[ntp:questions] Bizzare half second disagreement between ntp hosts

Luis Colorado luis.colorado at hispalinux.es
Fri Jan 11 02:14:15 UTC 2008

Just suppose you are downloading some great file what creates asymetries in the data flow bandwidth.  When that happens, you observe discrepances with external time sources in the form of sistematic offsets to  the clocks outside.  When you lose a local time source, your server gets the time reference from the couple of best stratum clocks disponible and can skip even more than a second.

If you need reliability and precission, you can mount two or more servers with gps timesources and connect them over ethernet media.  You'll have better than 1ms on the whole local net if you use PPS signals.

"Unruh" <unruh-spam at physics.ubc.ca> escribió en el mensaje news:FMfej.53259$5l3.36002 at edtnps82...
>I have a very weird situation. I am running a GPS PPS (Garmin GPS18LVM)
> with a few machines as a backup/initialization. 
> Sudeenly for about half and hour, my GPS failed for some reason ( still do
> not know what was wrong since it had come back on air by the time I noticed
> something wrong). Every hour I run a ntpq -p just to check that my gps is
> on air. I got this report.
>     remote           refid      st t when poll reach   delay   offset jitter
> ============================================================================ ==
> xtick.usask.ca   .GPS.            1 u 1003 1024  377   44.954    0.213 480.149
> +sanrail.com      2 u  993 1024  377    1.486  -479.03 479.917
> +raptor.tera-byt    2 u  322 1024  377   17.295  -480.35 0.766
> *zeus.yocum.org    2 u  390 1024  377   70.415  -481.02 1.230
> SHM(0)          .PPS.            0 l 1415   16    0    0.000   -0.002 0.001
> Now I believe the tick.usask.ca result, since all of the machines which use
> mine as a source suddenly noticed a .48 second jump when my GPS failed. But
> why in the world would three systems all suddenly be out by .48 sec? 
> Doing a peers on them, one has a GPS as its source, one a .WWVB. and one an
> .ACTS. Why should all three suddenly be out by half a second?

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