The torrent file can then be distributed to other users, often via email or placed on a website. The BitTorrent client is then started as a “seed node”, allowing other users to connect and commence downloading. When other users finish downloading the entire file, they can optionally “reseed” it–becoming an additional source for the file. One outcome of this approach is that if all seeds are taken offline, the file may no longer be available for download, even if the torrent file is possessed. However, even if there are no seeds, as long as there is at least one distributed copy of the file everyone can eventually get the complete file.

Downloading with BitTorrent is straightforward. Each person who wants to download the file first downloads the torrent and opens it in the BitTorrent client software. The torrent file tells the client the address of the tracker, which, in turn, maintains a log of which users are downloading the file and where the file and its fragments reside. For each available source, the client considers which blocks of the file are available and then requests the rarest block it does not yet have. This makes it more likely that peers will have blocks to exchange. As soon as the client finishes importing a block, it hashes it to make sure that the block matches what the torrent file said it should be. Then it begins looking for someone to upload the block to.

BitTorrent gives the best download performance to the people who upload the most, a property known as “leech resistance”, since it discourages “leechers” from trying to download the file without uploading it to anyone. (Although, confusingly, when used in opposition to “seeds” or “seeders” as in “S/L ratio” (meaning “seed/leech ratio”), “leecher” only means someone who hasn’t downloaded the full file yet.)

Though BitTorrent is a good protocol for a broadband user, it is less effective for dial up connections, where disconnections are common. On the other hand, many HTTP servers drop connections over several hours, while many torrents exist long enough to complete a multi-day download.

Terminology

Comparison to other file sharing systems

The method used by BitTorrent to distribute files parallels to a large extent the one used by the eDonkey2000 network, but nodes in eDonkey’s file sharing network usually share and download a much larger number of files, making the bandwidth available to each transfer much smaller. BitTorrent transfers are typically very fast, because all nodes in a group concentrate on transferring a single file or collection of files. While the original eDonkey2000 client provided little “leech resistance”, most new clients have some sort of system to encourage uploaders. eMule, for example, has a credits system whereby a client rewards other clients that upload to it by increasing their priority in its queue. However, the nature of the eDonkey2000 concept means download speeds tend to be much more variable, although the number of available files is far greater.

A similar method to BitTorrent was the Participation Level introduced in KaZaA in 2002. The Participation Level would increase when you upload and decrease when you download. Then when you upload a file to someone else the person with the highest Participation Level gets it first, then they upload it on to the person with the next highest Participation Level, and so on. This can be visualised as a pyramid, with the people who have the most upload bandwidth available at the top and people with less bandwidth on progressively lower levels. This is the most efficient way to distribute a file to a large number of users: it is probable that even the people at the bottom of the pyramid will get the file faster than if the file was served by a non-P2P method. Unfortunately, the system adopted by KaZaA is considered by some to be flawed as it relies on the client accurately reporting their Participation Level and therefore it is easy to cheat with the many “unofficial” clients.

Legal issues

BitTorrent, like any other file transfer protocol, can be used to distribute files without the permission of the copyright holder. BitTorrent has become famous for its ability to also share copyrighted files. Those who intend to distribute illegitimate files often have no or limited funds available, and BitTorrent’s efficiency provides a viable distribution method.

Source is Wikipedia, the free encyclopedia. The text of this article is licensed under the GFDL.