This data is provided from HP's Expert center in response to questions about XNTPD and leap second processing.
Leap Second processing in HP's xntpd
Purpose
The purpose of this document is to explain the processing of leap
seconds in HP's xntpd.
Leap Second Overview
For the benefit of readers who are not aware of the leap second
terminology, this section explains the concept of the leap
second. Readers familiar with the leap second concept may wish to
skip this section.
While the Earth is expected to make one complete rotation about its
own axis in 86400 seconds (one mean solar day), in reality it is not
the case. Earth takes slightly more than 86400 seconds to make one
complete rotation and hence one mean solar day is slightly longer than
86400 seconds. This introduces a slight error in the time calculations
used by the atomic clocks, because the atomic clocks were designed
to consider 86400 seconds as one day. In order to synchronize the
time derived from the atomic clocks with the astronomical time,
the concept of leap second was introduced.
Leap seconds were meant to synchronize the standard time (UTC)
with the astronomical time. However an exact synchronization is
not maintained. Leap seconds only guarantee that the UTC time does
not deviate more that 0.9 seconds from the astronomical time. Leap
seconds can be either positive or negative. A positive leap second
denotes a second to be added to the time on which it is applied,
while a negative leap second denotes a second to be removed.
Since a positive leap second adds an extra second to the time, the
UTC clock during midnight advances as follows :
23:59:58
23:59:59
23:59:60 <= leap second
00:00:00
00:00:01
Note the presence of an extra second - the positive leap second -
at 23:59:60.
Application of a negative leap second causes the UTC clock to advance
in the following fashion during midnight :
23:59:58
00:00:00
00:00:01
Note that the clock jumps directly from 23:59:58 to 00:00:00. The
59th second is skipped.
As of the date of publication of this document there has never been
an instance of negative leap second adjustment to UTC.
NTP and leap seconds
NTP has provisions in the packet to denote leap second adjustments. A
2-bit provision in NTP packet called the Leap Indicator (LI) can
denote the occurrence of either a positive leap second or a negative
leap second.
HP's xntpd and LI
HP's xntpd can handle LI in NTP packets and adjust the time
accordingly. Both positive and negative leap seconds are handled by
xntpd. xntpd implements positive leap seconds by going backward in
time and negative leap seconds by going forward in time. The default
behavior of xntpd is to step the clock in the right direction, when
xntpd needs to apply the leap second. Positive leap adjustments are
done by stepping the clock backward and negative leap adjustments
are done by stepping the clock forward. However this sudden step of
time can cause harm to certain applications. For example databases
can have problems when a positive leap second triggers xntpd to step
the system time backward. When this stepping of time is undesirable,
administrators may consider the option of starting xntpd with the
'-x' option. The '-x' option causes xntpd to always do a slew instead
of a step, to perform time adjustments. When the '-x' option is
set, xntpd slews the time in the right direction (either forward
or backward) when leap second occurs. However the consequences of
slewing must be completely understood before deciding to use the
'-x' option. See xntpd(1M) for more details.
When xntpd performs leap second adjustments it displays the following
messages in syslog ( at LOG_NOTICE level ) :
For positive leap adjustments, xntpd displays
"leap second occurred, stepped time back 1 second"
and for negative leap adjustments, xntpd displays
"leap second occurred, stepped time forward 1 second"
However xntpd logs the above mentioned messages only if sync events
or sys events are enabled in xntpd's conditional log mask. See the
logconfig command in ntp.conf(5) for more details. Unless altered,
xntpd by default logs sync events and hence logs the above mentioned
leap second related messages.
Note that irrespective of whether xntpd did a step or a slew, it
displays the message "..., stepped time ...". This message can cause
system administrators to wrongly believe that xntpd always performs
leap second adjustments by stepping. Note that with the -x option,
xntpd will perform leap second adjustments by slewing but displays
a wrong message.
Summary
This section summarizes the behavior of HP's xntpd with respect to
leap second adjustments.
o xntpd can perform both positive and negative leap second adjustments.
o xntpd can perform leap second adjustments by either stepping or
slewing the clock in the right direction. The default behavior of
xntpd is to the step the time, but the -x option causes xntpd to
slew the time to make leap second adjustments.
o xntpd displays the following messages in syslog. "leap second
occurred, stepped time back 1 second" is displayed for positive
leap second adjustments and "leap second occurred, stepped time
forward 1 second" is displayed for negative leap second adjustments.
o Positive leap second step adjustments can cause the clock to go
back slightly in time. If this is not desirable, customers may
consider starting xntpd with the -x option, which ensures that xntpd
performs time adjustments by slewing. The consequences of slewing
must be completely understood before deciding to use the '-x' option.
o A minor bug causes xntpd to always display the message "...,
stepped time ..." even if xntpd makes leap second adjustments by
slewing the clock.
