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-
- <h1 align="center">Content Negotiation</h1>
+<html><head><META http-equiv="Content-Type" content="text/html; charset=ISO-8859-1"><!--
+ XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
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+ --><title>Content Negotiation - Apache HTTP Server</title><link href="./style/manual.css" type="text/css" rel="stylesheet"></head><body><blockquote><div align="center"><img src="./images/sub.gif" alt="[APACHE DOCUMENTATION]"><h3>Apache HTTP Server Version 2.0</h3></div><h1 align="center">Content Negotiation</h1>
<p>Apache's supports content negotiation as described in
the HTTP/1.1 specification. It can choose the best
more intelligent handling of requests from browsers that send
incomplete negotiation information.</p>
- <p>Content negotiation is provided by the <a
- href="mod/mod_negotiation.html">mod_negotiation</a> module,
+ <p>Content negotiation is provided by the
+ <code><a href="./mod/mod_negotiation.html">mod_negotiation</a></code> module.
which is compiled in by default.</p>
- <hr />
-
- <h2>About Content Negotiation</h2>
+<ul><li><a href="#about">About Content Negotiation</a></li><li><a href="#negotiation">Negotiation in Apache</a><ul><li><a href="#type-map">Using a type-map file</a></li><li><a href="#multiviews">Multiviews</a></li></ul></li><li><a href="#methods">The Negotiation Methods</a><ul><li><a href="#dimensions">Dimensions of Negotiation</a></li><li><a href="#algorithm">Apache Negotiation Algorithm</a></li></ul></li><li><a href="#better">Fiddling with Quality
+ Values</a><ul><li><a href="#wildcards">Media Types and Wildcards</a></li><li><a href="#exceptions">Language Negotiation Exceptions</a></li></ul></li><li><a href="#extensions">Extensions to Transparent Content
+Negotiation</a></li><li><a href="#naming">Note on hyperlinks and naming conventions</a></li><li><a href="#caching">Note on Caching</a></li><li><a href="#more">More Information</a></li></ul><hr><h2><a name="about">About Content Negotiation</a></h2>
<p>A resource may be available in several different
representations. For example, it might be available in
if possible, else English will do. Browsers indicate their
preferences by headers in the request. To request only French
representations, the browser would send</p>
-<pre>
- Accept-Language: fr
-</pre>
+
+<blockquote><table cellpadding="10"><tr><td bgcolor="#eeeeee"><code>Accept-Language: fr</code></td></tr></table></blockquote>
<p>Note that this preference will only be applied when there is
a choice of representations and they vary by language.</p>
plain text or other text types, and preferring GIF or JPEG over
other media types, but also allowing any other media type as a
last resort:</p>
-<pre>
- Accept-Language: fr; q=1.0, en; q=0.5
- Accept: text/html; q=1.0, text/*; q=0.8, image/gif; q=0.6,
- image/jpeg; q=0.6, image/*; q=0.5, */*; q=0.1
-</pre>
- Apache supports 'server driven' content negotiation, as
+
+<blockquote><table cellpadding="10"><tr><td bgcolor="#eeeeee"><code>
+ Accept-Language: fr; q=1.0, en; q=0.5<br>
+ Accept: text/html; q=1.0, text/*; q=0.8, image/gif; q=0.6, image/jpeg; q=0.6, image/*; q=0.5, */*; q=0.1
+</code></td></tr></table></blockquote>
+
+ <p>Apache supports 'server driven' content negotiation, as
defined in the HTTP/1.1 specification. It fully supports the
Accept, Accept-Language, Accept-Charset and Accept-Encoding
request headers. Apache also supports 'transparent'
content negotiation, which is an experimental negotiation
protocol defined in RFC 2295 and RFC 2296. It does not offer
- support for 'feature negotiation' as defined in these RFCs.
+ support for 'feature negotiation' as defined in these RFCs. </p>
<p>A <strong>resource</strong> is a conceptual entity
identified by a URI (RFC 2396). An HTTP server like Apache
representations is termed a <strong>variant</strong>. The ways
in which the variants for a negotiable resource vary are called
the <strong>dimensions</strong> of negotiation.</p>
-
- <h2>Negotiation in Apache</h2>
+<h2><a name="negotiation">Negotiation in Apache</a></h2>
<p>In order to negotiate a resource, the server needs to be
given information about each of the variants. This is done in
results.</li>
</ul>
- <h3>Using a type-map file</h3>
+ <h3><a name="type-map">Using a type-map file</a></h3>
<p>A type map is a document which is associated with the
handler named <code>type-map</code> (or, for
use this feature, you must have a handler set in the
configuration that defines a file suffix as
<code>type-map</code>; this is best done with a</p>
-<pre>
- AddHandler type-map .var
-</pre>
- in the server configuration file.
+<blockquote><table cellpadding="10"><tr><td bgcolor="#eeeeee"><code>AddHandler type-map .var</code></td></tr></table></blockquote>
+ <p>in the server configuration file.</p>
<p>Type map files should have the same name as the resource
which they are describing, and have an entry for each available
present will be ignored). An example map file is shown below.
This file would be named <code>foo.var</code>, as it describes
a resource named <code>foo</code>.</p>
-<pre>
- URI: foo
-
- URI: foo.en.html
- Content-type: text/html
- Content-language: en
-
- URI: foo.fr.de.html
- Content-type: text/html;charset=iso-8859-2
- Content-language: fr, de
-</pre>
- Note also that a typemap file will take precedence over the
+
+<blockquote><table cellpadding="10"><tr><td bgcolor="#eeeeee"><code>
+ URI: foo<br>
+<br>
+ URI: foo.en.html<br>
+ Content-type: text/html<br>
+ Content-language: en<br>
+<br>
+ URI: foo.fr.de.html<br>
+ Content-type: text/html;charset=iso-8859-2<br>
+ Content-language: fr, de<br>
+</code></td></tr></table></blockquote>
+ <p>Note also that a typemap file will take precedence over the
filename's extension, even when Multiviews is on. If the
variants have different source qualities, that may be indicated
by the "qs" parameter to the media type, as in this picture
- (available as jpeg, gif, or ASCII-art):
-<pre>
- URI: foo
-
- URI: foo.jpeg
- Content-type: image/jpeg; qs=0.8
-
- URI: foo.gif
- Content-type: image/gif; qs=0.5
-
- URI: foo.txt
- Content-type: text/plain; qs=0.01
-</pre>
+ (available as jpeg, gif, or ASCII-art): </p>
+
+<blockquote><table cellpadding="10"><tr><td bgcolor="#eeeeee"><code>
+ URI: foo<br>
+<br>
+ URI: foo.jpeg<br>
+ Content-type: image/jpeg; qs=0.8<br>
+<br>
+ URI: foo.gif<br>
+ Content-type: image/gif; qs=0.5<br>
+<br>
+ URI: foo.txt<br>
+ Content-type: text/plain; qs=0.01<br>
+</code></td></tr></table></blockquote>
<p>qs values can vary in the range 0.000 to 1.000. Note that
any variant with a qs value of 0.000 will never be chosen.
variant depending on the nature of the resource it
represents.</p>
- <p>The full list of headers recognized is available in the <a
- href="mod/mod_negotiation.html#typemaps">mod_negotation</a>
- documentation.</p>
+ <p>The full list of headers recognized is available in the <a href="mod/mod_negotiation.html#typemaps">mod_negotation
+ typemap</a> documentation.</p>
- <h3>Multiviews</h3>
+<h3><a name="multiviews">Multiviews</a></h3>
- <p><code>MultiViews</code> is a per-directory option, meaning
- it can be set with an <code>Options</code> directive within a
- <code><Directory></code>, <code><Location></code>
- or <code><Files></code> section in
- <code>access.conf</code>, or (if <code>AllowOverride</code> is
- properly set) in <code>.htaccess</code> files. Note that
- <code>Options All</code> does not set <code>MultiViews</code>;
- you have to ask for it by name.</p>
+ <p><code>MultiViews</code> is a per-directory option, meaning it
+ can be set with an <a href="./mod/core.html#options" class="directive"><code class="directive">Options</code></a>
+ directive within a <a href="./mod/core.html#directory" class="directive"><code class="directive"><Directory></code></a>, <a href="./mod/core.html#location" class="directive"><code class="directive"><Location></code></a> or <a href="./mod/core.html#files" class="directive"><code class="directive"><Files></code></a> section in
+ <code>access.conf</code>, or (if <a href="./mod/core.html#allowoverride" class="directive"><code class="directive">AllowOverride</code></a> is properly set) in
+ <code>.htaccess</code> files. Note that <code>Options All</code>
+ does not set <code>MultiViews</code>; you have to ask for it by
+ name.</p>
<p>The effect of <code>MultiViews</code> is as follows: if the
server receives a request for <code>/some/dir/foo</code>, if
would have if the client had asked for one of them by name. It
then chooses the best match to the client's requirements.</p>
- <p><code>MultiViews</code> may also apply to searches for the
- file named by the <code>DirectoryIndex</code> directive, if the
- server is trying to index a directory. If the configuration
- files specify</p>
-<pre>
- DirectoryIndex index
-</pre>
- then the server will arbitrate between <code>index.html</code>
+ <p><code>MultiViews</code> may also apply to searches for the file
+ named by the <a href="./mod/mod_dir.html#directoryindex" class="directive"><code class="directive">DirectoryIndex</code></a> directive, if the
+ server is trying to index a directory. If the configuration files
+ specify</p>
+<blockquote><table cellpadding="10"><tr><td bgcolor="#eeeeee"><code>DirectoryIndex index</code></td></tr></table></blockquote>
+ <p>then the server will arbitrate between <code>index.html</code>
and <code>index.html3</code> if both are present. If neither
are present, and <code>index.cgi</code> is there, the server
- will run it.
+ will run it.</p>
<p>If one of the files found when reading the directory does not
have an extension recognized by <code>mod_mime</code> to designate
its Charset, Content-Type, Language, or Encoding, then the result
- depends on the setting of the <a
- href="mod/mod_mime.html#multiviewsmatch">MultiViewsMatch</a>
- directive. This directive determines whether handlers, filters,
- and other extension types can participate in MultiViews
- negotiation.</p>
-
- <h2>The Negotiation Methods</h2>
- After Apache has obtained a list of the variants for a given
+ depends on the setting of the <a href="./mod/mod_mime.html#multiviewsmatch" class="directive"><code class="directive">MultiViewsMatch</code></a> directive. This
+ directive determines whether handlers, filters, and other
+ extension types can participate in MultiViews negotiation.</p>
+
+<h2><a name="methods">The Negotiation Methods</a></h2>
+
+ <p>After Apache has obtained a list of the variants for a given
resource, either from a type-map file or from the filenames in
the directory, it invokes one of two methods to decide on the
'best' variant to return, if any. It is not necessary to know
any of the details of how negotiation actually takes place in
order to use Apache's content negotiation features. However the
rest of this document explains the methods used for those
- interested.
