計算機網(wǎng)絡(luò)問答Computernetworkquestionandanswer

1、計算機網(wǎng)絡(luò)問答(Computernetworkquestionandanswer)TrytodescribethesimilaritiesanddifferencesbetweentheTCP/IPreferencemodelandtheOSIreferencemodelSamepoint:Ahierarchicalprotocolmodelisadopted;Thefunctionsofthehierarchyareapproximatelythesame;DifferenceDifferentiationofservices,interfaces,protocols;Numberofpli

2、es:TCP/IPfour,OSIsevenTheversatilityofthemodel;Communicationservicetype:OSI:twocommunicationservices,connectionorientedandconnectionless,areprovidedonthenetworklayer,whileconnectionorientedservicesareprovidedonlyonthetransportlayer;TCP/IP:onlyconnectionlessservicesareprovidedonthenetworklayer,andtwo

3、services,connectionorientedandconnectionless,areofferedonthetransportlayerThecharacteristicsofdatagramandvirtualcircuitservicearebrieflydescribedVirtualcircuit:Ideas:reliablecommunicationsshouldbeguaranteedbytheInternetConnectionestablishment:musthaveDestinationaddress:usedonlyduringtheconnectionset

4、upphase,andeachgroupusesashortvirtualcircuitnumberPacketforwarding:packetsbelongingtothesamevirtualcircuitareforwardedinaccordancewiththesamerouteWhenthenodefails:allthevirtualcircuitthroughthefaultnodecannotworkPacketorder:alwaysarrivesatthedestinationintheorderofdeliveryEnd-to-enderrorhandlingandt

5、rafficcontrol:eitherbythenetworkorbytheusershostDatagramservice:Idea:reliablecommunicationsshouldbeguaranteedbytheusershostcomputerConnectionestablishment:notrequiredDestinationaddress:eachpackethasafulladdressforthedestinationPacketforwarding:eachpacketisroutedindependentlyWhenanodefails,thefaultyn

6、odemaylosepacketsandsomeroutesmaychangeGroupingorder:notnecessarilyinorderofdeliverywhenarrivingatthedestinationEnd-to-enderrorhandlingandflowcontrol:theuserhostisresponsibleBriefintroductionofTCPcongestioncontrolmethodWhenthehosthasjuststartedsendingamessagesegment,thecongestionwindowCWnd=1,whichis

7、thevalueofthemaximummessagesegmentMSS,canbefirstset.Aftereachreceivedaconfirmationofthenewmessagesegment,add1tothecongestionwindow,i.e.,toincreasethevalueofaMSS.Inthisway,thecongestionwindowCWndofthesenderisgraduallyincreased,sothattherateofpacketinjectionintothenetworkismorereasonable.Congestionavo

8、idance:Idea:letthecongestionwindowCWndslowlyincrease,thatis,aftereachroundtriptime,theRTTadds1tothesenderscongestionwindowCWndinsteadofdoublingthecongestionwindowCWndtoslowdownaccordingtothelinearrule.Fastretransmissionalgorithm:First,thereceiverisrequiredtosendarepeatedacknowledgementimmediatelyaft

9、erreceivinganoutofordermessagesegment.Thisallowsthesendertoknowearlythatamessagesegmenthasnotreachedthereceiver.Quickrecovery:(1)whenthesenderreceivesthreeconsecutiverepetitionsofconfirmation,themultiplicationreductionalgorithmisexecuted,andtheslowstartthresholdssthreshishalved.(2)buttheslowstartalg

10、orithmisnotexecuted,butthecongestionavoidancealgorithm(additiveaddition)isstartedtomakethecongestionwindowslowlyincreaselinearlyTrytoexplainwhythenetworkadoptshierarchicalstructurebenefitLargeandcomplexproblemscanbetranslatedintoanumberofsmallerlocalproblems,andthesesmallerlocalproblemsareeasiertost

11、udyanddealwith.Thelayersareindependent.Goodflexibility.Structurallydivisible.Easytoimplementandmaintain.CanpromotestandardizationworkWhatisCSMA/CD?ThemainpointsandworkingprincipleofCSMA/CDprotocolarebrieflydescribed.Itisshortforcarriersense,multipointaccess/collisiondetectionWorkingprinciple:Sendlis

