1 Independent Submission P. Resnick 2 Request for Comments: 5895 Qualcomm Incorporated 3 Category: Informational P. Hoffman 4 ISSN: 2070-1721 VPN Consortium 5 September 2010 6 7 8 Mapping Characters for 9 Internationalized Domain Names in Applications (IDNA) 2008 10 11 Abstract 12 13 In the original version of the Internationalized Domain Names in 14 Applications (IDNA) protocol, any Unicode code points taken from user 15 input were mapped into a set of Unicode code points that "made 16 sense", and then encoded and passed to the domain name system (DNS). 17 The IDNA2008 protocol (described in RFCs 5890, 5891, 5892, and 5893) 18 presumes that the input to the protocol comes from a set of 19 "permitted" code points, which it then encodes and passes to the DNS, 20 but does not specify what to do with the result of user input. This 21 document describes the actions that can be taken by an implementation 22 between receiving user input and passing permitted code points to the 23 new IDNA protocol. 24 25 Status of This Memo 26 27 This document is not an Internet Standards Track specification; it is 28 published for informational purposes. 29 30 This is a contribution to the RFC Series, independently of any other 31 RFC stream. The RFC Editor has chosen to publish this document at 32 its discretion and makes no statement about its value for 33 implementation or deployment. Documents approved for publication by 34 the RFC Editor are not a candidate for any level of Internet 35 Standard; see Section 2 of RFC 5741. 36 37 Information about the current status of this document, any errata, 38 and how to provide feedback on it may be obtained at 39 http://www.rfc-editor.org/info/rfc5895. 40 41 42 43 44 45 46 47 48 49 50 51 52 Resnick & Hoffman Informational [Page 1] 53 RFC 5895 IDNA Mapping September 2010 54 55 56 Copyright Notice 57 58 Copyright (c) 2010 IETF Trust and the persons identified as the 59 document authors. All rights reserved. 60 61 This document is subject to BCP 78 and the IETF Trust's Legal 62 Provisions Relating to IETF Documents 63 (http://trustee.ietf.org/license-info) in effect on the date of 64 publication of this document. Please review these documents 65 carefully, as they describe your rights and restrictions with respect 66 to this document. 67 68 1. Introduction 69 70 This document describes the operations that can be applied to user 71 input in order to get it into a form that is acceptable by the 72 Internationalized Domain Names in Applications (IDNA) protocol 73 [IDNA2008protocol]. It includes a general implementation procedure 74 for mapping. 75 76 It should be noted that this document does not specify the behavior 77 of a protocol that appears "on the wire". It describes an operation 78 that is to be applied to user input in order to prepare that user 79 input for use in an "on the network" protocol. As unusual as this 80 may be for a document concerning Internet protocols, it is necessary 81 to describe this operation for implementors who may have designed 82 around the original IDNA protocol (herein referred to as IDNA2003), 83 which conflates this user-input operation into the protocol. 84 85 It is very important to note that there are many potential valid 86 mappings of characters from user input. The mapping described in 87 this document is the basis for other mappings, and is not likely to 88 be useful without modification. Any useful mapping will have 89 features designed to reduce the surprise for users and is likely to 90 be slightly (or sometimes radically) different depending on the 91 locale of the user, the type of input being used (such as typing, 92 copy-and-paste, voice, and so on), the type of application used, etc. 93 Although most common mappings will probably produce similar results 94 for the same input, there will be subtle differences between 95 applications. 96 97 1.1. The Dividing Line between User Interface and Protocol 98 99 The user interface to applications is much more complicated than most 100 network implementers think. When we say "the user enters an 101 internationalized domain name in the application", we are talking 102 about a very complex process that encompasses everything from the 103 user formulating the name and deciding which symbols to use to 104 105 106 107 Resnick & Hoffman Informational [Page 2] 108 RFC 5895 IDNA Mapping September 2010 109 110 111 express that name, to the user entering the symbols into the computer 112 using some input method (be it a keyboard, a stylus, or even a voice 113 recognition program), to the computer interpreting that input (be it 114 keyboard scan codes, a graphical representation, or digitized sounds) 115 into some representation of those symbols, through finally 116 normalizing those symbols into a particular character repertoire in 117 an encoding recognizable to IDNA processes and the domain name 118 system. 