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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 1 Internet Draft Patrik Faltstrom 2 draft-ietf-idn-idna-06.txt Cisco 3 January 7, 2002 Paul Hoffman 4 Expires in six months IMC & VPNC 5 Adam M. Costello 6 UC Berkeley 8 Internationalizing Domain Names in Applications (IDNA) 10 Status of this Memo 12 This document is an Internet-Draft and is in full conformance with all 13 provisions of Section 10 of RFC2026. 15 Internet-Drafts are working documents of the Internet Engineering Task 16 Force (IETF), its areas, and its working groups. Note that other groups 17 may also distribute working documents as Internet-Drafts. 19 Internet-Drafts are draft documents valid for a maximum of six months 20 and may be updated, replaced, or obsoleted by other documents at any 21 time. It is inappropriate to use Internet-Drafts as reference material 22 or to cite them other than as "work in progress." 24 The list of current Internet-Drafts can be accessed at 25 http://www.ietf.org/ietf/1id-abstracts.txt 27 The list of Internet-Draft Shadow Directories can be accessed at 28 http://www.ietf.org/shadow.html. 30 Abstract 32 Until now, there has been no standard method for domain names to use 33 characters outside the ASCII repertoire. This document defines 34 internationalized domain names (IDNs) and a mechanism called IDNA for 35 handling them in a standard fashion. IDNs use characters drawn from a 36 large repertoire (Unicode), but IDNA allows the non-ASCII characters to 37 be represented using the same octets used in so-called hostnames today. 39 1. Introduction 41 IDNA works by allowing applications to use certain ASCII name labels 42 (beginning with a special prefix) to represent non-ASCII name labels. 43 Lower-layer protocols need not be aware of this; therefore IDNA does not 44 require changes to any infrastructure. In particular, IDNA does not 45 require any changes to DNS servers, resolvers, or protocol elements, 46 because the ASCII name service provided by the existing DNS is entirely 47 sufficient. 49 This document does not require any applications to conform to IDNA, 50 but applications can elect to use IDNA in order to support IDN while 51 maintaining interoperability with existing infrastructure. Adding IDNA 52 support to an existing application entails changes to the application 53 only, and leaves room for flexibility in the user interface. 55 A great deal of the discussion of IDN solutions has focused on 56 transition issues and how IDN will work in a world where not all of the 57 components have been updated. Other proposals would require that user 58 applications, resolvers, and DNS servers be updated in order for a user 59 to use an internationalized host name. Rather than require widespread 60 updating of all components, IDNA requires only user applications to be 61 updated; no changes are needed to the DNS protocol or any DNS servers or 62 the resolvers on user's computers. 64 This document is being discussed on the ietf-idna@mail.apps.ietf.org 65 mailing list. To subscribe, send a message to 66 ietf-idna-request@mail.apps.ietf.org with the single word "subscribe" in 67 the body of the message. 69 2 Terminology 71 The key words "MUST", "SHALL", "REQUIRED", "SHOULD", "RECOMMENDED", and 72 "MAY" in this document are to be interpreted as described in RFC 2119 73 [RFC2119]. 75 A code point is an integral value associated with a character in a coded 76 character set. 78 Unicode [UNICODE] is a coded character set containing tens of thousands 79 of characters. A single Unicode code point is denoted by "U+" followed 80 by four to six hexadecimal digits, while a range of Unicode code points 81 is denoted by two hexadecimal numbers separated by "..", with no 82 prefixes. 84 ASCII means US-ASCII, a coded character set containing 128 characters 85 associated with code points in the range 0..7F. Unicode is an extension 86 of ASCII: it includes all the ASCII characters and associates them with 87 the same code points. 89 The term "LDH code points" is defined in this document to mean the code 90 points associated with ASCII letters, digits, and the hyphen-minus; that 91 is, U+002D, 30..39, 41..5A, and 61..7A. "LDH" is an abbreviation for 92 "letters, digits, hyphen". 