115-441 Computer NetworkingInter-Domain RoutingBGP (Border Gateway Protocol)Routing Review• The Story So Far… •Routing protocols generate the forwarding table•Routing protocols generate the forwarding table• Two styles: distance vector, link state• Scalability issues: • Distance vector protocols suffer from count-to-infinity• Link state protocols must flood information through network• Today’s lecture10/4/07Lecture #11: Inter-Domain Routing 2• How to make routing protocols support large networks• How to make routing protocols support business policiesOutlineRouting hierarchy•Routing hierarchy• Internet structureEt lBGP(EBGP)10/4/07Lecture #11: Inter-Domain Routing 3•External BGP (E-BGP)Routing Hierarchies• Flat routing doesn’t scale• Storage Æ Each node cannot be expected to store routes to every destination (or destination network)• Convergence times increase• Communication Æ Total message count increases• Key observation• Need less information with increasing distance to 10/4/07Lecture #11: Inter-Domain Routing 4gdestination• Need lower diameters networks• Solution: area hierarchy2Areas• Divide network into areas• Areas can have nested sub-areasAreas can have nested subareas• 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 area10/4/07Lecture #11: Inter-Domain Routing 5Routing HierarchyBackbone AreasArea-BorderRouter• Partition Network into “Areas”• Within areaLower-level Areas10/4/07Lecture #11: Inter-Domain Routing 6• 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• Constraint: no path between two sub-areas of an area can exit that areaArea Hierarchy Addressing122231.11.22.12.22.2.12.2.21.2.11.2.210/4/07Lecture #11: Inter-Domain Routing 73.13.2Path Sub-optimality• Can result in sub-optimal paths121.11.22.12.22.2.1startend1.2.110/4/07Lecture #11: Inter-Domain Routing 833.13.23 hop red pathvs.2 hop green path3.2.13OutlineRti hi h•Routing hierarchy• Internet structure10/4/07Lecture #11: Inter-Domain Routing 9• External BGP (E-BGP)A Logical View of the Internet?•After looking at RRRR RgRIP/OSPF descriptions• End-hosts connected to routers• Routers exchange messages to determine connectivity10/4/07Lecture #11: Inter-Domain Routing 10connectivity• NOT TRUE!Internet’s Area Hierarchy• What is an Autonomous System (AS)?• A set of routers under a single technical administration, set o oute s u de a s g e tec ca ad st at o ,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• Each AS assigned unique ID•AS’s peer at network exchanges10/4/07Lecture #11: Inter-Domain Routing 11pgAS Numbers (ASNs)ASNs are 16 bit values 64512 through 65535 are “private”Currently over 15 000 in use• Genuity: 1 • MIT: 3•CMU: 9• UC San Diego: 7377•AT&T: 7018 6341 5074Currently over 15,000 in use10/4/07Lecture #11: Inter-Domain Routing 12AT&T: 7018, 6341, 5074, … • UUNET: 701, 702, 284, 12199, …• Sprint: 1239, 1240, 6211, 6242, …•…ASNs represent units of routing policy4Example122.12.2EGPIGPIGP31.11.22.2.144.14.2EGPEGPEGPIGPIGPEGPEGP10/4/07Lecture #11: Inter-Domain Routing 133.13.2455.15.2IGPIGPEGPA Logical View of the Internet?• RIP/OSPF not very scalable Æ area RRRR Rhierarchies• NOT TRUE EITHER!• ISP’s aren’t equal• Size•ConnectivityISPISP10/4/07Lecture #11: Inter-Domain Routing 14•ConnectivityA Logical View of the Internet• Tier 1 ISP• “Default-free” with global Tier 1 Tier 1Tier 2Tier 2Tier 3reachability info• Tier 2 ISP• Regional or country-wide• Tier 3 ISP• LocalCustomerProvider10/4/07Lecture #11: Inter-Domain Routing 15Tier 2Transit vs. PeeringTransit ($$ 1/2)Transit ($$$)ISP XISP YISP ZISP PTransit ($$$)Peering()Transit ($)Transit ($$$)10/4/07Lecture #11: Inter-Domain Routing 16Transit ($$)Transit ($$)Transit ($$)5Policy Impact• “Valley-free” routing• Number links as (+1, 0, -1) for provider, peer and 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• WHY?• Consider the economics of the situation10/4/07Lecture #11: Inter-Domain Routing 17OutlineRti hi h•Routing hierarchy• Internet structure10/4/07Lecture #11: Inter-Domain Routing 18• External BGP (E-BGP)Choices• Link state or distance vector?•No universal metric–policy decisions•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 largeentire Internet10/4/07Lecture #11: Inter-Domain Routing 19•LS database too large –entire Internet• May expose policies to other AS’sSolution: Distance Vector with Path• Each routing update carries the entire path•Loops are detected as follows:•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 localAS chooses path protocol ensures10/4/07Lecture #11: Inter-Domain Routing 20•Metrics are local -AS chooses path, protocol ensures no loops6Interconnecting BGP Peers• BGP uses TCP to connect peers•Advantages:•Advantages:• Simplifies BGP• No need for periodic refresh - routes are valid until withdrawn, or the connection is lost• Incremental updates•Disadvantages10/4/07Lecture #11: Inter-Domain Routing 21Disadvantages• Congestion control on a routing protocol?• Poor interaction during high loadHop-by-hop Model• BGP advertises to neighbors only those routes that it usesthat 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)• BGP enforces policies by choosing paths from 10/4/07Lecture #11: Inter-Domain Routing 22multiple alternatives and controlling advertisement to other AS’sExamples of BGP Policies• A multi-homed AS refuses to act as transit•Limit path advertisement•Limit path
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