158 lines
5.8 KiB
HTML
158 lines
5.8 KiB
HTML
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<HTML>
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<HEAD>
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<META HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=iso-8859-1">
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<META NAME="GENERATOR" CONTENT="Mozilla/4.01 [en] (Win95; I) [Netscape]">
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<TITLE>Undisciplined Local Clock
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</TITLE>
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</HEAD>
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<BODY>
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<H3>
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Undisciplined Local Clock</H3>
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<HR>
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<H4>
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Synopsis</H4>
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Address: 127.127.1.<I>u</I>
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<BR>Reference ID: <TT>LCL</TT>
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<BR>Driver ID: <TT>LOCAL</TT>
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<H4>
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Description</H4>
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This driver is intended for use in an isolated network where no external
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source of synchronization such as a radio clock or modem is available.
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It allows a designated time server to act as a primary server to provide
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synchronization to other clients on the network. Pick a machine that has
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a good clock oscillator (Digital machines are good, Sun machines are not)
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and configure it with this driver. Set the clock using the best means available,
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like 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.
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<P>Another application for this driver is if a particular server clock
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is to be used as the clock of last resort when all other normal synchronization
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sources have gone away. This is especially useful if that server has an
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ovenized oscillator. For this you would configure this driver at a stratum
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greater than any other likely sources of time (say 3 or 4) to prevent the
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server taking over when legitimate sources are still available.
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<P>A third application for this driver is when an external discipline source
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is available, such as the NIST <TT>lockclock</TT> program, which synchronizes
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the local clock via a telephone modem and the NIST Automated Computer Time
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Service (ACTS), or the Digital Time Synchronization Service (DTSS), which
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runs on DCE machines. In this case the stratum should be set at zero, indicating
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a bona fide stratum-1 source. In the case of DTSS, the local clock can
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have a rather large jitter, depending on the interval between corrections
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and the intrinsic frequency error of the clock oscillator. In extreme cases,
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this can cause clients to exceed the 128-ms slew window and drop off the
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NTP subnet.
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<P>In the case where a NTP time server is synchronized to some device or
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protocol that is not external to the NTP daemon itself, some means should
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be provided to pass such things as error and health values to the NTP daemon
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for dissemination to its clients. If this is not done, there is a very
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real danger that the device or protocol could fail and with no means to
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tell NTP clients of the mishap. When ordinary Unix system calls like <TT>adjtime()</TT>
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are used to discipline the kernel clock, there is no obvious way this can
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be done without modifying the code for each case. However, when a modified
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kernel with the <TT>ntp_adjtime()</TT> system call is available,
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that routine can be used for the same purpose as the <TT>adjtime()</TT>
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routine and in addition provided with the estimated error, maximum error,
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and leap-indicator values. This is the preferred way to synchronize the
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kernel clock and pass information to the NTP clients.
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<P>In the default mode the behavior of the clock selection algorithm is
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modified when this driver is in use. The algorithm is designed so that
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this driver will never be selected unless no other discipline source is
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available. This can be overridden with the <TT>prefer</TT> keyword of the
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<TT>server</TT> configuration command, in which case only this driver will
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be selected for synchronization and all other discipline sources will be
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ignored. 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">Mitigation
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Rules and the <TT>prefer</TT> Keyword </A>page for a detailed description
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of the exact behavior.
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<P>The stratum for this driver is set at 3 by default, but can be changed
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by the <TT>fudge</TT> configuration command and/or the <TT>ntpdc</TT> utility.
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The reference ID is <TT>LCL</TT> by default, but can be changed using the
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same mechanisms. <B>*NEVER*</B> configure this driver to operate at a stratum
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which might possibly disrupt a client with access to a bona fide primary
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server, unless the local clock oscillator is reliably disciplined by another
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source. <B>*NEVER NEVER*</B> configure a server which might devolve to
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an undisciplined local clock to use multicast mode.
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<P>This driver provides a mechanism to trim the local clock in both time
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and frequency, as well as a way to manipulate the leap bits. The <TT>fudge
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time1</TT> parameter adjusts the time (in seconds) and the <TT>fudge time2</TT>
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parameter adjusts the frequency (in parts per million). Both parameters
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are additive and operate only once; that is, each command (as from <TT>ntpdc</TT>)
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adds signed increments in time or frequency to the nominal local clock
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time and frequency.
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<H4>
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Monitor Data</H4>
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No <TT>filegen clockstats</TT> monitor data are produced by this driver.
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<H4>
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Fudge Factors</H4>
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<DL>
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<DT>
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<TT>time1 <I>time</I></TT></DT>
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<DD>
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Specifies the time offset calibration factor, in seconds and fraction,
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with default 0.0.</DD>
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<DT>
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<TT>time2 <I>time</I></TT></DT>
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<DD>
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Specifies the frequency offset calibration factor, in parts per million,
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with default 0.0.</DD>
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<DT>
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<TT>stratum <I>number</I></TT></DT>
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<DD>
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Specifies the driver stratum, in decimal from 0 to 15, with default 3.</DD>
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<DT>
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<TT>refid <I>string</I></TT></DT>
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<DD>
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Specifies the driver reference identifier, an ASCII string from one to
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four characters, with default <TT>LCL</TT>.</DD>
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<DT>
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<TT>flag1 0 | 1</TT></DT>
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<DD>
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Not used by this driver.</DD>
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<DT>
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<TT>flag2 0 | 1</TT></DT>
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<DD>
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Not used by this driver.</DD>
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<DT>
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<TT>flag3 0 | 1</TT></DT>
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<DD>
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Not used by this driver.</DD>
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<DT>
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<TT>flag4 0 | 1</TT></DT>
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<DD>
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Not used by this driver.</DD>
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<P>Additional Information
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<P><A HREF="refclock.htm">Reference Clock Drivers</A></DL>
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<HR>
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<ADDRESS>
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David L. Mills (mills@udel.edu)</ADDRESS>
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</BODY>
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</HTML>
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