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  • COS461:ComputerNetworksSpring2008(MW1:302:50inCOS105)

    MikeFreedman

    hEp://www.cs.princeton.edu/courses/archive/spring09/cos461/

    IPPacketSwitching

  • GoalsofTodaysLecture ConnecTvity

    Linksandnodes Circuitswitching Packetswitching

    IPservicemodel Besteffortpacketdelivery IPastheInternetsnarrowwaistDesignphilosophyofIP

    IPpacketstructure FieldsintheIPheader TracerouteusingTTLfield Sourceaddressspoofing

    2

  • SimpleNetwork:NodesandaLink

    Node:computer Endhost:generalpurposecomputer,cellphone,PDANetworknode:switchorrouter

    Link:physicalmediumconnecTngnodes Twistedpair:thewirethatconnectstotelephones Coaxialcable:thewirethatconnectstoTVsetsOpTcalfiber:highbandwidthlongdistancelinks Space:propagaTonofradiowaves,microwaves,

    3

    Node Link Node

  • NetworkComponents

    4

    Fibers

    Coaxial Cable

    Links Interfaces Switches/routers

    Ethernet card

    Wireless card

    Large router

    Telephone switch

  • Links:DelayandBandwidth Delay

    LatencyforpropagaTngdataalongthelinkCorrespondstothelengthofthelinkTypicallymeasuredinseconds

    BandwidthAmountofdatasent(orreceived)perunitTmeCorrespondstothewidthofthelinkTypicallymeasuredinbitspersecond

    5

    bandwidth

    delay

    delay x bandwidth

  • ConnecTngMoreThanTwoHosts MulTaccesslink:Ethernet,wireless

    Singlephysicallink,sharedbymulTplenodesLimitaTonsondistanceandnumberofnodes

    Pointtopointlinks:fiberopTccableOnlytwonodes(separatelinkperpairofnodes)LimitaTonsonthenumberofadapterspernode

    6

    multi-access link point-to-point links

  • BeyondDirectlyConnectedNetworks

    SwitchednetworkEndhostsattheedgeNetworknodesthatswitchtrafficLinksbetweenthenodes

    MulTplexingManyendhostscommunicateoverthenetworkTrafficsharesaccesstothesamelinks

    7

  • CircuitSwitching(e.g.,PhoneNetwork)

    SourceestablishesconnecTontodesTnaTonNodealongthepathstoreconnecToninfoNodesmayreserveresourcesfortheconnecTon

    SourcesendsdataovertheconnecTonNodesTnaTonaddress,sincenodesknowpath

    SourcetearsdownconnecTonwhendone

    8

  • CircuitSwitchingWithHumanOperator

    9

  • CircuitSwitching:MulTplexingaLink

    TimedivisionEachcircuitallocatedcertainTmeslots

    FrequencydivisionEachcircuitallocatedcertainfrequencies

    10

    time frequency

    time

  • AdvantagesofCircuitSwitching Guaranteedbandwidth

    PredictablecommunicaTonperformanceNotbesteffortdeliverywithnorealguarantees

    SimpleabstracTon ReliablecommunicaTonchannelbetweenhostsNoworriesaboutlostoroutoforderpackets

    Simpleforwarding ForwardingbasedonTmeslotorfrequencyNoneedtoinspectapacketheader

    Lowperpacketoverhead ForwardingbasedonTmeslotorfrequencyNoIP(andTCP/UDP)headeroneachpacket

    11

  • DisadvantagesofCircuitSwitching Wastedbandwidth

    BurstytrafficleadstoidleconnduringsilentperiodUnabletoachievegainsfromstaTsTcalmulTplexing

    BlockedconnecTons ConnecTonrefusedwhenresourcesarenotsufficientUnabletoofferokayservicetoeverybody

    ConnecTonsetupdelayNocommunicaTonunTltheconnecTonissetupUnabletoavoidextralatencyforsmalldatatransfers