+ interested. </p>
<p>There are two negotiation methods:</p>
variant selection algorithm' defined in RFC 2296.</li>
</ol>
- <h3>Dimensions of Negotiation</h3>
+<h3><a name="dimensions">Dimensions of Negotiation</a></h3>
<table>
<tr valign="top">
</tr>
</table>
- <h3>Apache Negotiation Algorithm</h3>
+
+<h3><a name="algorithm">Apache Negotiation Algorithm</a></h3>
<p>Apache can use the following algorithm to select the 'best'
variant (if any) to return to the browser. This algorithm is
dimensions of variance.</li>
</ol>
- <h2><a id="better" name="better">Fiddling with Quality
+<h2><a name="better">Fiddling with Quality
Values</a></h2>
<p>Apache sometimes changes the quality values from what would
sends full and correct information these fiddles will not be
applied.</p>
- <h3>Media Types and Wildcards</h3>
+<h3><a name="wildcards">Media Types and Wildcards</a></h3>
<p>The Accept: request header indicates preferences for media
types. It can also include 'wildcard' media types, such as
"image/*" or "*/*" where the * matches any string. So a request
including:</p>
-<pre>
- Accept: image/*, */*
-</pre>
- would indicate that any type starting "image/" is acceptable,
+
+<blockquote><table cellpadding="10"><tr><td bgcolor="#eeeeee"><code>Accept: image/*, */*</code></td></tr></table></blockquote>
+
+ <p>would indicate that any type starting "image/" is acceptable,
as is any other type (so the first "image/*" is redundant).
Some browsers routinely send wildcards in addition to explicit
- types they can handle. For example:
-<pre>
+ types they can handle. For example:</p>
+
+<blockquote><table cellpadding="10"><tr><td bgcolor="#eeeeee"><code>
Accept: text/html, text/plain, image/gif, image/jpeg, */*
-</pre>
- The intention of this is to indicate that the explicitly listed
+</code></td></tr></table></blockquote>
+ <p>The intention of this is to indicate that the explicitly listed
types are preferred, but if a different representation is
available, that is ok too. However under the basic algorithm,
as given above, the */* wildcard has exactly equal preference
to all the other types, so they are not being preferred. The
browser should really have sent a request with a lower quality
- (preference) value for *.*, such as:
-<pre>
+ (preference) value for *.*, such as:</p>
+<blockquote><table cellpadding="10"><tr><td bgcolor="#eeeeee"><code>
Accept: text/html, text/plain, image/gif, image/jpeg, */*; q=0.01
-</pre>
- The explicit types have no quality factor, so they default to a
+</code></td></tr></table></blockquote>
+ <p>The explicit types have no quality factor, so they default to a
preference of 1.0 (the highest). The wildcard */* is given a
low preference of 0.01, so other types will only be returned if
- no variant matches an explicitly listed type.
+ no variant matches an explicitly listed type.</p>
<p>If the Accept: header contains <em>no</em> q factors at all,
Apache sets the q value of "*/*", if present, to 0.01 to
<em>not</em> applied, so requests from browsers which send the
correct information to start with work as expected.</p>
- <h3>Language Negotiation Exceptions</h3>
+
+<h3><a name="exceptions">Language Negotiation Exceptions</a></h3>
<p>New in Apache 2.0, some exceptions have been added to the
negotiation algorithm to allow graceful fallback when language
Variant" or "Multiple Choices" response to the client. To avoid
these error messages, it is possible to configure Apache to ignore
the Accept-language in these cases and provide a document that
- does not explictly match the client's request. The <a
- href="mod/mod_negotiation.html#forcelanguagepriority">ForceLanguagePriority</a>
+ does not explictly match the client's request. The <a href="./mod/mod_negotiation.html#forcelanguagepriority" class="directive"><code class="directive">ForceLanguagePriority</code></a>
directive can be used to override one or both of these error
- messages and subsitute the servers judgement in the form of the <a
- href="mod/mod_negotiation.html#languagepriority">LanguagePriority</a>
+ messages and subsitute the servers judgement in the form of the
+ <a href="./mod/mod_negotiation.html#languagepriority" class="directive"><code class="directive">LanguagePriority</code></a>
directive.</p>
<p>The server will also attempt to match language-subsets when no
general. Unfortunately, many current clients have default
configurations that resemble this.) However, if no other language
match is possible and the server is about to return a "No
- Acceptable Variants" error or fallback to the
- <code>LanguagePriority</code>, the server will ignore the subset
- specification and match <code>en-GB</code> against <code>en</code>
- documents. Implicitly, Apache will add the parent language to
- the client's acceptable language list with a very low quality
- value. But note that if the client requests "en-GB; qs=0.9, fr;
- qs=0.8", and the server has documents designated "en" and "fr",
- then the "fr" document will be returned. This is necessary to
- maintain compliance with the HTTP/1.1 specification and to work
- effectively with properly configured clients.</p>
-
-
- <h2>Extensions to Transparent Content Negotiation</h2>
- Apache extends the transparent content negotiation protocol
- (RFC 2295) as follows. A new <code>{encoding ..}</code> element
- is used in variant lists to label variants which are available
- with a specific content-encoding only. The implementation of
- the RVSA/1.0 algorithm (RFC 2296) is extended to recognize
- encoded variants in the list, and to use them as candidate
- variants whenever their encodings are acceptable according to
- the Accept-Encoding request header. The RVSA/1.0 implementation
- does not round computed quality factors to 5 decimal places
- before choosing the best variant.
-
- <h2>Note on hyperlinks and naming conventions</h2>
+ Acceptable Variants" error or fallback to the <a href="./mod/mod_negotiation.html#languagepriority" class="directive"><code class="directive">LanguagePriority</code></a>, the server
+ will ignore the subset specification and match <code>en-GB</code>
+ against <code>en</code> documents. Implicitly, Apache will add
+ the parent language to the client's acceptable language list with
+ a very low quality value. But note that if the client requests
+ "en-GB; qs=0.9, fr; qs=0.8", and the server has documents
+ designated "en" and "fr", then the "fr" document will be returned.
