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HierarchicalRouting•  Introducealargerroutingunit–  IPprefix(hosts)ßfromonehost–  Region,e.g.,ISPnetwork

•  Routefirsttotheregion,thentotheIPprefixwithintheregion– Hidedetailswithinaregionfromoutsideoftheregion

HierarchicalRouting(2)

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HierarchicalRouting(3)

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HierarchicalRouting(4)•  Penaltyislongerpaths

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1Cisbestroutetoregion5,exceptfordestination5C

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Observations•  Outsidearegion,nodeshaveoneroutetoallhostswithintheregion–  Thisgivessavingsintablesize,messagesandcomputation

•  However,eachnodemayhaveadifferentroutetoanoutsideregion–  Routingdecisionsarestillmadebyindividualnodes;thereisnosingledecisionmadebyaregion

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Topic•  Howtoroutewithmultipleparties,eachwiththeirownroutingpolicies–  ThisisInternet-wideBGProuting

ISPA ISPC

Destination

ISPBSource

StructureoftheInternet•  Networks(ISPs,CDNs,etc.)grouphostsasIPprefixes•  Networksarerichlyinterconnected,oftenusingIXPs

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CDNC

PrefixC1

ISPAPrefixA1

PrefixA2NetF

PrefixF1

IXPIXP

IXP IXP

CDND

PrefixD1

NetE

PrefixE1

PrefixE2

ISPB

PrefixB1

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Internet-wideRoutingIssues•  Twoproblemsbeyondroutingwithinanindividualnetwork

1.  Scalingtoverylargenetworks–  TechniquesofIPprefixes,hierarchy,prefixaggregation

2.  Incorporatingpolicydecisions–  Lettingdifferentpartieschoosetheirroutestosuittheirownneeds Yikes!

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EffectsofIndependentParties•  Eachpartyselectsroutestosuititsowninterests–  e.g,shortestpathinISP

•  WhatpathwillbechosenforA2àB1andB1àA2?– Whatisthebestpath?

PrefixB2

PrefixA1ISPA ISPB

PrefixB1

PrefixA2

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EffectsofIndependentParties(2)•  Selectedpathsarelongerthanoverallshortestpath–  Andsymmetrictoo!

•  Thisisaconsequenceofindependentgoalsanddecisions,nothierarchy PrefixB2

PrefixA1ISPA ISPB

PrefixB1

PrefixA2

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RoutingPolicies•  Capturethegoalsofdifferentparties–couldbeanything–  E.g.,Internet2onlycarriesnon-commercialtraffic

•  Commonpolicieswe’lllookat:–  ISPsgiveTRANSITservicetocustomers–  ISPsgivePEERservicetoeachother

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RoutingPolicies–Transit•  Oneparty(customer)getsTRANSITservicefromanotherparty(ISP)–  ISPacceptstrafficforcustomerfromtherestofInternet

–  ISPsendstrafficfromcustomertotherestofInternet

–  CustomerpaysISPfortheprivilege

Customer1

ISP

Customer2

RestofInternetNon-

customer

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RoutingPolicies–Peer•  Bothparty(ISPsinexample)get

PEERservicefromeachother–  EachISPacceptstrafficfromtheotherISPonlyfortheircustomers

–  ISPsdonotcarrytraffictotherestoftheInternetforeachother

–  ISPsdon’tpayeachother

CustomerA1

ISPA

CustomerA2

CustomerB1

ISPB

CustomerB2

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RoutingwithBGP(BorderGatewayProtocol)•  BGPistheinterdomainroutingprotocolusedintheInternet–  Pathvector,akindofdistancevector

ISPAPrefixA1

PrefixA2NetFPrefixF1

IXP

ISPBPrefixB1 PrefixF1viaISP

B,NetFatIXP

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RoutingwithBGP(2)•  DifferentpartieslikeISPsarecalled

AS(AutonomousSystems)•  BorderroutersofASesannounce

BGProutestoeachother

•  RouteannouncementscontainanIPprefix,pathvector,nexthop–  PathvectorislistofASesonthewaytotheprefix;lististofindloops

•  Routeannouncementsmoveintheoppositedirectiontotraffic

RoutingwithBGP(3)

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Prefix

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RoutingwithBGP(4)Policyisimplementedintwoways:

1.  BorderroutersofISPannouncepathsonlytootherpartieswhomayusethosepaths–  Filteroutpathsotherscan’tuse

2.  BorderroutersofISPselectthebestpathoftheonestheyhearinany,non-shortestway

RoutingwithBGP(5)•  TRANSIT:AS1says[B,(AS1,AS3)],[C,(AS1,AS4)]toAS2

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RoutingwithBGP(6)•  CUSTOMER(othersideofTRANSIT):AS2says[A,(AS2)]toAS1

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RoutingwithBGP(7)•  PEER:AS2says[A,(AS2)]toAS3,AS3says[B,(AS3)]toAS2

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RoutingwithBGP(8)•  AS2hearstworoutestoB(viaAS1,AS3)andchoosesAS3(Free!)

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BGPThoughts•  Muchmorebeyondbasicstoexplore!

•  Policyisasubstantialfactor–  Canweevenbeindependentdecisionswillbesensibleoverall?

•  Otherimportantfactors:–  Convergenceeffects–  Howwellitscales–  Integrationwithintradomainrouting–  Andmore…