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2 Network Working Group J. Klensin
3 Internet-Draft July 12, 2009
4 Obsoletes: 3490, 3491
5 (if approved)
6 Updates: 3492 (if approved)
7 Intended status: Standards Track
8 Expires: January 13, 2010
10 Internationalized Domain Names in Applications (IDNA): Protocol
11 draft-ietf-idnabis-protocol-13.txt
13 Status of this Memo
15 This Internet-Draft is submitted to IETF in full conformance with the
16 provisions of BCP 78 and BCP 79. This document may contain material
17 from IETF Documents or IETF Contributions published or made publicly
18 available before November 10, 2008. The person(s) controlling the
19 copyright in some of this material may not have granted the IETF
20 Trust the right to allow modifications of such material outside the
21 IETF Standards Process. Without obtaining an adequate license from
22 the person(s) controlling the copyright in such materials, this
23 document may not be modified outside the IETF Standards Process, and
24 derivative works of it may not be created outside the IETF Standards
25 Process, except to format it for publication as an RFC or to
26 translate it into languages other than English.
28 Internet-Drafts are working documents of the Internet Engineering
29 Task Force (IETF), its areas, and its working groups. Note that
30 other groups may also distribute working documents as Internet-
31 Drafts.
33 Internet-Drafts are draft documents valid for a maximum of six months
34 and may be updated, replaced, or obsoleted by other documents at any
35 time. It is inappropriate to use Internet-Drafts as reference
36 material or to cite them other than as "work in progress."
38 The list of current Internet-Drafts can be accessed at
39 http://www.ietf.org/ietf/1id-abstracts.txt.
41 The list of Internet-Draft Shadow Directories can be accessed at
42 http://www.ietf.org/shadow.html.
44 This Internet-Draft will expire on January 13, 2010.
46 Copyright Notice
48 Copyright (c) 2009 IETF Trust and the persons identified as the
49 document authors. All rights reserved.
51 This document is subject to BCP 78 and the IETF Trust's Legal
52 Provisions Relating to IETF Documents in effect on the date of
53 publication of this document (http://trustee.ietf.org/license-info).
54 Please review these documents carefully, as they describe your rights
55 and restrictions with respect to this document.
57 Abstract
59 This document is the revised protocol definition for
60 internationalized domain names (IDNs). The rationale for changes,
61 the relationship to the older specification, and important
62 terminology are provided in other documents. This document specifies
63 the protocol mechanism, called Internationalizing Domain Names in
64 Applications (IDNA), for registering and looking up IDNs in a way
65 that does not require changes to the DNS itself. IDNA is only meant
66 for processing domain names, not free text.
68 Table of Contents
70 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5
71 1.1. Discussion Forum . . . . . . . . . . . . . . . . . . . . . 5
72 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
73 3. Requirements and Applicability . . . . . . . . . . . . . . . . 6
74 3.1. Requirements . . . . . . . . . . . . . . . . . . . . . . . 6
75 3.2. Applicability . . . . . . . . . . . . . . . . . . . . . . 6
76 3.2.1. DNS Resource Records . . . . . . . . . . . . . . . . . 7
77 3.2.2. Non-domain-name Data Types Stored in the DNS . . . . . 7
78 4. Registration Protocol . . . . . . . . . . . . . . . . . . . . 7
79 4.1. Input to IDNA Registration Process . . . . . . . . . . . . 8
80 4.2. Permitted Character and Label Validation . . . . . . . . . 8
81 4.2.1. Input Format . . . . . . . . . . . . . . . . . . . . . 8
82 4.2.2. Rejection of Characters that are not Permitted . . . . 8
83 4.2.3. Label Validation . . . . . . . . . . . . . . . . . . . 8
84 4.2.4. Registration Validation Summary . . . . . . . . . . . 9
85 4.3. Registry Restrictions . . . . . . . . . . . . . . . . . . 10
86 4.4. Punycode Conversion . . . . . . . . . . . . . . . . . . . 10
87 4.5. Insertion in the Zone . . . . . . . . . . . . . . . . . . 10
88 5. Domain Name Lookup Protocol . . . . . . . . . . . . . . . . . 10
89 5.1. Label String Input . . . . . . . . . . . . . . . . . . . . 11
90 5.2. Conversion to Unicode . . . . . . . . . . . . . . . . . . 11
91 5.3. Character Changes in Preprocessing or the User
92 Interface . . . . . . . . . . . . . . . . . . . . . . . . 11
93 5.4. A-label Input . . . . . . . . . . . . . . . . . . . . . . 12
94 5.5. Validation and Character List Testing . . . . . . . . . . 13
95 5.6. Punycode Conversion . . . . . . . . . . . . . . . . . . . 14
96 5.7. DNS Name Resolution . . . . . . . . . . . . . . . . . . . 15
97 6. Security Considerations . . . . . . . . . . . . . . . . . . . 15
98 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
99 8. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 15
100 9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 15
101 10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 16
102 10.1. Normative References . . . . . . . . . . . . . . . . . . . 16
103 10.2. Informative References . . . . . . . . . . . . . . . . . . 17
104 Appendix A. Local Mapping Alternatives . . . . . . . . . . . . . 18
105 A.1. Transitional Mapping Model . . . . . . . . . . . . . . . . 19
106 A.1.1. Fallback Lookup . . . . . . . . . . . . . . . . . . . 19
107 A.1.2. Two-step Lookup . . . . . . . . . . . . . . . . . . . 20
108 A.2. Internationalized Resource Identifier (IRI) Mapping
109 Model . . . . . . . . . . . . . . . . . . . . . . . . . . 20
110 Appendix B. Summary of Major Changes from IDNA2003 . . . . . . . 21
111 Appendix C. Change Log . . . . . . . . . . . . . . . . . . . . . 22
112 C.1. Changes between Version -00 and -01 of
113 draft-ietf-idnabis-protocol . . . . . . . . . . . . . . . 22
114 C.2. Version -02 . . . . . . . . . . . . . . . . . . . . . . . 22
115 C.3. Version -03 . . . . . . . . . . . . . . . . . . . . . . . 22
116 C.4. Version -04 . . . . . . . . . . . . . . . . . . . . . . . 23
117 C.5. Version -05 . . . . . . . . . . . . . . . . . . . . . . . 23
118 C.6. Version -06 . . . . . . . . . . . . . . . . . . . . . . . 23
119 C.7. Version -07 . . . . . . . . . . . . . . . . . . . . . . . 23
120 C.8. Version -08 . . . . . . . . . . . . . . . . . . . . . . . 24
121 C.9. Version -09 . . . . . . . . . . . . . . . . . . . . . . . 24
122 C.10. Version -10 . . . . . . . . . . . . . . . . . . . . . . . 24
123 C.11. Version -11 . . . . . . . . . . . . . . . . . . . . . . . 25
124 C.12. Version -12 . . . . . . . . . . . . . . . . . . . . . . . 25
125 C.13. Version -13 . . . . . . . . . . . . . . . . . . . . . . . 25
126 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 26
128 1. Introduction
130 This document supplies the protocol definition for internationalized
131 domain names. Essential definitions and terminology for
132 understanding this document and a road map of the collection of
133 documents that make up IDNA2008 appear in [IDNA2008-Defs].
