XL: An Efficient Network Routing Algorithm

1. XL: An Efficient NetworkRouting Algorithm • Kirill Levchenko • Geoffrey M. Voelker, Ramamohan Paturi, and Stefan Savage • University of California San Diego presented by Pierre-Élie Fauché 1

2. Routing • Getting from point A to point B • Need to know some state of the network • Today we do this by flooding 2

3. link changes here everyone gets update anyway does notneed toknow Flooding 3

4. Flooding • Number of updates per node grows with the number of links • Slowest link or node effectively limits sustainable size of an AS network • Is flooding inherent to routing? 4

5. Outline • Selective flooding • XL update propagation rules • Simulation results • Conclusion 5

6. Selective FloodingIdea A: Artificial Hierarchy • Manually restrict scope of updates • Example: OSPF areas • “Considered harmful” • Results in sub-optimal routing • Hard to adapt to growth 6

7. Reducing UpdatesIdea B: Bound Radius • Idea: limit update scope by distance • Drawback: not always correct • Distant links may be important • Greatly delays convergence 7

8. Goals • Automatically limit update scope • Formal correctness • Loop-free routing • All destinations reachable • Bounded stretch 8

9. Reducing UpdatesIdea XL: Selective Updates • Idea: Link state with selective update propagation • Need to know which updates are necessary and which can be suppressed 9

10. Propagation Rules • Always propagate a link cost increase. • Neighbor should know best cost to destination if it is the next hop. • Ensures distance estimate decreases along forwarding path • Path cost finite → no long-term loops 10

11. Propagation Rules • Propagate update if it significantly improves some route. • Guarantees all connected nodes are reachable and stretch is bounded • Bonus: stretch can be set per-destination or in response to network load • Under “normal” conditions set stretch to 1.0 (optimal) 11

12. Applying the Rules • Different link state tables (external views) for each neighbor • Internal view consists of most recent information from neighbors’ external views 12

13. Applying the Rules • Compute forwarding table using internal view 13

14. Applying the Rules • Propagate update to neighbor’s view if: • The link cost increases • Link cost decreased and neighbor is next hop to link • Cost decreased and new route much better 14

15. Examplestretch 1.5 Rule S1 (link increase must be propagated) {CD: 1 → ∞} 15

16. ☺ Examplestretch 1.5 No rules apply: update suppressed Rule C1 (significant improvement) {CD: ∞ → 1} A-B-C-D: 3 (actual path) 16

17. Goals • Loop-free routing • All destinations reachable • Bounded stretch 17

18. Simulation Model • No traffic or propagation delays • Poisson failure model • 1 day mean time to failure • 1 hour mean time to recovery • Flapping failure model • 2 days mean time to failure • High probability of repeat failure 18

19. Simulation Networks • Crown64 — crown-like ring (192 nodes) • H. 16 × 16 — honeycomb grid (289 nodes) • Q. 16 × 16 — square grid (576 nodes) • Abilene — Abilene backbone (11 nodes) • AS 1221 — Telstra (104 nodes) • AS 1239 — Sprint (315 nodes) 19

20. Simulated Algorithms • Distance Vector (e.g. RIP) • Link State (e.g. OSPF) • Distance Vector with Parent Pointer • fixes “counting-to-infinity” by sending shortest-path tree in addition to distances • Link Vector • SPT-based like Distance Vector with Parent Pointer 20

21. Updates per DayPoisson failure model □ Relative to Link State Crown64 H. 16 × 16 Q. 16 × 16 Abilene AS 1221 AS 1239 21

22. Updates per DayFlapping failure model □ Relative to Link State 10x Crown64 H. 16 × 16 Q. 16 × 16 Abilene AS 1221 AS 1239 22

23. Transient Loop DurationPoisson failure model □ Relative to Link State Crown64 H. 16 × 16 Q. 16 × 16 Abilene AS 1221 AS 1239 23

24. Time to Find New PathPoisson failure model □ Relative to Link State Crown64 H. 16 × 16 Q. 16 × 16 Abilene AS 1221 AS 1239 24

25. Actual Stretch Poisson link failure model Average (over all pairs of nodes) Maximum (over all pairs of nodes) Crown64 H. 16 × 16 median 1.0 Q. 16 × 16 Abilene AS 1221 AS 1239 1.0 1.5 25

26. OSPF Compatibility • Observation: classic link state algorithm is a “special case” of the XL algorithm • OSPF satisfies the XL rules • XL could be mixed with OSPF for incremental deployment 26

27. Conclusion • Provable correctness • Bounded (user-specified) stretch • Up to 3-10x fewer updates • Compatible with the favorite link-state protocol — OSPF 27