bittorrent-codex-docs/10 Notes/BitTorrent metadata files.md

tags	related-to
bittorrent	Learn BitTorrent Advertising BitTorrent content on Codex

#bittorrent

related-to	Learn BitTorrent, Advertising BitTorrent content on Codex

Called torrent files.

Although we are using trackerless torrents, still the information contained in torrent files is indispensable and will be provided by the peers using number of extension mechanism. We talk more about it in Advertising BitTorrent content on Codex.

BitTorrent v1

Files are split into fixed-size pieces of the same length except for maybe the last one that may be truncated. Almost always power of two, most commonly 2^18 = 256 KiB (older versions prior to v 3.2 used 2^20 = 1 MiB as default). For directories, the files are concatenated in the order in which they appear in the file list (see the files key in the info dictionary below).

Metainfo files, or .torrent files are bencoded dictionaries with the following keys:

• announce - the url of the tracker
• info - maps to a dictionaries with the following keys:
  • name - suggested name to save a file or dictionary - purely advisory
  • piece length - number of bytes each piece is split info

BitTorrent v2

Here the info dictionary, has the following keys:

• announce - the url of the tracker
• info
  • file tree - a three structure describing each file position in the file tree
    • length
    • pieces root - Merkle tree toor for the given file
  • piece length - as in v1, number of bytes each piece is split info
  • meta version - set to 2
  • name - as in v1 above
• piece layers - dictionary with entries for each file larger than one piece: key being the corresponding pieces root and value being string being a concatenation of piece layer hashes for that file.

We have extensive examples showing how the torrent files are constructed. They can be found under the following url: https://link.excalidraw.com/readonly/c9z3gLsV1qkTSQJUxEXk