ComputerNetworks:

ArchitectureandProtocols

CS4450

Lecture16THEInternetProtocolSwitchArchitecture

RachitAgarwal

Announcements

• PrelimonnextThursday

• Willcovereverythingdiscussedinclassandproblemsets

• ProblemSet3solutionsarereleased

• ProblemSet4isout(solutionswillbereleasedThursday)

• Practiceprelimisreleased(solutionswillbereleasedThursday)

• Wewillreleaseourfirstprojectthisweek(notincludedinprelim)

• Recall:notgraded,butwewillprovideallthehelp

• Youmusthavereceivedanemailformid-semesterfeedbackfortheclass

• Pleaseprovidefrankandconstructivefeedback• Recall:IalreadyknowIamanasshole;iteratingitmightnotbeuseful

• Whatyoulike?Wherecouldweimprove?3

Whatdoweknowsofar[1]…

• Networkperformancemetrics

• Transmissiondelay,propagationdelay,queueingdelay,bandwidth

• Sharingnetworks• Circuitswitching,packetswitching,andassociatedtradeoffs• WhyisInternetpacketswitched?

• Architecturalprinciplesanddesigngoals• Layeringprinciple,End-to-endprinciple,Fatesharingprinciple• ManyimportantdesigngoalsfromDavidClark’spaper

• Andmanyimportantmissinggoals

• Addressing• LinklayerMACnames,andscalabilitychallengesattheInternet

• NetworklayerIPaddresses:threerequirements,aggregation,CIDR

4

Whatdoweknowsofar[2]…• LinkLayer

• SharingaBroadcastmedium,associatedchallenges,CSMA/CD

• Linklayeraddressing:MACnames

• WhyFrames?WhySwitchedEthernet?

• TheSpanningTreeProtocol(STP)

• NetworkLayer

• WhyNetworkLayer?WhynotjustuseSTPacrosstheInternet?

• RoutingTables:Acollectionofspanningtrees,oneperdestination

• GeneratingValidRoutingtables(withinadomain):

• Globalview(Link-StateProtocol),andlimitations

• Localview(Distance-vectorProtocol)

• GeneratingValidRoutingtables(acrossdomains):

• BorderGatewayProtocol,Internetstructure,routingpolicies

5

Nextlecture

• Youmaynotrealizethisbut….

• Wehavelearntalotofmaterial!!!!!

• Nextlectureisveryveryvery….• veryveryveryvery….• important

• Pleaseattend

• IwilldiscusshoweverythingwehavecoveredsofarFITSTOGETHER…• …intoanend-to-enddesign

• Youwillfeelawesome—Ipromise!

6

GoalsforToday’sLecture

• UnderstandIP(theInternetProtocol)

• PacketHeaderasanetwork“interface”

• Understandswitcharchitecture

7

NetworkLayer

• THEfunctionality:deliveringthedata

• THEprotocol:InternetProtocol(IP)

• Achievesitsfunctionality(deliveringthedata),usingthreeideas:

• Addressing(IPaddressing)

• Routing(usingavarietyofprotocols)

• Packetheaderasaninterface(Encapsulatingdataintopackets)

InternetProtocol

• THEfunctionality:deliveringthedata

• THEprotocol:InternetProtocol(IP)

• Unifyingprotocol

WhatisDesigningIP?

• Syntax:formatofpacket

• Nontrivialpart:packet“header”

• Restisopaquepayload(whyopaque?)

• Semantics:meaningofheaderfields

• Requiredprocessing

Opaque PayloadHeader

PacketHeaderasInterface

• Thinkofpacketheaderasinterface

• Onlywayofpassinginformationfrompackettoswitch

• Designinginterfaces:

• Whattaskareyoutryingtoperform?

• Whatinformationdoyouneedtoaccomplishit?

• Headerreflectsinformationneededforbasictasks

WhatTasksDoWeNeedtoDo?

