1CS 640CS 640 Introduction to Computer NetworksLecture 23Based on slides by Tim GriffinCS 640Today’s lecture• Inter-domain routing– Architecture and relationships between networks–BGP• Introduction• Implementing peering relationships• Backups and multihoming• Hot potato/cold potatoCS 640Internet StructureTodayBackbone service providerPeeringpointPeeringpointLarge corporationLarge corporationSmallcorporation“Consumer”ISP“Consumer”ISP“Consumer”ISP2CS 640Autonomous Systems (ASes)An autonomous system is an autonomous routing domainthat has been assigned an Autonomous System Number (ASN).RFC 1930: Guidelines for creation, selection, and registration of an Autonomous System… the administration of an AS appears to other ASes to have a single coherent interior routing plan and presents a consistent picture of what networks are reachable through it.5How Are Forwarding Tables Populated to implement Routing?Statically DynamicallyRouters exchange network reachability information using ROUTING PROTOCOLS. Routers use this to compute best routesAdministrator manually configuresforwarding table entries In practice : a mix of these.Static routing mostly at the “edge”+ More control+ Not restricted to destination-based forwarding - Doesn’t scale- Slow to adapt to network failures+ Can rapidly adapt to changes in network topology+ Can be made to scale well- Complex distributed algorithms- Consume CPU, Bandwidth, Memory- Debugging can be difficult- Current protocols are destination-basedCS 640Architecture of Dynamic RoutingAS 1AS 2BGPEGP = Exterior Gateway ProtocolIGP = Interior Gateway ProtocolMetric based: OSPF, IS-IS, RIP, EIGRP (cisco)Policy based: BGP The Routing Domain of BGP is the entire InternetOSPFEIGRP3CS 640Customers and ProvidersCustomer pays provider for access to the InternetprovidercustomerIP trafficprovidercustomer$$$CS 640The “Peering” Relationshippeer peercustomerproviderPeers provide transit between their respective customersPeers do not provide transit between peersPeers (often) do not exchange $$$trafficallowedtraffic NOTallowed$$$$$$$$$$$$$$$CS 640Peering Provides ShortcutsPeering also allows connectivity betweenthe customers of “Tier 1” providers.peer peercustomerprovider4CS 640Peering Wars• Reduces upstream transit costs• Can increase end-to-end performance• May be the only way to connect your customers to some part of the Internet (“Tier 1”) • You would rather have customers• Peers are usually your competition• Peering 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 PeerCS 640Today’s lecture• Inter-domain routing– Architecture and relationships between networks–BGP• Introduction• Implementing peering relationships• Backups and multihoming• Hot potato/cold potatoCS 640BGP-4• BGP = Border Gateway Protocol • Aims to ensure reachability between ASes– “Doesn’t know” about internals of ASes– Not based on “shortest distance”– Based on business relationships• It is a path vector protocol (trivial to avoid loops)– Advertisements carry all ASes on the path to originator• Relatively simple protocol but– Configuration is complex (captures business relationships) – The entire world can be impacted by your mistakes5CS 640A real-world AS GraphThe subgraph showing all ASes that have more than 100 neighbors in full graph of 11,158 nodes. July 6, 2001. AT&T North AmericaWorldcomCS 640AS Graph != Internet TopologyThe AS graphmay look like this.Reality may be closer to this…BGP was designed to throw away information!CS 640Autonomous Routing Domains Don’t Always Need BGP or an ASN QwestYale UniversityNail up default routes 0.0.0.0/0pointing to QwestNail up routes 130.132.0.0/16pointing to Yale 130.132.0.0/16Static routing is the most common way of connecting anautonomous routing domain to the Internet. This helps explain why BGP is a mystery to many …616BGP Operations (Simplified)Establish session onTCP port 179Exchange allactive routes Exchange incrementalupdatesAS1AS2While connection is ALIVE exchangeroute UPDATE messagesBGP session17Two Types of BGP Neighbor Relations• External Neighbor (eBGP) in a different Autonomous Systems • Internal Neighbor (iBGP) in the same Autonomous System AS1AS2eBGPiBGPiBGP is routed (using IGP!) 18BGP announcement• Announcement = prefix + attributes• BGP has many attributes, not all present in each announcement–AS path– Local preference– Multi-exit discriminator MED– Community (unspecified meaning)192.0.2.0/24pick me!192.0.2.0/24pick me!192.0.2.0/24pick me!192.0.2.0/24pick me!Given multipleroutes to the sameprefix, a BGP speakermust pick at mostonebest route(Note: it could reject them all!)719BGP Route ProcessingBest RouteSelection Apply ImportPoliciesBest Route TableApply ExportPoliciesInstall forwardingEntries for bestRoutes. ReceiveBGPUpdatesBestRoutesTransmitBGP UpdatesApply Policy =filter routes & tweak attributesBased onAttributeValuesIP Forwarding TableApply Policy =filter routes & tweak attributesOpen ended programming.Constrained only by vendor configuration languageCS 640Route Selection (simplified)Highest Local PreferenceShortest ASPATHLowest MEDi-BGP < e-BGPLowest IGP cost to BGP egressLowest router IDtraffic engineering Enforce relationshipsThrow up hands andbreak tiesCS 640Forwarding TableIGPJoin EGP with IGP For ConnectivityAS 1 AS 2192.0.2.1135.207.0.0/1610.10.10.10EGP192.0.2.1135.207.0.0/16destination next hop10.10.10.10192.0.2.0/30destination next hop135.207.0.0/16Next Hop = 192.0.2.1192.0.2.0/30135.207.0.0/16destination next hop10.10.10.10+192.0.2.0/30 10.10.10.108CS 640Today’s lecture• Inter-domain routing– Architecture and relationships between networks–BGP• Introduction• Implementing peering relationships• Backups and multihoming• Hot potato/cold potatoCS 640Implementing Customer/Provider and Peer/Peer relationships• Enforce transit relationships – Outbound route filtering • Enforce order of route preference– provider < peer < customerCS 640Import Routes FrompeerFrompeerFromproviderFromproviderFrom customerFrom customerprovider route customer routepeer route ISP route9CS 640Export Routes TopeerTopeerTocustomerTocustomerToproviderFrom providerprovider route customer routepeer route ISP routefiltersblock CS 640Communities Example• 1:100– Customer routes•
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