    NetworkstateNetworknodesmuststoreperconnecToninformaTonUnabletoavoidperconnecTonstorageandstate

    12

  • PacketSwitching(e.g.,Internet) Datatrafficdividedintopackets

    Eachpacketcontainsaheader(withaddress) Packetstravelseparatelythroughnetwork

    PacketforwardingbasedontheheaderNetworknodesmaystorepacketstemporarily

    DesTnaTonreconstructsthemessage

    13

  • PacketSwitching:StaTsTcalMulTplexing

    14

    Packets

  • IPService:BestEffortPacketDelivery

    15

    PacketswitchingDividemessagesintoasequenceofpacketsHeaderswithsourceanddesTnaTonaddress

    BesteffortdeliveryPacketsmaybelostPacketsmaybecorruptedPacketsmaybedeliveredoutoforder

    source destination

    IP network

  • IPServiceModel:WhyPackets? Datatrafficisbursty

    LoggingintoremotemachinesExchangingemailmessages

    DontwanttowastebandwidthNotrafficexchangedduringidleperiods

    BeEertoallowmulTplexingDifferenttransfersshareaccesstosamelinks

    PacketscanbedeliveredbymostanythingRFC1149:IPDatagramsoverAvianCarriers

    sTll,packetswitchingcanbeinefficientExtraheaderbitsoneverypacket

    16

  • IPServiceModel:WhyBestEffort? IPmeansneverhavingtosayyouresorry

    DontneedtoreservebandwidthandmemoryDontneedtodoerrordetecTon&correcTonDontneedtorememberfromonepackettonext

    EasiertosurvivefailuresTransientdisrupTonsareokayduringfailover

    but,applicaTonsdowantefficient,accuratetransferofdatainorder,inaTmelyfashion

    17

  • IPService:BestEffortisEnough NoerrordetecTonorcorrecTon

    Higherlevelprotocolcanprovideerrorchecking Successivepacketsmaynotfollowthesamepath

    NotaproblemaslongaspacketsreachthedesTnaTon Packetscanbedeliveredoutoforder

    Receivercanputpacketsbackinorder(ifnecessary) Packetsmaybelostorarbitrarilydelayed

    Sendercansendthepacketsagain(ifdesired) NonetworkcongesToncontrol(beyonddrop)

    Sendercanslowdowninresponsetolossordelay18

  • LayeringintheIPProtocols

    19

    Internet Protocol

    Transmission Control Protocol (TCP)

    User Datagram Protocol (UDP)

    Telnet HTTP

    SONET ATM Ethernet

    RTP DNS FTP

  • History:WhyIPPackets? IPproposedintheearly1970s

    DefenseAdvancedResearchProjectAgency(DARPA) Goal:connectexisTngnetworks

    TodevelopaneffecTvetechniqueformulTplexeduTlizaTonofexisTnginterconnectednetworks

    E.g.,connectpacketradionetworkstotheARPAnet MoTvaTngapplicaTons

    Remotelogintoservermachines Inherentlyburstytrafficwithlongsilentperiods

    PriorARPAnetexperiencewithpacketswitching PreviousDARPAprojectDemonstratedstoreandforwardpacketswitching

    20

  • OtherMainDrivingGoals(InOrder)

    CommunicaTonshouldconTnuedespitefailures SurviveequipmentfailureorphysicalaEack TrafficbetweentwohostsconTnueonanotherpath

    SupportmulTpletypesofcommunicaTonservicesDifferingrequirementsforspeed,latency,&reliability BidirecTonalreliabledeliveryvs.messageservice

    Accommodateavarietyofnetworks BothmilitaryandcommercialfaciliTesMinimizeassumpTonsabouttheunderlyingnetwork

    21

  • OtherDrivingGoals,SomewhatMet Permitdistributedmanagementofresources

    NodesmanagedbydifferentinsTtuTonsthoughthisissTllratherchallenging

    CosteffecTveness StaTsTcalmulTplexingthroughpacketswitchingthoughpacketheadersandretransmissionswasteful

