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

Martin Burnicki <martin.burnicki@meinberg.de> Thu, 07 January 2021 15:55 UTC

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To: Miroslav Lichvar <mlichvar@redhat.com>, FUSTE Emmanuel <emmanuel.fuste@thalesgroup.com>
Cc: "ntp@ietf.org" <ntp@ietf.org>, Magnus Danielson <magnus@rubidium.se>
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From: Martin Burnicki <martin.burnicki@meinberg.de>
Organization: Meinberg Funkuhren GmbH & Co. KG, Bad Pyrmont, Germany
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Date: Thu, 07 Jan 2021 16:55:32 +0100
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Subject: Re: [Ntp] CLOCK_TAI (was NTPv5: big picture)
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Miroslav Lichvar wrote:
> On Wed, Jan 06, 2021 at 09:57:56AM +0000, FUSTE Emmanuel wrote:
>>
>> I clearly don't understand the resistance to go to a linear TAI like 
>> form for the core time-stamping format as long as the TAI-UTC offset is 
>> part of the core protocol frame.
> 
> That offset is not always available, e.g. with a DCF77 refclock.
> Relying on other sources is not always acceptable.
> 
>> You will have to do some conversions and adaptations in the OS 
>> adaptation layer, as today for all NTP implementations, and this layer 
>> will not be messier as the core is today. No need to wait that operating 
>> systems fully support a non-leaping timescale, or TAI-UTC offset natively.
> 
> No, it will be messier. That's the other problem.
> 
> If your OS doesn't have a TAI clock or it doesn't support timestamping
> with the TAI clock, you will have to resolve the ambiguity in your NTP
> implementation. That will not be 100% reliable as you cannot be sure
> if the timestamp is captured before or after the leap. You are only
> guessing. If the client doesn't know that the server is only guessing,
> it may get unexpected errors in measurements and its clock disrupted.
> 
> The best thing you can do on such an OS is to suspend the operation
> around leap second, the same thing as when using a leaping timescale.
> In other words, there is no advantage of using a non-leaping timescale
> for the time transfer. It would only make things even more messier
> than they already are.

Once more, I strongly agree.

Martin
-- 
Martin Burnicki

Senior Software Engineer

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