134 Appendix B discusses the relationship between this specification and
135 the earlier version of IDNA (referred to here as "IDNA2003") and the
136 rationale for these changes, along with considerable explanatory
137 material and advice to zone administrators who support IDNs is
138 provided in another documents, notably [IDNA2008-Rationale].
140 IDNA works by allowing applications to use certain ASCII string
141 labels (beginning with a special prefix) to represent non-ASCII name
142 labels. Lower-layer protocols need not be aware of this; therefore
143 IDNA does not changes any infrastructure. In particular, IDNA does
144 not depend on any changes to DNS servers, resolvers, or protocol
145 elements, because the ASCII name service provided by the existing DNS
146 can be used for IDNA.
148 IDNA applies only to DNS labels. The base DNS standards [RFC1034]
149 [RFC1035] and their various updates specify how to combine labels
150 into fully-qualified domain names and parse labels out of those
151 names.
153 This document describes two separate protocols, one for IDN
154 registration (Section 4) and one for IDN lookup (Section 5), that
155 share some terminology, reference data and operations. [[anchor2:
156 Note in draft: See the note in the introduction to.]]Section 5
158 1.1. Discussion Forum
160 [[anchor4: RFC Editor: please remove this section.]]
162 This work is being discussed in the IETF IDNABIS WG and on the
163 mailing list idna-update@alvestrand.no
165 2. Terminology
167 Terminology used in IDNA, but also in Unicode or other character set
168 standards and the DNS, appears in [IDNA2008-Defs]. Terminology that
169 is required as part of the IDNA definition, including the definitions
170 of "ACE", appears in that document as well. Readers of this document
171 are assumed to be familiar with [IDNA2008-Defs] and with the DNS-
172 specific terminology in RFC 1034 [RFC1034].
174 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
175 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
176 document are to be interpreted as described in BCP 14, RFC 2119
177 [RFC2119].
179 3. Requirements and Applicability
181 3.1. Requirements
183 IDNA makes the following requirements:
185 1. Whenever a domain name is put into an IDN-unaware domain name
186 slot (see Section 2 and [IDNA2008-Defs]), it MUST contain only
187 ASCII characters (i.e., must be either an A-label or an NR-LDH-
188 label), unless the DNS application is not subject to historical
189 recommendations for "hostname"-style names (see [RFC1034] and
190 Section 3.2.1).
192 2. Labels MUST be compared using equivalent forms: either both
193 A-Label forms or both U-Label forms. Because A-labels and
194 U-labels can be transformed into each other without loss of
195 information, these comparisons are equivalent. A pair of
196 A-labels MUST be compared as case-insensitive ASCII (as with all
197 comparisons of ASCII DNS labels). U-labels must be compared
198 as-is, without case-folding or other intermediate steps. Note
199 that it is not necessary to validate labels in order to compare
200 them. In many cases, validation may be important for other
201 reasons and SHOULD be performed.
203 3. Labels being registered MUST conform to the requirements of
204 Section 4. Labels being looked up and the lookup process MUST
205 conform to the requirements of Section 5.
207 3.2. Applicability
209 IDNA applies to all domain names in all domain name slots in
210 protocols except where it is explicitly excluded. It does not apply
211 to domain name slots which do not use the Letter/Digit/Hyphen (LDH)
212 syntax rules.
214 Because it uses the DNS, IDNA applies to many protocols that were
215 specified before it was designed. IDNs occupying domain name slots
216 in those older protocols MUST be in A-label form until and unless
217 those protocols and implementations of them are explicitly upgraded
218 to be aware of IDNs in Unicode. IDNs actually appearing in DNS
219 queries or responses MUST be A-labels.
221 IDNA is not defined for extended label types (see RFC 2671, Section 3
223 [RFC2671]).
225 3.2.1. DNS Resource Records
227 IDNA applies only to domain names in the NAME and RDATA fields of DNS
228 resource records whose CLASS is IN. See RFC 1034 [RFC1034] for
229 precise definitions of these terms.
231 The application of IDNA to DNS resource records depends entirely on
232 the CLASS of the record, and not on the TYPE except as noted below.
233 This will remain true, even as new types are defined, unless a new
234 type defines type-specific rules. Special naming conventions for SRV
235 records (and "underscore names" more generally) are incompatible with
236 IDNA coding. The first two labels on a SRV type record (the ones
237 required to start in "_") MUST NOT be A-labels or U-labels, because
238 conversion to an A-label would lose information (since the underscore
239 is not a letter, digit, or hyphen and is consequently DISALLOWED in
240 IDNs). Of course, those labels may be part of a domain that uses IDN
241 labels at higher levels in the tree.
