Warner Losh wrote:
From: James Maynard <[EMAIL PROTECTED]>
Subject: Re: [LEAPSECS]Comparing Time Scales
Date: Fri, 03 Feb 2006 15:37:40 -0800
Thanks, guys, for your feedback. Here's another iteration.
The numbering of NTP seconds in the vicinity of a leap second seems to
differ from one document to another. Here I follow the NTP (version 3)
specification, RFC 1305, in which the Leap Indicator (sys.leap,
peer.leap, pkt.leap) is 01 if a positive leap second is to occur at the
end of the current UTC day, and 00 if no leap second is pending.
UTC = 2005-12-31 23:59:59, NTP seconds = 3 345 062 399, LS_pending = 01
UTC = 2005-12-31 23:59:60, NTP seconds = 3 345 062 400, LS_pending = 01
UTC = 2006-01-01 00:00:00, NTP seconds = 3 345 062 400, LS_pending = 00
If you read different documents carefully, you'll see this sequence:
UTC = 2005-12-31 23:59:60.0, NTP seconds = 3 345 062 400.0, LS_pending = 01
UTC = 2005-12-31 23:59:60.5, NTP seconds = 3 345 062 399.5, LS_pending = 01
UTC = 2006-01-01 00:00:00.0, NTP seconds = 3 345 062 400.0, LS_pending = 00
Where the 399 second repeats. The documents say that just after time
is incremented to 400, the last second of the day is repeated....
Warner
This looks like something that needs to be resolved in a standards
committee, such as David Mills' working group on Version 4 of the NTP
protocol. Meanwhile, you have to cope with different implementations
that do it different ways!
If a device has leap second information available to it at all, it
should know at least an hour ahead of time that a positive leap second
is pending. Given that information, it should be able to arm itself for
the upcoming leap second. Once in "armed for leap second state," each
NTP server and client would count the one-second epochs in the last
61-second minute of the day, assigning NTP second numbers "correctly"
and displaying (if it has a display) the UTC time correctly. (Of course,
I think the method shown in my table should be the standard way of
assigning NTP second numbers. But that would be a matter for the
standards committee to resolve.)
So I *think* my table shows how the leap second *should* be handled on
the various time scales listed in the table. There remains the standards
effort -- electropolitical engineering -- of reaching agreement on an
unambiguous standard to describe how leap seconds should be handled.
--
James Maynard
Salem, Oregon, USA
