P2P file sharing systems have evolved from centralized models like Napster to decentralized models like BitTorrent. BitTorrent uses a peer-to-peer approach where users (peers) download and upload different pieces of a file from each other in a swarm. A tracker helps peers find each other but is not involved in the data transfer. BitTorrent has grown to account for a large percentage of Internet bandwidth and is used to distribute many open source and fan projects.

1. P2P: Grid-Oriented Distribution Systems KCCMG Fall Impact 2005 Lorin Olsen, Sprint Nextel

2. History of File Sharing 1980’s: Corporate file sharing (anyone remember IBM MVS/BDT???) and initial IP-based file transfer protocols for DoD use 1990’s: FTP used for point-to-point file transfers 1999: Shawn Fanning releases Napster March 2000: Justin Frankel releases Gnutella ; "See? AOL can bring you good things!" March 2001: Niklas Zennström and Janus Friis introduce KaZaA Defcon 2001 / CodeCon 2002: Bram Cohen shows off BitTorrent

3. Client-Server File Sharing

4. Napster You started the Napster software on your machine. Your machine became a small server able to make files available to other Napster users. Your machine connected to Napster's central servers. It told the central servers which files were available on your machine. So the Napster central servers had a complete list of every shared song available on every hard disk connected to Napster at that time. You typed in a query for a song. Let's say you were looking for the song "Roxanne" by The Police. Napster's central servers listed all of the machines storing that song. You picked a version of the song from the list. Your machine connected to the user's machine that had that song, and downloaded the song directly from that machine. “ Napster’s Architecture, http://computer.howstuffworks.com/file-sharing1.htm

5. P2P Philosophy “Many hands make light work” - Proverb “Single servants are less powerful then a single server but the collective of many servants is more powerful then any single server” - Daniel Stephen Rule.

6. P2P Evolution “ For the people that aim to stop P2P, they have turned a centralized system like Napster – easily controlled, easily monitored – into a fully decentralized system in the form of Kazaa, as well as a fragmented ecosystem of thousands of centralized servers through BitTorrent. This was probably a bad decision.” Monkey Methods Research Group

7. P2P Architecture

8. P2P Dark Side “ Too many cooks spoil the broth” - Proverb “ Arggghhhhh” – The sound Lorin makes when trying to coordinate the efforts of his four children

9. Gnutella Distributed query + lightweight content distribution Clients BearShare Gnucleus LimeWire Morpheus WinMX XoloX

10. BitTorrent You open a Web page and click on a link for the file you want. BitTorrent client software communicates with a tracker to find other computers running BitTorrent that have the complete file. The tracker identifies the swarm. The tracker helps the client software trade pieces of the file you want with other computers in the swarm. If you continue to run the BitTorrent client software after your download is complete, others can receive .torrent files from your computer; your future download rates improve because you are ranked higher in the "tit-for-tat" system. “ What BitTorrent Does”; How Stuff Works; http://computer.howstuffworks.com/bittorrent3.htm

11. BitTorrent Architecture A single source file within a group of BitTorrent users, called a swarm, spreads around pieces of a film or videogame or TV show so that everyone has a chunk to share. After the initial downloading, those pieces are then uploaded to other needy users in the swarm. Before long, the swarm has shared all the pieces, and everyone has their own complete source. Thompson, Clive; “The BitTorent Effect”; Wired Magazine; January 2005

12. BitTorrent Terms Peer: A peer is one instance of a BitTorrent client running on a computer on the Internet that you connect to and transfer data. Usually a peer does not have the complete file, but only parts of it. Seed: A seed is a peer that has a complete copy of the torrent and still offers it for upload. The more seeds there are, the better the chances are for completion of the file. Leech A leech is a peer that does not have a complete copy of the torrent yet. When downloading is complete, it may stay around and seed the file as a seed so that others can complete their download. The term leech is also used for peers that have very poor upload/download ratios or leave the swarm immediately after their downloads are complete. The leeches usually contribute a majority of the bandwidth in a swarm. Swarm: Together, all peers sharing a torrent are called a swarm . Six leeches and two seeds makes a swarm of eight. Tracker: A tracker is a broker service that mediates contacts between peers. The tracker is not directly involved in the data transfer and does not have a copy of the file.

13. The Role of the Tracker

14. Azureus: A Client in Action

15. File Distribution

16. BitTorrent Traffic Patterns BitTorrent has taken an enormous share of Internet bandwidth BitTorrent usage increases during “off-prime” hours (5PM-6AM)

17. Legal Uses of BitTorrent The demo of the flight sim X-Plane is offered via BitTorrent, as well as the World of Warcraft beta and its patches. PlaneShift is a free open-source MMORPG , which uses BitTorrent for its primary method of distribution. The fan-film Star Wars: Revelations is distributing two DVD images as well as the film by itself via BitTorrent. The NetBSD operating system version 1.6.2 and later as well as most major Linux distributions use BitTorrent as an alternative way of distributing ISO images of their releases. “ BitTorrent”; http://en.wikipedia.org/wiki/Bittorrent

18. References Cohen, Bram; “Incentives Build Robustness in BitTorrent”; 5/22/2003 Ewing, J & Lowry,T (2004); “It Seemed Like A Good Idea”; Business Week, New York; Aug 2, 2004, Iss. 3894; pg. 60 Pouwelse, Johan; “The BitTorrent P2P file-sharing system”; 12/18/2004; http://www.theregister.co.uk/2004/12/18/bittorrent_measurements_analysis/ Zihui Ge, Daniel R. Figueiredo, Sharad Jaiswal, Jim Kurose, Don Towsley; “Modeling Peer-Peer File Sharing Systems”; IEEE Infocom 2003;