243 3.2.2. Non-domain-name Data Types Stored in the DNS
245 Although IDNA enables the representation of non-ASCII characters in
246 domain names, that does not imply that IDNA enables the
247 representation of non-ASCII characters in other data types that are
248 stored in domain names, specifically in the RDATA field for types
249 that have structured RDATA format. For example, an email address
250 local part is stored in a domain name in the RNAME field as part of
251 the RDATA of an SOA record (hostmaster@example.com would be
252 represented as hostmaster.example.com). IDNA does not update the
253 existing email standards, which allow only ASCII characters in local
254 parts. Even though work is in progress to define
255 internationalization for email addresses [RFC4952], changes to the
256 email address part of the SOA RDATA would require action in, or
257 updates to, other standards, specifically those that specify the
258 format of the SOA RR.
260 4. Registration Protocol
262 This section defines the procedure for registering an IDN. The
263 procedure is implementation independent; any sequence of steps that
264 produces exactly the same result for all labels is considered a valid
265 implementation.
267 Note that, while the registration and lookup protocols (Section 5)
268 are very similar in most respects, they are different and
269 implementers should carefully follow the appropriate steps.
271 4.1. Input to IDNA Registration Process
273 Registration processes, especially processing by entities, such as
274 "registrars" who deal with registrants before the request actually
275 reaches the zone manager ("registry") are outside the scope of these
276 protocols and may differ significantly depending on local needs. By
277 the time a string enters the IDNA registration process as described
278 in this specification, it is expected to be in Unicode and MUST be in
279 Unicode Normalization Form C (NFC [Unicode-UAX15]). Entities
280 responsible for zone files ("registries") are expected to accept only
281 the exact string for which registration is requested, free of any
282 mappings or local adjustments. They SHOULD avoid any possible
283 ambiguity by accepting registrations only for A-labels, possibly
284 paired with the relevant U-labels so that they can verify the
285 correspondence.
287 4.2. Permitted Character and Label Validation
289 4.2.1. Input Format
291 The registry SHOULD permit submission of labels in A-label form and
292 is encouraged to accept both the A-label form and the U-label one.
293 If it does so, it MUST perform a conversion to a U-label, perform the
294 steps and tests described below, and verify that the A-label produced
295 by the step in Section 4.4 matches the one provided as input. In
296 addition, if a U-label was provided, that U-label and the one
297 obtained by conversion of the A-label MUST match exactly. If, for
298 some reason, these tests fail, the registration MUST be rejected. If
299 the conversion to a U-label is not performed, the registry MUST still
300 verify that the A-label is superficially valid, i.e., that it does
301 not violate any of the rules of Punycode [RFC3492] encoding such as
302 the prohibition on trailing hyphen-minus, appearance of non-basic
303 characters before the delimiter, and so on. Fake A-labels, i.e.,
304 invalid strings that appear to be A-labels but are not, MUST NOT be
305 placed in DNS zones that support IDNA.
307 4.2.2. Rejection of Characters that are not Permitted
309 The candidate Unicode string MUST NOT contain characters in the
310 "DISALLOWED" and "UNASSIGNED" lists specified in [IDNA2008-Tables].
312 4.2.3. Label Validation
314 The proposed label (in the form of a Unicode string, i.e., a string
315 that at least superficially appears to be a U-label) is then
316 examined, performing tests that require examination of more than one
317 character. Character order is considered to be the on-the-wire
318 order, not the display order.
320 4.2.3.1. Consecutive Hyphens
322 The Unicode string MUST NOT contain "--" (two consecutive hyphens) in
323 the third and fourth character positions.
325 4.2.3.2. Leading Combining Marks
327 The Unicode string MUST NOT begin with a combining mark or combining
328 character (see The Unicode Standard, Section 2.11 [Unicode] for an
329 exact definition).
331 4.2.3.3. Contextual Rules
333 The Unicode string MUST NOT contain any characters whose validity is
334 context-dependent, unless the validity is positively confirmed by a
335 contextual rule. To check this, each code-point marked as CONTEXTJ
336 and CONTEXTO in [IDNA2008-Tables] MUST have a non-null rule. If such
337 a code-point is missing a rule, it is invalid. If the rule exists
338 but the result of applying the rule is negative or inconclusive, the
339 proposed label is invalid.
341 NOTE: These contextual rules are required to support characters that
342 could be used, under some conditions, to produce misleading labels or
343 to cause unacceptable ambiguity in label matching and interpretation.
344 For example, labels containing zero-width characters may be permitted
345 in context with characters whose presentation forms are significantly
346 changed by the zero-width characters, while other labels in which
347 zero-width characters appear may be rejected.
348 [[anchor11: Note in draft: Should this note be moved to Rationale???
349 It has no normative consequences here.]]
351 4.2.3.4. Labels Containing Characters Written Right to Left
353 If the proposed label contains any characters that are written from
354 right to left it MUST meet the "bidi" criteria [IDNA2008-BIDI].
356 4.2.4. Registration Validation Summary
358 Strings that contain at least one non-ASCII character, have been
359 produced by the steps above, whose contents pass all of the tests in
360 Section 4.2, and are 63 or fewer characters long in ACE form (see
361 Section 4.4), are U-labels.
363 To summarize, tests are made in Section 4.2 for invalid characters,
364 invalid combinations of characters, for labels that are invalid even
365 if the characters they contain are valid individually, and for labels
366 that do not conform to the restrictions for strings containing right
367 to left characters.
369 4.3. Registry Restrictions
371 In addition to the rules and tests above, there are many reasons why
372 a registry could reject a label. Registries at all levels of the
373 DNS, not just the top level, establish policies about label
374 registrations. Policies are likely to be informed by the local
375 languages and may depend on many factors including what characters
376 are in the label (for example, a label may be rejected based on other
377 labels already registered). See [IDNA2008-Rationale] for a
378 discussion and recommendations about registry policies.
