[Cbor] extensible enumerations, ignoring unknown values in CDDL

[Rohan Mahy <[e-mail]>]

Hi,

In the MIMI content draft [1], I used C structs as a placeholder for a
concrete syntax, which I am now converting into TLS Presentation Language
and CBOR.

One of the enums we are using, "Disposition" should have a valid range from
0 to 255, but only the values from 0 to 8 have known values. The intent is
that new values between 9 and 255 inclusive will be valid, and if unknown
they will be treated as if they had the default disposition (render).

enum Disposition {
    unspecified = 0,
    render = 1,
    reaction = 2,
    profile = 3,
    inline = 4,
    icon = 5,
    attachment = 6,
    session = 7,
    preview = 8
};

I am currently just expressing this inside as "disposition: uint .size 1",
but I'd like to be more expressive if possible.

If I only wanted to allow these predefined values but allow them to be
extended later I could use a choice now (Disposition = render / reaction /
etc.) and add to that later with "Disposition /= foo / bar", but I still
want to allow unknown values <= 255. It doesn't look like I could define
this, then later redefine unknown-dispo:
Disposition = render / .. / preview / unknown-dispo
unknown-dispo = 9..255

Does anyone have a good way to accomplish this in CDDL?

(I have read draft-bormann-cbor-e-ref and while adjacent didn't seem to
solve this problem).

Thanks,
-rohan

[1]
https://www.ietf.org/archive/id/draft-ietf-mimi-content-02.html#section-4.3