Re: [Ntp] symmetricom tp1100 stratum and leap indicator behavior

Martin Burnicki <martin.burnicki@meinberg.de> Thu, 01 August 2019 07:16 UTC

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To: Andy Harrison <aharrison+ntp@gmail.com>, ntp@ietf.org
References: <CACJmLABdDsvzwv-xsXhLWmkYmuwp=4A+xirmbRy=AeCSsi0Jag@mail.gmail.com>
From: Martin Burnicki <martin.burnicki@meinberg.de>
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Date: Thu, 1 Aug 2019 09:16:31 +0200
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Archived-At: <https://mailarchive.ietf.org/arch/msg/ntp/B5lj9VqtyhIVyB1KHnnnhkg8sek>
Subject: Re: [Ntp] symmetricom tp1100 stratum and leap indicator behavior
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Andy,

basically, what Harlan wrote is correct: this is not the appropriate
discussion forum for this topic.

Andy Harrison wrote:
> Now for the hard part.  What we want to happen is that if the GPS of the
> primary Symmetricom goes offline, we want the clients to start getting
> time from the backup unit.
> 
> Ideally what would happen is the GPS goes offline, the Symmetricom would
> set itself to a higher stratum level and the clients abandon it for the
> device that has the better stratum, but this is not what happens.

The short answer is:

An NTP server server running ntpd, which has been accurately
synchronized before, does *not* change its stratum when its only time
source becomes unavailable. Instead, the root dispersion starts
increasing, so the client can see that the server's time is drifting.

The client can then decide whether to switch to a different time source
(if the client has another, better source available) or to stick with
the drifting server anyway.

In the latter case the clients would at least all drift at the same rate
as the server. If they clients ignored the server instead, the time on
each client would drift differently, which would be even worse.

This is how client mode is implemented in ntpd, but other client
implementations could behave differently. So you see the "best" behavior
of the server depends strongly on the software running on the clients.

You can find some more details here:

https://kb.meinbergglobal.com/kb/time_sync/ntp/ntp_basics

And specifically:

https://kb.meinbergglobal.com/kb/time_sync/ntp/ntp_basics#should_an_ntp_client_generally_discard_a_server_in_holdover_mode


Martin
-- 
Martin Burnicki

Senior Software Engineer

MEINBERG Funkuhren GmbH & Co. KG
Email: martin.burnicki@meinberg.de
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