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

Richard B. Gilbert rgilbert88 at comcast.net
Mon Jun 15 23:35:46 UTC 2009


Rich wrote:
> 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

The "Delay" values for some of the servers you have configured are large 
enough to suggest that they are poor choices!  The potential error in 
getting time from a distant server is limited to one half of the round 
trip delay.  It may and should be a lot better than that but it can't be 
worse.

The best choices, other things being equal, are the servers with the 
lowest round trip delays.




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