1 Network Working Group J. Klensin 2 Request for Comments: 4690 P. Faltstrom 3 Category: Informational Cisco Systems 4 C. Karp 5 Swedish Museum of Natural History 6 IAB 7 September 2006 8 9 10 Review and Recommendations for Internationalized Domain Names (IDNs) 11 12 Status of This Memo 13 14 This memo provides information for the Internet community. It does 15 not specify an Internet standard of any kind. Distribution of this 16 memo is unlimited. 17 18 Copyright Notice 19 20 Copyright (C) The Internet Society (2006). 21 22 Abstract 23 24 This note describes issues raised by the deployment and use of 25 Internationalized Domain Names. It describes problems both at the 26 time of registration and for use of those names in the DNS. It 27 recommends that IETF should update the RFCs relating to IDNs and a 28 framework to be followed in doing so, as well as summarizing and 29 identifying some work that is required outside the IETF. In 30 particular, it proposes that some changes be investigated for the 31 Internationalizing Domain Names in Applications (IDNA) standard and 32 its supporting tables, based on experience gained since those 33 standards were completed. 34 35 Table of Contents 36 37 1. Introduction ....................................................3 38 1.1. The Role of IDNs and This Document .........................3 39 1.2. Status of This Document and Its Recommendations ............4 40 1.3. The IDNA Standard ..........................................4 41 1.4. Unicode Documents ..........................................5 42 1.5. Definitions ................................................5 43 1.5.1. Language ............................................6 44 1.5.2. Script ..............................................6 45 1.5.3. Multilingual ........................................6 46 1.5.4. Localization ........................................7 47 1.5.5. Internationalization ................................7 48 49 50 51 52 Klensin, et al. Informational [Page 1] 53 RFC 4690 IAB -- IDN Next Steps September 2006 54 55 56 1.6. Statements and Guidelines ..................................7 57 1.6.1. IESG Statement ......................................8 58 1.6.2. ICANN Statements ....................................8 59 2. General Problems and Issues ....................................11 60 2.1. User Conceptions, Local Character Sets, and Input issues ..11 61 2.2. Examples of Issues ........................................13 62 2.2.1. Language-Specific Character Matching ...............13 63 2.2.2. Multiple Scripts ...................................13 64 2.2.3. Normalization and Character Mappings ...............14 65 2.2.4. URLs in Printed Form ...............................16 66 2.2.5. Bidirectional Text .................................17 67 2.2.6. Confusable Character Issues ........................17 68 2.2.7. The IESG Statement and IDNA issues .................19 69 3. Migrating to New Versions of Unicode ...........................20 70 3.1. Versions of Unicode .......................................20 71 3.2. Version Changes and Normalization Issues ..................21 72 3.2.1. Unnormalized Combining Sequences ...................21 73 3.2.2. Combining Characters and Character Components ......22 74 3.2.3. When does normalization occur? .....................23 75 4. Framework for Next Steps in IDN Development ....................24 76 4.1. Issues within the Scope of the IETF .......................24 77 4.1.1. Review of IDNA .....................................24 78 4.1.2. Non-DNS and Above-DNS Internationalization 79 Approaches .........................................25 80 4.1.3. Security Issues, Certificates, etc. ................25 81 4.1.4. Protocol Changes and Policy Implications ...........27 82 4.1.5. Non-US-ASCII in Local Part of Email Addresses ......27 83 4.1.6. Use of the Unicode Character Set in the IETF .......27 84 4.2. Issues That Fall within the Purview of ICANN ..............28 85 4.2.1. Dispute Resolution .................................28 86 4.2.2. Policy at Registries ...............................28 87 4.2.3. IDNs at the Top Level of the DNS ...................29 88 5. Specific Recommendations for Next Steps ........................29 89 5.1. Reduction of Permitted Character List .....................29 90 5.1.1. Elimination of All Non-Language Characters .........30 91 5.1.2. Elimination of Word-Separation Punctuation .........30 92 5.2. Updating to New Versions of Unicode .......................30 93 5.3. Role and Uses of the DNS ..................................31 94 5.4. Databases of Registered Names .............................31 95 6. Security Considerations ........................................31 96 7. Acknowledgements ...............................................32 97 8. References .....................................................32 98 8.1. Normative References ......................................32 99 8.2. Informative References ....................................33 100 101 102 103 104 105 106 107 Klensin, et al. Informational [Page 2] 108 RFC 4690 IAB -- IDN Next Steps September 2006 109 110 111 1. Introduction 112 113 1.1. The Role of IDNs and This Document 114 115 While IDNs have been advocated as the solution to a wide range of 116 problems, this document is written from the perspective that they are 117 no more and no less than DNS names, reflecting the same requirements 118 for use, stability, and accuracy as traditional "hostnames", but 119 using a much larger collection of permitted characters. In 120 particular, while IDNs represent a step toward an Internet that is 121 equally accessible from all languages and scripts, they, at best, 122 address only a small part of that very broad objective. There has 123 been controversy since IDNs were first suggested about how important 124 they will actually turn out to be; that controversy will probably 125 continue. Accessibility from all languages is an important 126 objective, hence it is important that our standards and definitions 127 for IDNs be smoothly adaptable to additional scripts as they are 128 added to the Unicode character set. 129 130 The utility of IDNs must be evaluated in terms of their application 131 by users and in protocols: the ability to simply put a name into the 132 DNS and retrieve it is not, in and of itself, important. From this 133 point of view, IDNs will be useful and effective if they provide 134 stable and predictable references -- references that are no less 135 stable and predictable, and no less secure, than their ASCII 136 counterparts. 137 138 This combination of objectives and criteria has proven very difficult 139 to satisfy. Experience in developing the IDNA standard and during 140 the initial years of its implementation and deployment suggests that 141 it may be impossible to fully satisfy all of them and that 142 engineering compromises are needed to yield a result that is 143 workable, even if not completely satisfactory. Based on that 144 experience and issues that have been raised, it is now appropriate to 145 review some of the implications of IDNs, the decisions made in 146 defining them, and the foundation on which they rest and determine 147 whether changes are needed and, if so, which ones. 148 149 The design of the DNS itself imposes some additional constraints. If 150 the DNS is to remain globally interoperable, there are specific 151 characteristics that no implementation of IDNs, or the DNS more 152 generally, can change. For example, because the DNS is a global 153 hierarchal administrative namespace with only a single name at any 154 given node, there is one and only one owner of each domain name. 155 Also, when strings are looked up in the DNS, positive responses can 156 only reflect exact matches: if there is no exact match, then one gets 157 an error reply, not a list of near matches or other supplemental 158 information. Searches and approximate matchings are not possible. 159 160 161 162 Klensin, et al. Informational [Page 3] 163 RFC 4690 IAB -- IDN Next Steps September 2006 164 165 166 Finally, because the DNS is a distributed system where any server 167 might cache responses, and later use those cached responses to 168 attempt to satisfy queries before a global lookup is done, every 169 server must use the same matching criteria. 170 171 1.2. Status of This Document and Its Recommendations 172 173 This document reviews the IDN landscape from an IETF perspective and 174 presents the recommendations and conclusions of the IAB, based 175 partially on input from an ad hoc committee charged with reviewing 176 IDN issues and the path forward (see Section 7). Its recommendations 177 are advice to the IETF, or in a few cases to other bodies, for topics 178 to be investigated and actions to be taken if those bodies, after 179 their examinations, consider those actions appropriate. 180 181 1.3. The IDNA Standard 182 183 During 2002, the IETF completed the following RFCs that, together, 184 define IDNs: 185 186 RFC 3454 Preparation of Internationalized Strings ("Stringprep") 187 [RFC3454]. 188 Stringprep is a generic mechanism for taking a Unicode string and 189 converting it into a canonical format. Stringprep itself is just 190 a collection of rules, tables, and operations. Any protocol or 191 algorithm that uses it must define a "Stringprep profile", which 192 specifies which of those rules are applied, how, and with which 193 characteristics. 194 195 RFC 3490 Internationalizing Domain Names in Applications (IDNA) 196 [RFC3490]. 197 IDNA is the base specification in this group. It specifies that 198 Nameprep is used as the Stringprep profile for domain names, and 199 that Punycode is the relevant encoding mechanism for use in 200 generating an ASCII-compatible ("ACE") form of the name. It also 201 applies some additional conversions and character filtering that 202 are not part of Nameprep. 203 204 RFC 3491 Nameprep: A Stringprep Profile for Internationalized Domain 205 Names (IDN) [RFC3491]. 206 Nameprep is designed to meet the specific needs of IDNs and, in 207 particular, to support case-folding for scripts that support what 208 are traditionally known as upper- and lowercase forms of the same 209 letters. The result of the Nameprep algorithm is a string 210 containing a subset of the Unicode Character set, normalized and 211 case-folded so that case-insensitive comparison can be made. 212 213 214 215 216 217 Klensin, et al. Informational [Page 4] 218 RFC 4690 IAB -- IDN Next Steps September 2006 219 220 221 RFC 3492 Punycode: A Bootstring encoding of Unicode for 222 Internationalized Domain Names in Applications (IDNA) [RFC3492]. 223 Punycode is a mechanism for encoding a Unicode string in ASCII 224 characters. The characters used are the same the subset of 225 characters that are allowed in the hostname definition of DNS, 226 i.e., the "letter, digit, and hyphen" characters, sometimes known 227 as "LDH". 228 229 1.4. Unicode Documents 230 231 Unicode is used as the base, and defining, character set for IDNs. 232 Unicode is standardized by the Unicode Consortium, and synchronized 233 with ISO to create ISO/IEC 10646 [ISO10646]. At the time the RFCs 234 mentioned earlier were created, Unicode was at Version 3.2. For 235 reasons explained later, it was necessary to pick a particular, 236 then-current, version of Unicode when IDNA was adopted. 237 Consequently, the RFCs are explicitly dependent on Unicode Version 238 3.2 [Unicode32]. There is, at present, no established mechanism for 239 modifying the IDNA RFCs to use newer Unicode versions (see 240 Section 3.1). 241 242 Unicode is a very large and complex character set. (The term 243 "character set" or "charset" is used in a way that is peculiar to the 244 IETF and may not be the same as the usage in other bodies and 245 contexts.) The Unicode Standard and related documents are created 246 and maintained by the Unicode Technical Committee (UTC), one of the 247 committees of the Unicode Consortium. 248 249 The Consortium first published The Unicode Standard [Unicode10] in 250 1991, and continues to develop standards based on that original work. 251 Unicode is developed in conjunction with the International 252 Organization for Standardization, and it shares its character 253 repertoire with ISO/IEC 10646. Unicode and ISO/IEC 10646 function 254 equivalently as character encodings, but The Unicode Standard 255 contains much more information for implementers, covering -- in depth 256 -- topics such as bitwise encoding, collation, and rendering. The 257 Unicode Standard enumerates a multitude of character properties, 258 including those needed for supporting bidirectional text. The 259 Unicode Consortium and ISO standards do use slightly different 260 terminology. 