Torrent file

Filename extension	.torrent
Internet media type	application/x-bittorrent
Standard(s)	BEP-0003[1]

A torrent file stores metadata used for BitTorrent. It is defined in the BitTorrent specification^[1].

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^[2]. These are known as "BitTorrent Enhancement Proposals". Examples of such proposals include metadata for stating who created the torrent, and when.

Contents 1 File structure 2 Extensions 2.1 Draft extensions 2.1.1 Distributed hash tables 2.1.2 Multiple trackers 2.1.3 HTTP seeds 2.1.4 Private torrents 2.1.5 Merkle trees 3 Examples 3.1 Single file 3.2 Multiple files 4 References 5 External Links

[edit] 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 dependant on whether one or more than one 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 = 256KB.
  • pieces - the concatenation of the SHA-1's of each piece. 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 to BitTorrent define more keys.

[edit] Extensions

[edit] Draft extensions

These extensions are under consideration for standardisation.

[edit] Distributed hash tables

BEP-0005^[3] 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."

[edit] Multiple trackers

BEP-0012^[4] 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 URLs of trackers:

 {   ...   'announce-list': [ "http://tracker.openbittorrent.com/announce", "http://bttracker.debian.org:6969/announce"]   ... } 

[edit] HTTP seeds

BEP-0017^[5] 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']   ... } 

[edit] Private torrents

BEP-0027^[6] extends BitTorrent to support private torrents.

A new key, private is placed in the top-most list (i.e. with announce and info). This key's value is 1 if the torrent is private:

 {   ...   'private': 1   ... } 

[edit] Merkle trees

BEP-0030^[7] 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 key': e6bdebcc5d55da0a77f4bb1b57d88de794838577             ...           }   ... } 

[edit] Examples

[edit] Single file

Here is what a de-bencoded torrent file (with piece length 256KB = 262144 bytes) for a file debian-503-amd64-CD-1.iso (whose size is 647MB = 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 = 51KB value.

[edit] Multiple files

Here is what a de-bencoded torrent file (with piece length 256KB = 262144) for a foo might look like:

 {'announce': 'http://tracker.openbittorrent.com/announce',  'info: {'name': 'directoryName',          'piece length': 262144',          'files': [ {'path': '111.txt', 'length': 111},                     {'path': '222.txt', 'length': 222}                   ],          'pieces:' '6a8af7eda90ba9f851831073c48ea6b7b7e9feeb ... 8a43d9d965a47f75488d3fb47d2c586337a20b9f'         } } 

[edit] References

[edit] External Links

• Official BitTorrent Specification
• Unofficial documentation of BitTorrent and its extensions

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