94 A label is an individual part of a domain name. Labels are usually shown 95 separated by dots; for example, the domain name "www.example.com" is 96 composed of three labels: "www", "example", and "com". In IDNA, not all 97 text strings can be labels. (The zero-length root label that is implied 98 in domain names, as described in [STD13], is not considered a label in 99 this specification.) 101 An "internationalized domain name" (IDN) is a domain name for which the 102 ToASCII operation (see section 4) can be applied to each label without 103 failing. 105 An internationalized label contains characters from the Unicode 106 character set. To allow such a label to be handled by existing 107 applications, an "ACE label" is defined to be a label that contains only 108 ASCII characters but represents an equivalent label containing non-ASCII 109 characters. For every internationalized label that cannot be directly 110 represented in ASCII, there is an equivalent ACE label. The conversion 111 of labels to and from the ACE form is specified in section 4. 113 The "ACE prefix" is defined in this document to be a string of ASCII 114 characters that appears at the beginning of every ACE label. It is 115 specified in section 5. 117 A "domain name slot" is defined in this document to be a protocol element 118 or a function argument or a return value (and so on) explicitly 119 designated for carrying a domain name. Examples of domain name slots 120 include: the QNAME field of a DNS query; the name argument of the 121 gethostbyname() library function; the part of an email address following 122 the at-sign (@) in the From: field of an email message header; and the host 123 portion of the URI in the src attribute of an HTML tag. 124 General text that just happens to contain a domain name is not a domain name 125 slot; for example, a domain name appearing in the plain text body of an 126 email message is not occupying a domain name slot. 128 An "internationalized domain name slot" is defined in this document to 129 be a domain name slot explicitly designated for carrying an 130 internationalized domain name as defined in this document. The 131 designation may be static (for example, in the specification of the 132 protocol or interface) or dynamic (for example, as a result of 133 negotiation in an interactive session). 135 A "generic domain name slot" is defined in this document to be any 136 domain name slot that is not an internationalized domain name slot. 137 Obviously, this includes any domain name slot whose specification 138 predates IDNA. 140 3. Requirements 142 IDNA conformance means adherence of the following three rules: 144 1) Whenever a domain name is put into a generic domain name slot, every 145 label MUST contain only ASCII characters. Given an internationalized 146 domain name (IDN), an equivalent domain name satisfying this requirement 147 can be obtained by applying the ToASCII operation (see section 4) 148 to each label. 150 2) ACE labels SHOULD be hidden from users whenever possible. Therefore, 151 before a domain name is displayed to a user or is output into a context 152 likely to be viewed by users, the ToUnicode operation (see section 4) 153 SHOULD be applied to each label. When requirements 1 and 2 both apply, 154 requirement 1 takes precedence. 156 3) Whenever two labels are compared, they MUST be considered to 157 match if and only if their ASCII forms (obtained by applying ToASCII) 158 match using a case-insensitive ASCII comparison. 160 4. Conversion operations 162 This section specifies the ToASCII and ToUnicode operations. Each one 163 operates on a sequence of Unicode code points (but remember that all 164 ASCII code points are also Unicode code points). When domain names are 165 represented using character sets other than Unicode and ASCII, they will 166 need to first be transcoded to Unicode before these operations can be 167 applied, and might need to be transcoded back afterwards. 169 4.1 ToASCII 171 The ToASCII operation takes a sequence of Unicode code points and 172 transforms it into a sequence of code points in the ASCII range (0..7F). 173 The original sequence and the resulting sequence are equivalent labels 174 (if the original is an internationalized label that cannot be directly 175 represented in ASCII, the result will be the equivalent ACE label). 177 ToASCII fails if any step of it fails. Failure means that the original 178 sequence cannot be used as a label in an IDN. 180 ToASCII never alters a sequence of code points that are all in the ASCII 181 range to begin with (although it may fail). 