• Readpacketcorrectly

• Getthepackettothedestination

• Getresponsestothepacketbacktosource

• Carrydata

• Tellhostwhattodowiththepacketoncearrived

• Specifyanyspecialnetworkhandlingofthepacket

• Dealwithproblemsthatarisealongthepath

ReadingPacketCorrectly

• Wheredoestheheaderend?

• Wherethethepacketend?

• Whatprotocolareweusing?

• Whyisthissoimportant?

GettingtotheDestination

• Providedestinationaddress

• Shouldthisbelocationoridentifier(name)?

• Andwhat’sthedifference?

• Ifahostmovesshoulditsaddresschange?

• Ifnot,howcanyoubuildscalableInternet?

• Ifso,thenwhatgoodisanaddressforidentification?

GettingResponseBacktoSource

• Sourceaddress

• Necessaryforrouterstorespondtosource

• Whenwouldtheyneedtorespondback?

• Failures!

• Dotheyreallyneedtorespondback?

• Howwouldthesourceknowifthepackethasreachedthe

destination?

CarryData

• Payload!

Questions?

ListofTasks

• Readpacketcorrectly

• Getthepackettothedestination

• Getresponsestothepacketbacktosource

• Carrydata

• Tellhostwhattodowithpacketoncearrived

• Specifyanyspecialnetworkhandlingofthepacket

• Dealwithproblemsthatarisealongthepath

TellingDestinationHowtoProcessPacket

• Indicatewhichprotocolsshouldhandlepacket

• Whatlayersshouldthisprotocolbein?

• Whataresomeoptionsforthistoday?

• Howdoesthesourceknowwhattoenterhere?

SpecialHandling

• Typeofservice,priority,etc.

• Options:discusslater

DealingWithProblems

• Ispacketcaughtinloop?

• TTL

• Headercorrupted:

• DetectwithChecksum

• Whataboutpayloadchecksum?

• Packettoolarge?

• Dealwithfragmentation

• Splitpacketapart

• Keeptrackofhowtoputtogether

AreWeMissingAnything?

• Readpacketcorrectly

• Getthepackettothedestination

• Getresponsestothepacketbacktosource

• Carrydata

• Tellhostwhattodowithpacketoncearrived

• Specifyanyspecialnetworkhandlingofthepacket

• Dealwithproblemsthatarisealongthepath

FromSemanticstoSyntax

• Thepastfewslidesdiscussedtheinformationtheheadermustprovide

• Willnowshowthesyntax(layout)ofIPv4header,anddiscussthe

semanticsinmoredetail

IPPacketStructure

4-bit Version4-bit Header

Length8-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

20BytesofStandardHeader,thenOptions

4-bit Version4-bit Header

Length8-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

NextSetofSlides

• Mappingbetweentasksandheaderfields

• Eachofthesefieldsisdevotedtoatask

• Let’sfindoutwhichonesandwhy…

GoThroughTasksOne-by-One

• Readpacketcorrectly

• Getthepackettothedestination

• Getresponsestothepacketbacktosource

• Carrydata

• Tellhostwhattodowithpacketoncearrived

• Specifyanyspecialnetworkhandlingofthepacket

• Dealwithproblemsthatarisealongthepath

ReadPacketCorrectly

• Versionnumber(4bits)

• IndicatestheversionoftheIPprotocol

• Necessarytoknowwhatotherfieldstoexpect

• Typically“4”(forIPv4),andsometimes“6”(forIPv6)

• Headerlength(4bits)

• Numberof32-bitwordsintheheader

• Typically“5”(fora20-byteIPv4header)

• CanbemorewhenIPoptionsareused

• Totallength(16bits)

• Numberofbytesinthepacket

• Maximumsizeis65,535bytes(2^16-1)

• …thoughunderlyinglinksmayimposesmallerlimits

FieldsforReadingPacketCorrectly

4-bit Version4-bit Header

Length8-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

GettingPackettoDestinationandBack

• TwoIPaddresses

• SourceIPaddress(32bits)

• DestinationIPaddress(32bits)

• DestinationAddress

• Uniquelocatorforthereceivinghost

• Allowseachnodetomakeforwardingdecisions

• SourceAddress

• Uniquelocatorforthesendinghost

• Recipientcandecidewhethertoacceptpacket

• Enablesrecipienttosendareplybacktothesource

FieldsforReadingPacketCorrectly

4-bit Version4-bit Header

Length8-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

Questions?

