[ntp:questions] Using ntpd with custom clock

Dennis Ferguson dennis.c.ferguson at gmail.com
Wed Oct 17 22:48:23 UTC 2012

On Oct 17, 2012, at 4:30 PM, David Woolley <david at ex.djwhome.demon.invalid> wrote:
> Kennedy, Paul wrote:
>> Forgive me if I am wrong, but this is a very odd request.  As far as I
>> can tell, the request is for the NTP corrections to the system clock to
>> be used to correct a different clock.
>> I cannot quite understand how this is of practical use.  It is like
>> diagnosing the faults on your car engine and then applying the
>> corrections to a train engine.
> The alternative clock has a much more stable oscillator than the motherboard, would be one good reason.

Another reason might be that packet timestamps are being taken
from a clock other than the system clock, e.g. from a clock residing
in the Ethernet interface hardware which is sampled in hardware
to obtain packet arrival and departure times (Ethernet interfaces
supporting PTP in hardware often have one like this).

In this case the best approach can be to let NTP synchronize the
(non-system) hardware clock that the packet timestamps are being
taken from, and then to separately discipline the system clock hardware
into synchronization with the NTP clock for the benefit of time-consuming
applications running on the same machine.

Dennis Ferguson

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