《计算机网络（第5版）》英文版教学课件

COMPUTERNETWORKS

Chapter01

Introduction(1)2022/12/19ComputerNetworks-1-Part11HistoricReview

ENIAC:1946,theveryfirstcomputerARPANET:1969,theveryfirstcomputernetworkforadvancedresearchprojectagentofdefensedepartment,USA.Internet:1986NationalInformationInfrastructure:1993CommercializedInternet:1995NGI:1995,nextgenerationinternetI2:1996,Internet2CNGI:Cernet2+…2022/12/19ComputerNetworks-1-Part12InternetHistory1961:Kleinrock-queueingtheoryshowseffectivenessofpacket-switching1964:Baran-packet-switchinginmilitarynets1967:ARPAnetconceivedbyAdvancedResearchProjectsAgency1969:firstARPAnetnodeoperational1972:

ARPAnetpublicdemonstrationNCP(NetworkControlProtocol)firsthost-hostprotocolfirste-mailprogramARPAnethas15nodes2022/12/19ComputerNetworks-1-Part131961-1972:Earlypacket-switchingprinciplesInternetHistory1970:ALOHAnetsatellitenetworkinHawaii1974:CerfandKahn-architectureforinterconnectingnetworks1976:EthernetatXeroxPARCearly70’s:proprietaryarchitectures:DECnet,SNA,XNAlate70’s:switchingfixedlengthpackets(ATMprecursor)1979:ARPAnethas200nodesCerfandKahn’sinternetworkingprinciples:minimalism,autonomy-nointernalchangesrequiredtointerconnectnetworksbesteffortservicemodelstatelessroutersdecentralizedcontroldefinetoday’sInternetarchitecture2022/12/19ComputerNetworks-1-Part141972-1980:Internetworking,newandproprietarynetsInternetHistory1983:deploymentofTCP/IP1982:smtpe-mailprotocoldefined1983:DNSdefinedforname-to-IP-addresstranslation1985:ftpprotocoldefined1988:TCPcongestioncontrolnewnationalnetworks:Csnet,BITnet,NSFnet,Minitel100,000hostsconnectedtoconfederationofnetworks2022/12/19ComputerNetworks-1-Part151980-1990:newprotocols,aproliferationofnetworksInternetHistoryEarly1990’s:ARPAnetdecommissioned1991:NSFliftsrestrictionsoncommercialuseofNSFnet(decommissioned,1995)early1990s:Webhypertext[Bush1945,Nelson1960’s]HTML,HTTP:Berners-Lee1994:Mosaic,laterNetscapelate1990’s:commercializationoftheWebLate1990’s–2000’s:morekillerapps:instantmessaging,P2Pfilesharingnetworksecuritytoforefrontest.50millionhost,100million+usersbackbonelinksrunningatGbps2022/12/19ComputerNetworks-1-Part161990,2000’s:commercialization,theWeb,newapps2022/12/19ComputerNetworks-1-Part17InternetHistory2007:~500millionhostsVoice,VideooverIPP2Papplications:BitTorrent(filesharing)Skype(VoIP),PPLive(video)moreapplications:YouTube,gamingwireless,mobilityWhyNetworking?Theintegrationofcomputersandcommunicationtechniques:(1)communicationnetworkistheinfrastructureforcomputernetwork;(2)computerprogressfacilitatestelecommunicationComputerNetwork=computer+communicationWhynetworking?

Theneedsforefficientlycollecting,storing,processing,distributingandmanaginginformationTheneedsforchangingmannerofusingcomputersTheneedsforchangingmannerofusingnetworks2022/12/19ComputerNetworks-1-Part18Whatisacomputernetwork?

