ComputerNetworks:ArchitectureandProtocols

CS4450

Lecture4-PacketDelays

-HowtheInternetworks-ThreeArchitecturalPrinciples

RachitAgarwal

ContextforandGoalsofToday’sLecture

• Context:• Today’slectureisgoingtobeoneofthehardestlectures• Ifyouunderstandeverything

• Thereissomethingwrong!

• Goals:• HowdoestheInternetwork?

• Anend-to-endview

• ThreePrinciples

But,asusual,letsstartwith:

whatwelearntlastlecture

• Handlingfailures• Resourceunderutilization• Blockedconnections• Connectionsetupoverheads• Per-connectionstateinswitches(scalabilityproblem)

Recap:ChallengeswithCircuitswitching(reservation)

• Breakdataintosmallerpieces

• Packets!

• Transmitthepacketswithoutanyreservations

• And,hopeforthebest

Recap:Solution:Packetswitching

• Goods:• Easiertohandlefailures• Noresourceunderutilization

• Asourcecansendmoreifothersdon’tuseresources

• Noblockedconnectionproblem

• Noper-connectionstate• Noset-upcost

• Not-so-goods:• Unpredictableperformance

• Highlatency• Packetheaderoverhead

Recap:Packetswitchingsummary

• Consistsofsixcomponents

• Linkproperties:• Transmissiondelay

• Propagationdelay

• OSinternals:• Processingdelay• Queueingdelay

• Trafficmatrixandswitchinternals:

• Processingdelay• Queueingdelay

• First,considertransmission,propagationdelays

• Queueingdelayandprocessingdelayslaterinthecourse

Recap:Deepdiveintoonelink:packetdelay/latency

• Transmissiondelay:

• Timetakentopushallthebitsofapacketintoalink

• =Packetsize/Linkbandwidth

• Propagationdelay:• Timetakentomoveonebitfromoneendofthelinktoother

• =Linklength/Speedoflight

Recap:Transmissionandpropagationdelay

Questions?

Today’slecture

1. Diveintoend-to-end:fromsourcetodestination

2. Firstlookintoswitches:routing,queueing,forwarding

3. Firstlookintonetworkstack:sockets,ports,“thestack”

4. Secondlookintothestack:layers

5. Whylayering?

Firstlookintoend-to-end

Source

Destination

Source

Destination

End-to-end:whatmechanismsdoweneed?

• Locatingthedestination:Naming,addressing

• Findingapathtothedestination:Routing

• Sendingdatatothedestination:Forwarding

• Reliability:Failurehandling

Fourfundamentalproblems!

Naming,Routing,Forwarding,Reliability

• Eachismotivatedbyaclearneed

• Thesolutionsarenotalwayscleanordeep

• Butifyoukeepinmindwhattheproblemis

• You’llbeabletounderstandthesolutions• Whentherighttimecomes:-)

Fourfundamentalproblems!

• NetworkAddress:wherehostislocated• Requiresanaddressforthedestinationhost

• canbemultipleheaders

• NetworkName:whichhostitis

• why?

• Whenyoumoveservertonewbuilding

• Namedoesn’tchange

• Addressdoeschange

• Samethingwithyourownnameandaddress!

• Remembertheanalogy:humannames,addresses,postoffice,letters

Fundamentalproblem#1:HostNamesandAddresses

• Considerwhenyouaccessawebpage• InsertURLintobrowser(eg,www.cornell.edu)• Packetssenttowebsite(reliably)• Packetreachapplicationondestinationhost

• Howdoyougettothewebsite?• URLisuser-levelname(eg,www.cornell.edu)

• Networkneedsaddress(eg,whereiswww.cornell.edu)?

• Mustmapnamestoaddresses

• Justlikeweuseanaddressbooktomaphumannamestoaddresses

Namesversusaddresses

• OntheInternet,weonlynamehosts(sortof)

• URLsarebasedonthenameofthehostcontainingthecontent(thatis,www.cornell.edunamesahost)

• Beforeyoucansendpacketstowww.cornell.edu,youmustresolvenamesintothehost’saddress

• DonebytheDomainNameSystem(DNS)

MappingNamestoAddresses

Thesourceknowsthename;

MapsthatnametoanaddressusingDNS!

Questions?