261 262 1.5. Definitions 263 264 The following terms and their meanings are critical to understanding 265 the rest of this document and to discussions of IDNs more generally. 266 These terms are derived from [RFC3536], which contains additional 267 discussion of some of them. 268 269 270 271 272 Klensin, et al. Informational [Page 5] 273 RFC 4690 IAB -- IDN Next Steps September 2006 274 275 276 1.5.1. Language 277 278 A language is a way that humans interact. The use of language occurs 279 in many forms, including speech, writing, and signing. 280 281 Some languages have a close relationship between the written and 282 spoken forms, while others have a looser relationship. RFC 3066 283 [RFC3066] discusses languages in more detail and provides identifiers 284 for languages for use in Internet protocols. Computer languages are 285 explicitly excluded from this definition. The most recent IETF work 286 in this area, and on script identification (see below), is documented 287 in [RFC4645] and [RFC4646]. 288 289 1.5.2. Script 290 291 A script is a set of graphic characters used for the written form of 292 one or more languages. This definition is the one used in 293 [ISO10646]. 294 295 Examples of scripts are Arabic, Cyrillic, Greek, Han (the so-called 296 ideographs used in writing Chinese, Japanese, and Korean), and 297 "Latin". Arabic, Greek, and Latin are, of course, also names of 298 languages. 299 300 Historically, the script that is known as "Latin" in Unicode and most 301 contexts associated with information technology standards is known in 302 the linguistic community as "Roman" or "Roman-derived". The latter 303 terminology distinguishes between the Latin language and the 304 characters used to write it, especially in Republican times, from the 305 much richer and more decorated script derived and adapted from those 306 characters. Since IDNA is defined using Unicode and that standard 307 used the term "LATIN" in its character names and descriptions, that 308 terminology will be used in this document as well except when 309 "Roman-derived" is needed for clarity. However, readers approaching 310 this document from a cultural or linguistic standpoint should be 311 aware that the use of, or references to, "Latin script" in this 312 document refers to the entire collection of Roman-derived characters, 313 not just the characters used to write the Latin language. Some other 314 issues with script identification and relationships with other 315 standards are discussed in [RFC4646]. 316 317 1.5.3. Multilingual 318 319 The term "multilingual" has many widely-varying definitions and thus 320 is not recommended for use in standards. Some of the definitions 321 relate to the ability to handle international characters; other 322 definitions relate to the ability to handle multiple charsets; and 323 still others relate to the ability to handle multiple languages. 324 325 326 327 Klensin, et al. Informational [Page 6] 328 RFC 4690 IAB -- IDN Next Steps September 2006 329 330 331 While this term has been deprecated for IETF-related uses and does 332 not otherwise appear in this document, a discussion here seemed 333 appropriate since the term is still widely used in some discussions 334 of IDNs. 335 336 1.5.4. Localization 337 338 Localization is the process of adapting an internationalized 339 application platform or application to a specific cultural 340 environment. In localization, the same semantics are preserved while 341 the syntax or presentation forms may be changed. 342 343 Localization is the act of tailoring an application for a different 344 language or script or culture. Some internationalized applications 345 can handle a wide variety of languages. Typical users understand 346 only a small number of languages, so the program must be tailored to 347 interact with users in just the languages they know. 348 349 Somewhat different definitions for localization and 350 internationalization (see below) are used by groups other than the 351 IETF. See [W3C-Localization] for one example. 352 353 1.5.5. Internationalization 354 355 In the IETF, the term "internationalization" is used to describe 356 adding or improving the handling of non-ASCII text in a protocol. 357 Other bodies use the term in other ways, often with subtle variation 358 in meaning. The term "internationalization" is often abbreviated 359 "i18n" (and localization as "l10n"). 360 361 Many protocols that handle text only handle the characters associated 362 with one script (often, a subset of the characters used in writing 363 English text), or leave the question of what character set is used up 364 to local guesswork (which leads to interoperability problems). 365 Adding non-ASCII text to such a protocol allows the protocol to 366 handle more scripts, with the intention of being able to include all 367 of the scripts that are useful in the world. It is naive (sic) to 368 believe that all English words can be written in ASCII, various 369 mythologies notwithstanding. 370 371 1.6. Statements and Guidelines 372 373 When the IDNA RFCs were published, the IESG and ICANN made statements 374 that were intended to guide deployment and future work. In recent 375 months, ICANN has updated its statement and others have also made 376 contributions. It is worth noting that the quality of understanding 377 of internationalization issues as applied to the DNS has evolved 378 379 380 381 382 Klensin, et al. Informational [Page 7] 383 RFC 4690 IAB -- IDN Next Steps September 2006 384 385 386 considerably over the last few years. Organizations that took 387 specific positions a year or more ago might not make exactly the same 388 statements today. 389 390 1.6.1. IESG Statement 391 392 The IESG made a statement on IDNA [IESG-IDN]: 393 394 IDNA, through its requirement of Nameprep [RFC3491], uses 395 equivalence tables that are based only on the characters 396 themselves; no attention is paid to the intended language (if any) 397 for the domain name. However, for many domain names, the intended 398 language of one or more parts of the domain name actually does 399 matter to the users. 400 401 Similarly, many names cannot be presented and used without 402 ambiguity unless the scripts to which their characters belong are 403 known. In both cases, this additional information should be of 404 concern to the registry. 405 406 The statement is longer than this, but these paragraphs are the 407 important ones. The rest of the statement consists of explanations 408 and examples. 409 410 1.6.2. ICANN Statements 411 412 184.108.40.206. Initial ICANN Guidelines 413 414 Soon after the IDNA standards were adopted, ICANN produced an initial 415 version of its "IDN Guidelines" [ICANNv1]. This document was 416 intended to serve two purposes. The first was to provide a basis for 417 releasing the Generic Top Level Domain (gTLD) registries that had 418 been established by ICANN from a contractual restriction on the 419 registration of labels containing hyphens in the third and fourth 420 positions. The second was to provide a general framework for the 421 development of registry policies for the implementation of IDNs. 422 423 One of the key components of this framework prescribed strict 424 compliance with RFCs 3490, 3491, and 3492. With the framework, ICANN 425 specified that IDNA was to be the sole mechanism to be used in the 426 DNS to represent IDNs. 427 428 Limitations on the characters available for inclusion in IDNs were 429 mandated by two mechanisms. The first was by requiring an 430 "inclusion-based approach (meaning that code points that are not 431 explicitly permitted by the registry are prohibited) for identifying 432 permissible 433 434 435 436 437 Klensin, et al. Informational [Page 8] 438 RFC 4690 IAB -- IDN Next Steps September 2006 439 440 441 code points from among the full Unicode repertoire." The second 442 mechanism required the association of every IDN with a specific 443 language, with additional policies also being language based: 444 445 "In implementing the IDN standards, top-level domain registries will 446 (a) associate each registered internationalized domain name with one 447 language or set of languages, 448 (b) employ language-specific registration and administration rules 449 that are documented and publicly available, such as the reservation 450 of all domain names with equivalent character variants in the 451 languages associated with the registered domain name, and, 452 (c) where the registry finds that the registration and administration 453 rules for a given language would benefit from a character variants 454 table, allow registrations in that language only when an appropriate 455 table is available. ... In implementing the IDN standards, top-level 456 domain registries should, at least initially, limit any given domain 457 label (such as a second-level domain name) to the characters 458 associated with one language or set of languages only." 459 460 It was left to each TLD registry to define the character repertoire 461 it would associate with any given language. This led to significant 462 variation from registry to registry, with further heterogeneity in 463 the underlying language-based IDN policies. If the guidelines had 464 made provision for IDN policies also being based on script, a 465 substantial amount of the resulting ambiguity could have been 466 avoided. However, they did not, and the sequence of events leading 467 to the present review of IDNA was thus triggered. 468 469 220.127.116.11. ICANN Version 2 Guidelines 470 471 One of the responses of the TLD registries to what was widely 472 perceived as a crisis situation was to invoke the mechanism described 473 in the initial guidelines: "As the deployment of IDNs proceeds, ICANN 474 and the IDN registries will review these Guidelines at regular 475 intervals, and revise them as necessary based on experience." 476 477 The pivotal requirement was the modification of the guidelines to 478 permit script-based policies for IDNs. Further concern was expressed 479 about the need for realistically implementable mechanisms for the 480 propagation of TLD registry policies into the lower levels of their 481 name trees. In addition to the anticipated increase of constraint on 482 the protocol level, one obvious additional approach would be to 483 replace the guidelines by an instrument that itself had clear status 484 in the IETF's normative framework. A BCP was therefore seen as the 485 appropriate focus for longer-term effort. The most pressing issues 486 would be dealt with in the interim by incremental modification to the 487 guidelines, but no need was seen for the detailed further development 488 of those guidelines once that incremental modification was complete. 489 490 491 492 Klensin, et al. Informational [Page 9] 493 RFC 4690 IAB -- IDN Next Steps September 2006 494 495 496 The outcome of this action was a version 2.0 of the guidelines 497 [ICANNv2], which was endorsed by the ICANN Board on November 8, 2005 498 for a period of nine months. The Board stated further that it "tasks 499 the IDN working group to continue its important work and return to 500 the board with specific IDN improvement recommendations before the 501 ICANN Meeting in Morocco" and "supports the working group's continued 502 action to reframe the guidelines completely in a manner appropriate 503 for further development as a Best Current Practices (BCP) document, 504 to ensure that the Guideline directions will be used deeper into the 505 DNS hierarchy and within TLD's where ICANN has a lesser policy 506 relationship." 507 508 Retaining the inclusion-based approach established in version 1.0, 509 the crucial addition to the policy framework is that: 510 511 "All code points in a single label will be taken from the same script 512 as determined by the Unicode Standard Annex #24: Script Names at 513 http://www.unicode.org/reports/tr24. Exception to this is 514 permissible for languages with established orthographies and 515 conventions that require the commingled use of multiple scripts. In 516 such cases, visually confusable characters from different scripts 517 will not be allowed to coexist in a single set of permissible 518 codepoints unless a corresponding policy and character table is 519 clearly defined." 