[ntp:questions] The "best" WAN clock is an outlyer?

Greg Troxel gdt at ir.bbn.com
Sat Nov 9 14:16:23 UTC 2013


Jos vd Ven <jacmvdven at gmail.com> writes:

> This is the output of ntpq -p
>
>      remote           refid      st t when poll reach   delay   offset  jitter
> ==============================================================================
> o127.127.20.0    .GPS.            0 l   15   16  377    0.000   -0.001   0.002
> +193.190.230.65  .PPS.            1 u   52   64  377   14.094    1.934   0.570
> +193.190.230.66  .PPS.            1 u    2   64  377   14.585    2.141   0.610
> +193.79.237.14   .PPS.            1 u   64   64  377   10.131    2.372   0.512
> +192.87.36.4     .GPS.            1 u   15   64  377   11.980    1.815   0.556
> -192.87.110.2    .GPS.            1 u   38   64  377    9.901   -0.399   0.742
> +195.169.124.100 .PPS.            1 u   63   64  377   11.958    1.806   1.396
> +194.171.167.130 .PPS.            1 u   48   64  377   12.612    2.165   0.439
> *192.87.106.3    .PPS.            1 u    9   64  377    8.953    1.849   1.577

It's really hard to say.  It does look like most of your peers are 2 ms
ahead of your GPS.  Likely explanations are:

  1) Your internet connection has some sort of asymmetric delay.

  2) Your ISP's peering introduces asymmetric delay.

  3) The way your GPS is hooked up to ntpd introduces delay.

Unfortunately these are all hard to debug.

If you can traceroute to the various servers, and then from those
servers traceroute back, and see if you spot anything odd, that can
help.

For example, I see paths from a Boston-area home ISP to a Boston office
that go through NYC, but return paths that go through Atlanta.  I see a
persistent -11ms offset from a home server to one at work over v4, and
over v6 I do not.  I believe the v6 path (which involves tunnels) ends
up being symmetric.


In your case, though, the outlier has a much lower delay, with less
opportunity for issues.  But still, 7 ms and 3 ms half-paths will
generate 2 ms of perceived offset.

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