183 ToASCII consists of the following steps: 185 1. If all code points in the sequence are in the ASCII range (0..7F) 186 then skip to step 3. 188 2. Perform the steps specified in [NAMEPREP]. 190 3. Host-specific restrictions: If the label is part of a host name 191 (or is subject to host name syntax rules) then perform these 192 checks: 194 * Verify the absence of non-LDH ASCII code points; that is, the 195 absence of 0..2C, 2E..2F, 3A..40, 5B..60, and 7B..7F. 197 * Verify the absence of leading and trailing hyphen-minus; that 198 is, the absence of U+002D at the beginning and end of the 199 sequence. 201 4. If all code points in the sequence are in the ASCII range (0..7F), 202 then skip to step 8. 204 5. Verify that the sequence does NOT begin with the ACE prefix. 206 6. Encode the sequence using the encoding algorithm in [PUNYCODE]. 208 7. Prepend the ACE prefix. 210 8. Verify that the number of code points is in the range 1 to 63 211 inclusive. 213 4.2 ToUnicode 215 The ToUnicode operation takes a sequence of Unicode code points and 216 returns a sequence of Unicode code points. If the input sequence is a 217 label in ACE form, then the result is an equivalent internationalized 218 label that is not in ACE form, otherwise the original sequence is 219 returned unaltered. 221 ToUnicode never fails. If any step fails, then the original input 222 sequence is returned immediately in that step. 224 1. If all code points in the sequence are in the ASCII range (0..7F) 225 then skip to step 3. 227 2. Perform the steps specified in [NAMEPREP]. (If step 3 228 of ToASCII is also performed here, it will not affect the 229 overall behavior of ToUnicode, but it is not necessary.) 231 3. Verify that the sequence begins with the ACE prefix, and save a 232 copy of the sequence. 234 4. Remove the ACE prefix. 236 5. Decode the sequence using decoding algorithm in [PUNYCODE]. Save 237 a copy of the result of this step. 239 6. Apply ToASCII. 241 7. Verify that the sequence matches the saved copy from step 3, using 242 a case-insensitive ASCII comparison. 244 8. Return the saved copy from step 5. 246 5. ACE prefix 248 The ACE prefix, used in the conversion operations (section 4), will be 249 specified in a future revision of this document. It will be two 250 alphanumeric ASCII characters followed by two hyphen-minuses. The 251 ToASCII and ToUnicode operations MUST recognize the ACE prefix in a 252 case-insensitive manner. 254 For example, the eventual ACE prefix might be the string "jk--". In this 255 case, an ACE label might be "jk--r3c2a-qc902xs", where "r3c2a-qc902xs" 256 is the part of the ACE label that is generated by the encoding steps in 257 [PUNYCODE]. 259 6. Implications for typical applications using DNS 261 In IDNA, applications perform the processing needed to input 262 internationalized domain names from users, display internationalized 263 domain names to users, and process the inputs and outputs from DNS and 264 other protocols that carry domain names. 266 The components and interfaces between them can be represented 267 pictorially as: 269 +------+ 270 | User | 271 +------+ 272 ^ 273 | Input and display: local interface methods 274 | (pen, keyboard, glowing phosphorus, ...) 275 +-------------------|-------------------------------+ 276 | v | 277 | +-----------------------------+ | 278 | | Application | | 279 | | (conversion between local | | 280 | | character set and Unicode | | 281 | | is done here) | | 282 | +-----------------------------+ | 283 | ^ ^ | End system 284 | | | | 285 | Call to resolver: | | Application-specific | 286 | ACE | | protocol: | 287 | v | predefined by the | 288 | +----------+ | protocol or defaults | 289 | | Resolver | | to ACE | 290 | +----------+ | | 291 | ^ | | 292 +-----------------|----------|----------------------+ 293 DNS protocol: | | 294 ACE | | 295 v v 296 +-------------+ +---------------------+ 297 | DNS servers | | Application servers | 298 +-------------+ +---------------------+ 300 6.1 Entry and display in applications 302 Applications can accept domain names using any character set or sets 303 desired by the application developer, and can display domain names in any 304 charset. That is, the IDNA protocol does not affect the interface 305 between users and applications. 