13章习题复习题参考答案.docx
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13章习题复习题参考答案.docx
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13章习题复习题参考答案
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.
4.1.Dial-upmodemovertelephoneline:
residential;2.DSLovertelephoneline:
residentialorsmalloffice;3.CabletoHFC:
residential;4.100MbpsswitchedEtherent:
company;5.WirelessLAN:
mobile;6.Cellularmobileaccess(forexample,WAP):
mobile
5.HFCbandwidthissharedamongtheusers.Onthedownstreamchannel,allpacketsemanatefromasinglesource,namely,theheadend.Thus,therearenocollisionsinthedownstreamchannel.
6.Currentpossibilitiesinclude:
dial-up;DSL;cablemodem;fiber-to-the-home.
7.EthernetLANshavetransmissionratesof10Mbps,100Mbps,1Gbpsand10Gbps.ForanXMbpsEthernet(whereX=10,100,1,000or10,000),ausercancontinuouslytransmitattherateXMbpsifthatuseristheonlypersonsendingdata.Iftherearemorethanoneactiveuser,theneachusercannotcontinuouslytransmitatXMbps.
8.Ethernetmostcommonlyrunsovertwisted-paircopperwireand“thin”coaxialcable.Italsocanrunoverfibersopticlinksandthickcoaxialcable.
9.Dialupmodems:
upto56Kbps,bandwidthisdedicated;ISDN:
upto128kbps,bandwidthisdedicated;ADSL:
downstreamchannelis.5-8Mbps,upstreamchannelisupto1Mbps,bandwidthisdedicated;HFC,downstreamchannelis10-30MbpsandupstreamchannelisusuallylessthanafewMbps,bandwidthisshared.
10.TherearetwomostpopularwirelessInternetaccesstechnologiestoday:
a)WirelessLAN
InawirelessLAN,wirelessuserstransmit/receivepacketsto/fromabasestation(wirelessaccesspoint)withinaradiusoffewtensofmeters.ThebasestationistypicallyconnectedtothewiredInternetandthusservestoconnectwirelessuserstothewirednetwork.
b)Wide-areawirelessaccessnetwork
Inthesesystems,packetsaretransmittedoverthesamewirelessinfrastructureusedforcellulartelephony,withthebasestationthusbeingmanagedbyatelecommunicationsprovider.Thisprovideswirelessaccesstouserswithinaradiusoftensofkilometersofthebasestation.
11.Acircuit-switchednetworkcanguaranteeacertainamountofend-to-endbandwidthforthedurationofacall.Mostpacket-switchednetworkstoday(includingtheInternet)cannotmakeanyend-to-endguaranteesforbandwidth.
12.Inapacketswitchednetwork,thepacketsfromdifferentsourcesflowingonalinkdonotfollowanyfixed,pre-definedpattern.InTDMcircuitswitching,eachhostgetsthesameslotinarevolvingTDMframe.
13.Attimet0thesendinghostbeginstotransmit.Attimet1=L/R1,thesendinghostcompletestransmissionandtheentirepacketisreceivedattherouter(nopropagationdelay).Becausetherouterhastheentirepacketattimet1,itcanbegintotransmitthepackettothereceivinghostattimet1.Attimet2=t1+L/R2,theroutercompletestransmissionandtheentirepacketisreceivedatthereceivinghost(again,nopropagationdelay).Thus,theend-to-enddelayisL/R1+L/R2.
14.Atier-1ISPconnectstoallothertier-1ISPs;atier-2ISPconnectstoonlyafewofthetier-1ISPs.Also,atier-2ISPisacustomerofoneormoretier-1.
15.a)2userscanbesupportedbecauseeachuserrequireshalfofthelinkbandwidth.
b)Sinceeachuserrequires1Mbpswhentransmitting,iftwoorfeweruserstransmitsimultaneously,amaximumof2Mbpswillberequired.Sincetheavailablebandwidthofthesharedlinkis2Mbps,therewillbenoqueuingdelaybeforethelink.Whereas,ifthreeuserstransmitsimultaneously,thebandwidthrequiredwillbe3Mbpswhichismorethantheavailablebandwidthofthesharedlink.Inthiscase,therewillbequeuingdelaybeforethelink.
c)Probabilitythatagivenuseristransmitting=0.2
d)Probabilitythatallthreeusersaretransmittingsimultaneously=
=(0.2)3=0.008.Sincethequeuegrowswhenalltheusersaretransmitting,thefractionoftimeduringwhichthequeuegrows(whichisequaltotheprobabilitythatallthreeusersaretransmittingsimultaneously)is0.008.
16.Thedelaycomponentsareprocessingdelays,transmissiondelays,propagationdelays,andqueuingdelays.Allofthesedelaysarefixed,exceptforthequeuingdelays,whicharevariable.
