BitTorrent
BitTorrent is a peer-to-peer (P2P) file distribution program and protocol where users connect to each other directly to send and receive portions of files. The key philosophy of BitTorrent is that users upload (transmit outbound) at the same time as they download (receiving inbound.) In this manner, network bandwidth is utilised as efficiently as possible. The more people interested in a certain file, the more efficiently the program works.
How it Works
BitTorrent breaks files down into smaller segments (around 256 KB), which users can download from each other and upload those that they already have to users that request them. This means that users start uploading segments to their peers before they have downloaded the entire file. Sharing begins when the first complete segment is downloaded; this can be uploaded as soon as another peer requests it.
These transfers are coordinated by special text files called torrent files (which use the extension .torrent), which help the BitTorrent clients (programs) coordinate the peer-based transfers.
Downloading files using BitTorrent consists of three parts: the torrent file, the tracker and the client (the program). The torrent file contains all the necessary information on what the file is and what tracker it uses to connect people. Each person who wants to download the file first downloads the torrent and opens it in the BitTorrent program. The torrent file tells the program the address of the tracker, which in turn keeps a log of which users are downloading the file and where the file and its segments reside. The program then requests segments from other users. Once a file fragment has been downloaded, the program then begins looking for someone to upload the fragment to.
Share Ratios
BitTorrent gives the best download performance to the people who upload the most, a behaviour known as 'leech resistance', since it discourages 'leechers' from trying to download a file without uploading it to anyone. A user's 'share ratio' is the number relating the amount of data uploaded to the amount downloaded.
A share ratio of 1.0 means that a user has uploaded as much data as they have downloaded. A share ratio of more than 1 means that a user has uploaded more than they have downloaded. Some trackers require users to have a minimum share ratio to register; users with a share ratio below the stated limit may be put into a restricted 'upload-only' mode, where they may not download until their share ratio reaches the minimum.
BitTorrent is both the name of a peer-to-peer (P2P) file distribution program and also the name of the file sharing protocol itself.
A growing number of individuals and organizations use BitTorrent to distribute their own or licensed material. For example, Blizzard's World of Warcraft video game utilizes the BitTorrent protocol to send game updates to clients. The use of BitTorrent dramatically reduces demands on networking hardware and bandwidth for the companies involved.
Finding Torrents
The BitTorrent protocol provides no way to index torrent files. As a result, a number of websites host a large majority of torrents, supporting the discovery and distribution of data on the BitTorrent network. Public tracker sites such as Mininova and The Pirate Bay allow users to search for and download from their collection of .torrent files; they also run BitTorrent trackers for those files.
Private tracker sites such as Demonoid operate like public ones except that they restrict access to registered users and keep track of the amount of data each user uploads and downloads, in an attempt to reduce leeching.
Search engines allow the discovery of .torrent files that are hosted and tracked on other sites; examples include TorrentSpy, Btjunkie and isoHunt.
Other Torrent Clients
There are many other BitTorrent clients, that download files using the BitTorrent protocol. These are written in a variety of programming languages, and run on a variety of computing platforms, and include clients such as Azureus, BitComet and µTorrent.
