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Add zstd compression #866

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12 changes: 12 additions & 0 deletions cellar-matroska5/matroska5_body.md
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All the new elements added in (#new-elements) have a `minver` attribute of "5".
If they are present in a Matroska file the EBML Header **MUST** have a `DocTypeVersion` of "5" or more, see [@!RFC8794, section 11.2.7]

# Zstandard Compression

This document adds value "4" to the `ContentCompAlgo` element ([@RFC9559, section 5.1.4.1.31.6]).
It corresponds to the Zstandard (zstd) compression algorithm [@!RFC8878].

A compressed chunk of data in Matroska, based on the `ContentEncodingScope` ([@RFC9559, section 5.1.4.1.31.3]),
consists of a Zstandard Frame, as defined in [@!RFC8878, section 3.1.1] without the 4 bytes `Magic_Number` 0xFD2FB528.

When the Zstandard compression algorithm is used, the `ContentCompSettings` element ([@RFC9559, section 5.1.4.1.31.7])
**MAY** optionally contain a dictionary to improve compression efficiency.
The dictionary Format correspond to the Dictionary Format described in [@!RFC8878, section 5] without the 4 bytes `Magic_Number` 0xEC30A437.

# New Elements

5 changes: 5 additions & 0 deletions cellar-matroska5/matroska5_iana.md
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# IANA Considerations

## Matroska Compression Algorithms Registry Additions

This document adds value "4" to the "Matroska Compression Algorithms" registry.
It corresponds to the Zstandard compression algorithm described in section (#zstandard-compression).

## Matroska Codec IDs Registry Additions

This document adds the following Codec IDs to the "Matroska Codec IDs" Registry.
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3 changes: 3 additions & 0 deletions ebml_matroska.xml
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Expand Up @@ -1202,6 +1202,9 @@ Each block **MUST** be decompressable, even if no previous block is available in
<enum value="3" label="Header Stripping">
<documentation lang="en" purpose="definition">Octets in `ContentCompSettings` ((#contentcompsettings-element)) have been stripped from each frame.</documentation>
</enum>
<enum value="4" label="Zstandard" minver="5">
<documentation lang="en" purpose="definition">Zstandard (zstd) compression [@!RFC8878].</documentation>
</enum>
</restriction>
<extension type="stream copy" keep="1"/>
</element>
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