[ntp:questions] Re: Best way to harness multiple stratum 1 servers

Richard B. Gilbert rgilbert88 at comcast.net
Wed Apr 5 00:11:41 UTC 2006

John Ackermann N8UR wrote:

> Ronan Flood wrote:
>> jra at febo.com (John Ackermann N8UR) wrote:
>>> After I finish some reconfiguration, I'll have the embarrassment of 
>>> riches of having five stratum one servers, each driven by an 
>>> independent refclock (two GPSDO, one WWVB, and two Cesium).
>> Good grief!  What Cesium and how do they interface to your servers?
> Old HP 5061As with the digital clock option; they provide a 1pps output 
> to use with the ATOM driver.
>>> Is there a preferred configuration that would let the downstream box 
>>> have the benefit of the multiple stratum 1s, without jumping around?
>> Pick "the best" ?
>>  server x.x.x.x prefer
> That's the easy answer, assuming I know which one is "best". :-)
> I guess my question may go to the fundamentals of ntp:  can an NTP 
> server ever do better than any of the individual servers it sees?  In 
> other words, does the algorithm blend the offset/jitter/delay 
> information it has to generate a concept of time based on not just one, 
> but the ensemble (or at least the survivors of the sanity checks), of 
> the available servers?  If so, how best to configure that?
> John
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Dave would probably refer you to the slide show!  I probably should too 
but I'll try to tackle this anyway.  Ntpd selects the "best" 
synchronization source.  The selected synchronization source is given 
great "weight" in the result but the other acceptable servers (marked 
with a "+" in the ntpq banner also have an influence.

I've found clock hopping to be a problem with internet servers.  I have 
yet to see any two internet servers deliver anything resembling the same 
time to my site.   The differences may be only a millisecond or two but 
a client hopping between sources that different is never going to settle 
down and track any one source accurately.

To the extent that your stratum 1 servers differ from each other in 
absolute time "as delivered to the client" and in stability, you will 
have problems with your client.

I'm inclined to suspect that your WWVB reflock will not look anywhere 
near as attractive to ntpd as your cesium clocks; the vagaries of LF 
radio propagation will probably introduce enough jitter that it will 
seldom, if ever, be selected.

You will be very dependent on the speed and latency of your local 
network; I'd be inclined to say that you'd be wasting your time with 
anything less than 100MB Full Duplex and switched.

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