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2 Network Working Group J. Klensin
3 Internet-Draft June 18, 2009
4 Intended status: Informational
5 Expires: December 20, 2009
7 Internationalized Domain Names for Applications (IDNA): Background,
8 Explanation, and Rationale
9 draft-ietf-idnabis-rationale-10.txt
11 Status of this Memo
13 This Internet-Draft is submitted to IETF in full conformance with the
14 provisions of BCP 78 and BCP 79. This document may contain material
15 from IETF Documents or IETF Contributions published or made publicly
16 available before November 10, 2008. The person(s) controlling the
17 copyright in some of this material may not have granted the IETF
18 Trust the right to allow modifications of such material outside the
19 IETF Standards Process. Without obtaining an adequate license from
20 the person(s) controlling the copyright in such materials, this
21 document may not be modified outside the IETF Standards Process, and
22 derivative works of it may not be created outside the IETF Standards
23 Process, except to format it for publication as an RFC or to
24 translate it into languages other than English.
26 Internet-Drafts are working documents of the Internet Engineering
27 Task Force (IETF), its areas, and its working groups. Note that
28 other groups may also distribute working documents as Internet-
29 Drafts.
31 Internet-Drafts are draft documents valid for a maximum of six months
32 and may be updated, replaced, or obsoleted by other documents at any
33 time. It is inappropriate to use Internet-Drafts as reference
34 material or to cite them other than as "work in progress."
36 The list of current Internet-Drafts can be accessed at
37 http://www.ietf.org/ietf/1id-abstracts.txt.
39 The list of Internet-Draft Shadow Directories can be accessed at
40 http://www.ietf.org/shadow.html.
42 This Internet-Draft will expire on December 20, 2009.
44 Copyright Notice
46 Copyright (c) 2009 IETF Trust and the persons identified as the
47 document authors. All rights reserved.
49 This document is subject to BCP 78 and the IETF Trust's Legal
50 Provisions Relating to IETF Documents in effect on the date of
51 publication of this document (http://trustee.ietf.org/license-info).
52 Please review these documents carefully, as they describe your rights
53 and restrictions with respect to this document.
55 Abstract
57 Several years have passed since the original protocol for
58 Internationalized Domain Names (IDNs) was completed and deployed.
59 During that time, a number of issues have arisen, including the need
60 to update the system to deal with newer versions of Unicode. Some of
61 these issues require tuning of the existing protocols and the tables
62 on which they depend. This document provides an overview of a
63 revised system and provides explanatory material for its components.
65 Table of Contents
67 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
68 1.1. Context and Overview . . . . . . . . . . . . . . . . . . . 4
69 1.2. Discussion Forum . . . . . . . . . . . . . . . . . . . . . 5
70 1.3. Terminology . . . . . . . . . . . . . . . . . . . . . . . 5
71 1.3.1. Documents and Standards . . . . . . . . . . . . . . . 5
72 1.3.2. DNS "Name" Terminology . . . . . . . . . . . . . . . . 5
73 1.3.3. New Terminology and Restrictions . . . . . . . . . . . 6
74 1.4. Objectives . . . . . . . . . . . . . . . . . . . . . . . . 7
75 1.5. Applicability and Function of IDNA . . . . . . . . . . . . 7
76 1.6. Comprehensibility of IDNA Mechanisms and Processing . . . 8
77 2. Processing in IDNA2008 . . . . . . . . . . . . . . . . . . . . 9
78 3. Permitted Characters: An Inclusion List . . . . . . . . . . . 9
79 3.1. A Tiered Model of Permitted Characters and Labels . . . . 10
80 3.1.1. PROTOCOL-VALID . . . . . . . . . . . . . . . . . . . . 10
81 3.1.2. CONTEXTUAL RULE REQUIRED . . . . . . . . . . . . . . . 11
82 3.1.2.2. Rules and Their Application . . . . . . . . . . . 12
83 3.1.3. DISALLOWED . . . . . . . . . . . . . . . . . . . . . . 12
84 3.1.4. UNASSIGNED . . . . . . . . . . . . . . . . . . . . . . 13
85 3.2. Registration Policy . . . . . . . . . . . . . . . . . . . 13
86 3.3. Layered Restrictions: Tables, Context, Registration,
87 Applications . . . . . . . . . . . . . . . . . . . . . . . 14
88 4. Issues that Constrain Possible Solutions . . . . . . . . . . . 15
89 4.1. Display and Network Order . . . . . . . . . . . . . . . . 15
90 4.2. Entry and Display in Applications . . . . . . . . . . . . 16
91 4.3. Linguistic Expectations: Ligatures, Digraphs, and
92 Alternate Character Forms . . . . . . . . . . . . . . . . 17
93 4.4. Case Mapping and Related Issues . . . . . . . . . . . . . 18
94 4.5. Right to Left Text . . . . . . . . . . . . . . . . . . . . 19
95 5. IDNs and the Robustness Principle . . . . . . . . . . . . . . 20
96 6. Front-end and User Interface Processing for Lookup . . . . . . 20
97 7. Migration from IDNA2003 and Unicode Version Synchronization . 24
98 7.1. Design Criteria . . . . . . . . . . . . . . . . . . . . . 24
99 7.1.1. Summary and Discussion of IDNA Validity Criteria . . . 24
100 7.1.2. Labels in Registration . . . . . . . . . . . . . . . . 25
101 7.1.3. Labels in Lookup . . . . . . . . . . . . . . . . . . . 26
102 7.2. Changes in Character Interpretations . . . . . . . . . . . 27
103 7.3. More Flexibility in User Agents . . . . . . . . . . . . . 28
104 7.4. The Question of Prefix Changes . . . . . . . . . . . . . . 30
105 7.4.1. Conditions Requiring a Prefix Change . . . . . . . . . 30
106 7.4.2. Conditions Not Requiring a Prefix Change . . . . . . . 31
107 7.4.3. Implications of Prefix Changes . . . . . . . . . . . . 31
108 7.5. Stringprep Changes and Compatibility . . . . . . . . . . . 31
109 7.6. The Symbol Question . . . . . . . . . . . . . . . . . . . 32
110 7.7. Migration Between Unicode Versions: Unassigned Code
111 Points . . . . . . . . . . . . . . . . . . . . . . . . . . 33
112 7.8. Other Compatibility Issues . . . . . . . . . . . . . . . . 35
113 8. Name Server Considerations . . . . . . . . . . . . . . . . . . 35
114 8.1. Processing Non-ASCII Strings . . . . . . . . . . . . . . . 36
115 8.2. DNSSEC Authentication of IDN Domain Names . . . . . . . . 36
116 8.3. Root and other DNS Server Considerations . . . . . . . . . 37
117 9. Internationalization Considerations . . . . . . . . . . . . . 37
118 10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 37
119 10.1. IDNA Character Registry . . . . . . . . . . . . . . . . . 38
120 10.2. IDNA Context Registry . . . . . . . . . . . . . . . . . . 38
121 10.3. IANA Repository of IDN Practices of TLDs . . . . . . . . . 38
122 11. Security Considerations . . . . . . . . . . . . . . . . . . . 38
123 11.1. General Security Issues with IDNA . . . . . . . . . . . . 38
124 12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 39
125 13. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 39
126 14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 40
127 14.1. Normative References . . . . . . . . . . . . . . . . . . . 40
128 14.2. Informative References . . . . . . . . . . . . . . . . . . 41
129 Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . 43
130 A.1. Changes between Version -00 and Version -01 of
131 draft-ietf-idnabis-rationale . . . . . . . . . . . . . . . 43
132 A.2. Version -02 . . . . . . . . . . . . . . . . . . . . . . . 44
133 A.3. Version -03 . . . . . . . . . . . . . . . . . . . . . . . 44
134 A.4. Version -04 . . . . . . . . . . . . . . . . . . . . . . . 44
135 A.5. Version -05 . . . . . . . . . . . . . . . . . . . . . . . 45
136 A.6. Version -06 . . . . . . . . . . . . . . . . . . . . . . . 45
137 A.7. Version -07 . . . . . . . . . . . . . . . . . . . . . . . 46
138 A.8. Version -08 . . . . . . . . . . . . . . . . . . . . . . . 46
139 A.9. Version -09 . . . . . . . . . . . . . . . . . . . . . . . 46
140 A.10. Version -10 . . . . . . . . . . . . . . . . . . . . . . . 47
141 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 47
143 1. Introduction
145 1.1. Context and Overview
147 Internationalized Domain Names in Applications (IDNA) is a collection
148 of standards that allow client applications to convert some Unicode
149 mnemonics to an ASCII-compatible encoding form ("ACE") which is a
150 valid DNS label containing only letters, digits, and hyphens. The
151 specific form of ACE label used by IDNA is called an "A-label". A
152 client can look up an exact A-label in the existing DNS, so A-labels
153 do not require any extensions to DNS, upgrades of DNS servers or
154 updates to low-level client libraries. An A-label is recognizable
155 from the prefix "xn--" before the characters produced by the Punycode
156 algorithm [RFC3492], thus a user application can identify an A-label
157 and convert it into Unicode (or some local coded character set) for
158 display.
160 [[anchor3: Note in draft: The above discussion, and the rest of the
161 text in this section, are very informal. In particular, the term
162 "A-label" is used to refer to some things that don't meet all of the
163 tests for A-labels. I have tightened it somewhat from the suggested
164 text I received, but not very much. Is the current form ok with
165 everyone???]]
167 On the registry side, IDNA allows a registry to offer
168 Internationalized Domain Names (IDNs) for registration as A-labels.
169 A registry may offer any subset of valid IDNs, and may apply any
170 restrictions or bundling (grouping of similar labels together in one
171 registration) appropriate for the context of that registry.
172 Registration of labels is sometimes discussed separately from lookup,
173 and is subject to a few specific requirements that do not apply to
174 lookup.
176 DNS clients and registries are subject to some differences in
177 requirements for handling IDNs. In particular, registries are urged
178 to register only exact, valid A-labels, while clients might do some
179 mapping to get from otherwise-invalid user input to a valid A-label.
181 The first version of IDNA was published in 2003 and is referred to
182 here as IDNA2003 to contrast it with the current version, which is
183 known as IDNA2008. The documents that made up both versions are
184 listed in Section 1.3.1. The characters that are valid in A-labels
185 are identified from rules listed in the Tables document
186 [IDNA2008-Tables], but validity can be derived from the Unicode
187 properties of those characters with a very few exceptions.
189 Traditionally, DNS labels are case-insensitive [RFC1034][RFC1035].
190 That pattern was preserved in IDNA2003, but if case rules are
191 enforced from one language, another language sometimes loses the
192 ability to treat two characters separately. Case-sensitivity is
193 treated slightly differently in IDNA2008.
195 IDNA2003 used Unicode version 3.2 only. In order to keep up with new
196 characters added in new versions of UNICODE, IDNA2008 decouples its
197 rules from any particular version of UNICODE. Instead, the
198 attributes of new characters in Unicode determines how and whether
199 the characters can be used in IDNA labels.
201 This document provides informational context for IDNA2008, including
202 terminology, background, and policy discussions.
204 1.2. Discussion Forum
206 [[ RFC Editor: please remove this section. ]]
208 IDNA2008 is being discussed in the IETF "idnabis" Working Group and
209 on the mailing list idna-update@alvestrand.no
211 1.3. Terminology
213 Terminology for IDNA2008 appears in [IDNA2008-Defs]. That document
214 also contains a roadmap to the IDNA2008 document collection. No
215 attempt should be made to understand this document without the
216 definitions and concepts that appear there.
218 1.3.1. Documents and Standards
220 This document uses the term "IDNA2003" to refer to the set of
221 standards published in 2003 to define IDNA: the IDNA base
222 specification [RFC3490], Nameprep [RFC3491], Punycode [RFC3492], and
223 Stringprep [RFC3454].
225 The term "IDNA2008" is used to refer to a new version of IDNA.
226 IDNA2008 is not dependent on any of the IDNA2003 specifications other
227 than the one for Punycode encoding. References to "these
228 specifications" or "these documents" are to the entire IDNA2008 set
229 listed in [IDNA2008-Defs].
231 1.3.2. DNS "Name" Terminology
233 In the context of IDNs, the DNS term 'name' has introduced some
234 confusion as people speak of DNS labels in terms of the words or
235 phrases of various natural languages. Historically, many of the
236 "names" in the DNS have been mnemonics to identify some particular
237 concept, object, or organization. They are typically rooted in some
238 language because most people think in language-based ways. But,
239 because they are mnemonics, they need not obey the orthographic
240 conventions of any language: it is not a requirement that it be
241 possible for them to be "words".
