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<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" lang="EN"><head><title>XML Base (Second Edition)</title><style type="text/css">
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</style><link type="text/css" rel="stylesheet" href="http://www.w3.org/StyleSheets/TR/W3C-REC.css"/></head><body><div class="head"><p><a href="http://www.w3.org/"><img width="72" height="48" alt="W3C" src="http://www.w3.org/Icons/w3c_home"/></a></p>
<h1><a id="title" name="title"/>XML Base (Second Edition)</h1>
<h2><a id="w3c-doctype" name="w3c-doctype"/>W3C Recommendation 28 January 2009</h2><dl><dt>This version:</dt><dd>
<a href="http://www.w3.org/TR/2009/REC-xmlbase-20090128/">http://www.w3.org/TR/2009/REC-xmlbase-20090128/</a>
</dd><dt>Latest version:</dt><dd>
<a href="http://www.w3.org/TR/xmlbase/">http://www.w3.org/TR/xmlbase/</a>
</dd><dt>Previous versions:</dt><dd>
<a href="http://www.w3.org/TR/2001/REC-xmlbase-20010627/">http://www.w3.org/TR/2001/REC-xmlbase-20010627/</a>
<a href="http://www.w3.org/TR/2008/PER-xmlbase-20080320/">http://www.w3.org/TR/2008/PER-xmlbase-20080320/</a>
</dd><dt>Editors:</dt><dd>Jonathan Marsh, Microsoft <a href="mailto:jmarsh@microsoft.com"><jmarsh@microsoft.com></a></dd><dd>Richard Tobin, University of Edinburgh <a href="mailto:richard@inf.ed.ac.uk"><richard@inf.ed.ac.uk></a></dd></dl><p>Please refer to the <a href="http://www.w3.org/2009/01/xmlbase-errata"><strong>errata</strong></a> for this document, which may
include normative corrections.</p><p>See also <a href="http://www.w3.org/2003/03/Translations/byTechnology?technology=xmlbase"><strong>translations</strong></a>.</p><p>This document is also available in these non-normative formats: <a href="http://www.w3.org/TR/2009/REC-xmlbase-20090128/Overview-review.html">HTML with diff markup</a> and <a href="http://www.w3.org/TR/2009/REC-xmlbase-20090128/Overview.xml">XML</a>.</p><p class="copyright"><a href="http://www.w3.org/Consortium/Legal/ipr-notice#Copyright">Copyright</a> © 2009 <a href="http://www.w3.org/"><acronym title="World Wide Web Consortium">W3C</acronym></a><sup>®</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 id="abstract" name="abstract"/>Abstract</h2><p>This document
<span>describes</span>
a facility, similar to that of HTML BASE,
for defining base URIs for parts of XML documents.</p></div><div>
<h2><a id="status" name="status"/>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 document has been produced by the
<a href="http://www.w3.org/XML/Core/">W3C XML Core Working Group</a>
as part of the
<a href="http://www.w3.org/XML/Activity.html">W3C XML Activity</a>.
The English version of this specification is the only normative version.
However, for translations of this document, see
<a href="http://www.w3.org/2003/03/Translations/byTechnology?technology=xmlbase">
http://www.w3.org/2003/03/Translations/byTechnology?technology=xmlbase</a>
</p><p>
This document is an Edited Recommendation of the W3C.
It supersedes the previous
<a href="http://www.w3.org/TR/2001/REC-xmlbase-20010627/">W3C Recommendation
of 27 June 2001</a>.
This second edition is not a new version of XML Base;
its purpose is to clarify a number of issues that
have become apparent since the first edition was published.
Some of these were first published as separate errata
(<a href="http://www.w3.org/2001/06/xmlbase-errata">
http://www.w3.org/2001/06/xmlbase-errata</a>),
others were published in a public editor's draft in November 2006
(<a href="http://www.w3.org/XML/2006/11/xmlbase-2e/Overview.html">http://www.w3.org/XML/2006/11/xmlbase-2e/Overview.html</a>),
and a PER in December 2006
(<a href="http://www.w3.org/TR/2006/PER-xmlbase-20061220/">http://www.w3.org/TR/2006/PER-xmlbase-20061220/</a>).
