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Re: [Ntp] Antwort: Re: The bump, or why NTP v5 must specify impulse response

Doug Arnold <doug.arnold@meinberg-usa.com> Tue, 21 April 2020 15:23 UTC

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From: Doug Arnold <doug.arnold@meinberg-usa.com>
To: "kristof.teichel@ptb.de" <kristof.teichel@ptb.de>, Hal Murray <hmurray@megapathdsl.net>
CC: "ntp@ietf.org" <ntp@ietf.org>, Kurt Roeckx <kurt@roeckx.be>
Thread-Topic: [Ntp] Antwort: Re: The bump, or why NTP v5 must specify impulse response
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Subject: Re: [Ntp] Antwort: Re: The bump, or why NTP v5 must specify impulse response
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The ITU does this for their profiles of PTP in wireless backhaul, but it is huge amount of work.  They define network traffic test cases, develop models for local clocks, and run quite sophisticated simulations.  In the case of NTP the use variety of network conditions are more varied.  I fear that we would either end up with a proof which is useless to network operators because of the simplifying assumptions we have to make, or a PhD thesis amount of work.

Doug

________________________________
From: ntp <ntp-bounces@ietf.org> on behalf of kristof.teichel@ptb.de <kristof.teichel@ptb.de>
Sent: Tuesday, April 21, 2020 3:25 AM
To: Hal Murray <hmurray@megapathdsl.net>
Cc: ntp@ietf.org <ntp@ietf.org>; Kurt Roeckx <kurt@roeckx.be>
Subject: [Ntp] Antwort: Re: The bump, or why NTP v5 must specify impulse response

Moreover, I think it's always good to ensure that if you demand a certain performance level, you can give at least a description of a proof that that performance level can in fact be reached.

I also agree with Hal that the specification of performance guarantees is going to be tricky, since you're going to have to assume a lot of parameters.
Easiest is probably to formulate guarantees dynamically depending on achieved message in-flight roundtrip time per exchange, but even then you're making assumptions about the drift and other behavior of the client's clock between the two timestamps that it takes.


Best regards,
Kristof



-----"ntp" <ntp-bounces@ietf.org<mailto:ntp-bounces@ietf.org>> schrieb: -----
An: "Kurt Roeckx" <kurt@roeckx.be<mailto:kurt@roeckx.be>>
Von: "Hal Murray"
Gesendet von: "ntp"
Datum: 17.04.2020 23:25
Kopie: ntp@ietf.org<mailto:ntp@ietf.org>, hmurray@megapathdsl.net<mailto:hmurray@megapathdsl.net>
Betreff: Re: [Ntp] The bump, or why NTP v5 must specify impulse response

kurt@roeckx.be<mailto:kurt@roeckx.be> said:
> Maybe it's useful to have performance guarantees specified, but leave the
> algorithm how to get there either to the implementations, or in a separate
> document.

If you are going to discuss performance guarantees, you have to start with a
description of the input data.

How would you describe the traffic parameters of the internet?

You also get to consider the effects of temperature on the system crystal.  I
think I could at least come up with some parameters for that.

Interrupt latency, Scheduler, ...


--
These are my opinions.  I hate spam.



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