Re: [Ntp] Antw: [EXT] comments on draft‑mlichvar‑ntp‑ntpv5‑03

[Miroslav Lichvar <[e-mail]>]

On Wed, Nov 24, 2021 at 08:32:09AM +0100, Ulrich Windl wrote:
> I wonder: Up to NTPv3 floating-point math was avoided, but starting in NTPv4
> floating-point math is used.

That is up to the implementation. The specification doesn't require
floating-point math, but it is easier to implement if you can use it.

> If so, are those many special fixed-point formats justified (outside of
> packets)?

For internal calculations, you could keep everything in the 64-bit
format. Even in the NTPv5 packet we could do that, if we don't mind
wasting 16 octets per packet. The specification would certainly be
simpler.

I implemented recently some of the NTPv5 ideas in chrony in an
experimental extension field. I noticed there is a minor compatibility
issue with the proposed time32 format and NTPv4. The maximum value is
not really 16, but slightly less than that. That means it cannot reach
the MAXDISP value from RFC5905. To avoid having to work with two
different MAXDISP values, we could specify an infinite value for the
maximum 32-bit value of the type.

-- 
Miroslav Lichvar