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2 Network Working Group J. Snell
3 Internet-Draft June 9, 2009
4 Updates: 4287 (if approved)
5 Intended status: Informational
6 Expires: December 11, 2009
8 Atom Bidirectional Attribute
9 draft-snell-atompub-bidi-08.txt
11 Status of this Memo
13 This Internet-Draft is submitted to IETF in full conformance with the
14 provisions of BCP 78 and BCP 79.
16 Internet-Drafts are working documents of the Internet Engineering
17 Task Force (IETF), its areas, and its working groups. Note that
18 other groups may also distribute working documents as Internet-
19 Drafts.
21 Internet-Drafts are draft documents valid for a maximum of six months
22 and may be updated, replaced, or obsoleted by other documents at any
23 time. It is inappropriate to use Internet-Drafts as reference
24 material or to cite them other than as "work in progress."
26 The list of current Internet-Drafts can be accessed at
27 http://www.ietf.org/ietf/1id-abstracts.txt.
29 The list of Internet-Draft Shadow Directories can be accessed at
30 http://www.ietf.org/shadow.html.
32 This Internet-Draft will expire on December 11, 2009.
34 Copyright Notice
36 Copyright (c) 2009 IETF Trust and the persons identified as the
37 document authors. All rights reserved.
39 This document is subject to BCP 78 and the IETF Trust's Legal
40 Provisions Relating to IETF Documents in effect on the date of
41 publication of this document (http://trustee.ietf.org/license-info).
42 Please review these documents carefully, as they describe your rights
43 and restrictions with respect to this document.
45 Abstract
47 This document adds a new attribute to the Atom Syndication Format
48 used to indicate the base directionality of directionally-neutral
49 characters.
51 Table of Contents
53 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
54 1.1. Namespace . . . . . . . . . . . . . . . . . . . . . . . . . 3
55 1.2. Notational Conventions . . . . . . . . . . . . . . . . . . 3
56 2. The "dir" Attribute . . . . . . . . . . . . . . . . . . . . . . 3
57 2.1. Direction Guessing . . . . . . . . . . . . . . . . . . . . 5
58 3. Security Considerations . . . . . . . . . . . . . . . . . . . . 5
59 4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5
60 5. References . . . . . . . . . . . . . . . . . . . . . . . . . . 6
61 5.1. Normative References . . . . . . . . . . . . . . . . . . . 6
62 5.2. Informative References . . . . . . . . . . . . . . . . . . 6
63 Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . . 6
64 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 7
66 1. Introduction
68 This document updates the Atom Syndication Format [RFC4287] by adding
69 a new "dir" attribute used to define the base directionality of
70 directionally-neutral characters contained within an Atom document.
72 1.1. Namespace
74 The XML Namespaces URI [W3C.REC-xml-names-19990114] for the Atom
75 Syndication Format [RFC4287] is:
76 http://www.w3.org/2005/Atom
78 1.2. Notational Conventions
80 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
81 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
82 document are to be interpreted as described in BCP 14, [RFC2119].
84 The Atom Syndication Format [RFC4287] is specified using terms from
85 the XML Infoset [W3C.REC-xml-infoset-20040204]. This specification
86 uses a shorthand form for two commons terms: The phrase "Information
87 Item" is omitted when naming Element and Attribute Information Items.
88 Therefore, when this specification uses the term "element," it is
89 referring to an Element Information Item in Infoset terms. Likewise,
90 when it uses the term "attribute," it is referring to an Attribute
91 Information Item.
93 Portions this specification are illustrated with fragments of a non-
94 normative RELAX NG Compact schema [RELAXNG]. However, the text of
95 this specification provides the sole definition of conformance.
97 2. The "dir" Attribute
99 The "dir" attribute specifies the base direction of directionally-
100 neutral text [ISO10646] in an Atom document. Possible values for the
101 attribute are "ltr" and "rtl" indicating "left-to-right" and "right-
102 to-left" respectively, or an empty string indicating that no base-
103 direction is specified. If a dir attribute is not provided, the
104 value MUST be assumed to be an empty string. The attribute can
105 appear on any element in an Atom document.
107 atomCommonAttributes =
108 attribute xml:base { atomUri }?,
109 attribute xml:lang { atomLanguageTag }?,
110 attribute dir { "ltr" | "rtl" | "" }?,
111 undefinedAttribute*
113 The direction specified by "dir" applies to elements and attributes
114 whose values are specified as being "Language-Sensitive" as defined
115 by Section 2 of [RFC4287]. The direction specified by the attribute
116 is inherited by descendant elements and attributes and may be
117 overridden.