Tanenbaum’sdefinitionComputernetwork-InterconnectedcollectionofautonomouscomputersOurdefinition:Asystemthatinterconnectingmultipleautonomouscomputersindifferentlocationswithcommunicationequipment,trunks,andcommunicationsoftware(OS,protocols,etc.),forresourcesharing,issocalledcomputernetwork.2022/12/19ComputerNetworks-1-Part19DifferencesbetweencomputernetworksanddistributedsystemKeydistinctionisthatinadistributedsystem,theexistenceofmultipleautonomouscomputersistransparenttousers.Itlookslikeavirtualmachinewithuniprocessor.Withanetwork,usersmustexplicitlylogontoonemachine,explicitlysubmitjobsremotely,explicitlymovefilesaroundandhandlenetworkpersonally.2022/12/19ComputerNetworks-1-Part110ImportantconclusionIneffect,adistributedsystemisasoftwaresystembuildontopofnetwork.Thesoftwaregivesitahighdegreeofcohesivenessandtransparency.Thusthedistinctionbetweenanetworkandadistributedsystemlieswiththesoftware(especiallytheoperatingsystem)ratherthanwiththehardware.2022/12/19ComputerNetworks-1-Part111ImportantconclusionTheInternetisnotasinglenetworkbutanetworkofnetworksandtheWebisadistributedsystemthatrunsontopoftheInternet.2022/12/19ComputerNetworks-1-Part1121.1UseofComputerNetworksBeforewestarttoexaminethetechnicalissuesindetail,itisworthdevotingsometimetopointingoutwhypeopleareinterestedincomputernetworksandwhattheycanbeusedfor.Afterall,ifnobodywereinterestedincomputernetworks,fewofthemwouldbebuilt.Wewillstartwithtraditionalusesatcompanies,thenmoveontohomenetworkingandrecentdevelopmentsregardingmobileusers,andfinishwithsocialissues.2022/12/19ComputerNetworks-1-Part1131.1.1BusinessapplicationsResourcesharing,informationsharingHighreliabilitySavingmoney–client-servermodelScalability2022/12/19ComputerNetworks-1-Part114BusinessApplicationsofNetworksFig.1-1Anetworkwithtwoclientsandoneserver.2022/12/19ComputerNetworks-1-Part115Fig.1-2Theclient-servermodelinvolvesrequestsandrepliesBusinessApplicationsofNetworksAcomputernetworkcanprovideapowerfulcommunicationmediumamongemployees.e-mail(electronicmail)IPtelephonyorVoiceoverIP(VoIP)Videoconferencingdoingbusinesselectronicallywithothercompanies,especiallysuppliersandcustomers.Thisnewmodeliscallede-commerce(electroniccommerce).2022/12/19ComputerNetworks-1-Part1161.1.2HomeNetworkApplicationsAccesstoremoteinformationPerson-to-personcommunicationInteractiveentertainmentElectroniccommerce2022/12/19ComputerNetworks-1-Part117HomeNetworkApplications(2)Fig.1-3Inapeer-to-peersystemtherearenofixedclientsandservers,suchasBitTorrent,andPPlive,Skype…2022/12/19ComputerNetworks-1-Part118HomeNetworkApplications(3)InstantMessagingAllowtwopeopletotypemessagesateachotherinrealtimeTwitterMulti-personmessagingserviceSocialnetworkBetweenperson-to-personcommunicationsandaccessinginformationFacebookWikiGroupsofpeoplecanworktogethertocreatecontentWikipedia:anencyclopediaanyonecanedit2022/12/19ComputerNetworks-1-Part119HomeNetworkApplications(4)IPTV(IPTeleVision)BasedonIPtechnologyinsteadofcableTVorradiotransmissionsGameplayingMultipersonreal-timesimulationgamesUbiquitouscomputingComputingisembeddedintoeverydaylifeAtechnologycalledRFID(RadioFrequencyIDentification)willpushideaevenfurtherinthefuture.TheymayturntherealworldintotheInternetofthings(IoT).2022/12/19ComputerNetworks-1-Part1201.1.3MobileusersWirelessLANFixedwirelessandmobilewirelessMobilephone:Textmessagingortexting(ShortMessageService),Smartphones,GPS(GlobalPositioningSystem)m-commerce:mobile-commerceSensornetworksWearablecomputers2022/12/19ComputerNetworks-1-Part121ComputerNetworksOverview(1)Hardware:Talkjustabitabouthowyoucanconfigureabunchofcomputersintoanetwork:LocalAreaNetworks(LAN)MetropolitanAreaNetworks(MAN)WideAreaNetworks(WAN)InternetworksSoftware:Thisiswhatactuallymakescomputernetworks–notthehardware!2022/12/19ComputerNetworks-1-Part122ComputerNetworksOverview(2)Protocols:describehowtwocommunicatingpartiesexchangeinformation.Services:describewhatanetworkofferstopartiesthatwanttocommunicate.Interfaces:describehowaclientcanmakeuseofnetworkservices,i.e.howtheservicescanbeaccessed.Referencemodels:DescribehowtheOSIandInternetnetworksareorganized.2022/12/19ComputerNetworks-1-Part1231.2NetworkHardwareTwotypesoftransmissiontechnologyareinwidespreaduse:broadcastlinksandpoint-to-pointlinksBroadcastnetwork:asinglecommunicationchannelissharedbyallcomputers,thatis,sendingapacketimpliesthatallothersreceiveit.Multicasting:transmissiontoasubnetofusersPoint-to-pointnetwork:Computersareconnectedinpairs,thatis,sendingapacketgoesstrictlyfromthesendertothereceiver,possiblyhavingtovisitintermediatemachines(routing).Unicasting:P2Pwithonlyonesenderandonereceiver2022/12/19ComputerNetworks-1-Part1242022/12/19ComputerNetworks-1-Part125Fig.1-6Classificationofinterconnectedprocessorsbyscale1.2.1PersonalAreaNetworks(PAN)ConnectdevicesovertherangeofapersonExampleofaBluetooth(wireless)PAN:2022/12/19ComputerNetworks-1-Part126Fig.1-7BluetoothPANconfiguration.1.2.2LocalAreaNetworks(LAN)Apartfromscale,LANsdistinguishthemselvesfromothernetworksby(generally)usingbroadcasttechnology,andhavingsimpletopologies:2022/12/19ComputerNetworks-1-Part127Fig.1-7Twobroadcastnetworks.(a)Bus.(b)Ring.

