3.5 Content Codings

Content coding values indicate an encoding transformation that has been or can be applied to an entity. Content codings are primarily used to allow a document to be compressed or otherwise usefully transformed without losing the identity of its underlying media type and without loss of information. Frequently, the entity is stored in coded form, transmitted directly, and only decoded by the recipient.

content-coding = token

All content-coding values are case-insensitive. HTTP/1.1 uses content-coding values in the Accept-Encoding (Section 14.3) and Content-Encoding (Section 14.11) header fields. Although the value describes the content-coding, what is more important is that it indicates what decoding mechanism will be required to remove the encoding.

The Internet Assigned Numbers Authority (IANA) acts as a registry for content-coding value tokens. Initially, the registry contains the following tokens:


An encoding format produced by the file compression program "gzip" (GNU zip) as described in RFC 1952 [25]. This format is a Lempel-Ziv coding (LZ77) with a 32 bit CRC.


The encoding format produced by the common UNIX file compression program "compress". This format is an adaptive Lempel-Ziv-Welch coding (LZW).

Use of program names for the identification of encoding formats is not desirable and is discouraged for future encodings. Their use here is representative of historical practice, not good design. For compatibility with previous implementations of HTTP, applications SHOULD consider "x-gzip" and "x-compress" to be equivalent to "gzip" and "compress" respectively.


The "zlib" format defined in RFC 1950 [31] in combination with the "deflate" compression mechanism described in RFC 1951 [29].


The default (identity) encoding; the use of no transformation whatsoever. This content-coding is used only in the Accept- Encoding header, and SHOULD NOT be used in the Content-Encoding header.

New content-coding value tokens SHOULD be registered; to allow interoperability between clients and servers, specifications of the content coding algorithms needed to implement a new value SHOULD be publicly available and adequate for independent implementation, and conform to the purpose of content coding defined in this section.