Table of contents
The Accept-Encoding request-header field is similar to Accept, but restricts the content-codings (section 3.5) that are acceptable in the response.
Accept-Encoding = "Accept-Encoding" ":" 1#( codings [ ";" "q" "=" qvalue ] ) codings = ( content-coding | "*" )
Examples of its use are:
Accept-Encoding: compress, gzip Accept-Encoding: Accept-Encoding: * Accept-Encoding: compress;q=0.5, gzip;q=1.0 Accept-Encoding: gzip;q=1.0, identity; q=0.5, *;q=0
A server tests whether a content-coding is acceptable, according to an Accept-Encoding field, using these rules:
- If the content-coding is one of the content-codings listed in the Accept-Encoding field, then it is acceptable, unless it is accompanied by a qvalue of 0. (As defined in section 3.9, a qvalue of 0 means "not acceptable.")
- The special "*" symbol in an Accept-Encoding field matches any available content-coding not explicitly listed in the header field.
- If multiple content-codings are acceptable, then the acceptable content-coding with the highest non-zero qvalue is preferred.
- The "identity" content-coding is always acceptable, unless specifically refused because the Accept-Encoding field includes "identity;q=0", or because the field includes "*;q=0" and does not explicitly include the "identity" content-coding. If the Accept-Encoding field-value is empty, then only the "identity" encoding is acceptable.
If an Accept-Encoding field is present in a request, and if the server cannot send a response which is acceptable according to the Accept-Encoding header, then the server SHOULD send an error response with the 406 (Not Acceptable) status code.
If no Accept-Encoding field is present in a request, the server MAY assume that the client will accept any content coding. In this case, if "identity" is one of the available content-codings, then the server SHOULD use the "identity" content-coding, unless it has additional information that a different content-coding is meaningful to the client.
Note: If the request does not include an Accept-Encoding field, and if the "identity" content-coding is unavailable, then content-codings commonly understood by HTTP/1.0 clients (i.e., "gzip" and "compress") are preferred; some older clients improperly display messages sent with other content-codings. The server might also make this decision based on information about the particular user-agent or client.
Note: Most HTTP/1.0 applications do not recognize or obey qvalues associated with content-codings. This means that qvalues will not work and are not permitted with x-gzip or x-compress.