switchTheStarType(a)(Bus-based):Allcomputersareconnectedtothesamewire.Whenoneofthemstartssending,thesignalispropagatedtoallothers.Iftwoofthemstartsendingatthesametime,packetscollideandrubbishistheresult.Type(b)(Token-based):atoken(whichisjustasmallpacket)continuouslycirculatesalongthering.Asendingcomputer:(1)waitsuntilthetokenpassesandremovesit(2)sendsitspacketalongthering(3)waitsuntilthepacketreturns(4)reinsertsthetokenThestaristhemostcommonlyusedtoday2022/12/19ComputerNetworks-1-Part128LocalAreaNetworksConnectdevicesinahomeorofficebuildingCalledenterprisenetworkinacompany2022/12/19ComputerNetworks-1-Part129MetropolitanAreaNetworksFig.1-8.AmetropolitanareanetworkbasedoncableTV.2022/12/19ComputerNetworks-1-Part1301.2.3MetropolitanAreaNetworks(MAN)1.2.4WideAreaNetworks(WAN)Note:LANsandMANsgenerallydon’thaveanyswitchingelements:thewiredoesallthework.Thismakesthemextremelyefficient,althoughhardertoscale.Here’swhereWANscomein.2022/12/19ComputerNetworks-1-Part131Fig.1-9.RelationbetweenhostsonLANsandthesubnetWideAreaNetworks(2)Fig.1-10.Astreamofpacketsfromsendertoreceiver.2022/12/19ComputerNetworks-1-Part132WideAreaNetworks(3)InaWAN,hostsareconnectedtoasubnet,whichinturnconsistsofrouters(switchingelements)andtrunks.Routersgenerallyadheretoastore-and-forwardprinciple:incomingpacketsarefirstbuffered(stored),theroutertakesadecisiononwherethepackethastogo,andforwardsthepacketacrosstheselectedoutputline.2022/12/19ComputerNetworks-1-Part133WideAreaNetworksConnectdevicesoveracountryExampleWANconnectingthreebranchoffices:2022/12/19ComputerNetworks-1-Part134WideAreaNetworksAnISP(InternetServiceProvider)networkisalsoaWAN.CustomersbuyconnectivityfromtheISPtouseit.2022/12/19ComputerNetworks-1-Part135WideAreaNetworksAVPN(VirtualPrivateNetwork)isaWANbuiltfromvirtuallinksthatrunontopoftheInternet.2022/12/19ComputerNetworks-1-Part136WANconsistsoftwosubnetsResourcesubnetIncludescomputers,terminals,programs,etc.ResponsibleforinformationprocessingCommunicationsubnet:Includestransportationlines,switchingelements(routersandswitches,etc.)Responsibleforinformationdeliveryanddistribution2022/12/19ComputerNetworks-1-Part137WANTopologiesNote:IncontrasttoLANsandMANs,theorganizationofaWANintermswhichhostsareinterconnectedisimportantObservation:Mostoftenyou’llseearbitrarytopologies;theothersareusedinapplication-specificways(mostlystar,ring,andtree)2022/12/19ComputerNetworks-1-Part1381.2.5Internetworks(1)Theassumptionsofaristhatanetworkishomogeneous:thereishardlyanyvariationinhardwareandsoftware.Inpractice,largenetworkscanonlybeconstructedbyinterconnectingdifferentkindsofnetworks,internet(work).Heterogeneoushardwareandsoftwarearevariable2022/12/19ComputerNetworks-1-Part1391.2.6Internetworks(2)Examples:ConnectingacollectionofdifferentkindsofLANs(bus-basedtotoken-based)withinadepartment.ConnectingLANstoeachotherthroughaWAN(thinkofenterprisenetworksformultinationals).TheWANactsasasubnet.ConnectingWANstoeachother(theInternet).2022/12/19ComputerNetworks-1-Part1401.3NetworkSoftware1.3.1.ProtocolHierarchiesToreducedesigncomplexity,mostnetworksareorganizedasaseriesoflayers,eachlayerofferscertainservicestoitsupperlayer.Layernononemachinecarriesonaconversationwithlayernonanothermachine.Therulesandconventionsusedinthisconversationarecollectivelyknownasthelayern