243 This distinction is important because the reasonable goal of an IDN
244 effort is not to be able to write the great Klingon (or language of
245 one's choice) novel in DNS labels but to be able to form a usefully
246 broad range of mnemonics in ways that are as natural as possible in a
247 very broad range of scripts.
249 1.3.3. New Terminology and Restrictions
251 These documents introduce new terminology, and precise definitions,
252 for the terms "U-label", "A-Label", LDH-label (to which all valid
253 pre-IDNA host names conformed), Reserved-LDH-label (R-LDH-label), XN-
254 label, Fake-A-Label, and Non-Reserved-LDH-label (NR-LDH-label).
256 In addition, the term "putative label" has been adopted to refer to a
257 label that may appear to meet certain definitional constraints but
258 has not yet been sufficiently tested for validity.
260 These definitions are illustrated in Figure 1 of the Definitions
261 Document [IDNA2008-Defs]. R-LDH-labels contain "--" in the third and
262 fourth character from the beginning of the label. In IDNA-aware
263 applications, only a subset of these reserved labels is permitted to
264 be used, namely the A-label subset. A-labels are a subset of the
265 R-LDH-labels that begin with the case-insensitive string "xn--".
266 Labels that bear this prefix but which are not otherwise valid fall
267 into the "Fake-A-label" category. The non-reserved labels (NR-LDH-
268 labels) are implicitly valid since they do not trigger any
269 resemblance to IDNA-landr NR-LDH-labels.
271 The creation of the Reserved-LDH category is required for three
272 reasons:
274 o to prevent confusion with pre-IDNA coding forms;
276 o to permit future extensions that would require changing the
277 prefix, no matter how unlikely those might be (see Section 7.4);
278 and
280 o to reduce the opportunities for attacks via the Punycode encoding
281 algorithm itself.
283 1.4. Objectives
285 These are the main objectives in revising IDNA.
287 o Use a more recent version of Unicode, and allow IDNA to be
288 independent of Unicode versions, so that IDNA2008 need not be
289 update for implementations to adopt codepoints from new Unicode
290 versions.
292 o Fix a very small number of code-point categorizations that have
293 turned out to cause problems in the communities that use those
294 code-points.
296 o Reduce the dependency on mapping, in order that the pre-mapped
297 forms (which are not valid IDNA labels) tend to appear less often
298 in various contexts, in favor of valid A-labels.
300 o Fix some details in the bidirectional codepoint handling
301 algorithms.
303 1.5. Applicability and Function of IDNA
305 The IDNA specification solves the problem of extending the repertoire
306 of characters that can be used in domain names to include a large
307 subset of the Unicode repertoire.
309 IDNA does not extend DNS. Instead, the applications (and, by
310 implication, the users) continue to see an exact-match lookup
311 service. Either there is a single exactly-matching (subject to the
312 base DNS requirement of case-insensitive ASCII matching) name or
313 there is no match. This model has served the existing applications
314 well, but it requires, with or without internationalized domain
315 names, that users know the exact spelling of the domain names that
316 are to be typed into applications such as web browsers and mail user
317 agents. The introduction of the larger repertoire of characters
318 potentially makes the set of misspellings larger, especially given
319 that in some cases the same appearance, for example on a business
320 card, might visually match several Unicode code points or several
321 sequences of code points.
323 The IDNA standard does not require any applications to conform to it,
324 nor does it retroactively change those applications. An application
325 can elect to use IDNA in order to support IDN while maintaining
326 interoperability with existing infrastructure. If an application
327 wants to use non-ASCII characters in domain names, IDNA is the only
328 currently-defined option. Adding IDNA support to an existing
329 application entails changes to the application only, and leaves room
330 for flexibility in front-end processing and more specifically in the
331 user interface (see Section 6).
333 A great deal of the discussion of IDN solutions has focused on
334 transition issues and how IDNs will work in a world where not all of
335 the components have been updated. Proposals that were not chosen by
336 the original IDN Working Group would have depended on updating of
337 user applications, DNS resolvers, and DNS servers in order for a user
338 to apply an internationalized domain name in any form or coding
339 acceptable under that method. While processing must be performed
340 prior to or after access to the DNS, IDNA requires no changes to the
341 DNS protocol or any DNS servers or the resolvers on user's computers.
343 IDNA allows the graceful introduction of IDNs not only by avoiding
344 upgrades to existing infrastructure (such as DNS servers and mail
345 transport agents), but also by allowing some rudimentary use of IDNs
346 in applications by using the ASCII-encoded representation of the
347 labels containing non-ASCII characters. While such names are user-
348 unfriendly to read and type, and hence not optimal for user input,
349 they can be used as a last resort to allow rudimentary IDN usage.
350 For example, they might be the best choice for display if it were
351 known that relevant fonts were not available on the user's computer.
352 In order to allow user-friendly input and output of the IDNs and
353 acceptance of some characters as equivalent to those to be processed
354 according to the protocol, the applications need to be modified to
355 conform to this specification.
357 This version of IDNA uses the Unicode character repertoire, for
358 continuity with the original version of IDNA.
360 1.6. Comprehensibility of IDNA Mechanisms and Processing
362 One goal of IDNA2008, which is aided by the main goal of reducing the
363 dependency on mapping, is to improve the general understanding of how
364 to users and registrants are important design goals for this effort.
365 End-user applications have an important role to play in increasing
366 this comprehensibility.
368 Any system that tries to handle international characters encounters
369 some common problems. For example, a UI cannot display a character
370 if no font for that character is available. In some cases,
371 internationalization enables effective localization while maintaining
372 some global uniformity but losing some universality.
374 It is difficult to even make suggestions for end-user applications to
375 cope when characters and fonts are not available. Because display
376 functions are rarely controlled by the types of applications that
377 would call upon IDNA, such suggestions will rarely be very effective.
379 Converting between local character sets and normalized Unicode, if
380 needed, is part of this set of user agent issues. This conversion
381 introduces complexity in a system that is not Unicode-native. If a
382 label is converted to a local character set that does not have all
383 the needed characters, the user agent may have to add special logic
384 to avoid or reduce loss of information.
386 The major difficulty may lie in accurately identifying the incoming
387 character set and applying the correct conversion routine. Even more
388 difficult, the local character coding system could be based on
389 conceptually different assumptions than those used by Unicode (e.g.,
390 choice of font encodings used for publications in some Indic
391 scripts). Those differences may not easily yield unambiguous
392 conversions or interpretations even if each coding system is
393 internally consistent and adequate to represent the local language
394 and script.
396 IDNA2008 shifts responsibility for character mapping and other
397 adjustments from the protocol (where it was located in IDNA2003) to
398 pre-processing before invoking IDNA. The intent is that this change
399 leads to greater usage of fully-valid A-Labels in display, transit
400 and storage, which should aid comprehensibility. A careful look at
401 pre-processing raises issues about what that pre-processing should do
402 and at what point pre-processing becomes harmful, how universally
403 consistent pre-processing algorithms can be, and how to be compatible
404 with labels prepared in a IDNA2003 context. Those issues are
405 discussed in Section 6. [[anchor9: Fix section reference.]]
407 2. Processing in IDNA2008
409 These specifications separate Domain Name Registration and Lookup in
410 the protocol specification. This separation reflects current
411 practice in which per-registry restrictions and special processing
412 are applied at registration time but not during lookup. Another
413 significant benefit is that separation facilitates incremental
414 addition of permitted character groups to avoid freezing on one
415 particular version of Unicode.
417 The actual registration and lookup protocols for IDNA2008 are
418 specified in [IDNA2008-Protocol].
420 3. Permitted Characters: An Inclusion List
422 IDNA2008 adopts the inclusion model. A code-point is assumed to be
423 invalid, unless it is included as part of a Unicode property-based
424 rule or in rare cases included individually by an exception. When an
425 implementation moves to a new version of Unicode, the rules may
426 indicate new valid code-points.
428 This section provides an overview of the model used to establish the
429 algorithm and character lists of [IDNA2008-Tables] and describes the
430 names and applicability of the categories used there. Note that the
431 inclusion of a character in the first category group (Section 3.1.1)
432 does not imply that it can be used indiscriminately; some characters
433 are associated with contextual rules that must be applied as well.
435 The information given in this section is provided to make the rules,
436 tables, and protocol easier to understand. The normative generating
437 rules that correspond to this informal discussion appear in
438 [IDNA2008-Tables] and the rules that actually determine what labels
439 can be registered or looked up are in [IDNA2008-Protocol].
441 3.1. A Tiered Model of Permitted Characters and Labels
443 Moving to an inclusion model involves a new specification for the
444 list of characters that are permitted in IDNs. In IDNA2003,
445 character validity is independent of context and fixed forever (or
446 until the standard is replaced). However, globally context-
447 independent rules have proved to be impractical because some
448 characters, especially those that are called "Join_Controls" in
449 Unicode, are needed to make reasonable use of some scripts but have
450 no visible effect in others. IDNA2003 prohibited those types of
451 characters entirely by discarding them. We now have a consensus that
452 under some conditions, these "joiner" characters are legitimately
453 needed to allow useful mnemonics for some languages and scripts. In
454 general, context-dependent rules help deal with characters that are
455 used differently across different scripts, and allow the standard to
456 be applied more appropriately in cases where a string is not
457 universally handled the same way.
459 IDNA2008 divides all possible Unicode code-points into four
460 categories: PROTOCOL-VALID, CONTEXTUAL RULE REQUIRED, DISALLOWED and
461 UNASSIGNED.
463 3.1.1. PROTOCOL-VALID
465 Characters identified as "PROTOCOL-VALID" (often abbreviated
466 "PVALID") are permitted in IDNs. Their use may be restricted by
467 rules about the context in which they appear or by other rules that
468 apply to the entire label in which they are to be embedded. For
469 example, any label that contains a character in this category that
470 has a "right-to-left" property must be used in context with the
471 "Bidi" rules (see [IDNA2008-Bidi]).
473 The term "PROTOCOL-VALID" is used to stress the fact that the
474 presence of a character in this category does not imply that a given
475 registry need accept registrations containing any of the characters
476 in the category. Registries are still expected to apply judgment
477 about labels they will accept and to maintain rules consistent with
478 those judgments (see [IDNA2008-Protocol] and Section 3.3).
480 Characters that are placed in the "PROTOCOL-VALID" category are
481 expected to never be removed from it or reclassified. While
482 theoretically characters could be removed from Unicode, such removal
483 would be inconsistent with the Unicode stability principles (see
484 [Unicode51], Appendix F) and hence should never occur.
486 3.1.2. CONTEXTUAL RULE REQUIRED
488 Some characters may be unsuitable for general use in IDNs but
489 necessary for the plausible support of some scripts. The two most
490 commonly-cited examples are the zero-width joiner and non-joiner
491 characters (ZWJ, U+200D and ZWNJ, U+200C).
493 3.1.2.1. Contextual Restrictions
495 Characters with contextual restrictions are identified as "CONTEXTUAL
496 RULE REQUIRED" and associated with a rule. The rule defines whether
497 the character is valid in a particular string, and also whether the
498 rule itself is to be applied on lookup as well as registration.
500 A distinction is made between characters that indicate or prohibit
501 joining and ones similar to them (known as "CONTEXT-JOINER" or
502 "CONTEXTJ") and other characters requiring contextual treatment
503 ("CONTEXT-OTHER" or "CONTEXTO"). Only the former require full
504 testing at lookup time.
506 It is important to note that these contextual rules cannot prevent
507 all uses of the relevant characters that might be confusing or
508 problematic. What they are expected do is to confine applicability
509 of the characters to scripts (and narrower contexts) where zone
510 administrators are knowledgeable enough about the use of those
511 characters to be prepared to deal with them appropriately. For
512 example, a registry dealing with an Indic script that requires ZWJ
513 and/or ZWNJ as part of the writing system is expected to understand
514 where the characters have visible effect and where they do not and to
515 make registration rules accordingly. By contrast, a registry dealing
516 with Latin or Cyrillic script might not be actively aware that the
517 characters exist, much less about the consequences of embedding them
518 in labels drawn from those scripts.