+ This is necessary to maintain compliance with the HTTP/1.1
+ specification and to work effectively with properly configured
+ clients.</p>
+
+<h2><a name="extensions">Extensions to Transparent Content
+Negotiation</a></h2>
+
+<p>Apache extends the transparent content negotiation protocol (RFC
+2295) as follows. A new <code>{encoding ..}</code> element is used in
+variant lists to label variants which are available with a specific
+content-encoding only. The implementation of the RVSA/1.0 algorithm
+(RFC 2296) is extended to recognize encoded variants in the list, and
+to use them as candidate variants whenever their encodings are
+acceptable according to the Accept-Encoding request header. The
+RVSA/1.0 implementation does not round computed quality factors to 5
+decimal places before choosing the best variant.</p>
+<h2><a name="naming">Note on hyperlinks and naming conventions</a></h2>
<p>If you are using language negotiation you can choose between
different naming conventions, because files can have more than
one extension, and the order of the extensions is normally
- irrelevant (see the <a
- href="mod/mod_mime.html#multipleext">mod_mime</a> documentation
+ irrelevant (see the <a href="mod/mod_mime.html#multipleext">mod_mime</a> documentation
for details).</p>
<p>A typical file has a MIME-type extension (<em>e.g.</em>,
- <samp>html</samp>), maybe an encoding extension (<em>e.g.</em>,
- <samp>gz</samp>), and of course a language extension
- (<em>e.g.</em>, <samp>en</samp>) when we have different
+ <code>html</code>), maybe an encoding extension (<em>e.g.</em>,
+ <code>gz</code>), and of course a language extension
+ (<em>e.g.</em>, <code>en</code>) when we have different
language variants of this file.</p>
<p>Examples:</p>
<tr>
<td><em>foo.html.en</em></td>
- <td>foo<br />
+ <td>foo<br>
foo.html</td>
<td>-</td>
<tr>
<td><em>foo.html.en.gz</em></td>
- <td>foo<br />
+ <td>foo<br>
foo.html</td>
- <td>foo.gz<br />
+ <td>foo.gz<br>
foo.html.gz</td>
</tr>
<td>foo</td>
- <td>foo.html<br />
- foo.html.gz<br />
+ <td>foo.html<br>
+ foo.html.gz<br>
foo.gz</td>
</tr>
<tr>
<td><em>foo.gz.html.en</em></td>
- <td>foo<br />
- foo.gz<br />
+ <td>foo<br>
+ foo.gz<br>
foo.gz.html</td>
<td>foo.html</td>
<tr>
<td><em>foo.html.gz.en</em></td>
- <td>foo<br />
- foo.html<br />
+ <td>foo<br>
+ foo.html<br>
foo.html.gz</td>
<td>foo.gz</td>
<p>Looking at the table above you will notice that it is always
possible to use the name without any extensions in an hyperlink
- (<em>e.g.</em>, <samp>foo</samp>). The advantage is that you
+ (<em>e.g.</em>, <code>foo</code>). The advantage is that you
can hide the actual type of a document rsp. file and can change
- it later, <em>e.g.</em>, from <samp>html</samp> to
- <samp>shtml</samp> or <samp>cgi</samp> without changing any
+ it later, <em>e.g.</em>, from <code>html</code> to
+ <code>shtml</code> or <code>cgi</code> without changing any
hyperlink references.</p>
<p>If you want to continue to use a MIME-type in your
- hyperlinks (<em>e.g.</em> <samp>foo.html</samp>) the language
+ hyperlinks (<em>e.g.</em> <code>foo.html</code>) the language
extension (including an encoding extension if there is one)
must be on the right hand side of the MIME-type extension
- (<em>e.g.</em>, <samp>foo.html.en</samp>).</p>
-
- <h2>Note on Caching</h2>
+ (<em>e.g.</em>, <code>foo.html.en</code>).</p>
+<h2><a name="caching">Note on Caching</a></h2>
<p>When a cache stores a representation, it associates it with
the request URL. The next time that URL is requested, the cache
responses.</p>
<p>For requests which come from a HTTP/1.0 compliant client
- (either a browser or a cache), the directive
- <tt>CacheNegotiatedDocs</tt> can be used to allow caching of
- responses which were subject to negotiation. This directive can
- be given in the server config or virtual host, and takes no
- arguments. It has no effect on requests from HTTP/1.1 clients.</p>
-
- <h2>More Information</h2>
+ (either a browser or a cache), the directive <a href="./mod/mod_negotiation.html#cachenegotiateddocs" class="directive"><code class="directive">CacheNegotiatedDocs</code></a> can be
+ used to allow caching of responses which were subject to
+ negotiation. This directive can be given in the server config or
+ virtual host, and takes no arguments. It has no effect on requests
+ from HTTP/1.1 clients.</p>
+<h2><a name="more">More Information</a></h2>
<p>For more information about content negotiation, see Alan
- J. Flavell's <a
- href="http://ppewww.ph.gla.ac.uk/~flavell/www/lang-neg.html">Language
+ J. Flavell's <a href="http://ppewww.ph.gla.ac.uk/~flavell/www/lang-neg.html">Language
Negotiation Notes</a>. But note that this document may not be
updated to include changes in Apache 2.0.</p>
-
- <!--#include virtual="footer.html" -->
- </body>
-</html>
-
+<hr></blockquote><h3 align="center">Apache HTTP Server Version 2.0</h3><a href="./"><img src="./images/index.gif" alt="Index"></a><a href="./"><img src="./images/home.gif" alt="Home"></a></body></html>
\ No newline at end of file
--- /dev/null
+<?xml version='1.0' encoding='UTF-8' ?>
+<!DOCTYPE manualpage SYSTEM "./style/manualpage.dtd">
+<?xml-stylesheet type="text/xsl" href="./style/manual.en.xsl"?>
+<manualpage>
+<relativepath href="."/>
+
+<title>Content Negotiation</title>
+
+<summary>
+
+ <p>Apache's supports content negotiation as described in
+ the HTTP/1.1 specification. It can choose the best
+ representation of a resource based on the browser-supplied
+ preferences for media type, languages, character set and
+ encoding. It also implements a couple of features to give
+ more intelligent handling of requests from browsers that send
+ incomplete negotiation information.</p>
+
+ <p>Content negotiation is provided by the
+ <module>mod_negotiation</module> module.