520 521 Additionally: 522 523 "Permissible code points will not include: (a) line symbol-drawing 524 characters (as those in the Unicode Box Drawing block), (b) symbols 525 and icons that are neither alphanumeric nor ideographic language 526 characters, such as typographic and pictographic dingbats, (c) 527 characters with well-established functions as protocol elements, (d) 528 punctuation marks used solely to indicate the structure of 529 sentences." 530 531 Attention has been called to several points that are not adequately 532 dealt with (if at all) in the version 2.0 guidelines but that ought 533 to be included in the policy framework without waiting for the 534 production and release of a document based on a "best practices" 535 model. The term "BCP" above does not necessarily refer to an IETF 536 consensus document. 537 538 The intention in November 2005 was for the recommended major revision 539 to be put to the ICANN Board prior to its meeting in Morocco (in late 540 June 2006), but for the changes to be collated incrementally and 541 appear in interim version 2.n releases of the guidelines. The IAB's 542 understanding is that, while there has been some progress with this, 543 544 545 546 547 Klensin, et al. Informational [Page 10] 548 RFC 4690 IAB -- IDN Next Steps September 2006 549 550 551 other issues relating to IDNs subsequently diverted much of the 552 energy that was intended to be devoted to the more extensive 553 treatment of the guidelines. 554 555 2. General Problems and Issues 556 557 This section interweaves problems and issues of several types. Each 558 subsection outlines something that is perceived to be a problem or 559 issue "with IDNs", therefore needing correction. Some of these 560 issues can be at least partially resolved by making changes to 561 elements of the IDNA protocol or tables. Others will exist as long 562 as people have expectations of IDNs that are inconsistent with the 563 basic DNS architecture. It is important to identify this entire 564 range of problems because users, registrants, and policy makers often 565 do not understand the protocol and other technical issues but only 566 the difference between what they believe happens or should happen and 567 what actually happens. As long as those differences exist, there 568 will be demands for functionality or policy changes for IDNs. Of 569 course, some of these demands will be less realistic than others, but 570 even the realistic ones should be understood in the same context as 571 the others. 572 573 Most of the issues that have been raised, and that are discussed in 574 this document, exist whether IDNA remains tied to Unicode 3.2 or 575 whether migration to new Unicode versions is contemplated. A 576 migration path is necessary to accommodate newly-coded scripts and to 577 permit the maximum number of languages and scripts to be represented 578 in domain names. However, the migration issues are largely separate 579 from those involving a single Unicode version or Version 3.2 in 580 particular, so they have been separated into this section and 581 Section 3. 582 583 2.1. User Conceptions, Local Character Sets, and Input issues 584 585 The labels of the DNS are just strings of characters that are not 586 inherently tied to a particular language. As mentioned briefly in 587 the Introduction, DNS labels that could not lexically be words in any 588 language are possible and indeed common. There appears to be no 589 reason to impose protocol restrictions on IDNs that would restrict 590 them more than all-ASCII hostname labels have been restricted. For 591 that reason, even describing DNS labels or strings of them as "names" 592 is something of a misnomer, one that has probably added to user 593 confusion about what to expect. 594 595 Ordinarily, people use "words" when they think of things and wish 596 others to think of them too, for example, "orange", "tree", 597 "restaurant" or "Acme Inc". Words are normally in a specific 598 language, such as English or Swedish. The character-string labels 599 600 601 602 Klensin, et al. Informational [Page 11] 603 RFC 4690 IAB -- IDN Next Steps September 2006 604 605 606 supported by the DNS are, as suggested above, not inherently "words". 607 While it is useful, especially for mnemonic value or to identify 608 objects, for actual words to be used as DNS labels, other constraints 609 on the DNS make it impossible to guarantee that it will be possible 610 to represent every word in every language as a DNS label, 611 internationalized or not. 612 613 When writing or typing the label (or word), a script must be selected 614 and a charset must be picked for use with that script. The choice of 615 charset is typically not under the control of the user on a per-word 616 or per-document basis, but may depend on local input devices, 617 keyboard or terminal drivers, or other decisions made by operating 618 system or even hardware designers and implementers. 619 620 If that charset, or the local charset being used by the relevant 621 operating system or application software, is not Unicode, a further 622 conversion must be performed to produce Unicode. How often this is 623 an issue depends on estimates of how widely Unicode is deployed as 624 the native character set for hardware, operating systems, and 625 applications. Those estimates differ widely, but it should be noted 626 that, among other difficulties: 627 628 o ISO 8859 versions [ISO.8859.2003] and even national variations of 629 ISO 646 [ISO.646.1991], are still widely used in parts of Europe; 630 631 o code-table switching methods, typically based on the techniques of 632 ISO 2022 [ISO.2022.1986] are still in general use in many parts of 633 the world, especially in Japan with Shift-JIS and its variations; 634 and 635 636 o computing, systems, and communications in China tend to use one or 637 more of the national "GB" standards rather than native Unicode. 638 639 Additionally, not all charsets define their characters in the same 640 way and not all preexisting coding systems were incorporated into 641 Unicode without changes. Sometimes local distinctions were made that 642 Unicode does not make or vice versa. Consequently, conversion from 643 other systems to Unicode may potentially lose information. 644 645 The Unicode string that results from this processing -- processing 646 that is trivial in a Unicode-native system but that may be 647 significant in others -- is then used as input to IDNA. 648 649 650 651 652 653 654 655 656 657 Klensin, et al. Informational [Page 12] 658 RFC 4690 IAB -- IDN Next Steps September 2006 659 660 661 2.2. Examples of Issues 662 663 While much of the discussion below is stated in terms of Unicode 664 codings and associated rules, the IAB believes that some of the 665 issues are actually not about the Unicode character set per se, but 666 about how distributed matching systems operate in reality, and about 667 what implications the distributed delayed search for stored data that 668 characterizes the DNS has on the mapping algorithms. 669 670 2.2.1. Language-Specific Character Matching 671 672 There are similar words that can be expressed in multiple languages. 673 Consider, for example, the name Torbjorn in Norwegian and Swedish. 674 In Norwegian it is spelled with the character U+00F8 (LATIN SMALL 675 LETTER O WITH STROKE) in the second syllable, while in Swedish it is 676 spelled with U+00F6 (LATIN SMALL LETTER O WITH DIAERESIS). Those 677 characters are not treated as equivalent according to the Unicode 678 Standard and its Annexes while most people speaking Swedish, Danish, 679 or Norwegian probably think they are equivalent. 680 681 It is neither possible nor desirable to make these characters 682 equivalent on a global basis. To do so would, for this example, 683 rationalize the situation in Sweden while causing considerable 684 confusion in Germany because the U+00F8 character is never used in 685 the German language. But the "variant" model introduced in [RFC3743] 686 and [RFC4290] can be used by a registry to prevent the worst 687 consequence of the possible confusion, by ensuring either that both 688 names are registered to the same party in a given domain or that one 689 of them is completely prohibited. 690 691 2.2.2. Multiple Scripts 692 693 There are languages in the world that can be expressed using multiple 694 scripts. For example, some Eastern European and Central Asian 695 languages can be expressed in either Cyrillic or Latin (see 696 Section 1.5.2) characters, or some African and Southeast Asian 697 languages can be expressed in either Arabic or Latin characters. A 698 few languages can even be written in three different scripts. In 699 other cases, the language is typically written in a combination of 700 scripts (e.g., Kanji, Kana, and Romaji for Japanese; Hangul and Hanji 701 for Korean). Because of this, the same word, in the same language, 702 can be expressed in different ways. For some languages, only a 703 single script is normally used to write a single word; for others, 704 mixed scripts are required; and, for still others, special 705 circumstances may dictate mixing scripts in labels although that is 706 not normally done for "words". For IDN purposes, these variations 707 make the definition of "script" extremely sensitive, especially since 708 ICANN is now recommending that it be used as the primary basis for 709 710 711 712 Klensin, et al. Informational [Page 13] 713 RFC 4690 IAB -- IDN Next Steps September 2006 714 715 716 registry policies. However essential it may be to prohibit mixed- 717 script labels, additional policy nuance is required for "languages 718 with established orthographies and conventions that require the 719 commingled use of multiple scripts". 720 721 2.2.3. Normalization and Character Mappings 722 723 Unicode contains several different models for representing 724 characters. The Chinese (Han)-derived characters of the "CJK" 725 (Chinese, Japanese, and Korean) languages are "unified", i.e., 726 characters with common derivation and similar appearances are 727 assigned to the same code point. European characters derived from a 728 Greek-Latin base are separated into separate code blocks for Latin, 729 Greek, and Cyrillic even when individual characters are identical in 730 both form and semantics. Separate code points based on font 731 differences alone are generally prohibited, but a large number of 732 characters for "mathematical" use have been assigned separate code 733 points even though they differ from base ASCII characters only by 734 font attributes such as "script", "bold", or "italic". Some 735 characters that often appear together are treated as typographical 736 digraphs with specific code points assigned to the combination, 737 others require that the two-character sequences be used, and still 738 others are available in both forms. Some Roman-derived letters that 739 were developed as decorated variations on the basic Latin letter 740 collection (e.g., by addition of diacritical marks) are assigned code 741 points as individual characters, others must be built up as two (or 742 more) character sequences using "combining characters". 743 744 Many of these differences result from the desire to maintain backward 745 compatibility while the standard evolved historically, and are hence 746 understandable. However, the DNS requires precise knowledge of which 747 codes and code sequences represent the same character and which ones 748 do not. Limiting the potential difficulties with confusable 749 characters (see Section 2.2.6) requires even more knowledge of which 750 characters might look alike in some fonts but not in others. These 751 variations make it difficult or impossible to apply a single set of 752 rules to all of Unicode and, in doing so, satisfy everyone and their 753 perceived needs. Instead, more or less complex mapping tables, 754 defined on a character-by-character basis, are required to 755 "normalize" different representations of the same character to a 756 single form so that matching is possible. 