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BitTorrentWe do recommend these programs. Please check them out first! BitTorrent is both the protocol and the name of the peer-to-peer (P2P) file distribution application created by programmer Bram Cohen that makes it possible to massively distribute files without incurring the corresponding massive consumption in server and bandwidth resources. The original BitTorrent application is written in Python and its source code has been released under the BitTorrent Open Source License (a modified version of the Jabber Open Source License), as of version 4.0. The name "BitTorrent" refers to the distribution protocol, the original client application, and the .torrent file type.
How BitTorrent worksBitTorrent greatly reduces the load on peers with complete copies, because peers with incomplete copies generally download the file from each other. As the colored bars below each client show, the file is downloaded in random order, instead of sequential order.The bittorrent protocol breaks the file(s) down into smaller fragments, typically a quarter of a megabyte (256 KB) in size. Peers download missing fragments from each other and upload those that they already have to peers that request them. The protocol is 'smart' enough to choose the peer with the best network connections for the fragments that it's requesting. To increase the overall efficiency of the swarm (the ad-hoc P2P network temporarily created to distribute a particular file), the bittorrent clients request from their peers the fragments that are most rare; in other words, the fragments that are available on the least number of peers, making most fragments available widely across many machines and avoiding bottlenecks. The file fragments are not usually downloaded in sequential order and need to be reassembled by the receiving machine. It is important to note that clients start uploading fragments to their peers before the entire file is downloaded. Sharing by each peer therefore begins when the first complete segment is downloaded and can begin to be uploaded if another peer requests it. This scheme is particularly useful for trading large files such as videos and operating systems. This is contrasted with conventional file serving where high demand can lead to saturation of the host's resources as the consumption of bandwidth to transfer the file to many requesting downloaders surges. With BitTorrent, high demand can actually increase throughput as more bandwidth and additional “seeds” of the file become available to the group. Cohen claims that for very popular files, BitTorrent can support about a thousand times as many downloads as HTTP. Sharing files To share a file using BitTorrent, a user creates a .torrent file, a small "pointer" file that contains: The torrent file is then distributed to users, often via email or placed on a website. The BitTorrent client is started as a "seed node", allowing other users to connect and begin 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 a client has a copy of the torrent file. However, everyone can eventually get the complete file as long as there is at least one distributed copy of the file, even if there are no seeds. 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.
Some parts from Wikipedia, the free encyclopedia. These parts licensed under the GFDL. Go to official BitTorrent site. |
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