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<h1>XML Linking Language (XLink) Version 1.0</h1>

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<h2>W3C Recommendation 27 June 2001</h2><dl><dt>This version:</dt><dd><a href="http://www.w3.org/TR/2001/REC-xlink-20010627/">http://www.w3.org/TR/2000/REC-xlink-20010627/</a>(available in <a href="http://www.w3.org/TR/2001/REC-xlink-20010627/Overview.html">HTML</a>, <a href="http://www.w3.org/TR/2001/REC-xlink-20010627/Overview.xml">XML</a>)</dd><dt>Latest version:</dt><dd><a href="http://www.w3.org/TR/xlink/">http://www.w3.org/TR/xlink/</a></dd><dt>Previous versions:</dt><dd><a href="http://www.w3.org/TR/2000/PR-xlink-20001220/">http://www.w3.org/TR/2000/PR-xlink-20001220/</a></dd><dt>Editors:</dt>

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<dd>Steve DeRose, Brown University Scholarly Technology

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Group</dd>

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<dd>Eve Maler, Sun Microsystems</dd>

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<dd>David Orchard, Jamcracker</dd>

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</dl><p class="copyright"><a href="http://www.w3.org/Consortium/Legal/ipr-notice-20000612#Copyright">Copyright</a> � 2001 <a href="http://www.w3.org/"><abbr title="World Wide Web Consortium">W3C</abbr></a><sup>�</sup> (<a href="http://www.lcs.mit.edu/"><abbr title="Massachusetts Institute of Technology">MIT</abbr></a>, <a href="http://www.inria.fr/"><abbr lang="fr" title="Institut National de Recherche en Informatique et Automatique">INRIA</abbr></a>, <a href="http://www.keio.ac.jp/">Keio</a>), All Rights Reserved. W3C <a href="http://www.w3.org/Consortium/Legal/ipr-notice-20000612#Legal_Disclaimer">liability</a>, <a href="http://www.w3.org/Consortium/Legal/ipr-notice-20000612#W3C_Trademarks">trademark</a>, <a href="http://www.w3.org/Consortium/Legal/copyright-documents-19990405">document use</a>, and <a href="http://www.w3.org/Consortium/Legal/copyright-software-19980720">software licensing</a> rules apply.</p></div><hr><div>

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<h2><a name="abstract">Abstract</a></h2>

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<p>This specification defines the XML Linking Language (XLink), which allows

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elements to be inserted into XML documents in order to create and describe

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links between resources. It uses XML syntax to create structures that can

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describe links similar to the simple unidirectional hyperlinks of today's

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HTML, as well as more sophisticated links.</p>

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</div><div>

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<h2><a name="status">Status of this Document</a></h2>

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<p>This document has been reviewed by W3C Members and other

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interested parties and has been endorsed by the Director

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as a W3C Recommendation. It is a stable document and may

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be used as reference material or cited as a normative

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reference from another document.

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W3C's role in making the Recommendation is to draw attention to the

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specification and to promote its widespread deployment. This

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enhances the functionality and interoperability of the Web.

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</p><p>For information about the XPointer language that <a title="Must, May, etc." href="#dt-must">may</a>

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be used with XLink, see <a href="#xptr">[XPTR]</a>.</p>

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<p>This document has been produced by the W3C XML Linking Working

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Group as part of the XML Activity in the W3C Architecture

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Domain. For background on this work, please see the <a href="http://www.w3.org/XML/Activity">XML

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Activity Statement</a>.</p>

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<p>Please report possible errors in this

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document to the public email list <a href="mailto:www-xml-linking-comments@w3.org">www-xml-linking-comments@w3.org</a> (archive

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at

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<a href="http://lists.w3.org/Archives/Public/www-xml-linking-comments/">http://lists.w3.org/Archives/Public/www-xml-linking-comments/</a>). Any

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confirmed errors will be documented in an list of errata available

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at <a href="http://www.w3.org/2001/06/xlink-errata">http://www.w3.org/2001/06/xlink-errata</a>.</p>

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<p>The English version of this specification is the only normative

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version. Information about translations of this document is available

