[LEAPSECS] Leap seconds have a larger context than POSIX

[Martin Burnicki]

Hal Murray wrote:
> 
> tvb said:
>> There's no ambiguity. Those are just bugs. No software should depend on  more
>> than 1 month notice of a leap second and no software should be  fooled if the
>> notice is months or even years in advance. 

Please keep in mind that e.g. GPS sends out leap second announcements
about 6 months in advance. This is not a problem if the receiver
firmware follows the specs, since the week number and day number are
specified. But, as we have seen, this can be a problem due to bugs in
the receiver firmware.

> There are plenty of quirks in ntp code along that line.  The APIs don't have 
> an explicit when.  The NTP-Kernal API for leap-pending is leap-tonight.  You 
> have most of the next day to turn it off.  The leap-pending on the wire is 
> leap-at-the-end-of-this-month.

I think this is some "best practice" that has evolved over many years.
For the kernel, the "end of this day" announcement is sufficient.

For the protocol, roughly 1 month is sufficient but short enough to
avoid ambiguous dates, i.e. at the end of which month.

If I remember correctly, the PTP specs even specify an announcement
interval of 12 hours only, the German DCF77 transmitter sends the
announcement only 1 hour in advance, and IRIG time codes (if they
support it at all, like IEEE1344 / C37.118) only 10 seconds or so in
advance.

So I wonder which spec should have been relevant for NTP/ntpd?

Martin
-- 
Martin Burnicki

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