Content-Encoding header
Baseline Widely available *
This feature is well established and works across many devices and browser versions. It’s been available across browsers since July 2015.
* Some parts of this feature may have varying levels of support.
The HTTPContent-Encodingrepresentation header lists the encodings and the order in which they have been applied to a resource.This lets the recipient know how to decode the data in order to obtain the original content format described in theContent-Type header.Content encoding is mainly used to compress content without losing information about the original media type.
Servers should compress data as much as possible, and should use content encoding where appropriate.Compressing already compressed media types, such as .zip or .jpeg, is usually not appropriate because it can increase the file size.If the original media is already encoded (e.g., as a .zip file), this information is not included in theContent-Encoding header.
When theContent-Encoding header is present, other metadata (e.g.,Content-Length) refer to the encoded form of the data, not the original resource, unless explicitly stated.Content encoding differs toTransfer-Encoding in thatTransfer-Encoding handles how HTTP messages themselves are delivered across the network on ahop-by-hop basis.
| Header type | Representation header |
|---|---|
| Forbidden request header | No |
Syntax
Content-Encoding: gzipContent-Encoding: compressContent-Encoding: deflateContent-Encoding: brContent-Encoding: zstdContent-Encoding: dcbContent-Encoding: dcz// Multiple, in the order in which they were appliedContent-Encoding: deflate, gzipDirectives
gzipA format using theLempel-Ziv coding (LZ77), with a 32-bit CRC.This is the original format of the UNIXgzip program.The HTTP/1.1 standard also recommends that the servers supporting this content-encoding should recognize
x-gzipas an alias, for compatibility purposes.compressA format using theLempel-Ziv-Welch (LZW) algorithm.The value name was taken from the UNIXcompress program, which implemented this algorithm.Like the compress program, which has disappeared from most UNIX distributions, this content-encoding is not used by many browsers today, partly because of a patent issue (it expired in 2003).
deflateUsing thezlib structure (defined inRFC 1950) with thedeflate compression algorithm (defined inRFC 1951).
brA format using theBrotli algorithm structure (defined inRFC 7932).
zstdA format using theZstandard algorithm structure (defined inRFC 8878).
dcbExperimentalA format that uses theDictionary-Compressed Brotli algorithm. SeeCompression Dictionary Transport.
dczExperimentalA format that uses theDictionary-Compressed Zstandard algorithm. SeeCompression Dictionary Transport.
Examples
Compressing with gzip
On the client side, you can advertise a list of compression schemes that will be sent along in an HTTP request. TheAccept-Encoding header is used for negotiating content encoding.
Accept-Encoding: gzip, deflateThe server responds with the scheme used, indicated by theContent-Encoding response header.
Content-Encoding: gzipWhether a server uses compression methods requested by the client depends on server configuration and capabilities.
Specifications
| Specification |
|---|
| HTTP Semantics # field.content-encoding |
| Zstandard Compression and the 'application/zstd' Media Type # name-content-encoding |