119 120 Considerations for a user interface for internationalized domain 121 names involves taking into account culture, context, and locale for 122 any given user. A simple and well-known example is the lowercasing 123 of the letter LATIN CAPITAL LETTER I (U+0049) when it is used in the 124 Turkish and other languages. A capital "I" in Turkish is properly 125 lowercased to a LATIN SMALL LETTER DOTLESS I (U+0131), not to a LATIN 126 SMALL LETTER I (U+0069). This lowercasing is clearly dependent on 127 the locale of the system and/or the locale of the user. Using a 128 single context-free mapping without considering the user interface 129 properties has the potential of doing exactly the wrong thing for the 130 user. 131 132 The original version of IDNA conflated user interface processing and 133 protocol. It took whatever characters the user produced in whatever 134 encoding the application used, assumed some conversion to Unicode 135 code points, and then without regard to context, locale, or anything 136 about the user's intentions, mapped them into a particular set of 137 other characters, and then re-encoded them in Punycode, in order to 138 have the entire operation be contained within the protocol. Ignoring 139 context, locale, and user preference in the IDNA protocol made life 140 significantly less complicated for the application developer, but at 141 the expense of violating the principle of "least user surprise" for 142 consumers and producers of domain names. 143 144 In IDNA2008, the dividing line between "user interface" and 145 "protocol" is clear. The IDNA2008 specification defines the protocol 146 part of IDNA: it explicitly does not deal with the user interface. 147 Mappings such as the one described in this document explicitly deal 148 with the user interface and not the protocol. That is, a mapping is 149 only to be applied before a string of characters is treated as a 150 domain name (in the "user interface") and is never to be applied 151 during domain name processing (in the "protocol"). 152 153 1.2. The Design of This Mapping 154 155 The user interface mapping in this document is a set of expansions to 156 IDNA2008 that are meant to be sensible and friendly and mostly 157 obvious to people throughout the world when using typical 158 applications with domain names that are entered by hand. It is also 159 160 161 162 Resnick & Hoffman Informational [Page 3] 163 RFC 5895 IDNA Mapping September 2010 164 165 166 designed to let applications be mostly backwards compatible with 167 IDNA2003. By definition, it cannot meet all of those design goals 168 for all people, and in fact is known to fail on some of those goals 169 for quite large populations of people. 170 171 A good mapping in the real world might use the "sensible and friendly 172 and mostly obvious" design goal but come up with a different 173 algorithm. Many algorithms will have results that are close to what 174 is described here, but will differ in assumptions about the users' 175 way of thinking or typing. Having said that, it is likely that some 176 mappings will be significantly different. For example, a mapping 177 might apply to a spoken user interface instead of a typed one. 178 Another example is that a mapping might be different for users that 179 are typing than for users that are copying-and-pasting from different 180 applications. Yet another example is that a user interface that 181 allows typed input that is transliterated from Latin characters could 182 have very different mappings than one that applies to typing in other 183 character sets; this would be typical in a Pinyin input method for 184 Chinese characters. 185 186 2. The General Procedure 187 188 This section defines a general algorithm that applications ought to 189 implement in order to produce Unicode code points that will be valid 190 under the IDNA protocol. An application might implement the full 191 mapping as described below, or it can choose a different mapping. 192 This mapping is very general and was designed to be acceptable to the 193 widest user community, but as stated above, it does not take into 194 account any particular context, culture, or locale. 195 196 The general algorithm that an application (or the input method 197 provided by an operating system) ought to use is relatively 198 straightforward: 199 200 1. Uppercase characters are mapped to their lowercase equivalents by 201 using the algorithm for mapping case in Unicode characters. This 202 step was chosen because the output will behave more like ASCII 203 host names behave. 204 205 2. Fullwidth and halfwidth characters (those defined with 206 Decomposition Types <wide> and <narrow>) are mapped to their 207 decomposition mappings as shown in the Unicode character 208 database. This step was chosen because many input mechanisms, 209 particularly in Asia, do not allow you to easily enter characters 210 in the form used by IDNA2008. Even if they do allow the correct 211 character form, the user might not know which form they are 212 entering. 213 214 215 216 217 Resnick & Hoffman Informational [Page 4] 218 RFC 5895 IDNA Mapping September 2010 219 220 221 3. All characters are mapped using Unicode Normalization Form C 222 (NFC). This step was chosen because it maps combinations of 223 combining characters into canonical composed form. As with the 224 fullwidth/halfwidth mapping, users are not generally aware of the 225 particular form of characters that they are entering, and 226 IDNA2008 requires that only the canonical composed forms from NFC 227 be used. 228 229 4. [IDNA2008protocol] is specified such that the protocol acts on 230 the individual labels of the domain name. If an implementation 231 of this mapping is also performing the step of separation of the 232 parts of a domain name into labels by using the FULL STOP 233 character (U+002E), the IDEOGRAPHIC FULL STOP character (U+3002) 234 can be mapped to the FULL STOP before label separation occurs. 