Impact of Incentives in BitTorrent

By Jenny Liu and Seth Cooper

Peer to Peer Networks

• Multiple independent peers connect to share resources with each other

Incentives

• But what reason does one peer have to share its resources with another?

Incentives

• Does a peer need something in return?

Incentives

• Or is simple altruism enough?

BitTorrent

• Torrent file directs peers to a tracker, which helps to coordinate peers

Torrent

Tracker

BitTorrent

• Initial peer, known as a “seed,” begins with the complete file.

Torrent

Tracker Seed

BitTorrent

• Peers trade pieces of the file amongst themselves, uploading pieces to the peers they are downloading the fastest from. When a peer finishes, it becomes a new seed.

Torrent

Tracker Seed

Motivation

• Other P2P networks (Napster, Kazaa) have become successful relying on altruism alone.

Motivation

• It is often claimed that freeloading on P2P networks is a problem, and many attempts have been made to find an effective incentive, but is one necessary?

Motivation

• How effective is BitTorrent? Are incentives necessary? How do greedy peers affect the network? Are incentives an unnecessary overheard?

System

• Modify BitTorrent source to remove incentives, and compare efficiency with unmodified BitTorrent

System

• Assign each peer a strategy: greedy or altruistic.

Metrics

• Client side: average download time

Metrics

• Server side: percent of unicast

Trials

• Measure data from several trials of one seed distributing a file to a set of peers. Start with a default network setting and vary parameters to see their effect on the two types of networks.

Seed

Trials

• We varied: delay between peer connections.

Trials

• We varied: minimum number of uploads for each peer.

Results

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Delay coefficient

Effect of delay coefficient on time to complete download

vanillano incentive

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Delay coefficient

Effect of delay coefficient on percent of unicast

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dMinimum number of uploads

Effect of minimum number of uploads on time to complete download

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Minimum number of uploads

Effect of minimum number of uploads on percent of unicast

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• Not much difference.

Trials

• We varied: total number of peers.

Results

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Number of peers

Effect of number of peers on time to complete download

vanillano incentive

• Number of peers has an effect on download time when the incentive is removed.

Trials

• We varied: percentage of greedy peers.

Results

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Percentage of greedy peers

Effect of percentage of greedy peers on percent of unicast

vanillano incentive

ideal

• Percentage of greedy peers has an effect on both metrics when the incentive is removed.

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Percentage of greedy peers

Effect of percentage of greedy peers on time to complete download

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Trials

• We varied: size of network and tried different percentages of greedy hosts

Results

• As the network gets bigger, a larger percentage of greedy hosts can be supported

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Percentage of greedy peers

Effect of percentage of greedy peers on time to complete download

vanillano incentive

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Percent of greedy peers

Effect of percent of greedy peers on time to complete download

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10 Peers 20 Peers

Conclusions

• Q: How efficient is BitTorrent?

• A: Very.

Conclusions

• Q: Are incentives necessary?

• A: It depends.

Conclusions

• Q: How do greedy peers affect the network?

• A1: Not much. • A2: Not much up to a point. • A3: Quite a lot after that.

Conclusions

• Q: Are incentives an unnecessary overhead?

A: No!

Future Work

• Conduct experiments on more heterogeneous networks.

• Try UDP.

• Address other negative aspects of BitTorrent, such as content lifetime.

The End