[ntp:questions] how many servers should I poll?

Brad Knowles brad.knowles at skynet.be
Fri Aug 8 16:46:42 UTC 2003


At 8:05 PM -0400 2003/08/07, Andrew Schulman wrote:

>  I have a small home network, with modest time accuracy requirements --
>  +/- 1 second is sufficient.  Balancing the desire for accurate time
>  against the desire to minimize use of public time servers, how many
>  source servers should I include in my ntp.conf?

	My understanding is that somewhere between five to ten servers is ideal.

>  I see the list of stratum 2 servers at
>  http://www.eecis.udel.edu/~mills/ntp/clock2a.html.  Are there public
>  stratum-3 or higher servers that I should use instead?

	That list is a bit dated.  I'd suggest checking out the servers 
in pool.ntp.org, or perhaps the country-code bases servers within 
pool.ntp.org (e.g., fr.pool.ntp.org, de.pool.ntp.org, etc...). 
Ideally, use "dig" to get a complete list of all known IP addresses 
for the set of servers you're looking at.

	Try to find a good selection of servers that are topologically 
close to you (so far the best solution seems to be to use "ping" but 
of course sometimes providers choose to block it), but which also 
have good Stratum ratings.  Also check with your provider -- in many 
cases they will have internal NTP servers that you can use.

	Once you have a selection of servers, put them in your ntp.conf 
and watch to see what their performance is like.  Once you find a 
good selection of servers that perform better than the others (low 
clock differential, low jitter, etc...), then you can drop the others 
from the configuration file.

	I would recommend re-checking out which servers are performing 
well versus which ones are not on a regular basis, perhaps once a 
month (you never know when old servers might go away, new servers 
might come along, etc...).


	We're working on developing some tools to try to help automate 
this process.  Stay tuned.

-- 
Brad Knowles, <brad.knowles at skynet.be>

"They that can give up essential liberty to obtain a little temporary
safety deserve neither liberty nor safety."
     -Benjamin Franklin, Historical Review of Pennsylvania.

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