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The Apache HTTP Server Reference Manual
by Apache Software Foundation
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ISBN 9781906966034
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15.3  The Negotiation Methods

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 section explains the methods used for those interested.

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.

15.3.1  Dimensions of Negotiation



Dimension

Notes



Media Type

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).

Language

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.

Encoding

Browser indicates preference with the Accept-Encoding header field. Each item can have a quality factor.

Charset

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.



15.3.2  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 variants 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.

ISBN 9781906966034The Apache HTTP Server Reference ManualSee the print edition