[ntp:questions] how to have offset < 1ms
david at ex.djwhome.demon.invalid
Mon Apr 12 22:27:17 UTC 2010
> 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 127.0.0.1
>>> restrict -6 ::1
>>> # Hosts on local network are less restricted.
>>> #restrict 192.168.1.0 mask 255.255.255.0 nomodify notrap
>>> # Use public servers from the pool.ntp.org project.
>>> # Please consider joining the pool (http://www.pool.ntp.org/
>>> #broadcast 192.168.1.255 key 42 # broadcast server
>>> #broadcastclient # broadcast client
>>> #broadcast 18.104.22.168 key 42 # multicast server
>>> #multicastclient 22.214.171.124 # multicast client
>>> #manycastserver 126.96.36.199 # manycast server
>>> #manycastclient 188.8.131.52 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 127.127.1.0
>>> fudge 127.127.1.0 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 255.255.255.255 nomodify notrap noquery
>> 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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