380 The string produced by the above steps is checked and processed as
381 appropriate to local registry restrictions. Application of those
382 registry restrictions may result in the rejection of some labels or
383 the application of special restrictions to others.
385 4.4. Punycode Conversion
387 The resulting U-label is converted to an A-label. The A-label, more
388 precisely defined elsewhere, is the encoding of the U-label according
389 to the Punycode algorithm [RFC3492] with the ACE prefix "xn--" added
390 at the beginning of the string. The resulting string must, of
391 course, conform to the length limits imposed by the DNS. This
392 document updates RFC 3492 only to the extent of replacing the
393 reference to the discussion of the ACE prefix. The ACE prefix is now
394 specified in this document rather than as part of RFC 3490 or
395 Nameprep [RFC3491] but is the same in both sets of documents.
397 The failure conditions identified in the Punycode encoding procedure
398 cannot occur if the input is a U-label as determined by the steps
399 above.
401 4.5. Insertion in the Zone
403 The A-label is registered in the DNS by insertion into a zone.
405 5. Domain Name Lookup Protocol
407 Lookup is different from registration and different tests are applied
408 on the client. Although some validity checks are necessary to avoid
409 serious problems with the protocol, the lookup-side tests are more
410 permissive and rely on the assumption that names that are present in
411 the DNS are valid. That assumption is, however, a weak one because
412 the presence of wild cards in the DNS might cause a string that is
413 not actually registered in the DNS to be successfully looked up.
415 The two steps in Section 5.2 and Section 5.3 are required.
417 [[anchor14: Note in Draft: Try to reorganize and renumber Section 5
418 (Lookup) so that it exactly parallels Section 4 (Registration). This
419 has not been done in drafts -10 through -13 because the task will be
420 much easier if the local mapping material is pulled from here (and
421 there is no point trying to align the section numbers twice).]]
423 5.1. Label String Input
425 The user supplies a string in the local character set, typically by
426 typing it or clicking on, or copying and pasting, a resource
427 identifier, e.g., a URI [RFC3986] or IRI [RFC3987] from which the
428 domain name is extracted. Alternately, some process not directly
429 involving the user may read the string from a file or obtain it in
430 some other way. Processing in this step and the next two are local
431 matters, to be accomplished prior to actual invocation of IDNA.
433 5.2. Conversion to Unicode
435 The string is converted from the local character set into Unicode, if
436 it is not already Unicode. A Unicode string may require
437 normalization as discussed in Section 4.1. The result MUST be a
438 Unicode string in NFC form.
440 5.3. Character Changes in Preprocessing or the User Interface
442 [[anchor15: Note in Draft -13. This entire section is likely to need
443 significant revision when we make final decisions about mapping. The
444 changes from -12 are intended simply to illustrate one of the ways in
445 which the mapping material might be incorporated if we keep it... and
446 to fix a possible ambiguity about the NFC requirement for mapping
447 output.]]
449 The Unicode string MAY then be processed to prevent confounding of
450 user expectations. For instance, it might be reasonable, at this
451 step, to convert all upper case characters to lower case, if this
452 makes sense in the user's environment, but even this should be
453 approached with caution due to some edge cases: in the long term, it
454 is probably better for users to understand IDNs strictly in lower-
455 case, U-label, form. More generally, preprocessing may be useful to
456 smooth the transition from IDNA2003, especially for direct user
457 input, but with similar cautions. In general, IDNs appearing in
458 files and those transmitted across the network as part of protocols
459 are expected to be in either ASCII form (including A-labels) or to
460 contain U-labels, rather than being in forms requiring mapping or
461 other conversions.
463 The mapping issue and some suggestions and tradeoffs are discussed in
464 [IDNA2008-Mapping]. Note that this specification does not require
465 that the processing into Unicode (See Section 5.2 above) be applied
466 as a separate step if it incorporated into some mapping process as
467 described in [IDNA2008-Mapping].
469 Other examples of processing for localization might be applied,
470 especially to direct user input, at this point. They include
471 interpreting various characters as separating domain name components
472 from each other (label separators) because they either look like
473 periods or are used to separate sentences, mapping halfwidth or
474 fullwidth East Asian characters to the common form permitted in
475 labels, or giving special treatment to characters whose presentation
476 forms are dependent only on placement in the label. Such
477 localization changes are also outside the scope of this
478 specification.
480 Recommendations for preprocessing for global contexts (i.e., when
481 local considerations do not apply or cannot be used) and for maximum
482 interoperability with labels that might have been specified under
483 liberal readings of IDNA2003 are given in [IDNA2008-Rationale]. It
484 is important to note that the intent of these specifications is that
485 labels in application protocols, files, or links are intended to be
486 in U-label or A-label form. Preprocessing MUST NOT map a character
487 that is valid in a label as specified elsewhere in this document or
488 in [IDNA2008-Tables] into another character. Excessively liberal use
489 of preprocessing, especially to strings stored in files, poses a
490 threat to consistent and predictable behavior for the user even if
491 not to actual interoperability.
493 Because these transformations are local, it is important that domain
494 names that might be passed between systems (e.g., in IRIs) be
495 U-labels or A-labels and not forms that might be accepted locally as
496 a consequence of this step. This step is not standardized as part of
497 IDNA, and is not further specified here.
499 If this step is applied, the results still MUST be in NFC form as
500 above. The step must not denormalize the characters.
502 5.4. A-label Input
504 If the input to this procedure appears to be an A-label (i.e., it
505 starts in "xn--"), the lookup application MAY attempt to convert it
506 to a U-label and apply the tests of Section 5.5 and the conversion of
507 Section 5.6 to that form. If the label is converted to Unicode
508 (i.e., to U-label form) using the Punycode decoding algorithm, then
509 the processing specified in those two sections MUST be performed, and
510 the label MUST be rejected if the resulting label is not identical to
511 the original. See the Name Server Considerations section of
512 [IDNA2008-Rationale] for additional discussion on this topic.
