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CS 268: Lecture 9 Inter-domain Routing Protocol

CS 268: Lecture 9 Inter-domain Routing Protocol. Scott Shenker and Ion Stoica Computer Science Division Department of Electrical Engineering and Computer Sciences University of California, Berkeley Berkeley, CA 94720-1776. (*slides from Timothy Griffin and Craig Labovitz). Overview.

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CS 268: Lecture 9 Inter-domain Routing Protocol

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  1. CS 268: Lecture 9Inter-domain Routing Protocol Scott Shenker and Ion Stoica Computer Science Division Department of Electrical Engineering and Computer Sciences University of California, Berkeley Berkeley, CA 94720-1776 (*slides from Timothy Griffin and Craig Labovitz)

  2. Overview • An Introduction to BGP • BGP and the Stable Paths problem • Convergence of BGP in the real world • The End-to-End Effects of Internet Path Selection

  3. 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

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

  5. Inter-domain Routing basics • Internet is composed of over 16000 autonomous systems • BGP = Border Gateway Protocol • Is a Policy-Based routing protocol • Is the de facto inter-domain routing protocol of today’s global Internet • Relatively simple protocol, but configuration is complex and the entire world can see, and be impacted by, your mistakes.

  6. Inter-domain Routing • Use TCP • Border Gateway Protocol (BGP), based on Bellman-Ford path vector • AS’s exchange reachability information through their BGP routers, only when routes change • BGP routing information – a sequence of AS’s indicating the path traversed by a route; next hop • General operations of a BGP router: • Learns multiple paths • Picks best path according to its AS policies • Install best pick in IP forwarding tables

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

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

  9. 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

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

  11. Reduces upstream transit costs Can increase end-to-end performance May be the only way to connect your customers to some part of the Internet (“Tier 1”) You would rather have customers Peers are usually your competition Peering relationships may require periodic renegotiation Peering Wars Peer Don’t Peer Peering struggles are by far the most contentious issues in the ISP world! Peering agreements are often confidential.

  12. Architecture of Dynamic Routing OSPF BGP AS 1 EIGRP IGP = Interior Gateway Protocol Metric based: OSPF, IS-IS, RIP, EIGRP (cisco) AS 2 EGP = Exterior Gateway Protocol Policy based: BGP The Routing Domain of BGP is the entire Internet

  13. AS-Path • Sequence of AS’s a route traverses • Used for loop detection and to apply policy AS-3 AS-4 130.10.0.0/16 120.10.0.0/16 AS-5 AS-2 110.10.0.0/16 120.10.0.0/16 AS-2 AS-3 AS-4 AS-1 130.10.0.0/16 AS-2 AS-3 110.10.0.0/16 AS-2 AS-5

  14. 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

  15. Two Types of BGP Neighbor Relationships • External Neighbor (eBGP) in a different Autonomous Systems • Internal Neighbor (iBGP) in the same Autonomous System AS1 iBGP is routed using Interior Gateway Protocol (IGP)! eBGP iBGP AS2

  16. eBGP update iBGP updates iBGP Peers Must be Fully Meshed • iBGP is needed to avoid routing loops within an AS • Injecting external routes into IGP does not scale and causes BGP policy information to be lost • BGP does not provide “shortest path” routing iBGP neighbors do not announce routes received via iBGP to other iBGP neighbors.

  17. Important BGP attributes • LocalPREF • Local preference policy to choose “most” preferred route • Multi-exit Discriminator (MED) • Which peering point to choose? • Import Rules • What route advertisements do I accept? • Export Rules • Which routes do I forward to whom?

  18. Route Selection Summary Highest Local Preference Enforce relationships Shortest ASPATH Lowest MED traffic engineering i-BGP < e-BGP Lowest IGP cost to BGP egress Throw up hands and break ties Lowest router ID

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

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

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

  22. Overview • An Introduction to BGP • BGP and the Stable Paths problem • Convergence of BGP in the real world • The End-to-End Effects of Internet Path Selection

  23. Underlying problem Distributed means of computing a solution. Shortest Paths RIP, OSPF, IS-IS X? BGP What Problem is BGP solving? Having an X can • aid in the design of policy analysis algorithms and heuristics, • aid in the analysis and design of BGP and extensions, • help explain some BGP routing anomalies, • provide a fun way of thinking about the protocol This talk

  24. 2 1 0 2 0 5 2 1 0 4 2 0 4 3 0 2 4 1 0 5 3 3 0 1 3 0 1 0 Q : How simple can X get? A: The Stable Paths Problem (SPP) 2 An instance of the SPP : • A graph of nodes and edges, • Node 0, called the origin, • For each non-zero node, a set or permitted paths to the origin. This set always contains the “null path”. • A ranking of permitted paths at each node. Null path is always least preferred. (Not shown in diagram) 1 most preferred … least preferred (not null) When modeling BGP : nodes represent BGP speaking border routers, and 0 represents a node originating some address block

  25. 5 2 1 0 5 1 0 2 4 3 A Solution to a Stable Paths Problem 2 A solution is an assignment of permitted paths to each node such that 2 1 0 2 0 • node u’s assigned path is either the null path or is a path uwP, where wP is assigned to node w and {u,w} is an edge in the graph, • each node is assigned the highest ranked path among those consistent with the paths assigned to its neighbors. 4 2 0 4 3 0 3 0 1 1 3 0 1 0 A Solution need not represent a shortest path tree, or a spanning tree.

