WD-exi-impacts-20080903 24.7 KB

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<h1><a name="title" id="title"></a>Efficient XML Interchange (EXI) Impacts</h1>
<h2><a name="w3c-doctype" id="w3c-doctype"></a>W3C Working Draft 03 September 2008</h2><dl><dt>This version:</dt><dd>
      <a href="http://www.w3.org/TR/2008/WD-exi-impacts-20080903">http://www.w3.org/TR/2008/WD-exi-impacts-20080903</a>
    </dd><dt>Latest version:</dt><dd>
      <a href="http://www.w3.org/TR/exi-impacts/">http://www.w3.org/TR/exi-impacts/</a>
    </dd><dt>Editor:</dt><dd>Jaakko Kangasharju, University of Helsinki</dd></dl><p>This document is also available in these non-normative formats: <a href="exi-impacts.xml">XML</a>.</p><p class="copyright"><a href="http://www.w3.org/Consortium/Legal/ipr-notice#Copyright">Copyright</a>&nbsp;&copy;&nbsp;2008&nbsp;<a href="http://www.w3.org/"><acronym title="World Wide Web Consortium">W3C</acronym></a><sup>&reg;</sup> (<a href="http://www.csail.mit.edu/"><acronym title="Massachusetts Institute of Technology">MIT</acronym></a>, <a href="http://www.ercim.org/"><acronym title="European Research Consortium for Informatics and Mathematics">ERCIM</acronym></a>, <a href="http://www.keio.ac.jp/">Keio</a>), All Rights Reserved. W3C <a href="http://www.w3.org/Consortium/Legal/ipr-notice#Legal_Disclaimer">liability</a>, <a href="http://www.w3.org/Consortium/Legal/ipr-notice#W3C_Trademarks">trademark</a> and <a href="http://www.w3.org/Consortium/Legal/copyright-documents">document use</a> rules apply.</p></div><hr><div>
<h2><a name="abstract" id="abstract"></a>Abstract</h2><p>
	The Efficient XML Interchange (EXI) format defines a new
	representation for the Extensible Markup Language (XML)
	Information Set. The introduction of such a format may cause
	disruption in systems that have so far been able to assume XML
	as the only representation of XML Information Set data. This
	document reviews areas where the introduction of EXI may
	disrupt or otherwise have an impact on existing XML
	technologies, XML processors, and applications. It also
	describes EXI design features and steps that may be taken by
	implementors to reduce or eliminate disruption and impacts.
      </p></div><div>
<h2><a name="status" id="status"></a>Status of this Document</h2><p>
	<em>
	  This section describes the status of this document at the
	  time of its publication. Other documents may supersede this
	  document. A list of current W3C publications and the latest
	  revision of this technical report can be found in the <a href="http://www.w3.org/TR/">W3C technical
	  reports index</a> at http://www.w3.org/TR/.
	</em>
      </p><p>
	This is a First Public Working Draft of &#8220;Efficient XML
	Interchange (EXI) Impacts.&#8221;
      </p><p>
	This document is intended to aid people in the XML community
	to determine whether their particular area of interest is
	affected by the introduction of EXI.  It currently contains
	the significant impacts identified by the Efficient XML Interchange
	Working Group, and
	the group would also appreciate hearing from the XML community
	if any potential impacts have been missed.
      </p><p>
	This document was developed by the <a href="http://www.w3.org/XML/EXI/">Efficient XML
	Interchange (EXI) Working Group</a>.
      </p><p>
	Please send comments about this document to <a href="mailto:public-exi@w3.org">public-exi@w3.org</a> (<a href="http://lists.w3.org/Archives/Public/public-exi/">public archive</a>).
      </p><p>
	Publication as a Working Draft does not imply endorsement by
	the W3C Membership. This is a draft document and may be
	updated, replaced or obsoleted by other documents at any
	time. It is inappropriate to cite this document as other than
	work in progress.
