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

David J Taylor david-taylor at blueyonder.not-this-part.nor-this.co.uk.invalid
Tue Jun 16 05:38:58 UTC 2009


Rich wrote:
[]
> 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:
[]
> 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

Rich,

Is it possible that your connection to one lot of servers is 
asymmetrical - in the sense of having more delay outbound than inbound or 
vice-versa?  NTP cannot compensate for such asymmetry, and could cause the 
apparent offsets you have seen.  You could also issue NTPQ commands 
against these remote servers.

I have also seen NTP make what appear to be unusual choices for sync 
servers, and reported it here.  As I now use my own stratum-1 servers and 
the "prefer" option, the problem has gone away.  I was never completely 
convinced about such behaviour.

Cheers,
David 




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