[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?
> 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.
>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
Email: davidtaylor at writeme.com
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