    EaseofaEachingnewhosts StandardimplementaTonsofendhostprotocolsthoughsTllneedafairamountofendhostsolware

    AccountabilityforuseofresourcesMonitoringfuncTonsinthenodesthoughthisissTllfairlylimitedandimmature

    22

  • IPPacketStructure

    4-bit Version

    4-bit Header Length

    8-bit Type of Service

    (TOS) 16-bit Total Length (Bytes)

    16-bit Identification 3-bit Flags 13-bit Fragment Offset

    8-bit Time to Live (TTL) 8-bit Protocol 16-bit Header Checksum

    32-bit Source IP Address

    32-bit Destination IP Address

    Options (if any)

    Payload

  • IPHeader:Version,Length,ToS Versionnumber(4bits)

    IndicatestheversionoftheIPprotocolNecessarytoknowwhatotherfieldstoexpect Typically4(forIPv4),andsomeTmes6(forIPv6)

    Headerlength(4bits)Numberof32bitwordsintheheader Typically5(fora20byteIPv4header) CanbemorewhenIPopTonsareused

    TypeofService(8bits)Allowpacketstobetreateddifferentlybasedonneeds E.g.,lowdelayforaudio,highb/wforbulktransfer

    24

  • IPHeader:Length,Fragments,TTL Totallength(16bits)

    NumberofbytesinthepacketMaximumsizeis63,535bytes(2161)thoughunderlyinglinksmayimposeharderlimits

    FragmentaToninformaTon(32bits) PacketidenTfier,flags,andfragmentoffset SupportsdividingalargeIPpacketintofragmentsincasealinkcannothandlealargeIPpacketsowhydowetypicallysendmaxMTUpackets?

    TimeToLive(8bits)UsedtoidenTfypacketsstuckinforwardingloopsandeventuallydiscardthemfromthenetwork

    25

  • IPHeader:MoreonTimetoLive(TTL)

    PotenTalrobustnessproblem Forwardingloopscancausepacketstocycleforever Confusingifthepacketarrivesmuchlater

    Timetolivefieldinpacketheader TTLfielddecrementedbyeachrouteronthepath PacketisdiscardedwhenTTLfieldreaches0andTmeexceededmessageissenttothesource

    26

  • IPHeader:UseofTTLinTraceroute TimeToLivefieldinIPpacketheader

    SourcesendsapacketwithaTTLofnEachrouteralongthepathdecrementstheTTLTTLexceededsentwhenTTLreaches0

    TraceroutetoolexploitsthisTTLbehavior

    27

    source destination

    TTL=1 Time

    exceeded

    TTL=2

    SendpacketswithTTL=1,2,andrecordsourceof;meexceededmessage

  • ExampleTraceroute:BerkeleytoCNN

    28

    1 169.229.62.1

    2 169.229.59.225

    3 128.32.255.169

    4 128.32.0.249

    5 128.32.0.66

    6 209.247.159.109

    7 *

    8 64.159.1.46

    9 209.247.9.170

    10 66.185.138.33

    11 *

    12 66.185.136.17

    13 64.236.16.52

    Hop number, IP address, DNS name

    inr-daedalus-0.CS.Berkeley.EDU

    soda-cr-1-1-soda-br-6-2

    vlan242.inr-202-doecev.Berkeley.EDU

    gigE6-0-0.inr-666-doecev.Berkeley.EDU

    qsv-juniper--ucb-gw.calren2.net

    POS1-0.hsipaccess1.SanJose1.Level3.net

    ?

    ?

    pos8-0.hsa2.Atlanta2.Level3.net

    pop2-atm-P0-2.atdn.net

    ?

    pop1-atl-P4-0.atdn.net

    www4.cnn.com

    No response from router

    No name resolution

  • TryRunningTracerouteYourself OnUNIXmachine

    TracerouteE.g.,traceroutecnn.comortraceroute12.1.1.1

    OnWindowsmachineTrace

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