Apache HTTP Server

Content Negotiation

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.

Content negotiation is provided by the mod_negotiation module, which is compiled in by default.

About Content Negotiation

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

  Accept-Language: fr

Note that this preference will only be applied when there is a choice of representations and they vary by language.

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:

  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

A resource is a conceptual entity identified by a URI (RFC 2396). An HTTP server like Apache provides access to representations 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 negotiable and each of its representations is termed a variant. The ways in which the variants for a negotiable resource vary are called the dimensions of negotiation.

Negotiation in Apache

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:

• Using a type map (i.e., a *.var file) which names the files containing the variants explicitly, or
• Using a 'MultiViews' search, where the server does an implicit filename pattern match and chooses from among the results.

Using a type-map file

A type map is a document which is associated with the handler named type-map (or, for backwards-compatibility with older Apache configurations, the mime type application/x-type-map). Note that to use this feature, you must have a handler set in the configuration that defines a file suffix as type-map; this is best done with a

  AddHandler type-map .var

Type map files 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:

  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

  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

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.

The full list of headers recognized is:

URI:

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.

Content-Type:

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

Content-Language:

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

Content-Encoding:

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 AddEncoding directive. This normally includes the encodings x-compress for compress'd files, and x-gzip for gzip'd files. The x- prefix is ignored for encoding comparisons.

Content-Length:

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

Description:

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.

Multiviews

MultiViews is a per-directory option, meaning it can be set with an Options directive within a <Directory>, <Location> or <Files> section in access.conf, or (if AllowOverride is properly set) in .htaccess files. Note that Options All does not set MultiViews; you have to ask for it by name.

The effect of MultiViews is as follows: if the server receives a request for /some/dir/foo, if /some/dir has MultiViews enabled, and /some/dir/foo does not 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.

MultiViews may also apply to searches for the file named by the DirectoryIndex directive, if the server is trying to index a directory. If the configuration files specify

  DirectoryIndex index

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.

The Negotiation Methods

There are two negotiation methods:

1. Server driven negotiation with the Apache algorithm 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.
2. Transparent content negotiation 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.

Dimensions of Negotiation

Apache Negotiation Algorithm

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:

1. First, for each dimension of the negotiation, check the appropriate Accept* header field and assign a quality to each variant. If the Accept* header for any dimension implies that this variant is not acceptable, eliminate it. If no variants remain, go to step 4.
2. 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.
   1. 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.
   2. Select the variants with the highest language quality factor.
   3. 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 LanguagePriority directive (if present).
   4. Select the variants with the highest 'level' media parameter (used to give the version of text/html media types).
   5. 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 text/* media type but not explicitly associated with a particular charset are assumed to be in ISO-8859-1.
   6. Select those variants which have associated charset media parameters that are not ISO-8859-1. If there are no such variants, select all variants instead.
   7. 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.
   8. Select the variants with the smallest content length.
   9. 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.
3. 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.

4. 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.

   You should be aware that the error message returned by Apache is neccessarily rather terse and might confuse some users (even though it lists the available alternatives). If you want to avoid users seeing this error page, you should organize your documents such that a document in a default language (or with a default encoding etc.) is always returned if a document is not available in any of the languages, encodings etc. the browser asked for.

   In particular, if you want a document in a default language to be returned if a document is not available in any of the languages a browser asked for, you should create a document with no language attribute set. See Variants with no Language below for details.

Fiddling with Quality Values

Apache sometimes changes the quality values from what would be expected by a strict interpretation of the Apache negotiationludes the encodings x-compress for compress'd files, and x-gzip for gzip'd files. The x- prefix is ignored for encoding comparisons.