15-441 Computer NetworkingSummaryOutlineRouting HierarchiesAreasRouting HierarchyArea Hierarchy AddressingPath Sub-optimalitySlide 9A Logical View of the Internet?Internet’s Area HierarchyAS Numbers (ASNs)ExampleSlide 14A Logical View of the InternetTransit vs. PeeringPolicy ImpactSlide 18ChoicesSolution: Distance Vector with PathInterconnecting BGP PeersHop-by-hop ModelPolicy with BGPExamples of BGP PoliciesBGP MessagesBGP UPDATE MessagePath Selection CriteriaLOCAL PREFLOCAL PREF – Common UsesAS_PATHMulti-Exit Discriminator (MED)MEDSlide 33Decision ProcessSlide 35Internal vs. External BGPInternal BGP (I-BGP)Slide 38Important ConceptsNext Lecture: DNS15-441 Computer NetworkingInter-Domain RoutingBGP (Border Gateway Protocol)10/6/05 Lecture #12: Inter-Domain Routing 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 policies10/6/05 Lecture #12: Inter-Domain Routing 3Outline•Routing hierarchy•Internet structure•External BGP (E-BGP)•Internal BGP (I-BGP)10/6/05 Lecture #12: Inter-Domain Routing 4Routing 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 hierarchy10/6/05 Lecture #12: Inter-Domain Routing 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 area10/6/05 Lecture #12: Inter-Domain Routing 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-BorderRouter10/6/05 Lecture #12: Inter-Domain Routing 7Area Hierarchy Addressing1 231.11.22.12.23.13.22.2.12.2.21.2.11.2.210/6/05 Lecture #12: Inter-Domain Routing 8Path 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 paths10/6/05 Lecture #12: Inter-Domain Routing 9Outline•Routing hierarchy•Internet structure•External BGP (E-BGP)•Internal BGP (I-BGP)10/6/05 Lecture #12: Inter-Domain Routing 10A 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!10/6/05 Lecture #12: Inter-Domain Routing 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 exchanges10/6/05 Lecture #12: Inter-Domain Routing 12AS 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 use10/6/05 Lecture #12: Inter-Domain Routing 13Example1 231.11.22.12.23.13.22.2.144.14.255.15.2EGPIGPEGPEGPIGPIGPIGPIGPEGPEGP10/6/05 Lecture #12: Inter-Domain Routing 14A Logical View of the Internet?RRRR R•RIP/OSPF not very scalable area hierarchies•NOT TRUE EITHER!•ISP’s aren’t equal•Size•ConnectivityISPISP10/6/05 Lecture #12: Inter-Domain Routing 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•LocalCustomerProvider10/6/05 Lecture #12: Inter-Domain Routing 16Transit vs. PeeringISP XISP YISP ZISP PTransit ($$)Transit ($$$)Transit ($$ 1/2)Transit ($$)PeeringTransit ($$$)Transit ($)Transit ($$)Transit ($$$)10/6/05 Lecture #12: Inter-Domain Routing 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 situation10/6/05 Lecture #12: Inter-Domain Routing 18Outline•Routing hierarchy•Internet structure•External BGP (E-BGP)•Internal BGP (I-BGP)10/6/05 Lecture #12: Inter-Domain Routing 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’s10/6/05 Lecture #12: Inter-Domain Routing 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 loops10/6/05 Lecture #12: Inter-Domain Routing 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 load10/6/05 Lecture #12: Inter-Domain Routing 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
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