[ntpwg] Testing NTP performance

[David L. Mills]

Danny,

With due respect, you guys are missing the point. The NTP design, 
specification and implementation have very specific performance 
objectives and metrics. The sources of error have been carefully 
modelled and justified, by analysis and experiment. You might argue 
about the specific error budget, but it is carefully exposed both in the 
literature and specification. Even the vulnerability to Byzantine 
attacks has been carefully considered and defended.

In your question below the measured offset is the maximum liklihood 
estimate of the client offset relative to the server, while the jitter 
represents the estimated error of the offset estimate itself. The 
synchronization distance represents the maximum error due all causes 
with the assumption that the maximum inherent clock frequency error is 
bounded. The assumed parameters for phase and frequency error estimate 
are represented by the limbs of the Allan deviation specific to each 
installation. There's not much more than can be said abount the 
probabilistics.

There is no quantum physics here, although there might be a dose of 
long-range depndency on oscillator frequency. You can of course argue 
that the specified performance metrics are flawed and incomplete. But, 
the basic metrics called out in the specifidation should in fact provide 
sufficient evidence for an informed judgement of performance.

Time and time again folks have requested absolute bounds on performance, 
such as required accuracy (sic) of one milliseoond within one minute 
after startup. I use as example an aircraft autopilot. Is it required to 
place the aircraft within one meter of the centerling of runway 27-right 
at Heathrow? Under all conditions? Even if the ILS is not operating? 
Does the landing gear survive an altitude error of one meter? Is NTP 
operating correctly if required to slew over one minute and the 
application reserves two passengers for the same seat or sells a stock 
before it is bought?

In short, there are no definitive answers to your questions, only 
statistics.

Dave

Danny Mayer wrote:

> Rob Seaman wrote:
>
>> Not to get too philosophical, but isn't this a description of the
>> precise problem that NTP itself is designed to solve?
>>
>> Rob Seaman
>> NOAO
>
>
> I think that the real questions that needs to be answered are:
> 1) How accurate is the estimated offset that the server is using at any
> point in time;
> 2) What is the error in the calculated frequency that the server has
> estimated;
> 3) What is the error of the jitter that the server has calculated?
>
> There are two possibilities here:
> 1) The server itself calculates these values so it will have an estimate
> of the error;
> 2) An external estimate of these errors introduces you to quantum
> physics issues, the measurement affects the values being measured.
>
> Does this restate the question better?
>
> Danny
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