Revision 1111749 of Accept-Encoding

<div>{{HTTPSidebar}}</div>

<p>The <strong><code>Accept-Encoding</code></strong> request HTTP header advertises which content encoding, usually a compression algorithm, the client is able to understand. Using <a href="/en-US/docs/Web/HTTP/Content_negotiation">content negotiation</a>, the server selects one of the proposals, uses it and informs the client of its choice with the {{HTTPHeader("Content-Encoding")}} response header.</p>

<p>Even if both the client and the server supports the same compression algorithms, the server may choose not to compress the body of a response, if the identity value is also acceptable. Two common cases lead to this:</p>

<ul>
 <li>The data to be sent is already compressed and a second compression won't lead to smaller data to be transmitted. This may the case with some image formats;</li>
 <li>The server is overloaded and cannot afford the computational overhead induced by the compression requirement. Typically, Microsoft recommends not to compress if a server use more than 80 % of its computational power.</li>
</ul>

<p>As long as the <code>identity</code> value, meaning no encryption, is not explicitly forbidden, by an <code>identity;q=0</code> or a <code>*;q=0</code> without another explicitly set value for identity, the server must never send back a {{HTTPStatus("406")}} <code>Not Acceptable</code> error.</p>

<div class="note"><strong>Notes:</strong>

<ul>
 <li>
  <p>An IANA registry maintains <a class="external" href="https://www.iana.org/assignments/http-parameters/http-parameters.xml#http-parameters-1" title="https://www.iana.org/assignments/http-parameters/http-parameters.xml#http-parameters-1">a complete list of official content encodings</a>.</p>
 </li>
 <li>Two others content encoding, <code>bzip</code> and <code>bzip2</code>, are sometimes used, though not standard. They implement the algorithm used by these two UNIX programs. Note that the first one was discontinued due to patent licensing problems.</li>
</ul>
</div>

<table class="properties">
 <tbody>
  <tr>
   <th scope="row">Header type</th>
   <td>{{Glossary("Request header")}}</td>
  </tr>
  <tr>
   <th scope="row">{{Glossary("Forbidden header name")}}</th>
   <td>yes</td>
  </tr>
 </tbody>
</table>

<h2 id="Syntax">Syntax</h2>

<pre class="syntaxbox">



Accept-Encoding: gzip
Accept-Encoding: compress
Accept-Encoding: deflate
Accept-Encoding: br
Accept-Encoding: identity
Accept-Encoding: *

// Multiple algorithms, weighted with the {{Glossary("Quality Values", "quality value")}} syntax:
Accept-Encoding: deflate, gzip;q=1.0, *;q=0.5</pre>

<h2 id="Directives">Directives</h2>

<dl>
 <dt><code>gzip</code></dt>
 <dd>A compression format using the <a class="external external-icon" href="https://en.wikipedia.org/wiki/LZ77_and_LZ78#LZ77">Lempel-Ziv coding</a> (LZ77), with a 32-bit CRC.</dd>
 <dt><code>compress</code></dt>
 <dd>A compression format using the <a class="external external-icon" href="https://en.wikipedia.org/wiki/LZW">Lempel-Ziv-Welch</a> (LZW) algorithm.</dd>
 <dt><code>deflate</code></dt>
 <dd>A compression format using the <a class="external external-icon" href="https://en.wikipedia.org/wiki/Zlib">zlib</a> structure, with the <a class="external external-icon" href="https://en.wikipedia.org/wiki/DEFLATE"><em>deflate</em></a> compression algorithm.</dd>
 <dt><code>br</code></dt>
 <dd>A compression format using the <a class="external external-icon" href="https://en.wikipedia.org/wiki/Brotli">Brotli</a> algorithm.</dd>
 <dt><code>identity</code></dt>
 <dd>Indicates the identity function (i.e. no compression, nor modification. This value is always considered as acceptable, even if not present.</dd>
 <dt><code>*</code></dt>
 <dd>Matches any content encoding not already listed in the header. This is the default value if the header is not present. It doesn't mean that any algorithm is supported; merely that no preference is expressed.</dd>
 <dt><code>;q=</code> (qvalues weighting)</dt>
 <dd>Any value is placed in an order of preference expressed using a relative <a href="/en-US/docs/Glossary/Quality_value">quality values</a> called <em>weight</em>.</dd>
</dl>

<h2 id="Examples">Examples</h2>

<pre>



Accept-Encoding: gzip

Accept-Encoding: gzip, compress, br

Accept-Encoding: br;q=1.0, gzip;q=0.8, *;q=0.1
</pre>

<h2 id="Specifications">Specifications</h2>

<table class="standard-table">
 <tbody>
  <tr>
   <th scope="col">Specification</th>
   <th scope="col">Title</th>
  </tr>
  <tr>
   <td>{{RFC("7231", "Accept-Encoding", "5.3.4")}}</td>
   <td>Hypertext Transfer Protocol (HTTP/1.1): Semantics and Context</td>
  </tr>
 </tbody>
</table>

<h2 id="Browser_compatibility">Browser compatibility</h2>

<p class="hidden">The compatibility table in this page is generated from structured data. If you'd like to contribute to the data, please check out <a href="https://github.com/mdn/browser-compat-data">https://github.com/mdn/browser-compat-data</a> and send us a pull request.</p>

<p>{{Compat}}</p>

<h2 id="See_also">See also</h2>

<ul>
 <li>HTTP <a href="/en-US/docs/Web/HTTP/Content_negotiation">content negotiation</a></li>
 <li>Header with the result of the content negotiation: {{HTTPHeader("Content-Encoding")}}</li>
 <li>Other similar headers: {{HTTPHeader("TE")}}, {{HTTPHeader("Accept")}}, {{HTTPHeader("Accept-Charset")}}, {{HTTPHeader("Accept-Language")}}</li>
</ul>