川大林锋计网第四次作业.docx

川大林锋计网第四次作业.docx

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川大林锋计网第四次作业

ReviewQuestions:

1.Let’sreviewsomeoftheterminologyusedinthistextbook.Recallthatthenameofatransport-layerpacketissegmentandthatthenameofalink-layerpacketisframe.传输层的包的名字是段，链路层的包的名字是帧Whatisthenameofanetwork-layerpacket?

Recallthatbothroutersandlink-layerswitchesarecalledpacketswitches.Whatisthefundamentaldifferencebetweenarouterandlink-layerswitch?

路由和链路层交换最基本的区别是？

RecallthatweusethetermroutersforbothdatagramnetworksandVCnetworks.

Anetwork-layerpacketisadatagram.Arouterforwardsitspacketbasedonthepacket’sIPaddress.Alink-layerswitchforwardsapacketbasedonthepacket’sMACaddress.

2.Whatisthedifferencebetweenroutingandforwarding?

Forwardingmovespacketsfromrouter’sinputtoappropriaterouteroutput,routingdeterminesrouterstakenbypacketfromresourcetodestinations.

3.Whatarethetwomostimportantnetwork-layerfunctionsinadatagramnetwork?

Whatarethethreemostimportantnetwork-layerfunctionsinavirtual-circuitnetwork?

虚电路网络中三个最重要的网络层功能是什么？

Themostimportantofdatagram-basednetworklayerareforwardingandrouting.It’sforwarding,routingandconnectionsetup.转发、选路和呼叫建立。

7.ThreetypesofswitchingfabricsarediscussedinSection4.3.Listandbrieflydescribeeachtype.

Switchingviamemory:

1.TraditionalcomputerswithswitchingunderdirectcontrolofCPU.

2.Packetcopiedtosystem’smemory.

3.Peedlimitedbymemorybandwidth.

Switchingviaabus:

1.Datagramfrominputportmemorytooutputportmemoryviaasharedbus.

2.Buscontention:

switchingspeedlimitedbybusbandwidth.

3.32Gbpsbus,Cisco5600:

sufficientspeedforaccessandenterpriserouters.

Switchingviaaninterconnectionnetwork:

1.Overcomebusbandwidthlimitations.

2.Banyannetworks,otherinterconnectionnetsinitiallydevelopedtoconnectprocessorsinmultiprocessor.

3.Advanceddesign:

fragmentingdatagramintofixedlengthcells,switchcellsthroughthefabric.

4.Cisco12000:

switches60Gbpsthroughtheinterconnectionnetwork.

10.Describehowpacketlosscanoccuratoutputports.

Packetlosscanoccurifqueuesizeattheinputportgrowslargebecauseofslowswitchingfabricspeedandthusexhaustingrouter’sbufferspace.Itcanbeeliminatediftheswitchingfabricspeedisnotsmallerthanthespeedofthenumberofinputportstime.

11.WhatisHOLblocking?

Doesitoccurininputportsoroutputports?

HOLmeanshead-of-the-line,在交换结构中，如果一个队列阻塞了，那么它后面的队列也跟着阻塞，这就是线上阻塞的意思。

大多数线上阻塞发生在输入端。

16.SupposeHostAsendsHostBaTCPsegmentencapsulatedinanIPdatagram.WhenHostBreceivesthedatagram,howdoesthenetworklayerinHostBknowitshouldpassthesegment（thatis,thepayloadofthedatagram）toTCPratherthantoUDPortosomethingelse?

因为TCP封装的时候会在封装的数据报中增加一个报头，报头中涵盖了传输的各种信息，各个转发的节点读取的时候就会知道它安排的转发方式和地址。

17.Supposeanapplicationgenerateschunksof40bytesofdataevery20msec,andeachchunkgetsencapsulatedinaTCPsegmentandthenanIPdatagram.Whatpercentageofeachdatagramwillbeoverhead,andwhatpercentagewillbeapplicationdata?

50%

19.IthasbeensaidthatwhenIPv6tunnelsthroughIPv4routers,IPv6treatstheIPv4tunnelsaslink-layerprotocols.Doyouagreewiththisstatement?

Whyorwhynot?

同意，因为IPv4把IPv6的全部信息当做一个包来传输，所以和链路层协议相似。

22.Isitnecessarythateveryautonomoussystem自制系统usethesameintra-ASroutingalgorithm?

Whyorwhynot?

不需要，因为自制系统中没有任何一个协议可以把所有的功能都实现。

24.ConsiderFigure4.37.StartingwiththeoriginaltableinD,supposethatDreceivesfromAthefollowingadvertisement:

WillthetableinDchange?

Ifsohow?

不会改变，算一下就知道了，没什么好说的。

26.Fillintheblank:

RIPadvertisementstypicallyannouncethenumberofhopstovariousdestinations.BGPupdates,ontheotherhand,announcethe_____sequenceofASsontherouters_____tothevariousdestinations.

30.Whyarepolicyconsiderationsasimportantforintra-ASprotocols,suchasOSPF开放最短路径优先openshortestpathfirstandRIP路由选择信息协议routinginformationprotocol,astheyareforaninter-ASroutingprotocollikeBGP边界网关协议bordergatewayprotocol?

内部网关协议是在一个自治系统内部使用的路由选择协议，与Internet中的其他自治系统选用什么路由选择协议无关。

内部以及外部网关协议的策略的重要性源于要考虑许多因素来决定到达目的地的最佳路由。

Problems:

1.Considersomeoftheprosandconsofvirtual-circuitanddatagramnetworks.虚电路网络和数据报网络优缺点

a.Supposethatinthenetworklayer,routersweresubjectedtostressfulconditionsthatmightcausethemtofailfairlyoften.Atahighlevel,whatactionswouldneedtobetakenonsuchrouterfailure?

DoesthisargueinfavorofVCordatagramarchitecture?

Withaconnection-orientednetwork,everyrouterfailurewillinvolvetheroutingofthatconnection.Ataminimum,thiswillrequiretherouterthatis“upstream”fromthefailedroutertoestablishanewdownstreampartofthepathtothedestinationnode,withalloftherequisitesignalinginvolvedinsettingupapath.Moreover,alloftheroutersontheinitialpaththataredownstreamfromthefailednodemusttakedownthefailedconnection,withalliftherequisitesignalinginvolvedtodothis.

Withaconnectionlessdatagramnetwork,nosignalingisrequiredtoeithersetupanewdownstreampathortakedowntheolddownstreampath.Wehaveseenthatroutingtableswillneedtobeupdatedtotakethefailedrouterintoaccount.Thus,adatagramnetworkwouldbepreferable.Interesting,thedesigncriteriathattheinitialARPAnetbeabletofunctionunderstressfulconditionswasoneofthereasonsthatdatagramarchitecturewaschosenforthisInternetancestor.

b.Supposethatinordertoprovideaguaranteeregardingthelevelofperformance（forexample,delay）thatwouldbeseenalongasource-to-destinationpath,thenetworkrequiresasendertodeclareitspeaktrafficrate.Ifthedeclaredpeaktrafficrateandtheexistingdeclaredtrafficratesthathavebeendeclaredaresuchthatthereisnowaytogettrafficfromthesourcetothedestinationthatmeetstherequireddelayrequirements,thesourceisnotallowedaccesstothenetwork.WouldsuchanapproachbemoreeasilyaccomplishedwithinaVCordatagramarchitecture?

Inorderforaroutertodeterminethedelayalonganoutgoinglink,itwouldneedtoknowthecharacteristicsofthetrafficfromallsessionspassingthroughthatlink.Thatis,theroutermusthaveper-sessionstateintherouter.Thisispossibleinaconnection-orientednetwork,butnotwithaconnectionlessnetwork.Thus,aconnection-orientednetworkwouldbepreferable.

2.Abare-bonesforwardingtableinaVCnetworkhasfourcolumns.Whatisthemeaningofthevaluesineachofthesescolumns?

