计算机网络自顶向下第4版课件Chapter1-4thed

Chapter1

IntroductionComputerNetworking:ATopDownApproach,

4thedition.

JimKurose,KeithRoss

Addison-Wesley,July2007.

Anoteontheuseofthesepptslides:We’remakingtheseslidesfreelyavailabletoall(faculty,students,readers).They’reinPowerPointformsoyoucanadd,modify,anddeleteslides(includingthisone)andslidecontenttosuityourneeds.Theyobviouslyrepresentalotofworkonourpart.Inreturnforuse,weonlyaskthefollowing:Ifyouusetheseslides(e.g.,inaclass)insubstantiallyunalteredform,thatyoumentiontheirsource(afterall,we’dlikepeopletouseourbook!)Ifyoupostanyslidesinsubstantiallyunalteredformonawwwsite,thatyounotethattheyareadaptedfrom(orperhapsidenticalto)ourslides,andnoteourcopyrightofthismaterial.Thanksandenjoy!JFK/KWRAllmaterialcopyright1996-2007J.FKuroseandK.W.Ross,AllRightsReserved1IntroductionChapter1:IntroductionOurgoal:

get“feel”andterminologymoredepth,detaillaterincourseapproach:useInternetasexampleOverview:what’stheInternet?what’saprotocol?networkedge;hosts,accessnet,physicalmedianetworkcore:packet/circuitswitching,Internetstructureperformance:loss,delay,throughputsecurityprotocollayers,servicemodelshistory2IntroductionChapter1:roadmap1.1WhatistheInternet?1.2Networkedgeendsystems,accessnetworks,links1.3Networkcorecircuitswitching,packetswitching,networkstructure1.4Delay,lossandthroughputinpacket-switchednetworks1.5Protocollayers,servicemodels1.6Networksunderattack:security1.7History3IntroductionWhat’stheInternet:“nutsandbolts”viewmillionsofconnectedcomputingdevices:hosts=endsystems

runningnetworkappsHomenetworkInstitutionalnetworkMobilenetworkGlobalISPRegionalISProuterPCserverwirelesslaptopcellularhandheldwiredlinksaccesspointscommunicationlinksfiber,copper,radio,satellitetransmissionrate=bandwidthrouters:forwardpackets(chunksofdata)4Introduction“Cool”internetappliancesWorld’ssmallestwebserver/~shri/iPic.htmlIPpictureframe/Web-enabledtoaster+weatherforecasterInternetphones5IntroductionWhat’stheInternet:“nutsandbolts”viewprotocols

controlsending,receivingofmsgse.g.,TCP,IP,HTTP,Skype,EthernetInternet:“networkofnetworks”looselyhierarchicalpublicInternetversusprivateintranetInternetstandardsRFC:RequestforcommentsIETF:InternetEngineeringTaskForceHomenetworkInstitutionalnetworkMobilenetworkGlobalISPRegionalISP6IntroductionWhat’stheInternet:aserviceviewcommunicationinfrastructureenablesdistributedapplications:Web,VoIP,email,games,e-commerce,filesharingcommunicationservicesprovidedtoapps:reliabledatadeliveryfromsourcetodestination“besteffort”(unreliable)datadelivery7IntroductionWhat’saprotocol?humanprotocols:“what’sthetime?”“Ihaveaquestion”introductions…specificmsgssent…specificactionstakenwhenmsgsreceived,orothereventsnetworkprotocols:machinesratherthanhumansallcommunicationactivityinInternetgovernedbyprotocolsprotocolsdefineformat,orderofmsgssentandreceivedamongnetworkentities,andactionstakenonmsgtransmission,receipt

8IntroductionWhat’saprotocol?ahumanprotocolandacomputernetworkprotocol:Q:Otherhumanprotocols?HiHiGotthetime?2:00TCPconnectionrequestTCPconnectionresponseGet/kurose-ross<file>time9IntroductionChapter1:roadmap1.1WhatistheInternet?1.2Networkedgeendsystems,accessnetworks,links1.3Networkcorecircuitswitching,packetswitching,networkstructure1.4Delay,lossandthroughputinpacket-switchednetworks1.5Protocollayers,servicemodels1.6Networksunderattack:security1.7History10IntroductionAcloserlookatnetworkstructure:networkedge:applicationsandhostsaccessnetworks,physicalmedia:wired,wirelesscommunicationlinks

