194 lines
9.4 KiB
HTML
194 lines
9.4 KiB
HTML
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<HTML><HEAD><TITLE>
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Reference Clock Options
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</TITLE></HEAD><BODY><H3>
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Reference Clock Options
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</H3><HR>
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<H4>Reference Clock Support</H4>
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The NTP Version 4 daemon supports many different radio, satellite and
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modem reference clocks plus a special pseudo-clock used for backup or
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when no other clock source is available. Detailed descriptions of
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individual device drivers and options can be found in the <A
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HREF="refclock.htm">Reference Clock Drivers </A>page. Additional
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information can be found in the pages referenced there, including the <A
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HREF="rdebug.htm">Debugging Hints for Reference Clock Drivers</A> and <A
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HREF="howto.html">How To Write a Reference Clock Driver</A> pages. In
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many drivers, support for a PPS signal is available as described in <A
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HREF="pps.htm">Pulse-per-second (PPS) Signal Interfacing</A> page. Many
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drivers support special line discipline/streams modules which can
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significantly improve the accuracy using the driver. These are described
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in the <A HREF="ldisc.htm">Line Disciplines and Streams Drivers</A>
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page.
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<P>A reference clock will generally (though not always) be a radio
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timecode receiver which is synchronized to a source of standard time
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such as the services offered by the NRC in Canada and NIST and USNO in
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the U.S. The interface between the computer and the timecode receiver is
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device dependent, but is usually a serial port. A device driver specific
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to each reference clock must be selected and compiled in the
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distribution; however, most common radio, satellite and modem clocks are
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included by default. Note that an attempt to configure a reference clock
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when the driver has not been included or the hardware port has not been
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appropriately configured results in a scalding remark to the system log
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file, but is otherwise non hazardous.
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<P>For the purposes of configuration, <TT>ntpd</TT> treats reference
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clocks in a manner analogous to normal NTP peers as much as possible.
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Reference clocks are identified by a syntactically correct but invalid
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IP address, in order to distinguish them from normal NTP peers.
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Reference clock addresses are of the form <TT>127.127.<I>t.u</I></TT>,
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where <I><TT>t</TT></I> is an integer denoting the clock type and
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<I><TT>u</TT></I> indicates the unit number. While it may seem overkill,
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it is in fact sometimes useful to configure multiple reference clocks of
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the same type, in which case the unit numbers must be unique.
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<P>The <TT>server</TT> command is used to configure a reference clock,
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where the <I><TT>address</TT></I> argument in that command is the clock
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address. The <TT>key</TT>, <TT>version</TT> and <TT>ttl</TT> options are
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not used for reference clock support. The <TT>mode</TT> option is added
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for reference clock support, as described below. The <TT>prefer</TT>
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option can be useful to persuade the server to cherish a reference clock
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with somewhat more enthusiasm than other reference clocks or peers.
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Further information on this option can be found in the <A
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HREF="prefer.htm">Mitigation Rules and the <TT>prefer</TT> Keyword
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</A>page. The <TT>minpoll</TT> and <TT>maxpoll</TT> options have meaning
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only for selected clock drivers. See the individual clock driver
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document pages for additional information.
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<P>The stratum number of a reference clock is by default zero. Since the
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<TT>ntpd</TT> daemon adds one to the stratum of each peer, a primary
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server ordinarily displays stratum one. In order to provide engineered
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backups, it is often useful to specify the reference clock stratum as
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greater than zero. The <TT>stratum</TT> option is used for this purpose.
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Also, in cases involving both a reference clock and a pulse-per-second
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(PPS) discipline signal, it is useful to specify the reference clock
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identifier as other than the default, depending on the driver. The
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<TT>refid</TT> option is used for this purpose. Except where noted,
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these options apply to all clock drivers.
