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. Get a .torrent file that contains the address of the tracker
BitTorrent Overview
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
Web server
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
610
192
Peers ordered by arriving time

7. Preemption Modification of When maximum peer set size reached
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. OK Preemption Tracker only Preemption
New peer
Accept me in your peer set
Tracker only
Preemption
Preemption enables rewiring of the graph

9. Connectivity Matrices Omax=40
Tracker
Preemption

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 Flash crowd phase
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
Tracker
Preemption

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 Diameter
Average number of neighbors for all peers
Diameter
Longest shortest path between any two peers

20. Results
Averages over 10 runs. Errorbars represent the min and max

21. Results
partition

22. Which Omax? Tracker strategy Preemption 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 at Stevens Le Blond, Arnaud Legout, Walid Dabbous. Pushing BitTorrent Locality to the Limit. Technical Report (inria , version May 2009), INRIA, Sophia Antipolis, May
Thank you!
Questions?
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