The changes are summarized in an
<a href="#changes">appendix</a>.
</p><p>
Please report errors in this document to the public mailing list
<a href="mailto:www-xml-linking-comments@w3.org">www-xml-linking-comments@w3.org</a>;
public
<a href="http://lists.w3.org/Archives/Public/www-xml-linking-comments/">archives</a>
are available.
</p><p>
There is no implementation report or test suite for this specification, but
there is a document describing
<a href="http://www.w3.org/XML/2006/12/xmlbase-testing.html">methods of
testing XML Base conformance</a>.
</p><p>
This document has been reviewed by W3C Members, by software
developers, and by other W3C groups and interested parties, and is
endorsed by the Director as a W3C Recommendation. It is a stable
document and may be used as reference material or cited from another
document. W3C's role in making the Recommendation is to draw attention
to the specification and to promote its widespread deployment. This
enhances the functionality and interoperability of the Web.
</p><p>
This document is governed by the
<a href="http://www.w3.org/TR/2002/NOTE-patent-practice-20020124">24 January 2002 CPP</a>
as amended by the
<a href="http://www.w3.org/2004/02/05-pp-transition">W3C Patent Policy Transition Procedure</a>.
W3C maintains a
<a href="http://www.w3.org/2004/01/pp-impl/18796/status">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 id="contents" name="contents"/>Table of Contents</h2><p class="toc">1 <a href="#introduction">Introduction</a><br/>
2 <a href="#terminology">Terminology</a><br/>
3 <a href="#syntax">xml:base Attribute</a><br/>
3.1 <a href="#escaping">URI Reference Encoding and Escaping</a><br/>
4 <a href="#resolution">Resolving Relative URIs</a><br/>
4.1 <a href="#rfc3986">Relation to RFC 3986</a><br/>
4.2 <a href="#granularity">Granularity of base URI information</a><br/>
4.3 <a href="#matching">Matching URIs with base URIs</a><br/>
4.4 <a href="#same-document">Interpretation of same-document references</a><br/>
5 <a href="#conformance">Conformance</a><br/>
</p>
<h3><a id="appendices" name="appendices"/>Appendices</h3><p class="toc">A <a href="#references">References</a><br/>
B <a href="#references2">References</a> (Non-Normative)<br/>
C <a href="#impacts">Impacts on Other Standards</a> (Non-Normative)<br/>
D <a href="#changes">Changes since the first edition</a> (Non-Normative)<br/>
</p></div><hr/><div class="body"><div class="div1">
<h2><a id="introduction" name="introduction"/>1 Introduction</h2><p>The XML Linking Language <a href="#XLink">[XLink]</a> defines Extensible
Markup Language (XML) 1.0 <a href="#XML">[XML]</a> constructs to describe
links between resources. One of the stated requirements on XLink is to support HTML
<a href="#HTML40">[HTML 4.01]</a> linking constructs in a generic way. The HTML BASE element is
one such construct which the XLink Working Group has considered. BASE allows authors
to explicitly specify a document's base URI for the purpose of resolving relative URIs
in links to external images, applets, form-processing programs, style sheets, and so
on.</p><p>This document describes a mechanism for providing base URI services to XLink, but
as a modular specification so that other XML applications benefiting from additional
control over relative URIs but not built upon XLink can also make use of it. The
syntax consists of a single XML attribute named <code>xml:base</code>.</p><p>The deployment of XML Base is through normative reference by new
specifications, for example XLink and the XML Infoset. Applications
and specifications built upon these new technologies will natively
support XML Base. The behavior of <code>xml:base</code> attributes
in applications based on specifications that do not have direct or
indirect normative reference to XML Base is undefined.</p><p>
This specification does not attempt to specify which text strings in
a document are to be interpreted as URIs. That is the responsibility of
each XML vocabulary. The question addressed by this specification
is: given a relative URI in an XML document, what base URI is
it resolved against?
