3.5 Content Codings

Content coding values are used to indicate an encoding transformation that has been applied to a resource. Content codings are primarily used to allow a document to be compressed or encrypted without losing the identity of its underlying media type. Typically, the resource is stored in this encoding and only decoded before rendering or analogous usage.

       content-coding = "x-gzip" | "x-compress" | token

       Note: For future compatibility, HTTP/1.0 applications should
       consider "gzip" and "compress" to be equivalent to "x-gzip"
       and "x-compress", respectively.

All content-coding values are case-insensitive. HTTP/1.0 uses content-coding values in the Content-Encoding (Section 10.3) header field. 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. Note that a single program may be capable of decoding multiple content-coding formats. Two values are defined by this specification:

x-gzip

An encoding format produced by the file compression program "gzip" (GNU zip) developed by Jean-loup Gailly. This format is typically a Lempel-Ziv coding (LZ77) with a 32 bit CRC.

x-compress

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

Note: Use of program names for the identification of encoding formats is not desirable and should be discouraged for future encodings. Their use here is representative of historical practice, not good design.