[ntp:questions] Re: How to handle leap second condition correctly?

David Woolley david at djwhome.demon.co.uk
Sat Jun 11 08:47:37 UTC 2005


In article <ywn9psutvr5k.fsf at ntp1.isc.org>,
Harlan Stenn <stenn at ntp1.isc.org> wrote:

> An SNTP server should not ordinarily be telling ANYBODY what time it is.

An SNTP server that isn't telling someone the time is a total waste of
resources.  Telling clients the time is the only reason for an SNTP
server to exist!

Are you possibly trying to say that an SNTP client should not also act
as a server?  Whilst true, this is only a SHOULD NOT, not a MUST NOT,
and Windows violates this rule anyway, which means it has become 
unenforceable (ignoring that Windows has more serious protocol violations).
(SHOULD NOTs are things that shouldn't normally be done but are permitted
in exceptional circumstances.)

In any case, the original article seems to have meant the server response
to an SNTP client request, not really that the server had to be an SNTP
server. Even if it were an SNTP server, there is nothing to indicate
that it isn't directly connected to a proper reference clock.

I would say that the answer was that an SNTP client isn't normally expected
to maintain the clock that well that it can't just ignore the leap warning
(it shouldn't ignore the alarm state) and just respond to the step in time
when it occurs.  However, if it did want to honour it, it would refrain 
from asking the time in the immediate vicinity of the two times a year
that leap seconds may be implemented, and should adjust its internal idea
of UTC time (which may or may not be able to represent second 59 minutes
and 60 seconds) at the nominal time at which leap seconds are allowed, and
in the direction indicated.  If it cannot represent 60 seconds, it may have
to compromise this to maintain monotonicity, if that is important.

The reason I suggest not asking the time, is that and SNTP implementation is
unlikely to have good filtering of outlyers, so may undo its automatic
adjustment when it gets a response that indicates that the time hasn't quite
passed, and then re-apply on the next sample.



More information about the questions mailing list