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at <a href="http://www.w3.org/2001/06/xlink-translations">http://www.w3.org/2001/06/xlink-translations</a>.</p>

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<p>See <a href="#xldp">[XLDP]</a> for additional background on the design principles

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informing XLink, and <a href="#xlreq">[XLREQ]</a> for the normative XLink requirements

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that this document attempts to satisfy. XLink does not support all HTML linking

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constructs as they stand; see <a href="#xlink-naming">[XLinkNaming]</a> for a discussion

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of this situation.</p>

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<p>A list of current W3C Recommendations and other technical documents can

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be found at <a href="http://www.w3.org/TR/">http://www.w3.org/TR/</a>.</p>

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</div>

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<h2><a name="contents">Table of Contents</a></h2><p class="toc">1 <a href="#intro">Introduction</a><br>    1.1 <a href="#origin-goals">Origin and Goals</a><br>2 <a href="#N781">XLink Concepts</a><br>    2.1 <a href="#N789">Links and Resources</a><br>    2.2 <a href="#N854">Arcs, Traversal, and Behavior</a><br>    2.3 <a href="#N899">Resources in Relation to the Physical Location of a Linking Element</a><br>3 <a href="#conformance">XLink Processing and Conformance</a><br>    3.1 <a href="#N1022">Processing Dependencies</a><br>    3.2 <a href="#markup-reqs">Markup Conformance</a><br>    3.3 <a href="#app-reqs">Application Conformance</a><br>4 <a href="#att-method">XLink Markup Design</a><br>    4.1 <a href="#N1238">XLink Attribute Usage Patterns</a><br>    4.2 <a href="#N1599">XLink Element Type Relationships</a><br>    4.3 <a href="#defaulting">Attribute Value Defaulting</a><br>    4.4 <a href="#integrating">Integrating XLink Usage with Other Markup</a><br>    4.5 <a href="#legacy">Using XLink with Legacy Markup</a><br>5 <a href="#linking-elements">XLink Elements and Attributes</a><br>    5.1 <a href="#extended-link">Extended Links (extended-Type Element)</a><br>        5.1.1 <a href="#local-resource">Local Resources for an Extended Link (resource-Type Element)</a><br>        5.1.2 <a href="#remote-resource">Remote Resources for an Extended Link (locator-Type Element)</a><br>        5.1.3 <a href="#xlink-arcs">Traversal Rules for an Extended Link (arc-Type Element)</a><br>        5.1.4 <a href="#title-element">Titles for Extended Links, Locators, and Arcs (title-Type Element)</a><br>        5.1.5 <a href="#xlg">Locating Linkbases (Special Arc Role)</a><br>    5.2 <a href="#simple-links">Simple Links (simple-Type Element)</a><br>    5.3 <a href="#link-types">XLink Element Type Attribute (type)</a><br>    5.4 <a href="#link-locators">Locator Attribute (href)</a><br>    5.5 <a href="#link-semantics">Semantic Attributes (role, arcrole, and title)</a><br>    5.6 <a href="#link-behaviors">Behavior Attributes (show and actuate) </a><br>        5.6.1 <a href="#show-att">show Attribute</a><br>        5.6.2 <a href="#actuate-att">actuate Attribute</a><br>    5.7 <a href="#traversal-atts">Traversal Attributes (label, from, and to)</a><br></p>

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<h3>Appendices</h3><p class="toc">A <a href="#references">References</a><br>    A.1 <a href="#N3954">Normative References</a><br>    A.2 <a href="#N4107">Non-Normative References</a><br>B <a href="#sample-dtd-appx">Sample DTD</a> (Non-Normative)<br>C <a href="#acknowledgements">Working Group Members and Acknowledgments</a> (Non-Normative)<br></p></div><hr><div class="body">

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<h2><a name="intro"></a>1 Introduction</h2>

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<p>This specification defines the XML Linking Language (XLink), which allows

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elements to be inserted into XML documents in order to create and describe <a title="Link" href="#dt-link">links</a> between resources.</p>

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<p>XLink provides a framework for creating both basic unidirectional links

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and more complex linking structures. It allows XML documents to:</p>

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<ul>

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<li><p>Assert linking relationships among more than two resources</p></li>

