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

Martin Burnicki <martin.burnicki@meinberg.de> Tue, 05 January 2021 16:12 UTC

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To: Miroslav Lichvar <mlichvar@redhat.com>, Hal Murray <hmurray@megapathdsl.net>
Cc: ntp@ietf.org
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From: Martin Burnicki <martin.burnicki@meinberg.de>
Organization: Meinberg Funkuhren GmbH & Co. KG, Bad Pyrmont, Germany
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Subject: Re: [Ntp] CLOCK_TAI (was NTPv5: big picture)
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Am 05.01.21 um 14:59 schrieb Miroslav Lichvar:
> On Tue, Jan 05, 2021 at 04:45:44AM -0800, Hal Murray wrote:
>>
>> mlichvar@redhat.com said:
>>> NTP is used for synchronization of various clocks. Most of them are based on
>>> UTC. NTPv5 cannot support only TAI. 
>>
>> This is a tangled area.
> 
> I'd say it's fairly simple.
> 
>> I think that a TAI only NTPv5 protocol would be much cleaner and easier to 
>> document.  Yes, I agree that we need a story for UTC, but the answer doesn't 
>> have to clutter up an RFC for NTPv5.
> 
> Much cleaner? It's just a single field needed to negotiate the
> timescale.
> 
>> All the kernel and/or libc need to convert TAI to/from UTC is the TAI-UTC 
>> offset (a small integer) and the date of the next leap second, a flag for 
>> insert/delete/none, and the valid-until date of the info.
> 
> Even if the kernel+libc fully supported TAI (they currently don't), it
> cannot work when the TAI-UTC is unknown, e.g. in an isolated network
> with a reference clock which doesn't provide this information. It
> doesn't matter how it is called or specified, but NTP has to support
> synchronization of UTC clocks when the TAI-UTC offset is unknown.
> Otherwise, it would be just an academic project of very limited value.

+10 (+1 wouldn't be enough, IMO ;-)

Martin
-- 
Martin Burnicki

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