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

[Doug Arnold <[e-mail]>]

The description of stratum in RFC 5905 is a little vague, so your case is open to interpretation.

I am assuming that the server receiving a PPS signal also receives ntp in order to know which second it is, and to know the health of its source of time.  The purpose of stratum numbers is to give a rough indication of the accuracy of the server.  If you calibrate the PPS cable delay, then the accuracy of your PPS receiving server should be nearly as good as the PPS sending one, and it would appropriate for the PPS receiving server to present itself as the same stratum as the PPS sending one.  It is not really different from an ntp server card in a modular time server which gets its time from the backplane of the chassis via pps and ntp.   You just have a long skinny chassis.

Doug

________________________________
From: Ulrich Windl <[e-mail]>
Sent: Friday, April 24, 2020 1:29 AM
To: [e-mail] <[e-mail]>; [e-mail] <[e-mail]>
Cc: [e-mail] <[e-mail]>; Doug Arnold <[e-mail]>; [e-mail] <[e-mail]>; [e-mail] <[e-mail]>
Subject: Antw: [EXT] Re: [Ntp] The bump, or why NTP v5 must specify impulse response

>>> Kurt Roeckx <[e-mail]> schrieb am 23.04.2020 um 22:18 in Nachricht
<[e-mail]>:
> On Thu, Apr 23, 2020 at 01:38:59PM -0600, Philip Prindeville wrote:
>> Not sure I agree…
>>
>>
>> > On Apr 23, 2020, at 12:37 PM, Doug Arnold <[e-mail]>

> wrote:
>> >
>> > User case A:
>> > A Stratum 2 server get it's time from a pool of stratum 1 servers through

> the public internet and distributes it to clients in computers, servers and

> routers in a large enterprise network.  The clients need time to about 1
> second for log file entries and security time out intervals.  Accuracy at
the
> stratum 2 server and clients is almost completely determined by queuing
noise
> from so many hops that the noise is almost Gaussian.  After removal of
> outliers the stratum 2 server and clients average clock corrections in a PLL

> based on a low pass filter.  Because accuracy requirements are loose,
clients
> request updates only every few minutes.
>>
>>
>> As you get into 10GBE networking, events can happen in extremely small
> intervals.
>>
>> Half a second of variance either way can completely obscure if two events
> are related, which happened first, what’s cause and what’s effect, etc.  I’d

> say that millisecond accuracy is the minimum useful precision in Enterprise

> and data center scenarios.
>
> I think your requirements seem to fall under use case B, not A.
>
> If you want millisecond accuracy, the stratum 1 servers should
> probably be in the order of 10 millisecond away, and always
> reachable, never congested, and you probably need an update rate
> that's smaller than 1 minute. And I'm not sure that 10 millisecond
> is actually good enough.

I'm just wondering (stupid idea following):
Can you configure a PPS-signal alone (meaning: time comes from higher (lower
number) stratum servers)? If so does it affect the stratum that server would
have? I one had the idea of reusing old "thin-wire" Ethernet cables with
BNC-T-adapters for distributing a PPS signal, but I never actually did it...
That PPS thing always had been some part of the monolithic NTP algorithm, and
some parts of the protocol rely on it (like "kerninfo"), while others don't. I
think a future NTP spec could modularize PPS much more...

>
>
> Kurt
>
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