</p><p>
It is expected that a future RFC for XML Media Types will specify XML Base
as the mechanism for establishing base URIs in the media types it defines.
</p></div><div class="div1">
<h2><a id="terminology" name="terminology"/>2 Terminology</h2><p>[<a title="Must, May, etc." id="dt-must" name="dt-must">Definition</a>: The key words
<b>must</b>, <b>must not</b>, <b>required</b>,
<b>shall</b>, <b>shall not</b>, <b>should</b>,
<b>should not</b>, <b>recommended</b>, <b>may</b>,
and <b>optional</b> in this specification are to be interpreted
as described in <a href="#RFC2119">[RFC 2119]</a>.]</p><p>The terms <b>base URI</b> and <b>relative URI</b>
are used in this specification as they are defined in <a href="#RFC3986">[RFC 3986]</a>.</p></div><div class="div1">
<h2><a id="syntax" name="syntax"/>3 <code>xml:base</code> Attribute</h2><p>The attribute <code>xml:base </code> <a title="Must, May, etc." href="#dt-must">may</a> be inserted in XML documents to
specify a base URI other than the base URI of the document or external
entity. The value of this attribute is interpreted as
<span>
a Legacy Extended IRI (LEIRI) as defined in
the W3C Note "Legacy extended IRIs for XML resource identification"
<a href="#LEIRI">[LEIRI]</a>
</span>.</p><p>In namespace-aware XML processors, the "xml" prefix is bound to the namespace
name <code>http://www.w3.org/XML/1998/namespace</code> as described in Namespaces in XML
<a href="#XMLNS">[XML Names]</a>. Note that <code>xml:base</code> can be still used by
non-namespace-aware processors.</p><p>An example of <code>xml:base</code> in a simple document containing
XLinks follows. XLink normatively references XML Base for interpretation
of relative URI references in <code>xlink:href</code> attributes.</p><div class="exampleInner"><pre><?xml version="1.0"?>
<doc xml:base="http://example.org/today/"
xmlns:xlink="http://www.w3.org/1999/xlink">
<head>
<title>Virtual Library</title>
</head>
<body>
<paragraph>See <link xlink:type="simple" xlink:href="new.xml">what's
new</link>!</paragraph>
<paragraph>Check out the hot picks of the day!</paragraph>
<olist xml:base="/hotpicks/">
<item>
<link xlink:type="simple" xlink:href="pick1.xml">Hot Pick #1</link>
</item>
<item>
<link xlink:type="simple" xlink:href="pick2.xml">Hot Pick #2</link>
</item>
<item>
<link xlink:type="simple" xlink:href="pick3.xml">Hot Pick #3</link>
</item>
</olist>
</body>
</doc></pre></div><p>The URIs in this example resolve to full URIs as follows:</p><ul><li><p>"what's new" resolves to the URI "http://example.org/today/new.xml"</p></li><li><p>"Hot Pick #1" resolves to the URI "http://example.org/hotpicks/pick1.xml"</p></li><li><p>"Hot Pick #2" resolves to the URI "http://example.org/hotpicks/pick2.xml"</p></li><li><p>"Hot Pick #3" resolves to the URI "http://example.org/hotpicks/pick3.xml"</p></li></ul><div class="note"><p class="prefix"><b>Note:</b></p><p>
This specification does not give the xml:base attribute any special status as
far as XML validity is concerned. In a valid document the attribute must be
declared in the DTD, and similar considerations apply to other schema languages.
</p></div><div class="div2">
<h3><a id="escaping" name="escaping"/>3.1 URI Reference Encoding and Escaping</h3><p>
The value of an <code>xml:base</code> attribute is a
Legacy Extended IRI (LEIRI)
and may contain characters not allowed in URIs.
