[ntpwg] Testing NTP performance

[Danny Mayer]

Dave,

I'm not missing the point at all. NTP does it's best to calculate these 
values and it's best estimates of error. If there were a way to get a 
better estimate NTP would use it. Noone has yet come up with a better 
solution. Installing a refclock on the server will give a better idea of 
how good the calculated values are but then you might as well just use 
the refclock.

Danny
David L. Mills wrote:
> 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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> 
> 
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