17.JavaApplet
18.10msec;d/s;no;no
19.a)500kbps
b)64seconds
c)100kbps;320seconds
20.EndsystemAbreaksthelargefileintochunks.Toeachchunk,itaddsheadergeneratingmultiplepacketsfromthefile.Theheaderineachpacketincludestheaddressofthedestination:
endsystemB.Thepacketswitchusesthedestinationaddresstodeterminetheoutgoinglink.Askingwhichroadtotakeisanalogoustoapacketaskingwhichoutgoinglinkitshouldbeforwardedon,giventhepacket’saddress.
21.JavaApplet
22.Fivegenerictasksareerrorcontrol,flowcontrol,segmentationandreassembly,multiplexing,andconnectionsetup.Yes,thesetaskscanbeduplicatedatdifferentlayers.Forexample,errorcontrolisoftenprovidedatmorethanonelayer.
23.ThefivelayersintheInternetprotocolstackare–fromtoptobottom–theapplicationlayer,thetransportlayer,thenetworklayer,thelinklayer,andthephysicallayer.TheprincipalresponsibilitiesareoutlinedinSection1.5.1.
24.Application-layermessage:
datawhichanapplicationwantstosendandpassedontothetransportlayer;transport-layersegment:
generatedbythetransportlayerandencapsulatesapplication-layermessagewithtransportlayerheader;network-layerdatagram:
encapsulatestransport-layersegmentwithanetwork-layerheader;link-layerframe:
encapsulatesnetwork-layerdatagramwithalink-layerheader.
25.Routersprocesslayers1through3.(Thisisalittlebitofawhitelie,asmodernrouterssometimesactasfirewallsorcachingcomponents,andprocesslayerfouraswell.)Linklayerswitchesprocesslayers1through2.Hostsprocessallfivelayers.
26.a)Virus
Requiressomeformofhumaninteractiontospread.Classicexample:
E-mailviruses.
b)Worms
Nouserreplicationneeded.WormininfectedhostscansIPaddressesandportnumbers,lookingforvulnerableprocessestoinfect.
c)Trojanhorse
Hidden,deviouspartofsomeotherwiseusefulsoftware.
27.Creationofabotnetrequiresanattackertofindvulnerabilityinsomeapplicationorsystem(e.g.exploitingthebufferoverflowvulnerabilitythatmightexistinanapplication).Afterfindingthevulnerability,theattackerneedstoscanforhoststhatarevulnerable.Thetargetisbasicallytocompromiseaseriesofsystemsbyexploitingthatparticularvulnerability.Anysystemthatispartofthebotnetcanautomaticallyscanitsenvironmentandpropagatebyexploitingthevulnerability.Animportantpropertyofsuchbotnetsisthattheoriginatorofthebotnetcanremotelycontrolandissuecommandstoallthenodesinthebotnet.Hence,itbecomespossiblefortheattackertoissueacommandtoallthenodes,thattargetasinglenode(forexample,allnodesinthebotnetmightbecommandedbytheattackertosendaTCPSYNmessagetothetarget,whichmightresultinaTCPSYNfloodattackatthetarget).
28.TrudycanpretendtobeBobtoAlice(andvice-versa)andpartiallyorcompletelymodifythemessage(s)beingsentfromBobtoAlice.Forexample,shecaneasilychangethephrase“Alice,Ioweyou$1000”to“Alice,Ioweyou$10,000”.Furthermore,TrudycanevendropthepacketsthatarebeingsentbyBobtoAlice(andvise-versa),evenifthepacketsfromBobtoAliceareencrypted.
Chapter1Problems:
Problem1.
Thereisnosinglerightanswertothisquestion.Manyprotocolswoulddothetrick.Here'sasimpleanswerbelow:
MessagesfromATMmachinetoServer
Msgnamepurpose
---------------
HELO
ATMcardtransmitsuserIDtoServer
PASSWD
BALANCEUserrequestsbalance
WITHDRAWL
BYEuseralldone
MessagesfromServertoATMmachine(display)
Msgnamepurpose
---------------
PASSWDAskuserforPIN(password)
OKlastrequestedoperation(PASSWD,WITHDRAWL)OK
ERRlastrequestedoperation(PASSWD,WITHDRAWL)inERROR
AMOUNT
BYEuserdone,displaywelcomescreenatATM
Correctoperation:
clientserver
HELO(userid)-------------->(checkifvaliduserid)
<-------------PASSWD
PASSWD
<-------------OK(passwordisOK)
BALANCE-------------->
<-------------AMOUNT
WITHDRAWL
<-------------OK
ATMdispenses$
BYE------------
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