答案Word文档下载推荐.docx

答案Word文档下载推荐.docx

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copyright1996-2005byJ.F.KuroseandK.W.Ross.Allrightsreserved

Chapter1ReviewQuestions

1.Thereisnodifference.Throughoutthistext,thewords“host”and“endsystem”areusedinterchangeably.EndsystemsincludePCs,workstations,Webservers,mailservers,Internet-connectedPDAs,WebTVs,etc.

2.SupposeAlice,anambassadorofcountryAwantstoinviteBob,anambassadorofcountryB,overfordinner.Alicedoesn’tsimplyjustcallBobonthephoneandsay,“cometoourdinnertablenow”.Instead,shecallsBobandsuggestsadateandtime.Bobmayrespondbysayinghe’snotavailablethatparticulardate,butheisavailableanotherdate.AliceandBobcontinuetosend“messages”backandforthuntiltheyagreeonadateandtime.Bobthenshowsupattheembassyontheagreeddate,hopefullynotmorethan15minutesbeforeoraftertheagreedtime.DiplomaticprotocolsalsoallowforeitherAliceorBobtopolitelycanceltheengagementiftheyhavereasonableexcuses.

3.Anetworkingprogramusuallyhastwoprograms,eachrunningonadifferenthost,communicatingwitheachother.Theprogramthatinitiatesthecommunicationistheclient.Typically,theclientprogramrequestsandreceivesservicesfromtheserverprogram.

4.TheInternetprovidesitsapplicationsaconnection-orientedservice（TCP）andaconnectionlessservice（UDP）.EachInternetapplicationmakesuseofonethesetwoservices.ThetwoserviceswillbediscussedindetailinChapter3.Someoftheprinciplecharacteristicsoftheconnection-orientedserviceare:

∙Twoend-systemsfirst“handshake”beforeeitherstartstosendapplicationdatatotheother.

∙Providesreliabledatatransfer,i.e.,allapplicationdatasentbyonesideoftheconnectionarrivesattheothersideoftheconnectioninorderandwithoutany

gaps.

∙Providesflowcontrol,i.e.,itmakessurethatneitherendofaconnectionoverwhelmsthebuffersintheotherendoftheconnectionbysendingtomanypacketstofast.

∙Providescongestioncontrol,i.e.,regulatestheamountofdatathatanapplicationcansendintothenetwork,helpingtopreventtheInternetfromenteringastateofgridlock.

Theprinciplecharacteristicsofconnectionlessserviceare:

∙Nohandshaking

∙Noguaranteesofreliabledatatransfer

∙Noflowcontrolorcongestioncontrol

5.Flowcontrolandcongestioncontrolaretwodistinctcontrolmechanismswithdistinctobjectives.Flowcontrolmakessurethatneitherendofaconnectionoverwhelmsthebuffersintheotherendoftheconnectionbysendingtomanypacketstofast.Congestioncontrolregulatestheamountofdatathatanapplicationcansendintothenetwork,helpingtopreventcongestioninthenetworkcore（i.e.,inthebuffersinthenetworkrouters）.

6.TheInternet’sconnection-orientedserviceprovidesreliabledatatransferbyusingacknowledgementsandretransmissions.Whenonesideoftheconnectiondoesn’treceiveanacknowledgement（fromtheothersideoftheconnection）forapacketittransmitted,itretransmitsthepacket.

7.Acircuit-switchednetworkcanguaranteeacertainamountofend-to-endbandwidth

forthedurationofacall.Mostpacket-switchednetworkstoday（includingthe

Internet）cannotmakeanyend-to-endguaranteesforbandwidth.

8.Inapacketswitchednetwork,thepacketsfromdifferentsourcesflowingonalinkdonotfollowanyfixed,pre-definedpattern.InTDMcircuitswitching,eachhostgetsthesameslotinarevolvingTDMframe.

9.Attimet0thesendinghostbeginstotransmit.Attimet1=L/R1,thesendinghostcompletestransmissionandtheentirepacketisreceivedattherouter（nopropagationdelay）.Becausetherouterhastheentirepacketattimet1,itcanbegintotransmitthepackettothereceivinghostattimet1.Attimet2=t1+L/R2,theroutercompletestransmissionandtheentirepacketisreceivedatthereceivinghost（again,nopropagationdelay）.Thus,theend-to-enddelayisL/R1+L/R2.

10.InaVCnetwork,eachpacketswitchinthenetworkcoremaintainsconnectionstateinformationforeachVCpassingthroughit.SomeofthisconnectionstateinformationismaintainedtoaVC-numbertranslationtable.（Seepage25）

11.TheconsofVCsinclude（i）theneedtohaveasignalingprotocoltoset-upandtear-downtheVCs;

（ii）theneedtomaintainconnectionstateinthepacketswitches.Forthepros,someresearchersandengineersarguethatitiseasiertoprovideQoSservices-suchasservicesthatguaranteeaminimumtransmissionrateorservices

thatguaranteemaximumend-to-endpacketdelay–whenVCsareused.