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<H4>Reference Clock Commands</H4>
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<DL><DT><TT>server 127.127.<I>t.u</I> [prefer] [mode <I>int</I>]
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[minpoll <I>int</I>] [maxpoll <I>int</I>]</TT></DT>
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<DD>This command can be used to configure reference clocks in special
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ways. The options are interpreted as follows:</DD>
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<DL><DT><TT>prefer</TT></DT>
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<DD>Marks the reference clock as preferred. All other things being
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equal, this host will be chosen for synchronization among a set of
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correctly operating hosts. See the <A HREF="prefer.htm">Mitigation Rules
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and the <TT>prefer</TT> Keyword </A>page for further information.</DD>
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<DT><TT>mode <I>int</I></TT></DT>
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<DD>Specifies a mode number which is interpreted in a device-specific
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fashion. For instance, it selects a dialing protocol in the ACTS driver
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and a device subtype in the <TT>parse</TT> drivers.</DD>
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<DT><TT>minpoll <I>int</I></TT></DT>
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<DT><TT>maxpoll<I> int</I></TT></DT>
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<DD>These options specify the minimum and maximum polling interval for
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reference clock messages, in seconds to the power of two. For most
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directly connected reference clocks, both <TT>minpoll</TT> and
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<TT>maxpoll</TT> default to 6 (64 s). For modem reference clocks,
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<TT>minpoll</TT> defaults to 10 (17.1 m) and <TT>maxpoll</TT> defaults
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to 14 (4.5 h). The allowable range is 4 (16 s) to 17 (36.4 h)
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inclusive.</DD>
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</DL>
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<DT><TT>fudge 127.127.<I>t.u</I> [time1 <I>sec</I>] [time2 <I>sec</I>]
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[stratum <I>int</I>] [refid <I>string</I>] [mode <I>int</I>] [flag1 0|1]
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[flag2 0|1] [flag3 0|1] [flag4 0|1]</TT></DT>
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<DD>This command can be used to configure reference clocks in special
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ways. It must immediately follow the <TT>server</TT> command which
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configures the driver. Note that the same capability is possible at run
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time using the <TT><A HREF="ntpdc.htm">ntpdc</A></TT> program. The
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options are interpreted as follows:</DD>
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<DL>
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<DT><TT>time1 <I>sec</I></TT></DT>
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<DD>Specifies a constant to be added to the time offset produced by the
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driver, a fixed-point decimal number in seconds. This is used as a
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calibration constant to adjust the nominal time offset of a particular
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clock to agree with an external standard, such as a precision PPS
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signal. It also provides a way to correct a systematic error or bias due
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to serial port latencies, different cable lengths or receiver internal
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delay. The specified offset is in addition to the propagation delay
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provided by other means, such as internal DIPswitches. Where a
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calibration for an individual system and driver is available, an
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approximate correction is noted in the driver documentation pages.</DD>
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<DT><TT>time2 <I>secs</I></TT></DT>
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<DD>Specifies a fixed-point decimal number in seconds, which is
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interpreted in a driver-dependent way. See the descriptions of specific
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drivers in the <A HREF="refclock.htm">reference clock drivers</A>
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page.</DD>
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<DT><TT>stratum <I>int</I></TT></DT>
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<DD>Specifies the stratum number assigned to the driver, an integer
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between 0 and 15. This number overrides the default stratum number
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ordinarily assigned by the driver itself, usually zero.</DD>
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<DT><TT>refid <I>string</I></TT></DT>
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<DD>Specifies an ASCII string of from one to four characters which
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defines the reference identifier used by the driver. This string
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overrides the default identifier ordinarily assigned by the driver
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itself.</DD>
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<DT><TT>mode <I>int</I></TT></DT>
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<DD>Specifies a mode number which is interpreted in a device-specific
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fashion. For instance, it selects a dialing protocol in the ACTS driver
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and a device subtype in the <TT>parse</TT> drivers.</DD>
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<DT><TT>flag1</TT> <TT>flag2</TT> <TT>flag3</TT> <TT>flag4</TT></DT>
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<DD>These four flags are used for customizing the clock driver. The
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interpretation of these values, and whether they are used at all, is a
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function of the particular clock driver. However, by convention
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<TT>flag4</TT> is used to enable recording monitoring data to the
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<TT>clockstats</TT> file configured with the <TT>filegen</TT> command.
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When a PPS signal is available, a special automatic calibration facility
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is provided. If the <tt>flag1</tt> switch is set and the PPS signal is
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actively disciplining the system time, the calibration value is
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automatically adjusted to maintain a residual offset of zero. Further
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information on the <TT>filegen</TT> command can be found in the <A
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HREF="monopt.htm">Monitoring Options </A>page.</DD>
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</DL>
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<DT><TT>pps <I>device</I> [assert|clear] [hardpps]</TT></DT>
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<DD>Specifies the name and options for the serial port device to which
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the PPS signal is connected. Note, this command replaces use of
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<TT>fudge flag3</TT>, which was used for the same purpose in NTPv3. Note
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that this command should preceed the <TT>server</TT> and <TT>fudge</TT>
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command for the same device. Note also that the <TT>assert</TT>,
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<TT>clear</TT> and <TT>hardpps</TT> options are only available if the
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<tt>ppsapi</tt> standard PPS interface is available.</DD>
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<DL>
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<DT><TT>device</TT></DT>
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<DD>Specify the device name associated with the PPS signal. The name
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must match exactly the link name specified in the driver documentation
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page.</DD>
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<DT><TT>assert</TT></DT>
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<DT><TT>clear</TT></DT>
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<DD>Using <TT>assert</TT> or <TT>clear</TT> specifies if the high going
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or low going edge of the signal must be used. The default is
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<TT>assert</TT>.</DD>
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<DT><TT>hardpps</TT></DT>
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<DD>This flag is used to tell the kernel that the signal from this
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device must be used to drive hardpps().</DD>
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<DD>The <TT>assert</TT>, <TT>clear</TT> and <TT>hardpps</TT> options
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are only available if the PPSAPI is used.</DD>
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</DL>
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<hr><a href=index.htm><img align=left src=pic/home.gif></a><address><a
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href=mailto:mills@udel.edu> David L. Mills <mills@udel.edu></a>
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</address></a></body></html>
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