Swarming Overlay Construction Strategies

Anwar Al Hamra1, Nikitas Liogkas2, Arnaud Legout3, Chadi Barakat3

1Hariri Canadian University, Damour, Lebanon2Knight Equity Markets, L.P., Jersey City, NJ, USA3 INRIA, Projet Planète, Sophia Antipolis, France

BitTorrent OverviewWeb

server

Tracker

Get a .torrent file that contains the address

of the tracker

random peer set

2

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

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

3

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

4

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?

5

Tracker Connectivity Matrix

1000 peers40 maximum

number of outgoing connections

Not a random graphStill well connected

2000 connections between the first 80 peers and the rest of the torrent

610

192

Peers ordered by arriving time

6

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

7

PreemptionNew peer

8

Tracker only

Accept me in your peer set

Preemption

OK

Preemption enables rewiring of the graph

Connectivity Matrices Omax=40

PreemptionTracker

9

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

10

Outline

BackgroundImpact of Omax on BitTorrent overlay

properties Connectivity matrices Metrics

11

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

12

Connectivity Matrices Omax=20

PreemptionTracker

13

Connectivity Matrices Omax=40

PreemptionTracker

14

Connectivity Matrices Omax=60

Tracker Preemption

15

Connectivity Matrices Omax=80

Tracker PreemptionPartition

16

Outline

BackgroundImpact of Omax on BitTorrent overlay

properties Connectivity matrices Metrics

17

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

18

Metrics

Average peer set size Average number of neighbors for all

peersDiameter

Longest shortest path between any two peers

19

Results

20

Results

partition

21

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

22

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 23

Thank you!

Questions?

Simulator available at:http://planete.inria.fr/software/BitSim/

Advertisement: If you are interested in P2P locality, have a look at

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http://hal.inria.fr/inria-00343822/en/