757 758 Unless normalization rules, such as those that underlie Nameprep, are 759 applied, characters that are essentially identical will not match in 760 the DNS, creating many opportunities for problems. The most common 761 of these problems is that, due to the processing applied (and 762 discussed above) before a word is represented as a Unicode string, a 763 single word can end up being expressed as several different Unicode 764 765 766 767 Klensin, et al. Informational [Page 14] 768 RFC 4690 IAB -- IDN Next Steps September 2006 769 770 771 strings. Even if normalization rules are applied, some strings that 772 are considered identical by users will not compare equal. That 773 problem is discussed in more detail elsewhere in this document, 774 particularly in Section 3.2.1. 775 776 IDNA attempts to compensate for these problems by using a 777 normalization algorithm defined by the Unicode Consortium. This 778 algorithm can change a sequence of one or more Unicode characters to 779 another set of characters. One example is that the base character 780 U+0061 (LATIN SMALL LETTER A) followed by U+0308 (COMBINING 781 DIAERESIS) is changed to the single Unicode character U+00E4 (LATIN 782 SMALL LETTER A WITH DIAERESIS). 783 784 This Unicode normalization process accounts only for simple character 785 equivalences, not equivalences that are language or script dependent. 786 For example, as mentioned above, the characters U+00F8 (LATIN SMALL 787 LETTER O WITH STROKE) and U+00F6 (LATIN SMALL LETTER O WITH 788 DIAERESIS) are considered to match in Swedish (and some other 789 languages), but not for all languages that use either of the 790 characters. Having these characters be treated as equivalent in some 791 contexts and not in others requires decisions and mechanisms that, in 792 turn, depend much more on context than either IDNA or the Unicode 793 character-based normalization tables can provide. 794 795 Additional complications occur if the sequences are more complicated 796 or if an attacker is making a deliberate effort to confuse the 797 normalization process. For example, if the sequence U+0069 U+0307 798 (LATIN SMALL LETTER I followed by COMBINING DOT ABOVE) appears, the 799 Unicode Normalization Method known as NFKC maps it into U+00EF (LATIN 800 SMALL LETTER I WITH DIAERESIS), which is what one would predict. But 801 consider U+0131 U+0308 (LATIN SMALL LETTER DOTLESS I and COMBINING 802 DIAERESIS): is that the same character? Is U+0131 U+0307 U+0307 803 (dotless i and two combining dot-above characters) equivalent to 804 U+00EF or U+0069, or neither? NFKC does not appear to tell us, nor 805 does the definition of U+0307 appear to tell us what happens when it 806 is combined with other "symbol above" arrangements (unlike some of 807 the "accent above" combining characters, which more or less specify 808 kerning). Similar issues arise when U+00EF is combined with various 809 dot-above combining characters. Each of these questions provides 810 some opportunities for spoofing if different display implementations 811 interpret the rules in different ways. 812 813 If we leave Latin scripts and examine those based on Chinese 814 characters, we see there is also an absence of specific, lexigraphic, 815 rules for transformations between Traditional and Simplified Chinese. 816 Even if there were such rules, unification of Japanese and Korean 817 818 819 820 821 822 Klensin, et al. Informational [Page 15] 823 RFC 4690 IAB -- IDN Next Steps September 2006 824 825 826 characters with Chinese ones would make it impossible to normalize 827 Traditional Chinese into Simplified Chinese ones without causing 828 problems in Japanese and Korean use of the same characters. 829 830 More generally, while some mappings, such as those between 831 precomposed Latin script characters and the equivalent multiple code 832 point composed character sequences, depend only on the characters 833 themselves, in many or most cases, such as the case with Swedish 834 above, the mapping is language or culturally dependent. There have 835 been discussions as to whether different canonicalization rules (in 836 addition to or instead of Unicode normalization) should be, or could 837 be, applied differently to different languages or scripts. The fact 838 that most scripts included in Unicode have been initially 839 incorporated by copying an existing standard more or less intact has 840 impact on the optimization of these algorithms and on forward 841 compatibility. Even if the language is known and language-specific 842 rules can be defined, dependencies on the language do not disappear. 843 Canonicalization operations are not possible unless they either 844 depend only on short sequences of text or have significant context 845 available that is not obvious from the text itself. DNS lookups and 846 many other operations do not have a way to capture and utilize the 847 language or other information that would be needed to provide that 848 context. 849 850 These variations in languages and in user perceptions of characters 851 make it difficult or impossible to provide uniform algorithms for 852 matching Unicode strings in a way that no end users are ever 853 surprised by the result. For closely-related scripts or characters, 854 surprises may even be frequent. However, because uniform algorithms 855 are required for mappings that are applied when names are looked up 856 in the DNS, the rules that are chosen will always represent an 857 approximation that will be more or less successful in minimizing 858 those user surprises. The current Nameprep and Stringprep algorithms 859 use mapping tables to "normalize" different representations of the 860 same text to a single form so that matching is possible. 861 862 More details on the creation of the normalization algorithms can be 863 found in the Unicode Specification and the associated Technical 864 Reports [UTR] and Annexes. Technical Report #36 [UTR36] and [UTR39] 865 are specifically related to the IDN discussion. 866 867 2.2.4. URLs in Printed Form 868 869 URLs and other identifiers appear, not only in electronic forms from 870 which they can (at least in principle) be accurately copied and 871 "pasted" but in printed forms from which the user must transcribe 872 them into the computer system. This is often known as the "side-of- 873 the-bus problem" because a particularly problematic version of it 874 875 876 877 Klensin, et al. Informational [Page 16] 878 RFC 4690 IAB -- IDN Next Steps September 2006 879 880 881 requires that the user be able to observe and accurately remember a 882 URL that is quickly glimpsed in a transient form -- a billboard seen 883 while driving, a sign on the side of a passing vehicle, a television 884 advertisement that is not frequently repeated or on-screen for a long 885 time, and so on. 886 887 The difficulty, in short, is that two Unicode strings that are 888 actually different might look exactly the same, especially when there 889 is no time to study them. This is because, for example, some glyphs 890 in Cyrillic, Greek, and Latin do look the same, but have been 891 assigned different code points in Unicode. Worse, one needs to be 892 reasonably familiar with a script and how it is used to understand 893 how much characters can reasonably vary as the result of artistic 894 fonts and typography. For example, there are a few fonts for Latin 895 characters that are sufficiently highly ornamented that an observer 896 might easily confuse some of the characters with characters in Thai 897 script. Uppercase ITC Blackadder (a registered trademark of 898 International Typeface Corporation) and Curlz MT are two fairly 899 obvious examples; these fonts use loops at the end of serifs, 900 creating a resemblance to Thai (in some fonts) for some characters. 901 902 2.2.5. Bidirectional Text 903 904 Some scripts (and because of that some words in some languages) are 905 written not left to right, but right to left. And, to complicate 906 things, one might have something written in Arabic script right to 907 left that includes some characters that are read from left to right, 908 such as European-style digits. This implies that some texts might 909 have a mixed left-to-right AND right-to-left order (even though in 910 most implementations, and in IDNA, all texts have a major direction, 911 with the other as an exception). 912 913 IDNA permits the inclusion of European digits in a label that is 914 otherwise a sequence of right-to-left characters, but prohibits most 915 other mixed-directional (or bidirectional) strings. This prohibition 916 can cause other problems such as the rejection of some otherwise 917 linguistically and culturally sensible strings. As Unicode and 918 conventions for handling so-called bidirectional ("BIDI") strings 919 evolve, the prohibition in IDNA should be reviewed and reevaluated. 920 921 2.2.6. Confusable Character Issues 922 923 Similar-looking characters in identifiers can cause actual problems 924 on the Internet since they can result, deliberately or accidentally, 925 in people being directed to the wrong host or mailbox by believing 926 that they are typing, or clicking on, intended characters that are 927 different from those that actually appear in the domain name or 928 reference. See Section 4.1.3 for further discussion of this issue. 929 930 931 932 Klensin, et al. Informational [Page 17] 933 RFC 4690 IAB -- IDN Next Steps September 2006 934 935 936 IDNs complicate these issues, not only by providing many additional 937 characters that look sufficiently alike to be potentially confused, 938 but also by raising new policy questions. For example, if a language 939 can be written in two different scripts, is a label constructed from 940 a word written in one script equivalent to a label constructed from 941 the same word written in the other script? Is the answer the same 942 for words in two different languages that translate into each other? 943 944 It is now generally understood that, in addition to the collision 945 problems of possibly equivalent words and hence labels, it is 946 possible to utilize characters that look alike -- "confusable" 947 characters -- to spoof names in order to mislead or defraud users. 948 That issue, driven by particular attacks such as those known as 949 "phishing", has introduced stronger requirements for registry efforts 950 to prevent problems than were previously generally recognized as 951 important. 952 953 One commonly-proposed approach is to have a registry establish 954 restrictions on the characters, and combinations of characters, it 955 will permit to be included in a string to be registered as a label. 956 Taking the Swedish top-level domain, .SE, as an example, a rule might 957 be adopted that the registry "only accepts registrations in Swedish, 958 using Latin script, and because of this, Unicode characters Latin-a, 959 -b, -c,...". But, because there is not a 1:1 mapping between country 960 and language, even a Country Code Top Level Domain (ccTLD) like .SE 961 might have to accept registrations in other languages. For example, 962 there may be a requirement for Finnish (the second most-used language 963 in Sweden). What rules and code points are then defined for Finnish? 964 Does it have special mappings that collide with those that are 965 defined for Swedish? And what does one do in countries that use more 966 than one script? (Finnish and Swedish use the same script.) In all 967 cases, the dispute will ultimately be about whether two strings are 968 the same (or confusingly similar) or not. That, in turn, will 969 generate a discussion of how one defines "what is the same" and "what 970 is similar enough to be a problem". 971 972 Another example arose recently that further illustrates the problem. 973 If one were to use Cyrillic characters to represent the country code 974 for Russia in a localized equivalent to the ccTLD label, the 975 characters themselves would be indistinguishable from the Latin 976 characters "P" and "Y" (in either lower- or uppercase) in most fonts. 977 We presume this might cause some consternation in Paraguay. 978 979 These difficulties can never be completely eliminated by algorithmic 980 means. Some of the problem can be addressed by appropriate tuning of 981 the protocols and their tables, other parts by registry actions to 982 reduce confusion and conflicts, and still other parts can be 983 984 985 986 987 Klensin, et al. Informational [Page 18] 988 RFC 4690 IAB -- IDN Next Steps September 2006 989 990 991 addressed by careful design of user interfaces in application 992 programs. But, ultimately, some responsibility to avoid being 993 tricked or harmfully confused will rest with the user. 994 995 Another registry technique that has been extensively explored 996 involves looking at confusable characters and confusion between 997 complete labels, restricting the labels that can be registered based 998 on relationships to what is registered already. Registries that 999 adopt this approach might establish special mapping rules such as: 1000 1001 1. If you register something with code point A, domain names with B 1002 instead of A will be blocked from registration by others (where B 1003 is a character at a separate code point that has a confusingly 1004 similar appearance to A). 