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<!-- This manual is last updated 4 March 2015 for version
3.4.8 of GnuTLS.
Copyright (C) 2001-2015 Free Software Foundation, Inc.\\
Copyright (C) 2001-2015 Nikos Mavrogiannopoulos
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<title>GnuTLS 3.4.8: DTLS sessions</title>
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<a name="DTLS-sessions"></a>
<div class="header">
<p>
Previous: <a href="Asynchronous-operation.html#Asynchronous-operation" accesskey="p" rel="prev">Asynchronous operation</a>, Up: <a href="Setting-up-the-transport-layer.html#Setting-up-the-transport-layer" accesskey="u" rel="up">Setting up the transport layer</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Function-and-Data-Index.html#Function-and-Data-Index" title="Index" rel="index">Index</a>]</p>
</div>
<hr>
<a name="DTLS-sessions-1"></a>
<h4 class="subsection">6.5.2 DTLS sessions</h4>
<p>Because datagram TLS can operate over connections where the client
cannot be reliably verified, functionality in the form of cookies, is available to prevent
denial of service attacks to servers. <acronym>GnuTLS</acronym> requires a server
to generate a secret key that is used to sign a cookie<a name="DOCF16" href="#FOOT16"><sup>16</sup></a>.
That cookie is sent to the client using <a href="Datagram-TLS-API.html#gnutls_005fdtls_005fcookie_005fsend">gnutls_dtls_cookie_send</a>, and
the client must reply using the correct cookie. The server side
should verify the initial message sent by client using <a href="Datagram-TLS-API.html#gnutls_005fdtls_005fcookie_005fverify">gnutls_dtls_cookie_verify</a>.
If successful the session should be initialized and associated with
the cookie using <a href="Datagram-TLS-API.html#gnutls_005fdtls_005fprestate_005fset">gnutls_dtls_prestate_set</a>, before proceeding to
the handshake.
</p>
<dl compact="compact">
<dt><code><var>int</var> <a href="Core-TLS-API.html#gnutls_005fkey_005fgenerate">gnutls_key_generate</a> (gnutls_datum_t * <var>key</var>, unsigned int <var>key_size</var>)</code></dt>
<dt><code><var>int</var> <a href="Datagram-TLS-API.html#gnutls_005fdtls_005fcookie_005fsend">gnutls_dtls_cookie_send</a> (gnutls_datum_t * <var>key</var>, void * <var>client_data</var>, size_t <var>client_data_size</var>, gnutls_dtls_prestate_st * <var>prestate</var>, gnutls_transport_ptr_t <var>ptr</var>, gnutls_push_func <var>push_func</var>)</code></dt>
<dt><code><var>int</var> <a href="Datagram-TLS-API.html#gnutls_005fdtls_005fcookie_005fverify">gnutls_dtls_cookie_verify</a> (gnutls_datum_t * <var>key</var>, void * <var>client_data</var>, size_t <var>client_data_size</var>, void * <var>_msg</var>, size_t <var>msg_size</var>, gnutls_dtls_prestate_st * <var>prestate</var>)</code></dt>
<dt><code><var>void</var> <a href="Datagram-TLS-API.html#gnutls_005fdtls_005fprestate_005fset">gnutls_dtls_prestate_set</a> (gnutls_session_t <var>session</var>, gnutls_dtls_prestate_st * <var>prestate</var>)</code></dt>
</dl>
<p>Note that the above apply to server side only and they are not mandatory to be
used. Not using them, however, allows denial of service attacks.
The client side cookie handling is part of <a href="Core-TLS-API.html#gnutls_005fhandshake">gnutls_handshake</a>.
</p>
<p>Datagrams are typically restricted by a maximum transfer unit (MTU). For that
both client and server side should set the correct maximum transfer unit for
the layer underneath <acronym>GnuTLS</acronym>. This will allow proper fragmentation
of DTLS messages and prevent messages from being silently discarded by the
transport layer. The “correct” maximum transfer unit can be obtained through
a path MTU discovery mechanism [<em>RFC4821</em>].
</p>
<dl compact="compact">
<dt><code><var>void</var> <a href="Datagram-TLS-API.html#gnutls_005fdtls_005fset_005fmtu">gnutls_dtls_set_mtu</a> (gnutls_session_t <var>session</var>, unsigned int <var>mtu</var>)</code></dt>
<dt><code><var>unsigned int</var> <a href="Datagram-TLS-API.html#gnutls_005fdtls_005fget_005fmtu">gnutls_dtls_get_mtu</a> (gnutls_session_t <var>session</var>)</code></dt>
<dt><code><var>unsigned int</var> <a href="Datagram-TLS-API.html#gnutls_005fdtls_005fget_005fdata_005fmtu">gnutls_dtls_get_data_mtu</a> (gnutls_session_t <var>session</var>)</code></dt>
</dl>
<div class="footnote">
<hr>
<h4 class="footnotes-heading">Footnotes</h4>
<h3><a name="FOOT16" href="#DOCF16">(16)</a></h3>
<p>A key of 128 bits or 16 bytes should be sufficient for this purpose.</p>
</div>
<hr>
<div class="header">
<p>
Previous: <a href="Asynchronous-operation.html#Asynchronous-operation" accesskey="p" rel="prev">Asynchronous operation</a>, Up: <a href="Setting-up-the-transport-layer.html#Setting-up-the-transport-layer" accesskey="u" rel="up">Setting up the transport layer</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Function-and-Data-Index.html#Function-and-Data-Index" title="Index" rel="index">Index</a>]</p>
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