  26. 1 2 4 3 0 Example: SHORTEST1 2 0 2 1 0 1 0 1 3 0 4 3 0 4 2 0 3 0

  27. 1 2 4 3 0 Example: SHORTEST1 (Solution) 2 0 2 1 0 1 0 1 3 0 4 3 0 4 2 0 3 0

  28. 1 2 4 3 0 Example: SHORTEST2 2 0 2 1 0 1 0 1 3 0 4 2 0 4 3 0 3 0

  29. 1 2 4 3 0 Example: SHORTEST2 (Solution) 2 0 2 1 0 1 0 1 3 0 4 2 0 4 3 0 3 0

  30. 1 2 4 3 0 Example: GOOD GADGET 2 1 0 2 0 1 3 0 1 0 4 3 0 4 2 0 3 0

  31. 1 2 4 3 0 Example: GOOD GADGET (Solution) 2 1 0 2 0 1 3 0 1 0 4 3 0 4 2 0 3 0

  32. 1 1 1 0 0 0 2 2 2 A Stable Paths Problem may have multiple solutions 1 2 0 1 0 1 2 0 1 0 1 2 0 1 0 2 1 0 2 0 2 1 0 2 0 2 1 0 2 0 First solution Second solution DISAGREE

  33. 1 2 4 3 0 Example: NAUGHTY GADGET 2 1 0 2 0 1 3 0 1 0 4 3 0 4 2 0 3 4 2 0 3 0

  34. 1 2 4 3 0 Example: NAUGHTY GADGET (Solution 1) 2 1 0 2 0 1 3 0 1 0 4 3 0 4 2 0 3 4 2 0 3 0

  35. 1 2 4 3 0 Example: NAUGHTY GADGET (Solution 2) 2 1 0 2 0 1 3 0 1 0 4 3 0 4 2 0 3 4 2 0 3 0

  36. Solvable Can Diverge The SPP view : must converge must diverge SPP helps explain possibility of BGP divergence • BGP is not guaranteed to converge to a stable routing. Policy inconsistencies can lead to “livelock” protocol oscillations. • See “Persistent Route Oscillations in Inter-domain Routing” by K. Varadhan, R. Govindan, and D. Estrin. ISI report, 1996

  37. 1 2 4 3 0 Example: NAUGHTY GADGET 2 1 0 2 0 1 3 0 1 0 4 3 0 4 2 0 3 4 2 0 3 0

  38. 1 2 4 3 0 Example: BAD GADGET 2 1 0 2 0 1 3 0 1 0 4 2 0 4 3 0 3 4 2 0 3 0

  39. 1 2 4 3 0 Example: BAD GADGET 2 1 0 2 0 1 3 0 1 0 4 2 0 4 3 0 3 4 2 0 3 0 • 2 chooses (2 0) • 4 chooses (4 2 0) • 3 chooses (3 4 2 0) • 1 chooses (1 0)

  40. 1 2 4 3 0 Example: BAD GADGET 2 1 0 2 0 1 3 0 1 0 4 2 0 4 3 0 3 4 2 0 3 0 … 2 chooses (2 1 0) …

  41. 1 2 4 3 0 Example: BAD GADGET 2 1 0 2 0 1 3 0 1 0 4 2 0 4 3 0 3 4 2 0 3 0 … 3 chooses (3 0) …

  42. 1 2 4 3 0 Example: BAD GADGET 2 1 0 2 0 1 3 0 1 0 4 2 0 4 3 0 3 4 2 0 3 0 … 1 chooses (1 3 0) …

  43. 1 2 4 3 0 Example: BAD GADGET 2 1 0 2 0 1 3 0 1 0 4 2 0 4 3 0 3 4 2 0 3 0 … 2 chooses (2 0) …

  44. 1 2 4 3 0 Example: BAD GADGET 2 1 0 2 0 1 3 0 1 0 4 2 0 4 3 0 3 4 2 0 3 0 … 3 chooses (3 4 2 0) … LOOP !

  45. BAD GADGET : No Solution With a BGP-like protocol, each node will do the best it can, so at least one node will always have the opportunity to improve its path. Result : persistent oscillation. 2 1 0 2 0 2 4 2 0 4 3 0 4 0 3 1 3 4 2 0 3 0 1 3 0 1 0

  46. SURPRISE : Beware of Backup Policies 2 1 0 2 0 Becomes BAD GADGET if link (4, 0) goes down. 2 4 0 4 2 0 4 3 0 BGP is not robust : it is not guaranteed to recover from network failures. 4 0 3 1 3 4 2 0 3 0 1 3 0 1 0

  47. 4 3 1 0 4 5 3 1 2 0 4 3 1 2 0 1 2 0 1 0 3 1 0 3 1 2 0 1 3 0 5 3 1 0 5 6 3 1 2 0 5 3 1 2 0 6 3 1 0 6 4 3 1 2 0 6 3 1 2 0 2 1 0 2 0 5 2 4 6 As with DISAGREE, this part has two distinct solutions This part has a solution only when node 1 is assigned the direct path (1 0). PRECARIOUS Has a solution, but can get “trapped”

  48. What is to be done? Static Approach Dynamic Approach Extend BGP with a dynamic means of detecting and suppressing policy-based oscillations? Inter-AS coordination Automated Analysis of Routing Policies (This is very hard). These approaches are complementary

  49. Theoretical Results • The problem of determining whether an instance of stable paths problem is solvable is NP-complete • Shortest path route selection is provably safe

  50. Overview • An Introduction to BGP • BGP and the Stable Paths problem • Convergence of BGP in the real world • The End-to-End Effects of Internet Path Selection

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