Re: [Cbor] Multiple ways of expressing the same time via draft-bormann-cbor-time-tag

[Carsten Bormann <[e-mail]>]

Hi Emile,

On 2021-04-06, at 22:35, Emile Cormier <[e-mail]> wrote:
> 
> I'm implementing tags 1001 and 1002 in my C++ library to exchange time points and durations (refer to https://www.ietf.org/archive/id/draft-bormann-cbor-time-tag-04.html).
> 
> I'm in the situation where I''m not sure which is the "expected" way to encode time as nanoseconds since the Unix epoch. I'm specifically referring to std::chrono::system_clock::time_point, which is a 64 bit integer. There are currently three ways I could encode such a value.
> 
> 1. The whole number of seconds under Key 1, with the remaining nanoseconds under Key -9.
> 
> 2. The total number of nanoseconds under Key -9, with a value of zero under Key 1.
> 
> 3. The total number of nanoseconds expressed as a Decimal Fraction under Key 4.

Oh.  You are pointing out a deficiency of the current text.  The *intention* was that Key 1 is an integer and Key -3*n is supplying the fractional part.  E.g., like UNIX struct timeval for -6 (old) and struct timespec for -9 (new) do.
So that would be your Option 1.

But since you raise it, we probably should entertain the question.

The issue really is whether the onus is on the sender/encoder or the recipient/decoder.

Restricting the value for key -3*n to be less than 10**(3*n) places the onus on the sender, who would need to do a modulo if that is not the way the data is already represented on the sender’s platform.
Not adding this restriction places the onus on the recipient, who would then need to do the modulo in case the recipient’s platform representation is like one of the UNIX ones.

> Option #2 is the easiest and most efficient because it doesn't involve a division/subtraction calculation.

… for the sender.

> However, a receiver that discards Key -9 will end up with zero seconds. At least with option #1, the receiver will know the number of whole seconds if Key -9 is ignored.

Which indeed maybe also is a concern.

> Option #3 requires that the sender and receiver understand Tag 4 (Decimal Fraction), or at least the concept of decimal fractions within the context of tags 1001-1003.

That would seem to be the easier case; “understanding” Tag 4 in this context is limited to being able to parse the 4([-6, microseconds]) arrangement, unless the sender’s code looks for trailing zeros and does stunts like 4([-5, tens-of-microseconds]) in these cases, which of course cannot be excluded.

> I'm guessing that option #1 is the "canonical" way, but I'm not entirely sure. Some guidance would be appreciated. Perhaps a few words concerning this should be added to the standard's text?

Definitely, either way.

I think I’m mildly in favor of placing the onus on the sender, i.e., Option 1.

etime = #6.1001({
  ...
  1: int,
  ? (
    -3: uint .lt 1000 //
    -6: uint .lt 1000000 //
    -9: uint .lt 1000000000 //
    -12: uint .lt 1000000000000 //
    -15: uint .lt 1000000000000000 //
    -18: uint .lt 1000000000000000000
  )
  ...
})

> On the receiving side, my library would ideally handle all three scenarios. It's on the sending side I'm more concerned about when I need to interoperate with CBOR decoders other than mine.

Right, be conservative…liberal…

Thanks for bringing this up!

One always learns a lot from implementation efforts.

Grüße, Carsten