Accept-Encoding

The 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 406 Not Acceptable error.

Notes:

  • An IANA registry maintains a complete list of official content encodings.

  • Two others content encoding, bzip and 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

Syntax

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

Directives

gzip
A compression format using the Lempel-Ziv coding (LZ77), with a 32-bit CRC.
compress
A compression format using the Lempel-Ziv-Welch (LZW) algorithm.
deflate
A compression format using the zlib structure, with the deflate compression algorithm.
br
A compression format using the Brotli algorithm.
identity
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.
;q= (qvalues weighting)
Any value is placed in an order of preference expressed using a relative quality value called weight.

Examples

Accept-Encoding: gzip

Accept-Encoding: gzip, compress, br

Accept-Encoding: br;q=1.0, gzip;q=0.8, *;q=0.1

Specifications

Specification Title
RFC 7231, section 5.3.4: Accept-Encoding Hypertext Transfer Protocol (HTTP/1.1): Semantics and Context

Browser compatibility

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See also