[ntp:questions] Query about NTP accuracy

Andy Yates andyy1234 at gmail.com
Fri May 22 22:51:51 UTC 2009


Dave Hart wrote:
> NTP includes error budget calculations at every level.  You should be
> able to demonstrate the difference between directly using a stratum 1
> appliance compared to using higher-strata intermediate NTP servers
> simply by observing the root dispersion on the client.
> 
> I suspect you are correct that the benefit is not worth the cost, but
> convince yourself with measurement.  Using FreeBSD instead of Linux
> for the intermediate servers will probably give better results, if
> that is a possibility, give that a test as well.
> 
> I would also encourage you to consider using NTPv4 in preference to
> NTPv3 where it is practical, but especially on the intermediate
> servers.  A lot has improved in NTP since version 3.
> 
> Finally, while it is true you can improve scalability using broadcast
> or multicast, it also requires extra configuration to provide matching
> keys on both sides, or using the "disable auth" option which is not
> recommended.  NTP uses very few resources even with hundreds or
> thousands of unicast clients, as each client polls at most once per
> minute and typically, after stabilizing, once per 17 minutes (1024
> seconds for default maxpoll 10).  Processing each packet and replying
> is very fast, so even after a power outage when many clients are
> starting at once, I would expect even relatively decrepit hardware to
> have no problem keeping up with the traffic.  With 10,000 clients
> polling a single server every 64 seconds, the average rate is 156
> packets per second in and out each 48 bytes long.  I bet your existing
> servers could handle that rate easily.
> 
> Cheers,
> Dave Hart

Many thanks for your help Dave

Your comments make sense. BTW the NTP v3 was only to take into account
end clients, many of which I believe are still v3 - I could be wrong, I
would use v4 for any intermediate strata.

Regards
Andy




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