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<li><p>Associate metadata with a link</p></li>

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<li><p>Express links that reside in a location separate from the linked

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resources</p></li>

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</ul>

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<p>An important application of XLink is in hypermedia systems that have <a title="Hyperlink" href="#dt-hyperlink">hyperlinks</a>. A simple case of a hyperlink is an

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HTML <code>A</code> element, which has these characteristics:</p>

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<ul>

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<li><p>The hyperlink uses URIs as its locator technology.</p></li>

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<li><p>The hyperlink is expressed at one of its two ends.</p></li>

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<li><p>The hyperlink identifies the other end (although a server may have

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great freedom in finding or dynamically creating that destination).</p></li>

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<li><p>Users can initiate traversal only from the end where the hyperlink

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is expressed to the other end.</p></li>

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<li><p>The hyperlink's effect on windows, frames, go-back lists, style sheets

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in use, and so on is determined by user agents, not by the hyperlink itself.

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For example, traversal of <code>A</code> links normally replaces the current

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view, perhaps with a user option to open a new window.</p></li>

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</ul>

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<p>This set of characteristics is powerful, but the model that underlies them

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limits the range of possible hyperlink functionality. The model defined in

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this specification shares with HTML the use of URI technology, but goes beyond

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HTML in offering features, previously available only in dedicated hypermedia

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systems, that make hyperlinking more scalable and flexible. Along with providing

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linking data structures, XLink provides a minimal link behavior model; higher-level

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applications layered on XLink will often specify alternate or more sophisticated

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rendering and processing treatments.</p>

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<p>Integrated treatment of specialized links used in other technical domains,

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such as foreign keys in relational databases and reference values in programming

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languages, is outside the scope of this specification.</p>

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<h3><a name="origin-goals"></a>1.1 Origin and Goals</h3>

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<p>The design of XLink has been informed by knowledge of established hypermedia

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systems and standards. The following standards have been especially influential:</p>

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<ul>

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<li><p><em>HTML</em> <a href="#html">[HTML]</a>: Defines several element types

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that represent links.</p></li>

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<li><p><em>HyTime</em> <a href="#iso10744">[ISO/IEC 10744]</a>: Defines inline and

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inbound and third-party link structures and some semantic features, including

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traversal control and presentation of objects.</p></li>

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<li><p><em>Text Encoding Initiative Guidelines</em> <a href="#tei">[TEI]</a>:

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Provides structures for creating links, aggregate objects, and link collections.</p>

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</li>

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</ul>

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<p>Many other linking systems have also informed the design of XLink, especially <a href="#dexter">[Dexter]</a>, <a href="#fress">[FRESS]</a>, <a href="#ohs">[OHS]</a>, <a href="#microcosm">[MicroCosm]</a>,

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and <a href="#intermedia">[Intermedia]</a>.</p>

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<p>See the XLink Requirements Document <a href="#xlreq">[XLREQ]</a> for a thorough

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explanation of requirements for the design of XLink.</p>

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</div>

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</div>

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<h2><a name="N781"></a>2 XLink Concepts</h2>

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<p>This section describes the terms and concepts that are essential to understanding

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XLink, without discussing the syntax used to create XLink constructs. A few

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additional terms are introduced in later parts of this specification.</p>

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<h3><a name="N789"></a>2.1 Links and Resources</h3>

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<p>[<a name="dt-link" title="Link">Definition</a>: An XLink <b>link</b> is an explicit

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relationship between resources or portions of resources.] [<a name="dt-linkel" title="Linking element">Definition</a>: It is made explicit by an XLink <b>linking

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element</b>, which is an XLink-conforming XML element that asserts the

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existence of a link.] There are six XLink elements; only two of them

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are considered linking elements. The others provide various pieces of information

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that describe the characteristics of a link. (The term "link"

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as used in this specification refers only to an XLink link, though nothing

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prevents non-XLink constructs from serving as links.)</p>

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<p>The notion of resources is universal to the World Wide Web. [<a name="dt-resource" title="Resource">Definition</a>: As discussed in <a href="#rfc2396">[IETF RFC 2396]</a>, a <b>resource</b>

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is any addressable unit of information or service.] Examples include

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files, images, documents, programs, and query results. The means used for

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addressing a resource is a URI (Uniform Resource Identifier) reference (described

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more in <a href="#link-locators"><b>5.4 Locator Attribute (href)</b></a>). It is possible to address a portion

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of a resource. For example, if the whole resource is an XML document, a useful

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portion of that resource might be a particular element inside the document.

