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
> ri...@richw.org, richwa...@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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