Request: Clarify extensibility rules

[bemasc]

cTLS enumerates keys in the template, but does not explain how templates can be extended.

Suggestion: Clients must reject the whole template if it contains any key they do not understand. The “optional” key holds a map of optional extensions that are safe to ignore.

[hannestschofenig]

Thanks for addressing your own issue with a PR. Creating an IANA registry for the keys used in the configuration data makes sense.

I would, however, like to point out (and maybe we need to make this explicit) that the configuration information is defined for convenience but is not a mandatory-to-implement functionality for a stack.

[bemasc]

Sure, some use cases can pre-arrange, or even hardcode, a specific configuration through a nonstandard channel. Some use cases (where the client and server are developed separately) obviously cannot.