[ntp:questions] Server offset included in served time?
martin.burnicki at meinberg.de
Mon Sep 15 08:04:58 UTC 2008
David Woolley wrote:
> Howard Barina wrote:
>> Does an NTP servers take into account it's estimated offset in serving
> 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.
But what about the behaviour shortly after startup? The NTP daemon tries to
determine the initial time offset from its upstream sources. Unless that
initial offset exceeds the 128 ms limit it starts to slew its system time
*very* slowly until the frequency drift has been compensated and the
estimated time offset has been minimized.
While the system time is being slewed it may be e.g. 120 ms off, and when
the daemon sends the system time to its clients then it will serve a time
which is 120 ms off.
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