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 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 use 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.

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

Browser compatibility

See also

• HTTP content negotiation
• Header with the result of the content negotiation: Content-Encoding
• Other similar headers: TE, Accept, Accept-Charset, Accept-Language