[Pool] Leap second is coming

Martin Burnicki martin.burnicki at meinberg.de
Wed Apr 15 17:09:02 UTC 2015


lst_hoe02 at 79365-rhs.de wrote:
> Sorry for maybe asking the obvious but
> 
> - How to check if ntp is configured with a leap second file? Must it be
> listed in ntp.conf or is there some default set?

If you run "ntpq -c rv" and the output shows a "tai" value then ntpd has
read a leap second file. For example:

pc-martin:/etc # ntpq -c rv
associd=0 status=0414 leap_none, sync_uhf_radio, 1 event, freq_mode,
version="ntpd 4.2.8p2 at 1.3316-o Wed Apr  8 08:20:36 UTC 2015 (3)",
processor="x86_64", system="Linux/3.16.7-7-desktop", leap=00, stratum=1,
precision=-23, rootdelay=0.000, rootdisp=7937.673, refid=shm0,
reftime=d8d9160e.2c2467ac  Wed, Apr 15 2015 18:53:34.172,
clock=d8d91616.da49a196  Wed, Apr 15 2015 18:53:42.852, peer=55606, tc=4,
mintc=3, offset=-0.053, frequency=0.000, sys_jitter=0.000000,
clk_jitter=0.019, clk_wander=0.000, tai=35, leapsec=201507010000,
expire=201512280000

As you can see in the "version", the output above is from ntpd 4.2.8*,
which also prints the "leapsec" date, and the expiration date of the
installed leap second file.

> - Without a leap second file ntp will simply decide based on the
> majority of upstream ntp servers it it will pass the information or will
> it pass the information from the "*" upstream?

ntpd 4.2.6 and newer requires a majority of configured upstream servers
to accept and send a leap second warning, *or* a refclock which can
provide it, *or* a current leap second file which supersedes both other
types of source.

> - What is the expected timeframe of leap second file changes
> (Month/Years/Decade)??

The IERS bulletin C is published twice a year, usually a few days after
a potential leap second date, i.e. in the first days of July or January.
Leap seconds files are updated afterwards, hopefully soon after the
bulletin.

The file should even be updated if *no* leap second has been scheduled,
with a new expiration date, so ntpd can possibly distinguish whether no
leap second has been scheduled, or the file has just not been updated.

> What happens if the upstream get it wrong and omit the leap second? Are
> we wrong by one second or do we simply switch later and slower?

I'm not sure what you mean here, and I think you need to distinguish
between several cases.

If your server has a leap second file and inserts the leap second, but
the upstream servers you have configured don't insert the leap second
then your server will observe a 1 s offset after the leap second and,
and will re-synchronize to the upstream server(s) a few minutes later,
i.e. step the time to be wrong like the servers.

> Thanks for explainig it to mortals ;-)

You're welcome. ;-)

Martin
-- 
Martin Burnicki

MEINBERG Funkuhren GmbH & Co. KG
Email: martin.burnicki at meinberg.de
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