networkcore:

interconnectedroutersnetworkofnetworks11IntroductionThenetworkedge:endsystems(hosts):runapplicationprogramse.g.Web,emailat“edgeofnetwork”client/serverpeer-peerclient/servermodelclienthostrequests,receivesservicefromalways-onservere.g.Webbrowser/server;emailclient/serverpeer-peermodel:minimal(orno)useofdedicatedserverse.g.Skype,BitTorrent12IntroductionAccessnetworksandphysicalmediaQ:Howtoconnectendsystemstoedgerouter?residentialaccessnetsinstitutionalaccessnetworks(school,company)mobileaccessnetworksKeepinmind:bandwidth(bitspersecond)ofaccessnetwork?sharedordedicated?13IntroductionResidentialaccess:pointtopointaccessDialupviamodemupto56Kbpsdirectaccesstorouter(oftenless)Can’tsurfandphoneatsametime:can’tbe“alwayson”DSL:digitalsubscriberlinedeployment:telephonecompany(typically)upto1Mbpsupstream(todaytypically<256kbps)upto8Mbpsdownstream(todaytypically<1Mbps)dedicatedphysicallinetotelephonecentraloffice14IntroductionResidentialaccess:cablemodemsHFC:hybridfibercoaxasymmetric:upto30Mbpsdownstream,2MbpsupstreamnetworkofcableandfiberattacheshomestoISProuterhomesshareaccesstorouterdeployment:availableviacableTVcompanies15IntroductionResidentialaccess:cablemodemsDiagram:/cmic/diagram.html16IntroductionCableNetworkArchitecture:Overviewhomecableheadendcabledistributionnetwork(simplified)Typically500to5,000homes17IntroductionCableNetworkArchitecture:Overviewhomecableheadendcabledistributionnetworkserver(s)18IntroductionCableNetworkArchitecture:Overviewhomecableheadendcabledistributionnetwork(simplified)19IntroductionCableNetworkArchitecture:OverviewhomecableheadendcabledistributionnetworkChannelsVIDEOVIDEOVIDEOVIDEOVIDEOVIDEODATADATACONTROL123456789FDM(moreshortly):20IntroductionCompanyaccess:localareanetworkscompany/univlocalareanetwork(LAN)connectsendsystemtoedgerouterEthernet:

10Mbs,100Mbps,1Gbps,10GbpsEthernetmodernconfiguration:endsystemsconnectintoEthernet

switchLANs:chapter521IntroductionWirelessaccessnetworkssharedwirelessaccessnetworkconnectsendsystemtorouterviabasestationaka“accesspoint”wirelessLANs:802.11b/g(WiFi):11or54Mbpswider-areawirelessaccessprovidedbytelcooperator~1Mbpsovercellularsystem(EVDO,HSDPA)nextup(?):WiMAX(10’sMbps)overwideareabasestationmobilehostsrouter22IntroductionHomenetworksTypicalhomenetworkcomponents:DSLorcablemodemrouter/firewall/NATEthernetwirelessaccesspointwirelessaccesspointwirelesslaptopsrouter/firewallcablemodemto/fromcableheadendEthernet23IntroductionPhysicalMediaBit:propagatesbetween

transmitter/rcvrpairsphysicallink:whatliesbetweentransmitter&receiverguidedmedia:

signalspropagateinsolidmedia:copper,fiber,coaxunguidedmedia:

signalspropagatefreely,e.g.,radioTwistedPair(TP)twoinsulatedcopperwiresCategory3:traditionalphonewires,10MbpsEthernetCategory5:

100MbpsEthernet24IntroductionPhysicalMedia:coax,fiberCoaxialcable:twoconcentriccopperconductorsbidirectionalbaseband:singlechanneloncablelegacyEthernetbroadband:multiplechannelsoncableHFCFiberopticcable:glassfibercarryinglightpulses,eachpulseabithigh-speedoperation:high-speedpoint-to-pointtransmission(e.g.,10’s-100’sGps)lowerrorrate:repeatersspacedfarapart;immunetoelectromagneticnoise25IntroductionPhysicalmedia:radiosignalcarriedinelectromagneticspectrumnophysical“wire”bidirectionalpropagationenvironmenteffects:reflectionobstructionbyobjectsinterferenceRadiolinktypes:terrestrialmicrowavee.g.upto45MbpschannelsLAN(e.g.,Wifi)11Mbps,54Mbpswide-area(e.g.,cellular)3Gcellular:~1MbpssatelliteKbpsto45Mbpschannel(ormultiplesmallerchannels)270msecend-enddelaygeosynchronousversuslowaltitude26IntroductionChapter1:roadmap1.1WhatistheInternet?1.2Networkedgeendsystems,accessnetworks,links1.3Networkcorecircuitswitching,packetswitching,networkstructure1.4Delay,lossandthroughputinpacket-switchednetworks1.5Protocollayers,servicemodels1.6Networksunderattack:security1.7History27IntroductionTheNetworkCoremeshofinterconnectedroutersthefundamentalquestion:howisdatatransferredthroughnet?circuitswitching:dedicatedcircuitpercall:telephonenetpacket-switching:datasentthrunetindiscrete“chunks”28IntroductionNetworkCore:CircuitSwitchingEnd-endresourcesreservedfor“call”linkbandwidth,switchcapacitydedicatedresources:nosharingcircuit-like(guaranteed)performancecallsetuprequired29IntroductionNetworkCore:CircuitSwitchingnetworkresources(e.g.,bandwidth)dividedinto“pieces”piecesallocatedtocallsresourcepieceidleifnotusedbyowningcall(nosharing)dividinglinkbandwidthinto“pieces”frequencydivisiontimedivision30IntroductionCircuitSwitching:FDMandTDMFDMfrequencytimeTDMfrequencytime4usersExample:31IntroductionNumericalexampleHowlongdoesittaketosendafileof640,000bitsfromhostAtohostBoveracircuit-switchednetwork?Alllinksare1.536MbpsEachlinkusesTDMwith24slots/sec500msectoestablishend-to-endcircuitLet’sworkitout!32IntroductionNetworkCore:PacketSwitchingeachend-enddatastreamdividedintopacketsuserA,Bpacketssharenetworkresources

eachpacketusesfulllinkbandwidthresourcesusedasneeded

resourcecontention:

aggregateresourcedemandcanexceedamountavailablecongestion:packetsqueue,waitforlinkusestoreandforward:packetsmoveonehopatatimeNodereceivescompletepacketbeforeforwardingBandwidthdivisioninto“pieces”DedicatedallocationResourcereservation33IntroductionPacketSwitching:StatisticalMultiplexingSequenceofA&Bpacketsdoesnothavefixedpattern,bandwidthsharedondemand

statisticalmultiplexing.TDM:eachhostgetssameslotinrevolvingTDMframe.ABC100Mb/sEthernet1.5Mb/sDEstatisticalmultiplexingqueueofpacketswaitingforoutputlink34IntroductionPacket-switching:store-and-forwardtakesL/Rsecondstotransmit(pushout)packetofLbitsontolinkatRbpsstoreandforward:entirepacketmustarriveatrouterbeforeitcanbetransmittedonnextlinkdelay=3L/R(assumingzeropropagationdelay)Example:L=7.5MbitsR=1.5Mbpstransmissiondelay=15secRRRLmoreondelayshortly…35IntroductionPacketswitchingversuscircuitswitching1Mb/slinkeachuser:100kb/swhen“active”active10%oftimecircuit-switching:

10userspacketswitching:

