Accept-Encoding request HTTP header advertises which content encoding, usually a compression algorithm, the client is able to understand. Using content negotiation, the server selects one of the proposals, uses it and informs the client of its choice with the
Content-Encoding response header.
Even if both the client and the server supports 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 and a second compression won't lead to smaller data to be transmitted. This may be the case with some image formats;
- The server is overloaded and cannot afford the computational overhead induced by the compression requirement. Typically, Microsoft recommends not to compress if a server uses more than 80% of its computational power.
As long as the
identity value, meaning no encoding, is not explicitly forbidden, by an
identity;q=0 or a
*;q=0 without another explicitly set value for identity, the server must never send back a
Not Acceptable error.
An IANA registry maintains a complete list of official content encodings.
- Two others content encoding,
bzip2, are sometimes used, though not standard. They implement the algorithm used by these two UNIX programs. Note that the first one was discontinued due to patent licensing problems.
|Header type||Request header|
|Forbidden header name||yes|
Accept-Encoding: gzip Accept-Encoding: compress Accept-Encoding: deflate Accept-Encoding: br Accept-Encoding: identity Accept-Encoding: * // Multiple algorithms, weighted with the quality value syntax: Accept-Encoding: deflate, gzip;q=1.0, *;q=0.5
- A compression format using the Lempel-Ziv coding (LZ77), with a 32-bit CRC.
- A compression format using the Lempel-Ziv-Welch (LZW) algorithm.
- A compression format using the zlib structure, with the deflate compression algorithm.
- A compression format using the Brotli algorithm.
- Indicates the identity function (i.e. no compression, nor modification). This value is always considered as acceptable, even if not present.
- Matches any content encoding not already listed in the header. This is the default value if the header is not present. It doesn't mean that any algorithm is supported; merely 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 Accept-Encoding: gzip, compress, br Accept-Encoding: br;q=1.0, gzip;q=0.8, *;q=0.1
|RFC 7231, section 5.3.4: Accept-Encoding||Hypertext Transfer Protocol (HTTP/1.1): Semantics and Context|
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