(However, some characters allowed in LEIRIs are not legal XML characters, and
cannot therefore appear in <code>xml:base</code> values.)
</p><p>
In accordance with the principle
that percent-encoding must occur as late as possible in the processing chain,
applications which provide access to the base URI of an element
<a title="Must, May, etc." href="#dt-must">should</a>
calculate and return the value without escaping.
</p><p>
In the example below, the base URI of element <code>e2</code> should be
returned as "http://example.org/wine/rosé".
</p><div class="exampleInner"><pre>
<?xml version="1.0"?>
<e1 xml:base="http://example.org/wine/">
<e2 xml:base="rosé"/>
</e1>
</pre></div></div></div><div class="div1">
<h2><a id="resolution" name="resolution"/>4 Resolving Relative URIs</h2><div class="div2">
<h3><a id="rfc3986" name="rfc3986"/>4.1 Relation to RFC 3986</h3><p>
<span>RFC 3986 <a href="#RFC3986">[RFC 3986]</a></span>
provides for base URI information to be
embedded within a document. The rules for determining the base URI can be
summarized as follows (highest priority to lowest):</p><ol class="enumar"><li><p>The base URI is embedded in the document's content.</p></li><li><p>The base URI is that of the encapsulating entity (message,
document, or none).</p></li><li><p>The base URI is the URI used to retrieve the entity.</p></li><li><p>The base URI is defined by the context of the application.</p></li></ol><div class="note"><p class="prefix"><b>Note:</b></p><p>The term "entity" in points #2 and #3 above uses the
<span>RFC 3986</span>
meaning of the term. Elsewhere in this document the term "entity"
is used in the XML sense.</p></div><p>This document specifies the details of rule #1 for embedding
base URI information in the specific case of XML documents.</p></div><div class="div2">
<h3><a id="granularity" name="granularity"/>4.2 Granularity of base URI information</h3><p>Relative URIs appearing in an XML document are always resolved
relative to either an element, a document entity, or an external
entity. There is no provision for finer granularity, such as
per-attribute, per-character, or per-entity base information.
Neither internal entities, whether declared in the internal
subset or in an external DTD, nor freestanding text (text not
enclosed in an element) in an external entity, are considered
to set a base URI separate from the base URI in scope for the
entity reference.</p><p>The base URI of a document entity or an external entity is
determined by
<span>RFC 3986</span>
rules, namely, that the base URI is
the URI used to retrieve the document entity or external
entity.</p><p>The base URI of an element is:</p><ol class="enumar"><li><p>the base URI specified by an <code>xml:base</code> attribute
on the element, if one exists, otherwise</p></li><li><p>the base URI of the element's parent element within the
document <span>entity</span> or external entity, if one exists, otherwise</p></li><li><p>the base URI of the document entity or external entity
containing the element.</p></li></ol><div class="note"><p class="prefix"><b>Note:</b></p><p>
It follows that the base URI specified by an <code>xml:base</code> attribute
is inherited by descendant elements within the same entity until another
element with an <code>xml:base</code> attribute is encountered.
</p></div><p>
The base URI of an element bearing an <code>xml:base</code> attribute with a value
that is not a valid Legacy Extended IRI is application dependent.
</p></div><div class="div2">
<h3><a id="matching" name="matching"/>4.3 Matching URIs with base URIs</h3><p>The base URI corresponding to a given relative URI appearing
in an XML document is determined as follows:</p><ul><li><p>The base URI for a URI reference appearing in text content
is the base URI of the element containing the text.</p></li><li><p>The base URI for a URI reference appearing in an
<code>xml:base</code> attribute is the base URI of the parent
element of the element bearing the <code>xml:base</code>
attribute, if one exists within the document entity or
external entity, otherwise the base URI of the
document entity or external entity containing the element.</p></li><li><p>The base URI for a URI reference appearing in any other
attribute value, including default attribute values, is the
base URI of the element bearing the attribute.</p></li><li><p>The base URI for a URI reference appearing in the content
of a processing instruction is the base URI of the parent
element of the processing instruction, if one exists within
the document entity or external entity, otherwise the base
URI of the document entity or external entity containing
the processing instruction.</p></li></ul><div class="note"><p class="prefix"><b>Note:</b></p><p>
The presence of <code>xml:base</code> attributes might
lead to unexpected results in the case where the attribute value
is provided, not directly in the XML document entity, but via a
default attribute.
