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<a name="Compression-algorithms-used-in-the-record-layer"></a>
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Next: <a href="Weaknesses-and-countermeasures.html#Weaknesses-and-countermeasures" accesskey="n" rel="next">Weaknesses and countermeasures</a>, Previous: <a href="Encryption-algorithms-used-in-the-record-layer.html#Encryption-algorithms-used-in-the-record-layer" accesskey="p" rel="prev">Encryption algorithms used in the record layer</a>, Up: <a href="The-TLS-record-protocol.html#The-TLS-record-protocol" accesskey="u" rel="up">The TLS record protocol</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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<hr>
<a name="Compression-algorithms-used-in-the-record-layer-1"></a>
<h4 class="subsection">3.3.2 Compression algorithms used in the record layer</h4>
<a name="index-compression-algorithms"></a>
<p>The TLS record layer also supports compression. The algorithms
implemented in <acronym>GnuTLS</acronym> can be found in the table below.
The included algorithms perform really good when text, or other
compressible data are to be transferred, but offer nothing on already
compressed data, such as compressed images, zipped archives etc.
These compression algorithms, may be useful in high bandwidth TLS
tunnels, and in cases where network usage has to be minimized. It
should be noted however that compression increases latency.
</p>
<p>The record layer compression in <acronym>GnuTLS</acronym> is implemented based
on [<em>RFC3749</em>]. The supported algorithms are shown below.
</p>
<div class="float"><a name="gnutls_005fcompression_005fmethod_005ft"></a>
<dl compact="compact">
<dt><code>GNUTLS_COMP_UNKNOWN</code></dt>
<dd><p>Unknown compression method.
</p></dd>
<dt><code>GNUTLS_COMP_NULL</code></dt>
<dd><p>The NULL compression method (no compression).
</p></dd>
<dt><code>GNUTLS_COMP_DEFLATE</code></dt>
<dd><p>The DEFLATE compression method from zlib.
</p></dd>
<dt><code>GNUTLS_COMP_ZLIB</code></dt>
<dd><p>Same as <code>GNUTLS_COMP_DEFLATE</code> .
</p></dd>
</dl>
<div class="float-caption"><p><strong>Figure 3.2: </strong>Supported compression algorithms</p></div></div>
<p>Note that compression enables attacks such as traffic analysis, or even
plaintext recovery under certain circumstances. To avoid some of these
attacks GnuTLS allows each record to be compressed independently (i.e.,
stateless compression), by using the "%STATELESS_COMPRESSION" priority string,
in order to be used in cases where the attacker controlled data are
pt in separate records.
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