Computer Networks
Lecture17:Inter-domainRoutingandBGP
RoutingontheInternetInthebeginningtherewastheARPANET:• routeusingGGP(Gateway-to-GatewayProtocol),adistancevectorroutingprotocol
Problems:• needed“flag-hour”toupdateroutingprotocol• incompatibilityacrossvendors
RoutingontheInternetSolution:hierarchicalrouting• administrativeautonomy:• eachnetworkadmincancontrolroutingwithinitsownnetwork
• internet:networkofnetworks• allowstheInternettoscale:• with200millionhosts,eachroutercan’tstorealldestinationsinitsroutingtable• routeupdatesalonewillswampthelinks
Aggregateroutersintoregionsof“autonomoussystems”(ASs)
1.1
2.1
3.1
2.2
4.1
4.4
4.3
4.2
3.4
3.3
3.2
border routers
AS1
AS2
AS3
AS4
HierarchicalRouting
Gateway/borderrouter• neighboringASsinteracttocoordinaterouting
• directlinktorouterinotherAS(s)
• keepsinitsroutingtable:
• nexthoptootherASs
• allhostswithinitsAS
• hostswithinanASonlykeepadefaultroutetotheborderrouter
3.1dest next1.* 1.12.* 2.14.* 2.13.2 3.23.3 3.33.4 3.4
1.1
2.1
3.1
2.2
4.1
4.4
4.3
4.2
3.4
3.3
3.2
border routers
AS1
AS2
AS3
AS4
HierarchicalRoutingRoutersinthesameASrunsameroutingprotocol• “intra-AS”routingprotocol
• eachASusesitsownlinkmetric
• routersindifferentASscanrundifferentintra-ASroutingprotocol
• internaltopologyisnotsharedbetweenASs
1.1
2.1
3.1
2.2
4.1
4.4
4.3
4.2
3.4
3.3
3.2
border routers
AS1
AS2
AS3
AS4
TheNSFNet1989 NSFNetbackbone
Regionalnetworks
Customernetworks
Users
pointofpresence(pop)
Areahierarchy:• backbone/core:NSFNet• regionalnetworks:MichNet,BARRNET,LosNettos,Cerfnet,JVCNet,NEARNet,etc.• campusnetworks
[Walrand]
[Halabi] [MeritNetworks]
Commercialization(1994)Roughlyhierarchical
Atcenter:“Tier-1”ISPs• Tier-1ASs:topoftheInternethierarchyof~10Ass:AOL,AT&T,GlobalCrossing,Level3,Verizon/UUNET,NTT,Qwest,SAVVIS(formerlyCable&Wireless),Sprint,etc.• full(N2)peeringrelationshipsbetweenTier-1providers
• hasnoupstreamprovider
• national/internationalcoverage
Tier-1providersalsointerconnectatpublicnetworkaccesspoints(NAPs)
[Halabi]
[Walrand]
Tier-1providersinterconnect(peer)privately
AT&T
Sprint
Verizon
ASStructure:OtherASsLowertierproviders• providetransitservicetodownstreamcustomers• but,needatleastoneprovideroftheirown• typicallyhavenationalorregionalscope• includesseveralthousandASs
StubASs• donotprovidetransitservicetoothers• connecttooneormoreupstreamproviders• includesthevastmajority(e.g.,85-90%)oftheASs
[Rexford]
“Tier-2”ISPs:Smaller(OftenRegional)ISPsConnecttooneormoretier-1ISPs,possiblyothertier-2ISPs
Tier1ISP
Tier-1ISP
Tier-1ISP
NAP
Tier-2ISPTier-2ISP
Tier-2ISP Tier-2ISP
Tier-2ISP
Tier-2ISPpaystier-1ISPforconnectivitytorestofInternet• tier-2ISPiscustomeroftier-1provider
Tier-2ISPsalsopeerprivatelywitheachother,andinterconnectatNAPs
Tier-1ISP
“Tier-3”ISPsandLocalISPsLasthop(“access”)network(closesttoendsystems)
Tier-1ISP
Tier-1ISP
Tier-1ISP
NAP
Tier-2ISPTier-2ISP
Tier-2ISP Tier-2ISP
