apache-http-server/docs/manual/content-negotiation.xml

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<manualpage metafile="content-negotiation.xml.meta">

<title>Content Negotiation</title>

<summary>

    <p>Apache HTTPD 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>httpd supports 'server driven' content negotiation, as
    defined in the HTTP/1.1 specification. It fully supports the
    <code>Accept</code>, <code>Accept-Language</code>,
    <code>Accept-Charset</code> and<code>Accept-Encoding</code> 
    request headers. httpd 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 HTTP Server
    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 httpd</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 httpd configurations, the <glossary>MIME-type</glossary>
    <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</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_negotiation
    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>httpd.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 httpd 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 httpd'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 httpd
      algorithm</strong> is used in the normal case. The httpd
      algorithm is explained in more detail below. When this
      algorithm is used, httpd can sometimes 'fiddle' the quality
      factor of a particular dimension to achieve a better result.
      The ways httpd 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 httpd to run the 'remote
      variant selection algorithm' defined in RFC 2296.</li>
    </ol>

<section id="dimensions"><title>Dimensions of Negotiation</title>

    <table>
      <columnspec><column width=".15"/><column width=".85"/></columnspec>
      <tr valign="top">
        <th>Dimension</th>

        <th>Notes</th>
      </tr>

      <tr valign="top">
        <td>Media Type</td>

        <td>Browser indicates preferences with the <code>Accept</code>
        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
        <code>Accept-Language</code> 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
        <code>Accept-Encoding</code> header field. Each item can have
        a quality factor.</td>
      </tr>

      <tr valign="top">
        <td>Charset</td>

        <td>Browser indicates preference with the
        <code>Accept-Charset</code> 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>httpd Negotiation Algorithm</title>

    <p>httpd 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 <code>Accept</code>
          header with the quality-of-source factor for this variants
          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
          <code>Accept-Language</code> 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 <code>Accept-Charset</code>
          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
      <code>Vary</code> 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 <code>Vary</code> header to
      indicate the dimensions of variance.</li>
    </ol>
</section>
</section>

<section id="better"><title>Fiddling with Quality
    Values</title>

    <p>httpd sometimes changes the quality values from what would
    be expected by a strict interpretation of the httpd
    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
    <code>Accept</code> 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 <code>Accept:</code> 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.
    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.  Using explicit quality values,
    what the browser really wants is something like:</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 <code>Accept:</code> header contains <em>no</em> q
    factors at all, httpd 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
    <code>Accept:</code> header contains a q factor, these special
    values are <em>not</em> applied, so requests from browsers which
    send the explicit information to start with work as expected.</p>
</section>

<section id="exceptions"><title>Language Negotiation Exceptions</title>

    <p>New in httpd 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
    <code>Accept-language</code> 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 httpd to ignore
    the <code>Accept-language</code> in these cases and provide a
    document that does not explicitly 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 substitute 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
    <code>Accept-Language</code> 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, httpd 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; q=0.9, fr; q=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>

    <p>In order to support advanced techniques (such as cookies or
    special URL-paths) to determine the user's preferred language,
    since httpd 2.0.47 <module>mod_negotiation</module> recognizes
    the <a href="env.html">environment variable</a>
    <code>prefer-language</code>. If it exists and contains an
    appropriate language tag, <module>mod_negotiation</module> will
    try to select a matching variant. If there's no such variant,
    the normal negotiation process applies.</p>

    <example><title>Example</title>
      SetEnvIf Cookie "language=(.+)" prefer-language=$1<br />
      Header append Vary cookie
    </example>
</section>
</section>

<section id="extensions"><title>Extensions to Transparent Content
Negotiation</title> 

<p>httpd 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 <code>Accept-Encoding</code> 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">
      <columnspec><column width=".2"/><column width=".2"/>
        <column width=".2"/></columnspec>
      <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 a 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, httpd normally
    marks all responses that are returned after content negotiation
    as non-cacheable by HTTP/1.0 clients. httpd 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>

    <p>For HTTP/1.1 clients, httpd sends a <code>Vary</code> HTTP
    response header to indicate the negotiation dimensions for the
    response.  Caches can use this information to determine whether a
    subsequent request can be served from the local copy.  To
    encourage a cache to use the local copy regardless of the
    negotiation dimensions, set the <code>force-no-vary</code> <a
    href="env.html#special">environment variable</a>.</p>

</section>

</manualpage>