interdomain roung policy€¦ · – diﬀerent languages for cisco, juniper, etc. • sll, all...

InterdomainRou.ngPolicy

COS461:ComputerNetworksSpring2011

MikeFreedmanh>p://www.cs.princeton.edu/courses/archive/spring11/cos461/

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GoalsofToday’sLecture

•  BusinessrelaKonshipsbetweenASes–  Customer‐provider:customerpaysprovider

–  Peer‐peer:typicallyse>lement‐free

•  RealizingrouKngpolicies–  Importandexportfiltering–  Assigningpreferencestoroutes

•  MulKplerouterswithinanAS–  DisseminatedBGPinformaKonwithintheAS–  CombiningwithintradomainrouKnginformaKon

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InterdomainRouKng•  AS‐leveltopology

– DesKnaKonsareIPprefixes(e.g.,12.0.0.0/8)– NodesareAutonomousSystems(ASes)

–  EdgesarelinksandbusinessrelaKonships

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Client Web server

1

2

3 4

5

6 7

BusinessRelaKonships

•  NeighboringASeshavebusinesscontracts– Howmuchtraffictocarry– WhichdesKnaKonstoreach– Howmuchmoneytopay

•  CommonbusinessrelaKonships– Customer‐provider:Customerpaysproviderfortransit

•  E.g.,PrincetonisacustomerofUSLEC•  E.g.,MITisacustomerofLevel3

– Peer‐peer:Nomoneychangeshands•  E.g.,UUNETisapeerofSprint•  E.g.,HarvardisapeerofHarvardBusinessSchool

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Customer‐ProviderRelaKonship•  Customerneedstobereachablefromeveryone

–  Providertellsallneighborshowtoreachthecustomer

•  Customerdoesnotwanttoprovidetransitservice–  Customerdoesnotletitsprovidersroutethroughit

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d

d

provider

customer

customer

provider

Traffictothecustomer Trafficfromthecustomer

announcements

traffic

CustomerConnecKngtoaProvider6

Provider Provider

1accesslink 2accesslinks

Provider

2accessrouters

Provider

2accessPoPs(PointsofPresence)

MulK‐Homing:TwoorMoreProviders

•  MoKvaKonsformulK‐homing– Extrareliability,survivesingleISPfailure– FinancialleveragethroughcompeKKon– Be>erperformancebyselecKngbe>erpath– Gamingthe95th‐percenKlebillingmodel

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Provider1 Provider2

PrincetonExample

•  Internet:customerofUSLECandPatriot•  ResearchuniversiKes/labs:customerofInternet2

•  Localnon‐profits:providerforseveralnon‐profits

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Patriot USLEC Internet2

Princeton

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Howmanylinksareenough?

Kupstream

ISPs

Notmuchbenefitbeyond4ISPs

Akellaetal.,“PerformanceBenefitsofMul.homing”,SIGCOMM2003

Peer‐PeerRelaKonship

•  Peersexchangetrafficbetweencustomers– ASexportsonlycustomerroutestoapeer– ASexportsapeer’sroutesonlytoitscustomers

– OcentherelaKonshipisse>lement‐free(i.e.,no$$$)

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peerpeer

Trafficto/fromthepeeranditscustomers

d

announcements

traffic

ASStructure:Tier‐1Providers•  Tier‐1provider

– Hasnoupstreamproviderofitsown–  TypicallyhasanaKonalorinternaKonalbackbone

•  TopoftheInternethierarchyof~10ASes– AOL,AT&T,GlobalCrossing,Level3,UUNET,NTT,Qwest,SAVVIS(formerlyCable&Wireless),andSprint

–  Fullpeer‐peerconnecKonsbetweenKer‐1providers

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ASStructure:OtherASes

•  Otherproviders– Providetransitservicetodownstreamcustomers

– …but,needatleastoneprovideroftheirown– TypicallyhavenaKonalorregionalscope–  IncludesseveralthousandASes

•  StubASes– Donotprovidetransitservicetoothers– Connecttooneormoreupstreamproviders–  Includesthevastmajority(e.g.,85‐90%)oftheASes

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TheBusinessGameandDepeering•  CooperaKvecompeKKon(brinksmanship)•  Muchmoredesirabletohaveyourpeer’scustomers

– Muchnicertogetpaidfortransit

•  Peering“Kffs”arerelaKvelycommon31Jul2005:Level3NoKfiesCogentofintenttodisconnect.16Aug2005:CogentbeginsmassivesaleseffortandmenKonsa15Sept.

expecteddepeeringdate.31Aug2005:Level3NoKfiesCogentagainofintenttodisconnect

(accordingtoLevel3)5Oct20059:50UTC:Level3disconnectsCogent.Masshysteriaensuesup

to,andincludingpolicymakersinWashington,D.C.7Oct2005:Level3reconnectsCogent

Duringthe“outage”,Level3andCogent’ssinglyhomedcustomerscouldnotreacheachother.(~4%oftheInternet’sprefixeswereisolatedfromeachother)

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DepeeringConKnuedResolu.on…

…butnotbeforeaCempttostealcustomers!

