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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>

 Content negotiation is provided by the
 <A HREF="mod/mod_negotiation.html">mod_negotiation</A> module,
 which is compiled in by default.

 <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

 <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>

 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:

 <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.
 <P>

 The terms used in content negotiation are: a <STRONG>resource</STRONG> is an
 item which can be requested of a server, which might be selected as
 the result of a content negotiation algorithm. If a resource is
 available in several formats, these are called <STRONG>representations</STRONG>
 or <STRONG>variants</STRONG>. The ways in which the variants for a particular
 resource vary are called the <STRONG>dimensions</STRONG> of negotiation.

 <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:

 <UL>
   <LI> Using a type map (i.e., a <CODE>*.var</CODE> file) which
        names the files containing the variants explicitly
   <LI> Or using a 'MultiViews' search, where the server does an implicit
        filename pattern match, and chooses from among the results.
 </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've got to have a <CODE>SetHandler</CODE> some place which defines a
 file suffix as <CODE>type-map</CODE>; this is best done with a
 <PRE>

   AddHandler type-map var

 </PRE>
 in <CODE>srm.conf</CODE>.  See comments in the sample config files for
 details. <P>

 Type map files have an entry for each available variant; these entries
 consist of contiguous RFC822-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:
 <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>

 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 between 0.000 and 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.  <P>

 The full list of headers recognized is:

 <DL>
   <DT> <CODE>URI:</CODE>
   <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.
   <DT> <CODE>Content-type:</CODE>
   <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>.
   <DT> <CODE>Content-language:</CODE>
   <DD> The languages of the variant, specified as an Internet standard
        language code (e.g., <CODE>en</CODE> for English,
        <CODE>kr</CODE> for Korean, etc.).
   <DT> <CODE>Content-encoding:</CODE>
   <DD> If the file is compressed, or otherwise encoded, rather than
        containing the actual raw data, this says how that was done.
        For compressed files (the only case where this generally comes
        up), content encoding should be
        <CODE>x-compress</CODE>, or <CODE>x-gzip</CODE>, as appropriate.
   <DT> <CODE>Content-length:</CODE>
   <DD> The size of the file.  Clients can ask to receive a given media
        type only if the variant isn't too big; specifying a content
        length in the map allows the server to compare against these
        thresholds without checking the actual file.
 </DL>

 <H3>Multiviews</H3>

 <P>
 This 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.  (Fixing this is a one-line change to
 <CODE>http_core.h</CODE>).

 <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, and forwards them along.

 <P>

 This applies 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
 <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.

 <H2>The Negotiation Algorithm</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
 applies a algorithm to decide on the 'best' variant to return, if
 any. To do this it calculates a quality value for each variant in each
 of the dimensions of variance. 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 in detail the algorithm used for those interested.  <P>

 In some circumstances, Apache can '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.

 <H3>Dimensions of Negotiation</H3>

 <TABLE>
 <TR><TH>Dimension
 <TH>Notes
 <TR><TD>Media Type
 <TD>Browser indicates preferences on Accept: header. Each item
 can have an associated quality factor. Variant description can also
 have a quality factor.
 <TR><TD>Language
 <TD>Browser indicates preferences on Accept-Language: header. Each
 item
 can have a quality factor. Variants can be associated with none, one
 or more languages.
 <TR><TD>Encoding
 <TD>Browser indicates preference with Accept-Encoding: header.
 <TR><TD>Charset
 <TD>Browser indicates preference with Accept-Charset: header. Variants
 can indicate a charset as a parameter of the media type.
 </TABLE>

 <H3>Apache Negotiation Algorithm</H3>

 <P>
 Apache uses an algorithm to select the 'best' variant (if any) to
 return to the browser. This algorithm is not configurable. It operates
 like this:

 <OL>
 <LI>
 Firstly, for each dimension of the negotiation, the appropriate
 Accept header is checked and a quality assigned to this each
 variant. If the Accept header for any dimension means that this
 variant is not acceptable, eliminate it. If no variants remain, go
 to step 4.

 <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 stage are eliminated. After each test, if only one variant
 remains, it is selected as the best match. If more than one variant
 remains, move onto 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>Select the variants with the highest language quality factor

 <LI>Select the variants with the best language match, using either the
   order of languages on the <CODE>LanguagePriority</CODE> directive (if
   present),
   else the order of languages on the Accept-Language header.

 <LI>Select the variants with the highest 'level' media parameter
   (used to give the version of text/html media types).

