[ntp:hackers] Leap second list file

Dave Hart hart at ntp.org
Mon Jan 16 08:02:57 UTC 2012

On Mon, Jan 16, 2012 at 02:30, Danny Mayer <mayer at ntp.org> wrote:
> On 1/15/2012 3:17 PM, Warner Losh wrote:
>> Also, it is only 'leap second at the end of today' nor 'leap second at end of the month'
> No, that's not what it says. See below:
> LI Leap Indicator (leap): 2-bit integer warning of an impending leap
>   second to be inserted or deleted in the last minute of the current
>   month with values defined in Figure 9.

You're both right.  Leap-add and leap-del LI values do indicate a leap
second at the end of the current month, on ntpd's input path.
However, in deciding whether to indicate a known upcoming leap second
(however known) to downstream clients, ntpd will not indicate a
pending leap second in LI until less than one day remains to the
scheduled insertion/deletion.  So in practice, LI means both at the
end of this UTC month at the end of today UTC.

Unless you're using the ACTS modem refclock with a poll interval
greater than 24h, a day is plenty of notice for the software to do its
job, though some human operators might appreciate the opportunity to
notice it sooner.  Given those two factors, I wonder if ntpd should be
changed to indicate pending leap seconds earlier, perhaps after the
first hour of the month to guard against accidentally misfiring the
change a month early.

Dave Hart

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