Asof5:30amEDT,October5th,Level(3)terminatedpeeringwithCogentwithoutcauses….CogenthaslecthepeeringcircuitsopeninthehopethatLevel(3)willchangeitsmindandallowtraffictobeexchangedbetweenournetworks.WeareextendingaspecialofferingtosinglehomedLevel3customers.

“CogentwillofferanyLevel3customer,whoissinglehomedtotheLevel3…,oneyearoffullInternettransitfreeofchargeatthesame

bandwidth….CogentwillprovidethisconnecKvityinover1,000locaKons.”

RealizingBGPRouKngPolicy

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BGPPolicy:ApplyingPolicytoRoutes

•  Importpolicy– Filterunwantedroutesfromneighbor

•  E.g.prefixthatyourcustomerdoesn’town

– Manipulatea>ributestoinfluencepathselecKon•  E.g.,assignlocalpreferencetofavoredroutes

•  Exportpolicy– Filterroutesyoudon’twanttotellyourneighbor

•  E.g.,don’ttellapeeraroutelearnedfromotherpeer

– Manipulatea>ributestocontrolwhattheysee•  E.g.,makeapathlookarKficiallylongerthanitis

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BGPPolicy:InfluencingDecisions17

BestRouteSelecKon

ApplyImportPolicies

BestRouteTable

ApplyExportPolicies

InstallforwardingEntriesforbestRoutes.

ReceiveBGPUpdates

BestRoutes

TransmitBGPUpdates

ApplyPolicy=filterroutes&tweaka>ributes

BasedonA>ributeValues

IPForwardingTable

ApplyPolicy=filterroutes&tweaka>ributes

Openendedprogramming.Constrainedonlybyvendorconfigura.onlanguage

ImportPolicy:LocalPreference•  Favoronepathoveranother

– OverridetheinfluenceofASpathlength– Applylocalpoliciestopreferapath

•  Example:prefercustomeroverpeer

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AT&T Sprint

Yale

Tier‐2

Tier‐3

Local‐pref=100

Local‐pref=90

ImportPolicy:Filtering•  Discardsomerouteannouncements

– DetectconfiguraKonmistakesanda>acks

•  Examplesonsessiontoacustomer– Discardrouteifprefixnotownedbythecustomer– DiscardroutethatcontainsotherlargeISPinASpath

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Patriot

Princeton

USLEC

128.112.0.0/16

ExportPolicy:Filtering•  Discardsomerouteannouncements

– LimitpropagaKonofrouKnginformaKon

•  Examples– Don’tannounceroutesfromonepeertoanother

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AT&T SprintUUNET

ExportPolicy:Filtering•  Discardsomerouteannouncements

– LimitpropagaKonofrouKnginformaKon

•  Examples– Don’tannounceroutesfornetwork‐managementhostsortheunderlyingroutersthemselves

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USLEC

network operator

Princeton

ExportPolicy:A>ributeManipulaKon

•  Modifya>ributesoftheacKveroute– ToinfluencethewayotherASesbehave

•  Example:ASprepending– ArKficiallyinflatetheASpathlengthseenbyothers– ToconvincesomeASestosendtrafficanotherway

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128.112.0.0/16

8888 88

Patriot

Princeton

USLECSprint

BGPPolicyConfiguraKon

•  RouKngpolicylanguagesarevendor‐specific– NotpartoftheBGPprotocolspecificaKon– DifferentlanguagesforCisco,Juniper,etc.

•  SKll,alllanguageshavesomekeyfeatures–  Policyasalistofclauses–  Eachclausematchesonroutea>ributes– …andeitherdiscardsormodifiesthematchingroutes

•  ConfiguraKondonebyhumanoperators–  ImplemenKngthepoliciesoftheirAS–  BusinessrelaKonships,trafficengineering,security,…

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WhyIsTheInternetGenerallyStable?

