Joseph Gwinn wrote:
On Thu, 26 Feb 2015 15:09:01 +0100, Martin Burnicki wrote:
Hi folks,
I've been asked off list to make the slides of my presentation at
FOSDEM 2015 in Brussels available and post the download link on this
list.
So here it is:
<https://fosdem.org/2015/schedule/event/technical_aspects_of_leap_second_propagation_and_evaluation/>
See the "Attachment" link.
Very interesting; thanks for posting this.
I have a few questions and comments:
1. Slide titled "Known Bugs (2)": Has support for negative leap
seconds been restored in NTP v4? It wasn't clear.
I have to admit that I wrote this before 4.2.8 had been released.
Support for negative leap second has been in older NTP versions, but had
been removed in 4.2.6.
Now in 4.2.8 the leap second code has been reworked and cleaned up, and
a very quick look at the source code seems to indicate that this might
work again. At least there's code which passes the announcement flag to
the kernel, if kernel support is enabled.
I think I'll give it a try soon. I'd expect that a negative leap second
might work if an appropriate announcement is received from a refclock or
upstream NTP server, but it will be interesting to find out if this
works with a NIST-style leap second file where the TAI offset decreases
at a given date.
2. Slide titled ""Possibilities for Future Improvements (2)": In the
wish list for a kernel call to ask if the kernel runs UTC or TAI, a
couple of issues come to mind. First, many systems set the GPS
receiver to emit GPS System Time via NTP (and IRIG), so a GPS System
Time option may be needed.
Yes.
Though I would prefer using TAI instead of raw GPS time if a linear time
scale is required. What if you use a different GNSS receiver, e.g. for
Galileo, or the Chinese Beidou?
GPS time (or whatever) is fine in closed projects/environments, but IMO
a UTC and TAI are the "global" time scales, while GPS is specific to the
U.S.
Second, we often have the GPS (or PTP 1588)
receiver to emit GPS System Time, but never share this with downstream
servers, who are configured for UTC (but strangely the leap seconds
never come). The difference between UTC and GPS System Time is handled
in applications code. The reason for this approach is so that the bulk
of the system is free from step discontinuities, and only the
interfaces need deal with UTC.
I agree, but as I've tried to point out above I think a better project
design would have been to use TAI instead of GPS time. PTP works
natively with TAI, and you can easily convert between he two scales.
Of course it's easy to convert GPS to TAI, and vice versa, but you have
to take care that more types of conversions are required and implemented
correctly.
Thanks for your feedback!
Martin
--
Martin Burnicki
MEINBERG Funkuhren GmbH & Co. KG
Email: [email protected]
Phone: +49 (0)5281 9309-14
Fax: +49 (0)5281 9309-30
Lange Wand 9, 31812 Bad Pyrmont, Germany
Amtsgericht Hannover 17HRA 100322
Geschäftsführer/Managing Directors: Günter Meinberg, Werner Meinberg,
Andre Hartmann, Heiko Gerstung
Web: http://www.meinberg.de
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