Torrent file
Encyclopedia
A torrent file stores metadata used for BitTorrent. It is defined in the BitTorrent specification.
Simply, a torrent is data about a target file, though it contains no information about the content of the file. The only data that the torrent holds is information about the location of different pieces of the target file. Torrents work by dividing the target file into small information chunks
, found on an unlimited number of different hosts. Through this method, torrents are able to download large files quickly. When a client (the recipient of a target file) has initiated a torrent download, the chunks of target file that are needed can be found easily, based on the data from the torrent itself. Once all the chunks are downloaded the client can assemble them into a usable form. Note: the torrent must be completed before it can be assembled into a usable form. This differs from conventional internet downloads which have only one host and are usable even if the file is incomplete.
A torrent file contains the URLs of many trackers
and integrity metadata about all the pieces. It 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.
d dictionary with the following keys:
All strings must be UTF-8 encoded.
the torrent file uses the extension is ".torrent"
s.
A trackerless torrent dictionary does not have an announce key. Instead, a trackerless torrent has a nodes key:
{
...
'nodes':", ], ["", ], ...]
...
}
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."
A new key, announce-list, is placed in the top-most list (i.e. with announce and info)
...
}
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 ']
...
}
A new key, private, is placed in the info dictionary. This key's value is 1 if the torrent is private:
{
...
'private': 1
...
}
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
...
}
...
}
{'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.
{'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'
}
}
Simply, a torrent is data about a target file, though it contains no information about the content of the file. The only data that the torrent holds is information about the location of different pieces of the target file. Torrents work by dividing the target file into small information chunks
Chunk (information)
A chunk is a fragment of information which is used in many multimedia formats, such as PNG, MP3 and AVI.Each chunk contains a header which indicates some parameters A chunk is a fragment of information which is used in many multimedia formats, such as PNG, MP3 and AVI.Each chunk contains a header...
, found on an unlimited number of different hosts. Through this method, torrents are able to download large files quickly. When a client (the recipient of a target file) has initiated a torrent download, the chunks of target file that are needed can be found easily, based on the data from the torrent itself. Once all the chunks are downloaded the client can assemble them into a usable form. Note: the torrent must be completed before it can be assembled into a usable form. This differs from conventional internet downloads which have only one host and are usable even if the file is incomplete.
A torrent file contains the URLs of many trackers
BitTorrent tracker
A BitTorrent tracker is a server that assists in the communication between peers using the BitTorrent protocol. It is also, in the absence of extensions to the original protocol, the only major critical point, as clients are required to communicate with the tracker to initiate downloads...
and integrity metadata about all the pieces. It 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.
File structure
A torrent file is a bencodeBencode
Bencode is the encoding used by the peer-to-peer file sharing system BitTorrent for storing and transmitting loosely structured data.It supports four different types of values:* byte strings,* integers,* lists, and* dictionaries ....
d 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 218 = 256 KiB = 262,144 B.
- pieces - a hash listHash listIn computer science, a hash list is typically a list of hashes of the data blocks in a file or set of files. Lists of hashes are used for many different purposes, such as fast table lookup and distributed databases...
. That is, a 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. - length - size of the file in bytes (only when one file is 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:
- 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.
Draft extensions
These extensions are under consideration for standardization.the torrent file uses the extension is ".torrent"
Distributed hash tables
BEP-0005 extends BitTorrent to support distributed hash tableDistributed hash table
A distributed hash table is a class of a decentralized distributed system that provides a lookup service similar to a hash table; pairs are stored in a DHT, and any participating node can efficiently retrieve the value associated with a given key...
s.
A trackerless torrent dictionary does not have an announce key. Instead, a trackerless torrent has a nodes key:
{
...
'nodes':
...
}
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 extends BitTorrent to support multiple trackers.A new key, announce-list, is placed in the top-most list (i.e. with announce and info)
...
}
HTTP seeds
BEP-0017 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': ['
...
}
Private torrents
BEP-0027 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 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
...
}
...
}
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'
}
}