[ntp:questions] Synchronize clocks without a reference clock

Spoon devnull at localhost.com
Mon Mar 19 10:12:45 UTC 2007


Maarten Wiltink wrote:

> Spoon wrote:
> 
>> Consider a cluster of computers on a LAN. These computers have no
>> acess to a reference clock (no Internet, no GPS, etc).
>>
>> One could still want to have all their clocks synchronized, even if
>> this means they are all slowly drifting away from the "real" time.
>>
>> I've read a FAQ which seems to describe this use case.
>> http://www.ntp.org/ntpfaq/NTP-s-refclk.htm#AEN4264
>>
>> All I need to do would be to add these lines:
>>
>> server 127.127.1.1 # LCL, local clock
>> fudge  127.127.1.1 stratum 12 # increase stratum
>>
>> to my configuration file? Did I understand correctly?
> 
> It would work, most likely.
> 
> You'd be following a single server's eccentricities, though, and there
> are two cases where this could be bad. First, if you've chosen a server
> that wanders around rather a lot (say, next to a door), your entire
> network will get time that also wanders around a lot. Not a very big
> deal, usually.

When you say that the clock in a server next to a door will wander a 
lot, are you referring to the effects of the sudden temperature swing 
when a cold draft comes in the server room?

> The other potential problem is that there is a maximum drift that can be
> corrected. If the difference between your server and any other host is
> larger than that, the client won't be able to follow the server, while
> both would be able to follow UTC. This is unlikely but possible.

I was wondering whether it is possible to reduce the maximum rate of 
change? Is that what the tinker dispersion option is for?

http://www.ee.udel.edu/~mills/ntp/html/miscopt.html

"The argument becomes the new value for the dispersion increase rate, 
normally .000015 s/s."

Does this mean that the maximum rate of change is 15 µs per second?

Could I safely change this value to 5 µs per second?

> Orphan mode may be better. It's in the docs, too.

Found it.

http://www.ee.udel.edu/~mills/ntp/html/manyopt.html#orphan

Thanks for the pointer. I will read it carefully.

Regards.




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