235 There are other characters that are used as "full stops" that one 236 could consider mapping as label separators, but their use as such 237 has not been investigated thoroughly. This step was chosen 238 because some input mechanisms do not allow the user to easily 239 enter proper label separators. Only the IDEOGRAPHIC FULL STOP 240 character (U+3002) is added in this mapping because the authors 241 have not fully investigated the applicability of other characters 242 and the environments where they should and should not be 243 considered domain name label separators. 244 245 Note that the steps above are ordered. 246 247 Definitions for the rules in this algorithm can be found in 248 [Unicode52]. Specifically: 249 250 o Unicode Normalization Form C can be found in Annex #15 of 251 [Unicode-UAX15]. 252 253 o In order to map uppercase characters to their lowercase 254 equivalents (defined in Section 3.13 of [Unicode52]), first map 255 characters to the "Lowercase_Mapping" property (the "<lower>" 256 entry in the second column) in 257 <http://www.unicode.org/Public/UNIDATA/SpecialCasing.txt>, if any. 258 Then, map characters to the "Simple_Lowercase_Mapping" property 259 (the fourteenth column) in 260 <http://www.unicode.org/Public/UNIDATA/UnicodeData.txt>, if any. 261 262 o In order to map fullwidth and halfwidth characters to their 263 decomposition mappings, map any character whose 264 "Decomposition_Type" (contained in the first part of the sixth 265 column) in <http://www.unicode.org/Public/UNIDATA/UnicodeData.txt> 266 is either "<wide>" or "<narrow>" to the "Decomposition_Mapping" of 267 that character (contained in the second part of the sixth column) 268 in <http://www.unicode.org/Public/UNIDATA/UnicodeData.txt>. 269 270 271 272 Resnick & Hoffman Informational [Page 5] 273 RFC 5895 IDNA Mapping September 2010 274 275 276 o The Unicode Character Database [TR44] has useful descriptions of 277 the contents of these files. 278 279 If the mappings in this document are applied to versions of Unicode 280 later than Unicode 5.2, the later versions of the Unicode Standard 281 should be consulted. 282 283 These form a minimal set of mappings that an application should 284 strongly consider doing. Of course, there are many others that might 285 be done. 286 287 3. Implementing This Mapping 288 289 If you are implementing a mapping for an application or operating 290 system by using exactly the four steps in Section 2, the authors of 291 this document have a request: please don't. We mean it. Section 2 292 does not describe a universal mapping algorithm because, as we said, 293 there is no universally-applicable mapping algorithm. 294 295 If you read the material in Section 2 without reading Section 1, go 296 back and carefully read all of Section 1; in many ways, Section 1 is 297 more important than Section 2. Further, you can probably think of 298 user interface considerations that we did not list in Section 1. If 299 you did read Section 1 but somehow decided that the algorithm in 300 Section 2 is completely correct for the intended users of your 301 application or operating system, you are probably not thinking hard 302 enough about your intended users. 303 304 4. Security Considerations 305 306 This document suggests creating mappings that might cause confusion 307 for some users while alleviating confusion in other users. Such 308 confusion is not covered in any depth in this document (nor in the 309 other IDNA-related documents). 310 311 5. Acknowledgements 312 313 This document is the product of many contributions from numerous 314 people in the IETF. 315 316 317 318 319 320 321 322 323 324 325 326 327 Resnick & Hoffman Informational [Page 6] 328 RFC 5895 IDNA Mapping September 2010 329 330 331 6. Normative References 332 333 [IDNA2008protocol] Klensin, J., "Internationalized Domain Names in 334 Applications (IDNA): Protocol", RFC 5891, 335 August 2010. 336 337 [TR44] The Unicode Consortium, "Unicode Technical Report 338 #44: Unicode Character Database", September 2009, 339 <http://www.unicode.org/reports/tr44/ 340 tr44-4.html>. 341 342 [Unicode-UAX15] The Unicode Consortium, "Unicode Standard Annex 343 #15: Unicode Normalization Forms, Revision 31", 344 September 2009, <http://www.unicode.org/reports/ 345 tr15/tr15-31.html>. 346 347 [Unicode52] The Unicode Consortium. The Unicode Standard, 348 Version 5.2.0, defined by: "The Unicode Standard, 349 Version 5.2.0", (Mountain View, CA: The Unicode 350 Consortium, 2009. ISBN 978-1-936213-00-9). 351 <http://www.unicode.org/versions/Unicode5.2.0/>. 352 353 Authors' Addresses 354 355 Peter W. Resnick 356 Qualcomm Incorporated 357 5775 Morehouse Drive 358 San Diego, CA 92121-1714 359 US 360 361 Phone: +1 858 651 4478 362 EMail: firstname.lastname@example.org 363 URI: http://www.qualcomm.com/~presnick/ 364 365 366 Paul Hoffman 367 VPN Consortium 368 127 Segre Place 369 Santa Cruz, CA 95060 370 US 371 372 Phone: 1-831-426-9827 373 EMail: email@example.com 374 375 376 377 378 379 380 381 382 Resnick & Hoffman Informational [Page 7] 383
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