ListofTasks

• Readpacketcorrectly

• Getthepackettothedestination

• Getresponsestothepacketbacktosource

• Carrydata

• Tellhostwhattodowithpacketoncearrived

• Specifyanyspecialnetworkhandlingofthepacket

• Dealwithproblemsthatarisealongthepath

TellingHostHowtoHandlePacket

• Protocol(8bits)

• Identifiesthehigherlevelprotocol

• Importantfordemultiplexingatreceivinghost

• Mostcommonexamples

• E.g.,“6”fortheTransmissionControlProtocol(TCP)

• E.g.,“17”fortheUserDatagramProtocol

IP HeaderTCP Header

IP HeaderTCP Header

Protocol = 6 Protocol = 17

FieldsforReadingPacketCorrectly

4-bit Version4-bit Header

Length8-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

SpecialHandling

• Type-of-Service(8-bits)

• Allowpacketstobetreateddifferentlybasedonneeds

• E.g.,lowdelayforaudio,highbandwidthforbulktransfer

• Hasbeenredefinedseveraltimes,nogeneraluse

• Options

• Abilitytospecifyotherfunctionality

• Extensibleformat

ExamplesofOptions

• RecordRoute

• StrictSourceRoute

• LooseSourceRoute

• Timestamp

• Traceroute

• RouterAlert

• …

PotentialProblems

• HeaderCorrupted:Checksum

• Loop:TTL

• Packettoolarge:Fragmentation

PreventingLoops

• Forwardingloopscausepacketstocycleforever

• Astheseaccumulate,eventuallyconsumeallcapacity

• Time-to-live(TTL)Field(8-bits)

• Decrementedateachhop,packetdiscardedifreaches0

• …and“timeexceeded”messageissenttothesource

• Using“ICMP”controlmessage;basisfortraceroute

TTLField

4-bit Version4-bit Header

Length8-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

HeaderCorruption

• Checksum(16bits)

• Particularformofchecksumoverpacketheader

• Ifnotcorrect,routerdiscardspackets

• Soitdoesn’tactinbogusinformation

• Checksumrecalculatedateveryrouter

• Why?

• WhyincludeTTL?

• Whyonlyheader?

ChecksumField

4-bit Version4-bit Header

Length8-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

PacketHeaderasaninterface

• Uselesstolearntheheaderformatbyheart

• Ifyourememberthetasksthatneedtobeperformed…

• Understandingwhyheaderformatiswhatitis…

• Ingeneral:ifyouunderstandtheproblem,solutioniseasy

• Astheproblemevolves,youwillknowwheretolookforasolution

• TransitionfromIPv4toIPv6

• Graduallyhappening…

• Ifyouwanttolearnabit,seebackupslides

Switch/RouterArchitecture

IPRoutersandSwitches(usedinterchangeablytoday)

• CorebuildingblockofInternetinfrastructure

• $120B+industry

• Vendors:Cisco,Huawei,Juniper,Alcatel-Lucent(accountfor>90%)

Recap:RoutersForwardPackets

Cornell

Harvard

MIT

Switch#1

Switch#2

Switch#3

RouterDefinitions

Rbits/s

• N=No.Ofexternalrouterports

• R=bandwidth(“linerate”)ofaport

• Routercapacity=NxR

12

3

45

…

N-1

N

AT&T BBN

MIT

Cornell

core

core

edge/border (ISP)

edge/border (enterprise)

home, small business

NetworksandRouters

ExamplesofRouters(core)

