwecansendtwopackets(22−1),
duringthethirdwecansendfour(23−1),andsoon.(Ajustificationfor
suchanexponentialincreasewillbegiveninChapter6.)
7.Considerapoint-to-pointlink2kminlength.Atwhatbandwidthwouldpropagationdelay(ataspeedof2×108m/sec)equaltransmitdelayfor100-bytepackets?
Whatabout512-bytepackets?
13.How“wide”isabitona1-Gbpslink?
Howlongisabitincopperwire,wherethespeedofpropagationis2.3×108m/s?
15.Supposea100-Mbpspoint-to-pointlinkisbeingsetupbetweenEarthandanewlunarcolony.ThedistancefromthemoontoEarthisapproximately385,000km,anddatatravelsoverthelinkatthespeedoflight—3×108m/s.
(a)CalculatetheminimumRTTforthelink.
(b)UsingtheRTTasthedelay,calculatethedelay×bandwidthproductfor
thelink.
(c)Whatisthesignificanceofhedelay×bandwidthproductcomputed
in(b)?
(d)AcameraonthelunarbasetakespicturesofEarthandsavesthemindigital
formattodisk.SupposeMissionControlonEarthwishestodownloadthe
mostcurrentimage,whichis25MB.Whatistheminimumamountof
timethatwillelapsebetweenwhentherequestforthedatagoesoutand
thetransferisfinished?
18.Calculatethelatency(fromfirstbitsenttolastbitreceived)forthefollowing:
(a)A10-MbpsEthernetwithasinglestore-and-forwardswitchinthepath,
andapacketsizeof5,000bits.Assumethateachlinkintroducesapropaga-
tiondelayof10µs,andthattheswitchbeginsretransmittingimmediately
afterithasfinishedreceivingthepacket.
(b)Sameas(a)butwiththreeswitches.
(c)Sameas(a)butassumetheswitchimplements“cut-through”switching:
it
isabletobeginretransmittingthepacketafterthefirst200bitshavebeen
received.
第二章
(除2.72.9节)
1.ShowtheNRZ,Manchester,andNRZIencodingsforthebitpatternshowninFigure2.46.AssumethattheNRZIsignalstartsoutlow.
23.ConsideranARQalgorithmrunningovera20-kmpoint-to-pointfiberlink.
(a)Computethepropagationdelayforthislink,assumingthatthespeedof
lightis2×108m/sinthefiber.
(b)SuggestasuitabletimeoutvaluefortheARQalgorithmtouse.
(c)WhymightitstillbepossiblefortheARQalgorithmtotimeoutand
retransmitaframe,giventhistimeoutvalue?
26.Thetextsuggeststhattheslidingwindowprotocolcanbeusedtoimplementflowcontrol.WecanimaginedoingthisbyhavingthereceiverdelayACKs,thatis,notsendtheACKuntilthereisfreebufferspacetoholdthenextframe.Indoingso,eachACKwouldsimultaneouslyacknowledgethereceiptofthelastframeandtellthesourcethatthereisnowfreebufferspaceavailabletoholdthenextframe.Explainwhyimplementingflowcontrolinthiswayisnotagoodidea.
44.LetAandBbetwostationsattemptingtotransmitonanEthernet.Eachhassteadyqueueofframesreadytosend;A’sframeswillbenumberedA1,A2,andsoon,andB’ssimilarly.LetT=51.2µsbetheexponentialbackoffbaseunit.SupposeAandBsimultaneouslyattempttosendframe1,collide,andhappentochoosebackofftimesof0×Tand1×T,respectively,meaningAwinstheraceandtransmitsA1whileBwaits.Attheendofthistransmission,BwillattempttoretransmitB1whileAwillattempttotransmitA2.Thesefirstattemptswillcollide,butnowAbacksoffforeither0×Tor1×T,whileBbacksofffortimeequaltooneof0×T,...,3×T.
(a)GivetheprobabilitythatAwinsthissecondbackoffraceimmediatelyafterthisfirst
collision,thatis,A’sfirstchoiceofbackofftimek×51.2islessthanB’s.
(b)SupposeAwinsthissecondbackoffrace.AtransmitsA3,andwhenitis
finished,AandBcollideagainasAtriestotransmitA4andBtriesonce
moretotransmitB1.GivetheprobabilitythatAwinsthisthirdbackoff
raceimmediatelyafterthefirstcollision.
(c)GiveareasonablelowerboundfortheprobabilitythatAwinsallthere-
mainingbackoffraces.
(d)WhatthenhappenstotheframeB1?
ThisscenarioisknownastheEthernetcaptureeffect.
48.Repeatthepreviousexercise,nowwiththeassumptionthatEthernetisp-persistentwithp=0.33(thatis,awaitingstationtransmitsimmediatelywithprobabilitypwhenthelinegoesidle,andotherwisedefersone51.2-µsslottimeandrepeatstheprocess).Yourtimelineshouldmeetcriterion
(1)ofthepreviousproblem,butinlieuofcriterion
(2),youshouldshowatleastonecollisionandatleastonerunoffourdeferralsonanidleline.Again,notethatmanysolutionsarepossible.