Routingtodestination

• Givendestinationaddress,howdoeseachswitch/routerknowwheretosendthepacketsothatthepacketreachesitsdestination

• Whenapacketarrivesatarouter

• aroutingtabledetermineswhichoutgoinglinkthepacketissenton

Fundamentalproblem#2

• Distributedalgorithmthatrunsbetweenrouters

• Distributedmeansnosinglerouterhas“full”viewofthenetwork

• Exchangeofmessagestogather“enough”information…

• …aboutthenetworktopology

• Computepathsthroughthattopology

• Storeforwardinginformationineachrouter

• IfpacketisdestinedforX,sendoutlinkl1• IfpacketisdestinedforY,sendoutlinkl2• Canpacketsgoingtodifferentdestinationssentouttosameport?

• Wecallthisaroutingtable

Routingprotocols(conceptually)

Questions?

QueueingandForwardingofpacketsatswitches/routers

• Queueing:Whenapacketarrives,storeitin“inputqueues”

• Eachincomingqueuedividedintomultiplevirtualoutputqueues

• Onevirtualoutputqueueperoutgoinglink• Whenapacketarrives:

• Lookupitsdestination’saddress(how?)• Findthelinkonwhichthepacketwillbeforwarded(how?)• Storethepacketincorrespondingvirtualoutputqueue

• Forwarding:Whentheoutgoinglinkfree

• Pickapacketfromthecorrespondingvirtualoutputqueue

• forwardthepacket!

Fundamentalproblem#3

• Packetsmustdescribewhereitshouldbesent

• Requiresanaddressforthedestination

• Packetsmustdescribewhereitscomingfrom

• Forhandlingfailures,etc.• Requiresanaddressforthesource

• Packetsmustcarrydata

• canbebitsinafile,image,whatever

Whatmustpacketscarrytoenableforwarding?

Header Data

Whatdoesaswitch/routerlooklike

Inputqueue

Virtualoutputqueue

Outputqueue

• Eachinputqueuecouldsendpacketstoeachoutputqueueatfullrate• Thatis,aswitcharchitectureisheavilyparallelized• Canalwaysfocusonasingleoutgoingqueuefordesign/analysis

Queueingandprocessingdelay:CaseI(lowload)

2packets/time

1packet/time

Queueingandprocessingdelay:CaseII(balancedload)

2packets/time

1packet/time

Queueingandprocessingdelay:CaseII(highload)

2packets/time

1packet/time

• Processingdelay• Easy;eachswitch/routerneedstodecidewheretoputpacket• Requirescheckingheader,etc.

• Queueingdelay• Dependsonnetworkload• Asloadincreases,queueingdelayincreases

• Inanextremecase,increaseinnetworkload

• resultsinpacketdrops

Queueingandprocessingdelay

Questions?

Howdoyoudeliverpacketsreliable?

• Packetscanbedroppedalongtheway• Buffersinroutercanoverflow• Routerscancrashwhilebufferingpackets• Linkscangarblepackets

• Howdoyoumakesurepacketsarrivesafelyonanunreliablenetwork?

• Or,atleast,knowiftheyaredelivered?• Wantnofalsepositives,andhighchangeofsuccess

Fundamentalproblem#4

• Whoisresponsibleforthis?(architecture)

• Network?• Host?

• Howisitimplemented?(engineering)

• Wewillconsiderbothperspectives

Twoquestionsaboutreliability

Questions?

• Wenowhavetheaddressofthewebsite

• And,aroute/pathtothedestination• And,mechanismsinplacetoforwardthepacketsateachswitch/router

• Inareliablemanner

• So,wecansendpacketsfromsourcetodestination

• Arewedone?

• Whenapacketarrivesatahost,whatdoesthehostdowithit?

• Towhichprocess(application)shouldthepacketbesent?

• Ifthepacketheaderonlyhasthedestinationaddress,howdoesthehostknowwheretodeliverpacket?

• Theremaybemultipleapplicationsonthatdestination

Finishingourstory

• Whoputsthesourceaddress,sourceport,destinationaddress,

destinationportinthepacketheader?

Andwhilewearefinishingourstory….