      </p><p>
	This document was produced by a group operating under the <a href="http://www.w3.org/Consortium/Patent-Policy-20040205/">5 February 2004 W3C Patent
	Policy</a>. The group does not expect this document to
	become a W3C Recommendation.  W3C maintains a <a href="http://www.w3.org/2004/01/pp-impl/38502/status#specs">public list of any patent
	disclosures</a> made in connection with the deliverables of
	the group; that page also includes instructions for disclosing
	a patent.  An individual who has actual knowledge of a patent
	which the individual believes contains <a href="http://www.w3.org/Consortium/Patent-Policy-20040205/#def-essential">Essential Claim(s)</a> must
	disclose the information in accordance with <a href="http://www.w3.org/Consortium/Patent-Policy-20040205/#sec-Disclosure">section 6 of the W3C Patent
	Policy</a>.
      </p></div><div class="toc">
<h2><a name="contents" id="contents"></a>Table of Contents</h2><p class="toc">1 <a href="#intro">Introduction</a><br>
2 <a href="#terminology">Terminology and Discussion</a><br>
3 <a href="#xml-processors">Existing XML Processors and Applications</a><br>
4 <a href="#xml-technologies">Existing XML Technologies</a><br>
&nbsp;&nbsp;&nbsp;&nbsp;4.1 <a href="#xml-security">XML Security</a><br>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;4.1.1 <a href="#xml-signature">XML Signature</a><br>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;4.1.2 <a href="#xml-encryption">XML Encryption</a><br>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;4.1.3 <a href="#s-xml-c14n">XML Canonicalization</a><br>
&nbsp;&nbsp;&nbsp;&nbsp;4.2 <a href="#api-impact">Existing XML Processing APIs</a><br>
&nbsp;&nbsp;&nbsp;&nbsp;4.3 <a href="#other-binary">XML and Binary Attachments</a><br>
5 <a href="#human-readability">Sacrificing Human Readability</a><br>
6 <a href="#other-impact">Other Impacts</a><br>
7 <a href="#conclusions">Conclusions</a><br>
8 <a href="#references">References</a><br>
</p>
<h3><a name="appendices" id="appendices"></a>Appendix</h3><p class="toc">A <a href="#acknowledgements">Acknowledgements</a><br>
</p></div><hr><div class="body"><div class="div1">
<h2><a name="intro" id="intro"></a>1 Introduction</h2><p>
	While the introduction of EXI has the potential to bring XML
	to new communities, it can also have adverse effects on the
	existing XML community.  The precise scope of these effects
	may not be fully knowable in advance, but based on experience
	with existing binary formats, educated estimates can be made.
      </p><p>
	The main goals of EXI in regards to existing systems are to
	provide maximally seamless compatibility with XML and to avoid
	disruption of existing XML technologies and specifications. In
	particular, EXI should not require modifications to existing
	XML systems, unless these systems are extended to adopt EXI.
	The purpose of this document is to identify any immediate
	impacts that require changes to existing XML-based
	specifications or XML-using applications. It also identifies
	cases where changes to existing specifications or applications
	are not required, but might be desirable to increase
	efficiency.
      </p></div><div class="div1">
<h2><a name="terminology" id="terminology"></a>2 Terminology and Discussion</h2><p>
	This section collects relevant definitions from the <a href="#xml">[XML]</a> and <a href="#exi-format">[EXI]</a> specifications.
      </p><dl><dt class="label">
	    <a href="http://www.w3.org/TR/2006/REC-xml-20060816#dt-xml-proc">XML Processor</a>
	  </dt><dd><p>
	      A module used to read XML documents and provide access
	      to their content and structure
	    </p></dd><dt class="label">
	    <a href="http://www.w3.org/TR/exi#key-exiprocessor">EXI Processor</a>
	  </dt><dd><p>
	      A module used to encode structured data into EXI streams
	      and/or to decode EXI streams to make structured data
	      accessible
	    </p></dd><dt class="label">
	    <a href="http://www.w3.org/TR/2006/REC-xml-20060816#dt-app">Application</a>
	  </dt><dd><p>
	      A module on behalf of which an XML processor or an EXI
	      processor does its work
	    </p></dd></dl><p id="app-structure">
	In a system containing both an XML and an EXI processor, the
	modules would normally be completely separate from each other.
