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'DNSSEC') (Obsoleted by RFC 4033, RFC 4034, RFC 4035) Summary: 3 errors (**), 0 flaws (~~), 3 warnings (==), 3 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 1 Internet Draft Patrik Faltstrom 2 draft-ietf-idn-idna-08.txt Cisco 3 May 22, 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 host names today. 38 This representation allows IDNs to be introduced with minimal changes to 39 the existing DNS infrastructure. IDNA is only meant for processing 40 domain names, not free text. 42 1. Introduction 44 IDNA works by allowing applications to use certain ASCII name labels 45 (beginning with a special prefix) to represent non-ASCII name labels. 46 Lower-layer protocols need not be aware of this; therefore IDNA does not 47 require changes to any infrastructure. In particular, IDNA does not 48 require any changes to DNS servers, resolvers, or protocol elements, 49 because the ASCII name service provided by the existing DNS is entirely 50 sufficient. 52 This document does not require any applications to conform to IDNA, but 53 applications can elect to use IDNA in order to support IDN while 54 maintaining interoperability with existing infrastructure. Adding IDNA 55 support to an existing application entails changes to the application 56 only, and leaves room for flexibility in the user interface. 58 A great deal of the discussion of IDN solutions has focused on 59 transition issues and how IDN will work in a world where not all of the 60 components have been updated. Proposals that were not chosen by the IDN 61 Working Group would require that user applications, resolvers, and DNS 62 servers be updated in order for a user to use an internationalized 63 domain name. Rather than require widespread updating of all components, 64 IDNA requires only user applications to be updated; no changes are 65 needed to the DNS protocol or any DNS servers or the resolvers on user's 66 computers. 68 1.1 Brief overview for application developers 70 Applications can use IDNA to support internationalized domain names 71 anywhere that ASCII domain names are already supported, including DNS 72 master files and resolver interfaces. (Applications can also define 73 protocols and interfaces that support IDNs directly using non-ASCII 74 representations. IDNA does not prescribe any particular representation 75 for new protocols, but it still defines which names are valid and how 76 they are compared.) 78 The IDNA protocol is contained completely within applications. It is not 79 a client-server or peer-to-peer protocol: everything is done inside the 80 application itself. When used with a DNS resolver library, IDNA is 81 inserted as a "shim" between the application and the resolver library. 82 When used for writing names into a DNS zone, IDNA is used just before 83 the name is committed to the zone. 85 There are two operations described in section 4 of this document: 87 - The ToASCII operation is used before sending an IDN to something that 88 expects ASCII names (such as a resolver) or writing an IDN into a place 89 that expects ASCII names (such as a DNS master file). 91 - The ToUnicode operation is used when displaying names to users, for 92 example names obtained from a DNS zone. 94 It is important to note that the ToASCII operation can fail. If it fails 95 when processing a domain name, that domain name cannot be used as an 96 internationalized domain name and the application has to have some 97 method of dealing with this failure. 99 IDNA requires that implementations process input strings with Nameprep 100 [NAMEPREP], which is a profile of Stringprep [STRINGPREP], and then with 101 Punycode [PUNYCODE]. Implementations of IDNA MUST fully implement 102 Nameprep and Punycode; neither Nameprep nor Punycode are optional. 104 2 Terminology 106 The key words "MUST", "SHALL", "REQUIRED", "SHOULD", "RECOMMENDED", and 107 "MAY" in this document are to be interpreted as described in RFC 2119 108 [RFC2119]. 110 A code point is an integral value associated with a character in a coded 111 character set. 113 Unicode [UNICODE] is a coded character set containing tens of thousands 114 of characters. A single Unicode code point is denoted by "U+" followed 115 by four to six hexadecimal digits, while a range of Unicode code points 116 is denoted by two hexadecimal numbers separated by "..", with no 117 prefixes. 119 ASCII means US-ASCII [USASCII], a coded character set containing 128 120 characters associated with code points in the range 0..7F. Unicode is an 121 extension of ASCII: it includes all the ASCII characters and associates 122 them with the same code points. 124 The term "LDH code points" is defined in this document to mean the code 125 points associated with ASCII letters, digits, and the hyphen-minus; that 126 is, U+002D, 30..39, 41..5A, and 61..7A. "LDH" is an abbreviation for 127 "letters, digits, hyphen". 129 [STD13] talks about "domain names" and "host names", but many people use 130 the terms interchangeably. Further, because [STD13] was not terribly 131 clear, many people who are sure they know the exact definitions of each 132 of these terms disagree on the definitions. In this document the term 133 "domain name" is used in general. When referring explicitly to the 134 syntax restrictions for host names in [STD3], the term "host name 135 syntax" is used. 137 A label is an individual part of a domain name. Labels are usually shown 138 separated by dots; for example, the domain name "www.example.com" is 139 composed of three labels: "www", "example", and "com". (The zero-length 140 root label described in [STD13], which can be explicit as in 141 "www.example.com." or implicit as in "www.example.com", is not 142 considered a label in this specification.) Throughout this document the 143 term "label" is shorthand for "text label", and "every label" means 144 "every text label". In IDNA, not all text strings can be labels. 146 An "internationalized domain name" (IDN) is a domain name for which the 147 ToASCII operation (see section 4) can be applied to each label without 148 failing. This document does not attempt to define an "internationalized 149 host name". It is expected that some name-handling bodies, such as large 150 zone administrators and groups of affiliated zone administrators, will 151 want to limit the characters allowed in IDNs further than what is 152 specified in this document, such as to prohibit additional characters 153 that they feel are unneeded or harmful in registered domain names. 