Thefinalpieceinthegame:End-hoststack

OfSocketsandPorts

• Whenaprocesswantsaccesstothenetwork,itopensasocket,whichisassociatedwithaport

• Socket:anOSmechanismthatconnectsprocessestothenetworkingstack

• Port:numberthatidentifiesthatparticularsocket

• TheportnumberisusedbytheOStodirectincomingpackets

• PacketHeadermustinclude:

• Destinationaddress(usedbynetwork)• Destinationport(usedbynetworkstack)• And?• Sourceaddress(usedbynetwork)• Sourceport(usedbynetworkstack)

• Whenapacketarrivesatthedestinationhost,packetisdeliveredtothesocketassociatedwiththedestinationport

• Moredetailslater

ImplicationsforPacketHeader

• Network:Deliverpacketsfromhosttohost(basedonaddress)

• Networkstack(OS):Deliverpacketstoappropriatesocket(basedonport)

• Applications:• Sendandreceivepackets• Understandcontentofpacketbodies

Separationofconcerns

SecretoftheInternet’ssuccessisgetting

theseandotherabstractionsright

• Whyisseparationofconcernsimportant?

• Separationofconcerns~Modularity

• Ifeachcomponent’staskwell-defined,onecanfocusdesignonthattask

• Andreplaceitwithanyotherimplementationthatdoesthattask

• Withoutchanginganythingelse

Whocares?

• Modularityisnothingmorethandecomposingprograms/systemsintosmallerunits.• Aclean“separationofconcerns”

• Playsacrucialroleincomputerscience…

• …andnetworking

WhatisModularity

“Modularitybasedonabstractionisthewaytogetthingsdone”--BarbaraLiskov

ModularityinComputerScience

• Partitionsystemintomodules

• Eachmodulehaswelldefinedinterface

• Interfacesgiveflexibilityinimplementation

• Changeshavelimitedscope

• Examples• Librariesencapsulatingsetoffunctionalities• ProgramminglanguageabstractsawayCPU

• Thetrickistofindtherightmodularity

• Theinterfacesshouldbelong-lasting• Ifinterfacesarechangingoften,modularityiswrong

ComputerSystemModularity

• Theneedformodularitystillapplies

• Andisevenmoreimportant!(why?)

• Networkimplementationsnotjustdistributedacrossmanylinesofcode

• Normalmodularity“organizes”thatcode

• Networkingisdistributedacrossmanymachines• Hosts• Routers

NetworkSystemModularity

• Howtobreaksystemintomodules?

• Classicdecompositionintotasks

• Wherearemodulesimplemented?

• Hosts?• Routers?• Both?

• Whereisstatestored?

• Hosts?• Routers?• Both?

NetworkModularityDecisions

• Howtobreaksystemintomodules

• Layering

• Wherearemodulesimplemented

• End-to-EndPrinciple

• Whereisstatestored?• Fate-Sharing

Leadstothreedesignprinciples

Layering

• Bitsonwire

• Packetsonwire

• Deliverpacketstohostsacrosslocalnetwork

• Deliverpacketstohostacrossnetworks

• Deliverpacketsreliably,tocorrectprocess

• Dosomethingwiththedata

Breakdownintotasks

• Bitsonwire(Physical)

• Packetsonwire

• Deliverpacketstohostsacrosslocalnetwork(Datalink)

• Deliverpacketstohostacrossnetworks(Network)

• Deliverpacketsreliably,tocorrectprocess(Transport)

• Dosomethingwiththedata(Application)

ResultingModules(Layers)

• Application:Providingnetworksupportforapps

• Transport(L4):(Reliable)end-to-enddelivery

• Network(L3):Globalbest-effortdelivery

• Datalink(L2):Localbest-effortdelivery

• Physical:Bitsonwire

FiveLayers(Top-Down)

• Akindofmodularity

• Functionalityseparatedintolayers• Layerninterfaceswithonlylayern-1

• Hidescomplexityofsurroundinglayers

• Evolutionof“modules”

• (IP)Connectivitybecomesacommodity

Layering

• Eachlayer:• Dependsonthelayerbelow• Supportslayerabove• Independentofothers

• Multipleversionsinlayer

• Interfacesdiffersomewhat

• Componentspickwhichlower-

levelprotocoltouse

• ButonlyoneIPlayer• Unifyingprotocol

ThreeObservations

Layering“modularized”theInternetarchitecturewithflexibleopeninterfaces

whichhelpedspurinnovagon

LayeringandInnovation

• Innovationatmostlevels:

• Applications(lots)• Transport(few)• Datalink(few)• Physical(lots)

• Innovationproceededlargelyinparallel

• Payoffofmodularity!

• Pursuedbyverydifferentcommunities

• Likesystemsandchipdesigners

LayeringcrucialtoInternet’ssuccess

Questions?

