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

Unruh unruh-spam at physics.ubc.ca
Tue Jun 16 10:31:30 UTC 2009


Rich <richwales at gmail.com> writes:

>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?

If the stanford machines all cluster around a time sever milliseconds
different from the rest of the world they are wrong. There may be some
problem with assymetric delays to the stanford machines.




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