XEP-0229: Stream Compression with LZW

NOTICE: The protocol defined herein is a Draft Standard of the XMPP Standards Foundation. Implementations are encouraged and the protocol is appropriate for deployment in production systems, but some changes to the protocol are possible before it becomes a Final Standard.

Document Information

Author Information

Peter Saint-Andre

JabberID: stpeter@jabber.org
URI: https://stpeter.im/

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## NOTE WELL: This Specification is provided on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, express or implied, including, without limitation, any warranties or conditions of TITLE, NON-INFRINGEMENT, MERCHANTABILITY, or FITNESS FOR A PARTICULAR PURPOSE. In no event shall the XMPP Standards Foundation or the authors of this Specification be liable for any claim, damages, or other liability, whether in an action of contract, tort, or otherwise, arising from, out of, or in connection with the Specification or the implementation, deployment, or other use of the Specification. ##

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Discussion Venue

Relation to XMPP

The Extensible Messaging and Presence Protocol (XMPP) is defined in the XMPP Core (RFC 3920) and XMPP IM (RFC 3921) specifications contributed by the XMPP Standards Foundation to the Internet Standards Process, which is managed by the Internet Engineering Task Force in accordance with RFC 2026. Any protocol defined in this document has been developed outside the Internet Standards Process and is to be understood as an extension to XMPP rather than as an evolution, development, or modification of XMPP itself.

Conformance Terms

The following keywords as used in this document are to be interpreted as described in RFC 2119: "MUST", "SHALL", "REQUIRED"; "MUST NOT", "SHALL NOT"; "SHOULD", "RECOMMENDED"; "SHOULD NOT", "NOT RECOMMENDED"; "MAY", "OPTIONAL".

1. Introduction
2. Background
3. Definition
4. Optionality
5. Security Considerations
6. IANA Considerations
7. XMPP Registrar Considerations
7.1. Compression Methods Registry
Notes
Revision History

1. Introduction

Stream Compression [1] specifies an extensible framework for XML stream compression and defines a registry for compression methods (see <http://www.xmpp.org/registrar/compress.html>). However, XEP-0138 registers only the ZLIB method (see RFC 1950 [2]). This document specifies usage of the LZW compression method.

2. Background

The "LZW" compression algorithm was originally developed by Abraham Lempel and Jacob Ziv, subsequently improved by Terry Welch [3], and patented by Sperry Corporation (later Unisys Corporation) as U.S. Patent Number 4,464,650 [4]. This patent expired on June 20, 2003 [5]. Therefore implementations of LZW are no longer patent-encumbered.

The algorithm is specified by Ecma International in Standard ECMA-151 [6] under the name "DCLZ".

3. Definition

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.

Example 1. Receiving Entity Offers Stream Compression Feature, Including LZW Method

<stream:features>
  <starttls xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>
  <compression xmlns='http://jabber.org/features/compress'>
    <method>zlib</method>
    <method>lzw</method>
  </compression>
</stream:features>

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

<compress xmlns='http://jabber.org/protocol/compress'>
  <method>lzw</method>
</compress>

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.

4. Optionality

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

5. Security Considerations

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

6. IANA Considerations

7. XMPP Registrar Considerations

7.1 Compression Methods Registry

The XMPP Registrar [8] maintains a registry of compression methods at <http://www.xmpp.org/registrar/compress.html>.

The LZW algorithm is already registered. This specification updates the registry submission as follows:

<method>
  <name>lzw</name>
  <desc>the LZW (DCLZ) compression method</desc>
  <doc>XEP-0229</doc>
</method>

Notes

1. XEP-0138: Stream Compression <http://www.xmpp.org/extensions/xep-0138.html>.

2. RFC 1950: ZLIB Compressed Data Format Specification version 3.3 <http://tools.ietf.org/html/rfc1950>.

3. See "A Technique for High-Performance Data Compression", Computer (June 1984), pp. 8-19.

4. See <http://patft.uspto.gov/netacgi/nph-Parser?patentnumber=4,464,650>.

5. See <http://www.unisys.com/about__unisys/lzw>.

6. Standard ECMA-151: Data Compression for Information Interchange - Adaptive Coding with Embedded Dictionary - DLCZ Algorithm <http://www.ecma-international.org/publications/standards/Ecma-151.htm>.

7. The Internet Assigned Numbers Authority (IANA) is the central coordinator for the assignment of unique parameter values for Internet protocols, such as port numbers and URI schemes. For further information, see <http://www.iana.org/>.

8. The XMPP Registrar maintains a list of reserved protocol namespaces as well as registries of parameters used in the context of XMPP extension protocols approved by the XMPP Standards Foundation. For further information, see <http://www.xmpp.org/registrar/>.