Re: [Ntp] NTPv5: big picture

[Magnus Danielson <[e-mail]>]

Hi,

On 2021-01-05 09:54, Miroslav Lichvar wrote:
> On Mon, Jan 04, 2021 at 01:20:55PM -0700, Philip Prindeville wrote:
>> PTP uses TAI.  It doesn’t seem to have been an impediment for them.  What am I missing?  And how did these “good technical reasons” not apply here?
> PTP doesn't support TAI only. It supports TAI and an "arbitrary"
> timescale. TAI is used for synchronization of hardware clocks, which
> is the primary use case of PTP. If you wanted to synchronize a system
> clock with PTP, the timestamps would typically be in the arbitrary
> timescale (UTC).

PTP actually support both providing a TAI and UTC replica, as it also
carries the TAI-UTC difference in addition to having the core time-stamp
format that derives from TAI.

Now, most (all?) PTP grand masters (a term that I expect to go away)
that I know of use GPS and other GNSSes to create a TAI-respecting
PTP-timescale replica with correct TAI-UTC difference.

For media clocks, it is the PTP time and thus TAI-based time that is
used, not the UTC based time.

> To anyone proposing NTPv5 to support only a non-leaping timescale I'd
> suggest them to try to implement it first.
>
Regardless how we turn, we can not push the leap-seconds completely off
the table. They need to be handled up front. If so, they can be handled
fairly OK, where as trying to ignore them just create a number of
problematic side-cases. I know what I prefer based out of my experience
of building timing systems.

Cheers,
Magnus