• Core:CiscoCRS

• R=10/40/100Gbps

• NR=922Tbps

• Netflix:0.7GB/hr(1.5Mb/s)

• ~600millionconcurrentNetflixusers

• Edge(ISP):CiscoASR• R=1/10/40Gbps• NR=120Gbps

• Edge(enterprise):Cisco3945E• R=10/100/1000Mbps

• NR<10Gbps

1

2

N

1

2

N

Linecards (input)

Interconnect(Switching)

Fabric

Route/Control Processor

Linecards (output)

Processespackets ontheirwayin

Processespackets beforetheyleave

Transferspacketsfrominputto outputports

InputandOutputforthesameportareonone

physicallinecard

What’sInsideaRouter?

1

2

N

1

2

N

Linecards (input)

Interconnect(Switching)

Fabric

Route/Control Processor

Linecards (output)

(1) Implement IGP and BGP protocols;

compute routing tables(2) Push forwarding

tables to the line cards

What’sInsideaRouter?

1

2

N

1

2

N

Linecards (input)

InterconnectFabric

Route/Control Processor

Linecards (output)

Constitutes the data plane

Constitutes the control plane

What’sInsideaRouter?

● Receiveincomingpackets(physicallayerstuff)

● UpdatetheIPheader● TTL,Checksum(maybesomeotherfields)

● LookuptheoutputportforthedestinationIPaddress

● Queuethepacketattheswitchfabric

InputLineCards:Tasks

● 100Bpackets@40Gbps=>packetevery20nanosecs!

● Typicallyimplementedwithspecializedhardware

● ASICs,specialized“networkprocessors”

54

Challenge:Speed!

● Uponreceivingapacket

● InspectthedestinationIPaddressintheheader

● Indexintotherouting/forwardingtable

● Ifnomatch,selectthedefaultroute● Forwardpacketoutappropriateinterface

● Defaultroute● Configuredtocovercaseswherenomatches

● Allowssmalltablesatedge(w/oroutingalgorithms)