1005 1006 2. If you register something with code point A, you also get domain 1007 name with B instead of A. 1008 1009 These approaches are discussed in more detail for "CJK" characters in 1010 RFC 3743 [RFC3743] and more generally in RFC 4290 [RFC4290]. 1011 1012 2.2.7. The IESG Statement and IDNA issues 1013 1014 The issues above, at least as they were understood at the time, 1015 provided the background for the IESG statement included in 1016 Section 1.6.1 (which, in turn, was part of the basis for the initial 1017 ICANN Guidelines) that a registry should have a policy about the 1018 scripts, languages, code points and text directions for which 1019 registrations will be accepted. While "accept all" might be an 1020 acceptable policy, it implies there is also a dispute resolution 1021 process that takes the problems listed above into account. This 1022 process must be designed for dealing with all types of potential 1023 disputes. For example, issues might arise between registrant and 1024 registry over a decision by the registry on collisions with already 1025 registered domain names and between registrant and trademark holder 1026 (that a domain name infringes on a trademark). In both cases, the 1027 parties disagreeing have different views on whether two strings are 1028 "equivalent" or not. They may believe that a string that is not 1029 allowed to be registered is actually different from one that is 1030 already registered. Or they might believe that two strings are the 1031 same, even though the rules adopted by the registry to prevent 1032 confusion define them as two different domain names. 1033 1034 1035 1036 1037 1038 1039 1040 1041 1042 Klensin, et al. Informational [Page 19] 1043 RFC 4690 IAB -- IDN Next Steps September 2006 1044 1045 1046 3. Migrating to New Versions of Unicode 1047 1048 3.1. Versions of Unicode 1049 1050 While opinions differ about how important the issues are in practice, 1051 the use of Unicode and its supporting tables for IDNA appears to be 1052 far more sensitive to subtle changes than it is in typical Unicode 1053 applications. This may be, at least in part, because many other 1054 applications are internally sensitive only to the appearance of 1055 characters and not to their representation. Or those applications 1056 may be able to take effective advantage of script, language, or 1057 character class identification. The working group that developed 1058 IDNA concluded that attempting to encode any ancillary character 1059 information into the DNS label would be impractical and unwise, and 1060 the IAB, based in part on the comments in the ad hoc committee, saw 1061 no reason to review that decision. 1062 1063 The Unicode Consortium has sometimes used the likelihood of a 1064 combination of characters actually appearing in a natural language as 1065 a criterion for the safety of a possible change. However, as 1066 discussed above, DNS names are often fabrications -- abbreviations, 1067 strings deliberately formed to be unusual, members of a series 1068 sequenced by numbers or other characters, and so on. Consequently, a 1069 criterion that considers a change to be safe if it would not be 1070 visible in properly-constructed running text is not helpful for DNS 1071 purposes: a change that would be safe under that criterion could 1072 still be quite problematic for the DNS. 1073 1074 This sensitivity to changes has made it quite difficult to migrate 1075 IDNA from one version of Unicode to the next if any changes are made 1076 that are not strictly additive. A change in a code point assignment 1077 or definition may be extremely disruptive if a DNS label has been 1078 defined using the earlier form and any of its previous components has 1079 been moved from one table position or normalization rule to another. 1080 Unicode normalization tables, tables of scripts or languages and 1081 characters that belong to them, and even tables of confusable 1082 characters as an adjunct to security recommendations may be very 1083 helpful in designing registry restrictions on registrations and 1084 applications provisions for avoiding or identifying suspicious names. 1085 Ironically, they also extend the sensitivity of IDNA and its 1086 implementations to all forms of change between one version of Unicode 1087 and the next. Consequently, they make Unicode version migration more 1088 difficult. 1089 1090 An example of the type of change that appears to be just a small 1091 correction from one perspective but may be problematic from another 1092 was the correction to the normalization definition in 2004 1093 [Unicode-PR29]. Community input suggested that the change would 1094 1095 1096 1097 Klensin, et al. Informational [Page 20] 1098 RFC 4690 IAB -- IDN Next Steps September 2006 1099 1100 1101 cause problems for Stringprep, but the Unicode Technical Committee 1102 decided, on balance, that the change was worthwhile. Because of 1103 difficulties with consistency, some deployed implementations have 1104 decided to adopt the change and others have not, leading to subtle 1105 incompatibilities. 1106 1107 This situation leads to a dilemma. On the one hand, it is completely 1108 unacceptable to freeze IDNA at a Unicode version level that excludes 1109 more recently-defined characters and scripts that are important to 1110 those who use them. On the other hand, it is equally unacceptable to 1111 migrate from one version of Unicode to the next if such migration 1112 might invalidate an existing registered DNS name or some of its 1113 registered properties or might make the string or representation of 1114 that name ambiguous. If IDNA is to be modified to accommodate new 1115 versions of Unicode, the IETF will need to work with the Unicode 1116 Consortium and other bodies to find an appropriate balance in this 1117 area, but progress will be possible only if all relevant parties are 1118 able to fairly consider and discuss possible decisions that may be 1119 very difficult and unpalatable. 1120 1121 It would also prove useful if, during the course of that dialog, the 1122 need for Unicode Consortium concern with security issues in 1123 applications of the Unicode character set could be clarified. It 1124 would be unfortunate from almost every perspective considered here, 1125 if such matters slowed the inclusion of as yet unencoded scripts. 1126 1127 3.2. Version Changes and Normalization Issues 1128 1129 3.2.1. Unnormalized Combining Sequences 1130 1131 One of the advantages of the Unicode model of combining characters, 1132 as with previous systems that use character overstriking to 1133 accomplish similar purposes, is that it is possible to use sequences 1134 of code points to generate characters that are not explicitly 1135 provided for in the character set. However, unless sequences that 1136 are not explicitly provided for are prohibited by some mechanism 1137 (such as the normalization tables), such combining sequences can 1138 permit two related dangers. 1139 1140 o The first is another risk of character confusion, especially if 1141 the relationship of the combining character with characters it 1142 combines with are not precisely defined or unexpected combinations 1143 of combining characters are used. That issue is discussed in more 1144 detail, with an example, in Section 2.2.3. 1145 1146 o These same issues also inherently impact the stability of the 1147 normalization tables. Suppose that, somewhere in the world, there 1148 is a character that looks like a Roman-derived lowercase "i", but 1149 1150 1151 1152 Klensin, et al. Informational [Page 21] 1153 RFC 4690 IAB -- IDN Next Steps September 2006 1154 1155 1156 with three (not one or two) dots above it. And suppose that the 1157 users of that character agree to represent it by combining a 1158 traditional "i" (U+0069) with a combining diaeresis (U+0308). So 1159 far, no problem. But, later, a broader need for this character is 1160 discovered and it is coded into Unicode either as a single 1161 precomposed character or, more likely under existing rules, by 1162 introducing a three-dot-above combining character. In either 1163 case, that version of Unicode should include a rule in NFKC that 1164 maps the "i"-plus-diaeresis sequence into the new, approved, one. 1165 If one does not do so, then there is arguably a normalization that 1166 should occur that does not. If one does so, then strings that 1167 were valid and normalized (although unanticipated) under the 1168 previous versions of Unicode become unnormalized under the new 1169 version. That, in turn, would impact IDNA comparisons because, 1170 effectively, it would introduce a change in the matching rules. 1171 1172 It would be useful to consider rules that would avoid or minimize 1173 these problems with the understanding that, for reasons given 1174 elsewhere, simply minimizing it may not be good enough for IDNA. One 1175 partial solution might be to ban any combination of a base character 1176 and a combining character that does not appear in a hypothetical 1177 "anticipated combinations" table from being used in a domain name 1178 label. The next subsection discusses a more radical, if impractical, 1179 view of the problem and its solutions. 1180 1181 3.2.2. Combining Characters and Character Components 1182 1183 For several reasons, including those discussed above, one thing that 1184 increases IDNA complexity and the need for normalization is that 1185 combining characters are permitted. Without them, complexity might 1186 be reduced enough to permit easier transitions to new versions. The 1187 community should consider the impact of entirely prohibiting 1188 combining characters from IDNs. While it is almost certainly 1189 unfeasible to introduce this change into Unicode as it is now defined 1190 and doing so would be extremely disruptive even if it were feasible, 1191 the thought experiment can be helpful in understanding both the 1192 issues and the implications of the paths not taken. For example, one 1193 consequence of this, of course, is that each new language or script, 1194 and several existing ones, would require that all of its characters 1195 have Unicode assignments to specific, precomposed, code points. 1196 1197 Note that this is not currently permitted within Unicode for Latin 1198 scripts. For non-Latin scripts, some such code points have been 1199 defined. The decisions that govern the assignment of such code 1200 points are managed entirely within the Unicode Consortium. Were the 1201 IETF to choose to reduce IDNA complexity by excluding combining 1202 characters, no doubt there would be additional input to the Unicode 1203 Consortium from users and proponents of scripts that precomposed 1204 1205 1206 1207 Klensin, et al. Informational [Page 22] 1208 RFC 4690 IAB -- IDN Next Steps September 2006 1209 1210 1211 characters be required. The IAB and the IETF should examine whether 1212 it is appropriate to press the Unicode Consortium to revise these 1213 policies or otherwise to recommend actions that would reduce the need 1214 for normalization and the related complexities. However, we have 1215 been told that the Technical Committee does not believe it is 1216 reasonable or feasible to add all possible precomposed characters to 1217 Unicode. If Unicode cannot be modified to contain the precomposed 1218 characters necessary to support existing languages and scripts, much 1219 less new ones, this option for IDN restrictions will not be feasible. 1220 1221 3.2.3. When does normalization occur? 1222 1223 In many Unicode applications, the preferred solution is to pick a 1224 style of normalization and require that all text that is stored or 1225 transmitted be normalized to that form. (This is the approach taken 1226 in ongoing work in the IETF on a standard Unicode text form 1227 [net-utf8]). IDNA does not impose this requirement. Text is 1228 normalized and case-reduced at registration time, and only the 1229 normalized version is placed in the DNS. However, there is no 1230 requirement that applications show only the native (and lower-case 1231 where appropriate) characters associated with the normalized form in 1232 discussions or references such as URLs. If conventions used for 1233 all-ASCII DNS labels are to be extended to internationalized forms, 1234 such a requirement would be unreasonable, since it would prohibit the 1235 use of mixed-case references for clarity or market identification. 