Fichier Torrent
A TRADUIRE
A torrent file stores metadata used for BitTorrent. It is defined in the BitTorrent specification. In a conventional Internet download, a file or group of files is transmitted from one computer (usually a server) to another computer (a client). Data is held in a central location and is retrieved per the request of a single user.
A torrent, however, distributes data differently on many levels. Firstly, a .torrent file is not the actual file being retrieved. The .torrent file is merely data which has information about the file that the user is seeking. The .torrent file is, in a sense, a sort of index of the file being retrieved. The torrent divides the target file into a series of equally sized pieces that are each assigned an identifying checksum.
The transfer of the torrent file is done between many different peers. Rather than the data being transferred from one machine to the next, the various pieces of the file are held on many different computers. The torrent client communicate with other peers in order to check which pieces they have and exchanges them for the ones they need. In this way, transferring files via a torrent is sort of like a lightning fast, digitally automated game of "Go Fish." Peers swap pieces of files with as many different computers as possible and eventually combine the various parts into the whole, requested file.
A torrent file contains the URLs of multiple trackers and integrity metadata about all the pieces. It can also contain additional metadata defined in extensions to the BitTorrent specification.<ref>Template:Cite web</ref> These are known as "BitTorrent Enhancement Proposals". Examples of such proposals include metadata for stating who created the torrent, and when.
File structure
A torrent file is a bencoded dictionary with the following keys:
- 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:
- name - suggested file/directory name where the file(s) is/are to be saved
- piece length - number of bytes per piece. This is commonly <math>2^{18}</math> = 256KiB = 262144B.
- pieces - concatenation of each piece's SHA-1 hash. As SHA-1 returns a 160-bit hash, pieces will be a string whose length is a multiple of 160-bits.
And exactly one of length (corresponds to when only one file is being shared) or files (corresponds to when multiple files are being shared):
- length - size of the file (in bytes)
- files - a list of dictionaries (each dictionary corresponds to a file) with the following keys:
- 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).
All strings must be UTF-8 encoded.
Extensions
Draft extensions
These extensions are under consideration for standardization.
Distributed hash tables
BEP-0005<ref>Template:Cite web</ref> 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:
{
...
'nodes': [["<host>", <port>], ["<host>", <port>], ...]
...
}
For example,
'nodes': [["127.0.0.1", 6881], ["your.router.node", 4804]]
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
BEP-0012<ref>Template:Cite web</ref> extends BitTorrent to support multiple trackers.
A new key, announce-list, is placed in the top-most list (i.e. with announce and info). This key's value is a list of tracker "tiers". Each tier is a list of URLs:
{
...
'announce-list': [["http://tracker.site1.com/announce"], ["http://tracker.site2.com/announce"]]
...
}
HTTP seeds
BEP-0017<ref>Template:Cite web</ref> extends BitTorrent to support HTTP seeds.
A new key, httpseeds, is placed in the top-most list (i.e. with announce and info). This key's value is a list of web addresses where torrent data can be retrieved:
{
...
'httpseeds': ['http://www.site1.com/source1.php', 'http://www.site2.com/source2.php']
...
}
Private torrents
BEP-0027<ref>Template:Cite web</ref> extends BitTorrent to support private torrents.
A new key, private, is placed in the info dictionary. This key's value is 1 if the torrent is private:
{
...
'private': 1
...
}
Merkle trees
BEP-0030<ref>Template:Cite web</ref> extends BitTorrent to support Merkle trees.
A torrent file using Merkle trees does not have a pieces key in the info list. Instead, such a torrent file has a root hash key in the info list. This key's value is the root hash of the Merkle hash:
{
...
'info': {
...
'root hash': e6bdebcc5d55da0a77f4bb1b57d88de794838577
...
}
...
}
Examples
Single file
Here is what a de-bencoded torrent file (with piece length 256KiB = 262144 bytes) for a file debian-503-amd64-CD-1.iso (whose size is 647MiB = 678301696 bytes) might look like:
{'announce': 'http://bttracker.debian.org:6969/announce'
'info': {'name': 'debian-503-amd64-CD-1.iso',
'piece length': 262144,
'length': 678301696,
'pieces': '841ae846bc5b6d7bd6e9aa3dd9e551559c82abc1 ... d14f1631d776008f83772ee170c42411618190a4'
}
}
Note: pieces here would be a ceil(length/piece length) * 160-bit = ceil(678301696/262144) * 160-bit = ceil(2587.515625) * 160-bit = 2588 * 160-bit = 414080-bit = 51KiB value.
Multiple files
Here is what a de-bencoded torrent file (with piece length 256KiB = 262144) for two files, 111.txt & 222.txt, might look like:
{'announce': 'http://tracker.site1.com/announce',
'info: {'name': 'directoryName',
'piece length': 262144,
'files': [ {'path': '111.txt', 'length': 111},
{'path': '222.txt', 'length': 222}
],
'pieces': '6a8af7eda90ba9f851831073c48ea6b7b7e9feeb...8a43d9d965a47f75488d3fb47d2c586337a20b9f'
}
}