Re: [Cbor] Invalid decimal fraction / big float?

[Laurence Lundblade <[e-mail]>]

> On Aug 25, 2019, at 6:19 AM, Carsten Bormann <[e-mail]> wrote:
> 
> On Aug 25, 2019, at 11:57, Laurence Lundblade <[e-mail]> wrote:
>> 
>> My inclination would be that they are both valid. If that is the choice, then decoders MUST handle both and that is not obvious from the text.
> 
> The intent (and the text) of RFC 7049 require that the second element of the array is either a basic integer (major type 0/1) or a tag 2/3 bignum.  
> 
> The meaning of your example with a byte string in that position may be clear to you, but it isn’t to me.  (It also isn’t allowed by RFC 7049.)  I would not propose that we start assigning that meaning to it after the fact.

OK. Tag 2/3 is required. 

It seemed worth checking to me because of 1) comments and emails during the recent Prague meeting about tags being hints and optional and 2) CWT expressly forbidding them. Wanted to be sure there was no assumptions being made. 

There seems to be no rule about when tags are present or not. Each protocol gets to decide between:
a) required — for example decimal fraction
b) forbidden — for example time CWT
c) optional — (I don’t know of an example)

A good generic decoders will handle all three, probably with some feature in the API to say which of the above scenarios to use.

But understood that a decimal fraction with an untagged big num is invalid.

LL