文本内容:
C卷答案
一、AACCDCBCCB
二、TFFTTTTFTF
三、
1、EthernetisapopularCSMA/CDprotocol.Explainhowitworks.Solution:Astationfirstsensesthewiretoseewhetheritisfree.Ifthereiscurrentlynootheractivitythestationwillstarttransmittingitsdatawhileitcontinuestolisten.Ifacollisionisdetectedtransmissionstopsimmediatelyandanextattemptismadesomerandomperiodlaterdeterminedbyanexponentialbackoffalgorithm.
2、WhatisthedifferencebetweenaswitchandarouterWhateffectdoesthishaveonthenumberoftimestheymustforwardamessage.Solution:Aswitchislayer2arouterislayer
3.Aswitchbuildsitsswitchtablebyeavesdroppingonpackets.Thustheswitchplugsandplaysbutwillneedtoforwardamessagestoalloutputlinksifithasnotheardfromthedestinationnode.Arouterbuildsitsroutingtablethroughroutingalgorithmseg.distancevectororlinkstate.Thusaroutercanforwardamessagetoalmostanyhostbysendingonlyasinglemessage.
3、WhatisthepurposeofARPDescribehowARPworks.Solution:ThepurposeofARPistoresolveIPaddressestoMACaddresseswithinasubnet.ARPusesMAC-levelbroadcastwhereonlythehostwiththematchingIPaddressrespondstothehostsendingtheARP.FromthisresponsethesendinghostcanlearntheMACaddressoftherespondinghost.WhatisperformanceGiveatleastthreeperformancemeasuresofinterestforcomputernetworks.Solution:Aquantitativemeasurementofasystem.Delaythroughputlossalsoresourceconsumptionutilizationreliabilityavailability.
5、WhatisthedifferencebetweenanetworkserviceandanetworkprotocolIsitpossibletohavedifferentimplementationsofthesameprotocolSolution:Aservicedescribeswhatthenetworkofferstoitsclientsbymeansofaninterface.Aprotocolisasetofrulesdescribingexactlyhowthatserviceistoberealizedthroughtheexchangeofmessagesbetweencomputers.Theserulesmaybeimplementeddifferentlye.g.usingdifferentprogramminglanguagesaslongasthoseimplementationscoincidewiththerules.
四、
1、Calculatehowlongittakestosenda1Mbfileovera15Mbpslinkacircuitswitchednetworkthatuses24dijferentfrequencyslotsanda500msecconnectionestablishmenttimealOhoppacketswitchednetworkwhereeachlinkisa
1.5Mbpslink.Assumenocongestionandnopacketsegmentation.RedoyourcalculationsforthepacketswitchednetworkaboveassumingyourmaximumsegmentsizeMSSis1KB.Solution:llatency=1/
1.5/24+.5=
16.5secondslatency=1/
1.5*10=
6.66secondslatency=timetosendpipeline+forwardingdelay=1/
1.5+.001/
1.5*10=.673seconds
2、Supposethatrootnameserverhasthetwofollowingrecords:xust.edudns.xust.eduNSdns.xust.edu
128.
119.
40.IllAAlsosupposethatmachinedns.umass.eduhasthefollowingresourcerecord:gciia.cs.xust.edu
128.
119.
40.200ASupposethatyouareworkingatXJTUandyourhostmachineneedstoresolvethenamegaia.cs.xust.edu.Describethesequenceofqueriesbetweenyourhostandtheserversinvolvedinthequeryprocess.WhatisthenumberofcommunicationsneededtoobtaintheIPaddressofgaicucs.xust.eduYoushouldassumethattheresolvedIPaddressisnotcachedatthelocalhostandthattheprocessingofallqueriesisrecursive.Solution:Localhostcontactsthelocalnameserver.Localnameservercontactstherootserver.Rootservercontactstheauthoritativeserver.AuthoritativeserversendstheresolvedIPaddress.Rootserverforwardstheanswertothelocalnameserver.Localnameserverforwardstheanswertothehost.。