Tier-2ISP
localISP
Tier-3ISPLocalandtier-3ISPsarecustomersofhighertierISPsconnectingthemtorestofInternet
localISP
localISPlocalISP
localISP
localISPlocalISPlocalISP
APacketPassesThroughManyNetworks
Tier-1ISP
Tier-1ISP
Tier-1ISP
NAP
Tier-2ISPTier-2ISP
Tier-2ISP Tier-2ISP
Tier-2ISP
localISP
Tier-3ISP
localISP
ASNumberTriviaASnumberisa16-bitquantity• 65,536uniqueASnumbers
Somearereservednumbers(e.g.,forprivateASs)• only64,510areavailableforpublicuse
ManagedbyInternetAssignedNumbersAuthority(IANA)• givesblocksof1,024toRegionalInternetRegistries• RIRsassignASnumberstoinstitutions• 49,649ASnumbersinvisibleuse(Feb’15)
In2007startedassigning32-bitAS#s
[Rexford]
GrowthofASnumbers
TolearnmoreaboutInternetASstatesee:• GeoffHuston’sCIDRReporthttp://www.cidr-report.org/as2.0/
• CAIDAskittermaps:http://www.caida.org/research/topology/as_core_network/AS_Network.xml
InterdomainRoutingAS-leveltopology• destinationsareCIDRaddressprefixes(APs,e.g.,12.0.0.0/8)• nodesareAutonomousSystems(ASs)• edgesarebusinessrelationships
1
2
3 4
5
6 7
ClientWebserver
[Rexford]
ChallengesforInterdomainRoutingScale• addressprefixes(APs):200,000andgrowing• ASs:~50,000visibleones,and60Kallocated• routers:atleastinthemillionsProprietaryinformation:• ASsdon’twanttodivulgeinternaltopologies• northeirbusinessrelationshipswithneighborsPolicy• noInternet-widenotionofalinkcostmetric• needcontroloverwhereyousendtraffic• andwhocansendtrafficthroughyou
[Rexford]
WhySPFisnotSuitableTopologyinformationisflooded• highbandwidthandstorageoverhead• nodesmustdivulgesensitivecommercialinformation
Entirepathcomputedlocallypernode• highprocessingoverheadinalargenetwork
Routecomputationminimizessomenotionoftotaldistance• alltrafficmusttravelonshortestpaths
[Rexford]
WhySPFisnotSuitableAllnodesneedcommonnotionoflinkcosts• worksonlyifpolicyissharedanduniform
Incompatiblewithcommercialrelationships
RegionalISP1
RegionalISP2
RegionalISP3
Cust1Cust3 Cust2
NationalISP1
NationalISP2
YES
NO
[Rexford]
WhyNotDistanceVector?
Advantages• hidesdetailsofthenetworktopology• nodesdetermineonly“nexthop”towardthedestination
Disadvantages• routecomputationstillentailsminimizationofsomenotionoftotaldistance,whichisdifficultinaninter-domainsetting• slowconvergenceduetorelianceoncounting-to-infinitytodetectroutingloop
Insteadusepathvector• easierloopdetection
[afterRexford]
Path-VectorRoutingAvoidcounting-to-infinitybyadvertisingentirepath• distancevector:senddistancemetricperdestination• pathvector:sendtheentirepathforeachdestination
Loopdetection:• eachnodelooksforitsownnodeidentifierinadvertisedpath• anddiscardspathswithloops• e.g.,node1seesitselfinthepath(3, 2, 1)anddiscardsthepath
3 2 1
d
“d: path (2,1)” “d: path (1)”
datatraffic datatraffic
[Rexford]
“d: path (3,2,1)”
OtherAdvantage:FlexiblePoliciesEachnodecanapplylocalpolicies• pathselection:whichpathtouse?• pathexport:whichpathstoadvertise?