520 3.1.2.2. Rules and Their Application
522 Rules have descriptions such as "Must follow a character from Script
523 XYZ", "Must occur only if the entire label is in Script ABC", or
524 "Must occur only if the previous and subsequent characters have the
525 DFG property". The actual rules may be DEFINED or NULL. If present,
526 they may have values of "True" (character may be used in any position
527 in any label), "False" (character may not be used in any label), or
528 may be a set of procedural rules that specify the context in which
529 the character is permitted.
531 Examples of descriptions of typical rules, stated informally and in
532 English, include "Must follow a character from Script XYZ", "Must
533 occur only if the entire label is in Script ABC", "Must occur only if
534 the previous and subsequent characters have the DFG property".
536 Because it is easier to identify these characters than to know that
537 they are actually needed in IDNs or how to establish exactly the
538 right rules for each one, a rule may have a null value in a given
539 version of the tables. Characters associated with null rules are not
540 permitted to appear in putative labels for either registration or
541 lookup. Of course, a later version of the tables might contain a
542 non-null rule.
544 The actual rules and their descriptions are in [IDNA2008-Tables].
545 [[anchor12: ??? Section number would be good here.]] That document
546 also creates a registry for future rules.
548 3.1.3. DISALLOWED
550 Some characters are inappropriate for use in IDNs and are thus
551 excluded for both registration and lookup (i.e., IDNA-conforming
552 applications performing name lookup should verify that these
553 characters are absent; if they are present, the label strings should
554 be rejected rather than converted to A-labels and looked up. Some of
555 these characters are problematic for use in IDNs (such as the
556 FRACTION SLASH character, U+2044), while some of them (such as the
557 various HEART symbols, e.g., U+2665, U+2661, and U+2765, see
558 Section 7.6) simply fall outside the conventions for typical
559 identifiers (basically letters and numbers).
561 Of course, this category would include code points that had been
562 removed entirely from Unicode should such removals ever occur.
564 Characters that are placed in the "DISALLOWED" category are expected
565 to never be removed from it or reclassified. If a character is
566 classified as "DISALLOWED" in error and the error is sufficiently
567 problematic, the only recourse would be either to introduce a new
568 code point into Unicode and classify it as "PROTOCOL-VALID" or for
569 the IETF to accept the considerable costs of an incompatible change
570 and replace the relevant RFC with one containing appropriate
571 exceptions.
573 There is provision for exception cases but, in general, characters
574 are placed into "DISALLOWED" if they fall into one or more of the
575 following groups:
577 o The character is a compatibility equivalent for another character.
578 In slightly more precise Unicode terms, application of
579 normalization method NFKC to the character yields some other
580 character.
582 o The character is an upper-case form or some other form that is
583 mapped to another character by Unicode casefolding.
585 o The character is a symbol or punctuation form or, more generally,
586 something that is not a letter, digit, or a mark that is used to
587 form a letter or digit.
589 3.1.4. UNASSIGNED
591 For convenience in processing and table-building, code points that do
592 not have assigned values in a given version of Unicode are treated as
593 belonging to a special UNASSIGNED category. Such code points are
594 prohibited in labels to be registered or looked up. The category
595 differs from DISALLOWED in that code points are moved out of it by
596 the simple expedient of being assigned in a later version of Unicode
597 (at which point, they are classified into one of the other categories
598 as appropriate).
600 The rationale for restricting the processing of UNASSIGNED characters
601 is simply that if such characters were permitted to be looked up, for
602 example, and were later assigned, but subject to some set of
603 contextual rules, un-updated instances of IDNA-aware software might
604 permit lookup of labels containing the previously-unassigned
605 characters while updated versions of IDNA-aware software might
606 restrict their use in lookup, depending on the contextual rules. It
607 should be clear that under no circumstance should an UNASSIGNED
608 character be permitted in a label to be registered as part of a
609 domain name.
611 3.2. Registration Policy
613 While these recommendations cannot and should not define registry
614 policies, registries should develop and apply additional restrictions
615 as needed to reduce confusion and other problems. For example, it is
616 generally believed that labels containing characters from more than
617 one script are a bad practice although there may be some important
618 exceptions to that principle. Some registries may choose to restrict
619 registrations to characters drawn from a very small number of
620 scripts. For many scripts, the use of variant techniques such as
621 those as described in RFC 3843 [RFC3743] and RFC 4290 [RFC4290], and
622 illustrated for Chinese by the tables described in RFC 4713 [RFC4713]
623 may be helpful in reducing problems that might be perceived by users.
625 In general, users will benefit if registries only permit characters
626 from scripts that are well-understood by the registry or its
627 advisers. If a registry decides to reduce opportunities for
628 confusion by constructing policies that disallow characters used in
629 historic writing systems or characters whose use is restricted to
630 specialized, highly technical contexts, some relevant information may
631 be found in Section 2.4 "Specific Character Adjustments", Table 4
632 "Candidate Characters for Exclusion from Identifiers" of
633 [Unicode-UAX31] and Section 3.1. "General Security Profile for
634 Identifiers" in [Unicode-Security].
636 It is worth stressing that these principles of policy development and
637 application apply at all levels of the DNS, not only, e.g., TLD or
638 SLD registrations and that even a trivial, "anything permitted that
639 is valid under the protocol" policy is helpful in that it helps users
640 and application developers know what to expect.
642 3.3. Layered Restrictions: Tables, Context, Registration, Applications
644 The character rules in IDNA2008 are based on the realization that
645 there is no single magic bullet for any of the issues associated with
646 IDNs. Instead, the specifications define a variety of approaches.
647 The character tables are the first mechanism, protocol rules about
648 how those characters are applied or restricted in context are the
649 second, and those two in combination constitute the limits of what
650 can be done in the protocol. As discussed in the previous section
651 (Section 3.2), registries are expected to restrict what they permit
652 to be registered, devising and using rules that are designed to
653 optimize the balance between confusion and risk on the one hand and
654 maximum expressiveness in mnemonics on the other.
656 In addition, there is an important role for user agents in warning
657 against label forms that appear problematic given their knowledge of
658 local contexts and conventions. Of course, no approach based on
659 naming or identifiers alone can protect against all threats.
661 4. Issues that Constrain Possible Solutions
663 4.1. Display and Network Order
665 Domain names are always transmitted in network order (the order in
666 which the code points are sent in protocols), but may have a
667 different display order (the order in which the code points are
668 displayed on a screen or paper). When a domain name contains
669 characters that are normally written right to left, display order may
670 be affected although network order is not. It gets even more
671 complicated if left to right and right to left labels are adjacent to
672 each other within a domain name. The decision about the display
673 order is ultimately under the control of user agents --including Web
674 browsers, mail clients, hosted Web applications and many more --
675 which may be highly localized. Should a domain name abc.def, in
676 which both labels are represented in scripts that are written right
677 to left, be displayed as fed.cba or cba.fed? Applications that are
678 in deployment today are already diverse, and one can find examples of
679 either choice.
681 The picture changes once again when an IDN appears in a
682 Internationalized Resource Identifier (IRI) [RFC3987]. An IRI or
683 Internationalized Email address contains elements other than the
684 domain name. For example, IRIs contain protocol identifiers and
685 field delimiter syntax such as "http://" or "mailto:" while email
686 addresses contain the "@" to separate local parts from domain names.
687 An IRI in network order begins with "http://" followed by domain
688 labels in network order, thus "http://abc.def".
690 User agents are not required to display and allow input of IRIs
691 directly but often do so. Implementors have to choose whether the
692 overall direction of these strings will always be left to right (or
693 right to left) for an IRI or email address. The natural order for a
694 user typing a domain name on a right to left system is fed.cba.
695 Should the R2L user agent reverse the entire domain name each time a
696 domain name is typed? Does this change if the user types "http://"
697 right before typing a domain name, thus implying that the user is
698 beginning at the beginning of the network order IRI? Experience in
699 the 1980s and 1990s with mixing systems in which domain name labels
700 were read in network order (left to right) and those in which those
701 labels were read right to left would predict a great deal of
702 confusion.
704 If each implementation of each application makes its own decisions on
705 these issues, users will develop heuristics that will sometimes fail
706 when switching applications. However, while some display order
707 conventions, voluntarily adopted, would be desirable to reduce
708 confusion, such suggestions are beyond the scope of these
709 specifications.
711 4.2. Entry and Display in Applications
713 Applications can accept and display domain names using any character
714 set or character coding system. That is, the IDNA protocol does not
715 necessarily affect the interface between users and applications. An
716 IDNA-aware application can accept and display internationalized
717 domain names in two formats: the internationalized character set(s)
718 supported by the application (i.e., an appropriate local
719 representation of a U-label), and as an A-label. Applications may
720 allow the display of A-labels, but are encouraged to not do so except
721 as an interface for special purposes, possibly for debugging, or to
722 cope with display limitations. In general, they should allow, but
723 not encourage, user input of A-labels. A-labels are opaque and ugly
724 and malicious variations on them are not easily detected by users.
725 Where possible, they should thus only be exposed when they are
726 absolutely needed. Because IDN labels can be rendered either as
727 A-labels or U-labels, the application may reasonably have an option
728 for the user to select the preferred method of display. Rendering
729 the U-label should normally be the default.
731 Domain names are often stored and transported in many places. For
732 example, they are part of documents such as mail messages and web
733 pages. They are transported in many parts of many protocols, such as
734 both the control commands of SMTP and associated the message body
735 parts, and in the headers and the body content in HTTP. It is
736 important to remember that domain names appear both in domain name
737 slots and in the content that is passed over protocols.
739 In protocols and document formats that define how to handle
740 specification or negotiation of charsets, labels can be encoded in
741 any charset allowed by the protocol or document format. If a
742 protocol or document format only allows one charset, the labels must
743 be given in that charset. Of course, not all charsets can properly
744 represent all labels. If a U-label cannot be displayed in its
745 entirety, the only choice (without loss of information) may be to
746 display the A-label.
748 Where a protocol or document format allows IDNs, labels should be in
749 whatever character encoding and escape mechanism the protocol or
750 document format uses at that place. This provision is intended to
751 prevent situations in which, e.g., UTF-8 domain names appear embedded
752 in text that is otherwise in some other character coding.
754 All protocols that use domain name slots (See Section 2.3.1.6
755 [[anchor15: ?? Verify this]] in [IDNA2008-Defs]) already have the
756 capacity for handling domain names in the ASCII charset. Thus,
757 A-labels can inherently be handled by those protocols.
759 4.3. Linguistic Expectations: Ligatures, Digraphs, and Alternate
760 Character Forms
762 Users have expectations about character matching or equivalence that
763 are based on their own languages and the orthography of those
764 languages. These expectations may not always be met in a global
765 system, especially if multiple languages are written using the same
766 script but using different conventions. Some examples:
768 o A Norwegian user might expect a label with the ae-ligature to be
769 treated as the same label as one using the Swedish spelling with
770 a-diaeresis even though applying that mapping to English would be
771 astonishing to users.
773 o A user in German might expect a label with an o-umlaut and a label
774 that had "oe" substituted, but was otherwise the same, treated as
775 equivalent even though that substitution would be a clear error in
776 Swedish.
778 o A Chinese user might expect automatic matching of Simplified and
779 Traditional Chinese characters, but applying that matching for
780 Korean or Japanese text would create considerable confusion.
782 o An English user might expect "theater" and "theatre" to match.
784 A number of languages use alphabetic scripts in which single phonemes
785 are written using two characters, termed a "digraph", for example,
786 the "ph" in "pharmacy" and "telephone". (Such characters can also
787 appear consecutively without forming a digraph, as in "tophat".)
788 Certain digraphs may be indicated typographically by setting the two
789 characters closer together than they would be if used consecutively
790 to represent different phonemes. Some digraphs are fully joined as
791 ligatures. For example, the word "encyclopaedia" is sometimes set
792 with a U+00E6 LATIN SMALL LIGATURE AE. When ligature and digraph
793 forms have the same interpretation across all languages that use a
794 given script, application of Unicode normalization generally resolves
795 the differences and causes them to match. When they have different
796 interpretations, matching must utilize other methods, presumably
797 chosen at the registry completely optional typographic convenience
798 for representing a digraph in one language (as in the above example
799 with some spelling conventions), while in another language it is a
800 single character that may not always be correctly representable by a
801 two-letter sequence (as in the above example with different spelling
802 conventions). This can be illustrated by many words in the Norwegian
803 language, where the "ae" ligature is the 27th letter of a 29-letter
804 extended Latin alphabet. It is equivalent to the 28th letter of the
805 Swedish alphabet (also containing 29 letters), U+00E4 LATIN SMALL
806 LETTER A WITH DIAERESIS, for which an "ae" cannot be substituted
807 according to current orthographic standards.
