Content-Encoding

The Content-Encoding entity header is used to compress the media-type. When present, its value indicates what additional content encoding has been applied to the entity-body. It lets the client know, how to decode in order to obtain the media-type referenced by the Content-Type header.

It is recommended to compress data as much as possible and therefore to use this field, but some type of ressources, like jpeg images, are already compressed and sometimes using an additional compression doesn't bring anything and can even make the payload longer.

Header type	Entity header
Forbidden header name	no

Syntax

Content-Encoding: gzip
Content-Encoding: compress
Content-Encoding: deflate
Content-Encoding: identity
Content-Encoding: br

Directives

gzip

A format using the Lempel-Ziv coding (LZ77), with a 32-bit CRC. This is originally the format of the UNIX gzip program. The HTTP/1.1 standard also recommends that the servers supporting this content-encoding should recognize x-gzip as an alias, for compatibility purposes.

compress

A format using the Lempel-Ziv-Welch (LZW) algorithm. The value name was taken from the UNIX compress program, which implemented this algorithm.
Like the compress program, which has disappeared from most UNIX distributions, this content-encoding is used by almost no browsers today, partly because of a patent issue (which expired in 2003).

deflate

Using the zlib structure (defined in RFC 1950), with the deflate compression algorithm (defined in RFC 1951).

identity

Indicates the identity function (i.e. no compression, nor modification). This token, except if explicitly specified, is always deemed acceptable.

br

A format using the Brotli algorithm.

Examples

Compressing with gzip

On the client side, you can advertise a list of compression schemes that will be send along in an HTTP request. The Accept-Encoding header is used for negotiating content encoding.

Accept-Encoding: gzip, deflate

The server responds with the scheme used, indicated by the Content-Encoding response header.

Content-Encoding: gzip

Note that the server is not obligated to use any compression method. Compression highly depends on server settings and used server modules.

Specifications

Specification	Title
RFC 7231, section 3.1.2.2: Content-Encoding	Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content
http://www.ietf.org/id/draft-alakuijala-brotli	Brotli Compressed Data Format

Browser compatibility

See also

• Accept-Encoding
• Transfer-Encoding