CS 268: Lecture 9 Inter-domain Routing ProtocolOverviewInternet RoutingExampleInter-domain Routing basicsInter-domain RoutingBGP Operations (Simplified)Customers and ProvidersThe “Peering” RelationshipPeering Provides ShortcutsPeering WarsArchitecture of Dynamic RoutingAS-PathFour Types of BGP MessagesTwo Types of BGP Neighbor RelationshipsiBGP Peers Must be Fully MeshedImportant BGP attributesRoute Selection SummaryImplementing Customer/Provider and Peer/Peer relationshipsImport RoutesExport RoutesSlide 22What Problem is BGP solving?Q : How simple can X get? A: The Stable Paths Problem (SPP)A Solution to a Stable Paths ProblemExample: SHORTEST1Example: SHORTEST1 (Solution)Example: SHORTEST2Example: SHORTEST2 (Solution)Example: GOOD GADGETExample: GOOD GADGET (Solution)A Stable Paths Problem may have multiple solutionsExample: NAUGHTY GADGETExample: NAUGHTY GADGET (Solution 1)Example: NAUGHTY GADGET (Solution 2)SPP helps explain possibility of BGP divergenceSlide 37Example: BAD GADGETSlide 39Slide 40Slide 41Slide 42Slide 43Slide 44BAD GADGET : No SolutionSURPRISE : Beware of Backup PoliciesPRECARIOUSWhat is to be done?Theoretical ResultsSlide 50Convergence in the real-world?BGP Convergence ExampleConvergence ResultOutline of the Today’s classEnd-to-end effects of Path SelectionApproximating the best pathMethodologySlide 58Slide 59Slide 60Slide 61Why Path Selection is imperfect?CS 268: Lecture 9Inter-domain Routing ProtocolScott Shenker and Ion StoicaComputer Science DivisionDepartment of Electrical Engineering and Computer SciencesUniversity of California, BerkeleyBerkeley, CA 94720-1776(*slides from Timothy Griffin and Craig Labovitz)2OverviewAn Introduction to BGPBGP and the Stable Paths problemConvergence of BGP in the real worldThe End-to-End Effects of Internet Path Selection3Internet RoutingInternet organized as a two level hierarchyFirst level – autonomous systems (AS’s)-AS – region of network under a single administrative domainAS’s run an intra-domain routing protocols-Distance Vector, e.g., RIP-Link State, e.g., OSPFBetween AS’s runs inter-domain routing protocols, e.g., Border Gateway Routing (BGP)-De facto standard today, BGP-44ExampleAS-1AS-2AS-3Interior routerBGP router5Inter-domain Routing basics Internet is composed of over 16000 autonomous systemsBGP = Border Gateway Protocol -Is a Policy-Based routing protocol -Is the de facto inter-domain routing protocol of today’s global InternetRelatively simple protocol, but configuration is complex and the entire world can see, and be impacted by, your mistakes.6Inter-domain RoutingUse TCPBorder Gateway Protocol (BGP), based on Bellman-Ford path vectorAS’s exchange reachability information through their BGP routers, only when routes changeBGP routing information – a sequence of AS’s indicating the path traversed by a route; next hopGeneral operations of a BGP router:-Learns multiple paths-Picks best path according to its AS policies-Install best pick in IP forwarding tables7BGP Operations (Simplified) Establish session on TCP port 179 Exchange all active routes Exchange incremental updatesAS1AS2While connection is ALIVE exchangeroute UPDATE messagesBGP session8Customers and ProvidersCustomer pays provider for access to the InternetprovidercustomerIP trafficprovidercustomer9The “Peering” Relationshippeer peercustomerproviderPeers provide transit between their respective customersPeers do not provide transit between peersPeers (often) do not exchange $$$trafficallowedtraffic NOTallowed10Peering Provides ShortcutsPeering also allows connectivity betweenthe customers of “Tier 1” providers.peer peercustomerprovider11Peering WarsReduces upstream transit costsCan increase end-to-end performanceMay be the only way to connect your customers to some part of the Internet (“Tier 1”) You would rather have customersPeers are usually your competitionPeering relationships may require periodic renegotiationPeering struggles are by far the most contentious issues in the ISP world!Peering agreements are often confidential.Peer Don’t Peer12Architecture of Dynamic RoutingAS 1AS 2 BGP EGP = Exterior Gateway Protocol IGP = Interior Gateway Protocol Metric based: OSPF, IS-IS, RIP, EIGRP (cisco)Policy based: BGP The Routing Domain of BGP is the entire InternetOSPFEIGRP13AS-PathSequence of AS’s a route traversesUsed for loop detection and to apply policy120.10.0.0/16130.10.0.0/16110.10.0.0/16AS-1AS-2AS-3AS-4AS-5120.10.0.0/16 AS-2 AS-3 AS-4130.10.0.0/16 AS-2 AS-3110.10.0.0/16 AS-2 AS-514Four Types of BGP MessagesOpen : Establish a peering session. Keep Alive : Handshake at regular intervals. Notification : Shuts down a peering session. Update : Announcing new routes or withdrawing previously announced routes. announcement = prefix + attributes values15Two Types of BGP Neighbor Relationships•External Neighbor (eBGP) in a different Autonomous Systems •Internal Neighbor (iBGP) in the same Autonomous System AS1AS2eBGPiBGPiBGP is routed using Interior Gateway Protocol(IGP)!16iBGP Peers Must be Fully MeshediBGP neighbors do not announce routes received via iBGP to other iBGPneighbors.eBGP updateiBGP updates•iBGP is needed to avoid routing loops within an AS•Injecting external routes into IGP does not scale and causes BGP policy information to be lost•BGP does not provide “shortest path” routing17Important BGP attributesLocalPREF-Local preference policy to choose “most” preferred routeMulti-exit Discriminator (MED)-Which peering point to choose?Import Rules-What route advertisements do I accept?Export Rules-Which routes do I forward to whom?18Route Selection SummaryHighest Local PreferenceShortest ASPATHLowest MEDi-BGP < e-BGPLowest IGP cost to BGP egressLowest router IDtraffic engineering Enforce relationshipsThrow up hands andbreak ties19Implementing Customer/Provider and Peer/Peer relationshipsEnforce transit relationships -Outbound route filtering Enforce order of route preference-provider < peer < customerTwo parts:20Import Routes FrompeerFrompeerFromproviderFromproviderFrom customerFrom customerprovider route customer routepeer route ISP route21Export Routes TopeerTopeerTocustomerTocustomerToproviderFrom providerprovider route customer routepeer route ISP routefiltersblock22OverviewAn Introduction to BGPBGP and the Stable Paths problemConvergence of BGP in the real
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