2001-08-29 14:35:15 +00:00
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<title>Undisciplined Local Clock</title>
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<h3>Undisciplined Local Clock</h3>
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<hr>
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<h4>Synopsis</h4>
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Address: 127.127.1.<i>u</i> <br>
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Reference ID: <tt>LCL</tt> <br>
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Driver ID: <tt>LOCAL</tt>
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<h4>Description</h4>
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<p>This driver is intended for use in an isolated network where no
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external source of synchronization such as a radio clock or modem
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is available. It allows a designated time server to act as a
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primary server to provide synchronization to other clients on the
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network. Pick a machine that has a good clock oscillator (Digital
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machines are good, Sun machines are not) and configure it with this
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driver. Set the clock using the best means available, like
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eyeball-and-wristwatch. Then, point all the other machines at this
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one or use broadcast (not multicast) mode to distribute time.</p>
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<p>Another application for this driver is if a particular server
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clock is to be used as the clock of last resort when all other
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normal synchronization sources have gone away. This is especially
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useful if that server has an ovenized oscillator. For this you
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would configure this driver at a stratum greater than any other
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likely sources of time (say 3 or 4) to prevent the server taking
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over when legitimate sources are still available.</p>
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<p>A third application for this driver is when an external
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discipline source is available, such as the NIST <tt>lockclock</tt>
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program, which synchronizes the local clock via a telephone modem
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and the NIST Automated Computer Time Service (ACTS), or the Digital
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Time Synchronization Service (DTSS), which runs on DCE machines. In
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this case the stratum should be set at zero, indicating a bona fide
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stratum-1 source. In the case of DTSS, the local clock can have a
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rather large jitter, depending on the interval between corrections
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and the intrinsic frequency error of the clock oscillator. In
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extreme cases, this can cause clients to exceed the 128-ms slew
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window and drop off the NTP subnet.</p>
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<p>In the case where a NTP time server is synchronized to some
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device or protocol that is not external to the NTP daemon itself,
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some means should be provided to pass such things as error and
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health values to the NTP daemon for dissemination to its clients.
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If this is not done, there is a very real danger that the device or
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protocol could fail and with no means to tell NTP clients of the
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mishap. When ordinary Unix system calls like <tt>adjtime()</tt> are
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used to discipline the kernel clock, there is no obvious way this
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can be done without modifying the code for each case. However, when
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a modified kernel with the <tt>ntp_adjtime()</tt> system call
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is available, that routine can be used for the same purpose as the
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<tt>adjtime()</tt> routine and in addition provided with the
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estimated error, maximum error, and leap-indicator values. This is
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the preferred way to synchronize the kernel clock and pass
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information to the NTP clients.</p>
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<p>In the default mode the behavior of the clock selection
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algorithm is modified when this driver is in use. The algorithm is
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designed so that this driver will never be selected unless no other
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discipline source is available. This can be overridden with the
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<tt>prefer</tt> keyword of the <tt>server</tt> configuration
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command, in which case only this driver will be selected for
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synchronization and all other discipline sources will be ignored.
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This behavior is intended for use when an external discipline
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source controls the system clock. See the <a href="prefer.htm">
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Mitigation Rules and the <tt>prefer</tt> Keyword</a> page for a
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detailed description of the exact behavior.</p>
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<p>The stratum for this driver is set at 3 by default, but can be
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changed by the <tt>fudge</tt> configuration command and/or the <tt>
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ntpdc</tt> utility. The reference ID is <tt>LCL</tt> by default,
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but can be changed using the same mechanisms. <b>*NEVER*</b>
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configure this driver to operate at a stratum which might possibly
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disrupt a client with access to a bona fide primary server, unless
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the local clock oscillator is reliably disciplined by another
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source. <b>*NEVER NEVER*</b> configure a server which might devolve
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to an undisciplined local clock to use multicast mode.</p>
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<p>This driver provides a mechanism to trim the local clock in both
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time and frequency, as well as a way to manipulate the leap bits.
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The <tt>fudge time1</tt> parameter adjusts the time (in seconds)
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and the <tt>fudge time2</tt> parameter adjusts the frequency (in
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parts per million). Both parameters are additive and operate only
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once; that is, each command (as from <tt>ntpdc</tt>) adds signed
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increments in time or frequency to the nominal local clock time and
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frequency.</p>
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<h4>Monitor Data</h4>
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No <tt>filegen clockstats</tt> monitor data are produced by this
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driver.
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<h4>Fudge Factors</h4>
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<dl>
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<dt><tt>time1 <i>time</i></tt></dt>
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<dd>Specifies the time offset calibration factor, in seconds and
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fraction, with default 0.0.</dd>
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<dt><tt>time2 <i>time</i></tt></dt>
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<dd>Specifies the frequency offset calibration factor, in parts per
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million, with default 0.0.</dd>
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<dt><tt>stratum <i>number</i></tt></dt>
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<dd>Specifies the driver stratum, in decimal from 0 to 15, with
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default 3.</dd>
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<dt><tt>refid <i>string</i></tt></dt>
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<dd>Specifies the driver reference identifier, an ASCII string from
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one to four characters, with default <tt>LCL</tt>.</dd>
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<dt><tt>flag1 0 | 1</tt></dt>
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<dd>Not used by this driver.</dd>
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<dt><tt>flag2 0 | 1</tt></dt>
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<dd>Not used by this driver.</dd>
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<dt><tt>flag3 0 | 1</tt></dt>
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<dd>Not used by this driver.</dd>
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<dt><tt>flag4 0 | 1</tt></dt>
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<dd>Not used by this driver.</dd>
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</dl>
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<p>Additional Information</p>
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<p><a href="refclock.htm">Reference Clock Drivers</a></p>
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<hr>
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<a href="index.htm"><img align="left" src="pic/home.gif" alt=
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"gif"></a>
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<address><a href="mailto:mills@udel.edu">David L. Mills
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<mills@udel.edu></a></address>
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</body>
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</html>
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