307 An IDNA-aware application can accept and display internationalized domain 308 names in two formats: the internationalized character set(s) supported 309 by the application, and as an ACE label. Applications MAY allow input 310 and display of ACE labels, but are not encouraged to do so except as an 311 interface for special purposes, possibly for debugging. ACE encoding is 312 opaque and ugly, and should thus only be exposed to users who absolutely 313 need it. The optional use, especially during a transition period, of ACE 314 encodings in the user interface is described in section 6.4. Because 315 name labels encoded as ACE name labels can be rendered either as the 316 encoded ASCII characters or the proper decoded characters, the 317 application MAY have an option for the user to select the preferred 318 method of display; if it does, rendering the ACE SHOULD NOT be the 319 default. 321 Domain names are often stored and transported in many places. For example, 322 they are part of documents such as mail messages and web pages. They are 323 transported in many parts of many protocols, such as both the 324 control commands and the RFC 2822 body parts of SMTP, and the headers 325 and the body content in HTTP. It is important to remember that domain 326 names appear both in domain name slots and in the content that is passed 327 over protocols. 329 In protocols and document formats that define how to handle 330 specification or negotiation of charsets, labels can be encoded in any 331 charset allowed by the protocol or document format. If a protocol or 332 document format only allows one charset, the labels MUST be given in 333 that charset. 335 In any place where a protocol or document format allows transmission of 336 the characters in internationalized labels, internationalized labels 337 SHOULD be transmitted using whatever character encoding and escape 338 mechanism that the protocol or document format uses at that place. 340 All protocols that use domain name slots already have the capacity for 341 handling domain names in the ASCII charset. Thus, ACE labels 342 (internationalized labels that have been processed with the ToASCII 343 operation) can inherently be handled by those protocols. 345 6.2 Applications and resolver libraries 347 Applications normally use functions in the operating system when they 348 resolve DNS queries. Those functions in the operating system are often 349 called "the resolver library", and the applications communicate with the 350 resolver libraries through a programming interface (API). 352 Because these resolver libraries today expect only domain names in 353 ASCII, applications MUST prepare labels that are passed to the resolver 354 library using the ToASCII operation. Labels received from the resolver 355 library contain only ASCII characters; internationalized labels that 356 cannot be represented directly in ASCII use the ACE form. 358 IDNA-aware applications MUST be able to work with both 359 non-internationalized labels (those that conform to [STD13] 360 and [STD3]) and internationalized labels. 362 It is expected that new versions of the resolver libraries in the future 363 will be able to accept domain names in other formats than ASCII, and 364 application developers might one day pass not only domain names in 365 Unicode, but also in local script to a new API for the resolver 366 libraries in the operating system. 368 6.3 DNS servers 370 An operating system might have a set of libraries for performing the 371 ToASCII operation. The input to such a library might be in one or more 372 charsets that are used in applications (UTF-8 and UTF-16 are likely 373 candidates for almost any operating system, and script-specific charsets 374 are likely for localized operating systems). 376 For internationalized labels that cannot be represented directly in 377 ASCII, DNS servers MUST use the ACE form produced by the ToASCII 378 operation. All IDNs served by DNS servers MUST contain only ASCII 379 characters. 