Re: [Ntp] CLOCK_TAI (was NTPv5: big picture)

[Martin Burnicki <[e-mail]>]

Magnus Danielson wrote:
> I agree that NTPv5 cannot support only TAI.
> 
> However, there comes the first challenge, do the core time-stamp format
> be that of a single format or of any timescale format. I think we agree
> that formats having the POSIX, Linux and NTPv4 type of leap-second jumps
> is to be avoided.

I really wonder what the "NTPv4 type of leap second jumps" is. If the
system supports leap seconds at all, at least ntpd just passes the leap
second announcement to the kernel. And the daemon relies on the
timestamps returned by the kernel.

If the kernel handles this in a way we don't consider "appropriate",
what could an NTP (or PTP or whatever) daemon do to fix it? Applications
will still suffer from limitations of the operating system.

> OK, so we want something more continuous and then
> concluding some variant of TAI-like property, similar to that of PTP is
> useful, not at all saying or suggesting it needs to be the PTP one.
> 
> Second challenge, if we have a TAI-like core format, and we indeed can
> rebuild a TAI replica, and we do not consider this enough, would a TAI
> format allows us to fairly cheaply adapt that to other formats? Turns
> out that this comes very cheap and is easy to do robust, if and only if,
> one has access to the TAI-UTC difference and information about an
> upcoming one as that eventually approaches.

Agreed.

> Will that satisfy all needs? Maybe not. OK, but will this provide a
> vehicle for more variants. Seems likely if we have a standard way of
> augmenting the core timing with additional parameters and users add the
> mapping.

That's also fine, but by default it should IMO be possible to provide
simple clients with time in a way as compatible as possible.

> Sure, I am advocating for a particular part of the solution space. But I
> think it is doable without any of the parts becoming cumbersomely
> complex to test and verify. An example is how PTP has extension fields
> extended by SMPTE 2059-2 and the various output mappings documented
> separately in SMPTE 2059-1 to show how the transported parameters
> generate all the legacy timing things. SMPTE 2110-10 then extends SMPTE
> 2059-2 and 2059-1 for the application within the SMPTE 2110 transport
> protocols and timing model of media used there. Anyway, I think it is a
> fair model that seems to work.

If a *simple* client needs to do this just to derive UTC (what's
probably the case for most not telco devices) than it's a wrong approach
to provide TAI and derive UTC from it.


Martin
-- 
Martin Burnicki

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