The Impact of Internet Policy and Topology on Dela

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TheImpatofInternetPoliyandTopologyonDelayedRoutingConvergeneCraigLabovitz,RogerWattenhofer,SrinivasanVenkataharyMirosoftResearhflabovit,rogerwa,heenugmirosoft.omAbhaAhujaMeritNetwork,In.ahujamerit.eduJuly2000TehnialReportMSR-TR-2000-74MirosoftResearhMirosoftCorporationOneMirosoftWayRedmond,WA98052’ssustainedexponentialgrowthandtheontinuedemergeneofnewandvariednetworkappliationsprovidestestamenttothesalabilityofthebakboneinfrastrutureandprotools.TheoriginalTCP/IPdeisiontoplaenetworkintelligeneandstatealmostexlusivelyonend-nodeshasenabledadiverseprogenyofappliationsrangingfromMP3leexhangetoollaborativelearning.Thissalability,however,omesataprie.Sineitsommerialin-eptionin1995,theInternethaslaggedbehindthepubliswithedtelephonenetwork(PSTN)inavailability,reliabilityandqualityofservie(QoS).Thisrel-ativelakofreliabilitystemsinpartfromtheabseneofintermediatebakbonestateandsynhronizationbetweenrouters.Despitetheremarkabletoleranedemonstratedbytoday’send-usersforfailuresanddelaysinemailandwebservies,therelativelakofInternetbakbonereliabilityposesasignianthallengeforemergingtransation-orientedandinterativeappliationslikeIn-ternettelephony,onlinebusinessandollaboratories.AlthoughreentadvanesintheIETF’sDierentiatedServiesworkinggrouppromisetoimprovetheperformaneofappliation-levelservieswithinsomenetworks,arossthewide-areaInternettheseQoSalgorithmsareusuallyprediatedontheexisteneofastableunderlyingforwardinginfrastruture.Inreentwork,weshowedthattheInternetlakseetiveinter-domainpathfail-over[1℄.Speially,wefoundthatmulti-homedInternetsitesmayexpe-rieneperiodsofdegradedperformaneaswellasompletelossofonnetivitypersistingfteenminutesormoreafterasinglefault.WeshowedthatmostofthelatenyinInternetfail-overstemsfromdelayedonvergene,orthetemporaryroutingtableosillationsformedduringtheop-erationofthepathseletionproessonInternetbakboneroutersafterafault.Unlikeswithesinthepublitelephonynetworkwhihexhibitfailoverontheor-derofmilliseonds,ouranalysisfoundthatinter-domainroutersinthepaketswithedInternetmaytakeseveralminutestoreahaonsistentviewofthenetworktopologyafterafault.TheurrentInternetinter-domainroutingprotool,BGP,evolvedfromear-lierdistanevetorroutingalgorithms.Theseprotools,inludingRIP[2℄,suerfromanumberofwell-doumentedproblems,inludingslowonvergenetimes[3℄.Distanevetorroutingrequiresthateahnodemaintainthedis-tanefromitselftoeahpossibledestinationandthevetor,orneighbor,tousetoreahthatdestination.Wheneverthisonnetivityinformationhanges,theroutertransmitsitsnewdistanevetortoeahofitsneighbors,allowingeahtorealulateitsroutingtable.Theount-to-innityproblem[2℄providestheanonialexampleusedtoillustratetheslowonvergeneindistanevetorrouting.TheadoptionofthepathvetorinBGPiswidelyandinorretlybelievedtohave\solvedtheroutingtableosillationproblemsexhibitedbyRIP.Instead,weshowedin[1℄thattheadoptionofthepathvetorexponentiallyexaerbates1thenumberofpotentialroutingtableosillations.Speially,wefoundthatadefaultonguration(i.e.onewithoutadditionaladministrativelyaddedpoli-iesorlters)ofnBGPautonomoussystemsonnetedinaompletegraphmaypotentiallyexploren!routes,orallpossiblepathsofallpossiblelengthsbetweeneahASafterafault.ThisuppertheoretiboundonBGPonvergeneomparespoorlywithearlierroutingprotools,suhasRIPwhihhavebeenshowntohaveO(n3)omputationalomplexity[4℄.Webasedourearlieranalysisonasimplied,abstratmodelofBGPinter-onnetivity.Thismodelnegletedtheimpatofroutingpoliies,morerealistitimingassumptionsandinter-ASonnetivityontheproessofdelayedonver-gene.AlthoughourinitialmodelprovidesausefultheoretiupperboundonBGPdistributedomputation,wenotethatthisboundisunlikelytoourinpratie.Inthiswork,weexpandonourearliereortbyexploringthemeasuredonvergenebehaviorsof\realtopologies,inludingmorethan20uniqueBGProuteadvertisementsbetweenmorethan200pairsofInternetservieproviders(ISPs).WealsoprovideanalysisofBGPbehavioringeneralnetworktopologiesandunderothermorerealistiassumptions.Ourmajorresultsinlude:ThetimeomplexityforInternetfail-overonvergeneis30(n)seonds,wherenisthelengthofthelongestbakuppathbetweenthesoureandanydestinationautonomoussystemforaroute.Onaverage,routesfromustomersoflargerISPsexhibitfasteronver-genethanroutesannounedbyustomersofsmallerInternetproviders.Errantpathsarefrequentlyexploredduringdelayedonvergene.These\vagabondpathslikelystemfrommisongurationorsoftwarebugs.MostInternetroutesexhibitmultiplebakuppathswhihtransitseveraltimesthenumberofInternetprovidersassteadystatepaths.Theremainderofthispaperisorganizedasfollows:InSetion0.2,wepro-videsomebakgroundandrelatedwork.Setion0.3disussesourexperimentaldataolletioninfrastruture.InSetion0.4,wepresentsurveyresultsonISPspoliymehanismsanddisusstheimpatofthesepoliiesontheowofrout-inginformation.InSetion0.5,wepresentbothempirialobservationsaswellasquantitativeanalysisoftherelationshipbetweenspeiInternettopologialongurationsandtherateofonvergene.Wedemonstratearelationshipbe-tweentheonvergenedelayofarouteannounedbetweentwoprovidersandthelongestp

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