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BGP Safety with Spurious Updates The Conditions of BGP Convergence

BGP Safety with Spurious Updates The Conditions of BGP Convergence. Martin Suchara in collaboration with: Alex Fabrikant and Jennifer Rexford. Local Control vs. Global Properties. The Internet is a “network of networks” ~35,000 independently administered ASes

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BGP Safety with Spurious Updates The Conditions of BGP Convergence

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  1. BGP Safety with Spurious UpdatesThe Conditions of BGP Convergence Martin Suchara in collaboration with: Alex Fabrikant and Jennifer Rexford

  2. Local Control vs. Global Properties • The Internet is a “network of networks” • ~35,000 independently administered ASes • Competitive cooperation to find routes Local Control Interdomain policies, Intradomainrouting Global Properties Stability, reliability, performance, etc.

  3. Interdomain Routing • Autonomous systems (AS) have different goals • Different views on which path is best • Interdomainrouting: agree on a set of paths 2 1 4 3 Prefer 5346 to 5326 5 Prefer 326 to 346 6 Has to use : 5 → 3 → 2 → 6 client

  4. The Border Gateway Protocol (BGP) • ASesexchange information about paths “I can reach d via AS 3” “I can reach d” 1 2 3 Data traffic d • Policy configurations provided by AS operators • Path selection: which path do I choose? • Path export: which neighbors do I tell?

  5. Business Driven Policies of ASes • Customer-Provider Relationship • Provider exports its customer’s routes to everybody • Customer exports provider’s routes only to downstream customers • Peer-Peer Relationship • Export only customer routers to a peer • Export peer routes only to customers

  6. BGP Safety Challenges • 35,000 ASes and 300,000 address blocks • Flexible AS policies • Routing convergence usually takes minutes • But the system does not always converge… Prefer 120 to 10 Prefer 210 to 20 1 2 Use 20 Use 210 Use 120 Use 10 0 d

  7. Results on BGP Safety • Safety verification important to network operators • Absence of a “dispute wheel” sufficient for safety (Griffin, Shepherd, Wilfong, 2002) • Necessary or sufficient conditions of safety (Gao and Rexford, 2001), (Gao, Griffin and Rexford, 2001), (Griffin, Jaggard and Ramachandran, 2003), (Feamster, Johari and Balakrishnan, 2005), (Sobrinho, 2005), (Fabrikant and Papadimitriou, 2008), (Cittadini, Battista, Rimondini and Vissicchio, 2009), …

  8. Models of BGP • Existing models (variants of SPVP) • Widely used to analyze BGP properties • Simple but do not capture spurious behavior of BGP • This work • A new model of BGP with spurious updates • Spurious updates have major consequences • More accurate model makes proofs easier!

  9. Overview • Classical model of BGP: the SPVP Spurious BGP updates: what are they? The surprise: networks believed to be safe oscillate! The consequences: applicability of earlier results Convergence conditions: polynomial time verifiable Conclusion

  10. SPVP– Traditional Model of BGP (Griffin and Wilfong, 2000) The higher the more preferred Permitted paths 120 10 ε 210 20 ε Always includes the empty path 1 2 0 The destination Network topology

  11. SPVP– Traditional Model of BGP (Griffin and Wilfong, 2000) 120 10 ε 210 20 ε Selected paths 1 2 0

  12. SPVP– Traditional Model of BGP (Griffin and Wilfong, 2000) 120 10 ε 210 20 ε Activation 1 2 0 • Activation models the processing of BGP update messages sent by neighbors • Vertex or edge activations

  13. SPVP– Traditional Model of BGP (Griffin and Wilfong, 2000) Switch to best available 120 10 ε 210 20 ε Activation 1 2 0 • Activation models the processing of BGP update messages sent by neighbors • Vertex or edge activations

  14. SPVP– Traditional Model of BGP (Griffin and Wilfong, 2000) Switch to best available 120 10 ε 210 20 ε Activation 1 2 0 • System is safe if all “fair” activation sequences lead to a stable path assignment

  15. Overview • Classical model of BGP: the SPVP Spurious BGP updates: what are they? The surprise: networks believed to be safe oscillate! The consequences: applicability of earlier results Convergence conditions: polynomial time verifiable Conclusion

  16. What are Spurious Updates? • A phenomenon: router announces a route other than the highest ranked one Spurious BGP update 230: 1230 10 210 20 230 230 Selected path: 20 1 2 30 3 0 • Behavior not allowed in SPVP