 <LI>Select only unencoded variants, if there is a mix of encoded
   and non-encoded variants. If either all variants are encoded
   or all variants are not encoded, select all.

 <LI>Select only variants with acceptable charset media parameters,
   as given on the Accept-Charset header line. Charset ISO-8859-1
   is always acceptable. Variants not associated with a particular
   charset are assumed to be in ISO-8859-1.

 <LI>Select the variants with the smallest content length

 <LI>Select the first variant of those remaining (this will be either the
 first listed in the type-map file, or the first read from the directory)
 and go to stage 3.

 </OL>

 <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>To get here means no variant was selected (because non 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.

 </OL>

 <H2><A 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 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:
 <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.

 <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>

 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:

 <UL>
 <LI>foo.en.html, language en
 <LI>foo.fr.html, language en
 <LI>foo.html, no language
 </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.

 <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 <A HREF="mod/mod_mime.html">mod_mime</A> documentation for details).
 <P>
 A typical file has a mime-type extension (e.g. <SAMP>html</SAMP>),
 maybe an encoding extension (e.g. <SAMP>gz</SAMP> and of course a
 language extension (e.g. <SAMP>en</SAMP>) when we have different
 language variants of this file.

 <P>
 Examples:
 <UL>
 <LI>foo.en.html
 <LI>foo.html.en
 <LI>foo.en.html.gz
 </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 (e.g. <SAMP>foo</SAMP>).
 The advantage is that you can hide the actual type of a
 document rsp. file and can change it later, e.g. from <SAMP>html</SAMP>
 to <SAMP>shtml</SAMP> or <SAMP>cgi</SAMP> without changing any
 hyperlink references.

 <P>
 If you want to continue to use a mime-type in your hyperlinks (e.g.
 <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
 (e.g. <SAMP>foo.html.en</SAMP>).


 <H2>Note on Caching</H2>

 <P>
 When a cache stores a document, it associates it with the request URL.
 The next time that URL is requested, the cache can use the stored
 document, provided it is still within date. But if the resource is
 subject to content negotiation at the server, this would result in
 only the first requested variant being cached, and subsequent cache
 hits could 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>

 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.

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	<HEAD>
	<TITLE>Apache Content Negotiation</TITLE>
	</HEAD>

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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>

	Content negotiation is provided by the
	<A HREF="mod/mod_negotiation.html">mod_negotiation</A> module,
	which is compiled in by default.

	<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

	<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>

	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:

	<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.
	<P>

	The terms used in content negotiation are: a <STRONG>resource</STRONG> is an
	item which can be requested of a server, which might be selected as
	the result of a content negotiation algorithm. If a resource is
	available in several formats, these are called <STRONG>representations</STRONG>
	or <STRONG>variants</STRONG>. The ways in which the variants for a particular
	resource vary are called the <STRONG>dimensions</STRONG> of negotiation.

	<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:

	<UL>
	<LI> Using a type map (i.e., a <CODE>*.var</CODE> file) which
	names the files containing the variants explicitly
	<LI> Or using a 'MultiViews' search, where the server does an implicit
	filename pattern match, and chooses from among the results.
	</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've got to have a <CODE>SetHandler</CODE> some place which defines a
	file suffix as <CODE>type-map</CODE>; this is best done with a
	<PRE>

	AddHandler type-map var

	</PRE>
	in <CODE>srm.conf</CODE>. See comments in the sample config files for
	details. <P>

	Type map files have an entry for each available variant; these entries
	consist of contiguous RFC822-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:
	<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>

	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 between 0.000 and 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. <P>

	The full list of headers recognized is:

	<DL>
	<DT> <CODE>URI:</CODE>
	<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.
	<DT> <CODE>Content-type:</CODE>
	<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; level=3</CODE>.
	<DT> <CODE>Content-language:</CODE>
	<DD> The languages of the variant, specified as an Internet standard
	language code (e.g., <CODE>en</CODE> for English,
	<CODE>kr</CODE> for Korean, etc.).
	<DT> <CODE>Content-encoding:</CODE>
	<DD> If the file is compressed, or otherwise encoded, rather than
	containing the actual raw data, this says how that was done.
	For compressed files (the only case where this generally comes
	up), content encoding should be
	<CODE>x-compress</CODE>, or <CODE>x-gzip</CODE>, as appropriate.
	<DT> <CODE>Content-length:</CODE>
	<DD> The size of the file. Clients can ask to receive a given media
	type only if the variant isn't too big; specifying a content
	length in the map allows the server to compare against these
	thresholds without checking the actual file.
	</DL>

	<H3>Multiviews</H3>

	<P>
	This 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. (Fixing this is a one-line change to
	<CODE>http_core.h</CODE>).

