Re: [Cbor] Martin Vigoureux's No Objection on draft-ietf-cbor-array-tags-07: (with COMMENT)

Carsten Bormann <cabo@tzi.org> Mon, 30 September 2019 12:49 UTC

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From: Carsten Bormann <cabo@tzi.org>
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Date: Mon, 30 Sep 2019 14:50:02 +0200
Cc: The IESG <iesg@ietf.org>, cbor@ietf.org, draft-ietf-cbor-array-tags@ietf.org, cbor-chairs@ietf.org, francesca.palombini@ericsson.com
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To: Martin Vigoureux <martin.vigoureux@nokia.com>
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Subject: Re: [Cbor] Martin Vigoureux's No Objection on draft-ietf-cbor-array-tags-07: (with COMMENT)
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On Sep 30, 2019, at 11:11, Martin Vigoureux via Datatracker <noreply@ietf.org> wrote:
> 
> […]
> The document, along with other ballot positions, can be found here:
> https://datatracker.ietf.org/doc/draft-ietf-cbor-array-tags/
> 
> 
> 
> ----------------------------------------------------------------------
> COMMENT:
> ----------------------------------------------------------------------
> 
> Hello,
> 
> thank you for this document. I have a minor question.
> 
>   IEEE 754 binary floating numbers are always signed.  Therefore, for
>   the float variants ("f" == 1), there is no need to distinguish
>   between signed and unsigned variants; the "s" bit is always zero.
> Since IEEE 754 binary floating numbers are always signed, I would have thought
> that s=1 would be used in conjunction with f=1. For my understanding, what was
> the reason for choosing s=0 instead?

Indeed, the current exposition makes s=0 for floating point this a little unexpected.
The structure of the assigments was worked out before that exposition was written.
(Essentially, it has the “usual” numbers under s=0 and “unusual” ones under s=1, as signed integers are a bit unusual in the constrained space. f=1 is floating point so we can use a contiguous range of 24 tag numbers with fs = 00, 01, 10.)
When we wrote up the current exposition and noticed the oddity, we didn’t really want to change the assignment again — this draft has a long history.  So we considered the benefit of keeping the arrangement stable more important than avoiding the oddity.

Thank you for reading this to the level of detail where this oddity became apparent!

Grüße, Carsten