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<h2>How the VPN works</h2>

<p>The VPN is extremely simple, based almost entirely on the standard
HTTPS and <a href="http://www.rfc-editor.org/rfc/rfc4347.txt">DTLS</a>
protocols. You connect to the secure web server, authenticate using
certificates and/or arbitrary web forms, and you are rewarded with a
standard HTTP cookie.</p>

<p>You then use this cookie in an HTTP <tt>CONNECT</tt> request, and can
then pass traffic over that connection. IP addresses and routing
information are passed back and forth in the headers of that
<tt>CONNECT</tt> request.</p>

<p>Since <a href="http://sites.inka.de/~W1011/devel/tcp-tcp.html">TCP
over TCP is very suboptimal</a>, the VPN also attempts to use UDP
datagrams, and will only <em>actually</em> pass traffic over the HTTPS
connection if that fails. The UDP connectivity is done using Datagram
TLS, which is supported by OpenSSL.</p>

<h2>DTLS compatibility</h2>

<p><i><b>Note: DTLS is optional and not required for basic connectivity, as explained above.</b></i></p>

<p>Unfortunately, Cisco used an old version of OpenSSL for their server,
which predates the official RFC and has a few differences in the
implementation of DTLS.
</p>
<h3>OpenSSL</h3>
<p>Compatibility support for their "speshul" version of the protocol is
in the 0.9.8m and later releases of OpenSSL (and 1.0.0-beta2 and later).
</p>

<p>If you are using an older version of OpenSSL, DTLS will
only work if you apply this patch from OpenSSL CVS:</p>
<ul>
  <li><a href="http://cvs.openssl.org/chngview?cn=18037">http://cvs.openssl.org/chngview?cn=18037</a> (OpenSSL <a href="http://rt.openssl.org/Ticket/Display.html?id=1751&amp;amp;user=guest&amp;amp;pass=guest">RT#1751</a>)</li>
</ul>

For versions older than 0.9.8j, some generic DTLS bug fixes are also required:
<ul>
  <li><a href="http://cvs.openssl.org/chngview?cn=17500">http://cvs.openssl.org/chngview?cn=17500</a>  (OpenSSL <a href="http://rt.openssl.org/Ticket/Display.html?id=1703&amp;amp;user=guest&amp;amp;pass=guest">RT#1703</a>)</li>
  <li><a href="http://cvs.openssl.org/chngview?cn=17505">http://cvs.openssl.org/chngview?cn=17505</a> (OpenSSL <a href="http://rt.openssl.org/Ticket/Display.html?id=1752&amp;amp;user=guest&amp;amp;pass=guest">RT#1752</a>) </li>
</ul>
The username/password for OpenSSL RT is 'guest/guest'

<h3>GnuTLS</h3>

<p>Support for Cisco's version of DTLS was included in GnuTLS in June 2012, in
<a href="http://git.savannah.gnu.org/gitweb/?p=gnutls.git;a=commitdiff;h=fd5ca1afb">
commit fd5ca1af</a> which will be part of GnuTLS 3.1.</p>

<p>The same patch will hopefully also be applied to the GnuTLS 3.0.x release branch
for 3.0.21, or it can be applied manually from <a href="http://git.infradead.org/users/dwmw2/gnutls.git/commitdiff_plain/436135d727cbfb1673f0c308869a6c15b2e17697">here</a>.</p>

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