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<table class="head">
<tr>
<td class="head-ltitle">CC_VEGAS(4)</td>
<td class="head-vol">Device Drivers Manual</td>
<td class="head-rtitle">CC_VEGAS(4)</td>
</tr>
</table>
<div class="manual-text">
<section class="Sh">
<h1 class="Sh" id="NAME"><a class="permalink" href="#NAME">NAME</a></h1>
<p class="Pp"><code class="Nm">cc_vegas</code> — <span class="Nd">Vegas
Congestion Control Algorithm</span></p>
</section>
<section class="Sh">
<h1 class="Sh" id="DESCRIPTION"><a class="permalink" href="#DESCRIPTION">DESCRIPTION</a></h1>
<p class="Pp">The Vegas congestion control algorithm uses what the authors term
the actual and expected transmission rates to determine whether there is
congestion along the network path i.e.</p>
<ul class="Bl-item Bd-indent">
<li>actual rate = (total data sent in a RTT) / RTT</li>
<li>expected rate = cwnd / RTTmin</li>
<li>diff = expected - actual</li>
</ul>
<p class="Pp">where RTT is the measured instantaneous round trip time and RTTmin
is the smallest round trip time observed during the connection.</p>
<p class="Pp">The algorithm aims to keep diff between two parameters alpha and
beta, such that:</p>
<ul class="Bl-item Bd-indent">
<li>alpha < diff < beta</li>
</ul>
<p class="Pp">If diff > beta, congestion is inferred and cwnd is decremented
by one packet (or the maximum TCP segment size). If diff < alpha, then
cwnd is incremented by one packet. Alpha and beta govern the amount of
buffering along the path.</p>
<p class="Pp">The implementation was done in a clean-room fashion, and is based
on the paper referenced in the <a class="Sx" href="#SEE_ALSO">SEE ALSO</a>
section below.</p>
</section>
<section class="Sh">
<h1 class="Sh" id="IMPLEMENTATION_NOTES"><a class="permalink" href="#IMPLEMENTATION_NOTES">IMPLEMENTATION
NOTES</a></h1>
<p class="Pp">The time from the transmission of a marked packet until the
receipt of an acknowledgement for that packet is measured once per RTT. This
implementation does not implement Brakmo's and Peterson's original duplicate
ACK policy since clock ticks in today's machines are not as coarse as they
were (i.e. 500ms) when Vegas was originally designed. Note that modern TCP
recovery processes such as fast retransmit and SACK are enabled by default
in the TCP stack.</p>
</section>
<section class="Sh">
<h1 class="Sh" id="MIB_Variables"><a class="permalink" href="#MIB_Variables">MIB
Variables</a></h1>
<p class="Pp">The algorithm exposes the following tunable variables in the
<var class="Va">net.inet.tcp.cc.vegas</var> branch of the
<a class="Xr">sysctl(3)</a> MIB:</p>
<dl class="Bl-tag">
<dt id="alpha"><var class="Va">alpha</var></dt>
<dd>Query or set the Vegas alpha parameter as a number of buffers on the path.
When setting alpha, the value must satisfy: 0 < alpha < beta.
Default is 1.</dd>
<dt id="beta"><var class="Va">beta</var></dt>
<dd>Query or set the Vegas beta parameter as a number of buffers on the path.
When setting beta, the value must satisfy: 0 < alpha < beta. Default
is 3.</dd>
</dl>
</section>
<section class="Sh">
<h1 class="Sh" id="SEE_ALSO"><a class="permalink" href="#SEE_ALSO">SEE
ALSO</a></h1>
<p class="Pp"><a class="Xr">cc_cdg(4)</a>, <a class="Xr">cc_chd(4)</a>,
<a class="Xr">cc_cubic(4)</a>, <a class="Xr">cc_dctcp(4)</a>,
<a class="Xr">cc_hd(4)</a>, <a class="Xr">cc_htcp(4)</a>,
<a class="Xr">cc_newreno(4)</a>, <a class="Xr">h_ertt(4)</a>,
<a class="Xr">mod_cc(4)</a>, <a class="Xr">tcp(4)</a>,
<a class="Xr">khelp(9)</a>, <a class="Xr">mod_cc(9)</a></p>
<p class="Pp"><cite class="Rs"><span class="RsA">L. S. Brakmo</span> and
<span class="RsA">L. L. Peterson</span>, <span class="RsT">TCP Vegas: end to
end congestion avoidance on a global internet</span>, <i class="RsJ">IEEE J.
Sel. Areas Commun.</i>, <span class="RsN">8</span>,
<span class="RsV">13</span>, <span class="RsP">1465-1480</span>,
<span class="RsD">October 1995</span>.</cite></p>
</section>
<section class="Sh">
<h1 class="Sh" id="ACKNOWLEDGEMENTS"><a class="permalink" href="#ACKNOWLEDGEMENTS">ACKNOWLEDGEMENTS</a></h1>
<p class="Pp">Development and testing of this software were made possible in
part by grants from the FreeBSD Foundation and Cisco University Research
Program Fund at Community Foundation Silicon Valley.</p>
</section>
<section class="Sh">
<h1 class="Sh" id="HISTORY"><a class="permalink" href="#HISTORY">HISTORY</a></h1>
<p class="Pp">The <code class="Nm">cc_vegas</code> congestion control module
first appeared in <span class="Ux">FreeBSD 9.0</span>.</p>
<p class="Pp">The module was first released in 2010 by David Hayes whilst
working on the NewTCP research project at Swinburne University of
Technology's Centre for Advanced Internet Architectures, Melbourne,
Australia. More details are available at:</p>
<p class="Pp">http://caia.swin.edu.au/urp/newtcp/</p>
</section>
<section class="Sh">
<h1 class="Sh" id="AUTHORS"><a class="permalink" href="#AUTHORS">AUTHORS</a></h1>
<p class="Pp">The <code class="Nm">cc_vegas</code> congestion control module and
this manual page were written by <span class="An">David Hayes</span>
<<a class="Mt" href="mailto:david.hayes@ieee.org">david.hayes@ieee.org</a>>.</p>
</section>
</div>
<table class="foot">
<tr>
<td class="foot-date">May 13, 2021</td>
<td class="foot-os">FreeBSD 15.0</td>
</tr>
</table>
|