● ifitisn’tonmysubnet,sendittomyISP

55

LookinguptheOutputPort

● Recall:Forscalability,addressesareaggregated

● LongestPrefixmatch

● Findtheentrywithmatching“longestprefix”withdestinationaddress

ScalingtheLookup

……

3 1128.16.120.xxx

1

2128.16.120.111

128.82.100.101

2128.82.xxx.xxx

128.82.100.xxx

● Incomingpacketdestination:201.143.7.0

57

Prefix Port

201.143.0.0/22 Port1

201.143.4.0.0/24 Port2

201.143.5.0.0/24 Port3

201.143.6.0/23 Port4

FindingaMatch

58

11001001 10001111 00000111 11010010

FindingaMatch:CoverttoBinary

● Incomingpacketdestination:201.143.7.0

11001001 10001111 000000-- --------

11001001 10001111 00000100 --------

11001001 10001111 00000101 --------

11001001 10001111 0000011- --------

RougngTable201.143.0.0/22

201.143.4.0/24

201.143.5.0/24

201.143.6.0/23

59

11001001 10001111 00000111 11010010

FindingaMatch:CoverttoBinary

● Incomingpacketdestination:201.143.7.0

11001001 10001111 000000-- --------

11001001 10001111 00000100 --------

11001001 10001111 00000101 --------

11001001 10001111 0000011- --------

RougngTable201.143.0.0/22

201.143.4.0/24

201.143.5.0/24

201.143.6.0/23

60

11001001 10001111 00000111 11010010

FindingaMatch:CoverttoBinary

● Incomingpacketdestination:201.143.7.0

11001001 10001111 000000-- --------

11001001 10001111 00000100 --------

11001001 10001111 00000101 --------

11001001 10001111 0000011- --------

RougngTable201.143.0.0/22

201.143.4.0/24

201.143.5.0/24

201.143.6.0/23

61

11001001 10001111 00000111 11010010

LongestPrefixMatch

● Incomingpacketdestination:201.143.7.0

11001001 10001111 000000 - - - - - - - - - -

11001001 10001111 00000100 - - - - - - - -

11001001 10001111 00000101 - - - - - - - -

11001001 10001111 0000011- - - - - - - - -

RougngTable201.143.0.0/22

201.143.4.0/24

201.143.5.0/24

201.143.6.0/23

Checkanaddressagainstalldesgnagonprefixesandselecttheprefixitmatcheswithonthemostbits

● Testingeachentrytofindamatchscalespoorly

● Roughly(numberofentries)×(numberofbits)

● Mustleveragetreestructureofbinarystrings

● Setuptree-likedatastructure

● CalledaTRIE● Wewillbrieflydiscussit;moredetailsintext

● Incaseyouareinterested….

62

FindingtheMatchEfficiently

● Justfocusingonthebitswherealltheactionis….

● 0**➔ Port1

● 100➔ Port2

● 101➔ Port3

● 11*➔ Port4

63

ConsiderFour3-BitPrefixes

64

00*

000 001

0 1 01*

010 011

0 111*

110 111

0 110*

100 101

0 1

0**0 1

1**0 1

***0 1

TreeStructure

65

00*

000 001

0 1 01*

010 011

0 111*

110 111

0 110*

100 101

0 1

0**0 1

1**0 1

***0 1

0** ➔ Port 1 100 ➔ Port 2 101 ➔ Port 3 11* ➔ Port 4

WalkTree:StopatPrefixEntries

66

00*

000 001

0 1 01*

010 011

0 111*

110 111

0 110*

100 101

0 1

0**0 1

1**0 1

***0 1

P1

P2 P3

P4

0** ➔ Port 1 100 ➔ Port 2 101 ➔ Port 3 11* ➔ Port 4

WalkTree:StopatPrefixEntries

walkingtreestakesO(#bits)

LongestPrefixMatchinRealRouters

• Realroutersusefarmoreadvanced/complexsolutions

• Butwhatwediscussedisthestartingpoint

• Withmanyheuristicsandoptimizationsthatleveragereal-worldpatterns

• Somedestinationsmorepopularthanothers

• Someportsleadtomoredestinations

• Typicalfixgranularities

Recap:InputLinecards

• Mainchallengeisprocessingspeed

• Butwhatwediscussedisthestartingpoint

• Tasksinvolved

• Updatepacketheader(easy)

• Longestprefixmatchlookupondestinationsaddress(harder)

• Mostlyimplementedwithspecializedhardware

OutputLinecard

• PacketClassification:mapeachpackettoa“flow”

• Flow(fornow):setofpacketsbetweentwoparticularendpoints

• BufferManagement:decidewhenandwhichpackettodrop

• Scheduler:decidewhenandwhichpackettotransmit

OutputLinecard

• PacketClassification:mapeachpackettoa“flow”

• Flow(fornow):setofpacketsbetweentwoparticularendpoints

• BufferManagement:decidewhenandwhichpackettodrop

• Scheduler:decidewhenandwhichpackettotransmit

• Usedtoimplementvariousformsofpolicy

• Denyalle-mailtrafficfromISPXtoY(accesscontrol)

• RouteIPtelephonytrafficfromXtoYviaPHY_CIRCUIT(policy)

• Ensurethatnomorethan50MbpsareinjectedfromISP-X(QoS)

SimplestFIFORouter

• Noclassification

• Droptailbuffermanagement:whenbufferisfulldropincomingpacket

• FirstInFirstOut(FIFO)Scheduling:schedulepacketsinorderofarrival

PacketClassification

• ClassifyanIPpacketbasedonthenumberoffieldsinthepacketheader

• Source/destinationIPaddress(32bits)• Source/destinationTCPportnumber(16bits)

• TypeofService(TOS)byte(8bits)• TypeofProtocol(8bits)

• Ingeneralfieldsarespecifiedbyrange

• Classificationrequiresamulti-dimensionalrangesearch

Scheduler

• Onequeueperflow

• Schedulerdecidesfromwhichqueuetosendapacket

• Goalsofschedulingalgorithm

• Fast!• Dependsonthepolicybeingimplemented(fairness,priority,etc.)

