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Re: [Ntp] Antw: Re: Antw: Re: Antw: Re: Calls for Adoption -- NTP Extension Field drafts -- Four separate drafts

Watson Ladd <watsonbladd@gmail.com> Thu, 12 September 2019 14:10 UTC

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From: Watson Ladd <watsonbladd@gmail.com>
Date: Thu, 12 Sep 2019 07:10:42 -0700
Message-ID: <CACsn0cnNRJjHmLwak0YThUO02bbEQ5rA9=97LQT5SaJiy6_t7g@mail.gmail.com>
To: Ulrich Windl <Ulrich.Windl@rz.uni-regensburg.de>
Cc: NTP WG <ntp@ietf.org>, Harlan Stenn <stenn@nwtime.org>
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Subject: Re: [Ntp] Antw: Re: Antw: Re: Antw: Re: Calls for Adoption -- NTP Extension Field drafts -- Four separate drafts
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On Thu, Sep 12, 2019, 5:33 AM Ulrich Windl <
Ulrich.Windl@rz.uni-regensburg.de> wrote:

> >>> Harlan Stenn <stenn@nwtime.org> schrieb am 12.09.2019 um 13:03 in
> Nachricht
> <edb8aa94-a667-a68a-b7e1-f18495c81aab@nwtime.org>:
>
> >
> > On 9/12/2019 3:15 AM, Ulrich Windl wrote:
> >>>>> Miroslav Lichvar <mlichvar@redhat.com> schrieb am 12.09.2019 um
> 11:46
> in
> >> Nachricht <20190912094603.GN21704@localhost>:
> >>> On Mon, Sep 02, 2019 at 05:52:33AM ‑0700, Harlan Stenn wrote:
> >>>> The thing is that NTP is a complete specification with a goal of time
> >>>> synchronization that includes *known behavior in the face of impulse
> >>>> response*.
> >>>>
> >>>> If one uses different algorithms with different impulse response
> >>>> behaviors, this can trigger oscillations and even unstable tracking
> (the
> >>>> wheels fly off the bus).
> >>>
> >>> This was discussed a bit on the last meeting.
> >>>
> >>> Does anyone have an example of a PLL that is unstable when cascaded
> >>> with a different loop? If something works well enough to be stable
> >>
> >> I think it was the old Dave Mills consensus like: "It's only NTP if it
> >> provides the same output for the same input". So if you think of some
> >> improvements, they still would not be allowed (I had some discussions
> while
> >> implementing PPS in the Linux kernel)
> >
> > It's not only about instability.
> >
> > It's about known and agreed-upon impulse behavior.
>
> But is the overshoot really well-defined NTP behavior? Are you saying that
> anything that doesn't overshoot is not NTP?
>

The problem is cascading can cause the overshoot to grow as I understand it.

>
> >
> > --
> > Harlan Stenn, Network Time Foundation
> > http://nwtime.org - be a Member!
> >
> > _______________________________________________
> > ntp mailing list
> > ntp@ietf.org
> > https://www.ietf.org/mailman/listinfo/ntp
>
>
>
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