  17. What Causes Spurious Updates? • Limited visibility to improve scalability • Internal structure of ASes • Cluster-based router architectures • Timers and delays to prevent instabilities and reduce overhead • Route flap damping • MRAI timers • Grouping updates to priority classes • Finite size message queues in routers

  18. Cause 1 – Limited Visibility • The internal structure of ASes improves scalability while reducing visibility Autonomous system (AS) RR 5 Route reflector RR r1 r2 r3 r1 r2 r3 Router A A B ri Route announcement r1 r3 r2 r1 r3 1 2 3 4 • After route r1 is withdrawn, router B temporarily announces r3

  19. Cause 1 – Limited Visibility • The internal structure of ASes improves scalability while reducing visibility Autonomous system (AS) RR 5 Route reflector RR r1 r2 r3 Router A A B ri Route announcement ε r2 r1 r3 1 2 3 4 • After withdrawals of routes r1 and r3, router B temporarily withdraws the route

  20. Cause 2 – Delays • Route flap damping temporarily suppresses all routes learned from a neighbor Autonomous system (AS) 4 Router A r1 r2 r3 r1 r2 r3 A ri Route announcement r2 r3 r3 r2 →r1 2 3 1 • After the update r2→r1 the less preferred route r3 is temporarily selected

  21. DPVP– A More General Model of BGP • DPVP = Dynamic Path Vector Protocol • Generalizes the earlier model (SPVP) • Spurious update with a less preferred route that was recently available • Spurious updates allowed in transient period τafter last route change • Safety is independent of numerical value τ

  22. DPVP– A More General Model of BGP The permitted paths and their ranking Spurious update Selected path: 210 120 10 ε 210 20 ε Remember all recently available paths (e.g. 20, 210) 20 1 2 StableTime = τ after last path change 0 • Spurious updates are allowed only if current time < StableTime • Spurious updates may include paths that were recently available or the empty path

  23. DPVP– A More General Model of BGP • Behavior captured irrespective of cause • Simple future-proof model independent of underlying network technologies • For every allowed spurious behavior in DPVP we can find a possible cause • Details in our technical report: • TR-881-10, Dept. of Comp. Sci., Princeton, July 2010 • www.cs.princeton.edu/~msuchara/publications.html

  24. Overview • Classical model of BGP:the SPVP Spurious BGP updates: what are they? The surprise: networks believed to be safe oscillate! The consequences: applicability of earlier results Convergence conditions: polynomial time verifiable Conclusion

  25. Consequences of Spurious Updates • Spurious behavior is temporary • Tempting to conclude that it cannot have long term consequences • The surprise: spurious behavior may trigger permanent oscillations!

  26. The Surprise: Spurious Announcements Trigger Permanent Oscillations! • Safe instance in all classical models of routing: 210 20 2 130 10 1 Stable outcome 0 3210 320 310 30 3 • Permanent oscillations with spurious behavior

  27. Example of Oscillation 210 20 RR 2 B 130 10 1 A 0 Autonomous system (AS) 1 3210 320 310 30 3 Route reflector RR Router A AS activation 1 ε Route announcement

  28. Example of Oscillation 210 20 RR 2 B 130 10 1 A 0 Autonomous system (AS) 1 3210 320 310 30 3 Route reflector RR Router A AS activation 1 ε Route announcement

  29. Example of Oscillation 210 Did not learn about withdrawal of 210 yet 210 20 RR 2 B 130 10 ε 1 Does not know any route 210 A ε 0 Autonomous system (AS) 1 3210 320 310 30 3 Route reflector RR Router A AS activation 1 ε Route announcement

  30. Example of Oscillation 210 Did not learn about withdrawal of 210 yet 210 20 RR 2 B 130 10 ε 1 Does not know any route 210 A ε 0 Autonomous system (AS) 1 3210 320 310 30 3 Route reflector RR Router A AS activation 1 ε Route announcement

  31. Example of Oscillation 210 Did not learn about withdrawal of 210 yet 210 20 RR 2 B 130 10 ε 1 Does not know any route 210 A ε 0 Autonomous system (AS) 1 3210 320 310 30 3 Route reflector RR Router A AS activation 1 ε Route announcement

  32. Example of Oscillation 20 210 20 RR 2 B 130 10 20 1 20 A 0 Autonomous system (AS) 1 3210 320 310 30 3 Route reflector RR Router A AS activation 1 ε Route announcement

  33. Example of Oscillation 210 20 RR 2 B 130 10 1 A 0 Autonomous system (AS) 1 3210 320 310 30 3 Route reflector RR Router A AS activation 1 ε Route announcement

