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94 lines
4.7 KiB
94 lines
4.7 KiB
<?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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