Chapter 4 NETWORK LAYER

1. Chapter 4 NETWORK LAYER Dynamic PowerPoint Slides Only Computer Networks Winter 2002 / 2003 DistributedComputing Group

2. Remember: Count to Infinity Problem c: 2 c: 1 a b c c: 3 c: 4 c: 5 c: 6 c: 7 c: 8 Distributed Computing Group Computer Networks R. Wattenhofer

3. BGP does not count to infinity a b c d e Zurich Distributed Computing Group Computer Networks R. Wattenhofer

4. BGP does not count to infinity “withdraw Zurich” a b c d e Zurich Distributed Computing Group Computer Networks R. Wattenhofer

5. BGP Basics Continued “announce bcdeZ” a b c d e Zurich Distributed Computing Group Computer Networks R. Wattenhofer

6. BGP Basics Continued “announce bfeZ” f 30s a b c d e Zurich backup active Distributed Computing Group Computer Networks R. Wattenhofer

7. BGP Basics Continued “announce bcdeZ” f a b c d e Zurich backup active Distributed Computing Group Computer Networks R. Wattenhofer

8. BGP (Border Gateway Protocol) • BGP is the Internet de-facto standard • Path Vector protocol • Receive BGP update (announce or withdrawal) from a neighbor. • Update routing table. • Does update affect active route? (Loop detection, policy, etc.) If yes, send update to all neighbors that are allowed by policy. • MinRouteAdver: At most 1 announce per neighbor per 30+jitter seconds. • Store the active routes of the neighbors. Distributed Computing Group Computer Networks R. Wattenhofer

9. BGP BGP BGP Internet Architecture • iBGP • Route flap dampening • Multipath • Soft configuration • … Distributed Computing Group Computer Networks R. Wattenhofer

10. Internet inter-AS routing: BGP • BGP messages exchanged using TCP. • BGP messages • OPEN: opens TCP connection to peer and authenticates sender • UPDATE: advertises new path (or withdraws old) • KEEPALIVE keeps connection alive in absence of UPDATES; also ACKs OPEN request • NOTIFICATION: reports errors in previous msg; also used to close connection • Policy • Even if two BGP routers are connected they may not announce all their routes or use all the routes of the other • Example: if AS A does not want to route traffic of AS B, then A should simply not announce anything to B. Distributed Computing Group Computer Networks R. Wattenhofer

11. Robustness of BGP • We are interested in routes to destination d. • Nodes a,b,c all have the policy to prefer a 2-hop route through their clockwise neighbor over a direct 1-hop route to destination d. a d b c Distributed Computing Group Computer Networks R. Wattenhofer

12. BGP Update Traffic (Mae-East) Cisco bug “withdraw loop” is fixed with IOS release. Distributed Computing Group Computer Networks R. Wattenhofer

13. Internet Evolution: NSFNet (1995) NSFNet Backbone Hello/EGP Hello/EGP Regional Regional Regional Campus Campus Campus Campus Distributed Computing Group Computer Networks R. Wattenhofer

14. Internet Evolution: Today AS1 AS2 AS5 BGP AS6 AS4 AS3 AS8 AS7 Distributed Computing Group Computer Networks R. Wattenhofer

15. Experimental Setup • Analyzed secondary paths of 20x20 AS pairs: • Inject and monitor BGP faults. • Survey providers on policies. Distributed Computing Group Computer Networks R. Wattenhofer

16. BGP Convergence Times 180 Distributed Computing Group Computer Networks R. Wattenhofer

17. BGP Convergence Results • If a link comes up, the convergence time is in the order of time to forward a message on the shortest path. • If a link goes down, the convergence time is in the order of time to forward a message on the longest path. Distributed Computing Group Computer Networks R. Wattenhofer

18. Intuition for Slow Convergence p a p:p b c d e f a:p a:p b:ap a:p c:ap a:p d:ap a:p e:ap Distributed Computing Group Computer Networks R. Wattenhofer

19. Intuition for Slow Convergence p Os a p:p b c d e f a:p a:p b:ap a:p c:ap a:p d:ap a:p e:ap Distributed Computing Group Computer Networks R. Wattenhofer