•  Mostlybecauseof$$

•  PolicyconfiguraKonsbasedonISPs’bilateralbusinessrelaKonships– Customer‐Provider

•  CustomerspayproviderforaccesstotheInternet

– Peer‐Peer•  Peersexchangetrafficfreeofcharge

•  Mostwell‐knownresultreflecKngthispracKce:“Gao‐Rexford”stabilitycondiKons

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The“Gao‐Rexford”StabilityCondiKons

•  PreferencecondiKon– Prefercustomerroutesoverpeerorproviderroutes

Node3prefers“3d”over“312d”

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•  ExportcondiKon– Exportonlycustomerroutestopeersorproviders

Validpaths: “12d”and“643d”Invalidpath:“58d”and“65d”

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•  TopologycondiKon(acyclic)– Nocycleofcustomer‐providerrelaKonships

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BGPandMulKpleRoutersinanAS

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AnASisNotaSingleNode•  ASpathlengthcanbemisleading

– AnASmayhavemanyrouter‐levelhops

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AS 4

AS 3

AS 2

AS 1

BGP says that path 4 1 is better than path 3 2 1

AnASisNotaSingleNode

•  MulKpleroutersinanAS– NeedtodistributeBGPinformaKonwithintheAS–  InternalBGP(iBGP)sessionsbetweenrouters

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AS1

AS2

eBGP

iBGP

InternalBGPandLocalPreference•  Example

– BothrouterspreferpaththroughAS100onthelec– …eventhoughrightrouterlearnsanexternalpath

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I‐BGP AS256

AS300

LocalPref=100 LocalPref=90

AS100

AS200

AnASisNotaSingleNode

•  MulKpleconnecKonstoneighboringASes– MulKpleborderroutersmaylearngoodroutes

– …withthesamelocal‐prefandASpathlength

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1

2

34

5

67

Multiple links

Early‐ExitorHot‐PotatoRouKng•  DiversepeeringlocaKons

•  Comparablecapacityatallpeeringpoints–  Canhandleevenload

•  Consistentroutes–  SamedesKnaKonsadverKsedatallpoints

–  SameASpathlengthforadesKnaKonatallpoints

•  Whynotpushwide‐arearouKngtopeer?

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CustomerA

CustomerB

multiple peering points

ProviderA

ProviderB

Early-exit routing

•  Hot‐potatorouKng– Eachrouterselectstheclosestegresspoint– …basedonthepathcostinintra‐domainprotocol

•  BGPdecisionprocess– Highestlocalpreference– ShortestASpath– Closestegresspoint– ArbitraryKebreak

RealizingHot‐PotatoRouKng34

A B

4 5

3 9

3 4

10 8

8

dst

C

D G

E F

A B

JoiningBGPandIGPInformaKon•  BorderGatewayProtocol(BGP)

– AnnouncesreachabilitytoexternaldesKnaKons– MapsadesKnaKonprefixtoanegresspoint

•  128.112.0.0/16reachedvia192.0.2.1

•  InteriorGatewayProtocol(IGP)– UsedtocomputepathswithintheAS– Mapsanegresspointtoanoutgoinglink

•  192.0.2.1reachedvia10.1.1.1

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192.0.2.1

10.1.1.1

JoiningBGPwithIGPInformaKon36

IGP

AS 7018 AS 88 192.0.2.1

128.112.0.0/16

10.10.10.10

BGP

192.0.2.1 128.112.0.0/16

destination next hop

10.10.10.10 192.0.2.0/30


128.112.0.0/16 Next Hop = 192.0.2.1

+ Forwarding Table

128.112.0.0/16


10.10.10.10 192.0.2.0/30 10.10.10.10

SomeRoutersDon’tNeedBGP

•  CustomerthatconnectstoasingleupstreamISP– TheISPcanintroducetheprefixesintoBGP– …andcustomercansimplydefault‐routetotheISP

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Qwest

Yale University

Nail up default routes 0.0.0.0/0 pointing to Qwest

Nail up routes 130.132.0.0/16 pointing to Yale

130.132.0.0/16

SomeRoutersDon’tNeedBGP

•  Routersinsidea“stub”network– BorderroutermayspeakBGPtoupstreamISPs

– But,internalrouterscansimply“defaultroute”

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Patriot

Princeton University 128.112.0.0/16

AS 88

BGP USLEC

Conclusions

•  BGPissolvingahardproblem–  RouKngprotocoloperaKngataglobalscale–  Tensofthousandsofindependentnetworks–  Eachhaveownpolicygoals;allwantconvergence

•  KeyfeaturesofBGP–  Prefix‐basedpath‐vectorprotocol–  Incrementalupdates(announcementsandwithdrawals)–  Policiesappliedatimportandexportofroutes–  InternalBGPtodistributeinformaKonwithinanAS–  InteracKonwiththeIGPtocomputeforwardingtables

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interdomain roung policy€¦ · – diﬀerent languages for cisco, juniper, etc. • sll, all...

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