1236 It might even be culturally inappropriate. However, without that 1237 restriction, the comparison that will ultimately be made in the DNS 1238 will be between strings normalized at different times and under 1239 different versions of Unicode. The assertion that a string in 1240 normalized form under one version of Unicode will still be in 1241 normalized form under all future versions is not sufficient. 1242 Normalization at different times also requires that a given source 1243 string always normalizes to the same target string, regardless of the 1244 version under which it is normalized. That criterion is much more 1245 difficult to fulfill. The discussion above suggests that it may even 1246 be impossible. 1247 1248 Ignoring these issues with combining characters entirely, as IDNA 1249 effectively does today, may leave us "stuck" at Unicode 3.2, leading 1250 either to incompatibility differences in applications that otherwise 1251 use a modern version of Unicode (while IDN remains at Unicode 3.2) or 1252 to painful transitions to new versions. If decisions are made 1253 quickly, it may still be possible to make a one-time version upgrade 1254 to Version 4.1 or Version 5 of Unicode. However, unless we can 1255 impose sufficient global restrictions to permit smooth transitions, 1256 upgrading to versions beyond that one are likely to be painful (e.g., 1257 potentially requiring changing strings already in the DNS or even a 1258 new Punycode prefix) or impossible. 1259 1260 1261 1262 Klensin, et al. Informational [Page 23] 1263 RFC 4690 IAB -- IDN Next Steps September 2006 1264 1265 1266 4. Framework for Next Steps in IDN Development 1267 1268 4.1. Issues within the Scope of the IETF 1269 1270 4.1.1. Review of IDNA 1271 1272 The IETF should consider reviewing RFCs 3454, 3490, 3491, and/or 1273 3492, and update, replace, or supplement them to meet the criteria of 1274 this paragraph (one or more of them may prove impractical after 1275 further study). Any new versions or additional specifications should 1276 be adapted to the version of Unicode that is current when they are 1277 created. Ideally, they should specify a path for adapting to future 1278 versions of Unicode (some suggestions below may facilitate this). 1279 The IETF should also consider whether there are significant 1280 advantages to mapping some groups of characters, such as code points 1281 assigned to font variations, into others or whether clarity and 1282 comprehensibility for the user would be better served by simply 1283 prohibiting those characters. More generally, it appears that it 1284 would be worthwhile for the IETF to review whether the Unicode 1285 normalization rules now invoked by the Stringprep profile in Nameprep 1286 are optimal for the DNS or whether more restrictive rules, or an even 1287 more restrictive set of permitted character combinations, would 1288 provide better support for DNS internationalization. 1289 1290 The IAB has concluded that there is a consensus within the broader 1291 community that lists of code points should be specified by the use of 1292 an inclusion-based mechanism (i.e., identifying the characters that 1293 are permitted), rather than by excluding a small number of characters 1294 from the total Unicode set as Stringprep and Nameprep do today. That 1295 conclusion should be reviewed by the IETF community and action taken 1296 as appropriate. 1297 1298 We suggest that the individuals doing the review of the code points 1299 should work as a specialized design team. To the extent possible, 1300 that work should be done jointly by people with experience from the 1301 IETF and deep knowledge of the constraints of the DNS and application 1302 design, participants from the Unicode Consortium, and other people 1303 necessary to be able to reach a generally-accepted result. Because 1304 any work along these lines would be modifications and updates to 1305 standards-track documents, final review and approval of any proposals 1306 would necessarily follow normal IETF processes. 1307 1308 It is worth noting that sufficiently extreme changes to IDNA would 1309 require a new Punycode prefix, probably with long-term support for 1310 both the old prefix and the new one in both registration arrangements 1311 and applications. An alternative, which is almost certainly 1312 impractical, would be some sort of "flag day", i.e., a date on which 1313 the old rules are simultaneously abandoned by everyone and the new 1314 1315 1316 1317 Klensin, et al. Informational [Page 24] 1318 RFC 4690 IAB -- IDN Next Steps September 2006 1319 1320 1321 ones adopted. However, preliminary analysis indicates that few, if 1322 any, of the changes recommended for consideration elsewhere in this 1323 document would require this type of version change. For example, 1324 suppose additional restrictions, such as those implied above, are 1325 imposed on what can be registered. Those restrictions might require 1326 policy decisions about how labels are to be disposed of if they 1327 conformed to the earlier rules but not to the new ones. But they 1328 would not inherently require changes in the protocol or prefix. 1329 1330 4.1.2. Non-DNS and Above-DNS Internationalization Approaches 1331 1332 The IETF should once again examine the extent to which it is 1333 appropriate to try to solve internationalization problems via the DNS 1334 and what place the many varieties of so-called "keyword systems" or 1335 other Internet navigational techniques might have. Those techniques 1336 can be designed to impose fewer constraints, or at least different 1337 constraints, than IDNA and the DNS. As discussed elsewhere in this 1338 document, IDNA cannot support information about scripts, languages, 1339 or Unicode versions on lookup. As a consequence of the nature of DNS 1340 lookups, characters and labels either match or do not match; a near- 1341 match is simply not a possible concept in the DNS. By contrast, 1342 observation of near-matching is common in human communication and in 1343 matching operations performed by people, especially when they have a 1344 particular script or language context in mind. The DNS is further 1345 constrained by a fairly rigid internal aliasing system (via CNAME and 1346 DNAME resource records), while some applications of international 1347 naming may require more flexibility. Finally, the rigid hierarchy of 1348 the DNS --and the tendency in practice for it to become flat at 1349 levels nearest the root-- and the need for names to be unique are 1350 more suitable for some purposes than others and may not be a good 1351 match for some purposes for which people wish to use IDNs. Each of 1352 these constraints can be relaxed or changed by one or more systems 1353 that would provide alternatives to direct use of the DNS by users. 1354 Some of the issues involved are discussed further in Section 5.3 and 1355 various ideas have been discussed in detail in the IETF or IRTF. 1356 Many of those ideas have even been described in Internet Drafts or 1357 other documents. As experience with IDNs and with expectations for 1358 them accumulates, it will probably become appropriate for the IETF or 1359 IRTF to revisit the underlying questions and possibilities. 1360 1361 4.1.3. Security Issues, Certificates, etc. 1362 1363 Some characters look like others, often as the result of common 1364 origins. The problem with these "confusable" characters, often 1365 incorrectly called homographs, has always existed when characters are 1366 presented to humans who interpret what is displayed and then make 1367 decisions based on what is seen. This is not a problem that exists 1368 only when working with internationalized domain names, but they make 1369 1370 1371 1372 Klensin, et al. Informational [Page 25] 1373 RFC 4690 IAB -- IDN Next Steps September 2006 1374 1375 1376 the problem worse. The result of a survey that would explain what 1377 the problems are might be interesting. Many of these issues are 1378 mentioned in Unicode Technical Report #36 [UTR36]. 1379 1380 In this and other issues associated with IDNs, precise use of 1381 terminology is important lest even more confusion result. The 1382 definition of the term 'homograph' that normally appears in 1383 dictionaries and linguistic texts states that homographs are 1384 different words that are spelled identically (for example, the 1385 adjective 'brief' meaning short, the noun 'brief' meaning a document, 1386 and the verb 'brief' meaning to inform). By definition, letters in 1387 two different alphabets are not the same, regardless of similarities 1388 in appearance. This means that sequences of letters from two 1389 different scripts that appear to be identical on a computer display 1390 cannot be homographs in the accepted sense, even if they are both 1391 words in the dictionary of some language. Assuming that there is a 1392 language written with Cyrillic script in which "cap" is a word, 1393 regardless of what it might mean, it is not a homograph of the 1394 Latin-script English word "cap". 1395 1396 When the security implications of visually confusable characters were 1397 brought to the forefront in 2005, the term homograph was used to 1398 designate any instance of graphic similarity, even when comparing 1399 individual characters. This usage is not only incorrect, but risks 1400 introducing even more confusion and hence should be avoided. The 1401 current preferred terminology is to describe these similar-looking 1402 characters as "confusable characters" or even "confusables". 1403 1404 Many people have suggested that confusable characters are a problem 1405 that must be addressed, at least in part, directly in the user 1406 interfaces of application software. While it should almost certainly 1407 be part of a complete solution, that approach creates it own set of 1408 difficulties. For example, a user switching between systems, or even 1409 between applications on the same system, may be surprised by 1410 different types of behavior and different levels of protection. In 1411 addition, it is unclear how a secure setup for the end user should be 1412 designed. Today, in the web browser, a padlock is a traditional way 1413 of describing some level of security for the end user. Is this 1414 binary signaling enough? Should there be any connection between a 1415 risk for a displayed string including confusable characters and the 1416 padlock or similar signaling to the user? 1417 1418 Many web browsers have adopted a convention, based on a "whitelist" 1419 or similar technique, of restricting the display of native characters 1420 to subdomains of top-level domains that are deemed to have safe 1421 practices for the registration of potentially confusable labels. 1422 IDNs in other domains are displayed as Punycode. These techniques 1423 may not be sufficiently sensitive to differences in policies among 1424 1425 1426 1427 Klensin, et al. Informational [Page 26] 1428 RFC 4690 IAB -- IDN Next Steps September 2006 1429 1430 1431 top-level domains and their subdomains and so, while they are clearly 1432 helpful, they may not be adequate. Are other methods of dealing with 1433 confusable characters possible? Would other methods of identifying 1434 and listing policies about avoiding confusing registrations be 1435 feasible and helpful? 1436 1437 It would be interesting to see a more coordinated effort in 1438 establishing guidelines for user interfaces. If nothing else, the 1439 current whitelists are browser specific and both can, and do, differ 1440 between implementations. 1441 1442 4.1.4. Protocol Changes and Policy Implications 1443 1444 Some potential protocol or table changes raise important policy 1445 issues about what to do with existing, registered, names. Should 1446 such changes be needed, their impact must be carefully evaluated in 1447 the IETF, ICANN, and possibly other forums. In particular, protocol 1448 or policy changes that would not permit existing names to be 1449 registered under the newer rules should be considered carefully, 1450 balancing their importance against possible disruption and the issues 1451 of invalidating older names against the importance of consistency as 1452 seen by the user. 1453 1454 4.1.5. Non-US-ASCII in Local Part of Email Addresses 1455 1456 Work is going on in the IETF related to the local part of email 1457 addresses. It should be noted that the local part of email addresses 1458 has much different syntax and constraints than a domain name label, 1459 so to directly apply IDNA on the local part is not possible. 