protocol.2022/12/19ComputerNetworks-1-Part1412022/12/19ComputerNetworks-1-Part142Fig.1.13Layers,protocols,andinterfacesWhat’saprotocol?humanprotocols:“what’sthetime?”“Ihaveaquestion”introductions…specificmsgssent…specificactionstakenwhenmsgsreceived,orothereventsnetworkprotocols:machinesratherthanhumansallcommunicationactivityinInternetgovernedbyprotocols2022/12/19ComputerNetworks-1-Part143protocolsdefineformat,orderofmsgssentandreceivedamongnetworkentities,andactionstakenonmsgtransmission,receipt

What’saprotocol?ahumanprotocolandacomputernetworkprotocol:2022/12/19ComputerNetworks-1-Part144HiHiGotthetime?2:00TCPconnectionrequestTCPconnectionresponseGet/kurose-ross<file>timeSometerminologies(1)LayersNetworksareorganizedasastackoflayersorlevels,eachonebuiltupontheonebelowitProtocolAnagreementbetweenthecommunicatingpartiesonhowcommunicationproceedPeerTheentitiescomprisingthecorrespondinglayersondifferentmachinesarecalledpeersInterfaceDefineswhichprimitiveoperationsandservicesthelowerlayermakesavailabletotheupperone2022/12/19ComputerNetworks-1-Part145InterfacesandServicesRelationbetweenlayersataninterface2022/12/19ComputerNetworks-1-Part146AServiceAccessPointisidentifiedbyanaddress,andformstheinterfacetoasetofservices.TheServiceDataUnitcontainsthedatayouwanttosend.TheInterfaceControlInformationcontainsinfoneededtosendtheSDU,e.g.numberofbytes.TheProtocolDataUnitisthedatathatissentacrossthenetwork,containingyourSDUaswellasprotocol-specificdata.2022/12/19ComputerNetworks-1-Part147Someterminologies(2)NetworkArchitectureAsetoflayersandprotocols,togetherwithinterfacesbetweenlayers,iscalledNetworkArchitecture.ProtocolstackAlistofprotocolsusedbyacertainsystem,oneprotocolperlayer,tocolhierarchies