809 That character (U+00E4) is also part of the German alphabet where,
810 unlike in the Nordic languages, the two-character sequence "ae" is
811 usually treated as a fully acceptable alternate orthography for the
812 "umlauted a" character. The inverse is however not true, and those
813 two characters cannot necessarily be combined into an "umlauted a".
814 This also applies to another German character, the "umlauted o"
815 (U+00F6 LATIN SMALL LETTER O WITH DIAERESIS) which, for example,
816 cannot be used for writing the name of the author "Goethe". It is
817 also a letter in the Swedish alphabet where, like the "a with
818 diaeresis", it cannot be correctly represented as "oe" and in the
819 Norwegian alphabet, where it is represented, not as "o with
820 diaeresis", but as "slashed o", U+00F8.
822 Some of the ligatures that have explicit code points in Unicode were
823 given special handling in IDNA2003 and now pose additional problems
824 in transition. See Section 7.2.
826 Additional cases with alphabets written right to left are described
827 in Section 4.5.
829 Matching and comparison algorithm selection often requires
830 information about the language being used, context, or both --
831 information that is not available to IDNA or the DNS. Consequently,
832 these specifications make no attempt to treat combined characters in
833 any special way. A registry that is aware of the language context in
834 which labels are to be registered, and where that language sometimes
835 (or always) treats the two- character sequences as equivalent to the
836 combined form, should give serious consideration to applying a
837 "variant" model [RFC3743] [RFC4290], or to prohibiting registration
838 of one the forms entirely, to reduce the opportunities for user
839 confusion and fraud that would result from the related strings being
840 registered to different parties.
842 [[anchor16: Placeholder: A discussion of the Arabic digit issue
843 should go here once it is resolved in some appropriate way.]]
845 4.4. Case Mapping and Related Issues
847 In the DNS, ASCII letters are stored with their case preserved.
848 Matching during the query process is case-independent, but none of
849 the information that might be represented by choices of case has been
850 lost. That model has been accidentally helpful because, as people
851 have created DNS labels by catenating words (or parts of words) to
852 form labels, case has often been used to distinguish among components
853 and make the labels more memorable.
855 Since DNS servers do not get involved in parsing IDNs, they cannot do
856 case-independent matching. Thus, keeping the cases separate in
857 lookup or registration, and doing matching at the server, is not
858 feasible with IDNA or any similar approach. Case-matching must be
859 done, if desired, by IDN clients even though it wasn't done by ASCII-
860 only DNS clients. That situation was recognized in IDNA2003 and
861 nothing in these specifications fundamentally changes it or could do
862 so. In IDNA2003, all characters are case-folded and mapped by
863 clients in a standardized step.
865 Some characters do not have upper case forms. For example the
866 Unicode case folding operation maps Greek Final Form Sigma (U+03C2)
867 to the medial form (U+03C3) and maps Eszett (German Sharp S, U+00DF)
868 to "ss". Neither of these mappings is reversible because the upper
869 case of U+03C3 is the Upper Case Sigma (U+03A3) and "ss" is an ASCII
870 string. IDNA2008 permits, at the risk of some incompatibility,
871 slightly more flexibility in this area by avoid case folding and
872 treating these characters as themselves. Approaches to handling one-
873 way mappings are discussed in Section 7.2.
875 Because IDNA2003 maps Final Sigma and Eszett to other characters, and
876 the reverse mapping is never possible, that in some sense means that
877 neither Final Sigma nor Eszett can be represented in a IDNA2003 IDN.
878 With IDNA2008, both characters can be used in an IDN and so the
879 A-label used for lookup for any U-label containing those characters,
880 is now different. See Section 7.1 for a discussion of what kinds of
881 changes might require the IDNA prefix to change; this case is clearly
882 worth discussing but the WG came to consensus not to make a prefix
883 change anyway.
885 4.5. Right to Left Text
887 In order to be sure that the directionality of right to left text is
888 unambiguous, IDNA2003 required that any label in which right to left
889 characters appear both starts and ends with them and that it not
890 include any characters with strong left to right properties (that
891 excludes other alphabetic characters but permits European digits).
892 Any other string that contains a right to left character and does not
893 meet those requirements is rejected. This is one of the few places
894 where the IDNA algorithms (both in IDNA2003 and in IDAN2008) examine
895 an entire label, not just individual characters. The algorithmic
896 model used in IDNA2003 rejects the label when the final character in
897 a right to left string requires a combining mark in order to be
898 correctly represented.
900 That prohibition is not acceptable for writing systems for languages
901 written with consonantal alphabets to which diacritical vocalic
902 systems are applied, and for languages with orthographies derived
903 from them where the combining marks may have different functionality.
904 In both cases the combining marks can be essential components of the
905 orthography. Examples of this are Yiddish, written with an extended
906 Hebrew script, and Dhivehi (the official language of Maldives) which
907 is written in the Thaana script (which is, in turn, derived from the
908 Arabic script). IDNA2008 removes the restriction on final combining
909 characters with a new set of rules for right to left scripts and
910 their characters. Those new rules are specified in [IDNA2008-Bidi].
912 5. IDNs and the Robustness Principle
914 The "Robustness Principle" is often stated as "Be conservative about
915 what you send and liberal in what you accept" (See, e.g., Section
916 1.2.2 of the applications-layer Host Requirements specification
917 [RFC1123]) This principle applies to IDNA. In applying the principle
918 to registries as the source ("sender") of all registered and useful
919 IDNs, registries are responsible for being conservative about what
920 they register and put out in the Internet. For IDNs to work well,
921 zone administrators (registries) must have and require sensible
922 policies about what is registered -- conservative policies -- and
923 implement and enforce them.
925 Conversely, lookup applications are expected to reject labels that
926 clearly violate global (protocol) rules (no one has ever seriously
927 claimed that being liberal in what is accepted requires being
928 stupid). However, once one gets past such global rules and deals
929 with anything sensitive to script or locale, it is necessary to
930 assume that garbage has not been placed into the DNS, i.e., one must
931 be liberal about what one is willing to look up in the DNS rather
932 than guessing about whether it should have been permitted to be
933 registered.
935 If a string cannot be successfully found in the DNS after the lookup
936 processing described here, it makes no difference whether it simply
937 wasn't registered or was prohibited by some rule at the registry.
938 Application implementors should be aware that where DNS wildcards are
939 used, the ability to successfully resolve a name does not guarantee
940 that it was actually registered.
942 6. Front-end and User Interface Processing for Lookup
944 [[anchor18: Note in Draft: While this section has been revised in
945 version -10 to improve clarity, a significant revision is expected
946 once the discussions of mapping stabilize.]]
947 Domain names may be identified and processed in many contexts. They
948 may be typed in by users either by themselves or embedded in an
949 identifier such as email addresses, URIs, or IRIs. They may occur in
950 running text or be processed by one system after being provided in
951 another. Systems may try to normalize URLs to determine (or guess)
952 whether a reference is valid or two references point to the same
953 object without actually looking the objects up (comparison without
954 lookup is necessary for URI types that are not intended to be
955 resolved). Some of these goals may be more easily and reliably
956 satisfied than others. While there are strong arguments for any
957 domain name that is placed "on the wire" -- transmitted between
958 systems -- to be in the zero-ambiguity forms of A-labels, it is
959 inevitable that programs that process domain names will encounter
960 U-labels or variant forms.
962 This section discusses these mapping and transformation issues among
963 names, contrasting IDNA2003 and IDNA2008 behavior. The discussion
964 applies only in operations that look up names or interpret files.
965 There are several reasons why registration activities should require
966 final names and verification of those names by the would-be
967 registrant.
969 One source of label forms that are neither A-labels nor U-labels will
970 be labels created under IDNA2003. That protocol allowed labels that
971 were transformed from native-character format by mapping some
972 characters into others before conversion into A-label format. One
973 consequence of the transformations was that conversion from the
974 A-label format back to native characters often did not produce the
975 original label. IDNA2008 explicitly defines A-labels and U-labels as
976 different forms of the same abstract label, forms that are stable
977 when conversions are performed between them (without mappings).
979 A different way of explaining this is that there are, today, domain
980 names in files on the Internet that use characters that cannot be
981 represented directly in, or recovered from, (A-label) domain names
982 but for which interpretations were provided by IDNA2003). There are
983 two major categories of characters irreversibly remapped by
984 Stringprep, those that are removed by NFKC normalization and those
985 upper-case characters that are mapped to lower-case (there are also a
986 few characters that are given special-case mapping treatment,
987 including lower-case characters that are case-folded into other
988 lower-case characters or strings and those that are simply
989 eliminated).
991 Other issues in domain name identification and processing arise
992 because IDNA2003 specified that several other characters be treated
993 as equivalent to the ASCII period (dot, full stop) character used as
994 a label separator. If a string that might be a domain name appears
995 in an arbitrary context (such as running text), it is difficult, even
996 with only ASCII characters, to know whether an actual domain name (or
997 a protocol parameter like a URI) is present and where it starts and
998 ends. When using Unicode, this gets even more difficult if treatment
999 of certain special characters (like the dot that separates labels in
1000 a domain name) depends on context (e.g., prior knowledge of whether
1001 the string represents a domain name or not). That knowledge is not
1002 available if the primary heuristic for identifying the presence of
1003 domain names in strings depends on the presence of dots separating
1004 groups of characters with no intervening spaces.
1006 [[anchor19: Placeholder: In serial efforts to move the mapping model
1007 out of the protocol and leave it unspecified here, this paragraph has
1008 become a complete botch. Rewrite when the mapping plan stabilizes.]]
1009 The IDNA2008 model removes all of these mappings and interpretations,
1010 including the equivalence of different forms of dots, from the
1011 protocol, discouraging such mappings and leaving them, when
1012 necessary, to local processing. This should not be taken to imply
1013 that local processing is optional or can be avoided entirely, even if
1014 doing so might have been desirable in a world without IDNA2003 IDNs
1015 in files and archives. Instead, unless the program context is such
1016 that it is known that any IDNs that appear will contain either
1017 U-label or A-label forms, or that other forms can safely be rejected,
1018 some local processing of apparent domain name strings will be
1019 required, both to maintain compatibility with IDNA2003 and to prevent
1020 user astonishment. Such local processing, while not specified in
1021 this document or the associated ones, will generally take one of two
1022 forms:
1024 o Generic Preprocessing.
1025 When the context in which the program or system that processes
1026 domain names operates is global, a reasonable balance must be
1027 found that is sensitive to the broad range of local needs and
1028 assumptions while, at the same time, not sacrificing the needs of
1029 one language, script, or user population to those of another.
1031 For this case, the best practice will usually be to apply NFKC and
1032 case-mapping (or, perhaps better yet, Stringprep itself), plus
1033 dot-mapping where appropriate, to the domain name string prior to
1034 applying IDNA. That practice will not only yield a reasonable
1035 compromise of user experience with protocol requirements but will
1036 be almost completely compatible with the various forms permitted
1037 by IDNA2003.
1039 o Highly Localized Preprocessing.
1040 Unlike the case above, there will be some situations in which
1041 software will be highly localized for a particular environment and
1042 carefully adapted to the expectations of users in that
1043 environment. The many discussions about using the Internet to
1044 preserve and support local cultures suggest that these cases may
1045 be more common in the future than they have been so far.
1047 In these cases, we should avoid trying to tell implementers what
1048 they should accept, if only because they are quite likely (and for
1049 good reason) to ignore us. We would assume that they would map
1050 characters that the intuitions of their users would suggest be
1051 mapped and would hope that they would do that mapping as early as
1052 possible, storing A-label or U-label forms in files and
1053 transporting only those forms between systems. One can imagine
1054 switches about whether some sorts of mappings occur, warnings
1055 before applying them or, in a slightly more extreme version of the
1056 approach taken in Internet Explorer version 7 (IE7), systems that
1057 utterly refuse to handle "strange" characters at all if they
1058 appear in U-label form. None of those local decisions are a
1059 threat to interoperability as long as (i) only U-labels and
1060 A-labels are used in interchange with systems outside the local
1061 environment, (ii) no character that would be valid in a U-label as
1062 itself is mapped to something else, (iii) any local mappings are
1063 applied as a preprocessing step (or, for conversions from U-labels
1064 or A-labels to presentation forms, postprocessing), not as part of
1065 IDNA processing proper, and (iv) appropriate consideration is
1066 given to labels that might have entered the environment in
1067 conformance to IDNA2003.
