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SPEC.md

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[Draft] IPLD UnixFS Spec

Basic Structure

A Unixfs is either a file or a directory.

This specification uses the term "path(s)" to refer to specific key/value pairs. Every path could be either an in-line value or a Link. This means that a single UnixFS tree could be entirely inlined into a single Block or could be many Blocks connected by CID Links.

A top level IPLD object has at least two paths: type and data and maybe a few other such as a version string or a set of flags.

The type path's value is either file or dir.

IPLD file

A file object has the following paths.

  • type: String with value of 'file'.
  • data: Array of file-data.
  • size: Integer. Cumulative size of data.

The type path must be set to file.

file-data

File data is an Array. Each element is an Array with only 2 elements (Tuple).

  • 0: Array. Tuple containing two integers, the start and length offsets of the content.
    • 0: Integer: start offset.
    • 1: Integer: length of part(s).
  • 1: Binary or file-data. Value can be either the binary data or additional nested file-data structures.
[
  [ [0, 1024], Link ],
  [ [1025, 1002], Link ]
]

Implementations are encouraged to use nested file-data array nodes through links for files with many chunks in order to limit the size of the serialized node. No strict boundary is set but it is typical to try and limit node size to less than one megabyte.

IPLD dir

A directory object represents a directory and has the following paths.

  • type: String with value of 'dir'.
  • data:
    • Object (or hamt) with keys as file/directory names that are string encoded in UTF-8.
    • Values must be file or dir nodes.
  • size: Integer. Cumulative size of all directories and files in data.

The type path must be set to dir.

When the keyspace is too large for a single node data should be a link to hamt node. hamt has not yet been specified but will follow the implementation here.