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Interdomain Routing (BGP)
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  1. Interdomain Routing (BGP) By Behzad Akbari These slides are based on the slides of Ion Stoica (UCB) and Shivkumar (RPI)

  2. Today Internet Structure Large ISP Large ISP Stub Small ISP Dial-Up ISP Stub Stub Stub

  3. Autonomous Systems (AS) • Internet is not a single network! • The Internet is a collection of networks, each controlled by different administrations • An autonomous system (AS) is a network under a single administrative control

  4. AS Numbers (ASNs) ASNs are 16 bit values. 64512 through 65535 are “private” Currently over 11,000 in use. • Genuity: 1 • AT&T: 7018, 6341, 5074, … • UUNET: 701, 702, 284, 12199, … • Sprint: 1239, 1240, 6211, 6242, … • …

  5. IP Address Allocation and Assignment: Internet Registries IANA APNIC ARIN RIPE Allocate to National and local registries and ISPs Addresses assigned to customers by ISPs RFC 2050 - Internet Registry IP Allocation Guidelines RFC 1918 - Address Allocation for Private Internets RFC 1518 - An Architecture for IP Address Allocation with CIDR

  6. Internet Routing • Internet organized as a two level hierarchy • First level – autonomous systems (AS’s) • AS – region of network under a single administrative domain • AS’s run an intra-domain routing protocols • Distance Vector, e.g., RIP • Link State, e.g., OSPF • Between AS’s runs inter-domain routing protocols, e.g., Border Gateway Routing (BGP) • De facto standard today, BGP-4

  7. Example Interior router BGP router AS-1 AS-3 AS-2

  8. Intra-Domain Interior router BGP router AS-1 AS-3 AS-2 Intra-domain routing protocol aka Interior Gateway Protocol (IGP), e.g. OSPF, RIP

  9. Inter-Domain Interior router BGP router AS-1 AS-3 AS-2 Inter-domain routing protocol aka Exterior Gateway Protocol (EGP), e.g. BGP

  10. Inter-Domain Routing • Global connectivity is at stake • Inevitably leads to one single protocol that everyone must speak • Unlike many choices in intra-domain routing • What are the requirements? • Scalability • Flexibility in choosing routes • If you were to choose, link state based or distance vector based? • BGP is sort of a hybrid: Path vector protocol

  11. R R2 R1 R3 Border Gateway Protocol Part I: E-BGP AS2 AS1 E-BGP border router internal router • Two types of routers • Border router (Edge), Internal router (Core)

  12. you can reachnet A via me R R2 R1 R3 table at R1: destnext hop A R2 Purpose of E-BGP AS2 AS1 E-BGP A border router internal router Share connectivity information across ASes

  13. I-BGP R R1 R4 R5 R2 R3 AS1 announce B AS3 border router internal router B Part II: I-BGP, Carrying Info within an AS A E-BGP AS2 E-BGP

  14. E-BGP update I-BGP updates I-BGP • Problem: Injecting external routes into IGP (e.g. OSPF) does not scale and causes BGP policy information to be lost • I-BGP can be used to disseminate BGP routes to all routers in AS • BGP route + IGP route suffice to create forwarding table I-BGP neighbors do not announce routes received via I-BGP to other I-BGP neighbors.

  15. I-BGP: Next Hop = IGP destination next hop Forwarding Table + destination next hop I-BGP destination next hop Join I-BGP with IGP to Create Forwarding Table E-BGP AS 1 AS 2

  16. OSPF Process OSPF Routing tables BGP Process BGP Routing tables Multiple Routing Processes on a Single Router BGP OS kernel OSPF Domain Forwarding Table Manager Forwarding Table

  17. Routing between ISPs • Routing protocol (BGP) contains reachability information (no metrics) • Not about optimizing anything • All about policy (business and politics) • Why? • Metrics optimize for a particular criteria • AT&T’s idea of a good route is not the same as UUnet’s • Scale • What a BGP speaker announces or not announces to a peer determines what routes may get used by whom

  18. Nontransit vs. Transit ASes ISP 2 Internet Service providers (often) have transit networks ISP 1 NET A Nontransit AS might be a corporate or campus network. Traffic NEVER flows from ISP 1 through NET A to ISP 2 IP traffic

  19. IP traffic Selective Transit NET B NET C NET A provides transit between NET B and NET C and between NET D and NET C NET A DOES NOT provide transit Between NET D and NET B NET A NET D Most transit networks transit in a selective manner…

  20. provider customer IP traffic Customers and Providers provider customer Customer pays provider for access to the Internet

  21. Customers Don’t Always Need BGP provider Configured route pointing to customer Default route pointing to provider. customer Static routing is the most common way of connecting an autonomous routing domain to the Internet. This helps explain why BGP is a mystery to many …

  22. Customer-Provider Hierarchy provider customer IP traffic

  23. peer peer provider customer The “Peering” Relationship Peers provide transit between their respective customers Peers do not provide transit between peers Peers (often) do not exchange $$$ traffic allowed traffic NOT allowed

  24. peer peer provider customer Peering Provides Shortcuts Peering also allows connectivity between the customers of “Tier 1” providers.

  25. BGP: Path Vector Protocol • Distance vector algorithm with extra information • For each route, store the complete path (ASs) • No extra computation, just extra storage • Advantages: • can make policy choices based on set of ASs in path • can easily avoid loops

  26. AS-Path • Sequence of AS’s a route traverses • Used for loop detection and to apply policy AS-3 AS-4 AS-5 AS-2 AS-2 AS-3 AS-4 AS-1 AS-2 AS-3 AS-2 AS-5

  27. BGP Operations (Simplified) Establish session on TCP port 179 AS1 BGP session Exchange all active routes AS2 While connection is ALIVE exchange route UPDATE messages Exchange incremental updates

  28. Four Types of BGP Messages • Open : Establish a peering session. • Keep Alive : Handshake at regular intervals. • Notification : Shuts down a peering session. • Update : Announcing new routes or withdrawing previously announced routes. Announcement = prefix + attributes values

  29. Attributes are Used to Select Best Routes pick me! pick me! pick me! Given multiple routes to the same prefix, a BGP speaker must pick at most one best route (Note: it could reject them all!) pick me!

  30. AS73 AS701 AS7018 AS1239 AS9 Example: Multiple AS Paths 128.2/16 128.2/16 9 701 128.2/16 9 7018 1239

  31. Shorter Doesn’t Always Mean Shorter Path 4 1 is “better” than path 3 2 1 AS 4 AS 3 AS 2 AS 1

  32. Implementing Customer/Provider and Peer/Peer relationships • Enforce transit relationships • Outbound route filtering • Enforce order of route preference • provider < peer < customer

  33. provider route peer route customer route ISP route Import Routes From provider From provider From peer From peer From customer From customer

  34. filters block Export Routes provider route peer route customer route ISP route To provider From provider To peer To peer To customer To customer

  35. Example AS Graph The subgraph showing all ASes that have more than 100 neighbors in full graph of 11,158 nodes. July 6, 2001. Point of view: AT&T route-server Does not reflect true topology