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Interdomain Routing

Explore the complexities of interdomain routing and the challenges faced in achieving stable paths. Discover potential research directions to improve protocol behavior and address issues like persistent oscillations and routing loops.

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Interdomain Routing

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  1. Interdomain Routing Gordon Wilfong Distinguished Member of Technical Staff Algorithms Research Department Mathematical and Algorithmic Sciences Center Bell Laboratories February, 2006

  2. Current Internet Routing • Autonomous Systems (ASes) • Subnetwork under single administrative control • Each AS has its own economic incentives to: • cooperate so as to achieve connectivity and to abide by economic contracts (SLAs) • to hide information about its internal network • Routing between ASes achieved by protocol where • AS selectively announces to neighbors its chosen route to destinations • AS selects highest ranked route announced to it

  3. Border Gateway Protocol (BGP) “The route I use to d goes through AS0” AS1 AS2 Data packets “The route I use to d goes through AS1 then AS0” “I route directly to d” AS3 AS0 d

  4. Routing Protocol Requirements • Protocol must scale • Routing table entries (destinations): ~150,000 and increasing • Autonomous Systems: ~20,000 and increasing • Administrators require expressiveness for routing policies • Local AS policy (based on SLAs, traffic engineering etc) determines • route selection (effects where AS sends packets) • route announcements (effects how/if packets from other ASes traverse it)

  5. E-BGP External BGP (E-BGP) is the mode of BGP that propagates routes between autonomous systems.

  6. Stable Paths Problem 3 2 0 3 1 0 3 2 1 0 310 3 1 0 210 2 1 0 2 0 1 0 1 2 0 1 2 0 0

  7. Independent policies can lead to nonconvergence 1 2 0 1 0 1 Each AS chooses “favorite” available route. Bad Gadget 0 0 3 1 0 3 0 2 3 0 2 0 3 2

  8. May Be Multiple Solutions 1 2 0 1 0 1 2 0 1 0 1 2 0 1 0 1 1 1 0 0 0 2 2 2 2 1 0 2 0 2 1 0 2 0 2 1 0 2 0 First solution Second solution DISAGREE

  9. What can be done? • Modify protocol • Design a protocol that has provably good properties. e.g., [Griffin, W.] • Constrain topology and/or policy • Prove good protocol behavior assuming constrained configurations. e.g., [Gao, Rexford] • These constraints give rise to interesting “valley-free” routing problems (from customer to local provider to regional provider to national provider and then down chain again to another customer). e.g. [Erlebach et al]

  10. Possible Research Directions • Complexity issues • What is the complexity of deciding if a configuration is “safe” (i.e., will it always converge)? • NP-complete to determine if there is a solution it can converge to [Griffin, W.] • Extending model • Model assumes destinations can be considered independently • Aggregation has not been taken into consideration • Trade-offs between protocol expressivity and safety • Formal model to study such trade-offs

  11. I-BGP Internal BGP (I-BGP) is the protocol used to propagate external routes within an autonomous system.

  12. Fully Meshed A router only announces its own routes RR RR RR RR RR RR

  13. RR RR RR RR RR RR Route Reflectors Route Reflectors must be fully meshed Route Reflectors pass along updates to client routers

  14. Highest Local Preference Enforce relationships Shortest ASPATH Lowest MED (if same next AS) Traffic engineering I-BGP < E-BGP Lowest IGP cost to BGP egress Throw up hands and break ties Lowest router ID Route Selection Summary

  15. What Are MEDs? AS1 CP MED=2 MED=0 MED=1 AS3 AS2 “Hot Potato Routing” vs. “Cold Potato Routing” Client

  16. Persistent Route Oscillations 1 B A 120 50 40 r1 r3 MED=1 r2 MED=1 MED=2 135.104.54.4

  17. Oscillations in I-BGP 1 RR3 AS0 RR1 RR2 1 1 5 5 5 C2 C3 C1 Physical link P2 P3 P1 I-BGP connection 135.104.54.4

  18. 135.104.54.4 Deflections Packets can be diverted out of a network unexpectedly. P AS0 RR 3 1 A B 1 Q