Fundamentaltoallsoftwarethatmakesacomputernetworkrun,isthenotionofprotocolhierarchies:structuringtheservicesthatanetworkmustofferintermsoflayers.2022/12/19ComputerNetworks-1-Part148Protocol“Layers”Networksarecomplex!many“pieces”:hostsrouterslinksofvariousmediaapplicationsprotocolshardware,softwareQuestion:

Isthereanyhopeoforganizingstructureofnetwork?Oratleastourdiscussionofnetworks?2022/12/19ComputerNetworks-1-Part149Organizationofairtravelaseriesofsteps2022/12/19ComputerNetworks-1-Part150ticket(purchase)baggage(check)gates(load)runwaytakeoffairplaneroutingticket(complain)baggage(claim)gates(unload)runwaylandingairplaneroutingairplaneroutingLayeringofairlinefunctionalityLayers:eachlayerimplementsaserviceviaitsowninternal-layeractionsrelyingonservicesprovidedbylayerbelow2022/12/19ComputerNetworks-1-Part151ticket(purchase)baggage(check)gates(load)runway(takeoff)airplaneroutingdepartureairportarrivalairportintermediateair-trafficcontrolcentersairplaneroutingairplaneroutingticket(complain)baggage(claimgates(unload)runway(land)airplaneroutingticketbaggagegatetakeoff/landingairplaneroutingWhylayering?Dealingwithcomplexsystems:explicitstructureallowsidentification,relationshipofcomplexsystem’spieceslayeredreferencemodelfordiscussionmodularizationeasesmaintenance,updatingofsystemchangeofimplementationoflayer’sservicetransparenttorestofsysteme.g.,changeingateproceduredoesn’taffectrestofsystemlayeringconsideredharmful?2022/12/19ComputerNetworks-1-Part1522022/12/19ComputerNetworks-1-Part153Fig.1.14Thephilosopher–translator–secretaryarchitectureLayering:TheConcepts(1)Twopartiesatdifferentsites,butatthesamelevel,alwaysagreeonhowtheywillexchangeinformation:specifiedinaprotocol.Example:weallagreetospeakChinese(butuseEnglishonthesheets).Inorderforonepartytosendandreceiveinformation,itcanonlymakeuseofthecommunicationservicesofferedbythelayerdirectlyunderneathit.Example:Theuseofinterpretersinnegotiationsbetweencountries.2022/12/19ComputerNetworks-1-Part154Layering:TheConcepts(2)Servicesofferedbyalayerarealwaysfullyspecifiedintermsofaninterfacethatmakesthoseservicesaccessible.Example:phoneshavebuttonsthatallowyouto“dial”anumber.Protocol:Asetofrulesandspecificationsforpeer-to-peervirtualcommunication,itis“horizontal”.Service:Asetofcommunicationabilitiesandoperationsprovidedbylowerlayertoitshigherlayer,itis“vertical”.2022/12/19ComputerNetworks-1-Part155Layering:AnExample2022/12/19ComputerNetworks-1-Part156Fig.1.15Informationflowsupportingvirtualcommunicationinlayer5Observation:Inaprotocolstack,LayerkputsitsentirepacketasdataintoaLayerk-1packet,thatis,thek-1packetpayload;thelattermayaddaheaderand/oratrailer.Note:ItmayevenoccurthatLayerkdatahastobesplitacrossseveralLayerk-1packets,i.e.,fragmentation.Atthedestination,reassemblyisneededtorecoverLayerkdata.2022/12/19ComputerNetworks-1-Part1571.3