	The application would be responsible for deciding which
	processor is to process each document.  It could use
	either out-of-band means, such as communication protocol
	metadata, or in-band means, such as the distinguishing bits of
	EXI, to make this decision.
      </p></div><div class="div1">
<h2><a name="xml-processors" id="xml-processors"></a>3 Existing XML Processors and Applications</h2><p>
	EXI offers two in-band means to distinguish it from other
	formats: the mandatory <a href="http://www.w3.org/TR/exi#DistinguishingBits">Distinguishing
	Bits</a> and the optional <a href="http://www.w3.org/TR/exi#EXICookie">EXI Cookie</a>. In
	particular, either of these is sufficient to distinguish EXI
	from XML when using any conventional character encoding (see
	<a href="#exi-bp">[EXI Best Practices]</a>, <a href="http://www.w3.org/TR/exi-best-practices#distinguishing-bits">section
	4.1.1</a>). Assuming such a conventional character encoding,
	the first octet of an EXI document, either one that includes
	the distinguishing bits or the first octet of the EXI cookie,
	can not appear as the first octet of a <a href="http://www.w3.org/TR/2006/REC-xml-20060816#dt-wellformed">well-formed</a>
	XML document. Therefore, an XML processor is required by the
	XML specification to reject any EXI document immediately upon
	reading that first octet.
      </p><p>
	XML is often used in conjunction with other protocols and
	technologies. In some such cases, in particular the World Wide
	Web and Web services where HTTP is common, the protocol
	supports content negotiation to allow applications to indicate
	which content types and encodings they are prepared to handle.
	<a href="#exi-bp">[EXI Best Practices]</a> describes <a href="http://www.w3.org/TR/exi-best-practices#http-content-negotiation">how
	such support can be used</a> to introduce EXI to such an
	environment with no impact to applications that have not
	adopted EXI.
      </p><p>
	More generally, in an environment consisting of multiple XML
	applications, where some but not all applications wish to
	adopt EXI, coordination is needed to avoid transmitting EXI to
	applications that are not prepared to handle it. Following the
	<a href="http://www.w3.org/TR/exi-best-practices#mixed-XML-EXI">EXI
	best practices</a>, the burden of such coordination should
	fall only on the applications that adopt EXI, as they should
	not send EXI to applications that are not known to understand
	it. In processing of incoming transmissions, an application
	adopting EXI will need to implement an internal mechanism for
	routing the incoming content to the appropriate processor (XML
	or EXI), but a non-EXI-aware application can continue using
	its XML processor for everything. If the communication
	protocol does not offer any method for content negotiation, it
	may be that a non-EXI-aware application occasionally gets sent
	EXI content. In such cases, the aforementioned immediate
	rejection should be communicated to the sender so that it can
	avoid sending EXI content to that receiver in the future.
      </p></div><div class="div1">
<h2><a name="xml-technologies" id="xml-technologies"></a>4 Existing XML Technologies</h2><p>
	Most existing XML technologies are specified based on the
	<a href="#infoset">[XML Infoset]</a>. EXI has been designed as an encoding
	format of the Infoset and is therefore immediately applicable
	to such technologies. Some technologies, however, are
	specified in terms of character or octet data, and therefore
	require further consideration on the impacts of EXI. This also
	means that applications requiring byte-for-byte preservation
	of XML documents cannot always use EXI, though EXI is capable
	of preserving all the information relevant to <a href="#xml-c14n">[Canonical XML]</a>. Other technologies may gain additional
	significant benefits if modified to support EXI. While such
	modifications are not required immediately, they may be
	desirable in future versions of the relevant specifications.
      </p><div class="div2">
<h3><a name="xml-security" id="xml-security"></a>4.1 XML Security</h3><p>
	  The XML security specifications <a href="#xml-sig">[XML Signature]</a> and
	  <a href="#xml-enc">[XML Encryption]</a> can be used as they currently exist
	  with EXI, so EXI has no immediate impact on them.  For
	  interoperability in current environments, this requires
	  computing signatures over an XML serialization and making
	  sure that any encrypted content has been serialized as XML.