381 If a signalling system which makes negotiation possible between old and 382 new DNS clients and servers is standardized in the future, the encoding 383 of the query in the DNS protocol itself can be changed from ACE to 384 something else, such as UTF-8. The question whether or not this should 385 be used is, however, a separate problem and is not discussed in this 386 memo. 388 6.4 Avoiding exposing users to the raw ACE encoding 390 All applications that might show the user a domain name obtained from a 391 domain name slot, such as from gethostbyaddr or part of a mail header, 392 SHOULD be updated as soon as possible in order to prevent users from 393 seeing the ACE. 395 If an application decodes an ACE name using ToUnicode but cannot show 396 all of the characters in the decoded name, such as if the name contains 397 characters that the output system cannot display, the application SHOULD 398 show the name in ACE format instead of displaying the name with the 399 replacement character (U+FFFD). This is to make it easier for the user 400 to transfer the name correctly to other programs. Programs that by 401 default show the ACE form when they cannot show all the characters in a 402 name label SHOULD also have a mechanism to show the name that is 403 produced by the ToUnicode operation with as many characters as possible 404 and replacement characters in the positions where characters cannot be 405 displayed. 407 The ToUnicode operation does not alter labels that are not valid ACE 408 labels, even if they begin with the ACE prefix. After ToUnicode has been 409 applied, if a label still begins with the ACE prefix, then it is not a 410 valid ACE label, and is not equivalent to any of the intermediate 411 Unicode strings constructed by ToUnicode. 413 6.5 Bidirectional text in domain names 415 The display of domain names that contain bidirectional text is not covered 416 in this document. It may be covered in a future version of this 417 document, or may be covered in a different document. 419 For developers interested in displaying host names that have 420 bidirectional text, the Unicode standard has an extensive discussion of 421 how to deal with reorder glyphs for display when dealing with 422 bidirectional text such as Arabic or Hebrew. See [UAX9] for more 423 information. In particular, all Unicode text is stored in logical order. 425 6.6 DNSSEC authentication of IDN domain names 427 DNS Security [DNSSEC] is a method for supplying cryptographic 428 verification information along with DNS messages. Public Key 429 Cryptography is used in conjunction with digital signatures to provide a 430 means for a requester of domain information to authenticate the source 431 of the data. This ensures that it can be traced back to a trusted 432 source, either directly, or via a chain of trust linking the source of 433 the information to the top of the DNS hierarchy. 435 IDNA specifies that all internationalized domain names served by DNS 436 servers that cannot be represented directly in ASCII must use the ACE 437 form produced by the ToASCII operation. This operation must be performed 438 prior to a zone being signed by the private key for that zone. Because 439 of this ordering, it is important to recognize that DNSSEC authenticates 440 the ASCII domain name, not the Unicode form or the mapping between the 441 Unicode form and the ASCII form. In other words, the output of ToASCII 442 is the canonical name. In the presence of DNSSEC, this is the name that 443 MUST be signed in the zone and MUST be validated against. It also SHOULD 444 be used for other name comparisons, such as when a browser wants to 445 indicate that a URL has been previously visited. 447 One consequence of this for sites deploying IDNA in the presence of 448 DNSSEC is that any special purpose proxies or forwarders used to 449 transform user input into IDNs must be earlier in the resolution flow 450 than DNSSEC authenticating nameservers for DNSSEC to work. 452 7. Name Server Considerations 454 Internationalized domain name data in zone files (as specified by section 455 5 of RFC 1035) MUST be processed with ToASCII before it is entered in 456 the zone files. 458 It is imperative that there be only one ASCII encoding for a particular 459 domain name. ACE is an encoding for domain name labels that use non-ASCII 460 characters. Thus, a primary master name server MUST NOT contain an 461 ACE-encoded label that decodes to an ASCII label. The ToASCII operation 462 assures that no such names are ever output from the operation. 464 Name servers MUST NOT serve records with domain names that contain 465 non-ASCII characters; such names MUST be converted to ACE form by the 466 ToASCII operation in order to be served. If names that are not processed 467 by ToASCII are passed to an application, it will result in unpredictable 468 behavior. Note that [NAMEPREP] describes how to handle versioning of 469 unallocated codepoints. 471 8. Root Server Considerations 473 Because there are no changes to the DNS protocols, adopting this 474 protocol has no effect on the DNS root servers. 