第三章
(3.13.2节)
1.UsingtheexamplenetworkgiveninFigure3.30,givethevirtualcircuittablesforalltheswitchesaftereachofthefollowingconnectionsisestablished.Assumethatthesequenceofconnectionsiscumulative,thatis,thefirstconnectionisstillupwhenthesecondconnectionisestablished,andsoon.AlsoassumethattheVCIassignmentalwayspicksthelowestunusedVCIoneachlink,startingwith0.
(a)HostAconnectstohostB.
(b)HostCconnectstohostG.
(c)HostEconnectstohostI.
(d)HostDconnectstohostB.
(e)HostFconnectstohostJ.
(f)HostHconnectstohostA.
3.ForthenetworkgiveninFigure3.31,givethedatagramforwardingtableforeachnode.Thelinksarelabeledwithrelativecosts;yourtablesshouldforwardeachpacketviathelowest-costpathtoitsdestination.
5.ConsiderthevirtualcircuitswitchesinFigure3.33.Table3.6lists,foreachswitch,whatport,VCI(orVCI,interface)pairsareconnectedtoother.Connectionsarebidirectional.Listallendpoint-to-endpointconnections.
13.GiventheextendedLANshowninFigure3.34,indicatewhichportsarenotselectedbythespanningtreealgorithm.
15.ConsiderthearrangementoflearningbridgesshowninFigure3.35.Assumingallareinitiallyempty,givetheforwardingtablesforeachofthebridgesB1–B4afterthefollowingtransmissions:
■AsendstoC.
■CsendstoA.
■DsendstoC.
Identifyportswiththeuniqueneighborreacheddirectlyfromthatport,that
is,theportsforB1aretobelabeled“A”and“B2.”
17.ConsiderhostsX,Y,Z,WandlearningbridgesB1,B2,B3,withinitiallyemptyforwardingtables,asinFigure3.36.
(a)SupposeXsendstoZ.WhichbridgeslearnwhereXis?
DoesY’snetwork
interfaceseethispacket?
(b)SupposeZnowsendstoX.WhichbridgeslearnwhereZ
is?
DoesY’snetworkinterfaceseethispacket?
(c)SupposeYnowsendstoX.WhichbridgeslearnwhereYis?
DoesZ’snet-
workinterfaceseethispacket?
(d)Finally,supposeZsendstoY.WhichbridgeslearnwhereZis?
DoesW’s
networkinterfaceseethispacket?
第四章
(4.14.24.3.14.3.54.5节)
4.SupposeaTCPmessagethatcontains2,048bytesofdataand20bytesofTCPheaderispassedtoIPfordeliveryacrosstwonetworksoftheInternet(i.e.,fromthesourcehosttoaroutertothedestinationhost).Thefirstnetworkuses14-byteheadersandhasanMTUof1,024bytes;theseconduses8-byteheaderswithanMTUof512bytes.Eachnetwork’sMTUgivesthesizeofthelargestIPdatagramthatcanbecarriedinalinklayerframe.Givethesizesandoffsetsofthesequenceoffragmentsdeliveredtothenetworklayeratthedestinationhost.AssumeallIPheadersare20bytes.
21.SupposearouterhasbuiltuptheroutingtableshowninTable4.14.Theroutercandeliverpacketsdirectlyoverinterfaces0and1,oritcanforwardpacketstoroutersR2,R3,orR4.Describewhattherouterdoeswithapacketaddressedtoeachofthefollowingdestinations:
(a)128.96.39.10.
(b)128.96.40.12.
(c)128.96.40.151.
(d)192.4.153.17.
(e)192.4.153.90.
45.Table4.16isaroutingtableusingCIDR.Addressbytesareinhexadecimal.Thenotation“/12”inC4.50.0.0/12denotesanetmaskwith12leading1bits,thatis,FF.F0.0.0.Notethatthelastthreeentriescovereveryaddressandthusserveinlieuofadefaultroute.Statetowhatnexthopthefollowingwillbedelivered.
(a)C4.5E.13.87.
(b)C4.5E.22.09.
(c)C3.41.80.02.
(d)5E.43.91.12.
(e)C4.6D.31.2E.
(f)C4.6B.31.2E.
第五章
(5.15.2节)
10.Youarehiredtodesignareliablebyte-streamprotocolthatusesaslidingwindow(likeTCP).Thisprotocolwillrunovera1-Gbpsnetwork.TheRTTofthenetworkis140ms,andthemaximumsegmentlifetimeis60seconds.HowmanybitswouldyouincludeintheAdvertisedWindowandSequenceNumfieldsofyourprotocolheader?
第六章
(6.5节)
第九章
(9.1节)
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