[ntp:questions] Accuracy of GPS device

Miguel Gonçalves mail at miguelgoncalves.com
Fri Sep 2 08:50:05 UTC 2011


On 2 September 2011 07:56, unruh <unruh at wormhole.physics.ubc.ca> wrote:

>
> Nope.
>
> It is completely unclear to me what your question is. Your 10.0.2.254 is
> an outside switch.


I had several questions in my first message. Your assumption is wrong.

You are telling me that a switch I installed in my rack and defined its many
IP addresses is outside my company? Uau!

10.0.2.254 is a local as any machine in the 10.0.0.0/8 network.

>> > $ ntpq -p 10.0.99.99
> >> >      remote           refid      st t when poll reach   delay   offset
> >> >  jitter
> >>
> >>
> >==============================================================================
> >> > *10.0.2.10       .GPS.            1 u   21  256  377    0.173    0.196
> >> > 0.008
> >> > +10.0.2.9        .GPS.            1 u   93  256  377    0.175    0.191
> >> > 0.014
> >> > +10.0.2.254      81.94.123.16     2 u  149  256  377    0.583   -6.884
> >> > 0.152
>
> This tells me that your two GPS receivers are consistant with each
> other, but I have no idea why the offset is larger than the delay, and
> why the offsets are so large. On a lan, the offsets should be a factor
> of 20 or so less than what you are getting here.
> That the external router is 7 ms out just tells me that it is really
> poorly synced with the outside world.
>

I found out the problem and just for the record I'll explain...

The offset is larger than the delay because NTPd is using 10.0.2.254 (more
on this switch later) as a time source and it shouldn't because it has two
local stratum 1 clocks that are closer (0.170 ms vs 0.583 ms) are show less
jitter. Anyway... to prove my point I removed 10.0.2.254 (the **internal**
switch) from the configuration and here's the result of ntpq -p as of now:

$ ntpq -p 10.0.2.2
     remote           refid      st t when poll reach   delay   offset
 jitter
==============================================================================
+10.0.2.10       .GPS.            1 u  889 1024  377    0.179   -0.066
0.083
*10.0.2.9        .GPS.            1 u  391 1024  377    0.166   -0.084
0.051

Oddly enough, FreeBSD embedded machine with roughly the same NTP
configuration shows better results

$ ntpq -p 10.0.99.99
     remote           refid      st t when poll reach   delay   offset
 jitter
==============================================================================
+10.0.2.10       .GPS.            1 u  864 1024  377    0.193    0.025
0.301
*10.0.2.9        .GPS.            1 u 1012 1024  377    0.192    0.030
0.004

Regarding 10.0.2.254 it is an internal switch it is getting time over a
cable connection from several sources

$ ntpq -p 10.0.2.254
     remote           refid      st t when poll reach   delay   offset
 jitter
==============================================================================
-ntp0.as34288.ne .PPS.            1 u  391 1024  377   71.960   -1.029
0.270
*canon.inria.fr  .GPSi.           1 u  707 1024  377   55.220    0.199
0.700
+ntp1.inrim.it   .CTD.            1 u  359 1024  377   65.860    0.091
1.830
+ntp-p1.obspm.fr .TS-3.           1 u  373 1024  377   55.110   -0.215
0.610
-metasweb01.admi .HBGi.           1 u  992 1024  377   73.290   -5.182
0.850

I can't control the network at my upstream provider and while my link is not
saturated (far from that) my upstream provider links be could and there's
nothing I can do about it except deploying local time sources as I did.

I am checking my local time sources with some remote NTP stratum 1 servers
at a fixed interval and plotting the results. I see that once in a while the
offset to external time servers increases and I agree this has to be with
network congestion but this happens at my ISP so there's not much I can do.

>> > This is a FreeBSD embedded PBX machine running Asterisk. The machine is
> >> > mostly idle. What kind of offsets should I get with local machines?
> >>
> >> in the 10s of usec range max. Certainly less than the delay.
> >>
> >
> > tens of usec is good... Anyone here which can explain why NTP isn't
> getting
> > that?
>
> How could we?
> Maybe you are running a virtual BSD machine, and thus the clocks are
> wonkey. Maybe you have lousy hardware. Who knows.
>

No lousy hardware here but I think posting detailed hardware specifications
won't help.

No virtualization here also.

>>  > Assuming ntp04, ntp05 and ntp06 are on the same LAN I see offsets
> higher
> >> > than 100 us. Is it possible to decrease these numbers?
> >>
> >> Sure. all my systems have offsets in the 10us range--  on the same lan
> >> as my time server.
> >> Mind you I do use chrony, not ntpd but even ntpd should be in a few 10s
> >> of usec.
> >
> >
> > Can ntpd really get there? I'll try to query some good public servers to
> see
> > what others are getting...
>
> Sure it can. It can get better than 30us. But why you are not getting it
> is impossible for us to say.



I got it... As you saw above.



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