How BitTorrent works?

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.

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