Code change:
"struct { Extension extensions<0..2^32-1>; } GroupContextExtensions; " <del> A member of the group applies a GroupContextExtensions proposal by removing all of the existing extensions from the GroupContext object for the group and replacing them with the list of extensions in the proposal.  (This is a wholesale replacement, not a merge.  An extension is only carried over if the sender of the proposal includes it in the new list.)  Note that once the GroupContext is updated, its inclusion in the confirmation_tag by way of the key schedule will confirm that all members of the group agree on the extensions in use. </del> <ins> A member of the group applies a GroupContextExtensions proposal with the following steps: If the new extensions include a "required_capabilities" extension, verify that all members of the group support the required capabilities (including those added in the same commit, and excluding those removed). Remove all of the existing extensions from the GroupContext object for the group and replacing them with the list of extensions in the proposal.  (This is a wholesale replacement, not a merge.  An extension is only carried over if the sender of the proposal includes it in the new list.) Note that once the GroupContext is updated, its inclusion in the confirmation_tag by way of the key schedule will confirm that all members of the group agree on the extensions in use. </ins> 11.1.9.

Related discussion:
text: Proposal: Preparatory PR: Rationalize layout Move "Cryptograhic Objects" to before "Ratchet Trees" Update "Ratchet Tree Nodes" to define ParentNode and LeafNode structs, with LeafNode initially just equal to KeyPackage Move subsections of "Key Packages" to under "Ratchet Trees" Parent Hash Tree Hashes Update Paths Move Group State to its own top-level section Main PR: Change LeafNode to have the required content: HPKE key for TreeKEM Credential Parent hash Capabilities Extensions Signature Change KeyPackage to refer to LeafNode for internals, plus: HPKE key for Welcome Lifetime Extensions Signature Add continues to send KeyPackage Update and Commit send LeafNode instead of KeyPackage Remove and Sender refer to LeafNodeRef instead of KeyPackageRef We should probably do the preparatory PR regardless of this issue.
Discussion on working call: General agreement on the approach Should sign group_id when possible Include in TBS if we at least have a flag for pre-published / in-group Or we can store it Or we could have independent structs for pre-published / in-group leaves Do we need extensions at both levels? Capabilities / lifetime should probably be promoted to fields
Many protocols which have an extensibility mechanism have a way to indicate that understanding an extension is mandatory for certain behaviors. For MLS, this would mean having a way to require a member understand an MLS extension before it can join a group. The three extension types defined currently are: MLS ciphersuites, MLS extension types, and MLS credential types. One logical way to add mandatory extensions would be to pick one of these types (the MLS extensions type) and use the high order bit or a specific range to indicate that a member must understand and implement the extension to join a group which uses that extension. The IANA values could be changed as follows: Values from 0x8000-0xffff indicate understanding the extension is required 0x7f00 - 0x7fff Reserved for Private Use (not required) 0xff00 - 0xfff Reserved for Private Use (required) If a required ciphersuite or credential becomes necessary, a new extension type could be created to add an extension in the KeyPackage or GroupInfo which would be required and would indicate support for the mandatory ciphersuites or credentials.