+ which is compiled in by default.</p>
+</summary>
+
+<section id="about"><title>About Content Negotiation</title>
+
+ <p>A resource may be available in several different
+ representations. For example, it might be available in
+ different languages or different media types, or a combination.
+ One way of selecting the most appropriate choice is to give the
+ user an index page, and let them select. However it is often
+ possible for the server to choose automatically. This works
+ because browsers can send as part of each request information
+ about what representations they prefer. For example, a browser
+ could indicate that it would like to see information in French,
+ if possible, else English will do. Browsers indicate their
+ preferences by headers in the request. To request only French
+ representations, the browser would send</p>
+
+<example>Accept-Language: fr</example>
+
+ <p>Note that this preference will only be applied when there is
+ a choice of representations and they vary by language.</p>
+
+ <p>As an example of a more complex request, this browser has
+ been configured to accept French and English, but prefer
+ French, and to accept various media types, preferring HTML over
+ plain text or other text types, and preferring GIF or JPEG over
+ other media types, but also allowing any other media type as a
+ last resort:</p>
+
+<example>
+ Accept-Language: fr; q=1.0, en; q=0.5<br />
+ Accept: text/html; q=1.0, text/*; q=0.8, image/gif; q=0.6, image/jpeg; q=0.6, image/*; q=0.5, */*; q=0.1
+</example>
+
+ <p>Apache supports 'server driven' content negotiation, as
+ defined in the HTTP/1.1 specification. It fully supports the
+ Accept, Accept-Language, Accept-Charset and Accept-Encoding
+ request headers. Apache also supports 'transparent'
+ content negotiation, which is an experimental negotiation
+ protocol defined in RFC 2295 and RFC 2296. It does not offer
+ support for 'feature negotiation' as defined in these RFCs. </p>
+
+ <p>A <strong>resource</strong> is a conceptual entity
+ identified by a URI (RFC 2396). An HTTP server like Apache
+ provides access to <strong>representations</strong> of the
+ resource(s) within its namespace, with each representation in
+ the form of a sequence of bytes with a defined media type,
+ character set, encoding, etc. Each resource may be associated
+ with zero, one, or more than one representation at any given
+ time. If multiple representations are available, the resource
+ is referred to as <strong>negotiable</strong> and each of its
+ representations is termed a <strong>variant</strong>. The ways
+ in which the variants for a negotiable resource vary are called
+ the <strong>dimensions</strong> of negotiation.</p>
+</section>
+
+<section id="negotiation"><title>Negotiation in Apache</title>
+
+ <p>In order to negotiate a resource, the server needs to be
+ given information about each of the variants. This is done in
+ one of two ways:</p>
+
+ <ul>
+ <li>Using a type map (<em>i.e.</em>, a <code>*.var</code>
+ file) which names the files containing the variants
+ explicitly, or</li>
+
+ <li>Using a 'MultiViews' search, where the server does an
+ implicit filename pattern match and chooses from among the
+ results.</li>
+ </ul>
+
+ <section id="type-map"><title>Using a type-map file</title>
+
+ <p>A type map is a document which is associated with the
+ handler named <code>type-map</code> (or, for
+ backwards-compatibility with older Apache configurations, the
+ mime type <code>application/x-type-map</code>). Note that to
+ use this feature, you must have a handler set in the
+ configuration that defines a file suffix as
+ <code>type-map</code>; this is best done with a</p>
+<example>AddHandler type-map .var</example>
+ <p>in the server configuration file.</p>
+
+ <p>Type map files should have the same name as the resource
+ which they are describing, and have an entry for each available
+ variant; these entries consist of contiguous HTTP-format header
+ lines. Entries for different variants are separated by blank
+ lines. Blank lines are illegal within an entry. It is
+ conventional to begin a map file with an entry for the combined
+ entity as a whole (although this is not required, and if
+ present will be ignored). An example map file is shown below.
+ This file would be named <code>foo.var</code>, as it describes
+ a resource named <code>foo</code>.</p>
+
+<example>
+ URI: foo<br />
+<br />
+ URI: foo.en.html<br />
+ Content-type: text/html<br />
+ Content-language: en<br />
+<br />
+ URI: foo.fr.de.html<br />
+ Content-type: text/html;charset=iso-8859-2<br />
+ Content-language: fr, de<br />
+</example>
+ <p>Note also that a typemap file will take precedence over the
+ filename's extension, even when Multiviews is on. If the
+ variants have different source qualities, that may be indicated
+ by the "qs" parameter to the media type, as in this picture
+ (available as jpeg, gif, or ASCII-art): </p>
+
+<example>
+ URI: foo<br />
+<br />
+ URI: foo.jpeg<br />
+ Content-type: image/jpeg; qs=0.8<br />
+<br />
+ URI: foo.gif<br />
+ Content-type: image/gif; qs=0.5<br />
+<br />
+ URI: foo.txt<br />
+ Content-type: text/plain; qs=0.01<br />
+</example>
+
+ <p>qs values can vary in the range 0.000 to 1.000. Note that
+ any variant with a qs value of 0.000 will never be chosen.