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Following a link to it might result, for example, in highlighting that element

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or scrolling to that point in the document.</p>

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<p>[<a name="dt-particip-resource" title="Participating resource">Definition</a>: When a

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link associates a set of resources, those resources are said to <b>participate</b>

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in the link.] Even though XLink links must appear in XML documents,

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they are able to associate all kinds of resources, not just XML-encoded ones.</p>

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<p>One of the common uses of XLink is to create hyperlinks. [<a name="dt-hyperlink" title="Hyperlink">Definition</a>: A <b>hyperlink</b> is a link that is intended primarily

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for presentation to a human user.] Nothing in XLink's design, however,

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prevents it from being used with links that are intended solely for consumption

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by computers.</p>

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</div>

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<h3><a name="N854"></a>2.2 Arcs, Traversal, and Behavior</h3>

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<p>[<a name="dt-traversal" title="Traversal">Definition</a>: Using or following a link for

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any purpose is called <b>traversal</b>.] Even though some kinds

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of link can associate arbitrary numbers of resources, traversal always involves

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a pair of resources (or portions of them); [<a name="dt-starting-resource" title="Starting resource">Definition</a>: the source from which traversal is begun is the <b>starting

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resource</b>] and [<a name="dt-ending-resource" title="Ending resource">Definition</a>: the

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destination is the <b>ending resource</b>]. Note that the term "resource"

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used in this fashion may at times apply to a resource portion, not a whole

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resource.</p>

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<p>[<a name="dt-arc" title="Arc">Definition</a>: Information about how to traverse a pair

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of resources, including the direction of traversal and possibly application

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behavior information as well, is called an <b>arc</b>]. If

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two arcs in a link specify the same pair of resources, but they switch places

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as starting and ending resources, then the link is multidirectional, which

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is not the same as merely "going back" after traversing a link.</p>

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</div>

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<h3><a name="N899"></a>2.3 Resources in Relation to the Physical Location of a Linking Element</h3>

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<p>[<a name="dt-local-resource" title="Local resource">Definition</a>: A <b>local resource</b>

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is an XML element that participates in a link by virtue of having as its parent,

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or being itself, a linking element]. [<a name="dt-remote-resource" title="Remote resource">Definition</a>: Any resource or resource portion that participates

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in a link by virtue of being addressed with a URI reference is considered

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a <b>remote resource</b>, even if it is in the same XML document as

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the link, or even inside the same linking element.] Put another way,

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a local resource is specified "by value," and a remote resource

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is specified "by reference."</p>

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<p>[<a name="dt-outbound" title="Outbound">Definition</a>: An arc that has a local starting

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resource and a remote ending resource goes <b>outbound</b>, that is,

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away from the linking element.] (Examples of links with such an arc

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are the HTML <code>A</code> element, HyTime "clinks," and Text Encoding

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Initiative <code>XREF</code> elements.) [<a name="dt-inbound" title="Inbound">Definition</a>: If

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an arc's ending resource is local but its starting resource is remote, then

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the arc goes <b>inbound</b>.] [<a name="dt-third-party" title="Third-party">Definition</a>: If neither the starting resource nor the ending resource

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is local, then the arc is a <b>third-party</b> arc.] Though

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it is not required, any one link typically specifies only one kind of arc

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throughout, and thus might be referred to as an inbound, outbound, or third-party

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link.</p>

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<p>To create a link that emanates from a resource to which you do not have

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(or choose not to exercise) write access, or from a resource that offers no

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way to embed linking constructs, it is necessary to use an inbound or third-party

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arc. When such arcs are used, the requirements for discovery of the link are

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greater than for outbound arcs. [<a name="dt-linkbase" title="Linkbase">Definition</a>: Documents

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containing collections of inbound and third-party links are called link databases,

