Incentives Build Robustness in BitTorrent

1
Incentives Build Robustness in BitTorrent
Bram Cohen May 22, 2003

• Presented by Eddie Kay on April 14, 2005

2
The Bandwidth Problem
Traditional HTTP
BitTorrent
BitTorrent speeds up file distribution by
harnessing previously unused bandwidth.
Graphics from http//www.bittorrent.com/introducti
on.html
3
Publishing with BitTorrent

• Start tracker, a server that facilitates
  discovery of peers.
• Create .torrent metadata file.
• File info (length, piece size, checksums)
• Address of the tracker
• Place .torrent file on web server.
• Connect to tracker from a computer with the file.

4
Downloading with BitTorrent

• Get .torrent file using web browser.
• Use BitTorrent client to connect to tracker.
• BitTorrent client then automatically
• requests pieces of the file from peers.
• sends pieces to other peers who need them.
• updates peers and tracker on file progress.

5
Making BitTorrent Work

• If a client at any time has a strict subset or
  superset of the information that its peers have,
  then performance will suffer.
• To ensure good performance, two questions must be
  answered
• What should I download, and from whom should I
  request it?
• What should I upload, and to whom?

6
What Should I Download?

• Random first piece.
• Partially-downloaded pieces.
• Rare pieces.

7
What Should I Upload?

• Clients upload what their peers ask for.
• Clients decide to whom they upload.
• Choking algorithm upload to the peers you can
  download from the fastest.
• Optimistic unchoking choose a peer at random and
  upload to him.
• If not downloading, just upload to the people to
  whom you can upload fastest.

8
BitTorrent in the Real World

• Many different clients, trackers, and publishing
  tools available that conform to the BitTorrent
  protocol.
• Widely deployed and used 35 of Internet
  traffic in November 2004.
• Success through ease of use simple for
  publishers and consumers.

Source http//www.cachelogic.com/research/slide
1.php