Exploiting BitTorrent For Fun (But Not Profit)

1
Exploiting BitTorrent For Fun(But Not Profit)

• Nikitas Liogkas, Robert Nelson, Eddie Kohler,
  Lixia Zhang
• University of California, Los Angeles

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Motivation

• Robustness system always provides useful
  service to all connected peers
• depends on peers willingness to contribute
• but uploading not in peers direct self interest
• Fairness those who do not contribute should not
  be able to receive good service
• Can fairness violations reduce robustness?

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Selfish peers

• BitTorrent fairness model
• rules for data exchange between peers
• tit-for-tat for upload decisions
• Selfishness peers violate fairness while
  evading detection
• abuse existing protocol mechanisms
• is it effective (fairness violation)?
• impact on honest peers (robustness reduction)?

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Contributions

• Designed and evaluated three selfish exploits
• tools for measuring robustness
• Despite selfishness, BitTorrent quite robust
• at most 29 higher rates for the selfish peer
• but no considerable degradation of service
• How is this robustness achieved?
• identify responsible protocol mechanisms
• propose five guiding design principles

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Presentation outline

• BitTorrent operation
• Implementation and methodology
• Design and evaluation of exploits
• Discussion
• Conclusions
• Future work

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BitTorrent joining a torrent
metadata file
peer list
join
datarequest

• Peers divided into
• seeds have the entire file
• leechers still downloading

1. obtain the metadata file
2. contact the tracker
3. obtain a peer list (contains seeds leechers)
4. contact peers from that list for data
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BitTorrent exchanging data
!
I have
? Verify pieces using hashes
? Download sub-pieces in parallel
? Advertise received pieces to the entire peer
list
? Look for the rarest pieces
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BitTorrent - unchoking
? Periodically calculate data-receiving rates
? Upload to (unchoke) the fastest downloaders
? Optimistic unchoking ? periodically select
a peer at random and upload to it ?
continuously look for the fastest partners
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Presentation outline

• BitTorrent operation
• Implementation and methodology
• Design and evaluation of exploits
• Discussion
• Conclusions
• Future work

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Implementation

• implemented all three exploits in the Ctorrent
  client
• ensured that our changes did not interfere with
  regular protocol operation
• exploits do not exhaust all possibleselfish
  behavior
• but prove to be valuable tools for evaluating
  robustness

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Experimental methodology

• Private torrents
• eight leechers one seed on Planetlab
• most torrents are small Guo et al., IMC05
• impose download and upload limits
• leechers join according to Poisson
• purpose measure benefit to selfish (fairness),
  impact on honest (robustness)
• Public torrents
• two clients join the same torrent together
• purpose reveal impact in real settings

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Exploit 1 Downloading only from seeds
new listrequest
peer list
? Download only from seeds no need to upload
? Repeatedly query the tracker for peer lists
? Distinguish the seeds, and receive data from
them
? Violates fairness model also harmful to honest
peers
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Evaluation Exploit 1in private torrents
Download rates for all peers

• Limit bandwidth of leechers 1 to 6. No limit on
  seed.
• Modest fairness violation (22 better rate)
  when selfish is fast
• Robustness does not suffer most honest slower by
  lt15

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Evaluation Exploit 1with modified seed
Download rates for all peers
155

• Seed only unchokes one leecher at a time
• Considerable fairness violation selfish peer
  faster by 155
• Reduces robustness honest peers slower by at
  least 32

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Evaluation Exploit 1in public torrents

• Tested with small (lt 20 peers) and large (gt 150
  peers) torrents
• Selfish leecher gets consistently higher
  download rates (7-20)
• Does particularly well in torrents with many
  seeds
• Greater incentive to cheat in popular torrents

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Exploit 2 Downloading only from the fastest
peers
!
I have
? Download only from fast peers, never waste
time on slow ones
? Do not perform optimistic unchokes
? Observe frequency of piece advertisements
to infer bandwidth of peers
? Especially harmful at the start of a peers
lifetime
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Evaluation Exploit 2in private torrents
Download rates for all peers

• Selfish peer interacts only with the two fastest
  leechers in its list
• Modest fairness violation selfish peer faster by
  29
• Robustness does not suffer impact on honest
  peers small

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Evaluation Exploit 2in public torrents

• Exploit fails in public torrents
• consistently lower download rates (1-30)
• Reasons
• optimistic unchoking aids in discovering the
  best partners
• short-term calculations beat our
  advertisement-based estimations
• Mechanism for continuous adaptation is valuable

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Exploit 3 Advertisingfalse pieces
2
1
1
2
4
!
I have
3
garbage
1
2
3
? Lie about the pieces you have
? Gradually advertise the rarest pieces
? Send garbage when you do not have a piece
? pollution is not primary objective
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Evaluation Exploit 3in private torrents
Download rates for all peers

• Modest fairness violation selfish peer faster by
  22
• Robustness does not suffer some of the honest
  peers even improve their rates!

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Evaluation Exploit 3in public torrents

• Exploit fails in public torrents
• modern implementations keep state about the
  origin of pieces
• lying leechers easily detected
• Remembering past interactions enables defending
  against false piece advertisements

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Presentation outline

• BitTorrent operation
• Implementation and methodology
• Design and evaluation of exploits
• Discussion
• Conclusions
• Future work

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Design principles

• Parallel downloading
• Memory of past interactions
• Problem partitioning
• Export minimal information
• Keep the network connected

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Principles 1 and 2

• Maintain parallel interactionswith multiple
  peers
• example limited impact of the download-only-from-
  seeds exploit
• Maintain memory of past interactions
• example failure of the false-advertisements
  exploit in public torrents
• trade-off between robustness and performance

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Principle 3

• Enforce problem partitioning Shneidman et al.,
  PODC04
• No peer should be able to influence another
  peers decision-making by declaring false
  information
• Not enforced by BitTorrent
• Should decouple data needs (pieces) from the
  provided service (unchoking)
• Could harm performance

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Principles 4 and 5

• Export minimal information necessary
• example hide that you are a seed
• super-seeding policy does just that
• Keep the network connected
• optimistic unchoking random choice that aids
  robustness
• value evident in the failure of the
  download-from-the fastest exploit in public
  torrents

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Related work

• Describes the basic BitTorrent mechanisms
  Cohen, P2PECON03
• Feasibility of selfish behavior in BitTorrent
  Shneidman et al., PINS04
• Theoretical analysis Qiu et al., SIGCOMM04
• Simulations Bharambe et al., MSR-TR-2005
• Measurement studies Izal et al., PAM04,
  Pouwelse et al., Delft TR 2004 and IPTPS05, Guo
  et al., IMC05

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Conclusions

• Presented three selfish exploits
• BitTorrent quite robust, despite fairness
  violations
• Identified protocol characteristics that enable
  robustness
• Proposed five guiding design principles

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Future work

• Investigate combinations of exploits
• Our exploits do not exhaust the complete space
  of selfish behavior
• how to methodically design other (possibly more
  successful) exploits?
• Selfish behavior in multi-torrent systems

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Exploiting BitTorrent For Fun(But Not Profit)

• Nikitas Liogkas, Robert Nelson, Eddie Kohler,
  Lixia Zhang
• Questions?

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Bonus slide Exploit 1all-selfish scenario
Download rates for all peers

• Appears as if everyone benefits when everyone is
  being selfish
• Artifact of the imposed bandwidth limits seed
  serving similar clients
• Degenerates into a client-server model