476 9. Security Considerations 478 Much of the security of the Internet relies on the DNS. Thus, any change 479 to the characteristics of the DNS can change the security of much of the 480 Internet. 482 This memo describes an algorithm which encodes characters that are not 483 valid according to STD3 and STD13 into octet values that are valid. No 484 security issues such as string length increases or new allowed values 485 are introduced by the encoding process or the use of these encoded 486 values, apart from those introduced by the ACE encoding itself. 488 Domain names are used by users to connect to Internet servers. The 489 security of the Internet would be compromised if a user entering a 490 single internationalized name could be connected to different servers 491 based on different interpretations of the internationalized domain name. 493 Because this document normatively refers to [NAMEPREP], it includes the 494 security considerations from that document as well. 496 A. References 498 [PUNYCODE] Adam Costello, "Punycode", draft-ietf-idn-punycode. 500 [DNSSEC] Don Eastlake, "Domain Name System Security Extensions", RFC 501 2535, March 1999. 503 [NAMEPREP] Paul Hoffman and Marc Blanchet, "Preparation of 504 Internationalized Host Names", draft-ietf-idn-nameprep. 506 [RFC2119] Scott Bradner, "Key words for use in RFCs to Indicate 507 Requirement Levels", March 1997, RFC 2119. 509 [STD3] Bob Braden, "Requirements for Internet Hosts -- Communication 510 Layers" (RFC 1122) and "Requirements for Internet Hosts -- Application 511 and Support" (RFC 1123), STD 3, October 1989. 513 [STD13] Paul Mockapetris, "Domain names - concepts and facilities" (RFC 514 1034) and "Domain names - implementation and specification" (RFC 1035, 515 STD 13, November 1987. 517 [UAX9] Unicode Standard Annex #9, The Bidirectional Algorithm. 518 http://www.unicode.org/unicode/reports/tr9/ 520 [UNICODE] The Unicode Standard, Version 3.1.0: The Unicode Consortium. 521 The Unicode Standard, Version 3.0. Reading, MA, Addison-Wesley 522 Developers Press, 2000. ISBN 0-201-61633-5, as amended by: Unicode 523 Standard Annex #27: Unicode 3.1 524 . 526 B. Design philosophy 528 Many proposals for IDN protocols have required that DNS servers be 529 updated to handle internationalized domain names. Because of this, a 530 person who wanted to use an internationalized domain name would have to be 531 sure that their request went to a DNS server that had been updated for 532 IDN. Further, that server could send queries only to other servers that 533 had been updated for IDN, because the queries contain new protocol 534 elements to differentiate IDN labels from current labels. In 535 addition, these proposals require that resolvers be updated to use the 536 new protocols, and in most cases the applications would need to be 537 updated as well. 539 These proposals would require changes to the application protocols that 540 use host names as protocol elements, because of the assumptions and 541 requirements made in those protocols about the characters that have 542 always been used for host names, and the encoding of those characters. 543 Other proposals for IDN protocols do not require changes to DNS servers 544 but still require changes to most application protocols to handle the 545 new names. 547 Updating all (or even a significant percentage) of the existing servers 548 in the world will be difficult, to say the least. Updating applications, 549 application gateways, and clients to handle changes to the application 550 protocols is also daunting. Because of this, we have designed a protocol 551 that requires no updating of any name servers. IDNA still requires the 552 updating of applications, but only for input and display of names, not 553 for changes to the protocols. Once users have updated the applications, 554 they can immediately start using internationalized host names. The cost 555 of implementing IDN may thus be much lower, and the speed of 556 implementation could be much higher. 558 C. Authors' Addresses 560 Patrik Faltstrom 561 Cisco Systems 562 Arstaangsvagen 31 J 563 S-117 43 Stockholm Sweden 564 paf@cisco.com 566 Paul Hoffman 567 Internet Mail Consortium and VPN Consortium 568 127 Segre Place 569 Santa Cruz, CA 95060 USA 570 phoffman@imc.org 572 Adam M. Costello 573 University of California, Berkeley 574 idna-spec.amc @ nicemice.net