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Understanding the Torrent File Format and Bencoding
A torrent file is an independent session of transfer. To distribute a file, say an Ubuntu image, we would create a torrent holding the meta information about it while the actual content is split into pieces and distributed in the network.
Lifecycle of a Torrent
Torrent is alive so long as there is at least a seeder, seeding the pieces in the network. Once all the nodes who can share the pieces have left the network, the download of the network stops, and the torrent ends until a new seeder pops up.
What torrent file holds
The .torrent file holds all critical meta yet static information about the content, like
announce- holds the URL of the trackercreated by- name and version of the program who created itcreation date- time at which the torrent was createdencoding- encoding of the strings ininfodictionarycomment- some additional commentsinfo- a dictionary that describes files in the torrent
The info dictionary holds the information about the file that is being shared through the torrent. If the torrent is about the Ubuntu image, it will hold info like name, length, and md5sum of the actual file.
Given that the file is split into equal-length pieces, the info dictionary also holds
pieces- SHA1 of every piece of the file concatenatedpiece length- number of bytes in each piece
This information is used in identifying and fetching pieces from the network.
Bencoding
Torrent files are encoded with a custom encoding called Bencoding. It supports data types such as Strings, Integers, List, and Dictionaries (which can only hold string keys).
Strings are encoded as <length>:<string>. Hence, a string arpit will be encoded as 5:arpit.
Integers are encoded as i<integer>e. Hence, an integer 10 will be encoded as i10e.
Lists are encoded as l<becoded values>e. Hence, the list ["a","b", 1 ] will be encoded as l1:a1:bi1e1.
Dictionaries are encoded as d<key1><value1><key2><value2>e. Hence, a dictionary {"a": 1, "b": 2} will be encoded as d1:ai1e1:bi2ee.
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