转发表四项，都是什么意思？

IntheVCforwardingtable:

incominginterface,incomingVCnumber,outgoinginterfaceandoutgoingVCnumber

Inthedatagramforwardingtable:

destinationaddress,outgoinginterface.

4.ConsideraVCnetworkwitha3-bitfieldfortheVCnumber.Supposethatthenetworkwantstosetupavirtualcircuitoverfourlinks:

linkA,linkB,linkC,andlinkD.Supposethateachoftheselinksiscurrentlycarryingtwoothervirtualcircuits,andtheVCnumbersoftheseotherVCsareasfollows:

LinkA

LinkB

LinkC

LinkD

000

001

010

011

001

010

011

000

111

101

110

100

Inansweringthefollowingquestions,keepinmindthateachoftheexistingVCsmayonlybetraversingoneofthefourlinks.

a.IfeachVCisrequiredtousethesameVCnumberonalllinksalongitspath,whatVCnumbercouldbeassignedtothenewVC?

考察VC的计算

没有新的VCnumber可以用，因为图中已经使用了所有的排序方法。

b.IfeachVCispermittedtohavedifferentVCnumbersinthedifferentlinksalongitspath（sothatforwardingtablesmustperformVCnumbertranslation）,howmanydifferentcombinationsoffourVCnumbers（oneforeachofthefourlinks）couldbeused?

2种。

看图数出来的。

7.Considerarouterwithaswitchfabric,2inputports（AandB）and2outputports（CandD）.Supposetheswitchfabricoperatesat1.5timesthelinespeed.交换机操作速度是linespeed的1.5倍

a.If,forsomereasons,allpacketsfromAaredesignedtoD,andallpacketsfromBaredestinedtoC,canaswitchfabricbedesignedsothatthereisnoinputportqueuing?

Explainwhyorwhynotinonesentence.

不可以，如果要求没有输入端口排队的话，交换机速度起码要为linespeed的两倍。

b.SupposenowpacketsfromAandBarerandomlydestinedtobothCandD.Canaswitchfabricbedesignedsothatthereisnoinputportqueuing?

Explainwhyorwhynotinonesentence.

不可以，原因和a中一样。

10.Consideradatagramnetworkusing8-bithostaddresses.Supposearouteruseslongestprefixmatchingandhasthefollowingforwardingtable:

假设一个使用8位主机地址的数据包网络，且使用最长前缀匹配，有如下转发表

Foreachofthefourinterfaces,givetheassociatedrangeofdestinationhostaddressesandthenumberofaddressesintherange.对每四个接口，给出目的主机地址相关联的范围和范围中的地址数

10000000–10111111对应端口0，64位地址

11000000–11011111对应端口1，32位地址

11100000–11111111对应端口2，32位地址

00000000–01111111对应端口3，128为地址

11.Considerarouterthatinterconnectsthreesubnets:

Subnet1,Subnet2,andSubnet3.Supposealloftheinterfacesineachofthesethreesubnetsarerequiredtohavetheprefix220.2.240/20.AlsosupposethatSubnet1isrequiredtosupportupto2000interfaces,Subnet2andSubnet3aretosupportupto1000interfaces.Providethreenetworkaddresses（oftheforma.b.c.d/x）thatsatisfytheseconstraints.

假设一个路由器连接着3个子网：

1,2,3.假设3个子网所有接口都需要前缀为XXX，同时子网1支持2000个接口，子网2、3支持1000个接口，求网络地址

子网3：

220.2.240.0/22

子网2：

220.2.244.0/22

子网1：

220.2.248.0/21

12.InSection4.2.2anexampleforwardingtable（usinglongestprefixmatching）isgiven.Rewritethisforwardingtableusingthea.b.c.d/xnotationinsteadofthebinarystringnotation.

目的地址链路端口

224/80

225.0/161

225/82

Otherwise3

14.Considerasubnetwithprefix101.101.101.64/26.GiveanexampleofoneIPaddress（offormxxx.xx