Post on 01-Feb-2016
description
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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
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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
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Connectivity Matrices Omax=20
PreemptionTracker
13
Connectivity Matrices Omax=40
PreemptionTracker
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Connectivity Matrices Omax=60
Tracker Preemption
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Connectivity Matrices Omax=80
Tracker PreemptionPartition
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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
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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
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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
Stevens 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/