[ntp:questions] NTP client configuration

Charles Elliott elliott.ch at verizon.net
Fri Sep 28 02:37:04 UTC 2012


I have 4 Windows client computers who get their time from a Windows Server
2003 machine, which in turn gets its time from 9 Stratum 2 servers, all but
one within 150 miles.  The clients easily experience offsets within +- 6 ms,
except when I download a large file.  In that case the offset immediately
goes up to 300 ms or more, says steady for the duration of the download, and
then declines to -300 ms, or vice versa; I don't remember exactly.  The time
offset will also occasionally become large when the ISP's router becomes
congested, say when half a dozen neighbors are all downloading movies.  The
surprising result is how quickly the clients recover the correct time:
within a minute or two.

I don't understand why you need the broadcast mode.  It seems to me that
NTPD is optimized for the client/server configuration.  

If you really want accurate time, the server should be synchronized to a
local time source, such as a GPS clock.

Charles Elliott

> -----Original Message-----
> From: questions-bounces+elliott.ch=verizon.net at lists.ntp.org
> [mailto:questions-bounces+elliott.ch=verizon.net at lists.ntp.org] On
> Behalf Of David Taylor
> Sent: Thursday, September 27, 2012 10:47 AM
> To: questions at lists.ntp.org
> Subject: Re: [ntp:questions] NTP client configuration
> 
> On 27/09/2012 07:08, Benjamin CABUT wrote:
> > Hello,
> >
> > We are using Meindberg NTP client and server.
> >
> > Our configuration is:
> > - 1 Computer under windows that is server server 127.127.1.0
> broadcast
> > 192.168.2.255 minpoll 4 maxpoll 4 iburst disable auth
> > - several computer under linux + computers under windows, client of
> > this
> > server:
> > server 192.168.2.250 minpoll 4 maxpoll 4 iburst broadcastclient
> > disable auth
> >
> >
> > My problem is the following:
> > Mainly on windows computer, sometime, depending what the computer is
> > doing, there is a big offset between clients and server.
> > ("Big" for me is more than 500ms).
> >
> > I would like to know if it is possible to setup client so as soon as
> > he see offset>500ms, then client do a "fast" synch. (STEP).
> >
> >
> > Another way to solve my problem could be for me, to modify the source
> > code of my application in order to get this "offset" information
> > inside my code (C or C++ langage). then I could add this offset to
> the
> > time of the computer in order to be as much accurate as possible
> > during the time the offset is bigger than 500ms.
> >
> > Do you have any solution?
> >
> > Regards.
> >
> > B CABUT
> 
> I haven't used "broadcast" or "auth", so I don't know if that affects
> things.  My understanding of NTP was that unless you configure it
> otherwise, an offset exceeding 128 milliseconds would force NTP to step
> the PC's clock rather than changing the rate, so I am surprised you are
> seeing 500 ms offsets.
> 
> I'm using NTP 4.2.7p285 (updated from the Meinberg installation) and
> seeing offsets in the millisecond range (except on a couple of PCs
> where it's the "few milliseconds" range).  This is with three local PCs
> as
> stratum-1 servers, with a GPS reference.
> 
> See:
>    http://www.satsignal.eu/mrtg/performance_ntp.php
> 
> I would not recommend using the "local clock", and if you need an
> isolated network I suggest looking into "orphan" mode (about which I
> know nothing).
> 
> Cheers,
> David
> --
> Cheers,
> David
> Web: http://www.satsignal.eu
> 
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