ip packet switching - cs. of today’s lecture • connecvity – links and nodes – circuit...

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