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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
0
5
10
15
20
25
30
35
40
45
50
0 2 4 6 8 10
Tim
e to
com
plet
e do
wnl
oad
Delay coefficient
Effect of delay coefficient on time to complete download
vanillano incentive
0
20
40
60
80
100
0 2 4 6 8 10
Per
cent
of u
nica
st
Delay coefficient
Effect of delay coefficient on percent of unicast
vanillano incentive
ideal
0
5
10
15
20
25
30
35
40
45
50
1 2 3 4 5 6 7Ti
me
to c
ompl
ete
dow
nloa
dMinimum number of uploads
Effect of minimum number of uploads on time to complete download
vanillano incentive
0
20
40
60
80
100
1 2 3 4 5 6 7
Per
cent
of u
nica
st
Minimum number of uploads
Effect of minimum number of uploads on percent of unicast
vanillano incentive
ideal
• Not much difference.
Trials
• We varied: total number of peers.
Results
0
5
10
15
20
25
30
35
40
45
50
2 4 6 8 10 12 14 16
Tim
e to
com
plet
e do
wnl
oad
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
0
20
40
60
80
100
0 20 40 60 80 100
Per
cent
of u
nica
st
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.
0
50
100
150
200
0 20 40 60 80 100
Tim
e to
com
plet
e do
wnl
oad
Percentage of greedy peers
Effect of percentage of greedy peers on time to complete download
vanillano incentive
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
0
50
100
150
200
0 20 40 60 80 100
Tim
e to
com
plet
e do
wnl
oad
Percentage of greedy peers
Effect of percentage of greedy peers on time to complete download
vanillano incentive
0
50
100
150
200
0 20 40 60 80 100Ti
m to
com
plet
e do
wnl
oad
Percent of greedy peers
Effect of percent of greedy peers on time to complete download
vanillano incentive
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