[ntp:questions] Bug 2341 - ntpd fails to keep up with clock drift at poll>7

Martin Burnicki martin.burnicki at meinberg.de
Mon Dec 2 12:02:27 UTC 2013


Mischanko, Edward T wrote:
> Martin,
> Reading your reply, it sounds like you understand and somewhat
> confirm this bug.

I *think* I have observed something like this some time ago.

> I agree this may be a Windows XP only bug.

I'm not sure whether it is. We'll have to find this out, but this can be 
very time-consuming.

We need to test both ntpd-stable (4.2.6p5) and ntpd-dev (4.2.7p???) both 
under Windows XP and Windows 7.

Under Win XP both ntpd-stable and ntpd-dev should converge fine at low 
polling intervals. We need to know if one of them or both are still 
stable when the polling interval increases beyond 7.

Under Win 7 ntpd-dev should also converge fine at low polling intervals, 
so another test could find out if it also stops to converge when the 
polling interval ramps up beyond 7.

ntp-staple might converge under Win 7, or not. If it doesn't at low 
polling intervals then it makes no sense to see what happens if the 
polling interval ramps up, if that happens at all.

Since this seems to happen under Windows only, the reason could be some 
calculation error or overflow, and it can have been unintentionally 
fixed or introduced between ntp-stable and ntp-dev.

So IMO we first need to find under which conditions this occurs, then we 
can try to find a bug in the source code.

> I have not the ability to confirm this bug on newer versions of
> Windows or newer versions of NTP at work.  The bug was seen on a
> stable corporate LAN, of which I am now locked out of by the IT
> Security Gestapo.

Huh, what have you done? ;-)

> I am no longer able to use your stable, or test your
> developmental software at my workplace.  I also am no longer able to
> provide constructive feedback on this software in this environment.
> At home, I found all the FreeBSD and Windows, XP and 7, ports to be
> unstable in the WAN / Internet environment.

Hm, if even the FreeBSD version is unstable I'd say this is due to you 
network environment or configuration.

As said above, a reliable test should first make sure that proper 
synchronization is possible, i.e. with a stable network connection and a 
good upstream NTP server.

> NTP polling increases
> regardless of offset and NTP appears not to keep up or sync the
> clock.  I have not observed any over-regulation that is feared with a
> shorter polling interval.  My primary goal is to have the smallest
> offset as possible; after that goal is met, I can worry about clock
> jitter.  Is that the goal of NTP?  If so, it is not working in the
> 100% network environment, with no PPS reference server use, and
> default software settings.
> I would be happy to answer specific questions if my observations are
> still not fully understood.

I've just set up some tests with XP/ntp-stable and Win7/ntp-dev to see 
how it gores and if I can confirm this bug.

Martin Burnicki

Meinberg Funkuhren
Bad Pyrmont

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