&xep0138; specifies an extensible framework for XML stream compression and defines a registry for compression methods (see &COMPRESSMETHODS;). However, XEP-0138 registers only the ZLIB method (see &rfc1950;). This document specifies usage of the LZW compression method.

The "LZW" compression algorithm was originally developed by Abraham Lempel and Jacob Ziv, subsequently improved by Terry Welch See "A Technique for High-Performance Data Compression", Computer (June 1984), pp. 8-19., and patented by Sperry Corporation (later Unisys Corporation) as U.S. Patent Number 4,464,650 See <http://patft.uspto.gov/netacgi/nph-Parser?patentnumber=4,464,650>.. This patent expired on June 20, 2003 See <http://compression-links.info/Link/1264_LZW_Patent_Expiration.htm> and See <http://compression-links.info/Link/1814_LZW_Patent_Expiration.htm>.. Therefore implementations of LZW are no longer patent-encumbered.

The algorithm is specified by Ecma International in &ecma151; under the name "DCLZ".

If the receiving entity (server) supports the LZW algorithm, it MUST include a <method/> element whose XML character data is "lzw" in the compression stream feature, as follows.

If the initiating entity wishes to use the LZW algorithm, then it MUST specify that method.

The initiating entity and receiving entity then MUST attempt to negotiate use of the LZW algorithm in accordance with XEP-0138.

If the use of the LZW algorithm is negotiated, the usage SHOULD follow the definition in ECMA-151.

The LZW algorithm is OPTIONAL to implement for XEP-0138 implementations and this specification does not define a mandatory-to-implement technology.

The security considerations specified in XEP-0138 apply to usage of the LZW algorithm.

This document requires no interaction with &IANA;.