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<title>s6-networking: the s6-taiclock program</title>
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<p>
<a href="index.html">s6-networking</a><br />
<a href="//skarnet.org/software/">Software</a><br />
<a href="//skarnet.org/">skarnet.org</a>
</p>
<h1> The <tt>s6-taiclock</tt> program </h1>
<p>
<tt>s6-taiclock</tt> is a client for the
<a href="https://cr.yp.to/proto/taiclock.txt">TAICLOCK</a> protocol.
It connects to a TAICLOCK server, computes an estimated discrepancy
between the local clock time and the absolute time given by the server,
and outputs it on stdout.
</p>
<h2> Interface </h2>
<pre>
s6-taiclock [ -f ] [ -v <em>verbosity</em> ] [ -r <em>roundtrips</em> ] [ -t <em>triptimeout</em> ] [ -h <em>throttle</em> ] [ -T <em>totaltimeout</em> ] [ -e <em>errmax</em> ] [ -p <em>port</em> ] <em>ipaddress</em> | s6-clockview
</pre>
<ul>
<li> s6-taiclock exchanges TAICLOCK messages with a server such as
<a href="s6-taiclockd.html">s6-taiclockd</a>
listening on <em>ipaddress</em>, UDP port 4014.
<em>ipaddress</em> can be IPv4 or IPv6. </li>
<li> It computes the mean difference between the absolute time
given by the system clock and the one given by the server. </li>
<li> It prints the difference to stdout in a format understood
by <a href="s6-clockadd.html">s6-clockadd</a> and
<a href="s6-clockview.html">s6-clockview</a>. It then exits 0. </li>
</ul>
<h2> Options </h2>
<ul>
<li> <tt>-f</tt> : force. Normally, s6-taiclock exits 111 if it cannot
compute a time with a smaller uncertainty than <em>errmax</em>. If this
option is set, it will output a time difference and exit 0 even if the
error is too big. </li>
<li> <tt>-v <em>verbosity</em></tt> : be more or less verbose.
By default, <em>verbosity</em> is 1. 0 means only print fatal error
messages; 2 means print advanced warnings. </li>
<li> <tt>-r <em>roundtrips</em></tt> : perform <em>roundtrips</em>
exchanges with the server. By default, <em>roundtrip</em> is 10. A lower
value yields a higher time uncertainty; a higher value puts more load on
the server. </li>
<li> <tt>-t <em>triptimeout</em></tt> : if a TAICLOCK exchange with
the server takes more than <em>triptimeout</em> milliseconds, abort this
exchange and move on to the next one. By default, <em>triptimeout</em>
is 2000. </li>
<li> <tt>-h <em>throttle</em></tt> : wait <em>throttle</em>
milliseconds between exchanges with the server. A lower value gets the
final result earlier, but exerts more load on the server. A higher
value puts a lighter load on the server, but delays the computation.
By default, <em>throttle</em> is 0. It is recommended to set it to a
reasonable nonzero value when increasing <em>roundtrips</em>. </li>
<li> <tt>-T <em>totaltimeout</em></tt> : if the whole
operation takes more than <em>totaltimeout</em> milliseconds, abort
and exit 1. By default, <em>totaltimeout</em> is 10000. </li>
<li> <tt>-e <em>errmax</em></tt> : accept a maximum time
uncertainty of <em>errmax</em> milliseconds. By default, <em>errmax</em>
is 100. </li>
<li> <tt>-p <em>port</em></tt> : contact a server on port
<em>port</em>. By default, <em>port</em> is 4014. </li>
</ul>
<h2> Notes </h2>
<h3> On the usage of NTP vs. TAICLOCK </h3>
<ul>
<li> TAICLOCK is not as generic or failproof as NTP. It is not as
resistant to network latency. It has been designed to broadcast
time on a local area network, whereas NTP has been designed to
broadcast time over the whole Internet. </li>
<li> TAICLOCK will produce faster results on a LAN; moreover, the
point of TAICLOCK is to broadcast TAI instead of UTC, so it is
more accurate around a leap second. </li>
<li> The Internet is much more reliable latency-wise today
than it was when dialout connections and broken routing protocols
were the norm. So it is possible to use TAICLOCK
across a WAN if the accuracy expectations are not too strict. </li>
<li> TAICLOCK is much easier to implement. The
<a href="s6-sntpclock.html">s6-sntpclock</a> client binary code
(statically linked on a i386) is 50% bigger than the s6-taiclock
client. Also, the <a href="s6-taiclockd.html">s6-taiclockd</a>
server is extremely small (close to 50% smaller than the client),
whereas NTP servers, even SNTP servers, are behemoths requiring
a project of their own. </li>
</ul>
<h3> Related work </h3>
<ul>
<li> The <a href="https://cr.yp.to/clockspeed.html">clockspeed</a> package
is the original inspiration for the clock management part of s6-networking.
Unfortunately, it is unmaintained. </li>
</ul>
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