	</p><div class="div3">
<h4><a name="xml-signature" id="xml-signature"></a>4.1.1 XML Signature</h4><p>
	    In current environments, XML Signature can be used with
	    EXI by specifying an existing XML canonicalization
	    algorithm, such as <a href="#xml-c14n">[Canonical XML]</a>. A signed
	    document can be transmitted using EXI, as long as the <a href="http://www.w3.org/TR/exi-best-practices#security">necessary
	    fidelity options</a> are enabled. As with XML, the
	    receiver will need to serialize the signed content using
	    the selected XML canonicalization algorithm to verify the
	    signatures. In the future, XML use could be avoided
	    completely by using a URI that designates a to-be-defined
	    EXI canonicalization algorithm, rather than an XML
	    canonicalization.
	  </p></div><div class="div3">
<h4><a name="xml-encryption" id="xml-encryption"></a>4.1.2 XML Encryption</h4><p>
	    Use of XML Encryption in mixed XML/EXI environments may
	    require using XML as the format for any data that is
	    encrypted, as the producer may not know whether the
	    ultimate recipient of the document is capable of
	    understanding EXI.  If it is known that the recipient
	    understands EXI, the <code>MimeType</code> attribute of
	    the <code><a href="http://www.w3.org/TR/2002/REC-xmlenc-core-20021210#sec-EncryptedData">EncryptedData</a></code>
	    element could be used to indicate EXI as the format of the
	    encrypted data (though this appears to require a minor
	    modification to <a href="#xml-enc">[XML Encryption]</a>).
	  </p></div><div class="div3">
<h4><a name="s-xml-c14n" id="s-xml-c14n"></a>4.1.3 XML Canonicalization</h4><p>
	    EXI has no impact on existing XML canonicalization
	    algorithms (<a href="#xml-c14n">[Canonical XML]</a>, <a href="#exc-xml-c14n">[Excl XML Canonicalization]</a>). For use in signatures, it may be
	    beneficial in the future to define a URI for
	    &#8220;canonical EXI&#8221; that defines a specific EXI
	    Options document to use in generating a canonical form,
	    but this consideration is completely separate from
	    existing specifications.
	  </p></div></div><div class="div2">
<h3><a name="api-impact" id="api-impact"></a>4.2 Existing XML Processing APIs</h3><p>
	  As EXI is an encoding of the XML Infoset, an EXI
	  implementation can support any of the commonly-used XML APIs
	  for XML processing, so EXI has no immediate impact on
	  existing XML APIs. However, using an existing XML API also
	  requires that all names and text appearing in the EXI
	  document be converted into strings. In the future, more
	  efficiency might be achievable if the higher layers could
	  directly use these data as typed values appearing in the EXI
	  document. For instance, if a higher layer needs typed data,
	  going through its string form can produce a performance
	  penalty, so an extended API that supports typed data
	  directly could improve performance when used with EXI.
	</p></div><div class="div2">
<h3><a name="other-binary" id="other-binary"></a>4.3 XML and Binary Attachments</h3><p>
	  Some use cases require the inclusion of binary data in XML
	  documents, and to avoid the required base64 conversions,
	  specifications such as <a href="#xop">[XOP]</a> exist to package
	  the binary data separately from XML.  Since EXI is capable
	  of encoding binary data directly, it is possible to simply
	  include the binary data inside an EXI document without a
	  loss in efficiency.  If a use case requires a packaging
	  where XML content is separated from the binary data, EXI can
	  still be used as the format for the XML part.
	</p></div></div><div class="div1">
<h2><a name="human-readability" id="human-readability"></a>5 Sacrificing Human Readability</h2><p>
	As a text-based format, XML allows direct editing with generic
	text editors as well as debugging generated XML by simply
	using &#8220;view source&#8221; features.  EXI, as a binary format,
	does not conveniently permit this, so generating and
	inspecting EXI is therefore mostly in the domain of specific
	tools that include an EXI processor.
      </p><p>
	Already many applications that support viewing and editing XML
	parse the XML to present a structured view of the data, more
	attractive than unformatted text. Such applications are
	usually easy to modify to include recognition of new data
	formats, so plugging in an existing EXI processor would have
	low cost and would provide the same data inspection
	opportunities as the application already provides for XML.
