[ntp:questions] time delta between clients

David Woolley david at ex.djwhome.demon.co.uk.invalid
Wed Dec 12 07:54:28 UTC 2007


In article <fjn9i7$4v8$2 at usenet01.boi.hp.com>,
Rick Jones <rick.jones2 at hp.com> wrote:

> taps with clue bats, is if I can take the difference in offset between
> each client and the time server and ass-u-me that is the difference in
> time between the two clients.  Or do I have to do something ntp-like

No.  If NTP is working properly, it is actually an indication of the
order of magnitude of the error you can expect in measuring the combination
of network and system timing.  The actual system timing variation should be
considerably less than this, but there may be a systematic bias, even if
offset is consistently almost zero.

The NTP clock discipline will strive to zero the offset, but cannot correct
for round trip asymmetry.  Any individual offset measurement will contain
a measurement error.  That measurement error will be of the same magnitude
as the measurement error you will get with your application.  If you are
interested in one way latency, you are interested in asymmetry!

To the extent that the above is not true, either you are making use of 
information not available to NTP (like recent temperature excursions), or
the NTP algorithms need fixing.

You need to use local, probably GPS, reference clocks, with pulse per 
second feeds, to remove network asymmetry.




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