A Measurement Study of Piece Population in BitTorrent

Cameron Dale and Jiangchuan Liu A Measurement Study of Piece Population in BitTorrent

Introduction BitTorrent Experiment Results Simulation Discussion

A Measurement Study ofPiece Population in BitTorrent

Cameron Dale and Jiangchuan LiuSimon Fraser University

Burnaby, BC, Canada

[email protected]

Globecom, November 29th, 2007, Washington, D.C.



Overview

● background on some BitTorrent features

● describe the experiment

● results for piece population snapshots and the evolution of the piece population for some real BitTorrent swarms*

● PlanetLab simulations of BitTorrent swarms

● discussion and future work*swarm: a group of peers connected to one another via the

BitTorrent file distribution protocol



Piece Population

● BitTorrent splits a large file into many pieces

● each piece will have a certain number of copies in all the peers in the swarm

● ideally, all pieces should have approximately the same number of copies

● local knowledge limitations prevent the ideal and introduce an imbalance

● too much variation can lead to inefficiencies and even starvation



Rarest-First Policy

● responsible for choosing the next piece that a peer will download

● each client chooses the piece that it believes is the rarest one

● choices are made based only on local knowledge of neighboring peers

● goal is to maintain an even distribution of pieces throughout the swarm



Piece Population Distribution

● expect the population to form a distribution around a mean value

● the mean will be determined solely by the macro-characteristics: peer arrival rate, download time, departure rate, etc.

● the width of the distribution will indicate the effectiveness of the rarest-first policy– the narrower, the better



Real Swarm Snapshots

● used a modified BitTorrent client to gather data from real swarms

● client constantly requests peers from the tracker* and connects to all returned peers

● client connects to most (90%) peers in a swarm and collects their piece information

● collection occurs very quickly and is terminated, usually after less than an hour

*tracker: a server that coordinates communication between peers attempting to download a file



Real Swarm Evolution

● collection was done as in the snapshots

● collection continued for many hours, or even days

● observe changes in the Piece Population as the swarm evolves

● focussed on the early stages of a swarm when there are a large number of leechers*

*leecher: a user who does not yet have a complete copy of the file, and so is currently downloading (and uploading)



Swarm Characteristics

*clients: the number of clients administered by us

Torrent Name Pieces Size (MB) Leechers Clients*

KNOPPIX 4125 4325 169 10

FreeBSD 5699 1494 34 10

mandriva 2803 735 89 9

openSUSE 14805 3881 398 9

feisty 1387 727 65-120 20

openSUSE-2 14977 3926 100-150 18

PlanetLab 1497 784 0-340 340

PlanetLab-2 1497 784 0-390 390



Piece Population Plots

● x-axis shows the number of copies within the downloaders in the swarm

● y-axis shows the number of pieces that have that number of copies

● x-axis is normalized to range from 0 to 1, by the total number of downloaders

● y-axis can be normalized by the number of pieces, or so that the area under the graph is 1 (to facilitate comparisons)



Real Internet Swarm Snapshots



Evolution: feisty



Evolution: feisty



Evolution: feisty



Evolution: feisty



Evolution: openSUSE-2












Simulated Swarms

● run on PlanetLab research network testbed

● all peers are controlled by us

● experiment starts with a single seed* and peers join randomly over first 4 hours

● PlanetLab: identical peers, no peers join after 4 hours

● PlanetLab-2: distribution of peers, new peers arriving continuously

*seed: a peer that has the complete file, and is only uploading



Simulation: PlanetLab

● all peers are identical

● peers stay in the system for about 9 hours, then leave forever

● each peer's maximum number of connections is limited to 40 to enhance the local effect

● it takes 9 hours for the initial seed to upload a single copy of the file















Simulation: PlanetLab-2

● peers have a distribution of download and upload speeds

● peers download the file, seed for a random time, then leave and rejoin as a new peer

● arrivals and departures are grouped together to increase the amount of churn*

● each peer's maximum number of connections is limited to 80 (default)

*churn: the arrival and departure of peers in a P2P system












Rarest-first Policy

● is effective– results in normal distributions that are narrow

● improves on random piece selection by 3-4x

– recovers quickly (exponentially) from events that cause a widened distribution

● could be improved– larger swarms show tails on the high end due to

the limited local knowledge of peers– churn causes increases in the width of the

population



Suggested Improvement

● peers knowledge of pieces could be extended through gossiping– share all piece information with neighbors

● increased communication cost

– share information on which pieces are the rarest●piece with the most votes gets downloaded next

● should reduce the size of the tails for the larger swarms



Future Work

● use these measurements to evaluate the effect of new piece selection strategies

● further analysis of swarms in the presence of churn to determine the cause of the increased population width

● work is already under way to create an analytical piece-level model describing the piece population of a swarm– complicated due to the complex interactions

between peers



A Measurement Study ofPiece Population in BitTorrent

Cameron Dale and Jiangchuan LiuSimon Fraser University

Burnaby, BC, Canada

[email protected]

Globecom, November 29th, 2007, Washington, D.C.

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A Measurement Study of Piece Population in BitTorrent

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