1069 In either case, it is vital that user interface designs and, where
1070 the interfaces are not sufficient, users, be aware that the only
1071 forms of domain names that this protocol anticipates will resolve
1072 globally or compare equal when crude methods (i.e., those not
1073 conforming to the strict definition of label equivalence given in
1074 [IDNA2008-Defs]) are used are those in which all native-script labels
1075 are in U-label form. Forms that assume mapping will occur,
1076 especially forms that were not valid under IDNA2003, may or may not
1077 function in predictable ways across all implementations.
1079 User interfaces involving Latin-based scripts should take special
1080 care when considering how to handle case mapping because small
1081 differences in label strings may cause behavior that is astonishing
1082 to users. Because case-insensitive comparison is done for ASCII
1083 strings by DNS-servers, an all-ASCII label is treated as case-
1084 insensitive. However, if even one of the characters of that string
1085 is replaced by one that requires the label to be given IDN treatment
1086 (e.g., by adding a diacritical mark), then the label effectively
1087 becomes case-sensitive because only lower-case characters are
1088 permitted in IDNs. Preprocessing in applications to handle case
1089 mapping for Latin-based scripts (and possibly other scripts with case
1090 distinctions) may be wise to prevent user astonishment. However, all
1091 applications may not do this and ambiguity in transport is not
1092 desirable. Consequently the case-dependent forms should not be
1093 stored in files.
1095 7. Migration from IDNA2003 and Unicode Version Synchronization
1097 7.1. Design Criteria
1099 As mentioned above and in RFC 4690, two key goals of the IDNA2008
1100 design are
1102 o to enable applications to be agnostic about whether they are being
1103 run in environments supporting any Unicode version from 3.2
1104 onward,
1106 o to permit incrementally adding new characters, character groups,
1107 scripts, and other character collections as they are incorporated
1108 into Unicode, doing so without disruption and, in the long term,
1109 without "heavy" processes (an IETF consensus process is required
1110 by the IDNA2008 specifications and is expected to be required and
1111 used until significant experience accumulates with IDNA operations
1112 and new versions of Unicode).
1114 7.1.1. Summary and Discussion of IDNA Validity Criteria
1116 The general criteria for a label to be considered IDNA-valid are (the
1117 actual rules are rigorously defined in the "Protocol" and "Tables"
1118 documents):
1120 o The characters are "letters", marks needed to form letters,
1121 numerals, or other code points used to write words in some
1122 language. Symbols, drawing characters, and various notational
1123 characters are intended to be permanently excluded. There is no
1124 evidence that they are important enough to Internet operations or
1125 internationalization to justify expansion of domain names beyond
1126 the general principle of "letters, digits, and hyphen".
1127 (Additional discussion and rationale for the symbol decision
1128 appears in Section 7.6).
1130 o Other than in very exceptional cases, e.g., where they are needed
1131 to write substantially any word of a given language, punctuation
1132 characters are excluded. The fact that a word exists is not proof
1133 that it should be usable in a DNS label and DNS labels are not
1134 expected to be usable for multiple-word phrases (although they are
1135 certainly not prohibited if the conventions and orthography of a
1136 particular language cause that to be possible).
1138 o Characters that are unassigned (have no character assignment at
1139 all) in the version of Unicode being used by the registry or
1140 application are not permitted, even on lookup. The issues
1141 involved in this decision are discussed in Section 7.7.
1143 o Any character that is mapped to another character by a current
1144 version of NFKC is prohibited as input to IDNA (for either
1145 registration or lookup). With a few exceptions, this principle
1146 excludes any character mapped to another by Nameprep [RFC3491].
1148 The principles above drive the design of rules that are specified
1149 exactly in [IDNA2008-Tables]. Those rules identify the characters
1150 that are IDNA-valid. The rules themselves are normative, and the
1151 tables are derived from them, rather than vice versa.
1153 7.1.2. Labels in Registration
1155 Any label registered in a DNS zone must be validated -- i.e., the
1156 criteria for that label must be met -- in order for applications to
1157 work as intended. This principle is not new. For example, since the
1158 DNS was first deployed, zone administrators have been expected to
1159 verify that names meet "hostname" requirements [RFC0952] where those
1160 requirements are imposed by the expected applications. Other
1161 applications contexts, such as the later addition of special service
1162 location formats [RFC2782] imposed new requirements on zone
1163 administrators. For zones that will contain IDNs, support for
1164 Unicode version-independence requires restrictions on all strings
1165 placed in the zone. In particular, for such zones:
1167 o Any label that appears to be an A-label, i.e., any label that
1168 starts in "xn--", must be IDNA-valid, i.e., they must be valid
1169 A-labels, as discussed in Section 2 above.
1171 o The Unicode tables (i.e., tables of code points, character
1172 classes, and properties) and IDNA tables (i.e., tables of
1173 contextual rules such as those that appear in the Tables
1174 document), must be consistent on the systems performing or
1175 validating labels to be registered. Note that this does not
1176 require that tables reflect the latest version of Unicode, only
1177 that all tables used on a given system are consistent with each
1178 other.
1180 Under this model, registry tables will need to be updated (both the
1181 Unicode-associated tables and the tables of permitted IDN characters)
1182 to enable a new script or other set of new characters. The registry
1183 will not be affected by newer versions of Unicode, or newly-
1184 authorized characters, until and unless it wishes to support them.
1185 The zone administrator is responsible for verifying IDNA-validity as
1186 well as its local policies -- a more extensive set of checks than are
1187 required for looking up the labels. Systems looking up or resolving
1188 DNS labels, especially IDN DNS labels, must be able to assume that
1189 applicable registration rules were followed for names entered into
1190 the DNS.
1192 7.1.3. Labels in Lookup
1194 Anyone looking up a label in a DNS zone is required to
1196 o Maintain IDNA and Unicode tables that are consistent with regard
1197 to versions, i.e., unless the application actually executes the
1198 classification rules in [IDNA2008-Tables], its IDNA tables must be
1199 derived from the version of Unicode that is supported more
1200 generally on the system. As with registration, the tables need
1201 not reflect the latest version of Unicode but they must be
1202 consistent.
1204 o Validate the characters in labels to be looked up only to the
1205 extent of determining that the U-label does not contain
1206 "DISALLOWED" code points or code points that are unassigned in its
1207 version of Unicode.
1209 o Validate the label itself for conformance with a small number of
1210 whole-label rules. In particular, it must verify that
1212 * there are no leading combining marks,
1214 * the "bidi" conditions are met if right to left characters
1215 appear,
1217 * any required contextual rules are available, and
1219 * any contextual rules that are associated with Joiner Controls
1220 are tested.
1222 o Do not reject labels based on other contextual rules about
1223 characters, including mixed-script label prohibitions. Such rules
1224 may be used to influence presentation decisions in the user
1225 interface, but not to avoid looking up domain names.
1227 Lookup applications that following these rules, rather than having
1228 their own criteria for rejecting lookup attempts, are not sensitive
1229 to version incompatibilities with the particular zone registry
1230 associated with the domain name except for labels containing
1231 characters recently added to Unicode.
1233 An application or client that processes names according to this
1234 protocol and then resolves them in the DNS will be able to locate any
1235 name that is registered, as long as those registrations are IDNA-
1236 value and its version of the IDNA tables is sufficiently up-to-date
1237 to interpret all of the characters in the label. Messages to users
1238 should distinguish between "label contains an unallocated code point"
1239 and other types of lookup failures. A failure on the basis of an old
1240 version of Unicode may lead the user to a desire to upgrade to a
1241 newer version, but will have no other ill effects (this is consistent
1242 with behavior in the transition to the DNS when some hosts could not
1243 yet handle some forms of names or record types).
1245 7.2. Changes in Character Interpretations
1247 [[anchor22: This subsection will need to be rewritten when the
1248 mapping decisions stabilize.]]
1250 In those scripts that make case distinctions, there are a few
1251 characters for which an obvious and unique upper case character has
1252 not historically been available to match a lower case one or vice
1253 versa. For those characters, the mappings used in constructing the
1254 Stringprep tables for IDNA2003, performed using the Unicode CaseFold
1255 operation (See Section 5.8 of the Unicode Standard [Unicode51]),
1256 generate different characters or sets of characters. Those
1257 operations are not reversible and lose even more information than
1258 traditional upper case or lower case transformations, but are more
1259 useful than those transformations for comparison purposes. Two
1260 notable characters of this type are the German character Eszett
1261 (Sharp S, U+00DF) and the Greek Final Form Sigma (U+03C2). The
1262 former is case-folded to the ASCII string "ss", the latter to a
1263 medial (Lower Case) Sigma (U+03C3).
1265 The decision to eliminate mappings, including case folding, from the
1266 IDNA2008 protocol in order to make A-labels and U-labels idempotent
1267 made these characters problematic. If they were to be disallowed,
1268 important words and mnemonics could not be written in
1269 orthographically reasonable ways. If they were to be permitted as
1270 distinct characters, there would be no information loss and
1271 registries would have more flexibility, but IDNA2003 and IDNA2008
1272 lookups might result in different A-labels.
1274 With the understanding that there would be incompatibility either way
1275 but a judgment that the incompatibility was not significant enough to
1276 justify a prefix change, the WG concluded that Eszett and Final Form
1277 Sigma should be treated as distinct and Protocol-Valid characters.
1279 Registries, especially those maintaining zones for third parties,
1280 must decide how to introduce a new service in a way that does not
1281 create confusion or significantly weaken or invalidate existing
1282 identifiers. This is not a new problem; registries were faced with
1283 similar issues when IDNs were introduced and when other new forms of
1284 strings have been permitted as labels.
1286 There are several approaches to problems of this type. Without any
1287 preference or claim to completeness, some of these, all of which have
1288 been used by registries in the past for similar transitions, are:
1290 o Do not permit use of the newly-available character at the registry
1291 level. This might cause lookup failures if a domain name were to
1292 be written with the expectation of the IDNA2003 mapping behavior,
1293 but would eliminate any possibility of false matches.
1295 o Hold a "sunrise"-like arrangement in which holders of labels
1296 containing "ss" in the Eszett case or Lower Case Sigma are given
1297 priority (and perhaps other benefits) for registering the
1298 corresponding string containing Eszett or Final Sigma
1299 respectively.
1301 o Adopt some sort of "variant" approach in which registrants obtain
1302 labels with both character forms.
1304 o Adopt a different form of "variant" approach in which registration
1305 of additional names is either not permitted at all or permitted
1306 only by the registrant who already has one of the names.
1308 7.3. More Flexibility in User Agents
1310 [[anchor23: Note in Draft: This section is mapping-related and may
1311 need to be revised after that issue settles down.]] Also, it is
1312 closely related to Section 4.2 and may need to be cross-referenced
1313 from it or consolidated into it.
1315 These documents do not specify mappings between one character or code
1316 point and others. Instead, IDNA2008 prohibits characters that would
1317 be mapped to others by normalization, upper case to lower case
1318 changes, or other rules. As examples, while mathematical characters
1319 based on Latin ones are accepted as input to IDNA2003, they are
1320 prohibited in IDNA2008. Similarly, double-width characters and other
1321 variations are prohibited as IDNA input.
1323 Since the rules in [IDNA2008-Tables] have the effect that only
1324 strings that are not transformed by NFKC are valid, if an application
1325 chooses to perform NFKC normalization before lookup, that operation
1326 is safe since this will never make the application unable to look up
1327 any valid string. However, as discussed above, the application
1328 cannot guarantee that any other application will perform that
1329 mapping, so it should be used only with caution and for informed
1330 users.
1332 In many cases these prohibitions should have no effect on what the
1333 user can type as input to the lookup process. It is perfectly
1334 reasonable for systems that support user interfaces to perform some
1335 character mapping that is appropriate to the local environment. This
1336 would normally be done prior to actual invocation of IDNA. At least
1337 conceptually, the mapping would be part of the Unicode conversions
1338 discussed above and in [IDNA2008-Protocol]. However, those changes
1339 will be local ones only -- local to environments in which users will
1340 clearly understand that the character forms are equivalent. For use
1341 in interchange among systems, it appears to be much more important
1342 that U-labels and A-labels can be mapped back and forth without loss
1343 of information.
