Re: [chrony-users] Isolated time domains

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On Tue, 3 Dec 2013, Miroslav Lichvar wrote:

On Tue, Dec 03, 2013 at 12:24:43AM +0000, Chris Dore wrote:
This client was previously synchronized with the external NTP servers which is why it is reporting that it is in synch with NTP time.  Since the master server is about 3 hours behind NTP time I need this client to say it is about 3 hours ahead of the master server.

While this example is a bit contrived, I think it illustrates that the chronyd running on the master server is telling the clients the external NTP time when I really want it to tell the clients the local system time.

Is it possible to configure chronyd to always serve it's local clock, regardless of the state of the external/remote servers?

This issue was discussed here some time ago. Currently, there is no
option in chronyd which would allow serving the raw system time.
It wouldn't be very difficult to implement, but I'm not sure if it
would meet your requirements on the maximum error between the clients.

As Bill wrote in his reply, the clients will not notice the change in
the frequency at the same time and their response may be different, so
their clocks won't be close until they all caught up with the server
slew and then the situation would repeat when the server stops the

I think a better solution to this problem would be an approach similar
to the Google's NTP leap second smearing. The jump in time is smeared
with a cosine function over very long interval, so that the frequency
observed in the NTP time changes slowly and the clients can stay in
sync during the whole correction.

But if one, say, put in a 10PPM limit on how fast the error was corrected
(instead of chrony's 100,000 PPM limit, but more likely about 1000PPM) then it
would take about 300,000 hrs, which is years, to fix a 3 hour error. That
would be absurd. Ie, I do not think limiting chrony to slew slowly enough that
the clients can keep up (with some error) is the answer. For me, having chrony report its best estimate of UTC rather than its local
time really is the only way to go. To slew the master slowly enough so that
the clients can keep up (presumably they would not have the slew limit
implimented) does not seem like a good option. ntpd of course gets around this
by jumping the clock (infinite slew). But that has its problems as well.

I think that Dore really needs to think his requirements through.

At this moment, I'm not sure how difficult it would be to implement
in chrony.

William G. Unruh   |  Canadian Institute for|     Tel: +1(604)822-3273
Physics&Astronomy  |     Advanced Research  |     Fax: +1(604)822-5324
UBC, Vancouver,BC  |   Program in Cosmology |     unruh@xxxxxxxxxxxxxx
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