Incentives Build Robustness in BitTorrent

1
Incentives Build Robustness in BitTorrent

• 1st Workshop on Economics of Peer-to-Peer Systems
  2003
• Bram Cohen
• bram_at_bitconjurer.org

2
Outline

• What BitTorrent Does
• Technical Framework
• Choking Algorithms

3
What BitTorrent Does

• How BitTorrent Works
• When multiple people are downloading the same
  file at the same time, they upload pieces of that
  file to each other
• Mathematically what must go down must go up
• High rate of churn

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What BitTorrent Does

• Interface
• Standard Save As dialog box

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What BitTorrent Does

• Deployment
• Known Tracker nodes
• .torrent file contains meta information
• length, name, hash, URL of tracker
• Incomplete downloaders increases rapidly
• Complete downloaders increase slowly
• Both fall off exponentially

6
Technical Framework

• Publishing Content
• Peer Distribution
• Pipelining
• Piece Selection

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Publishing Content

• Protocol layered on top of HTTP
• Publisher provides a .torrent file
• d8announce43http//torrent.linux.duke.edu6969/a
  nnounce13creation da
• tei1099856396e4infod5filesld6lengthi2639267840e
  4pathl18FC3-x86_6
• 4-DVD.isoeed6lengthi753e4pathl6MD5SUMeee4name2
  1heidelberg-x8
• 6_64-DVD12piece lengthi262144e6pieces201380gæGÓ
  gÈd1ÒñfMåcƒ
• !)UÁÍÂ??òA?,èZEoPLÂJÀLøÚT)?úUTúâO?Ëg
  µ?µ0
• ?poª'BêiJVBvÉßwäÅM??óøå?îÕµoKÜè .
  (200k file)

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Peer Distribution

• Tracker helps peers find each other.
• BitTorrent cuts the files into pieces
• Typically 256KB
• Uses SHA1 hash to verify integrity
• Peers download from any other peer
• Peers may not have what is wanted
• Peers may not allow them to download

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Pipelining

• BitTorrent uses TCP
• Avoid TCP slow start
• Have many pending requests
• Pieces get broken down
• Sub-pieces typically 16KB in size
• Try and keep some number (typically 5) of
  requests pending at a time.

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Piece Selection

• Strict Priority
• Other sub-pieces from same source
• Rarest First
• Common parts left for later
• Random First Piece
• Start-up need to get a complete piece
• Endgame Mode
• Broadcast for all remaining sub-pieces

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Choking Algorithms

• No central resource allocation
• choking is a temporary refusal to upload
• Pareto Efficiency
• BitTorrents Choking Algorithm
• Optimistic Unchoking
• Anti-snubbing
• Upload Only

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Pareto Efficiency

• An allocation is Pareto efficient if there is no
  other allocation in which some other individual
  is better off and no individual is worse off.
• In computer Science terms, seeking pareto
  efficiency is a local optimization algorithm in
  which pairs of counterparties see if they can
  improve their lot together, and such algoritms
  tend to lead to a global optima.

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BitTorrents Choking Algorithm

• Peers are unchoked based strictly on current
  download rate
• Leaves 4 peers unchoked
• Makes decision every 10 seconds
• Uses a 20-second rolling average

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Optimistic Unchoking

• Used to discover if a currently choked peer would
  be better.
• Done every third unchoke decision (30 seconds).

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Anti-snubbing

• After 60 seconds without a single piece, the
  client will assume it has been snubbed
• Does not upload to a peer that has snubbed it
  (except as an optimistic unchoke)

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Upload Only

• Once its done downloading it has no metric to
  determine who to upload to
• Prefers
• Peers that have a better upload rate
• Peers that no one else is currently uploading to

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Summary

• What BitTorrent Does
• Technical Framework
• Choking Algorithms