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Re: [Ntp] Temperature Compensation for NTP?

Kurt Roeckx <kurt@roeckx.be> Wed, 09 December 2020 19:45 UTC

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Date: Wed, 09 Dec 2020 20:45:46 +0100
From: Kurt Roeckx <kurt@roeckx.be>
To: Hal Murray <hmurray@megapathdsl.net>
Cc: ntp@ietf.org
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References: <stenn@nwtime.org> <d406413e-c539-cb74-daf4-5298a8a1db83@nwtime.org> <20201208233226.B32FA40605C@ip-64-139-1-69.sjc.megapath.net>
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Subject: Re: [Ntp] Temperature Compensation for NTP?
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On Tue, Dec 08, 2020 at 03:32:26PM -0800, Hal Murray wrote:
> kurt@roeckx.be said:
> > I think defining NTP as a impulse response is the wrong approach. It's an
> > implementation detail. If other systems use different algorithms, it should
> > only have an effect on the uncertainty. If it causes big problems, it's an
> > implementation problem. 
> 
> Not quite.  It's easy to get problems when chaining PLLs.  I don't understand 
> it well enough to explain and I don't have a handy reference.
> 
> The typical problem is that somebody tweaks a time constant so his system 
> recovers faster, then months later something crazy happens when triggered by 
> the right input data.

As far as I know, NTP's impulse response overshoots to have a
faster response. And gettings a chain to work with overshooting
complicates things, because it's possible that each next host
overshoots more. As I understand that, to avoid that problem,
NTP does not tell what it's best estimate of the time is, but
slowly moves to that time.


Kurt

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