BitTorrent is a peer-to-peer file sharing protocol used for distributing large amounts of data. BitTorrent is one of the most common protocols for transferring large files, and by Ipoque it is estimated that it accounts for approximately 27-55% of all Internet traffic (depending on geographical location) as of February 2009.
BitTorrent protocol allows users to receive large amounts of data without putting the level of strain on their computers that would be needed for standard Internet hosting. A standard host's servers can easily be brought to a halt if extreme levels of simultaneous data flow are reached. The protocol works as an alternative data distribution method that makes even small computers with low bandwidth capable of participating in large data transfers.
First, a user playing the role of file-provider makes a file available to the network. This first user's file is called a seed and its availability on the network allows other users, called peers, to connect and begin to download the seed file. As new peers connect to the network and request the same file, their computer receives a different piece of the data from the seed. Once multiple peers have multiple pieces of the seed, BitTorrent allows each to become a source for that portion of the file. The effect of this is to take on a small part of the task and relieve the initial user, distributing the file download task among the seed and many peers. With BitTorrent, no one computer needs to supply data in quantities which could jeopardize the task by overwhelming all resources, yet the same final result—each peer eventually receiving the entire file—is still reached.
After the file is successfully and completely downloaded by a given peer, the peer is able to shift roles and become an additional seed, helping the remaining peers to receive the entire file. The community of BitTorrent users frowns upon the practice of disconnecting from the network immediately upon success of a file download, and encourages remaining as another seed for as long as practical, which may be days.
This distributed nature of BitTorrent leads to a viral spreading of a file throughout peers. As more peers join the swarm, the likelihood of a successful download increases. Relative to standard Internet hosting, this provides a significant reduction in the original distributor's hardware and bandwidth resource costs. It also provides redundancy against system problems, reduces dependence on the original distributor and provides a source for the file which is generally temporary and therefore harder to trace than when provided by the enduring availability of a host in standard file distribution techniques.
Programmer Bram Cohen designed the protocol in April 2001 and released a first implementation on July 2, 2001. It is now maintained by Cohen's company BitTorrent, Inc. There are numerous BitTorrent clients available for a variety of computing platforms. According to isoHunt, the total amount of shared content was more than than 9.4 petabytes as of Sept 10 2009.