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

David Woolley david at ex.djwhome.demon.co.uk.invalid
Fri Sep 12 21:14:16 UTC 2008


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.

However, what one can certainly say is that combining "offset" with the 
local clock is very unlikely to give you a better time than just using 
the local time at all times except immediately after startup or after 
significant upsets.

"offset" is the difference between the clients best estimate of the true 
time and a weighted average of the most recent "best" measurements from 
its upstream servers.  Those measurements have a significant measurement 
error.




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