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or <b>linkbases</b>.]</p>

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</div>

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</div>

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<h2><a name="conformance"></a>3 XLink Processing and Conformance</h2>

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<p>This section details processing and conformance requirements on XLink applications

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and markup.</p>

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<p>[<a name="dt-must" title="Must, May, etc.">Definition</a>: The key words <b>must</b>, <b>must

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not</b>, <b>required</b>, <b>shall</b>, <b>shall not</b>, <b>should</b>, <b>should

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not</b>, <b>recommended</b>, <b>may</b>, and <b>optional</b>

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in this specification are to be interpreted as described in <a href="#rfc2119">[IETF RFC 2119]</a>.]</p>

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<h3><a name="N1022"></a>3.1 Processing Dependencies</h3>

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<p>XLink processing depends on <a href="#XML">[XML]</a>, <a href="#xname">[XML Names]</a>, <a href="#xbase">[XML Base]</a>, and <a href="#rfc2396">[IETF RFC 2396]</a> (as updated by <a href="#rfc2732">[IETF RFC 2732]</a>).</p>

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</div>

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<h3><a name="markup-reqs"></a>3.2 Markup Conformance</h3>

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<p>An XML element conforms to XLink if:</p>

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<ol>

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<li><p>it has a <code>type</code> attribute from the XLink namespace whose

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value is one of "simple", "extended", "locator", "arc", "resource", "title", or "none", and</p></li>

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<li><p>it adheres to the conformance constraints imposed by the chosen XLink

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element type, as prescribed in this specification.</p></li>

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</ol>

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<p>This specification imposes no particular constraints on DTDs; conformance

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applies only to elements and attributes.</p>

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</div>

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<h3><a name="app-reqs"></a>3.3 Application Conformance</h3>

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<p>An XLink application is any software module that interprets well-formed

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XML documents containing XLink elements and attributes, or XML information

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sets <a href="#infoset">[XIS]</a> containing information items and properties corresponding

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to XLink elements and attributes. (This document refers to elements and attributes,

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but all specifications herein apply to their information set equivalents as

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well.) Such an application is conforming if:</p>

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<ol>

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<li><p>it observes the mandatory conditions for applications ("must")

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set forth in this specification, and</p></li>

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<li><p>for any optional conditions ("should" and "may")

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it chooses to observe, it observes them in the way prescribed, and</p></li>

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<li><p>it performs markup conformance testing according to all the conformance

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constraints appearing in this specification.</p></li>

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</ol>

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</div>

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</div>

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<h2><a name="att-method"></a>4 XLink Markup Design</h2>

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<p>This section describes the design of XLink's markup vocabulary.</p>

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<p>Link markup needs to be recognized reliably by XLink applications in order

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to be traversed and handled properly. XLink uses the mechanism described in

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the Namespaces in XML Recommendation <a href="#xname">[XML Names]</a> to accomplish recognition

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of the constructs in the XLink vocabulary.</p>

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<p>The XLink namespace defined by this specification has the following URI:</p>

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<table class="eg" cellpadding="5" border="1" bgcolor="#99ffff" width="100%" summary="Example"><tr><td><pre>http://www.w3.org/1999/xlink</pre></td></tr></table>

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<p>As dictated by <a href="#xname">[XML Names]</a>, the use of XLink elements and attributes

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requires declaration of the XLink namespace. For example, the following declaration

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would make the prefix <code>xlink</code> available within the <code>myElement</code>

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element to represent the XLink namespace:</p>

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<table class="eg" cellpadding="5" border="1" bgcolor="#99ffff" width="100%" summary="Example"><tr><td><pre><myElement

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xmlns:xlink="http://www.w3.org/1999/xlink">

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...

