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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. - PowerPoint PPT Presentation
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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
