15-744: Computer NetworkingInter-Domain RoutingOutlineRouting HierarchiesAreasThe Area HierarchyRoutingPath Sub-optimalitySlide 9Internet’s Area HierarchyExampleHistoryA Logical View of the Internet?Slide 14A Logical View of the InternetTransit vs. PeeringPolicy ImpactSlide 18ChoicesSolution: Distance Vector with PathInterconnecting BGP PeersHop-by-hop ModelPolicy with BGPExamples of BGP PoliciesBGP Common HeaderBGP MessagesBGP UPDATE MessagePath Selection CriteriaLOCAL PREFLOCAL PREF – Common UsesAS_PATHCIDR and BGPOptionsSets and SequencesMulti-Exit Discriminator (MED)MEDSlide 37Other AttributesDecision ProcessSlide 40Internal vs. External BGPInternal BGP (I-BGP)Slide 43Link FailuresFailure on an E-BGP LinkFailure on an I-BGP LinkNext Lecture: New Routing Ideas15-744: Computer NetworkingL-5 Inter-Domain RoutingL -5; 9-24-02© Srinivasan Seshan, 2002 2Inter-Domain Routing•Border Gateway Protocol (BGP)•Assigned reading•MIT Lecture Notes•[Mah02] Understanding BGP MisconfigurationL -5; 9-24-02© Srinivasan Seshan, 2002 3Outline•Routing hierarchy•Internet structure•External BGP (E-BGP)•Internal BGP (I-BGP)L -5; 9-24-02© Srinivasan Seshan, 2002 4Routing Hierarchies•Flat routing doesn’t scale•Each node cannot be expected to have routes to every destination (or destination network)•Key observation•Need less information with increasing distance to destination•Two radically different approaches for routing•The area hierarchy•The landmark hierarchy (discuss in routing alternatives)L -5; 9-24-02© Srinivasan Seshan, 2002 5Areas•Divide network into areas•Areas can have nested sub-areas•Constraint: no path between two sub-areas of an area can exit that area•Hierarchically address nodes in a network•Sequentially number top-level areas•Sub-areas of area are labeled relative to that area•Nodes are numbered relative to the smallest containing areaL -5; 9-24-02© Srinivasan Seshan, 2002 6The Area Hierarchy1 231.11.22.12.23.13.22.2.12.2.21.2.11.2.2L -5; 9-24-02© Srinivasan Seshan, 2002 7Routing•Within area•Each node has routes to every other node•Outside area•Each node has routes for other top-level areas only•Inter-area packets are routed to nearest appropriate border router•Can result in sub-optimal pathsL -5; 9-24-02© Srinivasan Seshan, 2002 8Path Sub-optimality1 231.11.22.12.23.13.22.2.13 hop red pathvs.2 hop green pathstartend3.2.11.2.1L -5; 9-24-02© Srinivasan Seshan, 2002 9Outline•Routing hierarchy•Internet structure•External BGP (E-BGP)•Internal BGP (I-BGP)L -5; 9-24-02© Srinivasan Seshan, 2002 10Internet’s Area Hierarchy•What is an Autonomous System (AS)?•A set of routers under a single technical administration, using an interior gateway protocol (IGP) and common metrics to route packets within the AS and using an exterior gateway protocol (EGP) to route packets to other AS’s•Sometimes AS’s use multiple IGPs and metrics, but appear as single AS’s to other AS’s•Each AS assigned unique ID•AS’s peer at network exchangesL -5; 9-24-02© Srinivasan Seshan, 2002 11Example1 231.11.22.12.23.13.22.2.144.14.255.15.2EGPIGPEGPEGPIGPIGPIGPIGPEGPEGPL -5; 9-24-02© Srinivasan Seshan, 2002 12History•Mid-80s: EGP•Reachability protocol (no shortest path)•Did not accommodate cycles (tree topology)•Evolved when all networks connected to NSF backbone•Result: BGP introduced as routing protocol•Latest version = BGP 4•BGP-4 supports CIDR•Primary objective: connectivity not performanceL -5; 9-24-02© Srinivasan Seshan, 2002 13A Logical View of the Internet?RRRR R•After looking a RIP/OSPF descriptions•End-hosts connected to routers•Routers exchange messages to determine connectivity•NOT TRUE!L -5; 9-24-02© Srinivasan Seshan, 2002 14A Logical View of the Internet?RRRR R•RIP/OSPF not very scalable area hierarchies•But, ISP’s aren’t equal•Size•Connectivity•NOT TRUE EITHER!ISPISPL -5; 9-24-02© Srinivasan Seshan, 2002 15A Logical View of the InternetTier 1 Tier 1Tier 2Tier 2Tier 2Tier 3•Tier 1 ISP•“Default-free” with global reachability info•Tier 2 ISP•Regional or country-wide•Tier 3 ISP•LocalCustomerProviderL -5; 9-24-02© Srinivasan Seshan, 2002 16Transit vs. PeeringISP XISP YISP ZISP PTransit ($)Transit ($$$)Transit ($$ 1/2)Transit ($$)PeeringTransit ($$$)Transit ($)Transit ($$)Transit ($$$)L -5; 9-24-02© Srinivasan Seshan, 2002 17Policy Impact•“Valley-free” routing•Number links as (+1, 0, -1) for provider, peer and customer•In any path should only see sequence of +1, followed by at most one 0, followed by sequence of -1•Can we infer relationship from BGP•From routes taken•From size•From multiple views and BGP announcementsL -5; 9-24-02© Srinivasan Seshan, 2002 18Outline•Routing hierarchy•Internet structure•External BGP (E-BGP)•Internal BGP (I-BGP)L -5; 9-24-02© Srinivasan Seshan, 2002 19Choices•Link state or distance vector?•No universal metric – policy decisions•Problems with distance-vector:•Bellman-Ford algorithm may not converge•Problems with link state:•Metric used by routers not the same – loops•LS database too large – entire Internet•May expose policies to other AS’sL -5; 9-24-02© Srinivasan Seshan, 2002 20Solution: Distance Vector with Path•Each routing update carries the entire path•Loops are detected as follows:•When AS gets route check if AS already in path•If yes, reject route•If no, add self and (possibly) advertise route further•Advantage:•Metrics are local - AS chooses path, protocol ensures no loopsL -5; 9-24-02© Srinivasan Seshan, 2002 21Interconnecting BGP Peers•BGP uses TCP to connect peers•Advantages:•Simplifies BGP•No need for periodic refresh - routes are valid until withdrawn, or the connection is lost•Incremental updates•Disadvantages•Congestion control on a routing protocol?•Poor interaction during high loadL -5; 9-24-02© Srinivasan Seshan, 2002 22Hop-by-hop Model•BGP advertises to neighbors only those routes that it uses•Consistent with the hop-by-hop Internet paradigm•e.g., AS1 cannot tell AS2 to route to other AS’s in a manner different than what AS2 has chosen (need source routing for that)L -5; 9-24-02© Srinivasan Seshan, 2002 23Policy with BGP•BGP provides capability for enforcing various policies•Policies are not part of BGP: they are provided to BGP as configuration information•BGP enforces policies by choosing paths from
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