[ntp:questions] Re: NTP, no external time source, peering "Undisciplined Local Clocks" ?

David L. Mills mills at udel.edu
Wed Oct 19 20:52:09 UTC 2005


Eric,

As confirmed by experiment, having each client average all the others is 
not a good idea. The ensemble tends to do a random walk and display very 
poor statility. Better to pick one of them and let it time the rest. 
However, folks have tended to extend this idea to provide a multiple 
backup capability. This is a bad idea, as evil timing loops can form 
that take a long time to count to infinity.

There is a new wrinkle designed for isolated configurations with 
multiple backup requirements. It is called orphan mode and works best 
with broadcast/multicast networks or networks with multiple 
symmetric-mode networks. The networks self-organize, elect a leader and 
pecking order depending on the particular failure topology. It's been 
tested here in the ntp-dev version, but should still be considered 
experimental. See the documentation on the authentication options and 
miscellaneous commands pages in the online documentation.

Dave

Eric wrote:
> On Tue, 18 Oct 2005 14:19:11 GMT, mmolteni at cisco.com (Marco Molteni)
> wrote for the entire planet to see:
> 
> 
>>Say I don't know if a clock is better than the other, so by configuring
>>B and C to use A as server I risk to have a not so good time if A is 
>>unreliable.
>>
>>Can I configure A,B,C to peer with each other in a meshed fashion, so
>>to have each clock influenced by all the others? 
> 
> 
> The general feeling on this ng is that this is not a good application
> for NTP.
> 
> I would go further, and say it is a poor design using any type of
> software.  Sure, you may get some weird approximation of the average
> clock errors around your network, but it will still drift, by an
> unknown amount.  
> 
> Three PCs, all of which don't know the time, can't really help each
> other find it.
> 
> - Eric
> 




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