514 That conversion and testing SHOULD be performed if the domain name
515 will later be presented to the user in native character form (this
516 requires that the lookup application be IDNA-aware). If those steps
517 are not performed, the lookup process SHOULD at least make tests to
518 determine that the string is actually an A-label, examining it for
519 the invalid formats specified in the Punycode decoding specification.
520 Applications that are not IDNA-aware will obviously omit that
521 testing; others MAY treat the string as opaque to avoid the
522 additional processing at the expense of providing less protection and
523 information to users.
525 5.5. Validation and Character List Testing
527 As with the registration procedure described in Section 4, the
528 Unicode string is checked to verify that all characters that appear
529 in it are valid as input to IDNA lookup processing. As discussed
530 above and in [IDNA2008-Rationale], the lookup check is more liberal
531 than the registration one. Labels that have not been fully evaluated
532 for conformance to the applicable rules are referred to as "putative"
533 labels as discussed in [IDNA2008-Defs][[anchor16: ??? Insert section
534 number -- 2.2.3 as of Defs-09]]. Putative labels with any of the
535 following characteristics MUST BE rejected prior to DNS lookup:
537 o Labels containing code points that are unassigned in the version
538 of Unicode being used by the application, i.e.,in the UNASSIGNED
539 category of [IDNA2008-Tables].
541 o Labels that are not in NFC form as defined in [Unicode-UAX15].
543 o Labels containing prohibited code points, i.e., those that are
544 assigned to the "DISALLOWED" category in the permitted character
545 table [IDNA2008-Tables].
547 o Labels containing code points that are identified in
548 [IDNA2008-Tables] as "CONTEXTJ", i.e., requiring exceptional
549 contextual rule processing on lookup, but that do not conform to
550 that rule. Note that this implies that a rule much be defined,
551 not null: a character that requires a contextual rule but for
552 which the rule is null is treated in this step as having failed to
553 conform to the rule.
555 o Labels containing code points that are identified in
556 [IDNA2008-Tables] as "CONTEXTO", but for which no such rule
557 appears in the table of rules. Applications resolving DNS names
558 or carrying out equivalent operations are not required to test
559 contextual rules for "CONTEXTO" characters, only to verify that a
560 rule is defined (although they MAY make such tests to provide
561 better protection or give better information to the user).
563 o Labels whose first character is a combining mark (see
564 Section 4.2.3.2).
566 In addition, the application SHOULD apply the following test. The
567 test may be omitted in special circumstances, such as when the lookup
568 application knows that the conditions are enforced elsewhere, because
569 an attempt to look up and resolve such strings will almost certainly
570 lead to a DNS lookup failure except when wildcards are present in the
571 zone. However, applying the test is likely to give much better
572 information about the reason for a lookup failure -- information that
573 may be usefully passed to the user when that is feasible -- than DNS
574 resolution failure information alone. In any event, lookup
575 applications should avoid attempting to resolve labels that are
576 invalid under that test.
578 o Verification that the string is compliant with the requirements
579 for right to left characters, specified in [IDNA2008-BIDI].
581 For all other strings, the lookup application MUST rely on the
582 presence or absence of labels in the DNS to determine the validity of
583 those labels and the validity of the characters they contain. If
584 they are registered, they are presumed to be valid; if they are not,
585 their possible validity is not relevant. While a lookup application
586 may reasonably issue warnings about strings it believes may be
587 problematic, applications that decline to process a string that
588 conforms to the rules above (i.e., does not look it up in the DNS)
589 are not in conformance with this protocol.
591 5.6. Punycode Conversion
593 The string that has now been validated for lookup is converted to ACE
594 form using the Punycode algorithm (with the ACE prefix added). With
595 the understanding that this summary is not normative (the steps above
596 are), the string is either
598 o in Unicode NFC form that contains no leading combining marks,
599 contains no DISALLOWED or UNASSIGNED code points, has rules
600 associated with any code points in CONTEXTJ or CONTEXTO, and, for
601 those in CONTEXTJ, to satisfies the conditions of the rules; or
603 o in A-label form, was supplied under circumstances in which the
604 U-label conversions and tests have not been performed (see
605 Section 5.4).
607 5.7. DNS Name Resolution
609 That resulting validated string is looked up in the DNS, using normal
610 DNS resolver procedures. That lookup can obviously either succeed
611 (returning information) or fail.
613 6. Security Considerations
615 Security Considerations for this version of IDNA, except for the
616 special issues associated with right to left scripts and characters,
617 are described in [IDNA2008-Defs]. Specific issues for labels
618 containing characters associated with scripts written right to left
619 appear in [IDNA2008-BIDI].
621 7. IANA Considerations
623 IANA actions for this version of IDNA are specified in
624 [IDNA2008-Tables] and discussed informally in [IDNA2008-Rationale].
625 The components of IDNA described in this document do not require any
626 IANA actions.
628 8. Contributors
630 While the listed editor held the pen, the original versions of this
631 document represent the joint work and conclusions of an ad hoc design
632 team consisting of the editor and, in alphabetic order, Harald
633 Alvestrand, Tina Dam, Patrik Faltstrom, and Cary Karp. This document
634 draws significantly on the original version of IDNA [RFC3490] both
635 conceptually and for specific text. This second-generation version
636 would not have been possible without the work that went into that
637 first version and its authors, Patrik Faltstrom, Paul Hoffman, and
638 Adam Costello. While Faltstrom was actively involved in the creation
639 of this version, Hoffman and Costello were not and should not be held
640 responsible for any errors or omissions.
642 9. Acknowledgments
644 This revision to IDNA would have been impossible without the
645 accumulated experience since RFC 3490 was published and resulting
646 comments and complaints of many people in the IETF, ICANN, and other
647 communities, too many people to list here. Nor would it have been
648 possible without RFC 3490 itself and the efforts of the Working Group
649 that defined it. Those people whose contributions are acknowledged
650 in RFC 3490, [RFC4690], and [IDNA2008-Rationale] were particularly
651 important.