Example:PriorityScheduler

• Packetsinthehighestpriorityqueuearealwaysservedbeforethepacketsinthelowerpriorityqueues

Example:RoundRobinScheduler

• Packetsareservedfromeachqueueinturn

ConnectingInputtoOutput:SwitchFabric

• PriorityScheduler:packetsareservedfromeachqueueinturn

Today’sSwitchFabrics:MiniNetwork!

What’sHardAbouttheSwitchFabric?

Queueing!

ThirdGenerationRouter:SwitchedInterconnects

ThirdGenerationRouter:SwitchedInterconnects

RealityisMoreComplicated

• Commercialhigh-speedroutersuse

• Combinationofinputandoutputqueueing

• Complexmulti-stage“topologies”

• Distributedmulti-stageschedulers(forscalability)

IPRoutersRecap

• CorebuildingblockofInternetinfrastructure

• ScalableRouting->LongestPrefixMatching

• Needfastimplementationsfor

• Longestprefixmatching

• Switchfabricscheduling

Thisisitfortoday!

IPv6

IPv6

• Motivated(prematurely)byaddressexhaustion

• Addressfourtimesasbig

• SteveDeeringfocusedonsimplifyingIP

• Gotridofallfieldsthatwerenotabsolutelynecessary

• “SpringCleaning”forIP

• Resultisanelegant,ifunambitious,protocol

IPv4andIPv6HeaderComparison

Version IHLType of Service (TOS)

Total Length

Identification Flags Fragment Offset

Time to Live (TTL) Protocol Header Checksum

Source Address

Destination Address

Options

Version Traffic Class Flow Label

Payload Length Next Header Hop Limit

Source Address

Destination Address

Field name kept from IPv4 to IPv6

Fields not kept in IPv6

Name and position changed in IPv6

New field in IPv6

SummaryofChanges

• EliminatedFragmentation

• Eliminatedheaderlength

• EliminatedChecksum

• Newoptionsmechanism(nextheader)

• Expandedaddress

• AddedFlowLabel

IPv4andIPv6HeaderComparison

Version IHLType of Service (TOS)

Total Length

Identification Flags Fragment Offset

Time to Live (TTL) Protocol Header Checksum

Source Address

Destination Address

Options

Version Traffic Class Flow Label

Payload Length Next Header Hop Limit

Source Address

Destination Address

Field name kept from IPv4 to IPv6

Fields not kept in IPv6

Name and position changed in IPv6

New field in IPv6

PhilosophyofChanges

• Don’tdealwithproblems:leavetoends

• Eliminatedfragmentation

• Eliminatedchecksum

• WhyretainTTL?

• Simplifyhandling

• Newoptionsmechanism(usesnextheaderapproach)

• Eliminatedheaderlength

• Whycouldn’tIPv4dothis?

• Providegeneralflowlabelforpacket

• Nottiedtosemantics

• Providesgreatflexibility

Traffic Class

IPv4andIPv6HeaderComparison

IHLType of Service (TOS)

Total Length

Identification Flags Fragment Offset

Time to Live (TTL) Protocol Header Checksum

Source Address

Destination Address

Options

Version Flow Label

Payload Length Next Header Hop Limit

Source Address

Destination Address

To Destination and Back (expanded)

Deal with Problems (greatly reduced)

Read Correctly (reduced)

Special Handling (Similar)

Version