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W3C

XML Base (Second Edition)

W3C Recommendation 28 January 2009

This version:
http://www.w3.org/TR/2009/REC-xmlbase-20090128/
Latest version:
http://www.w3.org/TR/xmlbase/
Previous versions:
http://www.w3.org/TR/2001/REC-xmlbase-20010627/http://www.w3.org/TR/2008/PER-xmlbase-20080320/
Editors:
Jonathan Marsh, Microsoft<jmarsh@microsoft.com>
Richard Tobin, University of Edinburgh<richard@inf.ed.ac.uk>

Please refer to theerrata for this document, which may include normative corrections.

See alsotranslations.

This document is also available in these non-normative formats:HTML with diff markup and XML.

Copyright © 2009 W3C® (MIT,ERCIM,Keio), All Rights Reserved. W3Cliability,trademark anddocument use rules apply.

Abstract

This documentdescribes a facility, similar to that of HTML BASE, for defining base URIs for parts of XML documents.

Status of this Document

This section describes the status of this document at the time of itspublication. Other documents may supersede this document. A list ofcurrent W3C publications and the latest revision of this technicalreport can be found in theW3C technical reports indexat http://www.w3.org/TR/.

This document has been produced by theW3C XML Core Working Groupas part of theW3C XML Activity.The English version of this specification is the only normative version.However, for translations of this document, seehttp://www.w3.org/2003/03/Translations/byTechnology?technology=xmlbase

This document is an Edited Recommendation of the W3C. It supersedes the previousW3C Recommendationof 27 June 2001.This second edition is not a new version of XML Base;its purpose is to clarify a number of issues thathave become apparent since the first edition was published.Some of these were first published as separate errata(http://www.w3.org/2001/06/xmlbase-errata),others were published in a public editor's draft in November 2006(http://www.w3.org/XML/2006/11/xmlbase-2e/Overview.html),and a PER in December 2006(http://www.w3.org/TR/2006/PER-xmlbase-20061220/).The changes are summarized in anappendix.

Please report errors in this document to the public mailing listwww-xml-linking-comments@w3.org;publicarchivesare available.

There is no implementation report or test suite for this specification, butthere is a document describingmethods oftesting XML Base conformance.

This document has been reviewed by W3C Members, by softwaredevelopers, and by other W3C groups and interested parties, and isendorsed by the Director as a W3C Recommendation. It is a stabledocument and may be used as reference material or cited from anotherdocument. W3C's role in making the Recommendation is to draw attentionto the specification and to promote its widespread deployment. Thisenhances the functionality and interoperability of the Web.

This document is governed by the24 January 2002 CPPas amended by theW3C Patent Policy Transition Procedure.W3C maintains apubliclist of any patent disclosuresmade in connection with the deliverables ofthe group; that page also includes instructions for disclosing a patent.An individual who has actual knowledge of a patent which the individualbelieves containsEssential Claim(s)must disclose the information in accordance withsection 6 of the W3C Patent Policy.

Table of Contents

1Introduction
2Terminology
3xml:base Attribute
    3.1URI Reference Encoding and Escaping
4Resolving Relative URIs
    4.1Relation to RFC 3986
    4.2Granularity of base URI information
    4.3Matching URIs with base URIs
    4.4Interpretation of same-document references
5Conformance

Appendices

AReferences
BReferences (Non-Normative)
CImpacts on Other Standards (Non-Normative)
DChanges since the first edition (Non-Normative)

1 Introduction

The XML Linking Language[XLink] defines Extensible Markup Language (XML) 1.0[XML] constructs to describe links between resources. One of the stated requirements on XLink is to support HTML[HTML 4.01] 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.

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 namedxml:base.

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 ofxml:base attributes in applications based on specifications that do not have direct or indirect normative reference to XML Base is undefined.

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?

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.

2 Terminology

[Definition: The key wordsmust,must not,required,shall,shall not,should,should not,recommended,may, andoptional in this specification are to be interpreted as described in[RFC 2119].]

The termsbase URI andrelative URI are used in this specification as they are defined in[RFC 3986].

3xml:base Attribute

The attributexml:basemay 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 a Legacy Extended IRI (LEIRI) as defined in the W3C Note "Legacy extended IRIs for XML resource identification"[LEIRI].

In namespace-aware XML processors, the "xml" prefix is bound to the namespace namehttp://www.w3.org/XML/1998/namespace as described in Namespaces in XML[XML Names]. Note thatxml:base can be still used by non-namespace-aware processors.

An example ofxml:base in a simple document containing XLinks follows. XLink normatively references XML Base for interpretation of relative URI references inxlink:href attributes.

<?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>

The URIs in this example resolve to full URIs as follows:

Note:

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.

3.1 URI Reference Encoding and Escaping

The value of anxml:base 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 inxml:base values.)

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 elementshould calculate and return the value without escaping.

In the example below, the base URI of elemente2 should be returned as "http://example.org/wine/rosé".