653 Specific textual changes were incorporated into this document after
654 suggestions from the other contributors, Stephane Bortzmeyer, Vint
655 Cerf, Lisa Dusseault, Mark Davis, Paul Hoffman, Kent Karlsson, Erik
656 van der Poel, Marcos Sanz, Andrew Sullivan, Ken Whistler, and other
657 WG participants. Special thanks are due to Paul Hoffman for
658 permission to extract material from his Internet-Draft to form the
659 basis for Appendix B.
661 10. References
663 10.1. Normative References
665 [IDNA2008-BIDI]
666 Alvestrand, H. and C. Karp, "An updated IDNA criterion for
667 right-to-left scripts", July 2008, .
670 [IDNA2008-Defs]
671 Klensin, J., "Internationalized Domain Names for
672 Applications (IDNA): Definitions and Document Framework",
673 February 2009, .
676 [IDNA2008-Tables]
677 Faltstrom, P., "The Unicode Codepoints and IDNA",
678 July 2008, .
681 A version of this document is available in HTML format at
682 http://stupid.domain.name/idnabis/
683 draft-ietf-idnabis-tables-02.html
685 [RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
686 STD 13, RFC 1034, November 1987.
688 [RFC1035] Mockapetris, P., "Domain names - implementation and
689 specification", STD 13, RFC 1035, November 1987.
691 [RFC1123] Braden, R., "Requirements for Internet Hosts - Application
692 and Support", STD 3, RFC 1123, October 1989.
694 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
695 Requirement Levels", BCP 14, RFC 2119, March 1997.
697 [RFC3492] Costello, A., "Punycode: A Bootstring encoding of Unicode
698 for Internationalized Domain Names in Applications
699 (IDNA)", RFC 3492, March 2003.
701 [Unicode-PropertyValueAliases]
702 The Unicode Consortium, "Unicode Character Database:
703 PropertyValueAliases", March 2008, .
706 [Unicode-RegEx]
707 The Unicode Consortium, "Unicode Technical Standard #18:
708 Unicode Regular Expressions", May 2005,
709 .
711 [Unicode-Scripts]
712 The Unicode Consortium, "Unicode Standard Annex #24:
713 Unicode Script Property", February 2008,
714 .
716 [Unicode-UAX15]
717 The Unicode Consortium, "Unicode Standard Annex #15:
718 Unicode Normalization Forms", 2006,
719 .
721 10.2. Informative References
723 [ASCII] American National Standards Institute (formerly United
724 States of America Standards Institute), "USA Code for
725 Information Interchange", ANSI X3.4-1968, 1968.
727 ANSI X3.4-1968 has been replaced by newer versions with
728 slight modifications, but the 1968 version remains
729 definitive for the Internet.
731 [IDNA2008-Mapping]
732 Resnick, P., "Mapping Characters in IDNA", July 2009, .
736 [IDNA2008-Rationale]
737 Klensin, J., Ed., "Internationalizing Domain Names for
738 Applications (IDNA): Issues, Explanation, and Rationale",
739 February 2009, .
742 [RFC2136] Vixie, P., Thomson, S., Rekhter, Y., and J. Bound,
743 "Dynamic Updates in the Domain Name System (DNS UPDATE)",
744 RFC 2136, April 1997.
746 [RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS
747 Specification", RFC 2181, July 1997.
749 [RFC2535] Eastlake, D., "Domain Name System Security Extensions",
750 RFC 2535, March 1999.
752 [RFC2671] Vixie, P., "Extension Mechanisms for DNS (EDNS0)",
753 RFC 2671, August 1999.
755 [RFC3490] Faltstrom, P., Hoffman, P., and A. Costello,
756 "Internationalizing Domain Names in Applications (IDNA)",
757 RFC 3490, March 2003.
759 [RFC3491] Hoffman, P. and M. Blanchet, "Nameprep: A Stringprep
760 Profile for Internationalized Domain Names (IDN)",
761 RFC 3491, March 2003.
763 [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
764 Resource Identifier (URI): Generic Syntax", STD 66,
765 RFC 3986, January 2005.
767 [RFC3987] Duerst, M. and M. Suignard, "Internationalized Resource
768 Identifiers (IRIs)", RFC 3987, January 2005.
770 [RFC4690] Klensin, J., Faltstrom, P., Karp, C., and IAB, "Review and
771 Recommendations for Internationalized Domain Names
772 (IDNs)", RFC 4690, September 2006.
774 [RFC4952] Klensin, J. and Y. Ko, "Overview and Framework for
775 Internationalized Email", RFC 4952, July 2007.
777 [Unicode] The Unicode Consortium, "The Unicode Standard, Version
778 5.0", 2007.
780 Boston, MA, USA: Addison-Wesley. ISBN 0-321-48091-0
782 Appendix A. Local Mapping Alternatives
784 The subsections of this appendix are temporary and represent
785 different sketches of possible replacements for Section 5.3. They do
786 not represent an assertion of WG consensus or any assertion about the
787 possibility of including one of them as part of the WG's work
788 program. Instead, they are supplied only for purposes of comparison,
789 discussion, and, should it be relevant, refinement.
791 The first paragraph of each subsection describes how the material
792 would be placed relative to the existing main document text.
794 Subsequent paragraphs are the actual suggestions, although in
795 incomplete sketch form.
797 A.1. Transitional Mapping Model
799 If this subsection were adopted, Section 5.3 would be deleted and
800 this one would be inserted after, or integrated with, Section 5.7.
802 This specification does not support the extensive mappings from one
803 character to another, including Unicode Case Folding and
804 Compatibility Character mapping, of IDNA2003. It also changes the
805 interpretations of a small number of characters relative to IDNA2003.
