Add zstd compression

Based on ietf-wg-cellar#423.

Co-authored-by: rcombs <[email protected]>

Author: robUx4

cellar-matroska5/matroska5_body.md

@@ -3,5 +3,17 @@
 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
 

cellar-matroska5/matroska5_iana.md

@@ -1,5 +1,10 @@
 # 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.

ebml_matroska.xml

@@ -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>