1345 One specific, and very important, instance of this strategy arises
1346 with case-folding. In the ASCII-only DNS, names are looked up and
1347 matched in a case-independent way, but no actual case-folding occurs.
1348 Names can be placed in the DNS in either upper or lower case form (or
1349 any mixture of them) and that form is preserved, returned in queries,
1350 and so on. IDNA2003 approximated that behavior for non-ASCII strings
1351 by performing case-folding at registration time (resulting in only
1352 lower-case IDNs in the DNS) and when names were looked up.
1354 As suggested earlier in this section, it appears to be desirable to
1355 do as little character mapping as possible as long as Unicode works
1356 correctly (e.g., NFC mapping to resolve different codings for the
1357 same character is still necessary although the specifications require
1358 that it be performed prior to invoking the protocol) in order to make
1359 the mapping between A-labels and U-labels idempotent. Case-mapping
1360 is not an exception to this principle. If only lower case characters
1361 can be registered in the DNS (i.e., be present in a U-label), then
1362 IDNA2008 should prohibit upper-case characters as input. Some other
1363 considerations reinforce this conclusion. For example, in ASCII
1364 case-mapping for individual characters, uppercase(character) must be
1365 equal to uppercase(lowercase(character)). That may not be true with
1366 IDNs. In some scripts that use case distinctions, there are a few
1367 characters that do not have counterparts in one case or the other.
1368 The relationship between upper case and lower case may even be
1369 language-dependent, with different languages (or even the same
1370 language in different areas) expecting different mappings. User
1371 agents can meet the expectations of users who are accustomed to the
1372 case-insensitive DNS environment by performing case folding prior to
1373 IDNA processing, but the IDNA procedures themselves should neither
1374 require such mapping nor expect them when they are not natural to the
1375 localized environment.
1377 7.4. The Question of Prefix Changes
1379 The conditions that would require a change in the IDNA ACE prefix
1380 ("xn--" for the version of IDNA specified in [RFC3490]) have been a
1381 great concern to the community. A prefix change would clearly be
1382 necessary if the algorithms were modified in a manner that would
1383 create serious ambiguities during subsequent transition in
1384 registrations. This section summarizes our conclusions about the
1385 conditions under which changes in prefix would be necessary and the
1386 implications of such a change.
1388 7.4.1. Conditions Requiring a Prefix Change
1390 An IDN prefix change is needed if a given string would be looked up
1391 or otherwise interpreted differently depending on the version of the
1392 protocol or tables being used. An IDNA upgrade would require a
1393 prefix change if, and only if, one of the following four conditions
1394 were met:
1396 1. The conversion of an A-label to Unicode (i.e., a U-label) yields
1397 one string under IDNA2003 (RFC3490) and a different string under
1398 IDNA2008.
1400 2. In a significant number of cases, an input string that is valid
1401 under IDNA2003 and also valid under IDNA2008 yields two different
1402 A-labels with the different versions. This condition is believed
1403 to be essentially equivalent to the one above except for a very
1404 small number of edge cases which may not justify a prefix change
1405 (See Section 7.2).
1407 Note that if the input string is valid under one version and not
1408 valid under the other, this condition does not apply. See the
1409 first item in Section 7.4.2, below.
1411 3. A fundamental change is made to the semantics of the string that
1412 is inserted in the DNS, e.g., if a decision were made to try to
1413 include language or script information in the encoding in
1414 addition to the string itself.
1416 4. A sufficiently large number of characters is added to Unicode so
1417 that the Punycode mechanism for block offsets can no longer
1418 reference the higher-numbered planes and blocks. This condition
1419 is unlikely even in the long term and certain not to arise in the
1420 next several years.
1422 7.4.2. Conditions Not Requiring a Prefix Change
1424 As a result of the principles described above, none of the following
1425 changes require a new prefix:
1427 1. Prohibition of some characters as input to IDNA. This may make
1428 names that are now registered inaccessible, but does not change
1429 those names.
1431 2. Adjustments in IDNA tables or actions, including normalization
1432 definitions, that affect characters that were already invalid
1433 under IDNA2003.
1435 3. Changes in the style of the IDNA definition that does not alter
1436 the actions performed by IDNA.
1438 7.4.3. Implications of Prefix Changes
1440 While it might be possible to make a prefix change, the costs of such
1441 a change are considerable. Registries could not convert all IDNA2003
1442 ("xn--") registrations to a new form at the same time and synchronize
1443 that change with applications supporting lookup. Unless all existing
1444 registrations were simply to be declared invalid (and perhaps even
1445 then) systems that needed to support both labels with old prefixes
1446 and labels with new ones would first process a putative label under
1447 the IDNA2008 rules and try to look it up and then, if it were not
1448 found, would process the label under IDNA2003 rules and look it up
1449 again. That process could significantly slow down all processing
1450 that involved IDNs in the DNS especially since a fully-qualified name
1451 might contain a mixture of labels that were registered with the old
1452 and new prefixes. That would make DNS caching very difficult. In
1453 addition, looking up the same input string as two separate A-labels
1454 creates some potential for confusion and attacks, since the labels
1455 could map to different targets and then resolve to different entries
1456 in the DNS.
1458 Consequently, a prefix change is to be avoided if at all possible,
1459 even if it means accepting some IDNA2003 decisions about character
1460 distinctions as irreversible and/or giving special treatment to edge
1461 cases.
1463 7.5. Stringprep Changes and Compatibility
1465 The Nameprep [RFC3491] specification, a key part of IDNA2003, is a
1466 profile of Stringprep [RFC3454]. While Nameprep is a Stringprep
1467 profile specific to IDNA, Stringprep is used by a number of other
1468 protocols. Were Stringprep to be modified by IDNA2008, those changes
1469 to improve the handling of IDNs could cause problems for non-DNS
1470 uses, most notably if they affected identification and authentication
1471 protocols. Several elements of IDNA2008 give interpretations to
1472 strings prohibited under IDNA2003 or prohibit strings that IDNA2003
1473 permitted. Those elements include the proposed new inclusion tables
1474 [IDNA2008-Tables], the reduction in the number of characters
1475 permitted as input for registration or lookup (Section 3), and even
1476 the proposed changes in handling of right to left strings
1477 [IDNA2008-Bidi]. IDNA2008 does not use Nameprep or Stringprep at
1478 all, so there are no side-effect changes to other protocols.
1480 It is particularly important to keep IDNA processing separate from
1481 processing for various security protocols because some of the
1482 constraints that are necessary for smooth and comprehensible use of
1483 IDNs may be unwanted or undesirable in other contexts. For example,
1484 the criteria for good passwords or passphrases are very different
1485 from those for desirable IDNs: passwords should be hard to guess,
1486 while domain names should normally be easily memorable. Similarly,
1487 internationalized SCSI identifiers and other protocol components are
1488 likely to have different requirements than IDNs.
1490 7.6. The Symbol Question
1492 One of the major differences between this specification and the
1493 original version of IDNA is that the original version permitted non-
1494 letter symbols of various sorts, including punctuation and line-
1495 drawing symbols, in the protocol. They were always discouraged in
1496 practice. In particular, both the "IESG Statement" about IDNA and
1497 all versions of the ICANN Guidelines specify that only language
1498 characters be used in labels. This specification disallows symbols
1499 entirely. There are several reasons for this, which include:
1501 1. As discussed elsewhere, the original IDNA specification assumed
1502 that as many Unicode characters as possible should be permitted,
1503 directly or via mapping to other characters, in IDNs. This
1504 specification operates on an inclusion model, extrapolating from
1505 the original "hostname" rules (LDH, see [IDNA2008-Defs]) -- which
1506 have served the Internet very well -- to a Unicode base rather
1507 than an ASCII base.
1509 2. Symbol names are more problematic than letters because there may
1510 be no general agreement on whether a particular glyph matches a
1511 symbol; there are no uniform conventions for naming; variations
1512 such as outline, solid, and shaded forms may or may not exist;
1513 and so on. As just one example, consider a "heart" symbol as it
1514 might appear in a logo that might be read as "I love...". While
1515 the user might read such a logo as "I love..." or "I heart...",
1516 considerable knowledge of the coding distinctions made in Unicode
1517 is needed to know that there more than one "heart" character
1518 (e.g., U+2665, U+2661, and U+2765) and how to describe it. These
1519 issues are of particular importance if strings are expected to be
1520 understood or transcribed by the listener after being read out
1521 loud.
1522 [[anchor24: The above paragraph remains controversial as to
1523 whether it is valid. The WG will need to make a decision if this
1524 section is not dropped entirely.]]
1526 3. Consider the case of a screen reader used by blind Internet users
1527 who must listen to renderings of IDN domain names and possibly
1528 reproduce them on the keyboard.
1530 4. As a simplified example of this, assume one wanted to use a
1531 "heart" or "star" symbol in a label. This is problematic because
1532 those names are ambiguous in the Unicode system of naming (the
1533 actual Unicode names require far more qualification). A user or
1534 would-be registrant has no way to know -- absent careful study of
1535 the code tables -- whether it is ambiguous (e.g., where there are
1536 multiple "heart" characters) or not. Conversely, the user seeing
1537 the hypothetical label doesn't know whether to read it -- try to
1538 transmit it to a colleague by voice -- as "heart", as "love", as
1539 "black heart", or as any of the other examples below.
1541 5. The actual situation is even worse than this. There is no
1542 possible way for a normal, casual, user to tell the difference
1543 between the hearts of U+2665 and U+2765 and the stars of U+2606
1544 and U+2729 or the without somehow knowing to look for a
1545 distinction. We have a white heart (U+2661) and few black
1546 hearts. Consequently, describing a label as containing a heart
1547 hopelessly ambiguous: we can only know that it contains one of
1548 several characters that look like hearts or have "heart" in their
1549 names. In cities where "Square" is a popular part of a location
1550 name, one might well want to use a square symbol in a label as
1551 well and there are far more squares of various flavors in Unicode
1552 than there are hearts or stars.
1554 The consequence of these ambiguities is that symbols are a very poor
1555 basis for reliable communication. Consistent with this conclusion,
1556 the Unicode standard recommends that strings used in identifiers not
1557 contain symbols or punctuation [Unicode-UAX31]. Of course, these
1558 difficulties with symbols do not arise with actual pictographic
1559 languages and scripts which would be treated like any other language
1560 characters; the two should not be confused.
1562 7.7. Migration Between Unicode Versions: Unassigned Code Points
1564 In IDNA2003, labels containing unassigned code points are looked up
1565 on the assumption that, if they appear in labels and can be mapped
1566 and then resolved, the relevant standards must have changed and the
1567 registry has properly allocated only assigned values.
1569 In the protocol described in these documents, strings containing
1570 unassigned code points must not be either looked up or registered.
1571 In summary, the status of an unassigned character with regard to the
1572 DISALLOWED, PROTOCOL-VALID, and CONTEXTUAL RULE REQUIRED categories
1573 cannot be evaluated until a character is actually assigned and known.
1574 There are several reasons for this, with the most important ones
1575 being:
1577 o Tests involving the context of characters (e.g., some characters
1578 being permitted only adjacent to others of specific types) and
1579 integrity tests on complete labels are needed. Unassigned code
1580 points cannot be permitted because one cannot determine whether
1581 particular code points will require contextual rules (and what
1582 those rules should be) before characters are assigned to them and
1583 the properties of those characters fully understood.
1585 o It cannot be known in advance, and with sufficient reliability,
1586 that a no newly-assigned code point will associated with a
1587 character that would be disallowed by the rules in
1588 [IDNA2008-Tables] (such as a compatibility character). In
1589 IDNA2003, since there is no direct dependency on NFKC (many of the
1590 entries in Stringprep's tables are based on NFKC, but IDNA2003
1591 depends only on Stringprep), allocation of a compatibility
1592 character might produce some odd situations, but it would not be a
1593 problem. In IDNA2008, where compatibility characters are
1594 DISALLOWED unless character-specific exceptions are made,
1595 permitting strings containing unassigned characters to be looked
1596 up would violate the principle that characters in DISALLOWED are
1597 not looked up.
1599 o The Unicode Standard specifies that an unassigned code point
1600 normalizes (and, where relevant, case folds) to itself. If the
1601 code point is later assigned to a character, and particularly if
1602 the newly-assigned code point has a combining class that
1603 determines its placement relative to other combining characters,
1604 it could normalize to some other code point or sequence.