with35users,probability>10activeatsametimeislessthan.0004Packetswitchingallowsmoreuserstousenetwork!Nusers1MbpslinkQ:howdidwegetvalue0.0004?36IntroductionPacketswitchingversuscircuitswitchinggreatforburstydataresourcesharingsimpler,nocallsetupexcessivecongestion:packetdelayandlossprotocolsneededforreliabledatatransfer,congestioncontrolQ:Howtoprovidecircuit-likebehavior?bandwidthguaranteesneededforaudio/videoappsstillanunsolvedproblem(chapter7)Ispacketswitchinga“slamdunkwinner?”Q:humananalogiesofreservedresources(circuitswitching)versuson-demandallocation(packet-switching)?37IntroductionInternetstructure:networkofnetworksroughlyhierarchicalatcenter:“tier-1”ISPs(e.g.,Verizon,Sprint,AT&T,CableandWireless),national/internationalcoveragetreateachotherasequalsTier1ISPTier1ISPTier1ISPTier-1providersinterconnect(peer)privately38IntroductionTier-1ISP:e.g.,Sprint…to/fromcustomerspeeringto/frombackbone….………POP:point-of-presence39IntroductionInternetstructure:networkofnetworks“Tier-2”ISPs:smaller(oftenregional)ISPsConnecttooneormoretier-1ISPs,possiblyothertier-2ISPsTier1ISPTier1ISPTier1ISPTier-2ISPTier-2ISPTier-2ISPTier-2ISPTier-2ISPTier-2ISPpaystier-1ISPforconnectivitytorestofInternettier-2ISPiscustomeroftier-1providerTier-2ISPsalsopeerprivatelywitheachother.40IntroductionInternetstructure:networkofnetworks“Tier-3”ISPsandlocalISPslasthop(“access”)network(closesttoendsystems)Tier1ISPTier1ISPTier1ISPTier-2ISPTier-2ISPTier-2ISPTier-2ISPTier-2ISPlocalISPlocalISPlocalISPlocalISPlocalISPTier3ISPlocalISPlocalISPlocalISPLocalandtier-3ISPsarecustomersofhighertierISPsconnectingthemtorestofInternet41IntroductionInternetstructure:networkofnetworksapacketpassesthroughmanynetworks!Tier1ISPTier1ISPTier1ISPTier-2ISPTier-2ISPTier-2ISPTier-2ISPTier-2ISPlocalISPlocalISPlocalISPlocalISPlocalISPTier3ISPlocalISPlocalISPlocalISP42IntroductionChapter1:roadmap1.1WhatistheInternet?1.2Networkedgeendsystems,accessnetworks,links1.3Networkcorecircuitswitching,packetswitching,networkstructure1.4Delay,lossandthroughputinpacket-switchednetworks1.5Protocollayers,servicemodels1.6Networksunderattack:security1.7History43IntroductionHowdolossanddelayoccur?packetsqueueinrouterbuffers

packetarrivalratetolinkexceedsoutputlinkcapacitypacketsqueue,waitforturnABpacketbeingtransmitted(delay)packetsqueueing(delay)free(available)buffers:arrivingpacketsdropped(loss)ifnofreebuffers44IntroductionFoursourcesofpacketdelay1.nodalprocessing:

checkbiterrorsdetermineoutputlinkABpropagationtransmissionnodalprocessingqueueing2.queueingtimewaitingatoutputlinkfortransmissiondependsoncongestionlevelofrouter45IntroductionDelayinpacket-switchednetworks3.Transmissiondelay:R=linkbandwidth(bps)L=packetlength(bits)timetosendbitsintolink=L/R4.Propagationdelay:d=lengthofphysicallinks=propagationspeedinmedium(~2x108m/sec)propagationdelay=d/sABpropagationtransmissionnodalprocessingqueueingNote:sandRareverydifferentquantities!46IntroductionCaravananalogycars“propagate”at

100km/hrtollboothtakes12sectoservicecar(transmissiontime)car~bit;caravan~packetQ:Howlonguntilcaravanislinedupbefore2ndtollbooth?Timeto“push”entirecaravanthroughtollboothontohighway=12*10=120secTimeforlastcartopropagatefrom1stto2ndtollboth:100km/(100km/hr)=1hrA:62minutestollboothtollboothten-carcaravan100km100km47IntroductionCaravananalogy(more)Carsnow“propagate”at

1000km/hrTollboothnowtakes1mintoserviceacarQ:

Willcarsarriveto2ndboothbeforeallcarsservicedat1stbooth?Yes!After7min,1stcarat2ndboothand3carsstillat1stbooth.1stbitofpacketcanarriveat2ndrouterbeforepacketisfullytransmittedat1strouter!SeeEthernetappletatAWLWebsitetollboothtollboothten-carcaravan100km100km48IntroductionNodaldelaydproc=processingdelaytypicallyafewmicrosecsorlessdqueue=queuingdelaydependsoncongestiondtrans=transmissiondelay=L/R,significantforlow-speedlinksdprop=propagationdelayafewmicrosecstohundredsofmsecs49IntroductionQueueingdelay(revisited)R=linkbandwidth(bps)L=packetlength(bits)a=averagepacketarrivalratetrafficintensity=La/RLa/R~0:averagequeueingdelaysmallLa/R->1:delaysbecomelargeLa/R>1:more“work”arrivingthancanbeserviced,averagedelayinfinite!50Introduction“Real”InternetdelaysandroutesWhatdo“real”Internetdelay&losslooklike?Tracerouteprogram:providesdelaymeasurementfromsourcetorouteralongend-endInternetpathtowardsdestination.Foralli:sendsthreepacketsthatwillreachrouterionpathtowardsdestinationrouteriwillreturnpacketstosendersendertimesintervalbetweentransmissionandreply.3probes3probes3probes51Introduction“Real”Internetdelaysandroutes1cs-gw(54)1ms1ms2ms2(45)1ms1ms2ms3(30)6ms5ms5ms4(29)16ms11ms13ms5(36)21ms18ms18ms6()22ms18ms22ms7(6)22ms22ms22ms853(53)104ms109ms106ms9(29)109ms102ms104ms10(0)113ms121ms114ms11(4)112ms114ms112ms12nio-n2.cssi.renater.fr(3)111ms114ms116ms13nice.cssi.renater.fr(02)123ms125ms124ms14r3t2-nice.cssi.renater.fr(10)126ms126ms124ms15(4)135ms128ms133ms165(5)126ms128ms126ms17***18***19fantasia.eurecom.fr(42)132ms128ms136

mstraceroute:towww.eurecom.frThreedelaymeasurementsfromto*meansnoresponse(probelost,routernotreplying)trans-oceaniclink52IntroductionPacketlossqueue(akabuffer)precedinglinkinbufferhasfinitecapacitypacketarrivingtofullqueuedropped(akalost)lostpacketmayberetransmittedbypreviousnode,bysourceendsystem,ornotatallABpacketbeingtransmittedpacketarrivingtofullbuffer

islostbuffer(waitingarea)53IntroductionThroughputthroughput:rate(bits/timeunit)atwhichbitstransferredbetweensender/receiverinstantaneous:rateatgivenpointintimeaverage:rateoverlongerperiodoftimeserver,withfileofFbitstosendtoclientlinkcapacity

Rs

bits/seclinkcapacity

Rc

bits/secpipethatcancarryfluidatrate

Rs

bits/sec)pipethatcancarryfluidatrate

Rc

bits/sec)serversendsbits(fluid)intopipe54IntroductionThroughput(more)Rs<Rc

Whatisaverageend-endthroughput?

Rs

bits/secRc

bits/secRs>Rc

Whatisaverageend-endthroughput?Rs

bits/sec

Rc

bits/seclinkonend-endpaththatconstrainsend-endthroughputbottlenecklink55IntroductionThroughput:Internetscenario10connections(fairly)sharebackbonebottlenecklinkR

bits/secRsRsRsRcRcRcRper-connectionend-endthroughput:min(Rc,Rs,R/10)inpractice:RcorRsisoftenbottleneck56IntroductionChapter1:roadmap1.1WhatistheInternet?1.2Networkedgeendsystems,accessnetworks,links1.3Networkcorecircuitswitching,packetswitching,networkstructure1.4Delay,lossandthroughputinpacket-switchednetworks1.5Protocollayers,servicemodels1.6Networksunderattack:security1.7History57IntroductionProtocol“Layers”Networksarecomplex!many“pieces”:hostsrouterslinksofvariousmediaapplicationsprotocolshardware,softwareQuestion:

Isthereanyhopeoforganizingstructureofnetwork?Oratleastourdiscussionofnetworks?58IntroductionOrganizationofairtravelaseriesofstepsticket(purchase)baggage(check)gates(load)runwaytakeoffairplaneroutingticket(complain)baggage(claim)gates(unload)runwaylandingairplaneroutingairplanerouting59Introductionticket(purchase)baggage(check)gates(load)runway(takeoff)airplaneroutingdepartureairportarrivalairportintermediateair-trafficcontrolcentersairplaneroutingairplaneroutingticket(complain)baggage(claimgates(unload)runway(land)airplaneroutingticketbaggagegatetakeoff/landingairplaneroutingLayeringofairlinefunctionalityLayers:eachlayerimplementsaserviceviaitsowninternal-layeractionsrelyingonservicesprovidedbylayerbelow60IntroductionWhylayering?Dealingwithcomplexsystems:explicitstructureallowsidentification,relationshipofcomplexsystem’spieceslayeredreferencemodelfordiscussionmodularizationeasesmaintenance,updatingofsystemchangeofimplementationoflayer’sservicetransparenttorestofsysteme.g.,changeingateproceduredoesn’taffectrestofsystemlayeringconsideredharmful?61IntroductionInternetprotocolstackapplication:supportingnetworkapplicationsFTP,SMTP,HTTPtransport:process-processdatatransferTCP,UDPnetwork:routingofdatagramsfromsourcetodestinationIP,routingprotocolslink:datatransferbetweenneighboringnetworkelementsPPP,Ethernetphysical:bits“onthewire”applicationtransportnetworklinkphysical62IntroductionISO/OSIreferencemodelpresentation:allowapplicationstointerpretmeaningofdata,e.g.,encryption,compression,machine-specificconventionssession:synchronization,checkpointing,recoveryofdataexchangeInternetstack“missing”theselayers!theseservices,ifneeded,mustbeimplementedinapplicationneeded?applicationpresentationsessiontransportnetworklinkphysical63IntroductionsourceapplicationtransportnetworklinkphysicalHtHnMsegmentHtdatagramdestinationapplicationtransportnetworklinkphysicalHtHnHlMHtHnMHtMMnetworklinkphysicallinkphysicalHtHnHlMHtHnMHtHnMHtHnHlMrouterswitchEncapsulationmessageMHtMHnframe64IntroductionChapter1:roadmap1.1WhatistheInternet?1.2Networkedgeendsystems,accessnetworks,links1.3Networkcorecircuitswitching,packetswitching,networkstructure1.4Delay,lossandthroughputinpacket-switchednetworks1.5Protocollayers,servicemodels1.6Networksunderattack:security1.7History65IntroductionNetworkSecurityThefieldofnetworksecurityisabout:howbadguyscanattackcomputernetworkshowwecandefendnetworksagainstattackshowtodesignarchitecturesthatareimmunetoattacksInternetnotoriginallydesignedwith(much)securityinmindoriginalvision:“agroupofmutuallytrustingusersattachedtoatransparentnetwork”

Internetprotocoldesignersplaying“catch-up”Securityconsiderationsinalllayers!66IntroductionBadguyscanputmalwareintohostsviaInternetMalwarecangetinhostfromavirus,worm,ortrojanhorse.

Spyware

malwarecanrecordkeystrokes,websitesvisited,uploadinfotocollectionsite.

Infectedhostcanbeenrolledinabotnet,usedforspamandDDoSattacks.

Malwareisoftenself-replicating:fromaninfectedhost,seeksentryintootherhosts67IntroductionBadguyscanputmalwareintohostsviaInternetTrojanhorseHiddenpartofsomeotherwiseusefulsoftwareTodayoftenonaWebpage(Active-X,plugin)Virusinfectionbyreceivingobject(e.g.,e-mailattachment),activelyexecutingself-replicating:propagateitselftootherhosts,usersWorm:infectionbypassivelyreceivingobjectthatgetsitselfexecutedself-replicating:propagatestootherhosts,usersSapphireWorm:aggregatescans/secinfirst5minutesofoutbreak(CAIDA,UWiscdata)68IntroductionBadguyscanattackserversandnetworkinfrastructureDenialofservice(DoS):attackersmakeresources(server,bandwidth)unavailabletolegitimatetrafficbyoverwhelmingresourcewithbogustrafficselecttargetbreakintohostsaroundthenetwork(seebotnet)sendpacketstowardtargetfromcompromisedhoststarget69IntroductionThebadguyscansniffpacketsPacketsniffing:broadcastmedia(sharedEthernet,wireless)promiscuousnetworkinterfacereads/recordsallpackets(e.g.,includingpasswords!)passingbyABCsrc:B

dest:ApayloadWiresharksoftwareusedforend-of-chapterlabsisa(free)packet-sniffer70IntroductionThebadguyscanusefalsesourceaddressesIPspoofing:sendpacketwithfalsesourceaddressABCsrc:B

dest:Apayload71IntroductionThebadguyscanrecordandplaybackrecord-and-playback:sniffsensitiveinfo(e.g.,password),anduselaterpasswordholderisthatuserfromsystempointofviewABCsrc:B

dest:Auser:B;password:foo72IntroductionNetworkSecuritymorethroughoutthiscoursechapter8:focusonsecuritycrypographictechniques:obvioususesandnotsoobvioususes73IntroductionChapter1:roadmap1.1WhatistheInternet?1.2Networkedgeendsystems,accessnetworks,links1.3Networkcorecircuit