  19. 135.104.54.4 Routing Loops Badly configured networks can also experience routing loops 5 1 RR2 RR1 AS0 1 1 C1 C2

  20. A Real-world MED Oscillation Example • A functioning network breaks into a state • of persistent route oscillations when a BGP • session goes down • First thought to be a hardware problem • Analysis shows that route oscillations caused by • the use of the MED attribute • The example was actually predicted by the • theoretical analysis [Griffin, W.]

  21. Initial State Only AS 2 sends MEDs to AS 4 AS 4 reflector reflector A B 70000 1 2 1 IGP 2 C D E F (2) (2) (1) MEDs AS 2 AS 3 I-BGP AS 1 E-BGP

  22. Initial Routing AS 4 reflector reflector A B 70000 1 2 1 IGP 2 C D E F (2) (2) (1) MEDs AS2 AS 3 I-BGP D prefers AS 2 path due to router ID tie breaking AS 1 E-BGP

  23. B Changes Its Route The AS 4  AS 3 BGP Session is dropped AS 4 reflector reflector A B 70000 1 2 1 IGP 2 C D E F session down! (2) (2) (1) MEDs AS 2 AS 3 I-BGP AS 1 E-BGP

  24. A Changes Its Route The MED 1 route from B beats the MED 2 routes that A sees from its clients…. AS 4 reflector reflector A B 70000 1 2 1 IGP 2 C D E F session down! (2) (2) (1) MEDs AS 2 AS 3 I-BGP CALL THIS STATE ZERO AS 1 E-BGP

  25. C & D Change Routes The MED 1 route from A knocks both MED 2 routes out of the picture for C & D … AS 4 reflector reflector A B 70000 1 2 1 IGP 2 C D E F session down! (2) (2) (1) MEDs AS 2 AS 3 I-BGP AS 1 E-BGP

  26. A Changes Route Again A now sees the route from D through AS 3, and it is closer IGP-wise than the route from B… AS 4 reflector reflector A B 70000 1 2 1 IGP 2 C D E F session down! (2) (2) (1) MEDs AS 2 AS 3 I-BGP AS 1 E-BGP

  27. C&D Return to Initial Routes C & D no longer see MED 1 route from A, so they return to the eBGP routes with MED 2… AS 4 reflector reflector A B 70000 1 2 1 IGP 2 C D E F session down! (2) (2) (1) MEDs AS 2 AS 3 I-BGP AS 1 E-BGP

  28. Back to State Zero! A switches back to MED 1 route through B. AS 4 reflector reflector A B 70000 1 2 1 IGP 2 C D E F session down! (2) (2) (1) MEDs AS 2 AS 3 I-BGP AS 1 E-BGP

  29. What Can Be Done? • Possible Approaches: • Use only configurations that guarantee no problems • No modification to BGP required • Previous example shows this might be difficult • Prevent problems for any configuration • Modification to BGP required, e.g. [Basu et al] • Minimize overhead (number of messages, memory requirements, …)

  30. Conclusions • BGP is extremely flexible, allowing operators to easily make obscure errors that are difficult to find and correct. • Policy based routing is difficult to get right. • Defining a formal model of path vector protocols helps to understand what features can cause problems and what can be done to avoid errors.

  31. What about the future? • BGP is the routing protocol in today’s internet • What about the future? • new Internet architectures • new architectures within an AS • (e.g., Routing Control Platform [J. Rexford, et al]) • In any case, a formal theoretical approach is essential to getting it right.

  32. Extras

  33. r2r13 r1r2r3 r1r2r3 r2r1r3 r2r1rr3 r3r2 r1r3 r1r3 r3r2 r1r3 Persistent Route Oscillations 1 B A 120 50 40 r1 r3 MED=1 r2 MED=1 MED=2 135.104.54.4

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