Accept-Encoding request HTTP header indicates the content encoding (usually a compression algorithm) that the client can understand. The server uses content negotiation to select one of the proposals and informs the client of that choice with the
Content-Encoding response header.
Even if both the client and the server support the same compression algorithms, the server may choose not to compress the body of a response if the
identity value is also acceptable. Two common cases lead to this:
- The data to be sent is already compressed, therefore a second compression will not reduce the transmitted data size. This is true for pre-compressed image formats (JPEG, for instance);
- The server is overloaded and cannot allocate computing resources to perform the compression. For example, Microsoft recommends not to compress if a server uses more than 80% of its computational power.
As long as the
*;q=0 directives do not explicitly forbid the
identity value that means no encoding, the server must never return a
Not Acceptable error.
- An IANA registry maintains a complete list of official content encodings.
- Two other content encodings, namely
bzip2, are sometimes used, These non-standard encodings implement the algorithm that these two UNIX programs use. Note that
bzipwas discontinued due to patent licensing issues.
// Multiple algorithms, weighted with the quality value syntax:
Accept-Encoding: deflate, gzip;q=1.0, *;q=0.5
A compression format that uses the Lempel-Ziv coding (LZ77) with a 32-bit CRC.
A compression format that uses the Lempel-Ziv-Welch (LZW) algorithm.
A compression format that uses the Brotli algorithm.
Indicates the identity function (that is, without modification or compression). This value is always considered as acceptable, even if omitted.
Matches any content encoding not already listed in the header. This is the default value if the header is not present. This directive does not suggest that any algorithm is supported but indicates that no preference is expressed.
Any value is placed in an order of preference expressed using a relative quality value called weight.
Accept-Encoding: gzip, compress, br
Accept-Encoding: br;q=1.0, gzip;q=0.8, *;q=0.1
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