[ntp:questions] Single best NTP status indicator of clock accuracy?

unruh unruh at wormhole.physics.ubc.ca
Fri Dec 11 23:28:58 UTC 2009


On 2009-12-11, E-Mail Sent to this address will be added to the BlackLists <Null at BlackList.Anitech-Systems.invalid> wrote:
> unruh wrote:
>> The root dispersion is an estimate of the error in the remote
>>  clock's time.
>
> Only if you query the NTP server your NTP is peered with,
>  instead of querying your NTP server.
>
>
>> It is not an estimate of the difference in time between
>>  your clock and the true time.
>
> References?
>
> I contend root dispersion is, the diff to UTC.

It is were the difference to UTC then ntp would use it, rather than the
offset to discipline the clock. It is clearly stupid to use something
which is a bad estimate of the difference from true time to discipline
the clock. 


>
>
>> The best estimate of the difference between your clock
>>  time and the true time is the offset.
>
> References?
>
> I contend it is not.
>
>  That only gets you the diff to the NTP server(s) your NTP
>   server is syncing to.  Not the diff to UTC.

And unfortunately that is the best estimate you have. since the server
has no estimate of its offset from true time, neither do you. 

It has an estimate of the error from true time, but that is a) a very
conservative ( ie large) estimate, and b) you have no idea where in that
range from - to + the discrepancy lies. 

>
>
>>> <http://alumni.media.mit.edu/~nelson/research/ntp-survey99/html/>
>>> "dispersion of that root time: these measurements are useful
>>>   for determining the final accuracy of a host's clock"
>

It is good for estimating the accuracy, but not the offset. 




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