[ntp:questions] FW: Clock Offset

David Woolley david at ex.djwhome.demon.invalid
Sun Dec 23 00:14:04 UTC 2012


unruh wrote:
> On 2012-12-22, Mischanko, Edward T <Edward.Mischanko at arcelormittal.com> wrote:

>> I do not doubt you, but if they are measuring different things
>> to represent the same thing, then there is a formula off
>> somewhere?
> 
> No, they do not "represent the same thing". They are different things.
> One is the instantaneously measured offset between the local clock and
> the servers. The other is the best estimate ntpd has for the difference
> between the local clock and true UTC time. 

I don't believe he is now comparing the overall estimate with the most 
recent individual measurements.  I think he is comparing the best 
estimate for an individual source, with the ntpq peers, most recent 
offset for that source.

The formula for an individual filtered offset is something like:

(z  ^ n) * Offset

where z is the Z-Transform operator and n is an natural number <= 7, 
such that (z ^ n) * estimated-offset-error  is minimal.   Of course, it 
is not described that way in the actual code, and I don't think it is 
described that way in the documentation.

I believe the error estimate includes the round trip delay, and the 
stratum, but it is that sort of thing that is subject to change.



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