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<xep>
  <header>
    <title>Stream Compression with LZW</title>
    <abstract>This document specifies how to use the LZW algorithm in XML stream compression.</abstract>
    &LEGALNOTICE;
    <number>0229</number>
    <status>Draft</status>
    <type>Standards Track</type>
    <sig>Standards</sig>
    <dependencies>
      <spec>XMPP Core</spec>
      <spec>XEP-0138</spec>
    </dependencies>
    <supersedes/>
    <supersededby/>
    <shortname>N/A</shortname>
    &stpeter;
    <revision>
      <version>1.0</version>
      <date>2007-09-26</date>
      <initials>psa</initials>
      <remark><p>Per a vote of the XMPP Council, published as Draft.</p></remark>
    </revision>
    <revision>
      <version>0.0.1</version>
      <date>2007-08-30</date>
      <initials>psa</initials>
      <remark><p>Copied text from XEP-0138.</p></remark>
    </revision>
  </header>

  <section1 topic='Introduction' anchor='intro'>
    <p>&xep0138; specifies an extensible framework for XML stream compression and defines a registry for compression methods (see &COMPRESSMETHODS;). However, <cite>XEP-0138</cite> registers only the ZLIB method (see &rfc1950;). This document specifies usage of the LZW compression method.</p>
  </section1>

  <section1 topic='Background' anchor='background'>
    <p>The "LZW" compression algorithm was originally developed by Abraham Lempel and Jacob Ziv, subsequently improved by Terry Welch <note>See "A Technique for High-Performance Data Compression", <cite>Computer</cite> (June 1984), pp. 8-19.</note>, and patented by Sperry Corporation (later Unisys Corporation) as U.S. Patent Number 4,464,650 <note>See &lt;<link url='http://patft.uspto.gov/netacgi/nph-Parser?patentnumber=4,464,650'>http://patft.uspto.gov/netacgi/nph-Parser?patentnumber=4,464,650</link>&gt;.</note>. This patent expired on June 20, 2003 <note>See &lt;<link url='http://compression-links.info/Link/1264_LZW_Patent_Expiration.htm'>http://compression-links.info/Link/1264_LZW_Patent_Expiration.htm</link>&gt;</note> and <note>See &lt;<link url='http://compression-links.info/Link/1814_LZW_Patent_Expiration.htm'>http://compression-links.info/Link/1814_LZW_Patent_Expiration.htm</link>&gt;.</note>. Therefore implementations of LZW are no longer patent-encumbered.</p>
    <p>The algorithm is specified by Ecma International in &ecma151; under the name "DCLZ".</p>
  </section1>

  <section1 topic='Definition' anchor='definition'>
    <p>If the receiving entity (server) supports the LZW algorithm, it MUST include a &lt;method/&gt; element whose XML character data is "lzw" in the compression stream feature, as follows.</p>
    <example caption='Receiving Entity Offers Stream Compression Feature, Including LZW Method'><![CDATA[
<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>
]]></example>
    <p>If the initiating entity wishes to use the LZW algorithm, then it MUST specify that method.</p>
    <example caption='Initiating Entity Requests Stream Compression Using LZW'><![CDATA[
<compress xmlns='http://jabber.org/protocol/compress'>
  <method>lzw</method>
</compress>
]]></example>
    <p>The initiating entity and receiving entity then MUST attempt to negotiate use of the LZW algorithm in accordance with <cite>XEP-0138</cite>.</p>
    <p>If the use of the LZW algorithm is negotiated, the usage SHOULD follow the definition in <cite>ECMA-151</cite>.</p>
  </section1>

  <section1 topic='Optionality' anchor='optionality'>
    <p>The LZW algorithm is OPTIONAL to implement for <cite>XEP-0138</cite> implementations and this specification does not define a mandatory-to-implement technology.</p>
  </section1>

  <section1 topic='Security Considerations' anchor='security'>
    <p>The security considerations specified in <cite>XEP-0138</cite> apply to usage of the LZW algorithm.</p>
  </section1>

  <section1 topic='IANA Considerations' anchor='iana'>
    <p>This document requires no interaction with &IANA;. </p>
  </section1>

  <section1 topic='XMPP Registrar Considerations' anchor='registrar'>
    <section2 topic='Compression Methods Registry' anchor='registrar-methods'>
      <p>The &REGISTRAR; maintains a registry of compression methods at &COMPRESSMETHODS;.</p>
      <p>The LZW algorithm is already registered. This specification updates the registry submission as follows:</p>
      <code><![CDATA[
<method>
  <name>lzw</name>
  <desc>the LZW (DCLZ) compression method</desc>
  <doc>XEP-0229</doc>
</method>
]]></code>
    </section2>
  </section1>

</xep>
