Content negotiation
Encyclopedia
Content negotiation is a mechanism defined in the HTTP specification that makes it possible to serve different versions of a document (or more generally, a resource representation) at the same URI
, so that user agent
s can specify which version fit their capabilities the best. One classical use of this mechanism is to serve an image in GIF
or PNG format, so that a browser that cannot display PNG images (e.g. MS Internet Explorer 4) will be served the GIF version. To summarize how this works, when a user agent submits a request to a server, the user agent informs the server what media types
it understands with ratings of how well it understands them. More precisely, the user agent provides an
that lists acceptable media types and associated quality factors. The server is then able to supply the version of the resource that best fits the user agent's needs.
So, 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 information as part of each request about the representations they prefer. For example, a browser could indicate that it would like to see information in German, if possible, else English will do. Browsers indicate their preferences by headers in the request. To request only German representations, the browser would send:
Accept-Language: de
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 German and English, but prefer German, 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: de; 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
Uniform Resource Identifier
In computing, a uniform resource identifier is a string of characters used to identify a name or a resource on the Internet. Such identification enables interaction with representations of the resource over a network using specific protocols...
, so that user agent
User agent
In computing, a user agent is a client application implementing a network protocol used in communications within a client–server distributed computing system...
s can specify which version fit their capabilities the best. One classical use of this mechanism is to serve an image in GIF
GIF
The Graphics Interchange Format is a bitmap image format that was introduced by CompuServe in 1987 and has since come into widespread usage on the World Wide Web due to its wide support and portability....
or PNG format, so that a browser that cannot display PNG images (e.g. MS Internet Explorer 4) will be served the GIF version. To summarize how this works, when a user agent submits a request to a server, the user agent informs the server what media types
Internet media type
An Internet media type, originally called a MIME type after MIME and sometimes a Content-type after the name of a header in several protocols whose value is such a type, is a two-part identifier for file formats on the Internet.The identifiers were originally defined in RFC 2046 for use in email...
it understands with ratings of how well it understands them. More precisely, the user agent provides an
Accept HTTP headerHeader (information technology)
In information technology, header refers to supplemental data placed at the beginning of a block of data being stored or transmitted. In data transmission, the data following the header are sometimes called the payload or body....
that lists acceptable media types and associated quality factors. The server is then able to supply the version of the resource that best fits the user agent's needs.
So, 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 information as part of each request about the representations they prefer. For example, a browser could indicate that it would like to see information in German, if possible, else English will do. Browsers indicate their preferences by headers in the request. To request only German representations, the browser would send:
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 German and English, but prefer German, and to accept various media types, preferring HTML over plain text or other text types, and preferring GIF or JPEG
JPEG
In computing, JPEG . The degree of compression can be adjusted, allowing a selectable tradeoff between storage size and image quality. JPEG typically achieves 10:1 compression with little perceptible loss in image quality....
over other media types, but also allowing any other media type as a last resort:
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
External links
- RFC 2616 – Hypertext Transfer Protocol -- HTTP/1.1 – (Section 12: Content Negotiation)
- RFC 2295 – Transparent Content Negotiation in HTTP
- Apache 1.3 Content Negotiation
- Open source PHP content negotiation library (supports wildcards and q values)
- Discussion about XHTML serving with content negotiation and browser concerns requiring this
- The variant button
- Parse Accept-Language to detect a user's language
- Apache 2.0 Content Negotiation Info