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</myElement></pre></td></tr></table>

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<p>Most code examples in this specification do not show an XLink namespace

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declaration. The <code>xlink</code> prefix is used throughout to stand for the

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declaration of the XLink namespace on elements in whose scope the so-marked

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attribute appears (on the same element that bears the attribute or on some

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ancestor element), whether or not an XLink namespace declaration is present

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in the example.</p>

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</div>

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<p>XLink's namespace provides <b>global attributes</b> for use on elements

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that are in any arbitrary namespace. The global attributes are <code>type</code>, <code>href</code>, <code>role</code>, <code>arcrole</code>, <code>title</code>, <code>show</code>, <code>actuate</code>, <code>label</code>, <code>from</code>,

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and <code>to</code>. Document creators use the XLink global attributes to make

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the elements in their own namespace, or even in a namespace they do not control,

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recognizable as XLink elements. The <code>type</code> attribute indicates the

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XLink element type (simple, extended, locator, arc, resource, or title); the

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element type dictates the XLink-imposed constraints that such an element <a title="Must, May, etc." href="#dt-must">must</a> follow and the behavior of XLink applications

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on encountering the element.</p>

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<p>Following is an example of a <code>crossReference</code> element from a non-XLink

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namespace that has XLink global attributes:</p>

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<table class="eg" cellpadding="5" border="1" bgcolor="#99ffff" width="100%" summary="Example"><tr><td><pre><my:crossReference

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xmlns:my="http://example.com/"

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xmlns:xlink="http://www.w3.org/1999/xlink"

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xlink:type="simple"

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xlink:href="students.xml"

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xlink:role="http://www.example.com/linkprops/studentlist"

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xlink:title="Student List"

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xlink:show="new"

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xlink:actuate="onRequest">

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Current List of Students

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</my:crossReference></pre></td></tr></table>

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<p>Using global attributes always requires the use of namespace prefixes on

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the individual attributes and the use of the <code>type</code> attribute on

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the element.</p>

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<h3><a name="N1238"></a>4.1 XLink Attribute Usage Patterns</h3>

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<p>While the XLink attributes are considered global by virtue of their use

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of the namespace mechanism, their allowed combinations on any one XLink element

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type depend greatly on the value of the special <code>type</code> attribute

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(see <a href="#link-types"><b>5.3 XLink Element Type Attribute (type)</b></a> for more information) for the element on

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which they appear. The conformance constraint notes in this specification

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detail their allowed usage patterns. Following is a summary of the element

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types (columns) on which the global attributes (rows) are allowed, with an

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indication of whether a value is required (R) or optional (O):</p>

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<table border="1" frame="border"><thead><tr><th rowspan="1" colspan="1"></th><th rowspan="1" colspan="1"><code>simple</code></th>

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<th rowspan="1" colspan="1"><code>extended</code></th><th rowspan="1" colspan="1"><code>locator</code></th><th rowspan="1" colspan="1"><code>arc</code></th><th rowspan="1" colspan="1"><code>resource</code></th>

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<th rowspan="1" colspan="1"><code>title</code></th></tr></thead><tbody><tr><td rowspan="1" colspan="1"><code>type</code></td><td rowspan="1" colspan="1">R</td>

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<td rowspan="1" colspan="1">O</td><td rowspan="1" colspan="1">O</td><td rowspan="1" colspan="1">O</td><td rowspan="1" colspan="1"></td><td rowspan="1" colspan="1">O</td><td rowspan="1" colspan="1"></td></tr><tr><td rowspan="1" colspan="1"><code>arcrole</code></td>

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<td rowspan="1" colspan="1">O</td><td rowspan="1" colspan="1"></td><td rowspan="1" colspan="1"></td><td rowspan="1" colspan="1">O</td><td rowspan="1" colspan="1"></td><td rowspan="1" colspan="1"></td></tr><tr><td rowspan="1" colspan="1"><code>title</code></td>

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<td rowspan="1" colspan="1">O</td><td rowspan="1" colspan="1"></td><td rowspan="1" colspan="1"></td><td rowspan="1" colspan="1">O</td><td rowspan="1" colspan="1"></td><td rowspan="1" colspan="1"></td></tr><tr><td rowspan="1" colspan="1"><code>actuate</code></td>

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<td rowspan="1" colspan="1">O</td><td rowspan="1" colspan="1"></td><td rowspan="1" colspan="1"></td><td rowspan="1" colspan="1">O</td><td rowspan="1" colspan="1"></td><td rowspan="1" colspan="1"></td></tr><tr><td rowspan="1" colspan="1"><code>label</code></td>

