> My concern is not with "new extensions" per se.  The hidden assumption here 
> is that "extensions" are the only way TLS can evolve.  In fact, future TLS 
> versions are not constrained to evolve in any particular way.  For example, 
> future versions can introduce entirely new messages in the handshake, or 
> remove messages that are currently visible in the handshake.  QUIC is 
> arguably just an extreme version of this observation.

I understand.  I used TLS extensions merely as an example.

> 
> Even within the realm of ClientHello extensions, there is significant 
> inflexibility here.  For example, consider the handling of GREASE extensions. 
>  GREASE uses a variety of reserved extension codepoints, specifically to make 
> sure that no entity is attempting to restrict use of unrecognized extensions. 
>  This proposal therefore has to add a flag declaring whether the client uses 
> GREASE, because otherwise the set of extensions is dynamic, and the number of 
> potential codepoints is impractically large.  Any change to the way GREASE 
> selects and rotates extension codepoints would therefore require a revision 
> of this YANG model first.  There has also been discussion of adding 
> GREASE-type behavior to the "supported_versions" extension; that would 
> similarly require a revised YANG model here.
> 

Probably greasing is something that needs a certain special handling.  Indeed 
that’s a form of fingerprinting (greases field XYZ).

Eliot

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