115-441 Computer NetworkingInter-Domain RoutingBGP (Border Gateway Protocol)Lecture #13: 02-24-04 2Summary• The Story So Far…• 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 lecture• How to make routing protocols support large networks• How to make routing protocols support business policiesLecture #13: 02-24-04 3Outline• Routing hierarchy• Internet structure• External BGP (E-BGP)• Internal BGP (I-BGP)Lecture #13: 02-24-04 4Routing 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 destination• Need lower diameters networks• Solution: area hierarchy2Lecture #13: 02-24-04 5Areas• Divide network into areas• Areas can have nested sub-areas• 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 areaLecture #13: 02-24-04 6Routing Hierarchy• Partition Network into “Areas”• 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• Constraint: no path between two sub-areas of an area can exit that areaBackbone AreasLower-level AreasArea-BorderRouterLecture #13: 02-24-04 7Area Hierarchy Addressing1231.11.22.12.23.13.22.2.12.2.21.2.11.2.2Lecture #13: 02-24-04 8Path Sub-optimality1231.11.22.12.23.13.22.2.13 hop red pathvs.2 hop green pathstartend3.2.11.2.1• Can result in sub-optimal paths3Lecture #13: 02-24-04 9Outline• Routing hierarchy• Internet structure• External BGP (E-BGP)• Internal BGP (I-BGP)Lecture #13: 02-24-04 10A 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!Lecture #13: 02-24-04 11Internet’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 exchangesLecture #13: 02-24-04 12AS Numbers (ASNs)ASNs are 16 bit values 64512 through 65535 are “private”• Genuity: 1 • MIT: 3• JANET: 786• UC San Diego: 7377• AT&T: 7018, 6341, 5074, …• UUNET: 701, 702, 284, 12199, …• Sprint: 1239, 1240, 6211, 6242, …•…ASNs represent units of routing policyCurrently over 15,000 in use4Lecture #13: 02-24-04 13Example1231.11.22.12.23.13.22.2.144.14.255.15.2EGPIGPEGPEGPIGPIGPIGPIGPEGPEGPLecture #13: 02-24-04 14A Logical View of the Internet?RRRR R• RIP/OSPF not very scalable Æ area hierarchies• NOT TRUE EITHER!• ISP’s aren’t equal• Size• ConnectivityISPISPLecture #13: 02-24-04 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• LocalCustomerProviderLecture #13: 02-24-04 16Transit vs. PeeringISP XISP YISP ZISP PTransit ($)Transit ($$$)Transit ($$ 1/2)Transit ($$)PeeringTransit ($$$)Transit ($)Transit ($$)Transit ($$$)5Lecture #13: 02-24-04 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• WHY?• Consider the economics of the situationLecture #13: 02-24-04 18Outline• Routing hierarchy• Internet structure• External BGP (E-BGP)• Internal BGP (I-BGP)Lecture #13: 02-24-04 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’sLecture #13: 02-24-04 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 loops6Lecture #13: 02-24-04 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 loadLecture #13: 02-24-04 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)Lecture #13: 02-24-04 23Policy with BGP• BGP provides capability for enforcing various policies• Policies are notpart of BGP: they are provided to BGP as configuration information• BGP enforces policies by choosing paths from multiple alternatives and controlling advertisement to other AS’sLecture #13: 02-24-04 24Examples of BGP Policies• A multi-homed AS refuses to act as transit• Limit path advertisement• A multi-homed AS can become transit for some AS’s• Only advertise paths to some AS’s• An AS can favor or disfavor certain AS’s for traffic transit from itself7Lecture #13: 02-24-04 25BGP Messages• Open• Announces AS ID• Determines hold timer – interval between keep_alive or update messages, zero interval implies no keep_alive• Keep_alive• Sent periodically (but before hold timer expires) to peers to ensure connectivity.• Sent in place of an UPDATE message• Notification• Used for error notification• TCP
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