119 Values other than "ltr", "rtl" and "" MUST be ignored and processed
120 as if the dir attribute was not present; Atom processors MUST NOT
121 stop processing or signal an error. The value of the attribute is
122 not case-sensitive.
124 The "dir" attribute is an extension to the Atom vocabulary that will
125 be treated as unknown foreign markup by existing Atom processors that
126 have not been explicitly implemented to support the "dir" attribute.
127 As per the rules specified in [RFC4287], such processors are required
128 to ignore unknown foreign markup and continue processing as if the
129 markup does not exist.
131 Example atom:feed with right-to-left directionality
133
134
135 ٹٺٻ
136 ...
137
139 If an Atom document contains bidirectional text, the Unicode
140 Bidirectional Algorithm [UAX9] SHOULD be used to render that text.
141 Because consumers of Atom documents vary broadly in the way they
142 display text, the "ltr" and "rtl" values do not necessarily open an
143 additional level of embedding or override with respect to the
144 bidirectional algorithm. Consuming applications that render
145 bidirectional text are responsible for determining the appropriate
146 level of embedding. If the dir attribute value is "rtl", Atom
147 processors that display affected text MAY choose to right-align that
148 text as per the rules described in Section 8 of
149 [W3C.REC-html401-19991224].
151 When Atom Text Constructs or the atom:content elements contain
152 bidirectional text and the type attribute value is either "html" or
153 "xhtml", the bidirectional markup mechanisms specific to each format
154 SHOULD be used. The value of the "dir" attribute does define the
155 base directionality of Language-Sensitive text within Text Constructs
156 and atom:content elements regardless of the value of the type
157 attribute.
159 Example atom:feed with bidirectional XHTML:
161
162
163 ...
164
165
168
169 ...
170
172 Unicode bidirectional control characters MAY also be used within
173 attributes and element values to indicate the directionality of text
174 or to modify the default operation of the Bidirectional Algorithm.
175 Implementers are reminded that unexpected results could occur when
176 using both the "dir" attribute and the Unicode control characters
177 within a single document.
179 2.1. Direction Guessing
181 In Atom documents that do not contain a "dir" attribute, it is
182 possible to apply heuristics to guess the base directionality of text
183 in the document. Such heuristics can take into consideration the in-
184 scope language context established by the use of the xml:lang
185 attribute or an analysis of the directional properties of the Unicode
186 characters used within the text. Such guessing algorithms can
187 produce reasonably acceptable results in many cases but cannot be
188 guaranteed to produce correct results in every case. For this
189 reason, explicit determination of text direction using the "dir"
190 attribute is preferred over any guessing algorithm.
192 For compatibility with existing Atom documents that rely on direction
193 guessing, user agents MAY perform direction guessing in documents
194 that do not contain a "dir" attribute but they SHOULD NOT do so when
195 an in-scope "dir" attribute is provided.
197 3. Security Considerations
199 The security considerations discussed in [RFC4287] Section 8 apply.
201 4. IANA Considerations
203 No IANA actions are required by this document.
205 5. References
206 5.1. Normative References
208 [ISO10646]
209 International Organization for Standardization, "ISO/IEC
210 10646:2003: Information Technology - Universal Multiple-
211 Octet Coded Character Set (UCS)", December 2003.
213 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
214 Requirement Levels", BCP 14, RFC 2119, March 1997.
216 [RFC4287] Nottingham, M., Ed. and R. Sayre, Ed., "The Atom
217 Syndication Format", RFC 4287, December 2005.
219 [UAX9] Davis, M., "Unicode Standard Annex #9: The Bidirectional
220 Algorithm", September 2006.
222 [W3C.REC-xml-infoset-20040204]
223 Tobin, R. and J. Cowan, "XML Information Set (Second
224 Edition)", World Wide Web Consortium Recommendation REC-
225 xml-infoset-20040204, February 2004,
226 .
228 [W3C.REC-xml-names-19990114]
229 Bray, T., Hollander, D., and A. Layman, "Namespaces in
230 XML", World Wide Web Consortium FirstEdition REC-xml-
231 names-19990114, January 1999,
232 .
234 5.2. Informative References
236 [RELAXNG] Clark, J., "RELAX NG Compact Syntax", December 2001, .
240 [W3C.REC-html401-19991224]
241 Raggett, D., Hors, A., and I. Jacobs, "HTML 4.01
242 Specification", World Wide Web Consortium
243 Recommendation REC-html401-19991224, December 1999,
244 .
246 Appendix A. Acknowledgements
248 The author gratefully acknowledges the feedback from the Atom
249 Publishing Format and Protocol Working Group.
251 Author's Address
253 James M Snell
255 Email: jasnell@us.ibm.com
256 URI: http://www.ibm.com