[ntp:questions] Server offset included in served time?

Unruh unruh-spam at physics.ubc.ca
Fri Sep 12 22:58:39 UTC 2008


"Richard B. Gilbert" <rgilbert88 at comcast.net> writes:

>David Woolley wrote:
>> Howard Barina wrote:
>>>
>>> Does an NTP servers take into account it's estimated offset in serving 
>>> time
>> 
>> There seem to have been a lot of questions asked in the last month that 
>> are based on the false assumption that "offset" measures the difference 
>> between the local clock and true time.  Has someone published a 
>> misleading document, somewhere?
>> 
>>> to others?  If I am a server and think I am 1.5 milliseconds off from 
>>> true
>>> time, will I include this in the timestamps of my ntp replies to others?
>> 
>> An NTP client never thinks that its time is wrong.  If it did, it would 
>> be admitting that the NTP algorithms are wrong.  Therefore the server 
>> always serves its client's idea of the time.
>> 
>> The "offset" should always be within the statistical error from the 
>> current measurement history (if ntpd suspects otherwise, it steps, 
>> and/or reduces the poll interval, to try to rapidly re-acquire that 
>> condition).  Immediately after startup, there will be little history, so 
>> quite large offsets will still be consistent with that history.
>> 
>> There are people who who argue that the NTP algorithms are fundamentally 
>> flawed and don't give the statistically best time in real world 
>> situations.  I think they have some credibility, but NTP's inventor, 
>> does not.
>> 

>I hope you did not mean to say that David Mills has no credibility but
>what you wrote sure reads that way!!

Just because A has credibility does not mean that B does not. credibility
is not an exclusive property. There can be disagreements between honest men
without one having "no credibility".

Note that the NTP algorithms are NOT fundamentally flawed. They are great.
They are just, in my opinion and measurements, not the best.




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