+ Variants with no 'qs' parameter value are given a qs factor of
+ 1.0. The qs parameter indicates the relative 'quality' of this
+ variant compared to the other available variants, independent
+ of the client's capabilities. For example, a jpeg file is
+ usually of higher source quality than an ascii file if it is
+ attempting to represent a photograph. However, if the resource
+ being represented is an original ascii art, then an ascii
+ representation would have a higher source quality than a jpeg
+ representation. A qs value is therefore specific to a given
+ variant depending on the nature of the resource it
+ represents.</p>
+
+ <p>The full list of headers recognized is available in the <a
+ href="mod/mod_negotiation.html#typemaps">mod_negotation
+ typemap</a> documentation.</p>
+</section>
+
+<section id="multiviews"><title>Multiviews</title>
+
+ <p><code>MultiViews</code> is a per-directory option, meaning it
+ can be set with an <directive module="core">Options</directive>
+ directive within a <directive module="core"
+ type="section">Directory</directive>, <directive module="core"
+ type="section">Location</directive> or <directive module="core"
+ type="section">Files</directive> section in
+ <code>access.conf</code>, or (if <directive
+ module="core">AllowOverride</directive> is properly set) in
+ <code>.htaccess</code> files. Note that <code>Options All</code>
+ does not set <code>MultiViews</code>; you have to ask for it by
+ name.</p>
+
+ <p>The effect of <code>MultiViews</code> is as follows: if the
+ server receives a request for <code>/some/dir/foo</code>, if
+ <code>/some/dir</code> has <code>MultiViews</code> enabled, and
+ <code>/some/dir/foo</code> does <em>not</em> exist, then the
+ server reads the directory looking for files named foo.*, and
+ effectively fakes up a type map which names all those files,
+ assigning them the same media types and content-encodings it
+ would have if the client had asked for one of them by name. It
+ then chooses the best match to the client's requirements.</p>
+
+ <p><code>MultiViews</code> may also apply to searches for the file
+ named by the <directive
+ module="mod_dir">DirectoryIndex</directive> directive, if the
+ server is trying to index a directory. If the configuration files
+ specify</p>
+<example>DirectoryIndex index</example>
+ <p>then the server will arbitrate between <code>index.html</code>
+ and <code>index.html3</code> if both are present. If neither
+ are present, and <code>index.cgi</code> is there, the server
+ will run it.</p>
+
+ <p>If one of the files found when reading the directory does not
+ have an extension recognized by <code>mod_mime</code> to designate
+ its Charset, Content-Type, Language, or Encoding, then the result
+ depends on the setting of the <directive
+ module="mod_mime">MultiViewsMatch</directive> directive. This
+ directive determines whether handlers, filters, and other
+ extension types can participate in MultiViews negotiation.</p>
+</section>
+</section>
+
+<section id="methods"><title>The Negotiation Methods</title>
+
+ <p>After Apache has obtained a list of the variants for a given
+ resource, either from a type-map file or from the filenames in
+ the directory, it invokes one of two methods to decide on the
+ 'best' variant to return, if any. It is not necessary to know
+ any of the details of how negotiation actually takes place in
+ order to use Apache's content negotiation features. However the
+ rest of this document explains the methods used for those
+ interested. </p>
+
+ <p>There are two negotiation methods:</p>
+
+ <ol>
+ <li><strong>Server driven negotiation with the Apache
+ algorithm</strong> is used in the normal case. The Apache
+ algorithm is explained in more detail below. When this
+ algorithm is used, Apache can sometimes 'fiddle' the quality
+ factor of a particular dimension to achieve a better result.
+ The ways Apache can fiddle quality factors is explained in
+ more detail below.</li>
+
+ <li><strong>Transparent content negotiation</strong> is used
+ when the browser specifically requests this through the
+ mechanism defined in RFC 2295. This negotiation method gives
+ the browser full control over deciding on the 'best' variant,
+ the result is therefore dependent on the specific algorithms
+ used by the browser. As part of the transparent negotiation
+ process, the browser can ask Apache to run the 'remote
+ variant selection algorithm' defined in RFC 2296.</li>
+ </ol>
+
+<section id="dimensions"><title>Dimensions of Negotiation</title>
+
+ <table>
+ <tr valign="top">
+ <th>Dimension</th>
+
+ <th>Notes</th>
+ </tr>
+
+ <tr valign="top">
+ <td>Media Type</td>
+
+ <td>Browser indicates preferences with the Accept header
+ field. Each item can have an associated quality factor.
+ Variant description can also have a quality factor (the
+ "qs" parameter).</td>
+ </tr>
+
+ <tr valign="top">
+ <td>Language</td>
+
+ <td>Browser indicates preferences with the Accept-Language
+ header field. Each item can have a quality factor. Variants
+ can be associated with none, one or more than one
+ language.</td>
+ </tr>
+
+ <tr valign="top">
+ <td>Encoding</td>
+
+ <td>Browser indicates preference with the Accept-Encoding
+ header field. Each item can have a quality factor.</td>
+ </tr>
+
+ <tr valign="top">
+ <td>Charset</td>
+
+ <td>Browser indicates preference with the Accept-Charset
+ header field. Each item can have a quality factor. Variants
+ can indicate a charset as a parameter of the media
+ type.</td>
+ </tr>
+ </table>
+</section>
+
+<section id="algorithm"><title>Apache Negotiation Algorithm</title>
+
+ <p>Apache can use the following algorithm to select the 'best'
+ variant (if any) to return to the browser. This algorithm is
+ not further configurable. It operates as follows:</p>
+
+ <ol>
+ <li>First, for each dimension of the negotiation, check the
+ appropriate <em>Accept*</em> header field and assign a
+ quality to each variant. If the <em>Accept*</em> header for
+ any dimension implies that this variant is not acceptable,
+ eliminate it. If no variants remain, go to step 4.</li>
+
+ <li>
+ Select the 'best' variant by a process of elimination. Each
+ of the following tests is applied in order. Any variants
+ not selected at each test are eliminated. After each test,
+ if only one variant remains, select it as the best match
+ and proceed to step 3. If more than one variant remains,
+ move on to the next test.
