docs/manual/content-negotiation.html

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    <h1 align="CENTER">Content Negotiation</h1>

    <p>Apache's support for content negotiation has been updated to
    meet 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 is also implements a couple of features to give
    more intelligent handling of requests from browsers which send
    incomplete negotiation information.</p>

    <p>Content negotiation is provided by the <a
    href="mod/mod_negotiation.html">mod_negotiation</a> module,
    which is compiled in by default.</p>
    <hr />

    <h2>About Content Negotiation</h2>

    <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>
<pre>
  Accept-Language: fr
</pre>

    <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>
<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 1.2 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 1.3.4 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>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>

    <h2>Negotiation in Apache</h2>

    <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>

    <h3>Using a type-map file</h3>

    <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>
<pre>
  AddHandler type-map .var
</pre>
    in the server configuration file. 

    <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.html</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
    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>

    <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:</p>

    <dl>
      <dt><code>URI:</code></dt>

      <dd>uri of the file containing the variant (of the given
      media type, encoded with the given content encoding). These
      are interpreted as URLs relative to the map file; they must
      be on the same server (!), and they must refer to files to
      which the client would be granted access if they were to be
      requested directly.</dd>

      <dt><code>Content-Type:</code></dt>

      <dd>media type --- charset, level and "qs" parameters may be
      given. These are often referred to as MIME types; typical
      media types are <code>image/gif</code>,
      <code>text/plain</code>, or
      <code>text/html;&nbsp;level=3</code>.</dd>

      <dt><code>Content-Language:</code></dt>

      <dd>The languages of the variant, specified as an Internet
      standard language tag from RFC 1766 (<em>e.g.</em>,
      <code>en</code> for English, <code>kr</code> for Korean,
      <em>etc.</em>).</dd>

      <dt><code>Content-Encoding:</code></dt>

      <dd>If the file is compressed, or otherwise encoded, rather
      than containing the actual raw data, this says how that was
      done. Apache only recognizes encodings that are defined by an
      <a href="mod/mod_mime.html#addencoding">AddEncoding</a>
      directive. This normally includes the encodings
      <code>x-compress</code> for compress'd files, and
      <code>x-gzip</code> for gzip'd files. The <code>x-</code>
      prefix is ignored for encoding comparisons.</dd>

      <dt><code>Content-Length:</code></dt>

      <dd>The size of the file in bytes. Specifying content lengths
      in the type-map allows the server to compare file sizes
      without checking the actual files.</dd>

      <dt><code>Description:</code></dt>

      <dd>A human-readable textual description of the variant. If
      Apache cannot find any appropriate variant to return, it will
      return an error response which lists all available variants
      instead. Such a variant list will include the human-readable
      variant descriptions.</dd>
    </dl>
    Using a type map file is preferred over <code>MultiViews</code>
    because it requires less CPU time, and less file access, to
    parse a file explicitly listing the various resource variants,
    than to have to look at every matching file, and parse its file
    extensions. 

    <h3>Multiviews</h3>

    <p><code>MultiViews</code> is a per-directory option, meaning
    it can be set with an <code>Options</code> directive within a
    <code>&lt;Directory&gt;</code>, <code>&lt;Location&gt;</code>
    or <code>&lt;Files&gt;</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>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 <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>
    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>If one of the files found when reading the directive is a
    CGI script, it's not obvious what should happen. The code gives
    that case special treatment --- if the request was a POST, or a
    GET with QUERY_ARGS or PATH_INFO, the script is given an
    extremely high quality rating, and generally invoked; otherwise
    it is given an extremely low quality rating, which generally
    causes one of the other views (if any) to be retrieved.</p>

    <h2>The Negotiation Methods</h2>
    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>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>

    <h3>Dimensions of Negotiation</h3>

    <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>

    <h3>Apache Negotiation Algorithm</h3>

    <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>

    <h2><a id="better" name="better">Fiddling with Quality
    Values</a></h2>

    <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>

    <h3>Media Types and Wildcards</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,
    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>
  Accept: text/html, text/plain, image/gif, image/jpeg, */*
</pre>
    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>
  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
    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>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>

    <h3>Variants with no Language</h3>

    <p>If some of the variants for a particular resource have a
    language attribute, and some do not, those variants with no
    language are given a very low language quality factor of
    0.001.</p>

    <p>The reason for setting this language quality factor for
    variant with no language to a very low value is to allow for a
    default variant which can be supplied if none of the other
    variants match the browser's language preferences. For example,
    consider the situation with three variants:</p>

    <ul>
      <li>foo.en.html, language en</li>

      <li>foo.fr.html, language en</li>

      <li>foo.html, no language</li>
    </ul>

    <p>The meaning of a variant with no language is that it is
    always acceptable to the browser. If the request
    Accept-Language header includes either en or fr (or both) one
    of foo.en.html or foo.fr.html will be returned. If the browser
    does not list either en or fr as acceptable, foo.html will be
    returned instead.</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>

    <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>,
    <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
    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>, <samp>foo</samp>). 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
    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
    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>

    <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
    <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. 
    <!--#include virtual="footer.html" -->
    </p>
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