.2DesignIssuesfortheLayers(1)AddressingMechanismforidentifyingsendersandreceivers:addressingtospecifyapartyRulesfordatatransferSimplexcommunication,SDXHalf-duplexcommunication,HDXFull-duplexcommunication,FDXErrorcontrolPhysicalcommunicationcircuitsarenotperfect,sothaterror-detectinganderror-correctingcodeareused.Inaddition,receivermusthavesomewayoftellingthesenderwhichmessageshavebeencorrectlyreceivedandwhichhavenot.2022/12/19ComputerNetworks-1-Part1581.3.2DesignIssuesfortheLayers(2)SequencingKeepthemessagesorderFlowcontrolKeepafastsenderfromswampingaslowreceiverwithdataDisassemblingandreassemblingAllowallprocessestoacceptarbitrarilylongmessagesMultiplexingUsethesameconnectionformultiple,unrelatedconversationsRoutingSelectaroutefrommultiplepathsbetweensourceanddestination2022/12/19ComputerNetworks-1-Part1591.3.3Connection-orientedandConnectionlessServices(1)Whatisservice?Asetofcommunicationabilitiesandoperationsprovidedbylowerlayertoitshigherlayer.Connection-orientedServiceLikethetelephonemodel:youfirstestablishaconnection,thendoalotcommunication,andfinallyreleasetheconnection.Threephases.2022/12/19ComputerNetworks-1-Part160Services:ConnectionsorNot(2)Connectionless:Likethepostalmodel:yourdataisputintosomekindofenvelopecalledpacket,onwhichthedestinationaddresshasbeenwritten.Theenvelope+contents(packet)isindependentlyroutedincommunicationsubnet,untilgetstothedestination,andthat’sit.2022/12/19ComputerNetworks-1-Part161Services:ConnectionsorNot(3)Eachservicecanprovidesomequality:Isdatadeliveredintheorderitwassent?Withconnections,thisisgenerallythecase.Isdatatransmissionreliable?Generallyofferedwithconnections,butnotalwayswithconnectionlessservices.Reliabilityrequiressendingacknowledgements,sothatperformancemaydegrade.2022/12/19ComputerNetworks-1-Part162Connection-OrientedandConnectionlessServices(4)Sixdifferenttypesofservice2022/12/19ComputerNetworks-1-Part1631.3.4ServicePrimitivesServicesaregenerallyspecifiedbyasetofprimitives(operations),whichtelltheservicetoperformsomeactionorreportonanactiontakenbyapeerentity.Primitivesarenormallysystemcalls.2022/12/19ComputerNetworks-1-Part1641.CONNECT.request:requestforestablishingaconnection(dialaphonenumber).2.CONNECT.indication:signalthecallee(phonerings).3.CONNECT.response:reactionbycalleetoindication(pickupthephone).4.CONNECT.confirm:tellcallerwhethercallwasaccepted(callerhearsringingstop).5.DATA.request:requestdatatobesent(saysomething)6.DATA.indication:signalarrivalofdata(calleehearsyou)7.DISCONNECT.request:requestreleaseofconnection(callerhangsup)8.DISCONNECT.indication:signalreleaseofconnection(calleehearsbusytone)2022/12/19ComputerNetworks-1-Part165ServicePrimitives(2)Packetssentinasimpleclient-serverinteractiononaconnection-orientednetwork.2022/12/19ComputerNetworks-1-Part1661.3.5TheRelationshipofServicestoProtocolsService

