Re: [Cbor] Eric Rescorla's Discuss on draft-ietf-cbor-cddl-06: (with DISCUSS and COMMENT)

[Benjamin Kaduk <[e-mail]>]

On Wed, Nov 21, 2018 at 02:10:24PM +0100, Carsten Bormann wrote:
> Hi Eric,
> 
> you probably won’t have time to read this before the telechat, but I would like to keep the ball rolling.

Jumping in on one specific point...

> (For some reason, my mail reader doesn’t pick up the quoting correctly in this reply; I hope it is still readable enough.)
> 
> Grüße, Carsten
> 
> 
> > On Nov 20, 2018, at 20:15, Eric Rescorla <[e-mail]> wrote:
> > 
> > On Tue, Nov 20, 2018 at 8:14 AM Carsten Bormann <[e-mail]> wrote:
> > 
> > > On Nov 20, 2018, at 02:46, Eric Rescorla <[e-mail]> wrote:
> > > 
[...]
> > > S 3.7.
> > >>                              buuid = #6.37(bstr)
> > >> 
> > >>     In the following example, usage of the tag 32 for URIs is optional:
> > >> 
> > >>                          my_uri = #6.32(tstr) / tstr
> > >> 
> > > 
> > > I am basically unable to make sense of this section. Your previous
> > > example of tags used #7.25 but here you are specifying everything as
> > > using 6.
> > 
> > 6 is CBOR’s major type for tags.  #7.25 is not a tag, but a (16-bit) floating point value.
> > 
> > Yes, I understand that.
> > 
> > 
> > The example shows how to construct tagged items by using the #6.xxx(ttt) syntax, where xxx is a tag value and ttt is the type of the data item enclosed by the tag.
> > 
> > > It seems like the semantics here are something to the effect of:
> > > 
> > > X = #6.Y(Z)
> > > 
> > > means: act as if this were a thing of type Z but it’s tagged by Y.
> 
> That is not how I would describe what tags are about.
> 
> E.g., using CBOR diagnostic notation, h’123456789a’ is a byte string, but 2(h’123456789a’) is the number 78187493520.
> This does no longer act as if it were a byte string.  It is tagged by the tag number 2 (which stands for unsigned bignum).
> 
> A type for tagged data items like this is described in the prelude by:
> 
> biguint = #6.2(bstr)

I'm still a bit confused on where the #M.N(x) syntax is formally defined.
Is it supposed to be part of CDDL's ABNF?

-Benjamin