Examples• node2maypreferthepath“2, 3, 1”over“2, 1”• node1maynotwantnode3tohearofthepath“1, 2”
2 3
1
2 3
1
[Rexford]
✗
Internetinter-ASRouting:BGP
BGP(BorderGatewayProtocol)isthedefactostandardforinter-ASrouting• 06/89 v.1• 06/90 v.2EGP(ExteriorGatewayProtocol)toBGPtransition• 10/91 v.3BGPinstalled• 07/94 v.4defactostandard
Internetinter-ASRouting:BGPBGPprovideseachASameansto:• useprefix-basedpath-vectorprotocol• propagatesAPreachabilitytoallroutersinsidetheAS• obtainsAPreachabilityfromneighboringASs• determines“good”routestoAPsbasedonreachabilityinformationandpolicy• Inter-ASroutingispolicydriven,notload-sensitive,generallynotQoS-based
WhenanASadvertisesanAPtoanotherAS,itispromisingtoforwardanypacketstheotherASsendstotheAP• anAScanaggregateCIDRAPsinitsadvertisement
BGPrunsoverTCPPairsofBGProuters(BGPpeers)establishsemi-permanentTCPconnections:BGPsessions• advantageofusingTCP:reliabletransmissionallowsforincrementalupdates:updatesonlywhenchangesoccur• disadvantage:TCPcongestioncontrolmechanismslowsdownrouteupdatesthatcoulddecongestlink!
Failuredetection:• TCPdoesn’tdetectlostconnectivityonitsown• instead,BGPmustdetectfailure• sendsKEEPALIVEpacketsevery60seconds• holdtimer:180seconds
BGPsessionsdonotcorrespondtophysicallinks,butratherbusinessrelationship
BGPMessages
BGPmessages:• OPEN:opensTCPconnectiontopeerandauthenticatessender• UPDATE:advertisesanewactivepath(orwithdrawsonenolongeravailable)• KEEPALIVE:keepsconnectionaliveintheabsenceofUPDATEs;alsoacknowledgesOPENrequest• NOTIFICATION:reportserrorsinpreviousmessage;alsousedtocloseconnection
[afterRexford]
BGPOperations
EstablishsessiononTCPport179
Exchangeallactiveroutes
Exchangeincrementalupdates
AS1
AS2
WhileconnectionisALIVE,exchangerouteUPDATEmessages
BGPsession
[Rexford]
PathAttributes&BGPRoutes
WhenadvertisinganAP,advertisementincludesBGPattributes
Twoimportantattributes:• AS-PATH:thepathvectorofASsthroughwhichtheadvertisementforanAPpassedthrough• NEXT-HOP:thespecificinternal-ASroutertonext-hopAS(theremaybemultipleexitsfromcurrentAStonext-hop-AS)
PathAttributes&BGPRoutesSampleBGPentry:destination NEXT-HOP AS-PATH 198.32.163.0/24 202.232.1.8 2497 2914 3582 4600
• addressrange198.32.163.0/24isinAS4600 • togetthere,sendtonexthoprouterataddress202.232.1.8 • thepaththeregoesthroughASs2497,2914,3582,inorder
ASpathchosenmaynotbetheshortestASpath
RouterpathmaybelongerthanASpath
2AShops,11routerhops
s d
3AShops,7routerhops
[afterRexford]
CausesofBGPRoutingChangesTopologychanges• equipmentsgoingupordown• deploymentofnewroutersorsessions
BGPsessionfailures• duetoequipmentfailures,maintenance,etc.• or,duetocongestiononthephysicalpath
Changesinroutingpolicy• changesinpreferencesintheroutes• changesinwhethertherouteisexported
Persistentprotocoloscillation• conflictsbetweenpoliciesofdifferentASs
[Rexford]
BGPSessionFailure
AS1
AS2
Reactingtoafailure• discardallrouteslearnedfromtheneighbor
• sendnewupdatesforanyroutesthatchange
• overheadincreaseswith#ofroutes• reasonwhymanyTier-1ASsfilteroutprefixeslongerthan/24
[Rexford]
RoutingChange:BeforeandAfter
0
1 2
3
(1, 0) (2, 0)
(3, 1, 0)
(1, 2, 0)
(3, 2, 0)
[Rexford]
AS1• deletetheroute(1, 0)• switchtonextroute(1, 2, 0)• sendroute(1, 2, 0)toAS3
AS3• sees(1, 2, 0)replace(1,0)• comparestoroute(2, 0)• switchestousingAS2
1