12.1.Dial-upmodemovertelephoneline:

residential;

2.DSLovertelephoneline:

residentialorsmalloffice;

3.CabletoHFC:

4.100MbpsswitchedEtherent:

company;

5.WirelessLAN:

mobile;

6.Cellularmobileaccess（forexample,WAP）:

mobile

13.Atier-1ISPconnectstoallothertier-1ISPs;

atier-2ISPconnectstoonlyafewofthetier-1ISPs.Also,atier-2ISPisacustomerofoneormoretier-1

14.APOPisagroupofoneormoreroutersinanISPsnetworkatwhichroutersinotherISPscanconnect.NAPsarelocalizednetworksatwhichmanyISPs（tier-1,tier-2andlower-tierISPs）caninterconnect.

15.HFCbandwidthissharedamongtheusers.Onthedownstreamchannel,allpacketsemanatefromasinglesource,namely,theheadend.Thus,therearenocollisionsinthedownstreamchannel.

16.EthernetLANshavetransmissionratesof10Mbps,100Mbps,1Gbpsand10Gbps.

ForanXMbpsEthernet（whereX=10,100,1,000or10,000）,ausercancontinuouslytransmitattherateXMbpsifthatuseristheonlypersonsendingdata.

Iftherearemorethanoneactiveuser,theneachusercannotcontinuouslytransmitat

XMbps.

17.Ethernetmostcommonlyrunsovertwisted-paircopperwireand“thin”coaxialcable.

Italsocanrunoverfibersopticlinksandthickcoaxialcable.

18.Dialupmodems:

upto56Kbps,bandwidthisdedicated;

ISDN:

upto128kbps,bandwidthisdedicated;

ADSL:

downstreamchannelis.5-8Mbps,upstreamchannel

isupto1Mbps,bandwidthisdedicated;

HFC,downstreamchannelis10-30MbpsandupstreamchannelisusuallylessthanafewMbps,bandwidthisshared.

19.Thedelaycomponentsareprocessingdelays,transmissiondelays,propagationdelays,andqueuingdelays.Allofthesedelaysarefixed,exceptforthequeuingdelays,whicharevariable.

20.Fivegenerictasksareerrorcontrol,flowcontrol,segmentationandreassembly,multiplexing,andconnectionsetup.Yes,thesetaskscanbeduplicatedatdifferentlayers.Forexample,errorcontrolisoftenprovidedatmorethanonelayer.

21.ThefivelayersintheInternetprotocolstackare–fromtoptobottom–theapplicationlayer,thetransportlayer,thenetworklayer,thelinklayer,andthephysicallayer.TheprincipalresponsibilitiesareoutlinedinSection1.7.1.

22.application-layermessage:

datawhichanapplicationwantstosendandpassedontothetransportlayer;

transport-layersegment:

generatedbythetransportlayerandencapsulatesapplication-layermessagewithtransportlayerheader;

network-layerdatagram:

encapsulatestransport-layersegmentwithanetwork-layerheader;

link-

layerframe:

encapsulatesnetwork-layerdatagramwithalink-layerheader.

23.Routersprocesslayers1through3.（Thisisalittlebitofawhitelie,asmodernrouterssometimesactasfirewallsorcachingcomponents,andprocesslayerfouraswell.）Linklayerswitchesprocesslayers1through2.Hostsprocessallfivelayers.

Chapter1Problems

Problem1.

Thereisnosinglerightanswertothisquestion.Manyprotocolswoulddothetrick.Here'

sasimpleanswerbelow:

MessagesfromATMmachinetoServer

Msgnamepurpose

---------------

HELO<

userid>

Letserverknowthatthereisacardinthe

ATMmachine

ATMcardtransmitsuserIDtoServerPASSWD<

passwd>

UserentersPIN,whichissenttoserverBALANCEUserrequestsbalance

WITHDRAWL<

amount>

Useraskstowithdrawmoney

BYEuseralldone

MessagesfromServertoATMmachine（display）

PASSWDAskuserforPIN（password）

OK

last

requested

operation

（PASSWD,

WITHDRAWL）

ERR

inERROR

AMOUNT<

amt>

sentinresponsetoBALANCErequest

BYEuserdone,displaywelcomescreenatATM

Correctoperation:

clientserver

HELO（userid）-------------->

（checkifvaliduserid）

<

-------------PASSWD

PASSWD<

-------------->

（checkpassword）

-------------OK（passwordisOK）BALANCE-------------->

-------------AMOUNT<

checkifenough$tocover

withdrawl

-------------OK

ATMdispens