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dadaef51ba
git-svn-id: file:///home/ksmith/gitmigration/svn/xmpp/trunk@3425 4b5297f7-1745-476d-ba37-a9c6900126ab
95 lines
4.7 KiB
XML
95 lines
4.7 KiB
XML
<?xml version='1.0' encoding='UTF-8'?>
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<!DOCTYPE xep SYSTEM 'xep.dtd' [
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<!ENTITY % ents SYSTEM 'xep.ent'>
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%ents;
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]>
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<?xml-stylesheet type='text/xsl' href='xep.xsl'?>
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<xep>
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<header>
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<title>Stream Compression with LZW</title>
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<abstract>This document specifies how to use the LZW algorithm in XML stream compression.</abstract>
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&LEGALNOTICE;
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<number>0229</number>
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<status>Draft</status>
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<type>Standards Track</type>
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<sig>Standards</sig>
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<dependencies>
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<spec>XMPP Core</spec>
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<spec>XEP-0138</spec>
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</dependencies>
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<supersedes/>
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<supersededby/>
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<shortname>N/A</shortname>
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&stpeter;
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<revision>
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<version>1.0</version>
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<date>2007-09-26</date>
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<initials>psa</initials>
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<remark><p>Per a vote of the XMPP Council, published as Draft.</p></remark>
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</revision>
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<revision>
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<version>0.0.1</version>
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<date>2007-08-30</date>
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<initials>psa</initials>
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<remark><p>Copied text from XEP-0138.</p></remark>
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</revision>
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</header>
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<section1 topic='Introduction' anchor='intro'>
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<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>
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</section1>
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<section1 topic='Background' anchor='background'>
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<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 <<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>>.</note>. This patent expired on June 20, 2003 <note>See <<link url='http://compression-links.info/Link/1264_LZW_Patent_Expiration.htm'>http://compression-links.info/Link/1264_LZW_Patent_Expiration.htm</link>></note> and <note>See <<link url='http://compression-links.info/Link/1814_LZW_Patent_Expiration.htm'>http://compression-links.info/Link/1814_LZW_Patent_Expiration.htm</link>>.</note>. Therefore implementations of LZW are no longer patent-encumbered.</p>
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<p>The algorithm is specified by Ecma International in &ecma151; under the name "DCLZ".</p>
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</section1>
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<section1 topic='Definition' anchor='definition'>
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<p>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.</p>
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<example caption='Receiving Entity Offers Stream Compression Feature, Including LZW Method'><![CDATA[
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<stream:features>
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<starttls xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>
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<compression xmlns='http://jabber.org/features/compress'>
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<method>zlib</method>
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<method>lzw</method>
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</compression>
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</stream:features>
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]]></example>
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<p>If the initiating entity wishes to use the LZW algorithm, then it MUST specify that method.</p>
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<example caption='Initiating Entity Requests Stream Compression Using LZW'><![CDATA[
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<compress xmlns='http://jabber.org/protocol/compress'>
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<method>lzw</method>
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</compress>
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]]></example>
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<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>
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<p>If the use of the LZW algorithm is negotiated, the usage SHOULD follow the definition in <cite>ECMA-151</cite>.</p>
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</section1>
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<section1 topic='Optionality' anchor='optionality'>
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<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>
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</section1>
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<section1 topic='Security Considerations' anchor='security'>
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<p>The security considerations specified in <cite>XEP-0138</cite> apply to usage of the LZW algorithm.</p>
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</section1>
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<section1 topic='IANA Considerations' anchor='iana'>
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<p>This document requires no interaction with &IANA;. </p>
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</section1>
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<section1 topic='XMPP Registrar Considerations' anchor='registrar'>
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<section2 topic='Compression Methods Registry' anchor='registrar-methods'>
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<p>The ®ISTRAR; maintains a registry of compression methods at &COMPRESSMETHODS;.</p>
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<p>The LZW algorithm is already registered. This specification updates the registry submission as follows:</p>
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<code><![CDATA[
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<method>
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<name>lzw</name>
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<desc>the LZW (DCLZ) compression method</desc>
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<doc>XEP-0229</doc>
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</method>
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]]></code>
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</section2>
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</section1>
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</xep>
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