For instance, such a declaration in an external entity might
not be read by software which is based on
a non-validating XML processor. Defaulting attributes
through an external mechanism such as XML Schema may also lead to
unexpected results; even if a validating processor is used by the
application, the addition of defaulted attributes subsequent to
creation of the infoset can cause xml:base attributes to get out of
sync with the [base URI] infoset property. For these reasons,
<code>xml:base</code> values
<a title="Must, May, etc." href="#dt-must">should</a>
be provided either directly in the XML document instance
or via default attributes declared
in the internal subset of the DTD.
</p></div></div><div class="div2">
<h3><a id="same-document" name="same-document"/>4.4 Interpretation of same-document references</h3><p>
RFC 3986 defines certain relative URI references, in particular the
empty string and those of the form <code>#fragment</code>,
as <em>same-document references</em>. Dereferencing
of same-document references is handled specially. However, their use as
the value of an <code>xml:base</code> attribute does not involve
dereferencing, and XML Base processors should resolve them in
the usual way. In particular, <code>xml:base=""</code> does not
reset the base URI to that of the containing document.
</p><div class="note"><p class="prefix"><b>Note:</b></p><p>
Some existing processors <em>do</em> treat these <code>xml:base</code>
values as resetting the base URI to that of the containing document,
so the use of such values is strongly discouraged.
</p></div></div></div><div class="div1">
<h2><a id="conformance" name="conformance"/>5 Conformance</h2><p>An application conforms to XML Base if it calculates base URIs
in accordance with the conditions set forth in this specification.</p></div></div><div class="back"><div class="div1">
<h2><a id="references" name="references"/>A References</h2><dl><dt class="label"><a id="RFC2119" name="RFC2119"/>RFC 2119</dt><dd>
<a href="http://www.ietf.org/rfc/rfc2119.txt"><cite>RFC 2119: Key words for use in RFCs to Indicate Requirement Levels</cite></a>.
<em>Internet Engineering Task Force, 1997.</em>
</dd><dt class="label"><a name="RFC3986"/>RFC 3986</dt><dd>
<a href="http://www.ietf.org/rfc/rfc3986.txt"><cite>RFC 3986: Uniform Resource Identifier (URI): Generic Syntax</cite></a>.
<em>Internet Engineering Task Force, 2005.</em>
</dd><dt class="label"><a name="LEIRI"/>LEIRI</dt><dd>
<a href="http://www.w3.org/TR/leiri/"><cite>Legacy extended IRIs for XML resource identification</cite></a>.
Henry S. Thompson, Richard Tobin, and Norman Walsh, editors.
<em>World Wide Web Consortium.</em>
</dd><dt class="label"><a name="XML"/>XML</dt><dd>
<a href="http://www.w3.org/TR/xml/"><cite>Extensible Markup Language (XML) 1.0.</cite></a>
Tim Bray et al.
<em>World Wide Web Consortium.</em>
</dd><dt class="label"><a name="XMLNS"/>XML Names</dt><dd>
<a href="http://www.w3.org/TR/xml-names/"><cite>Namespaces in XML 1.0.</cite></a>.
Tim Bray et al.
<em>World Wide Web Consortium.</em>
</dd></dl></div><div class="div1">
<h2><a id="references2" name="references2"/>B References (Non-Normative)</h2><dl><dt class="label"><a id="HTML40" name="HTML40"/>HTML 4.01</dt><dd>
<a href="http://www.w3.org/TR/1999/REC-html401-19991224/"><cite>HTML 4.01 Specification</cite></a>.
