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Swarming Overlay Construction Strategies

Swarming Overlay Construction Strategies. Anwar Al Hamra 1 , Nikitas Liogkas 2 , Arnaud Legout 3 , Chadi Barakat 3 1 Hariri Canadian University, Damour, Lebanon 2 Knight Equity Markets, L.P., Jersey City, NJ, USA 3 INRIA, Projet Planète, Sophia Antipolis, France. BitTorrent Overview.

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Swarming Overlay Construction Strategies

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  1. Swarming Overlay Construction Strategies Anwar Al Hamra1, Nikitas Liogkas2, Arnaud Legout3, Chadi Barakat3 1Hariri Canadian University, Damour, Lebanon 2Knight Equity Markets, L.P., Jersey City, NJ, USA 3 INRIA, Projet Planète, Sophia Antipolis, France

  2. BitTorrent Overview Web server • Peer set (neighbor set) as defined in mainline 4.0.2 • Maximum 80 connections in total • Maximum 40 outgoing connections • Results in a well connected graph • Recent peers get a chance to be connected to old peers Get a .torrent file that contains the address of the tracker random peer set Tracker

  3. Peer Bootstrap Strategies • Three strategies • Tracker • Default method, used by all clients • DHT • Distributed tracker • Leads to the same overlay as the tracker strategy • PEX (Peer EXchange) • Implemented in most popular clients

  4. Peer Bootstrap: Current Practice • Current clients use a mix of tracker + DHT + PEX • We focus on the tracker only strategy • Classical bootstrap strategy • Can be used in another context • Fundamental to understand its properties

  5. BitTorrent Overlay Structure • Common wisdom is that BT builds random graphs with the tracker strategy • Robust to churn • Low diameter D • Important for piece selection • Does BT build a random graph?

  6. Tracker Connectivity Matrix • 1000 peers • 40 maximum number of outgoing connections • Not a random graph • Still well connected • 2000 connections between the first 80 peers and the rest of the torrent 192 610 Peers ordered by arriving time

  7. Preemption • Modification of • Tracker strategy • Just requires client support • Backward compatible • When maximum peer set size reached • Always allows incoming connections from peers redirected by the tracker

  8. Preemption OK New peer Accept me in your peer set Tracker only Preemption • Preemption enables rewiring of the graph

  9. Connectivity Matrices Omax=40 • Preemption • Tracker

  10. Focus of the Presentation • Impact of Omax during the flash crowd phase • Churn might improve overlay structure • But, flash crowd is the most critical phase • Single seed • Overlay structure dictates pieces propagation • Impact of Omax during flash crowd on the overlay structure obtained with • Tracker • Preemption

  11. Outline • Background • Impact of Omax on BitTorrent overlay properties • Connectivity matrices • Metrics

  12. Methodology • Simulations with 1000 peers • Validated with experiments • Omax: 5 to 80 • Max peer set size: 80 • Tracker returns 50 peers • Flash crowd phase • Before the first peer leaves the torrent

  13. Connectivity Matrices Omax=20 • Preemption • Tracker

  14. Connectivity Matrices Omax=40 • Preemption • Tracker

  15. Connectivity Matrices Omax=60 • Tracker • Preemption

  16. Connectivity Matrices Omax=80 • Tracker • Preemption Partition

  17. Outline • Background • Impact of Omax on BitTorrent overlay properties • Connectivity matrices • Metrics

  18. Metrics • Bottleneck index • Ratio of the number of connections between the first 80 peers and the rest of the peers, to the maximum possible number of such connections (80 ¤ 80 = 6400 connections) • Indication of the presence of a bottleneck between the initial seed peer set and the rest of the torrent

  19. Metrics • Average peer set size • Average number of neighbors for all peers • Diameter • Longest shortest path between any two peers

  20. Results

  21. Results partition

  22. Which Omax? • Tracker strategy • 20 to 40 gives a good tradeoff • 40 to 80 is used in current clients • Preemption • 80 is optimal for the three metrics • With preemption no more the need to define a constraint on Omax

  23. Conclusion • The BitTorrent overlay is not random with tracker only strategy • Specific structure • Omax is an important parameter • Preemption strategy improves overlay structure • No more need to set Omax • Outperforms tracker strategy for all 3 metrics

  24. Advertisement: If you are interested in P2P locality, have a look atStevens Le Blond, Arnaud Legout, Walid Dabbous. Pushing BitTorrent Locality to the Limit. Technical Report (inria-00343822, version 2 - 12 May 2009), INRIA, Sophia Antipolis, May 2009.http://hal.inria.fr/inria-00343822/en/ Thank you! Questions? Simulator available at: http://planete.inria.fr/software/BitSim/

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