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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 <div xmlns="http://www.w3.org/1999/xhtml"> 166 <p dir="rtl">ٹٺٻ</p> 167 </div> 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