12、tenerchannel;Ifthechannelisbusy,persistlisteninguntilthechannelisidle;Oncethechannelisfoundfree,sendimmediately;Continuetomonitorchannelswhensending.Ifacollisionisdetected,stopthetransmissionframeimmediatelyandsendablockingsignal;Intotheexponentialbackoffphase,randomlywaitingforaperiodoftimetorelist

13、enthechannelMainpoints:Multipointaccessmeansthatthisisabustypenetwork,andmanycomputersconnecttoonebusinamultipointconnection.Theessenceoftheprotocoliscarriermonitoringandcollisiondetection.Carriermonitoringistolistenbeforesending,Thatis,beforeeachstationissendingdata,itisnecessarytocheckwhetherthebu

14、shasotherstationstosenddata,andifso,donotsenddatatemporarily.Waituntilthechannelbecomesidle.Infact,thecarrierisnotexplainedonthebus.Carriermonitoringistheuseofelectronictechnologytodetectwhetherothercomputerstransmitdatasignalsonthebus.Collisiondetectionistosendandlistenwhiletheadaptersendsthedataan

15、ddetectsthechangeofthesignalvoltageonthechannel,andthenjudgeswhethertheotherstationsaretransmittingthedatawhentheysendthedata.Whenseveralstationssimultaneouslysenddataonthebus,thesignalvoltagechangesonthebuswillincrease(overlapeachother).Whentheadapterdetectsthatthesignalvoltagechangesaboveacertaint

16、hreshold,itisassumedthatatleasttwostationsonthebusaretransmittingdataatthesametime,indicatingthatacollisionoccurred.BrieflydescribethedifferencesandconnectionsbetweenservicesandagreementsEntitiesareoftenrepresentedbyamoreabstractname,anyhardwareorsoftwareprocessthatcansendandreceiveinformation.Anagr

17、eementisasetofrulesthatcontrolthecommunicationbetweentwopeerentities(ormultipleentities)Underthecontroloftheprotocol,communicationbetweentwopeerentitiesenablesthelayertoprovideservicestothenextlevel.Toimplementthislayerprotocol,youalsoneedtousetheservicesprovidedatthefollowinglayer.Entitiesthatuseth

18、islayerservicecanonlyseeserviceprovidersandcannotseethefollowingprotocols.Thefollowingprotocolistransparenttotheentitiesabove.Theprotocolislevel,thatis,theprotocolistocontrolcommunicationrulesbetweenpeerentities.Theserviceisvertical,andtheserviceisprovidedbythelowerlevelandtheupperlayerthroughtheint

19、erlayerinterfaceHowmanylayersarethereintheTCP/IPmodel?Whatarethelayers?Therearefourfloors.Applicationlayer.Transportlayer.Internetlayer.NetworkinterfacelayerosimodelSevenlayersofapplication,presentation,session,transportation,network,datalink,physicsBriefdescriptionofmeasurestosolvetheexhaustionofIP

20、address.TheuseofnonclassifiedaddressingCIDRmakestheallocationofIPaddressesmorereasonable.UsingthenetworkaddresstranslationNATmethodtosavetheglobalIPaddress.IPprotocolIPv6withanewversionwithalargeraddressspaceDescribetheprocessofestablishingconnectioninTCP.A:thefirsttwoTCPprocessesareintheCLOSEDstate

21、(off).NotethatAistheinitiativetoopentheconnection,Bispassivetoopentheconnection.TheTCPBserverprocessfirstcreatesthetransportcontrolblockTCB,readytoaccepttheconnectionrequestfromtheclientprocess.TheserverthenenterstheLISTEN(listening)stateandwaitsfortheconnectionrequestfromtheclient.TheTCPAclientproc

22、essalsocreatesthetransportcontrolblockTCBfirst,andthensendstheconnectionrequestmessagesegmenttotheB,whentheTCPclientprocessenterstheSYN-SENT(synchronoussent)state.BsendsaconfirmationtoAifitagreestoestablishaconnectionafterreceiptofthemessage.Atthispoint,theTCPserverprocessenterstheSYN-RCVD(synchrono