<?xml version="1.0"?><e1 xml:base="http://example.org/wine/">  <e2 xml:base="rosé"/></e1>

4 Resolving Relative URIs

4.1 Relation to RFC 3986

RFC 3986[RFC 3986] 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):

  1. The base URI is embedded in the document's content.

  2. The base URI is that of the encapsulating entity (message, document, or none).

  3. The base URI is the URI used to retrieve the entity.

  4. The base URI is defined by the context of the application.

Note:

The term "entity" in points #2 and #3 above uses theRFC 3986 meaning of the term. Elsewhere in this document the term "entity" is used in the XML sense.

This document specifies the details of rule #1 for embedding base URI information in the specific case of XML documents.

4.2 Granularity of base URI information

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.

The base URI of a document entity or an external entity is determined byRFC 3986 rules, namely, that the base URI is the URI used to retrieve the document entity or external entity.

The base URI of an element is:

  1. the base URI specified by anxml:base attribute on the element, if one exists, otherwise

  2. the base URI of the element's parent element within the documententity or external entity, if one exists, otherwise

  3. the base URI of the document entity or external entity containing the element.

Note:

It follows that the base URI specified by anxml:base attribute is inherited by descendant elements within the same entity until another element with anxml:base attribute is encountered.

The base URI of an element bearing anxml:base attribute with a valuethat is not a valid Legacy Extended IRI is application dependent.

4.3 Matching URIs with base URIs

The base URI corresponding to a given relative URI appearing in an XML document is determined as follows:

Note:

The presence ofxml:base attributes mightlead to unexpected results in the case where the attribute valueis provided, not directly in the XML document entity, but via adefault attribute.For instance, such a declaration in an external entity mightnot be read by software which is based ona non-validating XML processor. Defaulting attributesthrough an external mechanism such as XML Schema may also lead tounexpected results; even if a validating processor is used by theapplication, the addition of defaulted attributes subsequent tocreation of the infoset can cause xml:base attributes to get out ofsync with the [base URI] infoset property. For these reasons,xml:base valuesshouldbe provided either directly in the XML document instanceor via default attributes declaredin the internal subset of the DTD.

4.4 Interpretation of same-document references

RFC 3986 defines certain relative URI references, in particular theempty string and those of the form#fragment,assame-document references. Dereferencingof same-document references is handled specially. However, their use asthe value of anxml:base attribute does not involvedereferencing, and XML Base processors should resolve them inthe usual way. In particular,xml:base="" does notreset the base URI to that of the containing document.

Note:

Some existing processorsdo treat thesexml:basevalues as resetting the base URI to that of the containing document,so the use of such values is strongly discouraged.

5 Conformance

An application conforms to XML Base if it calculates base URIs in accordance with the conditions set forth in this specification.

A References

RFC 2119
RFC 2119: Key words for use in RFCs to Indicate Requirement Levels.Internet Engineering Task Force, 1997.
RFC 3986
RFC 3986: Uniform Resource Identifier (URI): Generic Syntax.Internet Engineering Task Force, 2005.
LEIRI
Legacy extended IRIs for XML resource identification. Henry S. Thompson, Richard Tobin, and Norman Walsh, editors.World Wide Web Consortium.
XML
Extensible Markup Language (XML) 1.0. Tim Bray et al.World Wide Web Consortium.
XML Names
Namespaces in XML 1.0.. Tim Bray et al.World Wide Web Consortium.

B References (Non-Normative)

HTML 4.01
HTML 4.01 Specification. Dave Raggett, Arnaud Le Hors, Ian Jacobs, editors.World Wide Web Consortium, 1999.
XLink
XML Linking Language (XLink). Steve DeRose, Eve Maler, David Orchard, and Ben Trafford, editors.World Wide Web Consortium, 2000.
XML Datatypes
XML Schema Part 2: Datatypes. Paul V. Biron, Ashok Malhotra, editors.World Wide Web Consortium Working Draft.
XHTML
XHTML(TM) 1.0: The Extensible HyperText Markup Language. Steven Pemberton, et al.World Wide Web Consortium, 2000.
XML Infoset
XML Information Set. John Cowan and Richard Tobin, editors.World Wide Web Consortium, 1999.
XPath
XML Path Language James Clark and Steven DeRose, editors.World Wide Web Consortium, 1999.
XSLT
XSL Transformations. James Clark, editor.World Wide Web Consortium, 1999.

C Impacts on Other Standards (Non-Normative)

This section has been deleted.

D Changes since the first edition (Non-Normative)

  1. The published errata(seehttp://www.w3.org/2001/06/xmlbase-errata)have been incorporated;

  2. The definition of URI reference has been switched from RFC2396 to 3986;

  3. The xml:base attribute has been redescribed as a Legacy Extended IRI,but this does not change its syntax(the December 2006 PER usedthe term "XML Resource Identifier" which was to be defined in an XLinkrevision, but that plan has been superseded by the definition of LEIRIin the W3C Note);

  4. Implementations are now encouraged to return base “URIs” without escapingnon-URI characters;

  5. The meanings of xml:base="" and xml:base="#frag" have been clarified;

  6. The expected reference to XML Base in the forthcoming XML Media Types RFC(“son of 3023”) has been noted;

  7. It has been clarified that normal validity rules apply to the xml:base attribute;

  8. The out-of-date appendix describing effects on other standards has been removed;

  9. Various minor editorial changes have been made.

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