Multicodec is an agreed-upon codec table. It is designed for use in binary representations, such as keys or identifiers (i.e [CID](https://github.com/ipld/cid)).
The code of a multicodec is usually encoded as unsigned varint as defined by [multiformats/unsigned-varint](https://github.com/multiformats/unsigned-varint). It is then used as a prefix to identify the data that follows.
Multicodec is used in various [Multiformats](https://github.com/multiformats/multiformats). In [Multihash](https://github.com/multiformats/multihash) it is used to identify the hashes, in the machine-readable [Multiaddr](https://github.com/multiformats/multiaddr) to identify components such as IP addresses, domain names, identities, etc.
Find the canonical table of multicodecs at [table.csv](/table.csv). There's also a sortable [viewer](https://ipfs.io/ipfs/QmXec1jjwzxWJoNbxQF5KffL8q6hFXm9QwUGaa3wKGk6dT/#title=Multicodecs&src=https://raw.githubusercontent.com/multiformats/multicodec/master/table.csv).
NOTE: Just because a codec is marked draft, don't assume that it can be re-assigned. Check to see if it ever gained wide adoption and, if so, mark it as permanent.
This ["first come, first assign"](https://github.com/multiformats/multicodec/pull/16#issuecomment-260146609) policy is a way to assign codes as they are most needed, without increasing the size of the table (and therefore the size of the multicodecs) too rapidly.
The first 127 bits are encoded as a single-byte varint, hence they are reserved for the most widely used multicodecs. So if you are adding your own codec to the table, you most likely would want to ask for a codec bigger than `0x80`.
Codec names should be easily convertible to constants in common programming languages using basic transformation rules (e.g. upper-case, conversion of `-` to `_`, etc.). Therefore they should contain alphanumeric characters, with the first character being alphabetic. The primary delimiter for multi-part names should be `-`, with `_` reserved for cases where a secondary delimiter is required. For example: `bls12_381-g1-pub` contains 3 parts: `bls_381`, `g1` and `pub`, where `bls_381` is "BLS 381" which is not commonly written as "BLS381" and therefore requires a secondary separator.
The `validate.py` script can be used to validate the table once it's edited.
Yes, but we already have to agree on what protocols themselves are, so this is not so hard. The table even leaves some room for custom protocol paths, or you can use your own tables. The standard table is only for common things.
For a period of time, the [multibase](https://github.com/multiformats/multibase) prefixes lived in this table. However, multibase prefixes are *symbols* that may map to *multiple* underlying byte representations (that may overlap with byte sequences used for other multicodecs). Including them in a table for binary/byte identifiers lead to more confusion than it solved.
Sure, you can use multicodec whenever you have the need for self-identifiable data. Just prefix your own data with the corresponding varint encodec multicodec.
Contributions welcome. Please check out [the issues](https://github.com/multiformats/multicodec/issues).
Check out our [contributing document](https://github.com/multiformats/multiformats/blob/master/contributing.md) for more information on how we work, and about contributing in general. Please be aware that all interactions related to multiformats are subject to the IPFS [Code of Conduct](https://github.com/ipfs/community/blob/master/code-of-conduct.md).