Dave Raggett, Arnaud Le Hors, Ian Jacobs, editors.
<em>World Wide Web Consortium, 1999.</em>
</dd><dt class="label"><a id="XLink" name="XLink"/>XLink</dt><dd>
<a href="http://www.w3.org/TR/2001/REC-xlink-20010627/"><cite>XML Linking Language (XLink).</cite></a>
Steve DeRose, Eve Maler, David Orchard, and Ben Trafford, editors.
<em>World Wide Web Consortium, 2000.</em>
</dd><dt class="label"><a id="XMLDT" name="XMLDT"/>XML Datatypes</dt><dd>
<a href="http://www.w3.org/TR/2001/REC-xmlschema-2-20010502/"><cite>XML Schema Part 2: Datatypes.</cite></a>
Paul V. Biron, Ashok Malhotra, editors.
<em>World Wide Web Consortium Working Draft.</em>
</dd><dt class="label"><a id="XHTML" name="XHTML"/>XHTML</dt><dd>
<a href="http://www.w3.org/TR/2000/REC-xhtml1-20000126/"><cite>XHTML(TM) 1.0: The Extensible HyperText Markup Language.</cite></a>
Steven Pemberton, et al.
<em>World Wide Web Consortium, 2000.</em>
</dd><dt class="label"><a id="XMLIS" name="XMLIS"/>XML Infoset</dt><dd>
<a href="http://www.w3.org/TR/2001/CR-xml-infoset-20010514/"><cite>XML Information Set</cite></a>.
John Cowan and Richard Tobin, editors.
<em>World Wide Web Consortium, 1999.</em>
</dd><dt class="label"><a id="XPath" name="XPath"/>XPath</dt><dd>
<a href="http://www.w3.org/TR/1999/REC-xpath-19991116"><cite>XML Path Language</cite></a>
James Clark and Steven DeRose, editors.
<em>World Wide Web Consortium, 1999.</em>
</dd><dt class="label"><a id="XSLT" name="XSLT"/>XSLT</dt><dd>
<a href="http://www.w3.org/TR/1999/REC-xslt-19991116"><cite>XSL Transformations</cite></a>.
James Clark, editor.
<em>World Wide Web Consortium, 1999.</em>
</dd></dl></div><div class="div1">
<h2><a id="impacts" name="impacts"/>C Impacts on Other Standards (Non-Normative)</h2><p>This section has been deleted.</p></div><div class="div1">
<h2><a id="changes" name="changes"/>D Changes since the first edition (Non-Normative)</h2><ol class="enumar"><li><p>
The published errata
(see <a href="http://www.w3.org/2001/06/xmlbase-errata">
http://www.w3.org/2001/06/xmlbase-errata</a>)
have been incorporated;
</p></li><li><p>
The definition of URI reference has been switched from RFC2396 to 3986;
</p></li><li><p>
The xml:base attribute has been redescribed as a
Legacy Extended IRI,
but this does not change its syntax
(the December 2006 PER used
the term "XML Resource Identifier" which was to be defined in an XLink
revision, but that plan has been superseded by the definition of LEIRI
in the W3C Note);
</p></li><li><p>
Implementations are now encouraged to return base “URIs” without escaping
non-URI characters;
</p></li><li><p>
The meanings of xml:base="" and xml:base="#frag" have been clarified;
</p></li><li><p>
The expected reference to XML Base in the forthcoming XML Media Types RFC
(“son of 3023”) has been noted;
</p></li><li><p>
It has been clarified that normal validity rules apply to the xml:base attribute;
</p></li><li><p>
The out-of-date appendix describing effects on other standards has been removed;
</p></li><li><p>
Various minor editorial changes have been made.
</p></li></ol></div></div></body></html>