806 Most applications, especially those with which IDNs have been used
807 for some time, will need to maintain reasonable compatibility with
808 files created under IDNA2003 and user interfaces designed for it.
809 This section specifies additional steps to be taken to provide
810 maximum IDNA2003 compatibility.
812 If an application requires IDNA2003 backward compatibility, it MUST
813 execute the steps in one of the two subsections that immediately
814 follow.
816 A.1.1. Fallback Lookup
818 If the string validates and the resolution attempt in Section 5.7
819 successfully returns a result, the lookup process terminates with
820 that result. If validation succeeds but resolution fails, the
821 validated string is proceeded through the ToASCII operation specified
822 in IDNA2003 [RFC3490]. Assuming it produces a valid result, the
823 resulting string is compared to the previous validated one. If they
824 are not identical, a resolution attempt is made with the ToASCII
825 output and the result of that attempt is returned as the result of
826 the lookup operation.
828 Should IDNA2008 validation fail, the string is processed through
829 ToASCII and, assuming the result is valid, the resulting string is
830 resolved and the result of that attempt returned as the result of the
831 lookup operation.
833 If ToASCII (IDNA2003) conversion is attempted and fails, the lookup
834 operation behaves as if no name was found in the DNS.
836 Note that this procedure involves, at most, one DNS lookup
837 (resolution attempt). If IDNA2008 string validation, conversion, and
838 resolution succeed, no attempt is made to use IDNA2003 mechanisms.
839 The procedure does, however, require that lookup applications fully
840 support both IDNA2008 and IDNA2003 lookup operations so that the
841 fallback can occur.
843 A.1.2. Two-step Lookup
845 Prior to the resolution attempt in Section 5.7, ACE strings are
846 computed using both IDNA2003 (ToASCII) and IDNA2008 methods (as
847 specified here). Assuming both validate, those strings are compared.
848 If they are identical, or only one was valid, then a single DNS
849 resolution is performed and its result is the result of the lookup
850 operation. If both are valid but they are not identical, one
851 resolution attempt is made with each of the two ACE strings.
853 If neither string is valid as an IDN, then the lookup operation
854 fails.
856 When two resolutions are attempted, if one of the two is successful
857 and the other is not, the successful value is used as the result of
858 the lookup. If both are successful, the user or calling application
859 must be presented with a choice in some way.
861 This procedure will require two DNS lookups (resolution attempts) in
862 all cases except those in which the label string fails IDNA2008
863 validation, neither IDNA2003 or IDNA2008 can validate the string and
864 translate it to ACE form, or the strings obtained from the two
865 conversions are identical. As with the prior option, IDNA
866 implementations will need to support both the IDNA2003 algorithm and
867 tables and the IDNA2008 one. The question of how multiple results
868 from different interpretations of the same input string should be
869 handled by applications is a difficult one, with potential false
870 positive and security attack vector implications as well as the
871 possibility of general confusion.
873 In particular, if both interpretations of the name return values, the
874 lookup application has no practical way to tell whether the relevant
875 registry has applied "variant" or "bundling" techniques to ensure
876 that both domain names are under the same control or not. From that
877 perspective, the approach in the previous subsection assumes that has
878 been done (if the IDNA2003-interpretation label is present at all)
879 while this one assumes that such bundling is unlikely to have
880 occurred.
882 [[anchor26: Note in Draft: If this appendix is used, RFC3490 must be
883 moved from Informative to Normative.]]
885 A.2. Internationalized Resource Identifier (IRI) Mapping Model
887 This subsection is intended to be descriptive of an approach that
888 lies outside IDNA, rather than a normative component of it. If it
889 were adopted, Section 5.3 would be deleted and the material below
890 would be referenced, either as a non-normative Appendix in Protocol
891 or, more reasonably, as a section of Rationale.
893 IDNA2003 supported extensive mappings from one character to another,
894 including Unicode Case Folding and Compatibility Character mapping.
895 Those mappings are no longer supported on registration and are
896 inconsistent with the "exact match" lookups that people expect from
897 the DNS. Some mapping should still be supported, both for
898 compatibility with applications that assume IDNA2003 and to avoid
899 confounding user expectations. The specific mappings involved are
900 not part of IDNA, but are expected to be specified as part of a
901 revision to the IRI specification [RFC3987] and the conversion from
902 IRI form to URI form. That change leaves mapping unspecified and
903 prohibited for actual domain names, however, in practice, most domain
904 names, especially in the web applications that appear to have been
905 most important for IDNs between the publication of IDNA2003 and the
906 release of this specification, are not interpreted as themselves but
907 as abbreviated form of URIs or IRIs and hence subject to the
908 transformation rules of the latter.
910 Appendix B. Summary of Major Changes from IDNA2003
912 1. Update base character set from Unicode 3.2 to Unicode version-
913 agnostic.
915 2. Separate the definitions for the "registration" and "lookup"
916 activities.
918 3. Disallow symbol and punctuation characters except where special
919 exceptions are necessary.
921 4. Remove the mapping and normalization steps from the protocol and
922 have them instead done by the applications themselves, possibly
923 in a local fashion, before invoking the protocol.
925 5. Change the way that the protocol specifies which characters are
926 allowed in labels from "humans decide what the table of
927 codepoints contains" to "decision about codepoints are based on
928 Unicode properties plus a small exclusion list created by
929 humans".
931 6. Introduce the new concept of characters that can be used only in
932 specific contexts.
934 7. Allow typical words and names in languages such as Dhivehi and
935 Yiddish to be expressed.
937 8. Make bidirectional domain names (delimited strings of labels,
938 not just labels standing on their own) display in a less
939 surprising fashion whether they appear in obvious domain name
940 contexts or as part of running text in paragraphs.
942 9. Remove the dot separator from the mandatory part of the
943 protocol.
945 10. Make some currently-valid labels that are not actually IDNA
946 labels invalid.
948 Appendix C. Change Log
950 [[anchor29: RFC Editor: Please remove this appendix.]]
