[ntp:hackers] Re: NTP and leap-seconds

David J Taylor david-taylor at blueyonder.co.uk
Sun Jul 2 14:40:20 UTC 2006

David L. Mills wrote:
> David J Taylor wrote:
>> - is there some sort of sanity check in NTP about leap-seconds, such
>> as requiring a majority of servers to say that a leap second is due
>> before a client actually inserts the leap second?
> David,
> The usual behavior is that servers do not realize a leapsecond is
> pending, either because they have not been told using ntpdc or the
> radios or drivers do not implement the warning. So the code sifts from
> among the (usually) three survivors of the mitigation algorithms and
> sets the leap if one or more show leap. You suggestion would change
> that to require two out of the three to set the leap. What if there
> are only two survivors?
> The most reliable and secure solution to the false alarm problem is to
> run Autokey and use the leapsecond table as distributed from the
> primary servers.
> Dave

>From your response, I seem to have identified a weakness in the otherwise 
excellent hands-off, automated operation of NTP!  Something which requires 
manual intervention is error prone, as we have now seen twice this year.

I don't know much about the Autokey method you mention - it sounds as if 
it may be too complex for the average end-use client software.  I am 
unsure if it is the Windows port.

Considering your comments on the basic algorithm, I would consider 
requiring a majority of servers, and if you are down to just two then I 
guess you have to decide between requiring one or both servers to be 
showing a leap-second.  I would go for two, personally.  Can you anything 
before mitigation like noting that a majority of servers suggested the 
leap second, to aid the after-mitigation decision?

One other thing I an unsure of - if four weeks ago a server says leap, but 
on June 30 it does not, does ntp recognise the change, or does the 
erroneous "leap" command get kept?

SatSignal software - quality software written to your requirements
Web:  http://www.satsignal.net
Email:  davidtaylor at writeme.com 

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