	Thus the sacrifice of human readability is not as large a
	concern as it might initially seem due to the XML
	compatibility that EXI provides.
      </p></div><div class="div1">
<h2><a name="other-impact" id="other-impact"></a>6 Other Impacts</h2><p>
	Content negotiation in protocols like HTTP is based on peers
	informing each other what content types and encodings they
	support. While this is sufficient for basic usage of EXI, many
	use cases also require information on common schemas and
	datatype representation maps. Negotiation of such additional
	parameters might be accomplished through a variety of methods,
	and it is not yet clear which methods are best suited for the
	task.
      </p></div><div class="div1">
<h2><a name="conclusions" id="conclusions"></a>7 Conclusions</h2><p>
	EXI has been designed to be compatible with XML and can be
	introduced into the existing family of XML technologies
	without immediate disruption to XML-using applications.
	However, with certain modifications to existing XML-related
	specifications in the future it may be possible to achieve
	additional benefits when using EXI, still without disruption
	to existing XML-based applications. Furthermore, in a
	multi-application system where only some applications adopt
	EXI, sending EXI data to the other applications can
	potentially cause disruption, so care is needed to account for
	differing format support among the participating applications.
      </p></div><div class="div1">
<h2><a name="references" id="references"></a>8 References</h2><dl><dt class="label"><a name="exi-format" id="exi-format"></a>EXI</dt><dd>
	  <a href="http://www.w3.org/TR/exi/"><cite>Efficient XML Interchange (EXI) Format 1.0
	  (Working Draft)</cite></a>, John Schneider and Takuki
	  Kamiya, Editors. World Wide Web Consortium.
	  (See http://www.w3.org/TR/exi/.)</dd><dt class="label"><a name="xml" id="xml"></a>XML</dt><dd>
	  <a href="http://www.w3.org/TR/2006/REC-xml-20060816/"><cite>Extensible Markup Language (XML) 1.0 (Fourth
	  Edition)</cite></a>, Tim Bray, Jean Paoli,
	  C. M. Sperberg-McQueen, Eve Maier, and Fran&ccedil;ois Yergeau,
	  Editors. World Wide Web Consortium, 16 August 2006.
	  (See http://www.w3.org/TR/2006/REC-xml-20060816/.)</dd><dt class="label"><a name="infoset" id="infoset"></a>XML Infoset</dt><dd>
	  <a href="http://www.w3.org/TR/2004/REC-xml-infoset-20040204"><cite>XML Information Set (Second Edition)</cite></a>,
	  John Cowan and Richard Tobin, Editors.  World Wide Web
	  Consortium, 4 February 2004.
	  (See http://www.w3.org/TR/2004/REC-xml-infoset-20040204.)</dd><dt class="label"><a name="xml-sig" id="xml-sig"></a>XML Signature</dt><dd>
	  <a href="http://www.w3.org/TR/2002/REC-xmldsig-core-20020212/"><cite>XML-Signature Syntax and Processing</cite></a>,
	  Donald Eastlake, Joseph Reagle, and David Solo, Editors.
	  World Wide Web Consortium, 12 February 2002.
	  (See http://www.w3.org/TR/2002/REC-xmldsig-core-20020212/.)</dd><dt class="label"><a name="xml-enc" id="xml-enc"></a>XML Encryption</dt><dd>
	  <a href="http://www.w3.org/TR/2002/REC-xmlenc-core-20021210/"><cite>XML Encryption Syntax and Processing</cite></a>,
	  Donald Eastlake and Joseph Reagle, Editors.  World Wide Web
	  Consortium, 10 December 2002.
	  (See http://www.w3.org/TR/2002/REC-xmlenc-core-20021210/.)</dd><dt class="label"><a name="xml-c14n" id="xml-c14n"></a>Canonical XML</dt><dd>
	  <a href="http://www.w3.org/TR/2001/REC-xml-c14n-20010315"><cite>Canonical XML Version 1.0</cite></a>, John Boyer,
	  Editor.  World Wide Web Consortium, 15 March 2001.