1606 It is possible to argue that the issues above are not important and
1607 that, as a consequence, it is better to retain the principle of
1608 looking up labels even if they contain unassigned characters because
1609 all of the important scripts and characters have been coded as of
1610 Unicode 5.1 and hence unassigned code points will be assigned only to
1611 obscure characters or archaic scripts. Unfortunately, that does not
1612 appear to be a safe assumption for at least two reasons. First, much
1613 the same claim of completeness has been made for earlier versions of
1614 Unicode. The reality is that a script that is obscure to much of the
1615 world may still be very important to those who use it. Cultural and
1616 linguistic preservation principles make it inappropriate to declare
1617 the script of no importance in IDNs. Second, we already have
1618 counterexamples in, e.g., the relationships associated with new Han
1619 characters being added (whether in the BMP or in Unicode Plane 2).
1621 Independent of the technical transition issues identified above, it
1622 can be observed that any addition of characters to an existing script
1623 to make it easier to use or to better accommodate particular
1624 languages may lead to transition issues. Such changes may change the
1625 preferred form for writing a particular string, changes that may be
1626 reflected, e.g., in keyboard transition modules that would
1627 necessarily be different from those for earlier versions of Unicode
1628 where the newer characters may not exist. This creates an inherent
1629 transition problem because attempts to access labels may use either
1630 the old or the new conventions, requiring registry action whether the
1631 older conventions were used in labels or not. The need to consider
1632 transition mechanisms is inherent to evolution of Unicode to better
1633 accommodate writing systems and is independent of how IDNs are
1634 represented in the DNS or how transitions among versions of those
1635 mechanisms occur. The requirement for transitions of this type is
1636 illustrated by the addition of Malayalam Chillu in Unicode 5.1.0.
1638 7.8. Other Compatibility Issues
1640 The 2003 IDNA model includes several odd artifacts of the context in
1641 which it was developed. Many, if not all, of these are potential
1642 avenues for exploits, especially if the registration process permits
1643 "source" names (names that have not been processed through IDNA and
1644 Nameprep) to be registered. As one example, since the character
1645 Eszett, used in German, is mapped by IDNA2003 into the sequence "ss"
1646 rather than being retained as itself or prohibited, a string
1647 containing that character but that is otherwise in ASCII is not
1648 really an IDN (in the U-label sense defined above) at all. After
1649 Nameprep maps the Eszett out, the result is an ASCII string and so
1650 does not get an xn-- prefix, but the string that can be displayed to
1651 a user appears to be an IDN. The newer version of the protocol
1652 eliminates this artifact. A character is either permitted as itself
1653 or it is prohibited; special cases that make sense only in a
1654 particular linguistic or cultural context can be dealt with as
1655 localization matters where appropriate.
1657 8. Name Server Considerations
1658 8.1. Processing Non-ASCII Strings
1660 Existing DNS servers do not know the IDNA rules for handling non-
1661 ASCII forms of IDNs, and therefore need to be shielded from them.
1662 All existing channels through which names can enter a DNS server
1663 database (for example, master files (as described in RFC 1034) and
1664 DNS update messages [RFC2136]) are IDN-unaware because they predate
1665 IDNA. Other sections of this document provide the needed shielding
1666 by ensuring that internationalized domain names entering DNS server
1667 databases through such channels have already been converted to their
1668 equivalent ASCII A-label forms.
1670 Because of the distinction made between the algorithms for
1671 Registration and Lookup in [IDNA2008-Protocol] (a domain name
1672 containing only ASCII codepoints can not be converted to an A-label),
1673 there can not be more than one A-label form for any given U-label.
1675 As specified in RFC 2181 [RFC2181], the DNS protocol explicitly
1676 allows domain labels to contain octets beyond the ASCII range
1677 (0000..007F), and this document does not change that. However,
1678 although the interpretation of octets 0080..00FF is well-defined in
1679 the DNS, many application protocols support only ASCII labels and
1680 there is no defined interpretation of these non-ASCII octets as
1681 characters and, in particular, no interpretation of case-independent
1682 matching for them (see, e.g., [RFC4343]). If labels containing these
1683 octets are returned to applications, unpredictable behavior could
1684 result. The A-label form, which cannot contain those characters, is
1685 the only standard representation for internationalized labels in the
1686 DNS protocol.
1688 8.2. DNSSEC Authentication of IDN Domain Names
1690 DNS Security (DNSSEC) [RFC2535] is a method for supplying
1691 cryptographic verification information along with DNS messages.
1692 Public Key Cryptography is used in conjunction with digital
1693 signatures to provide a means for a requester of domain information
1694 to authenticate the source of the data. This ensures that it can be
1695 traced back to a trusted source, either directly or via a chain of
1696 trust linking the source of the information to the top of the DNS
1697 hierarchy.
1699 IDNA specifies that all internationalized domain names served by DNS
1700 servers that cannot be represented directly in ASCII MUST use the
1701 A-label form. Conversion to A-labels MUST be performed prior to a
1702 zone being signed by the private key for that zone. Because of this
1703 ordering, it is important to recognize that DNSSEC authenticates a
1704 domain name containing A-labels or conventional LDH-labels, not
1705 U-labels. In the presence of DNSSEC, no form of a zone file or query
1706 response that contains a U-label may be signed or the signature
1707 validated.
1709 One consequence of this for sites deploying IDNA in the presence of
1710 DNSSEC is that any special purpose proxies or forwarders used to
1711 transform user input into IDNs must be earlier in the lookup flow
1712 than DNSSEC authenticating nameservers for DNSSEC to work.
1714 8.3. Root and other DNS Server Considerations
1716 IDNs in A-label form will generally be somewhat longer than current
1717 domain names, so the bandwidth needed by the root servers is likely
1718 to go up by a small amount. Also, queries and responses for IDNs
1719 will probably be somewhat longer than typical queries historically,
1720 so EDNS0 [RFC2671] support may be more important (otherwise, queries
1721 and responses may be forced to go to TCP instead of UDP).
1723 9. Internationalization Considerations
1725 DNS labels and fully-qualified domain names provide mnemonics that
1726 assist in identifying and referring to resources on the Internet.
1727 IDNs expand the range of those mnemonics to include those based on
1728 languages and character sets other than Western European and Roman-
1729 derived ones. But domain "names" are not, in general, words in any
1730 language. The recommendations of the IETF policy on character sets
1731 and languages, (BCP 18 [RFC2277]) are applicable to situations in
1732 which language identification is used to provide language-specific
1733 contexts. The DNS is, by contrast, global and international and
1734 ultimately has nothing to do with languages. Adding languages (or
1735 similar context) to IDNs generally, or to DNS matching in particular,
1736 would imply context dependent matching in DNS, which would be a very
1737 significant change to the DNS protocol itself. It would also imply
1738 that users would need to identify the language associated with a
1739 particular label in order to look that label up. That knowledge is
1740 generally not available because many labels are not words in any
1741 language and some may be words in more than one.
1743 10. IANA Considerations
1745 This section gives an overview of IANA registries required for IDNA.
1746 The actual definitions of, and specifications for, the first two,
1747 which must be newly-created for IDNA2008, appear in
1748 [IDNA2008-Tables]. This document describes the registries but does
1749 not specify any IANA actions.
1751 10.1. IDNA Character Registry
1753 The distinction among the major categories "UNASSIGNED",
1754 "DISALLOWED", "PROTOCOL-VALID", and "CONTEXTUAL RULE REQUIRED" is
1755 made by special categories and rules that are integral elements of
1756 [IDNA2008-Tables]. While not normative, an IANA registry of
1757 characters and scripts and their categories, updated for each new
1758 version of Unicode and the characters it contains, will be convenient
1759 for programming and validation purposes. The details of this
1760 registry are specified in [IDNA2008-Tables].
1762 10.2. IDNA Context Registry
1764 IANA will create and maintain a list of approved contextual rules for
1765 characters that are defined in the IDNA Character Registry list as
1766 requiring a Contextual Rule (i.e., the types of rule described in
1767 Section 3.1.2). The details for those rules appear in
1768 [IDNA2008-Tables].
1770 10.3. IANA Repository of IDN Practices of TLDs
1772 This registry, historically described as the "IANA Language Character
1773 Set Registry" or "IANA Script Registry" (both somewhat misleading
1774 terms) is maintained by IANA at the request of ICANN. It is used to
1775 provide a central documentation repository of the IDN policies used
1776 by top level domain (TLD) registries who volunteer to contribute to
1777 it and is used in conjunction with ICANN Guidelines for IDN use.
1779 It is not an IETF-managed registry and, while the protocol changes
1780 specified here may call for some revisions to the tables, these
1781 specifications have no direct effect on that registry and no IANA
1782 action is required as a result.
1784 11. Security Considerations
1786 11.1. General Security Issues with IDNA
1788 This document is purely explanatory and informational and
1789 consequently introduces no new security issues. It would, of course,
1790 be a poor idea for someone to try to implement from it; such an
1791 attempt would almost certainly lead to interoperability problems and
1792 might lead to security ones. A discussion of security issues with
1793 IDNA, including some relevant history, appears in [IDNA2008-Defs].
1795 12. Acknowledgments
1797 The editor and contributors would like to express their thanks to
1798 those who contributed significant early (pre-WG) review comments,
1799 sometimes accompanied by text, especially Mark Davis, Paul Hoffman,
1800 Simon Josefsson, and Sam Weiler. In addition, some specific ideas
1801 were incorporated from suggestions, text, or comments about sections
1802 that were unclear supplied by Vint Cerf, Frank Ellerman, Michael
1803 Everson, Asmus Freytag, Erik van der Poel, Michel Suignard, and Ken
1804 Whistler, although, as usual, they bear little or no responsibility
1805 for the conclusions the editor and contributors reached after
1806 receiving their suggestions. Thanks are also due to Vint Cerf, Lisa
1807 Dusseault, Debbie Garside, and Jefsey Morfin for conversations that
1808 led to considerable improvements in the content of this document.
1810 A meeting was held on 30 January 2008 to attempt to reconcile
1811 differences in perspective and terminology about this set of
1812 specifications between the design team and members of the Unicode
1813 Technical Consortium. The discussions at and subsequent to that
1814 meeting were very helpful in focusing the issues and in refining the
1815 specifications. The active participants at that meeting were (in
1816 alphabetic order as usual) Harald Alvestrand, Vint Cerf, Tina Dam,
1817 Mark Davis, Lisa Dusseault, Patrik Faltstrom (by telephone), Cary
1818 Karp, John Klensin, Warren Kumari, Lisa Moore, Erik van der Poel,
1819 Michel Suignard, and Ken Whistler. We express our thanks to Google
1820 for support of that meeting and to the participants for their
1821 contributions.
1823 Useful comments and text on the WG versions of the draft were
1824 received from many participants in the IETF "IDNABIS" WG and a number
1825 of document changes resulted from mailing list discussions made by
1826 that group. Marcos Sanz provided specific analysis and suggestions
1827 that were exceptionally helpful in refining the text, as did Vint
1828 Cerf, Mark Davis, Martin Duerst, Andrew Sullivan, and Ken Whistler.
1829 Lisa Dusseault provided extensive editorial suggestions during the
1830 spring of 2009, most of which were incorporated.
1832 13. Contributors
1834 While the listed editor held the pen, the core of this document and
1835 the initial WG version represents the joint work and conclusions of
1836 an ad hoc design team consisting of the editor and, in alphabetic
1837 order, Harald Alvestrand, Tina Dam, Patrik Faltstrom, and Cary Karp.
1838 In addition, there were many specific contributions and helpful
1839 comments from those listed in the Acknowledgments section and others
1840 who have contributed to the development and use of the IDNA
1841 protocols.
1843 14. References
1845 14.1. Normative References
1847 [ASCII] American National Standards Institute (formerly United
1848 States of America Standards Institute), "USA Code for
1849 Information Interchange", ANSI X3.4-1968, 1968.
1851 ANSI X3.4-1968 has been replaced by newer versions with
1852 slight modifications, but the 1968 version remains
1853 definitive for the Internet.
1855 [IDNA2008-Bidi]
1856 Alvestrand, H. and C. Karp, "An updated IDNA criterion for
1857 right to left scripts", July 2008, .