20. Intuition for Slow Convergence p a p:p W W W W W b c d e f a:p a:p b:ap a:p c:ap a:p d:ap a:p e:ap Distributed Computing Group Computer Networks R. Wattenhofer

21. Intuition for Slow Convergence a W c d e a:p c:ap Distributed Computing Group Computer Networks R. Wattenhofer

22. Intuition for Slow Convergence a dcap c d e a:p c:ap Distributed Computing Group Computer Networks R. Wattenhofer

23. Intuition for Slow Convergence p O.1s a p:p W cbap dcap edap b c d e f a:p a:p b:ap a:p c:ap a:p d:ap a:p e:ap Distributed Computing Group Computer Networks R. Wattenhofer

24. Intuition for Slow Convergence p O.2s a - W cbap dcap edap b c d e f - b:ap c:ap d:ap e:ap d:cap e:dap c:bap Distributed Computing Group Computer Networks R. Wattenhofer

25. Intuition for Slow Convergence p 30s!!! a - W dcbap edcap b c d e f - - c:bap d:cap e:dap Distributed Computing Group Computer Networks R. Wattenhofer

26. Intuition for Slow Convergence p 60s a - W edcbap b c d e f - - - d:cbap e:dcap Distributed Computing Group Computer Networks R. Wattenhofer

27. Intuition for Slow Convergence p 90s a - W b c d e f - - - - e:dcbap Distributed Computing Group Computer Networks R. Wattenhofer

28. Intuition for Slow Convergence p a b c d e f Convergence in the time to forward a message on the longest path. Distributed Computing Group Computer Networks R. Wattenhofer

29. The longest path… p a g e b h f d c j i Distributed Computing Group Computer Networks R. Wattenhofer

30. … is NP-complete (APX) p a g e b h f d c j i Distributed Computing Group Computer Networks R. Wattenhofer

31. Example of BGP Convergence Time BGP Message/Event 10:40:30 2129 withdraws p 10:41:08 2117 announces 5696 2129 p 10:41:32 2117 announces 1 5696 2129 p 10:41:50 2117 announces 2041 3508 3508 4540 7037 1239 5696 2129 p 10:42:17 2117 announces 1 2041 3508 3508 4540 7037 1239 5696 2129 p 10:43:05 2117 announces 2041 3508 3508 4540 7037 1239 6113 5696 2129 p 10:43:35 2117 announces 1 2041 3508 3508 4540 7037 1239 6113 5696 2129 p 10:43:59 2117 withdraws p Distributed Computing Group Computer Networks R. Wattenhofer

32. Remember the Example p a W b c d e f edap edcap edcbap Distributed Computing Group Computer Networks R. Wattenhofer W

33. What might help? • Idea: Attach a “cause tag” to the withdrawal message identifying the failed link/node (for a given prefix). • It can be shown that a cause tag reduces the convergence time to the shortest path • Problems • Since BGP is widely deployed, it cannot be changed easily • ISP’s (AS’s) don’t like the world to know that it is their link that is not stable, and cause tags do exactly that. • Race conditions make the cause tags protocol intricate Distributed Computing Group Computer Networks R. Wattenhofer

34. Example with BGP-CT (Cause Tags) p a p:p b c d e f a:p a:p b:ap a:p c:ap a:p d:ap a:p e:ap Distributed Computing Group Computer Networks R. Wattenhofer

35. Example with BGP-CT p Os a p:p b c d e f a:p a:p b:ap a:p c:ap a:p d:ap a:p e:ap Distributed Computing Group Computer Networks R. Wattenhofer

36. Example with BGP-CT p O.1s a p:p W(ap) W(ap) W(ap) W(ap) W(ap) b c d e f a:p a:p b:ap a:p c:ap a:p d:ap a:p e:ap Distributed Computing Group Computer Networks R. Wattenhofer

37. Convergence Time using Cause Tags b c p x e f Distributed Computing Group Computer Networks R. Wattenhofer

38. Convergence Time using Cause Tags b c p x e f Distributed Computing Group Computer Networks R. Wattenhofer

39. Convergence Time using Cause Tags b c p x e f Convergence in the time to forward a message on the new shortest path (instead of the longest). Distributed Computing Group Computer Networks R. Wattenhofer