[ntp:questions] Syncing to nearby vs. faraway servers

Rich richwales at gmail.com
Mon Jun 15 21:38:48 UTC 2009


I'm running ntpd 4.2.4p4 on several Ubuntu 9.04 ("Jaunty") servers.
I'm associated with Stanford University and have been depending
primarily on Stanford's own pool of stratum-2 servers.

Recently, in order to spread out my time base somewhat, I tried adding
some outside servers (using the *.pool.ntp.org DNS names) to my NTP
configurations.  Since doing this, I've noticed that the nearby
(Stanford) servers are uniformly "off" by several milliseconds, in
comparison to more distant servers.  Here, for example, is some output
from the ntpq "peers" command (with host names turned off) on one of
my servers:

     remote           refid      st t when poll reach   delay
offset  jitter
==============================================================================
 10.0.229.29     10.0.229.53      3 u   25   64  376    0.109
-3.955   0.130
-10.0.229.114    171.64.7.89      3 u  103  256  377    6.058
-5.277   3.545
-10.0.229.117    209.167.68.100   3 u   78  256  377   22.338
-8.460   5.624
+171.64.7.61     171.64.7.87      2 u  392 1024  377    5.614
-5.418   0.923
+171.64.7.55     171.64.7.87      2 u  393 1024  377    4.998
-5.405   0.977
-171.64.7.111    171.64.7.87      2 u  394 1024  377    5.598
-5.499   1.000
-207.150.167.80  209.51.161.238   2 u  387 1024  377   79.606
6.888   1.485
-72.36.170.170   132.163.4.102    2 u  368 1024  377   51.702
6.867   0.297
-66.254.57.165   18.26.4.105      2 u  386 1024  377   95.627
3.567   1.373
*131.234.137.24  .DCF.            1 u  449 1024  377  171.784
0.923   0.978
-89.16.178.36    195.66.241.3     2 u  442 1024  377  157.082
1.874   0.443

(The 10.0.229.* servers are on my home LAN; the 171.64.7.* servers are
at Stanford; and the others are from various places around the world
and have much larger delays than the nearby servers.)

I also see that the above machine is currently syncing to a server in
Germany (delay = 171 msec) -- possibly because it's on stratum 1.  (I
submitted a separate posting questioning whether stratum-1 servers
should really be in the pools, but that's a separate issue.)

Is this sort of behaviour to be expected?  Does this mean that the NTP
algorithm ought to be giving more weight to servers with shorter
delays?  Or, perhaps, does it suggest that there might be something
wrong with the Stanford servers that is making them all cluster around
a time that is several milliseconds different from the rest of the
world?

Rich Wales
richw at richw.org, richwales at gmail.com




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