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Understanding BitTorrent: An Efficient P2P File Distribution System

This presentation by Fan Bin delves into the necessity and workings of BitTorrent, a powerful P2P (peer-to-peer) file distribution protocol. It highlights traditional limitations in client/server sharing such as performance degradation as user count rises and the challenge of free-riding, where some users download without contributing. BitTorrent addresses these issues by enhancing scalability and implementing strong incentives for user contribution, thus allowing better performance with increased peers. Key components like torrent files, web servers, and trackers are also discussed, outlining how they function to improve data distribution efficiency.

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Understanding BitTorrent: An Efficient P2P File Distribution System

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  1. A P2P file distribution system ——BitTorrent Fan Bin bfan@cse.cuhk.edu.hk Sep,25,2004

  2. Outline • 1 Why do we need BitTorrent • 2 What’s it • 3 How does it work • 4 Why does it work so well

  3. Problems • Traditional Client/Server Sharing • Performance deteriorates rapidly as the number of clients increases • Free-riding in P2P network • Free riders only download without contributing to the network.

  4. BitTorrent • Scalability of BitTorrent is very good. Even more peers means better performance. • Strong incentives to prevent free-riding.

  5. Outline • 1 Why do we need BitTorrent • 2 What’s it • 3 How does it work • 4 Why does it work so well

  6. Critical Elements • 1 A web server • To provide the ‘metainfo’ file by HTTP • For example: • http://bt.btchina.net • http://bt.ydy.com/ Web Server The Lord of Ring.torrent Troy.torrent

  7. Matrix.torrent Critical Elements • 2 The .torrent file • Static ‘metainfo’ file to contain necessary information : • Name • Size • Checksum • IP address of the Tracker

  8. Critical Elements • 3 A BitTorrent tracker • Non-content-sharing node • Track peers • For example: • http://bt.cnxp.com:8080/announce • http://btfans.3322.org:6969/announce

  9. Critical Elements • 4 An end user (peer) • Guys who want to use BitTorrent must install corresponding software or plug-in for web browsers. • Downloader (leecher) : Peer has only a part ( or none ) of the file. • Seeder: Peer has the complete file, and chooses to stay in the system to allow other peers to download

  10. Outline • 1 Why do we need BitTorrent • 2 What’s it • 3 How does it work • 4 Why does it work so well

  11. Matrix.torrent Connectivity Fan Bin Web Server Tracker Downloader: Joe Seeder: John Downloader: Haibin

  12. Piece • A file is cut into pieces of fixed size, typically 256Kb • Each downloader reports to all of its peers what pieces it has. • To verify data, Hash codes are used for all the pieces, included in .torrent files.

  13. A trivial example {1,2,3,4,5,6,7,8,9,10} Seeder: John {} {1,2,3} {1,2,3,5} {} {1,2,3} {1,2,3,4} {1,2,3,4,5} Downloader Joe Downloader Fan Bin

  14. Outline • 1 Why do we need BitTorrent • 2 What’s it • 3 How does it work • 4 Why does it work so well

  15. Piece Selection • Strict Priority • First Priority • Rarest First • General rules • Random First Piece • Special case, at the beginning • Endgame Mode • Special case, in the end

  16. Peer Selection • Built-in incentive mechanism (where all the magic happens): • Choking Algorithm • Optimistic Unchoking • Anti-snubbing

  17. Choking Algorithm • Choking is a temporal refusal to upload • Choking evaluation is performed every 10 seconds. • Each peer unchokes a fixed number of peers (default = 4) • The decision on which peers to un/choke is based solely on download rate, which is evaluated on a rolling, 20-second average

  18. Reasons for Choking • TCP congestion control. • To ensure the peers to get a consistent download rate. Joe Choked Choked Fan Bin

  19. Optimistic Unchoking • a BitTorrent peer has a single ‘optimistic unchoke’ which is uploaded regardless of the current download rate from it. This peer rotates severy 30s • Reason: • To discover currently unused connections are better than the ones being used

  20. Anti-snubbing • When a peer received no data in 60s, we assume it is choked by all other peers, and refuse to upload to it except for the optimistic unchoking. • Reason • It may cause several concurrent optimistic unchokes.

  21. 100kb/s 70kb/s 20kb/s 15kb/s Example Fan Bin 40kb/s 70kb/s 70kb/s 110kb/s 10kb/s Downloader Joe 10kb/s 30kb/s 5kb/s Downloader: Haibin Downloader: Wang Hui Downloader: Gao yan Downloader: Zhang Yu

  22. Conclusion • BitTorrent is a well thought-out protocol that embraces aspects of cooperation and self-optimizing mechanisms. • BitTorrent propose solutions for current optimization and scalability problems

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