[Cbor] CDDL style questions

[Jim Hague <[e-mail]>]

A couple of CDDL style questions arising from the progress of our C-DNS
draft, which includes a CDDL spec. I'm not asking for any 'right'
answer, more curious to know whether there is such a thing as an
emerging consensus on CDDL style. Or just whether CDDL users out there
Have Opinions.

o Specifying fields with constant values. Should field types be explicit
or implicit?

FileHeader = {
    "identifier1" => "A string",
    "identifier2" => tstr .eq "Another string",
    "value1" => 1,
    "value2" => uint .eq 2,
}

I'm torn on this at the moment. Assuming the implementer of the format
is not a CDDL/CBOR sophisticate, including explicit types may be
valuable. But implicit is pleasingly succinct.

o Use of .size and ranges to restrict contents of fields.

The C-DNS file format obviously includes a lot of IP and DNS specific
quantities. In the main, I have not attempted to specify value
ranges/storage sizes for these - apart from anything else, I started on
this before the necessary facilities were added to CDDL:

    ? server-port           => uint,
    ? query-opcode          => uint,
    ? query-rcode           => uint,

The exception is for IP addresses where we take our lead from the CDDL
draft:

    IPv4Address = bstr .size 4
    IPv6Address = bstr .size 16
    IPAddress = IPv4Address / IPv6Address

I think my motivation here is to try minimise 'magic numbers' and define
a type rather than have 'bstr .size 16' all over the place.

It's been suggested, though, that we could:

    Port = uint .size 2
    OpCode = 0..15
    RCode = 0..15
    ....
    ? server-port           => Port,
    ? query-opcode          => OpCode,
    ? query-rcode           => RCode,

My initial reaction was that this isn't a clear-cut win; it feels like
I'm duplicating IP/DNS protocol definitions which are specified
elsewhere. However, the more I look at it, the more I wonder if the
extra info is in fact valuable to an implementer.
-- 
Jim Hague - [e-mail]          Never trust a computer you can't lift.