23、usreceive)state.AftertheTCPclientprocessreceivestheBconfirmation,italsogivesconfirmationtotheB.Atthispoint,theTCPconnectionhasbeenestablishedandtheAenterstheESTABLISHED(establishedconnection)state.BrieflydescribetheworkingprocessofARP.Answer:whenthehostAtotheLANinaBhostIPdatatransmissiontime,firstin

24、theARPcachetoseewhetherthehostBIP.hasputthehardwareaddressintotheMACframe,throughtheLANtothehardware.WhentheIPaddressofBisnotfoundinthehost,AautomaticallyrunsARP,andthenthefollowingstepsaremadetofindthehardwareaddressoftheBtheARPprocessbroadcastsaARPrequestpacketonthelocalareanetwork.theARPprocesses

25、runningonallhostsonthelocalnetworkreceivethisARPrequestpacketthehostBseesitsownIPaddressintheARPrequestpacketandsendstheARPresponsepackettothehostAandwritestoitsownhardwareaddress.TherestofthehostignoredtheARPrequestpacket.thehostAreceivestheARPresponsepacketofthehostB,WritestheIPaddressofthehostBto

26、thehardwareaddressinitsARPcache.Ofcourse,forfutureconvenience,BsendsdatagramstotheA(whichislikelytobesent),andwhentheAsendstherequestpacket,ithaswrittenitsownIPaddresstothehardwareaddressintotheARPrequestpacket2.brieflydescribethecharacteristicsandshortcomingsofpacketswitchingInadequate:packetqueuin

27、gisrequiredwhenroutingisstoredandforwarded,resultingindelay;Thecontrolinformationthateachpacketmustcarryalsoresultsinsomeoverhead;Thewholepacketswitchingnetworkalsoneedsspecialmanagementandcontrolmechanism.Characteristic：Highefficiency:intheprocessofpackettransmission,thetransmissionbandwidthisdynam

28、icallyallocated,andthecommunicationlinkisoccupiedbysegments.FlexibleroutingforeachpacketindependentselectionroutingQuickly,ingroupsastransportunits,ucansendpacketstootherhostswithoutfirstsettingupaconnectionReliableandreliablenetworkprotocol;thedistributedmultiroutepacketswitchingnetworkmakesthenetw

29、orkhavegoodsurvivability1.whatarethethreeelementsoftheagreement?Whatarethemaindifferencesbetweenservicesandprotocols?Networkprotocol:arule,standard,orconventionfortheexchangeofdatainanetwork.Consistsofthefollowingthreeelements:syntax:thestructureorformatofdataandcontrolinformation.semantics:whatcont

30、rolinformationneedstobesent,whatactiontobecompletedandwhatresponsetobemade.synchronization:adetaileddescriptionoftheorderofeventimplementation.Anagreementisasetofrulesthatcontrolthecommunicationbetweentwopeerentities.Underthecontroloftheprotocol,communicationbetweentwopeerentitiesenablesthelayertopr

31、ovideservicestothenextlayer,andtoimplementthislayerprotocol,itisalsonecessarytoprovideservicesusingthefollowinglayer.Thedistinctionbetweenprotocolsandservices:Differencesbetweenprotocolsandservices:Theprotocolishorizontal,thatis,theprotocolisarulethatcontrolscommunicationbetweenpeerentities.Theservi

32、ceisvertical,thatis,theserviceisprovidedfromthelowerlayertotheupperlevelthroughtheinterlayerinterface.Therelationshipbetweenprotocolandservice:Underthecontroloftheprotocol,theupperlayercallsthelowerlayer,thelowerlayerservestheupperlayer,andtheupperandlowerlayersexchangeinformationwiththeexchangeprimitive.TherearesometimesconnectionsbetweentwoentitiesonthesamelayerHowmanybinarydigitsdoes3.IPV6have?HowtoachieveexcessiveIPV4toIPV6?Thereare128binarydigits.Dual(stack):me