[LEAPSECS] My FOSDEM slides

[Martin Burnicki]

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

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