Friday, February 27, 2009

Info - Bencoding (and XPCOM component)

Bencoding is used in the Bittorrent protocol to encode simple structures containing dictionaries, strings, integers, booleans, and lists in .torrent files and tracker responses.

.torrent (Metainfo file)
The contents of a basic .torrent file will identify the tracker server, that is later used to find peers, and also describes the info dictionary (or hash table, or associative array). The info dictionary plays a couple of roles.
It lists the files in the torrent, and their lengths. Secondly, it describes the concatenated binary data as specific sized pieces, and provides a checksum (in the form of a SHA1 hash) for each piece. Thirdly, the dictionary is re-bencoded and a SHA1 hash of the resulting byte array is used to identify the torrent.

Tracker response
The tracker response should either identify some error, such as the tracker not handling the SHA1 torrent identifier, or specifies information about the tracker and a list of peers who are also part of the identified torrent's swarm.

Implementation of Bencoding XPCOM component
IDL for Bencoding XPCOM component

var bencoding = Components.
// Encode an object into a string
var data = bencoding.bencode({
,info: {}
// Decode a string into an object
var obj = bencoding.bdecode(data);
More information on bencoding ( Bittorrent Protocol Spec (BEP0003)
More information on the .torrent structure ( Bittorrent Protocol Specification (BEP0003)
More information on the tracker response ( Bittorrent Protocol Spec (BEP0003)

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