952 C.1. Changes between Version -00 and -01 of draft-ietf-idnabis-protocol
954 o Corrected discussion of SRV records.
956 o Several small corrections for clarity.
958 o Inserted more "open issue" placeholders.
960 C.2. Version -02
962 o Rewrote the "conversion to Unicode" text in Section 5.2 as
963 requested on-list.
965 o Added a comment (and reference) about EDNS0 to the "DNS Server
966 Conventions" section, which was also retitled.
968 o Made several editorial corrections and improvements in response to
969 various comments.
971 o Added several new discussion placeholder anchors and updated some
972 older ones.
974 C.3. Version -03
976 o Trimmed change log, removing information about pre-WG drafts.
978 o Incorporated a number of changes suggested by Marcos Sanz in his
979 note of 2008.07.17 and added several more placeholder anchors.
981 o Several minor editorial corrections and improvements.
983 o "Editor" designation temporarily removed because the automatic
984 posting machinery does not accept it.
986 C.4. Version -04
988 o Removed Contextual Rule appendices for transfer to Tables.
990 o Several changes, including removal of discussion anchors, based on
991 discussions at IETF 72 (Dublin)
993 o Rewrote the preprocessing material (Section 5.3) somewhat.
995 C.5. Version -05
997 o Updated part of the A-label input explanation (Section 5.4) per
998 note from Erik van der Poel.
1000 C.6. Version -06
1002 o Corrected a few typographical errors.
1004 o Incorporated the material (formerly in Rationale) on the
1005 relationship between IDNA2003 and IDNA2008 as an appendix and
1006 pointed to the new definitions document.
1008 o Text modified in several places to recognize the dangers of
1009 interaction between DNS wildcards and IDNs.
1011 o Text added to be explicit about the handling of edge and failure
1012 cases in Punycode encoding and decoding.
1014 o Revised for consistency with the new Definitions document and to
1015 make the text read more smoothly.
1017 C.7. Version -07
1019 o Multiple small textual and editorial changes and clarifications.
1021 o Requirement for normalization clarified to apply to all cases and
1022 conditions for preprocessing further clarified.
1024 o Substantive change to Section 4.2.1, turning a SHOULD to a MUST
1025 (see note from Mark Davis, 19 November, 2008 18:14 -0800).
1027 C.8. Version -08
1029 o Added some references and altered text to improve clarity.
1031 o Changed the description of CONTEXTJ/CONTEXTO to conform to that in
1032 Tables. In other words, these are now treated as distinction
1033 categories (again), rather than as specially-flagged subsets of
1034 PROTOCOL VALID.
1036 o The discussion of label comparisons has been rewritten to make it
1037 more precise and to clarify that one does not need to verify that
1038 a string is a [valid] A-label or U-label in order to test it for
1039 equality with another string. The WG should verify that the
1040 current text is what is desired.
1042 o Other changes to reflect post-IETF discussions or editorial
1043 improvements.
1045 C.9. Version -09
1047 o Removed Security Considerations material to Defs document.
1049 o Removed the Name Server Considerations material to Rationale.
1050 That material is not normative and not needed to implement the
1051 protocol itself.
1053 o Adjusted terminology to match new version of Defs.
1055 o Removed all discussion of local mapping and option for it from
1056 registration protocol. Such mapping is now completely prohibited
1057 on Registration.
1059 o Removed some old placeholders and inquiries because no comments
1060 have been received.
1062 o Small editorial corrections.
1064 C.10. Version -10
1066 o Rewrote the registration input material slightly to further
1067 clarify the "no mapping on registration" principle.
1069 o Added placeholder notes about several tasks, notably reorganizing
1070 Section 4 and Section 5 so that subsection numbers are parallel.
1072 o Cleaned up an incorrect use of the terms "A-label" and "U-label"
1073 in the lookup phase that was spotted by Mark Davis. Inserted a
1074 note there about alternate ways to deal with the resulting
1075 terminology problem.
1077 o Added a temporarily appendix (above) to document alternate
1078 strategies for possible replacements for Section 5.3.
1080 C.11. Version -11
1082 o Removed dangling reference to "C-label" (editing error in prior
1083 draft).
1085 o Recast the last steps of the Lookup description to eliminate
1086 "apparent" (previously "putative") terminology.
1088 o Rewrote major portions of the temporary appendix that describes
1089 transitional mappings to improve clarity and add context.
1091 o Did some fine-tuning of terminology, notably in Section 3.2.1.
1093 C.12. Version -12
1095 o Extensive editorial improvements, mostly due to suggestions from
1096 Lisa Dusseault.
1098 o Conformance statements have been made consistent, especially in
1099 Section 4.2.1 and subsequent text, which said "SHOULD" in one
1100 place and then said "MAY" as the result of incomplete removal of
1101 registration-time mapping. Also clarified the definition of
1102 "registration processes" in Section 4.1 -- the previous text had
1103 confused several people.
1105 o A few new "question to the WG notes have been added about
1106 appropriateness or placement of text. If there are no comments on
1107 the mailing list, the editor will apply his own judgment.
1109 o Several of the usual small typos and other editorial errors have
1110 been corrected.
1112 o Section 5 has still not been reorganized to match Section 4 in
1113 structure and subsection numbering -- will be done as soon as the
1114 mapping decisions and references are final.
1116 C.13. Version -13
1118 o Modified the "putative label" text to better explain the term and
1119 explicitly point back to Defs.
1121 o Slight rewrite of Section 5.3 to clarify the NFC requirement and
1122 to start the transition toward having some of the explanation in
1123 the Mapping document. The latter might need to be undone as WG
1124 consensus evolves.
1126 Author's Address
1128 John C Klensin
1129 1770 Massachusetts Ave, Ste 322
1130 Cambridge, MA 02140
1131 USA
1133 Phone: +1 617 245 1457
1134 Email: john+ietf@jck.com