1860 [IDNA2008-Defs]
1861 Klensin, J., "Internationalized Domain Names for
1862 Applications (IDNA): Definitions and Document Framework",
1863 November 2008, .
1866 [IDNA2008-Protocol]
1867 Klensin, J., "Internationalized Domain Names in
1868 Applications (IDNA): Protocol", November 2008, .
1871 [IDNA2008-Tables]
1872 Faltstrom, P., "The Unicode Code Points and IDNA",
1873 July 2008, .
1876 A version of this document is available in HTML format at
1877 http://stupid.domain.name/idnabis/
1878 draft-ietf-idnabis-tables-02.html
1880 [RFC3490] Faltstrom, P., Hoffman, P., and A. Costello,
1881 "Internationalizing Domain Names in Applications (IDNA)",
1882 RFC 3490, March 2003.
1884 [RFC3492] Costello, A., "Punycode: A Bootstring encoding of Unicode
1885 for Internationalized Domain Names in Applications
1886 (IDNA)", RFC 3492, March 2003.
1888 [Unicode-UAX15]
1889 The Unicode Consortium, "Unicode Standard Annex #15:
1890 Unicode Normalization Forms", March 2008,
1891 .
1893 [Unicode51]
1894 The Unicode Consortium, "The Unicode Standard, Version
1895 5.1.0", 2008.
1897 defined by: The Unicode Standard, Version 5.0, Boston, MA,
1898 Addison-Wesley, 2007, ISBN 0-321-48091-0, as amended by
1899 Unicode 5.1.0
1900 (http://www.unicode.org/versions/Unicode5.1.0/).
1902 14.2. Informative References
1904 [BIG5] Institute for Information Industry of Taiwan, "Computer
1905 Chinese Glyph and Character Code Mapping Table, Technical
1906 Report C-26", 1984.
1908 There are several forms and variations and a closely-
1909 related standard, CNS 11643. See the discussion in
1910 Chapter 3 of Lunde, K., CJKV Information Processing,
1911 O'Reilly & Associates, 1999
1913 [GB18030] "Chinese National Standard GB 18030-2000: Information
1914 Technology -- Chinese ideograms coded character set for
1915 information interchange -- Extension for the basic set.",
1916 2000.
1918 [RFC0810] Feinler, E., Harrenstien, K., Su, Z., and V. White, "DoD
1919 Internet host table specification", RFC 810, March 1982.
1921 [RFC0952] Harrenstien, K., Stahl, M., and E. Feinler, "DoD Internet
1922 host table specification", RFC 952, October 1985.
1924 [RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
1925 STD 13, RFC 1034, November 1987.
1927 [RFC1035] Mockapetris, P., "Domain names - implementation and
1928 specification", STD 13, RFC 1035, November 1987.
1930 [RFC1123] Braden, R., "Requirements for Internet Hosts - Application
1931 and Support", STD 3, RFC 1123, October 1989.
1933 [RFC2136] Vixie, P., Thomson, S., Rekhter, Y., and J. Bound,
1934 "Dynamic Updates in the Domain Name System (DNS UPDATE)",
1935 RFC 2136, April 1997.
1937 [RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS
1938 Specification", RFC 2181, July 1997.
1940 [RFC2277] Alvestrand, H., "IETF Policy on Character Sets and
1941 Languages", BCP 18, RFC 2277, January 1998.
1943 [RFC2535] Eastlake, D., "Domain Name System Security Extensions",
1944 RFC 2535, March 1999.
1946 [RFC2671] Vixie, P., "Extension Mechanisms for DNS (EDNS0)",
1947 RFC 2671, August 1999.
1949 [RFC2673] Crawford, M., "Binary Labels in the Domain Name System",
1950 RFC 2673, August 1999.
1952 [RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
1953 specifying the location of services (DNS SRV)", RFC 2782,
1954 February 2000.
1956 [RFC3454] Hoffman, P. and M. Blanchet, "Preparation of
1957 Internationalized Strings ("stringprep")", RFC 3454,
1958 December 2002.
1960 [RFC3491] Hoffman, P. and M. Blanchet, "Nameprep: A Stringprep
1961 Profile for Internationalized Domain Names (IDN)",
1962 RFC 3491, March 2003.
1964 [RFC3743] Konishi, K., Huang, K., Qian, H., and Y. Ko, "Joint
1965 Engineering Team (JET) Guidelines for Internationalized
1966 Domain Names (IDN) Registration and Administration for
1967 Chinese, Japanese, and Korean", RFC 3743, April 2004.
1969 [RFC3987] Duerst, M. and M. Suignard, "Internationalized Resource
1970 Identifiers (IRIs)", RFC 3987, January 2005.
1972 [RFC4290] Klensin, J., "Suggested Practices for Registration of
1973 Internationalized Domain Names (IDN)", RFC 4290,
1974 December 2005.
1976 [RFC4343] Eastlake, D., "Domain Name System (DNS) Case Insensitivity
1977 Clarification", RFC 4343, January 2006.
1979 [RFC4690] Klensin, J., Faltstrom, P., Karp, C., and IAB, "Review and
1980 Recommendations for Internationalized Domain Names
1981 (IDNs)", RFC 4690, September 2006.
1983 [RFC4713] Lee, X., Mao, W., Chen, E., Hsu, N., and J. Klensin,
1984 "Registration and Administration Recommendations for
1985 Chinese Domain Names", RFC 4713, October 2006.
1987 [Unicode-Security]
1988 The Unicode Consortium, "Unicode Technical Standard #39:
1989 Unicode Security Mechanisms", August 2008,
1990 .
1992 [Unicode-UAX31]
1993 The Unicode Consortium, "Unicode Standard Annex #31:
1994 Unicode Identifier and Pattern Syntax", March 2008,
1995 .
1997 [Unicode-UTR36]
1998 The Unicode Consortium, "Unicode Technical Report #36:
1999 Unicode Security Considerations", July 2008,
2000 .
2002 Appendix A. Change Log
2004 [[ RFC Editor: Please remove this appendix. ]]
2006 A.1. Changes between Version -00 and Version -01 of
2007 draft-ietf-idnabis-rationale
2009 o Clarified the U-label definition to note that U-labels must
2010 contain at least one non-ASCII character. Also clarified the
2011 relationship among label types.
2013 o Rewrote the discussion of Labels in Registration (Section 7.1.2)
2014 and related text about IDNA-validity (in the "Defs" document as of
2015 -04 of this one) to narrow its focus and remove more general
2016 restrictions. Added a temporary note in line to explain the
2017 situation.
2019 o Changed the "IDNA uses Unicode" statement to focus on
2020 compatibility with IDNA2003 and avoid more general or
2021 controversial assertions.
2023 o Added a discussion of examples to Section 7.1
2025 o Made a number of other small editorial changes and corrections
2026 suggested by Mark Davis.
2028 o Added several more discussion anchors and notes and expanded or
2029 updated some existing ones.
2031 A.2. Version -02
2033 o Trimmed change log, removing information about pre-WG drafts.
2035 o Adjusted discussion of Contextual Rules to match the new location
2036 of the tables and some conceptual material.
2038 o Rewrote the material on preprocessing somewhat.
2040 o Moved the material about relationships with IDNA2003 to be part of
2041 a single section on transitions.
2043 o Removed several placeholders and made editorial changes in
2044 accordance with decisions made at IETF 72 in Dublin and not
2045 disputed on the mailing list.
2047 A.3. Version -03
2049 This special update to the Rationale document is intended to try to
2050 get the discussion of what is normative or not under control. While
2051 the IETF does not normally annotate individual sections of documents
2052 with whether they are normative or not, concerns that we don't know
2053 which is which, claims that some material is normative that would be
2054 problematic if so classified, etc., argue that we should at least be
2055 able to have a clear discussion on the subject.
2057 Two annotations have been applied to sections that might reasonably
2058 be considered normative. One annotation is based on the list of
2059 sections in Mark Davis's note of 29 September (http://
2060 www.alvestrand.no/pipermail/idna-update/2008-September/002667.html).
2061 The other is based on an elaboration of John Klensin's response on 7
2062 October (http://www.alvestrand.no/pipermail/idna-update/2008-October/
2063 002691.html). These should just be considered two suggestions to
2064 illuminate and, one hopes, advance the Working Group's discussions.
2066 Some additional editorial changes have been made, but they are
2067 basically trivial. In the editor's judgment, it is not possible to
2068 make significantly more progress with this document until the matter
2069 of document organization is settled.
2071 A.4. Version -04
2073 o Definitional and other normative material moved to new document
2074 (draft-ietf-idnabis-defs). Version -03 annotations removed.
2076 o Material on differences between IDNA2003 and IDNA2008 moved to an
2077 appendix in Protocol.
2079 o Material left over from the origins of this document as a
2080 preliminary proposal has been removed or rewritten.
2082 o Changes made to reflect consensus call results, including removing
2083 several placeholder notes for discussion.
2085 o Added more material, including discussion of historic scripts, to
2086 Section 3.2 on registration policies.
2088 o Added a new section (Section 7.2) to contain specific discussion
2089 of handling of characters that are interpreted differently in
2090 input to IDNA2003 and 2008.
2092 o Some material, including this section/appendix, rearranged.
2094 A.5. Version -05
2096 o Many small editorial changes, including changes to eliminate the
2097 last vestiges of what appeared to be 2119 language (upper-case
2098 MUST, SHOULD, or MAY) and small adjustments to terminology.
2100 A.6. Version -06
2102 o Removed Security Considerations material and pointed to Defs,
2103 where it now appears as of version 05.
2105 o Started changing uses of "IDNA2008" in running text to "in these
2106 specifications" or the equivalent. These documents are titled
2107 simply "IDNA"; once they are standardized, "the current version"
2108 may be a more appropriate reference than one containing a year.
2109 As discussed on the mailing list, we can and should discuss how to
2110 refer to these documents at an appropriate time (e.g., when we
2111 know when we will be finished) but, in the interim, it seems
2112 appropriate to simply start getting rid of the version-specific
2113 terminology where it can naturally be removed.
2115 o Additional discussion of mappings, etc., especially for case-
2116 sensitivity.
2118 o Clarified relationship to base DNS specifications.
2120 o Consolidated discussion of lookup of unassigned characters.
2122 o More editorial fine-tuning.
2124 A.7. Version -07
2126 o Revised terminology by adding terms: NR-LDH-label, Invalid-A-label
2127 (or False-A-label), R-LDH-label, valid IDNA-label in
2128 Section 1.3.3.
2130 o Moved the "name server considerations" material to this document
2131 from Protocol because it is non-normative and not part of the
2132 protocol itself.
2134 o To improve clarity, redid discussion of the reasons why looking up
2135 unassigned code points is prohibited.
2137 o Editorial and other non-substantive corrections to reflect earlier
2138 errors as well as new definitions and terminology.
2140 A.8. Version -08
2142 o Slight revision to "contextual" discussion (Section 3.1.2) and
2143 moving it to a separate subsection, rather than under "PVALID",
2144 for better parallelism with Tables. Also reflected Mark's
2145 comments about the limitations of the approach.
2147 o Added placeholder notes as reminders of where references to the
2148 other documents need Section numbers. More of these will be added
2149 as needed (feel free to identify relevant places), but the actual
2150 section numbers will not be inserted until the documents are
2151 completely stable, i.e., on their way to the RFC Editor.
2153 A.9. Version -09
2155 o Small editorial changes to clarify transition possibilities.
2157 o Small clarification to the description of DNS "exact match".
2159 o Added discussion of adding characters to an existing script to the
2160 discussion of unassigned code point transitions in Section 7.7.
2162 o Tightened up the discussion of non-ASCII string processing
2163 (Section 8.1) slightly.
2165 o Removed some placeholders and comments that have been around long
2166 enough to be considered acceptable or that no longer seem
2167 necessary for other reasons.
2169 A.10. Version -10
2171 o Extensive editorial improvements, mostly due to suggestions from
2172 Lisa Dusseault.
2174 o Changes required for the new "mapping" approach and document have,
2175 in general, not been incorporated despite several suggestions.
2176 The editor intends to wait until the mapping model is stable, or
2177 at least until -11 of this document, before trying to incorporate
2178 those suggestions.
2180 Author's Address
2182 John C Klensin
2183 1770 Massachusetts Ave, Ste 322
2184 Cambridge, MA 02140
2185 USA
2187 Phone: +1 617 245 1457
2188 Email: john+ietf@jck.com