1460 1461 4.1.6. Use of the Unicode Character Set in the IETF 1462 1463 Unicode and the closely-related ISO 10646 are the only coded 1464 character sets that aspire to include all of the world's characters. 1465 As such, they permit use of international characters without having 1466 to identify particular character coding standards or tables. The 1467 requirement for a single character set is particularly important for 1468 use with the DNS since there is no place to put character set 1469 identification. The decision to use Unicode as the base for IETF 1470 protocols going forward is discussed in [RFC2277]. The IAB does not 1471 see any reason to revisit the decision to use Unicode in IETF 1472 protocols. 1473 1474 1475 1476 1477 1478 1479 1480 1481 1482 Klensin, et al. Informational [Page 27] 1483 RFC 4690 IAB -- IDN Next Steps September 2006 1484 1485 1486 4.2. Issues That Fall within the Purview of ICANN 1487 1488 4.2.1. Dispute Resolution 1489 1490 IDNs create new types of collisions between trademarks and domain 1491 names as well as collisions between domain names. These have impact 1492 on dispute resolution processes used by registries and otherwise. It 1493 is important that deployment of IDNs evolve in parallel with review 1494 and updating of ICANN or registry-specific dispute resolution 1495 processes. 1496 1497 4.2.2. Policy at Registries 1498 1499 The IAB recommends that registries use an inclusion-based model when 1500 choosing what characters to allow at the time of registration. This 1501 list of characters is in turn to be a subset of what is allowed 1502 according to the updated IDNA standard. The IAB further recommends 1503 that registries develop their inclusion-based models in parallel with 1504 dispute resolution process at the registry itself. 1505 1506 Most established policies for dealing with claimed or apparent 1507 confusion or conflicts of names are based on dispute resolution. 1508 Decisions about legitimate use or registration of one or more names 1509 are resolved at or after the time of registration on a case-by-case 1510 basis and using policies that are specific to the particular DNS zone 1511 or jurisdiction involved. These policies have generally not been 1512 extended below the level of the DNS that is directly controlled by 1513 the top-level registry. 1514 1515 Because of the number of conflicts that can be generated by the 1516 larger number of available and confusable characters in Unicode, we 1517 recommend that registration-restriction and dispute resolution 1518 policies be developed to constrain registration of IDNs and zone 1519 administrators at all levels of the DNS tree. Of course, many of 1520 these policies will be less formal than others and there is no 1521 requirement for complete global consistency, but the arguments for 1522 reduction of confusable characters and other issues in TLDs should 1523 apply to all zones below that specific TLD. 1524 1525 Consistency across all zones can obviously only be accomplished by 1526 changes to the protocols. Such changes should be considered by the 1527 IETF if particular restrictions are identified that are important and 1528 consistent enough to be applied globally. 1529 1530 Some potential protocol changes or changes to character-mapping 1531 tables might, if adopted, have profound registry policy implications. 1532 See Section 4.1.4. 1533 1534 1535 1536 1537 Klensin, et al. Informational [Page 28] 1538 RFC 4690 IAB -- IDN Next Steps September 2006 1539 1540 1541 4.2.3. IDNs at the Top Level of the DNS 1542 1543 The IAB has concluded that there is not one issue with IDNs at the 1544 top level of the DNS (IDN TLDs) but at least three very separate 1545 ones: 1546 1547 o If IDNs are to be entered in the root zone, decisions must first 1548 be made about how these TLDs are to be named and delegated. These 1549 decisions fall within the traditional IANA scope and are ICANN 1550 issues today. 1551 1552 o There has been discussion of permitting some or all existing TLDs 1553 to be referenced by multiple labels, with those labels presumably 1554 representing some understanding of the "name" of the TLD in 1555 different languages. If actual aliases of this type are desired 1556 for existing domains, the IETF may need to consider whether the 1557 use of DNAME records in the root is appropriate to meet that need, 1558 what constraints, if any, are needed, whether alternate 1559 approaches, such as those of [RFC4185], are appropriate or whether 1560 further alternatives should be investigated. But, to the extent 1561 to which aliases are considered desirable and feasible, decisions 1562 presumably must be made as to which, if any, root IDN labels 1563 should be associated with DNAME records and which ones should be 1564 handled by normal delegation records or other mechanisms. That 1565 decision is one of DNS root-level namespace policy and hence falls 1566 to ICANN although we would expect ICANN to pay careful attention 1567 to any technical, operational, or security recommendations that 1568 may be produced by other bodies. 1569 1570 o Finally, if IDN labels are to be placed in the root zone, there 1571 are issues associated with how they are to be encoded and 1572 deployed. This area may have implications for work that has been 1573 done, or should be done, in the IETF. 1574 1575 5. Specific Recommendations for Next Steps 1576 1577 Consistent with the framework described above, the IAB offers these 1578 recommendations as steps for further consideration in the identified 1579 groups. 1580 1581 5.1. Reduction of Permitted Character List 1582 1583 Generalize from the original "hostname" rules to non-ASCII 1584 characters, permitting as few characters as possible to do that job. 1585 This would involve a restrictive model for characters permitted in 1586 IDN labels, thus contrasting with the approach used to develop the 1587 original IDNA/Nameprep tables. That approach was to include all 1588 Unicode characters that there was not a clear reason to exclude. 1589 1590 1591 1592 Klensin, et al. Informational [Page 29] 1593 RFC 4690 IAB -- IDN Next Steps September 2006 1594 1595 1596 The specific recommendation here is to specify such internationalized 1597 hostnames. Such an activity would fall to the IETF, although the 1598 task of developing the appropriate list of permitted characters will 1599 require effort both in the IETF and elsewhere. The effort should be 1600 as linguistically and culturally sensitive as possible, but smooth 1601 and effective operation of the DNS, including minimizing of 1602 complexity, should be primary goals. The following should be 1603 considered as possible mechanisms for achieving an appropriate 1604 minimum number of characters. 1605 1606 5.1.1. Elimination of All Non-Language Characters 1607 1608 Unicode characters that are not needed to write words or numbers in 1609 any of the world's languages should be eliminated from the list of 1610 characters that are appropriate in DNS labels. In addition to such 1611 characters as those used for box-drawing and sentence punctuation, 1612 this should exclude punctuation for word structure and other 1613 delimiters. While DNS labels may conveniently be used to express 1614 words in many circumstances, the goal is not to express words (or 1615 sentences or phrases), but to permit the creation of unambiguous 1616 labels with good mnemonic value. 1617 1618 5.1.2. Elimination of Word-Separation Punctuation 1619 1620 The inclusion of the hyphen in the original hostname rules is a 1621 historical artifact from an older, flat, namespace. The community 1622 should consider whether it is appropriate to treat it as a simple 1623 legacy property of ASCII names and not attempt to generalize it to 1624 other scripts. We might, for example, not permit claimed equivalents 1625 to the hyphen from other scripts to be used in IDNs. We might even 1626 consider banning use of the hyphen itself in non-ASCII strings or, 1627 less restrictively, strings that contained non-Latin characters. 1628 1629 5.2. Updating to New Versions of Unicode 1630 1631 As new scripts, to support new languages, continue to be added to 1632 Unicode, it is important that IDNA track updates. If it does not do 1633 so, but remains "stuck" at 3.2 or some single later version, it will 1634 not be possible to include labels in the DNS that are derived from 1635 words in languages that require characters that are available only in 1636 later versions. Making those upgrades is difficult, and will 1637 continue to be difficult, as long as new versions require, not just 1638 addition of characters, but changes to canonicalization conventions, 1639 normalization tables, or matching procedures (see Section 3.1). 1640 Anything that can be done to lower complexity and simplify forward 1641 transitions should be seriously considered. 1642 1643 1644 1645 1646 1647 Klensin, et al. Informational [Page 30] 1648 RFC 4690 IAB -- IDN Next Steps September 2006 1649 1650 1651 5.3. Role and Uses of the DNS 1652 1653 We wish to remind the community that there are boundaries to the 1654 appropriate uses of the DNS. It was designed and implemented to 1655 serve some specific purposes. There are additional things that it 1656 does well, other things that it does badly, and still other things it 1657 cannot do at all. No amount of protocol work on IDNs will solve 1658 problems with alternate spellings, near-matches, searching for 1659 appropriate names, and so on. Registration restrictions and 1660 carefully-designed user interfaces can be used to reduce the risk and 1661 pain of attempts to do some of these things gone wrong, as well as 1662 reducing the risks of various sort of deliberate bad behavior, but, 1663 beyond a certain point, use of the DNS simply because it is available 1664 becomes a bad tradeoff. The tradeoff may be particularly unfortunate 1665 when the use of IDNs does not actually solve the proposed problem. 1666 For example, internationalization of DNS names does not eliminate the 1667 ASCII protocol identifiers and structure of URIs [RFC3986] and even 1668 IRIs [RFC3987]. Hence, DNS internationalization itself, at any or 1669 all levels of the DNS tree, is not a sufficient response to the 1670 desire of populations to use the Internet entirely in their own 1671 languages and the characters associated with those languages. 1672 1673 These issues are discussed at more length, and alternatives 1674 presented, in [RFC2825], [RFC3467], [INDNS], and [DNS-Choices]. 1675 1676 5.4. Databases of Registered Names 1677 1678 In addition to their presence in the DNS, IDNs introduce issues in 1679 other contexts in which domain names are used. In particular, the 1680 design and content of databases that bind registered names to 1681 information about the registrant (commonly described as "whois" 1682 databases) will require review and updating. For example, the whois 1683 protocol itself [RFC3912] has no standard capability for handling 1684 non-ASCII text: one cannot search consistently for, or report, either 1685 a DNS name or contact information that is not in ASCII characters. 1686 This may provide some additional impetus for a switch to IRIS 1687 [RFC3981] [RFC3982] but also raises a number of other questions about 1688 what information, and in what languages and scripts, should be 1689 included or permitted in such databases. 1690 1691 6. Security Considerations 1692 1693 This document is simply a discussion of IDNs and IDNA issues; it 1694 raises no new security concerns. However, if some of its 1695 recommendations to reduce IDNA complexity, the number of available 1696 characters, and various approaches to constraining the use of 1697 confusable characters, are followed and prove successful, the risks 1698 of name spoofing and other problems may be reduced. 