[ntp:questions] Making a clock tell the wrong time?

Steve Kostecke kostecke at ntp.org
Tue Mar 10 14:29:23 UTC 2009

On 2009-03-10, Mike K Smith <mks-usenet at dsl.pipex.com> wrote:

>Steve Kostecke wrote:
>>Mike K Smith <mks-usenet at dsl.pipex.com> wrote:
>>> For testing purposes I want to configure an NTP server to run with
>>> a small but known offset. I would like to test at the following
>>> offset values 25, 90, 180, 350 and 2500ms.
>> What is this test intended to prove?
> I want to understand how long it takes in practice for an offset to be
> propagated from stratum 1 servers through a set of stratum 2 servers
> to a stratum 3 client.


>>> with a fudge line to set the time1 parameter to 90ms.
>>> The server showed a 90ms offset after the first poll of the local
>>> clock, but within a few poll cycles it had discarded the offset.
>> That's because ntpd disciplined the clock to amortize that offset.
>> Once this has happened, the clock is now operating at the 90ms offset.
>> Isn't this what you wanted?
> What you describe is what I expected. What I wanted was for that
> offset to be reflected in the time output by the ntpd.

Now that I thing about it, what happened when you applied that offset is
that you "moved" the clock 90ms.

> What I saw was a very slow slew, which generated about 3ms offset in
> an hour - this is just under 1ppm which I would expect from a free-
> running local clock which has a fairly current drift correction..

The place to see view offset is from the client. Is that where you were
looking from?

Steve Kostecke <kostecke at ntp.org>
NTP Public Services Project - http://support.ntp.org/

More information about the questions mailing list