[ntp:questions] how to have offset < 1ms

David Woolley david at ex.djwhome.demon.invalid
Mon Apr 12 22:27:17 UTC 2010

lhommedumatch wrote:
> On 12 avr, 22:35, "Richard B. Gilbert" <rgilber... at comcast.net> wrote:
>> lhommedumatch wrote:
>>> I have my PC clock synchronised with a ntp server (ntpgmtaceb).
>>> With ntpq -p, I see an offset of 6ms.
>>> How can I be sure to have an offset < 1ms
>>> ntpq -p
>>>      remote           refid      st t when poll reach   delay
>>> offset  jitter
>>> ==============================================================================
>>>  LOCAL(0)        .LOCL.          10 l   61   64  377    0.000
>>> 0.000   0.001
>>> *ntpgmtaceb      .1PPS.           1 u  709 1024  377   29.431
>>> -6.450   0.917
>>> Here is my ntp.conf:
>>> # Permit time synchronization with our time source, but do not
>>> # permit the source to query or modify the service on this system.
>>> restrict default kod nomodify notrap nopeer noquery
>>> # Permit all access over the loopback interface.  This could
>>> # be tightened as well, but to do so would effect some of
>>> # the administrative functions.
>>> restrict
>>> restrict -6 ::1
>>> # Hosts on local network are less restricted.
>>> #restrict mask nomodify notrap
>>> # Use public servers from the pool.ntp.org project.
>>> # Please consider joining the pool (http://www.pool.ntp.org/
>>> join.html).
>>> #broadcast key 42         # broadcast server
>>> #broadcastclient                        # broadcast client
>>> #broadcast key 42             # multicast server
>>> #multicastclient              # multicast client
>>> #manycastserver         # manycast server
>>> #manycastclient key 42  # manycast client
>>> # Undisciplined Local Clock. This is a fake driver intended for backup
>>> # and when no outside source of synchronized time is available.
>>> server
>>> fudge stratum 10
>>> # Drift file.  Put this in a directory which the daemon can write to.
>>> # No symbolic links allowed, either, since the daemon updates the file
>>> # by creating a temporary in the same directory and then rename()'ing
>>> # it to the file.
>>> driftfile /var/lib/ntp/drift
>>> # Key file containing the keys and key identifiers used when operating
>>> # with symmetric key cryptography.
>>> keys /etc/ntp/keys
>>> # Specify the key identifiers which are trusted.
>>> #trustedkey 4 8 42
>>> # Specify the key identifier to use with the ntpdc utility.
>>> #requestkey 8
>>> # Specify the key identifier to use with the ntpq utility.
>>> #controlkey 8
>>> server ntpgmtaceb
>>> restrict ntpgmtaceb mask nomodify notrap noquery
>>> Regards,
>>> Ludovic
>> Read the documentation CAREFULLY!  I think that you will find that
>> "offset" is NOT the difference between your clock and the correct time!
>> There IS a number that does represent the difference between your clock
>> and the correct time but I have forgotten what it's called.
> Sorry, but I don't see where offset is NOT the difference time between
> my clock and the ntp server

Offset is the difference between the local clock and the estimated time 
on the immediate upstream server.  It includes the estimation error, 
which in certain circumstances can have a large systematic component 
(particularly if the round trip is not symmetric.  Once ntpd has 
stabilised, and assuming no perturbations (e.g. temperature jumps), the 
non-systematic part of the error in the local clock should be 
considerably smaller than the peak to peak offset.  This is because ntpd 
low pass filters the measurement noise.

> And what is this number that does represent the difference between my
> clock and the ntp server

That is not measurable, because, if you could measure it, you could set 
the local clock with less jitter and again end up with a smaller, but 
still not measurable, value.

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