1699 1700 1701 1702 Klensin, et al. Informational [Page 31] 1703 RFC 4690 IAB -- IDN Next Steps September 2006 1704 1705 1706 7. Acknowledgements 1707 1708 The contributions to this report from members of the IAB-IDN ad hoc 1709 committee are gratefully acknowledged. Of course, not all of the 1710 members of that group endorse every comment and suggestion of this 1711 report. In particular, this report does not claim to reflect the 1712 views of the Unicode Consortium as a whole or those of particular 1713 participants in the work of that Consortium. 1714 1715 The members of the ad hoc committee were: Rob Austein, Leslie Daigle, 1716 Tina Dam, Mark Davis, Patrik Faltstrom, Scott Hollenbeck, Cary Karp, 1717 John Klensin, Gervase Markham, David Meyer, Thomas Narten, Michael 1718 Suignard, Sam Weiler, Bert Wijnen, Kurt Zeilenga, and Lixia Zhang. 1719 1720 Thanks are due to Tina Dam and others associated with the ICANN IDN 1721 Working Group for contributions of considerable specific text, to 1722 Marcos Sanz and Paul Hoffman for careful late-stage reading and 1723 extensive comments, and to Pete Resnick for many contributions and 1724 comments, both in conjunction with his former IAB service and 1725 subsequently. Olaf M. Kolkman took over IAB leadership for this 1726 document after Patrik Faltstrom and Pete Resnick stepped down in 1727 March 2006. 1728 1729 Members of the IAB at the time of approval of this document were: 1730 Bernard Aboba, Loa Andersson, Brian Carpenter, Leslie Daigle, Patrik 1731 Faltstrom, Bob Hinden, Kurtis Lindqvist, David Meyer, Pekka Nikander, 1732 Eric Rescorla, Pete Resnick, Jonathan Rosenberg and Lixia Zhang. 1733 1734 8. References 1735 1736 8.1. Normative References 1737 1738 [ISO10646] International Organization for Standardization, 1739 "Information Technology - Universal Multiple- 1740 Octet Coded Character Set (UCS) - Part 1: 1741 Architecture and Basic Multilingual Plane"", 1742 ISO/IEC 10646-1:2000, October 2000. 1743 1744 [RFC3454] Hoffman, P. and M. Blanchet, "Preparation of 1745 Internationalized Strings ("stringprep")", 1746 RFC 3454, December 2002. 1747 1748 [RFC3490] Faltstrom, P., Hoffman, P., and A. Costello, 1749 "Internationalizing Domain Names in Applications 1750 (IDNA)", RFC 3490, March 2003. 1751 1752 1753 1754 1755 1756 1757 Klensin, et al. Informational [Page 32] 1758 RFC 4690 IAB -- IDN Next Steps September 2006 1759 1760 1761 [RFC3491] Hoffman, P. and M. Blanchet, "Nameprep: A 1762 Stringprep Profile for Internationalized Domain 1763 Names (IDN)", RFC 3491, March 2003. 1764 1765 [RFC3492] Costello, A., "Punycode: A Bootstring encoding of 1766 Unicode for Internationalized Domain Names in 1767 Applications (IDNA)", RFC 3492, March 2003. 1768 1769 [Unicode32] The Unicode Consortium, "The Unicode Standard, 1770 Version 3.0", 2000. 1771 (Reading, MA, Addison-Wesley, 2000. ISBN 1772 0-201-61633-5). Version 3.2 consists of the 1773 definition in that book as amended by the Unicode 1774 Standard Annex #27: Unicode 3.1 1775 (http://www.unicode.org/reports/tr27/) and by the 1776 Unicode Standard Annex #28: Unicode 3.2 1777 (http://www.unicode.org/reports/tr28/). 1778
The IETF is responsible for the creation and maintenance of the DNS RFCs. The ICANN DNS RFC annotation project provides a forum for collecting community annotations on these RFCs as an aid to understanding for implementers and any interested parties. The annotations displayed here are not the result of the IETF consensus process.
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1779 8.2. Informative References 1780 1781 [DNS-Choices] Faltstrom, P., "Design Choices When Expanding 1782 DNS", Work in Progress, June 2005. 1783 1784 [ICANNv1] ICANN, "Guidelines for the Implementation of 1785 Internationalized Domain Names, Version 1.0", 1786 March 2003, <http://www.icann.org/general/ 1787 idn-guidelines-20jun03.htm>. 1788 1789 [ICANNv2] ICANN, "Guidelines for the Implementation of 1790 Internationalized Domain Names, Version 2.0", 1791 November 2005, <http://www.icann.org/general/ 1792 idn-guidelines-20sep05.htm>. 1793 1794 [IESG-IDN] Internet Engineering Steering Group (IESG), "IESG 1795 Statement on IDN", IESG Statements IDN Statement, 1796 February 2003, <http://www.ietf.org/IESG/ 1797 STATEMENTS/IDNstatement.txt>. 1798 1799 [INDNS] National Research Council, "Signposts in 1800 Cyberspace: The Domain Name System and Internet 1801 Navigation", National Academy Press ISBN 0309- 1802 09640-5 (Book) 0309-54979-5 (PDF), 2005, <http:// 1803 www7.nationalacademies.org/cstb/pub_dns.html>. 1804 1805 [ISO.2022.1986] International Organization for Standardization, 1806 "Information Processing: ISO 7-bit and 8-bit 1807 coded character sets: Code extension techniques", 1808 ISO Standard 2022, 1986. 1809 1810 1811 1812 Klensin, et al. Informational [Page 33] 1813 RFC 4690 IAB -- IDN Next Steps September 2006 1814 1815 1816 [ISO.646.1991] International Organization for Standardization, 1817 "Information technology - ISO 7-bit coded 1818 character set for information interchange", 1819 ISO Standard 646, 1991. 1820 1821 [ISO.8859.2003] International Organization for Standardization, 1822 "Information processing - 8-bit single-byte coded 1823 graphic character sets - Part 1: Latin alphabet 1824 No. 1 (1998) - Part 2: Latin alphabet No. 2 1825 (1999) - Part 3: Latin alphabet No. 3 (1999) - 1826 Part 4: Latin alphabet No. 4 (1998) - Part 5: 1827 Latin/Cyrillic alphabet (1999) - Part 6: Latin/ 1828 Arabic alphabet (1999) - Part 7: Latin/Greek 1829 alphabet (2003) - Part 8: Latin/Hebrew alphabet 1830 (1999) - Part 9: Latin alphabet No. 5 (1999) - 1831 Part 10: Latin alphabet No. 6 (1998) - Part 11: 1832 Latin/Thai alphabet (2001) - Part 13: Latin 1833 alphabet No. 7 (1998) - Part 14: Latin alphabet 1834 No. 8 (Celtic) (1998) - Part 15: Latin alphabet 1835 No. 9 (1999) - Part 16: Part 16: Latin alphabet 1836 No. 10 (2001)", ISO Standard 8859, 2003. 1837 1838 [RFC2277] Alvestrand, H., "IETF Policy on Character Sets 1839 and Languages", BCP 18, RFC 2277, January 1998. 1840 1841 [RFC2825] IAB and L. Daigle, "A Tangled Web: Issues of 1842 I18N, Domain Names, and the Other Internet 1843 protocols", RFC 2825, May 2000. 1844 1845 [RFC3066] Alvestrand, H., "Tags for the Identification of 1846 Languages", BCP 47, RFC 3066, January 2001. 1847 1848 [RFC3467] Klensin, J., "Role of the Domain Name System 1849 (DNS)", RFC 3467, February 2003. 1850 1851 [RFC3536] Hoffman, P., "Terminology Used in 1852 Internationalization in the IETF", RFC 3536, 1853 May 2003. 1854 1855 [RFC3743] Konishi, K., Huang, K., Qian, H., and Y. Ko, 1856 "Joint Engineering Team (JET) Guidelines for 1857 Internationalized Domain Names (IDN) Registration 1858 and Administration for Chinese, Japanese, and 1859 Korean", RFC 3743, April 2004. 1860 1861 [RFC3912] Daigle, L., "WHOIS Protocol Specification", 1862 RFC 3912, September 2004. 1863 1864 1865 1866 1867 Klensin, et al. Informational [Page 34] 1868 RFC 4690 IAB -- IDN Next Steps September 2006 1869 1870 1871 [RFC3981] Newton, A. and M. Sanz, "IRIS: The Internet 1872 Registry Information Service (IRIS) Core 1873 Protocol", RFC 3981, January 2005. 1874 1875 [RFC3982] Newton, A. and M. Sanz, "IRIS: A Domain Registry 1876 (dreg) Type for the Internet Registry Information 1877 Service (IRIS)", RFC 3982, January 2005. 1878 1879 [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, 1880 "Uniform Resource Identifier (URI): Generic 1881 Syntax", STD 66, RFC 3986, January 2005. 1882 1883 [RFC3987] Duerst, M. and M. Suignard, "Internationalized 1884 Resource Identifiers (IRIs)", RFC 3987, 1885 January 2005. 1886 1887 [RFC4185] Klensin, J., "National and Local Characters for 1888 DNS Top Level Domain (TLD) Names", RFC 4185, 1889 October 2005. 1890 1891 [RFC4290] Klensin, J., "Suggested Practices for 1892 Registration of Internationalized Domain Names 1893 (IDN)", RFC 4290, December 2005. 1894 1895 [RFC4645] Ewell, D., "Initial Language Subtag Registry", 1896 RFC 4645, September 2006. 1897 1898 [RFC4646] Phillips, A. and M. Davis, "Tags for Identifying 1899 Languages", BCP 47, RFC 4646, September 2006. 1900 1901 [UTR] Unicode Consortium, "Unicode Technical Reports", 1902 <http://www.unicode.org/reports/>. 1903 1904 [UTR36] Davis, M. and M. Suignard, "Unicode Technical 1905 Report #36: Unicode Security Considerations", 1906 November 2005, <http://www.unicode.org/draft/ 1907 reports/tr36/tr36.html>. 1908 1909 [UTR39] Davis, M. and M. Suignard, "Unicode Technical 1910 Standard #39 (proposed): Unicode Security 1911 Considerations", July 2005, <http:// 1912 www.unicode.org/draft/reports/tr39/tr39.html>. 1913 1914 [Unicode-PR29] The Unicode Consortium, "Public Review Issue #29: 1915 Normalization Issue", Unicode PR 29, 1916 February 2004. 1917 1918 [Unicode10] The Unicode Consortium, "The Unicode Standard, 1919 1920 1921 1922 Klensin, et al. Informational [Page 35] 1923 RFC 4690 IAB -- IDN Next Steps September 2006 1924 1925 1926 Version 1.0", 1991. 1927 1928 [W3C-Localization] Ishida, R. and S. Miller, "Localization vs. 1929 Internationalization", W3C International/ 1930 questions/qa-i18n.txt, December 2005. 1931 1932 [net-utf8] Klensin, J. and M. Padlipsky, "Unicode Format for 1933 Network Interchange", Work in Progress, 1934 April 2006. 1935 1936 Authors' Addresses 1937 1938 John C Klensin 1939 1770 Massachusetts Ave, #322 1940 Cambridge, MA 02140 1941 USA 1942 1943 Phone: +1 617 491 5735 1944 EMail: email@example.com 1945 1946 1947 Patrik Faltstrom 1948 Cisco Systems 1949 1950 EMail: firstname.lastname@example.org 1951 1952 1953 Cary Karp 1954 Swedish Museum of Natural History 1955 Box 50007 1956 Stockholm SE-10405 1957 Sweden 1958 1959 Phone: +46 8 5195 4055 1960 EMail: email@example.com 1961 1962 1963 IAB 1964 1965 EMail: firstname.lastname@example.org 1966 1967 1968 1969 1970 1971 1972 1973 1974 1975 1976 1977 Klensin, et al. Informational [Page 36] 1978 RFC 4690 IAB -- IDN Next Steps September 2006 1979 1980 1981 Full Copyright Statement 1982 1983 Copyright (C) The Internet Society (2006). 1984 1985 This document is subject to the rights, licenses and restrictions 1986 contained in BCP 78, and except as set forth therein, the authors 1987 retain all their rights. 1988 1989 This document and the information contained herein are provided on an 1990 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS 1991 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET 1992 ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, 1993 INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE 1994 INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED 1995 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. 1996 1997 Intellectual Property 1998 1999 The IETF takes no position regarding the validity or scope of any 2000 Intellectual Property Rights or other rights that might be claimed to 2001 pertain to the implementation or use of the technology described in 2002 this document or the extent to which any license under such rights 2003 might or might not be available; nor does it represent that it has 2004 made any independent effort to identify any such rights. Information 2005 on the procedures with respect to rights in RFC documents can be 2006 found in BCP 78 and BCP 79. 2007 2008 Copies of IPR disclosures made to the IETF Secretariat and any 2009 assurances of licenses to be made available, or the result of an 2010 attempt made to obtain a general license or permission for the use of 2011 such proprietary rights by implementers or users of this 2012 specification can be obtained from the IETF on-line IPR repository at 2013 http://www.ietf.org/ipr. 2014 2015 The IETF invites any interested party to bring to its attention any 2016 copyrights, patents or patent applications, or other proprietary 2017 rights that may cover technology that may be required to implement 2018 this standard. Please address the information to the IETF at 2019 email@example.com. 2020 2021 Acknowledgement 2022 2023 Funding for the RFC Editor function is provided by the IETF 2024 Administrative Support Activity (IASA). 2025 2026 2027 2028 2029 2030 2031 2032 Klensin, et al. Informational [Page 37] 2033
[IESG-IDN] Internet Engineering Steering Group (IESG), "IESG Statement on IDN", IESG Statements IDN Statement, February 2003, <http://www.ietf.org/IESG/ STATEMENTS/IDNstatement.txt>.
[IESG-IDN] Internet Engineering Steering Group (IESG), "IESG Statement on IDN", IESG Statements IDN Statement, February 2003, <https://www.ietf.org/iesg/statement/ idn.html>.
URL of resource has changed. Original gives 'Not found'. --VERIFIER NOTES-- The right thing to do here is make sure the original URL redirects to the right place, which is now happening.