torrent-fetcher

In the BitTorrent file distribution system, a torrent file or meta-info file is a computer file that contains metadata about files and folders to be distributed, and usually also a list of the network locations of trackers, which are computers that help participants in the system find each other and form efficient distribution groups called swarms . A torrent file does not contain the content to be distributed; it only contains information about those files, such as their names, folder structure, and sizes obtained via cryptographic hash values for verifying file integrity. The term torrent may refer either to the metadata file or to the files downloaded, depending on the context. A torrent file acts like a table of contents (index) that allows computers to find information through the use of a Bittorrent client. A torrent file gives addresses identifying computers that can send parts of the requested file. With the help of a torrent file, one can download small parts of the origina...

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 dist...

A torrent file contains a list of files and integrity metadata about all the pieces, and optionally contains a list of trackers. A torrent file is a bencoded dictionary with the following keys (the keys in any bencoded dictionary are lexicographically ordered): announce —the URL of the tracker info —this maps to a dictionary whose keys are dependent on whether one or more files are being shared: files —a list of dictionaries each corresponding to a file (only when multiple files are being shared). Each dictionary has the following keys: length —size of the file in bytes. path —a list of strings corresponding to subdirectory names, the last of which is the actual file name length —size of the file in bytes (only when one file is being shared) name —suggested filename where the file is to be saved (if one file)/suggested directory name where the files are to be saved (if multiple files) piece length —number of bytes per piece. This is commonly 28 KiB = 256 KiB = 262,144 B. pieces —...

A torrent file can also contain additional metadata defined in extensions to the BitTorrent specification. These are known as "BitTorrent Enhancement Proposals." Examples of such proposals include metadata for stating who created the torrent, and when. Draft extensions edit These extensions are under consideration for standardization. Distributed hash tables edit BEP-0005 extends BitTorrent to support distributed hash tables. A trackerless torrent dictionary does not have an announce key. Instead, a trackerless torrent has a nodes key: For example, The specification recommends that nodes "should be set to the K closest nodes in the torrent generating client's routing table. Alternatively, the key could be set to a known good node such as one operated by the person generating the torrent." Multiple trackers edit BEP-0012 extends BitTorrent to support multiple trackers. A new key, announce-list , is placed in the top-most dictionary (i.e. with announce and inf...

Single file edit Here is what a de-bencoded torrent file (with piece length 256 KiB = 262,144 bytes) for a file debian-503-amd64-CD-1.iso (whose size is 678 301 696 bytes) might look like: Note: pieces here would be a 51 KiB value ( ⌈ l e n g t h p i e c e   l e n g t h ⌉ × 160 = 414080   b i t s {\displaystyle {\color {Blue}\left\lceil {\color {Black}{\frac {\mathtt {length}}{\mathtt {piece\ length}}}}\right\rceil }\times 160=414080\ \mathrm {bits} } ). Multiple files edit Here is what a de-bencoded torrent file (with 'piece length' 256 KiB = 262144 B) for two files, 111.txt and 222.txt , might look like: