How Does Bit Torrent Work ?






















BitTorrent is the name of a client application for the torrent peer-to-peer (P2P) file distribution protocol. created by programmer Bram Cohen. BitTorrent is designed to widely distribute large amounts of data without incurring the corresponding consumption in server and bandwidth resources (and typically, monetary fees attracted as a result of that).

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.

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.

BitTorrent-etiquette

Because BitTorrent relies on the upstream bandwidth of its users — and the more users, the more aggregate bandwidth is available for sharing the files — it is considered good etiquette to leave one's BitTorrent client open after downloading has completed so that others may continue to gain from the file that has been distributed.

It is not clear, however, how long one should leave their client open after downloading has finished. Many clients report the byte traffic upstream as well as down, so the user can see how much they have contributed back to the network. Some clients also report the "share ratio", a number relating the amount of data uploaded to the amount downloaded. It is generally considered good form to at least share back the equivalent amount of traffic as the original file size.

It is worth noting that the requirement of a "1.00" share ratio (uploading as much data as you have downloaded) is rather hotly contested given its relative impossibility to achieve for every person. On any given torrent, the best possible outcome is the original seeder with an infinite ratio (having only uploaded data and never downloaded any data), a number of peers with 1.00 ratios (having downloaded the file, uploaded just as much data, and then promptly logged off), and two users with a .50 ratio (the last two having each downloaded a separate half of the file and then shared their half with the other). This is highly unlikely to be achieved due to the very small chance of the last two peers downloading completely opposite halves and finishing just as the last seeder logged off and the fact that not all people will upload the same amount of data they downloaded as some will upload less and others will upload more. Ultimately, a perfect torrent would leave two end users with only a .50 ratio for the torrent, which means every user would have to provide new content at least equal to the portion of data they did not get to upload in the last torrent to maintain an overall ratio of 1.00.

While it's highly unlikely that all users who download a given torrent will achieve a 1.0 ratio on it (because the net ratio of all users is 1.0, if any user uploads past 1.0 some other user will have to sustain a lower ratio), it is more of a guideline to encourage the average upstream of a given network. Some networks, for example, prevent access to new torrents for the first 24-48 hours that the torrent is active to people with overall ratios of less than 1.0 and a certain amount of data uploaded.

The amount of time the client is left open may be more important than the amount of traffic contributed, since new users attempting to download a file will first need to find peers hosting the file.

Many advanced trackers now track statistics such as how many seeders and downloaders were on a torrent at the time of a user's disconnect as many consider this information more important than just the user's ratio of downloaded/uploaded.

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