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

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

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Flooding link changes here everyone gets update anyway does not need to know 3

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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

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Outline Selective flooding XL update propagation rules Simulation results Conclusion 5

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

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

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

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

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

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Propagation Rules 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) Propagate update if it significantly improves some route. 11

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

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Applying the Rules Compute forwarding table using internal view 13

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

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

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

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Goals ✓ Loop-free routing ✓ All destinations reachable ✓ Bounded stretch 17

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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

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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

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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

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Updates per Day Poisson failure model □ Relative to Link State Crown64H. 16 × 16Q. 16 × 16AbileneAS 1221AS

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Updates per Day Flapping failure model □ Relative to Link State Crown64H. 16 × 16Q. 16 × 16AbileneAS 1221AS x 22

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Transient Loop Duration Poisson failure model □ Relative to Link State Crown64H. 16 × 16Q. 16 × 16AbileneAS 1221AS

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Time to Find New Path Poisson failure model □ Relative to Link State Crown64H. 16 × 16Q. 16 × 16AbileneAS 1221AS

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

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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

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

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