	  (See http://www.w3.org/TR/2001/REC-xml-c14n-20010315.)</dd><dt class="label"><a name="exc-xml-c14n" id="exc-xml-c14n"></a>Excl XML Canonicalization</dt><dd>
	  <a href="http://www.w3.org/TR/2002/REC-xml-exc-c14n-20020718/"><cite>Exclusive XML Canonicalization Version
	  1.0</cite></a>, John Boyer, Donald E. Eastlake, and Joseph
	  Reagle, Editors.  World Wide Web Consortium, 18 July 2002.
	  (See http://www.w3.org/TR/2002/REC-xml-exc-c14n-20020718/.)</dd><dt class="label"><a name="xop" id="xop"></a>XOP</dt><dd>
	  <a href="http://www.w3.org/TR/2005/REC-xop10-20050125/"><cite>XML-binary Optimized Packaging</cite></a>, Martin
	  Gudgin, Noah Mendelsohn, Mark Nottingham, and Herv&eacute; Ruellan,
	  Editors.  World Wide Web Consortium, 25 January 2005.
	  (See http://www.w3.org/TR/2005/REC-xop10-20050125/.)</dd><dt class="label"><a name="exi-bp" id="exi-bp"></a>EXI Best Practices</dt><dd>
	  <a href="http://www.w3.org/TR/exi-best-practices"><cite>Efficient XML Interchange (EXI) Best Practices
	  (Working Draft)</cite></a>, Mike Cokus and Daniel Vogelheim,
	  Editors.  World Wide Web Consortium.
	  (See http://www.w3.org/TR/exi-best-practices.)</dd></dl></div></div><div class="back"><div class="div1">
<h2><a name="acknowledgements" id="acknowledgements"></a>A Acknowledgements</h2><p>
	This document is the work of the <a href="http://www.w3.org/XML/EXI/">Efficient XML Interchange
	(EXI) WG</a>.
      </p><p>
	Members of the Working Group are (at the time of publication,
	sorted alphabetically by last name):
      </p><blockquote><p>Carine Bournez, W3C/ERCIM (<em>staff contact</em>)<br>Don Brutzman, Web3D Consortium<br>Alex Ceponkus, AgileDelta, Inc.<br>Michael Cokus, MITRE Corporation (<em>chair</em>)<br>Roger Cutler, Chevron<br>Ed Day, Objective Systems, Inc.<br>Philippe de Cuetos, Expway<br>Joerg Heuer, Siemens AG<br>Alan Hudson, Web3D Consortium<br>Takuki Kamiya, Fujitsu Limited<br>Jaakko Kangasharju, University of Helsinki<br>Richard Kuntschke, Siemens AG<br>Don McGregor, Web3D Consortium<br>Daniel Peintner, Siemens AG<br>Santiago Pericas-Geertsen, Sun Microsystems, Inc.<br>Liam Quin, W3C/MIT<br>Rich Rollman, AgileDelta, Inc.<br>Paul Sandoz, Sun Microsystems, Inc.<br>John Schneider, AgileDelta, Inc.<br>Cedric Thienot, Expway<br>Yun Wang, Intel Corporation<br>Greg White, Stanford University (<em>former co-chair</em>)</p></blockquote><p>
	The EXI Working Group would like to acknowledge the following
	former members of the group for their leadership, guidance and
	expertise they provided throughout their individual tenure in
	the WG. (sorted alphabetically by last name)
      </p><blockquote><p>Robin Berjon, Expway (<em>former co-chair</em>) (until 17 October 2006)<br>Oliver Goldman, Adobe Systems, Inc. (<em>former co-chair</em>) (until 08 June 2006)<br>Peter Haggar, IBM (until 07 March 2007)<br>Kimmo Raatikainen, Nokia (until 18 March 2008)<br>Paul Thorpe, OSS Nokalva, Inc. (until 11 September 2007)<br>Daniel Vogelheim, Invited Expert (<em>former co-chair</em> then from Siemens AG) (until 28 February 2008)<br>Stephen Williams, High Performance Technologies, Inc. (until 30 June 2008)</p></blockquote></div></div></body></html>