  34. Example of Oscillation 210 20 RR 2 B 130 10 1 A 0 Autonomous system (AS) 1 3210 320 310 30 3 Route reflector RR Router A AS activation 1 ε Route announcement

  35. Example of Oscillation 210 20 RR 2 B 130 10 1 A 0 Autonomous system (AS) 1 3210 320 310 30 3 Route reflector RR Router A AS activation 1 ε Route announcement

  36. Example of Oscillation 210 20 RR 2 B 130 10 1 A 0 Autonomous system (AS) 1 3210 320 310 30 3 Route reflector RR Router A AS activation 1 ε Route announcement

  37. Consequences of Spurious Updates • Temporary behavior may cause permanent oscillations • The number of oscillating nodes and / or frequency of oscillations may increase • Which results do not hold in the new model?

  38. Overview • Classical model of BGP:the SPVP Spurious BGP updates: what are they? The surprise: networks believed to be safe oscillate! The consequences: applicability of earlier results Convergence conditions: polynomial time verifiable Conclusion

  39. Which SPVP Results Hold in DPVP? • Most previous results in SPVP also hold for DPVP • Formal justification later in the talk • Some results cannot be extended • Slightly different conditions of convergence • Exponentially slower convergence possible

  40. Case Study IDifferent Conditions of Convergence • Safety under filtering: is instance safe under any filtering? 3210 320 310 30 Filter arbitrary subset of routes 3 Subgraph with specific properties • Absence of a “dispute reel” necessary and sufficient for safety under filtering in SPVP (Cittadini et al., 2009) • Our result: permanent oscillations in DPVP even without a reel

  41. Case Study I Different Conditions of Convergence • Example of a “safe” topology, Cittadini et al.: 320 1320 10 130 3210 30 320 1 3 130 0 210 2130 20 210 2 • Spurious updates cause oscillations

  42. Case Study IIExponential Slowdown of Convergence • BGP converges in 2l + 2 phases(Sami et al., 2009) • l: length of longest customer-provider chain • Phase: each node processes and sends updates • Assumes standard business relationships • With spurious updates exponential slowdown to (2k + 1)l-2phases • k: max. # of spurious updates after route change

  43. Overview • Classical model of BGP:the SPVP Spurious BGP updates: what are they? The surprise: networks believed to be safe oscillate! The consequences: applicability of earlier results Convergence conditions: polynomial time verifiable Conclusion

  44. Convergence Conditions • Absence of a “dispute wheel” sufficient for safety in SPVP (Griffin, Shepherd, Wilfong, 2002) • One of the most cited results • Absence of a “dispute wheel” is still sufficient for safety in DPVP • Most of the previous results of the past decade still hold under DPVP! • Other stronger results in DPVP next

  45. Why are Proofs Easier in DPVP? • No need to prove that: • Announced route is the highest ranked one • Announced route is the last one learned from the downstream neighbor • We changed the problem • PSPACE complete vs. NP complete • Next the necessary and sufficient conditions

  46. Necessary and Sufficient Conditions • How can we prove a system may oscillate? • Classify each node as “stable” or “coy” • At least one “coy” node exists • Prove that “stable” nodes must be stable • Prove that “coy” nodes may oscillate Easy in a model with spurious announcements • Next: a formal definition of a construction that captures this intuition

  47. Necessary and Sufficient Conditions • Definition:CoyOTEisatriple(C, S, Π) satisfyingseveralconditions 1230 10 Coy nodes may make spurious announcements 210 20 230 1 2 30 Stable nodes have a permanent path 3 One path assigned to each node proves if the node is coy or stable 0 • Theorem: DPVP oscillates if and only if it has a CoyOTE

  48. Necessary and Sufficient Conditions Definition: CoyOTE satisfies these conditions: The best stable path assigned to each stable node 1230 10 210 20 230 1 2 • Coy node is assigned a coy path: • more preferred than the best stable path • consistent with the paths of stable nodes 30 3 0 = coy Origin is stable = stable

  49. Verifying the Convergence Conditions = Finding a CoyOTE • In general an NP-hard problem • Compact inputs with regular expressions • Can be checked in polynomial time for most “reasonable” network configurations! e.g.

  50. DeCoy – Safety Verification Algorithm • Goal: verify safety in polynomial time • Main idea: find the maximal stable set S by expanding it in a greedy fashion • If all nodes are stable system is safe • Otherwise system may oscillate

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