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
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