Immutable Extensions — MoQT draft-16 §11.2.

The Immutable Extensions (Extension Header Type 0xB) contains a sequence of Key-Value-Pairs (see Figure 2) which are also Track or Object Extension Headers.

Immutable Extensions {
  Type (0xB),
  Length (i),
  Key-Value-Pair (..) ...
}

This extension can be added by the Original Publisher, but MUST NOT be added by Relays. This extension MUST NOT be modified or removed. Relays MUST cache this extension if the Object is cached and MUST forward this extension if the enclosing Object is forwarded. Relays MAY decode and view these extensions.

A Track is considered malformed (see Section 2.4.2) if any of the following conditions are detected:

An Object contains an Immutable Extensions header that contains another Immutable Extensions key.
A Key-Value-Pair cannot be parsed.

The following figure shows an example Object structure with a combination of mutable and immutable extensions and end to end encrypted metadata in the Object payload.

                   Object Header                      Object Payload
<------------------------------------------------> <------------------->
+--------+-------+------------+-------+-----------+--------------------+
| Object | Ext 1 | Immutable  | Ext N | [Payload] | Private Extensions |
| Fields |       | Extensions |       | [Length]  | App Payload        |
+--------+-------+------------+-------+-----------+--------------------+
                  xxxxxxxxxxxx                     xxxxxxxxxxxxxxxxxxxx
                                                   yyyyyyyyyyyyyyyyyyyy
x = e2e Authenticated Data
y = e2e Encrypted Data
EXT 1 and EXT N can be modified or removed by Relays

An Object MUST NOT contain more than one instance of this extension header.