+
+ <ol>
+ <li>Multiply the quality factor from the Accept header
+ with the quality-of-source factor for this variant's
+ media type, and select the variants with the highest
+ value.</li>
+
+ <li>Select the variants with the highest language quality
+ factor.</li>
+
+ <li>Select the variants with the best language match,
+ using either the order of languages in the
+ Accept-Language header (if present), or else the order of
+ languages in the <code>LanguagePriority</code> directive
+ (if present).</li>
+
+ <li>Select the variants with the highest 'level' media
+ parameter (used to give the version of text/html media
+ types).</li>
+
+ <li>Select variants with the best charset media
+ parameters, as given on the Accept-Charset header line.
+ Charset ISO-8859-1 is acceptable unless explicitly
+ excluded. Variants with a <code>text/*</code> media type
+ but not explicitly associated with a particular charset
+ are assumed to be in ISO-8859-1.</li>
+
+ <li>Select those variants which have associated charset
+ media parameters that are <em>not</em> ISO-8859-1. If
+ there are no such variants, select all variants
+ instead.</li>
+
+ <li>Select the variants with the best encoding. If there
+ are variants with an encoding that is acceptable to the
+ user-agent, select only these variants. Otherwise if
+ there is a mix of encoded and non-encoded variants,
+ select only the unencoded variants. If either all
+ variants are encoded or all variants are not encoded,
+ select all variants.</li>
+
+ <li>Select the variants with the smallest content
+ length.</li>
+
+ <li>Select the first variant of those remaining. This
+ will be either the first listed in the type-map file, or
+ when variants are read from the directory, the one whose
+ file name comes first when sorted using ASCII code
+ order.</li>
+ </ol>
+ </li>
+
+ <li>The algorithm has now selected one 'best' variant, so
+ return it as the response. The HTTP response header Vary is
+ set to indicate the dimensions of negotiation (browsers and
+ caches can use this information when caching the resource).
+ End.</li>
+
+ <li>To get here means no variant was selected (because none
+ are acceptable to the browser). Return a 406 status (meaning
+ "No acceptable representation") with a response body
+ consisting of an HTML document listing the available
+ variants. Also set the HTTP Vary header to indicate the
+ dimensions of variance.</li>
+ </ol>
+</section>
+</section>
+
+<section id="better"><title>Fiddling with Quality
+ Values</title>
+
+ <p>Apache sometimes changes the quality values from what would
+ be expected by a strict interpretation of the Apache
+ negotiation algorithm above. This is to get a better result
+ from the algorithm for browsers which do not send full or
+ accurate information. Some of the most popular browsers send
+ Accept header information which would otherwise result in the
+ selection of the wrong variant in many cases. If a browser
+ sends full and correct information these fiddles will not be
+ applied.</p>
+
+<section id="wildcards"><title>Media Types and Wildcards</title>
+
+ <p>The Accept: request header indicates preferences for media
+ types. It can also include 'wildcard' media types, such as
+ "image/*" or "*/*" where the * matches any string. So a request
+ including:</p>
+
+<example>Accept: image/*, */*</example>
+
+ <p>would indicate that any type starting "image/" is acceptable,
+ as is any other type (so the first "image/*" is redundant).
+ Some browsers routinely send wildcards in addition to explicit
+ types they can handle. For example:</p>
+
+<example>
+ Accept: text/html, text/plain, image/gif, image/jpeg, */*
+</example>
+ <p>The intention of this is to indicate that the explicitly listed
+ types are preferred, but if a different representation is
+ available, that is ok too. However under the basic algorithm,
+ as given above, the */* wildcard has exactly equal preference
+ to all the other types, so they are not being preferred. The
+ browser should really have sent a request with a lower quality
+ (preference) value for *.*, such as:</p>
+<example>
+ Accept: text/html, text/plain, image/gif, image/jpeg, */*; q=0.01
+</example>
+ <p>The explicit types have no quality factor, so they default to a
+ preference of 1.0 (the highest). The wildcard */* is given a
+ low preference of 0.01, so other types will only be returned if
+ no variant matches an explicitly listed type.</p>
+
+ <p>If the Accept: header contains <em>no</em> q factors at all,
+ Apache sets the q value of "*/*", if present, to 0.01 to
+ emulate the desired behavior. It also sets the q value of
+ wildcards of the format "type/*" to 0.02 (so these are
+ preferred over matches against "*/*". If any media type on the
+ Accept: header contains a q factor, these special values are
+ <em>not</em> applied, so requests from browsers which send the
+ correct information to start with work as expected.</p>
+</section>
+
+<section id="exceptions"><title>Language Negotiation Exceptions</title>
+
+ <p>New in Apache 2.0, some exceptions have been added to the
+ negotiation algorithm to allow graceful fallback when language
+ negotiation fails to find a match.</p>
+
+ <p>When a client requests a page on your server, but the server
+ cannot find a single page that matches the Accept-language sent by
+ the browser, the server will return either a "No Acceptable
+ Variant" or "Multiple Choices" response to the client. To avoid
+ these error messages, it is possible to configure Apache to ignore
+ the Accept-language in these cases and provide a document that
+ does not explictly match the client's request. The <directive
+ module="mod_negotiation">ForceLanguagePriority</directive>
+ directive can be used to override one or both of these error
+ messages and subsitute the servers judgement in the form of the
+ <directive module="mod_negotiation">LanguagePriority</directive>
+ directive.</p>
+
+ <p>The server will also attempt to match language-subsets when no
+ other match can be found. For example, if a client requests
+ documents with the language <code>en-GB</code> for British
+ English, the server is not normally allowed by the HTTP/1.1
+ standard to match that against a document that is marked as simply
+ <code>en</code>. (Note that it is almost surely a configuration
+ error to include <code>en-GB</code> and not <code>en</code> in the
+ Accept-Language header, since it is very unlikely that a reader
+ understands British English, but doesn't understand English in
+ general. Unfortunately, many current clients have default
+ configurations that resemble this.) However, if no other language
+ match is possible and the server is about to return a "No
+ Acceptable Variants" error or fallback to the <directive
+ module="mod_negotiation">LanguagePriority</directive>, the server
+ will ignore the subset specification and match <code>en-GB</code>
+ against <code>en</code> documents. Implicitly, Apache will add
+ the parent language to the client's acceptable language list with
+ a very low quality value. But note that if the client requests
+ "en-GB; qs=0.9, fr; qs=0.8", and the server has documents
+ designated "en" and "fr", then the "fr" document will be returned.