	<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, and forwards them along.

	<P>

	This applies 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
	<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.

	<H2>The Negotiation Algorithm</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
	applies a algorithm to decide on the 'best' variant to return, if
	any. To do this it calculates a quality value for each variant in each
	of the dimensions of variance. 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 in detail the algorithm used for those interested. <P>

	In some circumstances, Apache can '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.

	<H3>Dimensions of Negotiation</H3>

	<TABLE>
	<TR><TH>Dimension
	<TH>Notes
	<TR><TD>Media Type
	<TD>Browser indicates preferences on Accept: header. Each item
	can have an associated quality factor. Variant description can also
	have a quality factor.
	<TR><TD>Language
	<TD>Browser indicates preferences on Accept-Language: header. Each
	item
	can have a quality factor. Variants can be associated with none, one
	or more languages.
	<TR><TD>Encoding
	<TD>Browser indicates preference with Accept-Encoding: header.
	<TR><TD>Charset
	<TD>Browser indicates preference with Accept-Charset: header. Variants
	can indicate a charset as a parameter of the media type.
	</TABLE>

	<H3>Apache Negotiation Algorithm</H3>

	<P>
	Apache uses an algorithm to select the 'best' variant (if any) to
	return to the browser. This algorithm is not configurable. It operates
	like this:

	<OL>
	<LI>
	Firstly, for each dimension of the negotiation, the appropriate
	Accept header is checked and a quality assigned to this each
	variant. If the Accept header for any dimension means that this
	variant is not acceptable, eliminate it. If no variants remain, go
	to step 4.

	<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 stage are eliminated. After each test, if only one variant
	remains, it is selected as the best match. If more than one variant
	remains, move onto 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>Select the variants with the highest language quality factor

	<LI>Select the variants with the best language match, using either the
	order of languages on the <CODE>LanguagePriority</CODE> directive (if
	present),
	else the order of languages on the Accept-Language header.

	<LI>Select the variants with the highest 'level' media parameter
	(used to give the version of text/html media types).

	<LI>Select only unencoded variants, if there is a mix of encoded
	and non-encoded variants. If either all variants are encoded
	or all variants are not encoded, select all.

	<LI>Select only variants with acceptable charset media parameters,
	as given on the Accept-Charset header line. Charset ISO-8859-1
	is always acceptable. Variants not associated with a particular
	charset are assumed to be in ISO-8859-1.

	<LI>Select the variants with the smallest content length

	<LI>Select the first variant of those remaining (this will be either the
	first listed in the type-map file, or the first read from the directory)
	and go to stage 3.

	</OL>

	<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>To get here means no variant was selected (because non 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.

	</OL>

	<H2><A 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 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:
	<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.

	<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>

	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:

	<UL>
	<LI>foo.en.html, language en
	<LI>foo.fr.html, language en
	<LI>foo.html, no language
	</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.

	<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 <A HREF="mod/mod_mime.html">mod_mime</A> documentation for details).
	<P>
	A typical file has a mime-type extension (e.g. <SAMP>html</SAMP>),
	maybe an encoding extension (e.g. <SAMP>gz</SAMP> and of course a
	language extension (e.g. <SAMP>en</SAMP>) when we have different
	language variants of this file.

	<P>
	Examples:
	<UL>
	<LI>foo.en.html
	<LI>foo.html.en
	<LI>foo.en.html.gz
	</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 (e.g. <SAMP>foo</SAMP>).
	The advantage is that you can hide the actual type of a
	document rsp. file and can change it later, e.g. from <SAMP>html</SAMP>
	to <SAMP>shtml</SAMP> or <SAMP>cgi</SAMP> without changing any
	hyperlink references.

	<P>
	If you want to continue to use a mime-type in your hyperlinks (e.g.
	<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
	(e.g. <SAMP>foo.html.en</SAMP>).


	<H2>Note on Caching</H2>

	<P>
	When a cache stores a document, it associates it with the request URL.
	The next time that URL is requested, the cache can use the stored
	document, provided it is still within date. But if the resource is
	subject to content negotiation at the server, this would result in
	only the first requested variant being cached, and subsequent cache
	hits could 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>

	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" -->
	</BODY>
	</HTML>