isasetofprimitives(operations)thatalayerprovidestothelayeraboveit,definingwhatoperationsthelayerispreparedtoperformonbehalfofitsusers,butnothingabouthowtheseoperationsareimplemented.Protocolisasetofrulesgoverningtheformatandmeaningofthepackets,ormessagesthatareexchangedbythepeerentitieswithinalayer.Entitiesuseprotocolstoimplementtheirservicedefinitions.2022/12/19ComputerNetworks-1-Part1671.3.5TheRelationshipofServicestoProtocolsFig.1.19Therelationshipbetweenaserviceandaprotocol2022/12/19ComputerNetworks-1-Part168Endofpart12022/12/19ComputerNetworks-1-Part169COMPUTERNETWORKS

Chapter01

Introduction(2)2022/12/19ComputerNetworks-1-Part2701.4ReferenceModels1.4.1OSIReferenceModel1983,InternationalStandardsOrganization(ISO)proposedthe“ISOOSI(OpenSystemsInterconnection)ReferenceModel”,OSI-rmTheprinciplesoflayeringAlayershouldbecreatedwhereadifferentabstractionisneededEachlayershouldperformawell-definedfunctionThefunctionofeachlayershouldbechosentowardsdefininginternationallystandardizedprotocolsThelayerboundariesshouldbechosentominimizetheinformationflowacrosstheinterfacesThenumberoflayersshouldbelargeenoughthatdistinctfunctionsandsmallenoughthatthearchitecturedoesnotbecomeunwieldy2022/12/19ComputerNetworks-1-Part271TheOSIModel2022/12/19ComputerNetworks-1-Part272ThePhysicalLayerEssence:Describesthetransmissionofrawbitsintermsofmechanical,electrical,functionalandtiminginterfacesissues:Typicalquestionsherearewhatelectricalsignalsshouldbeusedtorepresenta1anda0HowmanynanosecondsabitlastswhethertransmissionmayproceedsimultaneouslyinbothdirectionshowtheinitialconnectionisestablishedhowitistorndownwhenbothsidesarefinishedhowmanypinsthenetworkconnectorhasWhateachpinisusedfor.Example:Connecttwocomputersbymeansofawire:Setting-3Vto-12Vonthewirecorrespondstoabinary1;+3Vto+12Visabinary0Thewireisnottobelongerthan15metersYoumaychangethevoltageatmost20,000timespersecond(Question:what’sthetransferrate?)2022/12/19ComputerNetworks-1-Part273TheDataLinkLayerDatalinklayeristotransformarawtransmissionfacilityintoalinethatappearsfree

ofundetectedtransmissionerrorstothenetworklayerObservation:Weneedtoatleastdetectbittransmissionerrors,sendbitsinframesthataddredundancytodetectsomethingwentwrongExamples:

Addaparitybittoevery7transmittedbits:1saystherewereoddnumberof1’s;0saystherewereanevennumberof1’sAddachecksum(cyclicredundancycheck)thatshouldmatchthebitsbeforeitAlso:Providethemechanismssothatfastsendersdon’toverwhelmslowreceivers(flowcontrol)2022/12/19ComputerNetworks-1-Part274DataLinkLayerObservation:Wealsoneedtospecifyhowanumberofcomputerscanshareacommonchannel(e.g.wire),thatismediumaccesscontrolsublayer(MAC):1.Specifieshowoneoutofseveralcompetingsenders,iseventuallyallowedexclusiveaccesstothewire2.Commonapproach1:listentoeachother;retreatwhenyouhearsomeoneelse,andtryagainlater3.Commonapproach2:waityourturnbypassingatokenbetweenallstationsWell-knownprotocols:Ethernet,tokenring,tokenbus,FDDI2022/12/19ComputerNetworks-1-Part275TheNetworkLayerEssence:Describeshowrouting(andcongestion)istobedone.Mostlyneededinsubnets.Networklayercontrolssubnetoperations.1.Howdowefindoutwhichcomputers/routersareinthenetwork?2.HowdowecalculatethebestroutefromAtoB?3.Whathappenswhenacomputer/routergoesdown?4.Shouldmulticasting/broadcastingbesupported?5.Whathappensifarouterbecomesoverloadedandstartsdroppingpackets?6.Canwedetectandavoid“hotspots?”2022/12/19ComputerNetworks-1-Part276TheTransportLayerObservation:Thetransportlayeristoacceptdatafromabove,splititupintosmallerunitsifneedbe,

passthesetonetworklayer,andensurethatthepiecesallarrivecorrectlyattheotherend.

Generallyoffersconnection-orientedaswellasconnectionlessservices,andvaryingdegreesofreliability.Thislayerprovidestheactualnetworkinterfacetoapplications1.Oftenprovidesnetworkinterfacethroughsockets(UNIX,Windows)2.Allowstosetupaconnectiontoanotherapplication,andsubsequentlydeliverdatareliably,andintheorderthatitwassent3.Oftenalsosupportforsecureconnections4.Alsosupportfordatagrams:unreliablemessagepassingonaper-messagebasis2022/12/19ComputerNetworks-1-Part277TheTransportLayerThetransportlayerisatrueend-to-endlayer;itcarriesd