--- /usr/share/doc/ntp-doc/html/ntpd.html 2011-07-11 05:18:26.000000000
+0300
+++ ntpd.html 2012-10-15 22:49:39.000000000 +0200
@@ -39,7 +39,7 @@
<p>Once the NTP software distribution has been compiled and
installed and the configuration file constructed, the next step is to verify
correct operation and fix any bugs that may result. Usually, the command line
that starts the daemon is included in the system startup file, so it is
executed only at system boot time; however, the daemon can be stopped and
restarted from root at any time. Once started, the daemon will begin sending
and receiving messages, as specified in the configuration file.</p>
<h4 id="time">Setting the Time and Frequency</h4>
<p>The <tt>ntpd</tt> program operates by exchanging messages
with one or more servers at designated intervals ranging from about one minute
to about 17 minutes. When started, the program requires several exchanges while
the algorithms accumulate and groom the data before setting the clock. The
initial delay to set the clock can be reduced using options on the <a
href="confopt.html">Server Options</a> page.</p>
- <p>Most compters today incorporate a time-of-year (TOY) chip to
maintain the time during periods when the power is off. When the machine is
booted, the chip is used to initialize the operating system time. In case there
is no TOY chip or the TOY time is more than 1000 s from the
server time, <tt>ntpd</tt> assumes something must be terribly wrong and
exits with a panic message to the system operator. With the <tt>-g</tt> option
the clock will be initially set to the server time regardless of the chip time.
However, once the clock has been set, an error greater than 1000 s will cause
<tt>ntpd</tt> to exit anyway.</p>
+ <p>Most computers today incorporate a time-of-year (TOY) chip
to maintain the time during periods when the power is off. When the machine is
booted, the chip is used to initialize the operating system time. In case there
is no TOY chip or the TOY time is more than 1000 s from the
server time, <tt>ntpd</tt> assumes something must be terribly wrong and
exits with a panic message to the system operator. With the <tt>-g</tt> option
the clock will be initially set to the server time regardless of the chip time.
However, once the clock has been set, an error greater than 1000 s will cause
<tt>ntpd</tt> to exit anyway.</p>
<p>Under ordinary conditions, <tt>ntpd</tt> slews the clock so
that the time is effectively continuous and never runs backwards. If due to
extreme network congestion an error spike exceeds the <i>step threshold</i>, by
default 128 ms, the spike is discarded. However, if the error persists for more
than the <i>stepout threshold</i>, by default 900 s, the system clock is
stepped to the correct value. In practice the need for a step has is extremely
rare and almost always the result of a hardware failure. With the <tt>-x</tt>
option the step threshold is increased to 600 s. Other options are available
using the <tt>tinker</tt> command on the <a href="miscopt.html">Miscellaneous
Options</a> page.</p>
<p>The issues should be carefully considered before using these
options. The maximum slew rate possible is limited to 500 parts-per-million
(PPM) by the Unix kernel. As a result, the clock can take 2000 s for each
second the clock is outside the acceptable range. During this interval the
clock will not be consistent with any other network clock and the system cannot
be used for distributed applications that require correctly synchronized
network time.</p>
<p>The frequency file, usually called <tt>ntp.drift</tt>,
contains the latest estimate of clock frequency. If this file does not exist
when <tt>ntpd</tt> is started, it enters a special mode designed to measure the
particular frequency directly. The measurement takes 15 minutes, after which
the frequency is set and <tt>ntpd</tt> resumes normal mode where the time and
frequency are continuously adjusted. The frequency file is updated at intervals
of an hour or more depending on the measured clock stability.</p>
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