计算机网络课程作业.docx
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计算机网络课程作业.docx
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计算机网络课程作业
Chapter1.
R1,R3,R8,R11,R12,R13,R15,R16,R18,R19,R23,R25,R26(R4,R7,R10,R14,R20,R22,R24)
p2,p5,p7,p8,p9,p12,p13,p22,p24,p26(p3,p6,p10,p11,p14,p15,p17,p18,p19,p20,p21)
R1.
None.
Serversandclients.
Yes.
R3.
Aclientprogramisaprogramrunningononeendsystemthatrequestsandreceivesaservicefromaserverprogramrunningonanotherendsystem.
Yes.
R8.
Dial-upmodems:
upto56kbps;dedicated.
HFC:
upto30Mbps,2Mbpsupstream;shared.
DSL:
upto1Mbpsupstream,upto8Mbpsdownstream;dedicated.
R11.
Moresuitableforreal-timeservices;providingastablebandwidth;noworryaboutdelayandloss;noneedofanalysiswhiledatatransfer
Moresuitablefordigitalsignaltransfer.
R12.
L/R1+L/R2
R13.
Becausepacketswitchingprovideson-demandsharingofresourcesinsteadofpre-allocated.WhileTDMdecidesthebandwidthofeachsender-receiverpairinadvance.
R15.
Tier-1ISPsareeachdirectlyconnectedonlytoeachoftheothertier-1ISPs,whilecanbeeachconnectedtoalargenumberoftier-2ISPs.
R16.
Processingdelay,transmissiondelay,propagationdelay,andqueuingdelay.Thequeuingdelayisvariable(dependsoncongestionlevelofrouter)andtheotherthreeareallconstant.
R18.
Rate:
RPropagationdelay:
DPacketsize:
L
WhenL/R
R19.
a.250kbps
b.2M*8/250k=64s
c.200kbps;80s
R23.
Applicationlayer,transportlayer,networklayer,linklayer,physicallayer.
Application:
coversthereactionbetweendifferentapplications
Transport:
transferringmessagesbetweenapplicationendpoints
Network:
movingnetwork-layerpacketsfromonehosttoanother
Link:
routingadatagramthroughaseriesofroutersbetweenthesourcesanddestinations
Physical:
movingentireframesfromonenetworkelementtoanadjacentnetworkelement
R25.
Theyarethepacketdataunitsofeachlayer.
R26.
Wormsaremalwarethatcanenteradevicewithoutanyexplicituserinteraction.
Virusesaremalwarethatrequiresomeformofuserinteractiontoinfecttheuser’sdevice.
Trojanhorseismalwarethatisahiddenpartofsomeotherwiseusefulsoftware.
(?
)P2.
a.6
b.4
P5.
a.m/s
b.L/R
c.m/s+L/R
d.justlefthostA
e.adistanceofs*L/RfarfromhostA
f.hasreachedhostB
g.m=s*L/R=250M*100/28k=25,000km
P7.
a.10
b.0.1
c.0.1ⁿ*0.9⁽⁴⁰⁻ⁿ⁾*C₄₀ⁿ
d.λ=40*0.1=4,P(X≥11)=0.002840
P8.
a.1G/100k=1000
b.λ=p*M,P(X≥N+1)
P9.
a.L/R₁+d₁/s₁+L/R₂+d₂/s₂
b.2*1000*8bits/1Mbps+2*0.002second+(6000+3000)km/250000(km/s)=0.056second
P12.
a.3.5*1250*8bits/1Mbps=0.035second
b.(n*L-x)/R
P13.
(0+L/R+2L/R+…+(N-1)L/R)/N=(N-1)L/2R
P22.
a.(h=36000km)dprop=36000km/2*10⁸(km/s)=0.00018s
b.R∙dprop=10Mbps*0.00018s=1800Mb
c.x=R∙dprop=1800Mb
P24.
a.tothefirstpacketswitch:
8∙10⁶bits/2Mbps=4s
todestinationhost:
3*4s=12s
b.2000bits/2Mbps=0.001s
whent=0.001s
whent=0.002s
c.4000*0.001s+0.002s=4.002s
comment:
Messagesegmentationsavetimewhilepacketswitch
d.Ifoneofthedatasegmentsgetslost,mistakewilloccurinthereceivedfile,sothesegmentshavetobeaddedaordermark.
P26.
F/S*L/R+L/R=FL/SR+L/R
Chapter2.
R1,R3,R4,R5,R10,R11,R13,R15,R16,R20,R21,R22,R27,R28
P1,P4,P5,P7,P8,P16,P24,P25
R1.
TheWeb:
HTTP;Filetransfer:
FTP;ElectronicmailintheInternet:
SMTP;Remoteterminalaccess:
Telnet;Streamingmultimedia:
HTTP
R3.
Fromtheapplicationdeveloper’sperspective,thenetworkarchitectureisfixedandprovidesaspecificsetofservicestoapplications.Theapplicationarchitecture,ontheotherhand,isdesignedbytheapplicationdeveloperanddictateshowtheapplicationisstructuredoverthevariousendsystems.
R4.
Theportnumber.
R5.
Notexactly.Althoughthehostscanallperformaseitherclientsandservers,thereisstilladifferencebetweenthesenderandreceiverofthefileduringacommunicationsession.Thesendercouldbeseenastheserversideandthereceivercouldbeseenastheclientside.
R10.
Transportlayer.
Inparticular,ifanapplicationwantstousetheservicesofSSL,itneedstoincludeSSLcode(existing,highlyoptimizedlibrariesandclasses)inboththeclientandserversidesoftheapplication.
R11.
Ahandshakingprotocolisaprotocolwhichsendsacontrolmessagetothereceiverbeforestartingtotransferringtherequesteddata.(likeSMTP)
R13.
R15.
a.Aliceinvokesheruseragentfore-mail,providesBob’se-mailaddress,composesamessage,andinstructstheuseragenttosendthemessage.
b.Alice’suseraentsendsthemessagetohermailserver,whereitisplacedinamessagequeue.
c.TheclientsideofSMTP
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