15-441 Computer NetworkingImportant Lessons From Last LectureRouting ReviewOutlineA Logical View of the Internet?Routing HierarchiesAreasRouting HierarchyArea Hierarchy AddressingPath Sub-optimalitySlide 11Slide 12Internet’s Area HierarchyAS Numbers (ASNs)ExampleSlide 16A Logical View of the InternetTransit vs. PeeringPolicy ImpactSlide 20HistoryChoicesSolution: Distance Vector with PathInterconnecting BGP PeersHop-by-hop ModelExamples of BGP PoliciesBGP MessagesBGP UPDATE MessagePath Selection CriteriaLOCAL PREFLOCAL PREF – Common UsesAS_PATHMulti-Exit Discriminator (MED)MEDSlide 35Decision ProcessInternal vs. External BGPImportant Concepts15-441 Computer NetworkingInter-Domain RoutingBGP (Border Gateway Protocol)Copyright ©, 2007-10 Carnegie Mellon University2Important Lessons From Last Lecture•Every router needs to be able to forward towards any destination•Forwarding table must be complete•Can rely on friends to tell you how to get there (DV)•Can get an entire map of the network (LS)•Key challenges•What if a router fails or is added? need to inform everyone•Soft-state recovery•What if people have inconsistent/different views?•Loops, count to infinity3Routing Review•The Story So Far… •IP forwarding requires next-hop information•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 policies4Outline•Routing hierarchy•Internet structure•External BGP (E-BGP)5A Logical View of the Internet?RRRR R•After looking at RIP/OSPF descriptions•End-hosts connected to routers•Routers exchange messages to determine connectivity•NOT TRUE!6Routing Hierarchies•Flat routing doesn’t scale•Storage Each node cannot be expected to store routes to every destination (or destination network)•Convergence times increase•Communication Total message count increases•Key observation•Need less information with increasing distance to destination•Need lower diameters networks•Solution: area hierarchy7Areas•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 area8Routing 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-BorderRouter9Area Hierarchy Addressing1 231.11.22.12.23.13.22.2.12.2.21.2.11.2.210Path Sub-optimality1 231.11.22.12.23.13.22.2.13 hop red pathvs.2 hop green pathstartend3.2.11.2.1•Can result in sub-optimal paths11Outline•Routing hierarchy•Internet structure•External BGP (E-BGP)12A Logical View of the Internet?RRRR R•After looking at RIP/OSPF descriptions•End-hosts connected to routers•Routers exchange messages to determine connectivity•NOT TRUE!13Internet’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•Each AS assigned unique ID•AS’s peer at network exchanges14AS Numbers (ASNs)ASNs are 16 bit values 64512 through 65535 are “private”•Genuity: 1 •MIT: 3•CMU: 9•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 use15Example1 231.11.22.12.23.13.22.2.144.14.255.15.2EGPIGPEGPEGPIGPIGPIGPIGPEGPEGP16A Logical View of the Internet?RRRR R•RIP/OSPF not very scalable area hierarchies•NOT TRUE EITHER!•ISP’s aren’t equal•Size•ConnectivityISPISP17A 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•LocalCustomerProvider18Transit vs. PeeringISP XISP YISP ZISP PTransit ($$)Transit ($$$)Transit ($$ 1/2)Transit ($$)PeeringTransit ($$$)Transit ($)Transit ($$)Transit ($$$)19Policy 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 situation20Outline•Routing hierarchy•Internet structure•External BGP (E-BGP)21History•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 performance22Choices•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’s23Solution: 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 loops24Interconnecting 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 load25Hop-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)•BGP enforces policies
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