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<title>Kernel Model for Precision Timekeeping</title>
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<h3>Kernel Model for Precision Timekeeping</h3>
<hr>
<img align="left" src="pic/alice61.gif" alt="gif"> <a href=
"http://www.eecis.udel.edu/~mills/pictures.htm">from <i>Pogo</i>,
Walt Kelly</a>
<p>Exploding kernel<br clear="left">
</p>
<hr>
<p>The technical report [2], which is a major revision and update
of an earlier report [3], describes an engineering model for a
precision time-of-day function for a generic operating system. The
model is based on the principles of disciplined oscillators using
phase-lock loops (PLL) and frequency-lock loops (FLL) often found
in the engineering literature. The model uses a hybrid PLL/FLL
discipline algorithm implemented in the kernel. The hybrid loop
provides automatic time and frequency steering with update
intervals from a few seconds to over one day.</p>
<p>The hybrid PLL/FLL has been implemented in the Unix kernels for
several operating systems, including FreeBSD and Linux and those
made by Sun Microsystems, Digital/Compaq and Hewlett Packard. The
modifications are currently included in the licensed kernels for
Digital Unix 4.0 (aka Tru64) and Sun Solaris 2.8. Since the
modifications involve proprietary kernel interface code, they
cannot be provided for other licensed kernels directly. Inquiries
should be directed to the manufacturer's representatives. The
software and documentation, including a simulator with code
segments almost identical to the implementations, but not involving
licensed code, is called <tt>nanokernel.tar.gz</tt> and available
via the web at <a href="http://www.ntp.org">www.ntp.org</a> or by
anonymous FTP from ftp.udel.edu in the <tt>pub/ntp/software</tt>
directory.</p>
<p>Recently [1], the model has been re-implemented to support a
nanosecond system clock. The <tt>/usr/include/sys/timex.h</tt>
header file defines the applications programming interface (API)
routines and data structures. Implementations are available for
Linux, FreeBSD, SunOS and Tru64; however, only the Linux and
FreeBSD implementations, which are included in recent system
versions, are directly available. The software and documentation,
including a simulator with code segments almost identical to the
implementations, but not involving licensed code, is called <tt>
nanokernel.tar.gz</tt> and available via the web at <a href=
"http://www.ntp.org">www.ntp.org</a> or by anonymous FTP from
ftp.udel.edu in the <tt>pub/ntp/software</tt> directory.</p>
<p>The model changes the way the system clock is adjusted in time
and frequency, as well as provides mechanisms to discipline its
time and frequency to an external precision timing source, such as
described in the <a href="pps.htm">Pulse-per-second (PPS) Signal
Interfacing</a> page. The model incorporates a generic system call
interface for use with the NTP or similar time synchronization
protocol. The NTP software daemons for Version 3 <tt>xntpd</tt> and
Version 4 <tt>ntpd</tt> use this API to provide synchronization
limited in principle only by the accuracy and stability of the
external timing source. There are two new system calls defined in
<tt>timex.h</tt>, <tt>ntp_gettime()</tt>, which returns a structure
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including the current time, estimated error and maximum error, and
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<tt>ntp_adjtime()</tt>, which provides a means to adjust kernel
variables, including the current time and frequency offsets.</p>
<p>These kernel modifications are normally used in conjunction with
a kernel hardware interface such as described in the <a href=
"kernpps.htm">Kernel Programming Interface for Precision Time
Signals</a> page.</p>
<h4>References</h4>
<ol>
<li><p>Mills, D.L., and P.-H. Kamp. The nanokernel. <i>Proc. Precision
Time and Time Interval (PTTI) Applications and Planning Meeting</i>
(Reston VA, November 2000). Paper: <a href=
"database/papers/nano/nano2.ps">PostScript</a> | <a href=
"http://www.eecis.udel.edu/~mills/database/papers/nano/nano2.pdf">
PDF</a>, Slides: <a href=
"http://www.eecis.udel.edu/~mills/database/brief/nano/nano.htm">
HTML</a> | <a href=
"http://www.eecis.udel.edu/~mills/database/brief/nano/nano.ps">
PostScript</a> | <a href=
"http://www.eecis.udel.edu/~mills/database/brief/nano/nano.pdf">
PDF</a> | <a href=
"http://www.eecis.udel.edu/~mills/database/brief/nano/nano.ppt">
PowerPoint</a></p></li>
<li><p>Mills, D.L. Unix kernel modifications for precision time
synchronization. Electrical Engineering Department Report 94-10-1,
University of Delaware, October 1994, 24 pp. Abstract: <a href=
"http://www.eecis.udel.edu/~mills/database/reports/kern/kerna.ps">
PostScript</a> | <a href="database/reports/kern/kerna.pdf">PDF</a>,
Body: <a href=
"http://www.eecis.udel.edu/~mills/database/reports/kern/kernb.ps">
PostScript</a> | <a href=
"http://www.eecis.udel.edu/~mills/database/reports/kern/kernb.pdf">
PDF</a></p></li>
<li><p>Mills, D.L. A kernel model for precision timekeeping. Network
Working Group Report RFC-1589, University of Delaware, March 1994.
31 pp. <a href=
"http://www.eecis.udel.edu/~mills/database/rfc/rfc1589.txt">
ASCII</a></p></li>
</ol>
<hr>
<a href="index.htm"><img align="left" src="pic/home.gif" alt=
"gif"></a>
<address><a href="mailto:mills@udel.edu">David L. Mills
&lt;mills@udel.edu&gt;</a></address>
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