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<p>(See also <a href="#sample-dtd-appx"><b>B Sample DTD</b></a> for a non-normative DTD that

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illustrates the allowed patterns of attributes.)</p>

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<p>This specification uses the convention "<var>xxx</var>-type element"

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to refer to elements that <a title="Must, May, etc." href="#dt-must">must</a> adhere to

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a named set of constraints associated with an XLink element type, no matter

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what name the element actually has. For example, "<code>locator</code>-type

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element" would refer to all of the following elements:</p>

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<my:pointer xlink:type="locator" ... /></pre></td></tr></table>

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</div>

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<h3><a name="N1599"></a>4.2 XLink Element Type Relationships</h3>

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<p>Various XLink element types have special meanings dictated by this specification

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when they appear as direct children of other XLink element types. Following

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is a summary of the child element types that play a significant role in particular

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parent element types. (Other combinations have no XLink-dictated significance.)</p>

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<table border="1"><thead><tr><th rowspan="1" colspan="1">Parent type</th><th rowspan="1" colspan="1">Significant child types</th>

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</tr></thead><tbody><tr><td rowspan="1" colspan="1"><code>simple</code></td><td rowspan="1" colspan="1">none</td></tr><tr><td rowspan="1" colspan="1"><code>extended</code></td>

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<td rowspan="1" colspan="1"><code>locator</code>, <code>arc</code>, <code>resource</code>, <code>title</code></td>

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</tr><tr><td rowspan="1" colspan="1"><code>locator</code></td><td rowspan="1" colspan="1"><code>title</code></td></tr><tr><td rowspan="1" colspan="1"><code>arc</code></td>

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<td rowspan="1" colspan="1"><code>title</code></td></tr><tr><td rowspan="1" colspan="1"><code>resource</code></td><td rowspan="1" colspan="1">none</td></tr>

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<tr><td rowspan="1" colspan="1"><code>title</code></td><td rowspan="1" colspan="1">none</td></tr></tbody></table></div>

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<h3><a name="defaulting"></a>4.3 Attribute Value Defaulting</h3>

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<p>Using XLink potentially involves using a large number of attributes for

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supplying important link information. In cases where the values of the desired

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XLink attributes are unchanging across individual instances in all the documents

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of a certain type, attribute value defaults (fixed or not) <a title="Must, May, etc." href="#dt-must">may</a>

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be added to a DTD so that the attributes do not have to appear physically

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on element start-tags. For example, if attribute defaults were provided for

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the <code>xmlns:xlink</code>, <code>xmlns:my</code>, <code>type</code>, <code>show</code>,

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and <code>actuate</code> attributes in the example in the introduction to <a href="#att-method"><b>4 XLink Markup Design</b></a>, the example would look as follows:</p>

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<table class="eg" cellpadding="5" border="1" bgcolor="#99ffff" width="100%" summary="Example"><tr><td><pre><my:crossReference

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xlink:href="students.xml"

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xlink:role="http://www.example.com/linkprops/studentlist"

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xlink:title="Student List">

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Current List of Students

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</my:crossReference></pre></td></tr></table>

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<p>Information sets that have been created under the control of a DTD have

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all attribute values filled in.</p>

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</div>

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<h3><a name="integrating"></a>4.4 Integrating XLink Usage with Other Markup</h3>

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<p>This specification defines only attributes and attribute values in the

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XLink namespace. There is no restriction on using non-XLink attributes alongside

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XLink attributes. In addition, most XLink attributes are optional and the

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choice of simple or extended link is up to the markup designer or document

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creator, so a DTD that uses XLink features need not use or declare the entire

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set of XLink's attributes. Finally, while this specification identifies the

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minimum constraints on XLink markup, DTDs that use XLink are free to tighten

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these constraints. The use of XLink does not absolve a valid document from

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conforming to the constraints expressed in its governing DTD.</p>

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<p>Following is an example of a <code>crossReference</code> element with both

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XLink and non-XLink attributes:</p>

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<table class="eg" cellpadding="5" border="1" bgcolor="#99ffff" width="100%" summary="Example"><tr><td><pre><my:crossReference

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xmlns:my="http://example.com/"

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my:lastEdited="2000-06-10"

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xmlns:xlink="http://www.w3.org/1999/xlink"

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xlink:type="simple"

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xlink:href="students.xml">

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Current List of Students

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</my:crossReference></pre></td></tr></table>

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</div>

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<h3><a name="legacy"></a>4.5 Using XLink with Legacy Markup</h3>

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<p>Because XLink's global attributes require the use of namespace prefixes,

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non-XLink-based links in legacy documents generally do not serve as conforming

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XLink constructs as they stand, even if attribute value defaulting is used.