• Howtobreaksystemintomodules?

• Layering

• Wherearemodulesimplemented?

• End-to-EndPrinciple

• Whereisstatestored?

• Fate-Sharing

ThreeInternetDesignPrinciples

• Layersaresimpleifonlyonasimplemachine

• Juststackofmodulesinteractingwiththoseabove/below

• Butweneedtoimplementlayersacrossmachines

• Hosts• Routers(Switches)

• Whatgetsimplementedwhere?

DistributingLayersacrossNetwork

• Bitsarriveonwire,mustmakeituptoapplication

• Therefore,alllayersmustexistathost!

WhatgetsimplementedonHost?

• Bitsarriveonwire• Physicallayernecessary

• Packetsmustbedeliveredtonexthop

• Datalinklayernecessary

• Routersparticipateinglobaldelivery• Networklayernecessary

• Routersdonotsupportreliabledelivery• Transportlayer(andabove)notsupported

WhatgetsimplementedonRouter?

• Lowerthreelayersimplementedeverywhere

• Toptwolayersonlyimplementedathosts

SimpleDiagram

• Layeringdoesn'ttellyouwhatserviceseachlayershouldprovide

• Whatisaneffectivedivisionofresponsibilitybetweenvariouslayers?

Butwhyimplementedthisway?

If a function can completely and correctly be implemented only with the knowledge and help of the application standing at the endpoints of the communication system,

then providing that function as a feature of the communication system itself is not possible.

Sometimes providing an incomplete version of that function as a feature of the communication system itself may be useful as a performance enhancement.

End-to-endPrinciple

Supposethelinklayerisreliable.Doesthatensurereliabledatatransfer?

Supposethenetworklayerisreliable.Doesthatensurereliabledata

transfer?

machine machine

Example: file transfer

Suppose the link layer is reliable. Does that ensure reliable file transfer?

Suppose the network layer is reliable. Does that ensure reliable file transfer?

disk disk

file transferapplication

file transferapplication

network

flow of data

End-to-endPrinciple:anexample

Assumethecondigon(IF)holds.Then,

• End-to-endimplementation• Correct• Generalized,andsimplifieslowerlayers

• In-networkimplementation• Insufficient• Mayhelp—orhurt—performance

Examples?Contradicfons?

End-to-endPrinciple(Interpretation)

Whatdoestheendmean?

End-to-endPrinciple(Interpretation)

• Failureavoidance?

• Failurereaction?

• Routing?

• Topologydiscovery?

• PathSelection?

• Security?

• Networkmanagement?

• Resourcemanagement?

GroupExercise4

Whereshallweimplementthefollowing?

• Wheretoimplementfunctionalityiscomplicated

• Norightorwronganswer

• Buteveryoneagreesthatreliabilitydoesnotbelonginthenetwork

• Multicastisagoodtestcase

Summary

Questions?

• Howtobreaksystemintomodules?

• Layering

• Wherearemodulesimplemented?

• End-to-EndPrinciple

• Whereisthestatestored?

• Fate-sharing

ThreeInternetDesignPrinciples

• NotethatE2Eprinciplereliedon“fate-sharing”• Invariantsonlybreakwhenendpointsthemselvesbreak

• Minimizethedependenceonothernetworkelements

• Thisshoulddictateplacementofstorage

Fate-Sharing

• Whenstoringstateinadistributedsystem,colocateitwithentitiesthat

relyonthatstate

• Onlywayfailurecancauselossofthecriticalstateisiftheentitythatcaresaboutitalsofails…

• …inwhichcaseitdoesn’tmatter

• Oftenarguesforkeepingnetworkstateatendhostsratherthaninsiderouters

• E.g.,packet-switchingratherthancircuit-switching

GeneralPrinciple:Fate-Sharing

• Howtobreaksystemintomodules

• Dictatedbylayering

• Wheremodulesareimplemented

• DictatedbyEnd-to-EndPrinciple

• Wherestateisstored

• DictatedbyFateSharing

DecisionsandtheirPrinciples

• TheInternetisahuge,complicatedsystem

• Onecanstudythepartsinisolation• Routing• Ports,sockets• Networkstack• …

• Butthepiecesallfittogetherinaparticularway

• Todaywasquickoverviewofhowpiecesfit…• Don’tworryifyoudidn’tunderstandmuchofit

• Youprobablyabsorbedmorethanyourealize

Today’slecture