A torrent file acts like a table of contents (index) that allows computers to find information through the use of a BitTorrent client. With the help of a torrent file, one can download small parts of the original file from computers that have already downloaded it. These \"peers\" allow for downloading of the file in addition to, or in place of, the primary server.
The BitTorrent system has been created to ease the load on central servers, as instead of having individual clients fetch files from the server, BitTorrent can crowd-source the bandwidth needed for the file transfer and reduce the time needed to download large files. Many free/freeware programs and operating systems, such as the various Linux distributions offer a torrent download option for users seeking the aforementioned benefits. Other large downloads, such as media files, are often torrented as well.
Typically, Internet access is asymmetrical, supporting greater download speeds than upload speeds, limiting the bandwidth of each download, and sometimes enforcing bandwidth caps and periods where systems are not accessible. This creates inefficiency when many people want to obtain the same set of files from a single source; the source must always be online and must have massive outbound bandwidth. The BitTorrent protocol addresses this by decentralizing the distribution, leveraging the ability of people to network \"peer-to-peer\", among themselves.
Each file to be distributed is divided into small information chunks called pieces. Downloading peers achieve high download speeds by requesting multiple pieces from different computers simultaneously in the swarm. Once obtained, these pieces are usually immediately made available for download by others in the swarm. In this way, the burden on the network is spread among the downloaders, rather than concentrating at a central distribution hub or cluster. As long as all the pieces are available, peers (downloaders and uploaders) can come and go; no one peer needs to have all the chunks or to even stay connected to the swarm in order for distribution to continue among the other peers.
A small torrent file is created to represent a file or folder to be shared. The torrent file acts as the key to initiating downloading of the actual content. Someone interested in receiving the shared file or folder first obtains the corresponding torrent file, either by directly downloading it or by using a magnet link. The user then opens that file in a BitTorrent client, which automates the rest of the process. In order to learn the internet locations of peers who may be sharing pieces, the client connects to the trackers named in the torrent file, and/or achieves a similar result through the use of distributed hash tables. Then the client connects directly to the peers in order to request pieces and otherwise participate in a swarm. The client may also report progress to trackers, to help the tracker with its peer recommendations.
A core feature of the new format is its application of merkle trees, allowing for 16KiB blocks of a piece to be individually verified and re-downloaded. Each file now always occupy whole piece sizes and have an independent merkle root hash, so that it's possible to find duplicate files across unrelated torrent files of any piece length. The file size is not reduced, but the info dictionary required for magnet links are (only in v2-only torrents). 59ce067264