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Cameron Dale and Jiangchuan LiuA Measurement Study of Piece Population in BitTorrent Introduction BitTorrent Experiment Results Simulation Discussion A Measurement Study of Piece Population in BitTorrent Cameron Dale and Jiangchuan Liu Simon Fraser University Burnaby, BC, Canada camerond@cs.sfu.ca Globecom, November 29 th, 2007, Washington, D.C.
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Cameron Dale and Jiangchuan LiuA Measurement Study of Piece Population in BitTorrent Introduction BitTorrent Experiment Results Simulation Discussion 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
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Cameron Dale and Jiangchuan LiuA Measurement Study of Piece Population in BitTorrent Introduction BitTorrent Experiment Results Simulation Discussion 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
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Cameron Dale and Jiangchuan LiuA Measurement Study of Piece Population in BitTorrent Introduction BitTorrent Experiment Results Simulation Discussion 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
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Cameron Dale and Jiangchuan LiuA Measurement Study of Piece Population in BitTorrent Introduction BitTorrent Experiment Results Simulation Discussion 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
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Cameron Dale and Jiangchuan LiuA Measurement Study of Piece Population in BitTorrent Introduction BitTorrent Experiment Results Simulation Discussion 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
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Cameron Dale and Jiangchuan LiuA Measurement Study of Piece Population in BitTorrent Introduction BitTorrent Experiment Results Simulation Discussion 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)
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Cameron Dale and Jiangchuan LiuA Measurement Study of Piece Population in BitTorrent Introduction BitTorrent Experiment Results Simulation Discussion Swarm Characteristics *clients: the number of clients administered by us
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Cameron Dale and Jiangchuan LiuA Measurement Study of Piece Population in BitTorrent Introduction BitTorrent Experiment Results Simulation Discussion 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)
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Cameron Dale and Jiangchuan LiuA Measurement Study of Piece Population in BitTorrent Introduction BitTorrent Experiment Results Simulation Discussion Real Internet Swarm Snapshots
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Cameron Dale and Jiangchuan LiuA Measurement Study of Piece Population in BitTorrent Introduction BitTorrent Experiment Results Simulation Discussion Evolution: feisty
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Cameron Dale and Jiangchuan LiuA Measurement Study of Piece Population in BitTorrent Introduction BitTorrent Experiment Results Simulation Discussion Evolution: feisty
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Cameron Dale and Jiangchuan LiuA Measurement Study of Piece Population in BitTorrent Introduction BitTorrent Experiment Results Simulation Discussion Evolution: feisty
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Cameron Dale and Jiangchuan LiuA Measurement Study of Piece Population in BitTorrent Introduction BitTorrent Experiment Results Simulation Discussion Evolution: feisty
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Cameron Dale and Jiangchuan LiuA Measurement Study of Piece Population in BitTorrent Introduction BitTorrent Experiment Results Simulation Discussion Evolution: openSUSE-2
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Cameron Dale and Jiangchuan LiuA Measurement Study of Piece Population in BitTorrent Introduction BitTorrent Experiment Results Simulation Discussion Evolution: openSUSE-2
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Cameron Dale and Jiangchuan LiuA Measurement Study of Piece Population in BitTorrent Introduction BitTorrent Experiment Results Simulation Discussion Evolution: openSUSE-2
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Cameron Dale and Jiangchuan LiuA Measurement Study of Piece Population in BitTorrent Introduction BitTorrent Experiment Results Simulation Discussion Evolution: openSUSE-2
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Cameron Dale and Jiangchuan LiuA Measurement Study of Piece Population in BitTorrent Introduction BitTorrent Experiment Results Simulation Discussion 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
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Cameron Dale and Jiangchuan LiuA Measurement Study of Piece Population in BitTorrent Introduction BitTorrent Experiment Results Simulation Discussion 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
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Cameron Dale and Jiangchuan LiuA Measurement Study of Piece Population in BitTorrent Introduction BitTorrent Experiment Results Simulation Discussion Simulation: PlanetLab
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Cameron Dale and Jiangchuan LiuA Measurement Study of Piece Population in BitTorrent Introduction BitTorrent Experiment Results Simulation Discussion Simulation: PlanetLab
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Cameron Dale and Jiangchuan LiuA Measurement Study of Piece Population in BitTorrent Introduction BitTorrent Experiment Results Simulation Discussion Simulation: PlanetLab
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Cameron Dale and Jiangchuan LiuA Measurement Study of Piece Population in BitTorrent Introduction BitTorrent Experiment Results Simulation Discussion Simulation: PlanetLab
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Cameron Dale and Jiangchuan LiuA Measurement Study of Piece Population in BitTorrent Introduction BitTorrent Experiment Results Simulation Discussion 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
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Cameron Dale and Jiangchuan LiuA Measurement Study of Piece Population in BitTorrent Introduction BitTorrent Experiment Results Simulation Discussion Simulation: PlanetLab-2
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Cameron Dale and Jiangchuan LiuA Measurement Study of Piece Population in BitTorrent Introduction BitTorrent Experiment Results Simulation Discussion Simulation: PlanetLab-2
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Cameron Dale and Jiangchuan LiuA Measurement Study of Piece Population in BitTorrent Introduction BitTorrent Experiment Results Simulation Discussion Simulation: PlanetLab-2
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Cameron Dale and Jiangchuan LiuA Measurement Study of Piece Population in BitTorrent Introduction BitTorrent Experiment Results Simulation Discussion 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
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Cameron Dale and Jiangchuan LiuA Measurement Study of Piece Population in BitTorrent Introduction BitTorrent Experiment Results Simulation Discussion 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
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Cameron Dale and Jiangchuan LiuA Measurement Study of Piece Population in BitTorrent Introduction BitTorrent Experiment Results Simulation Discussion 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
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Cameron Dale and Jiangchuan LiuA Measurement Study of Piece Population in BitTorrent Introduction BitTorrent Experiment Results Simulation Discussion A Measurement Study of Piece Population in BitTorrent Cameron Dale and Jiangchuan Liu Simon Fraser University Burnaby, BC, Canada camerond@cs.sfu.ca Globecom, November 29 th, 2007, Washington, D.C.
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