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For example, XHTML 1.0 has an <code>a</code> element with an <code>href</code> attribute,

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but because the attribute is a local one attached to the <code>a</code> element

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in the XHTML namespace, it is not the same as an <code>xlink:href</code> global

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attribute in the XLink namespace.</p>

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</div>

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</div>

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<h2><a name="linking-elements"></a>5 XLink Elements and Attributes</h2>

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<p>XLink offers two kinds of links:</p>

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<dl>

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<dt class="label">Extended links</dt>

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<dd>

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<p>Extended links offer full XLink functionality, such as inbound and third-party

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arcs, as well as links that have arbitrary numbers of participating resources.

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As a result, their structure can be fairly complex, including elements for

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pointing to remote resources, elements for containing local resources, elements

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for specifying arc traversal rules, and elements for specifying human-readable

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resource and arc titles.</p>

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<p>XLink defines a way to give an extended link special semantics for finding

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linkbases; used in this fashion, an extended link helps an XLink application

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process other links.</p>

426

</dd>

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<dt class="label">Simple links</dt>

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<dd>

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<p>Simple links offer shorthand syntax for a common kind of link, an outbound

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link with exactly two participating resources (into which category HTML-style <code>A</code>

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and <code>IMG</code> links fall). Because simple links offer less functionality

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than extended links, they have no special internal structure.</p>

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<p>While simple links are conceptually a subset of extended links, they are

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syntactically different. For example, to convert a simple link into an extended

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link, several structural changes would be needed.</p>

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</dd>

437

</dl>

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<p>The following sections define the XLink elements and attributes.</p>

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<h3><a name="extended-link"></a>5.1 Extended Links (<code>extended</code>-Type Element)</h3>

442

<p>[<a name="dt-extendedlink" title="Extended link">Definition</a>: An <b>extended link</b>

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is a link that associates an arbitrary number of resources. The participating

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resources <a title="Must, May, etc." href="#dt-must">may</a> be any combination of remote

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and local.]</p>

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<p>The only kind of link that is able to have inbound and third-party arcs

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is an extended link. Typically, extended linking elements are stored separately

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from the resources they associate (for example, in entirely different documents).

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Thus, extended links are important for situations where the participating

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resources are read-only, or where it is expensive to modify and update them

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but inexpensive to modify and update a separate linking element, or where

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the resources are in formats with no native support for embedded links (such

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as many multimedia formats).</p>

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<p>The following diagram shows an extended link that associates five remote

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resources. This could represent, for example, information about a student's

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course load: one resource being a description of the student, another being

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a description of the student's academic advisor, two resources representing

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courses that the student is attending, and the last resource representing

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a course that the student is auditing.</p>

460

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<p>Without the extended link, the resources might be entirely unrelated; for

462

example, they might be in five separate documents. The lines emanating from

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the extended link represent the association it creates among the resources.

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However, notice that the lines do not have directionality. Directionality

465

is expressed with traversal rules; without such rules being provided, the

466

resources are associated in no particular order, with no implication as to

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whether and how individual resources are accessed.</p>

468

<p>The following diagram shows an extended link that associates five remote

469

resources and one local resource (a special element inside the extended link

470

element). This could represent the same sort of course-load example as described

471

above, with the addition of the student's grade point average stored locally.

472

Again, the lines represent mere association of the six resources, without

473

traversal directions or behaviors implied.</p>

474

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<p>The XLink element type for extended links is any element with an attribute

476

in the XLink namespace called <code>type</code> with a value of "extended".</p>

477

<p>The <code>extended</code>-type element <a title="Must, May, etc." href="#dt-must">may</a>

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contain a mixture of the following elements in any order, possibly along with

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