+ This is necessary to maintain compliance with the HTTP/1.1
+ specification and to work effectively with properly configured
+ clients.</p>
+</section>
+</section>
+
+<section id="extensions"><title>Extensions to Transparent Content
+Negotiation</title>
+
+<p>Apache extends the transparent content negotiation protocol (RFC
+2295) as follows. A new <code>{encoding ..}</code> element is used in
+variant lists to label variants which are available with a specific
+content-encoding only. The implementation of the RVSA/1.0 algorithm
+(RFC 2296) is extended to recognize encoded variants in the list, and
+to use them as candidate variants whenever their encodings are
+acceptable according to the Accept-Encoding request header. The
+RVSA/1.0 implementation does not round computed quality factors to 5
+decimal places before choosing the best variant.</p>
+</section>
+
+<section id="naming"><title>Note on hyperlinks and naming conventions</title>
+
+ <p>If you are using language negotiation you can choose between
+ different naming conventions, because files can have more than
+ one extension, and the order of the extensions is normally
+ irrelevant (see the <a
+ href="mod/mod_mime.html#multipleext">mod_mime</a> documentation
+ for details).</p>
+
+ <p>A typical file has a MIME-type extension (<em>e.g.</em>,
+ <code>html</code>), maybe an encoding extension (<em>e.g.</em>,
+ <code>gz</code>), and of course a language extension
+ (<em>e.g.</em>, <code>en</code>) when we have different
+ language variants of this file.</p>
+
+ <p>Examples:</p>
+
+ <ul>
+ <li>foo.en.html</li>
+
+ <li>foo.html.en</li>
+
+ <li>foo.en.html.gz</li>
+ </ul>
+
+ <p>Here some more examples of filenames together with valid and
+ invalid hyperlinks:</p>
+
+ <table border="1" cellpadding="8" cellspacing="0">
+ <tr>
+ <th>Filename</th>
+
+ <th>Valid hyperlink</th>
+
+ <th>Invalid hyperlink</th>
+ </tr>
+
+ <tr>
+ <td><em>foo.html.en</em></td>
+
+ <td>foo<br />
+ foo.html</td>
+
+ <td>-</td>
+ </tr>
+
+ <tr>
+ <td><em>foo.en.html</em></td>
+
+ <td>foo</td>
+
+ <td>foo.html</td>
+ </tr>
+
+ <tr>
+ <td><em>foo.html.en.gz</em></td>
+
+ <td>foo<br />
+ foo.html</td>
+
+ <td>foo.gz<br />
+ foo.html.gz</td>
+ </tr>
+
+ <tr>
+ <td><em>foo.en.html.gz</em></td>
+
+ <td>foo</td>
+
+ <td>foo.html<br />
+ foo.html.gz<br />
+ foo.gz</td>
+ </tr>
+
+ <tr>
+ <td><em>foo.gz.html.en</em></td>
+
+ <td>foo<br />
+ foo.gz<br />
+ foo.gz.html</td>
+
+ <td>foo.html</td>
+ </tr>
+
+ <tr>
+ <td><em>foo.html.gz.en</em></td>
+
+ <td>foo<br />
+ foo.html<br />
+ foo.html.gz</td>
+
+ <td>foo.gz</td>
+ </tr>
+ </table>
+
+ <p>Looking at the table above you will notice that it is always
+ possible to use the name without any extensions in an hyperlink
+ (<em>e.g.</em>, <code>foo</code>). The advantage is that you
+ can hide the actual type of a document rsp. file and can change
+ it later, <em>e.g.</em>, from <code>html</code> to
+ <code>shtml</code> or <code>cgi</code> without changing any
+ hyperlink references.</p>
+
+ <p>If you want to continue to use a MIME-type in your
+ hyperlinks (<em>e.g.</em> <code>foo.html</code>) the language
+ extension (including an encoding extension if there is one)
+ must be on the right hand side of the MIME-type extension
+ (<em>e.g.</em>, <code>foo.html.en</code>).</p>
+</section>
+
+<section id="caching"><title>Note on Caching</title>
+
+ <p>When a cache stores a representation, it associates it with
+ the request URL. The next time that URL is requested, the cache
+ can use the stored representation. But, if the resource is
+ negotiable at the server, this might result in only the first
+ requested variant being cached and subsequent cache hits might
+ return the wrong response. To prevent this, Apache normally
+ marks all responses that are returned after content negotiation
+ as non-cacheable by HTTP/1.0 clients. Apache also supports the
+ HTTP/1.1 protocol features to allow caching of negotiated
+ responses.</p>
+
+ <p>For requests which come from a HTTP/1.0 compliant client
+ (either a browser or a cache), the directive <directive
+ module="mod_negotiation">CacheNegotiatedDocs</directive> can be
+ used to allow caching of responses which were subject to
+ negotiation. This directive can be given in the server config or
+ virtual host, and takes no arguments. It has no effect on requests
+ from HTTP/1.1 clients.</p>
+</section>
+
+<section id="more"><title>More Information</title>
+
+ <p>For more information about content negotiation, see Alan
+ J. Flavell's <a
+ href="http://ppewww.ph.gla.ac.uk/~flavell/www/lang-neg.html">Language
+ Negotiation Notes</a>. But note that this document may not be
+ updated to include changes in Apache 2.0.</p>
+</section>
+
+</manualpage>
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