BitTorrent

BitTorrent
BitTorrent is a peer-to-peer file sharing protocol used to distribute large amounts of data. BitTorrent is one of the most common protocols for transferring large files, and by some estimates it accounts for about 35% of all traffic on the entire Internet. The initial distributor of the complete file or collection acts as the first seed. Each peer who downloads the data also uploads it to other peers. Because of this, BitTorrent is extremely efficient. One seed is needed to begin spreading files between many users (peers). The additions of more seeds increases the likelihood of a successful connection exponentially. 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 and reduces dependence on the original distributor.

A BitTorrent client is any program that implements the BitTorrent protocol. Each client is capable of preparing, requesting, and transmitting any type of computer file over a network, using the protocol. A peer is any computer running an instance of a client.

To share a file or group of files, a peer first creates a small file called a “torrent” (e.g. MyFile.torrent). This file contains metadata about the files to be shared and about the tracker, the computer that coordinates the file distribution. Peers that want to download the file must first obtain a torrent file for it, and connect to the specified tracker, which tells them from which other peers to download the pieces of the file.

Though both ultimately transfer files over a network, a BitTorrent download differs from a classic full-file HTTP request in several fundamental ways:

* BitTorrent makes many small data requests over different TCP sockets, while web-browsers typically make a single HTTP GET request over a single TCP socket.
* BitTorrent downloads in a random or in a “rarest-first” approach that ensures high availability, while HTTP downloads in a sequential manner.

Taken together, these differences allow BitTorrent to achieve much lower cost to the content provider, much higher redundancy, and much greater resistance to abuse or to “flash crowds” than a regular HTTP server. However, this protection comes at a cost: downloads can take time to rise to full speed because it may take time for enough peer connections to be established, and it takes time for a node to receive sufficient data to become an effective uploader. As such, a typical BitTorrent download will gradually rise to very high speeds, and then slowly fall back down toward the end of the download. This contrasts with an HTTP server that, while more vulnerable to overload and abuse, rises to full speed very quickly and maintains this speed throughout.

In general, BitTorrent’s non-contiguous download methods have prevented it from supporting “progressive downloads” or “streaming playback”. But comments made by Bram Cohen in January 2007 suggest that streaming torrent downloads will soon be commonplace and ad supported streaming appears to be the result of those comments.