155 In IDNA, equivalence of labels is defined in terms of the ToASCII 156 operation, which constructs an ASCII form for a given label. Labels are 157 defined to be equivalent if and only if their ASCII forms produced by 158 ToASCII match using a case-insensitive ASCII comparison. Traditional 159 ASCII labels already have a notion of equivalence: upper case and lower 160 case are considered equivalent. The IDNA notion of equivalence is an 161 extension of the old notion. Equivalent labels in IDNA are treated as 162 alternate forms of the same label, just as "foo" and "Foo" are treated 163 as alternate forms of the same label. 165 An "internationalized label" is a label composed of characters from the 166 Unicode character set; note, however, that not every string of Unicode 167 characters can be an internationalized label. 169 To allow internationalized labels to be handled by existing 170 applications, IDNA uses an "ACE label" (ACE stands for ASCII Compatible 171 Encoding), which can be represented using only ASCII characters but is 172 equivalent to a label containing non-ASCII characters. More rigorously, 173 an ACE label is defined to be any label that the ToUnicode operation 174 would alter. For every internationalized label that cannot be directly 175 represented in ASCII, there is an equivalent ACE label. An ACE label 176 always begins with the ACE prefix defined in section 5. The conversion 177 of labels to and from the ACE form is specified in section 4. 179 The "ACE prefix" is defined in this document to be a string of ASCII 180 characters that appears at the beginning of every ACE label. It is 181 specified in section 5. 183 A "domain name slot" is defined in this document to be a protocol 184 element or a operation argument or a return value (and so on) explicitly 185 designated for carrying a domain name. Examples of domain name slots 186 include: the QNAME field of a DNS query; the name argument of the 187 gethostbyname() library function; the part of an email address following 188 the at-sign (@) in the From: field of an email message header; and the 189 host portion of the URI in the src attribute of an HTML tag. 190 General text that just happens to contain a domain name is not a domain 191 name slot; for example, a domain name appearing in the plain text body 192 of an email message is not occupying a domain name slot. 194 An "IDN-aware domain name slot" is defined in this document to be a 195 domain name slot explicitly designated for carrying an internationalized 196 domain name as defined in this document. The designation may be static 197 (for example, in the specification of the protocol or interface) or 198 dynamic (for example, as a result of negotiation in an interactive 199 session). 201 An "IDN-unaware domain name slot" is defined in this document to be any 202 domain name slot that is not an IDN-aware domain name slot. Obviously, 203 this includes any domain name slot whose specification predates IDNA. 205 3. Requirements 207 IDNA conformance means adherence to the following four requirements: 209 1) Whenever dots are used as label separators, the following characters 210 MUST be recognized as dots: U+002E (full stop), U+3002 (ideographic full 211 stop), U+FF0E (fullwidth full stop), U+FF61 (halfwidth ideographic full 212 stop). 214 2) Whenever a domain name is put into an IDN-unaware domain name slot 215 (see section 2), it MUST contain only ASCII characters, and, if dots are 216 used as label separators, changing all the label separators to U+002E. 217 Given an internationalized domain name (IDN), an equivalent domain name 218 satisfying this requirement can be obtained by applying the ToASCII 219 operation (see section 4) to each label. 221 3) ACE labels obtained from domain name slots SHOULD be hidden from 222 users except when the use of the non-ASCII form would cause problems or 223 when the ACE form is explicitly requested. Given an internationalized 224 domain name, an equivalent domain name containing no ACE labels can be 225 obtained by applying the ToUnicode operation (see section 4) to each 226 label. When requirements 2 and 3 both apply, requirement 1 takes 227 precedence. 229 4) Whenever two labels are compared, they MUST be considered to match if 230 and only if they are equivalent, that is, their ASCII forms (obtained by 231 applying ToASCII) match using a case-insensitive ASCII comparison. 232 Whenever two names are compared, they MUST be considered to match if and 233 only if their corresponding labels match, regardless of whether the 234 names use the same forms of label separators. 236 4. Conversion operations 238 An application converts a domain name put into an IDN-unaware slot or 239 displayed to a user. This section specifies the steps to perform in the 240 conversion, and the ToASCII and ToUnicode operations. 242 The input to ToASCII or ToUnicode is a single label that is a sequence 243 of Unicode code points (remember that all ASCII code points are also 244 Unicode code points). If a domain name is represented using a character 245 set other than Unicode or US-ASCII, it will first need to be transcoded 246 to Unicode. 248 Starting from a whole domain name, the steps that an application takes 249 to do the conversions are: 251 1) Decide whether the domain name is a "stored string" or a "query 252 string" as described in [STRINGPREP]. If this conversion follows the 253 "queries" rule from [STRINGPREP], set the flag called "AllowUnassigned". 255 2) Split the domain name into individual labels as described in section 256 3. The labels do not include the separator. 258 3) Decide whether or not to enforce the restrictions on ASCII characters 259 in host names [STD3]. If the restrictions are to be enforced, set the 260 flag called "UseSTD3ASCIIRules". 262 4) Process each label with either the ToASCII or the ToUnicode 263 operation. Use the ToASCII operation/function if you are about to put 264 the name into an IDN-unaware slot. Use the ToUnicode operation if you 265 are displaying the name to a user. 267 5) If ToASCII was applied in step 4 and dots are used as label 268 separators, change all the label separators to U+002E (full stop). 270 The following two subsections define the ToASCII and ToUnicode 271 operations that are used in step 4. 273 4.1 ToASCII 275 The ToASCII operation takes a sequence of Unicode code points that make 276 up one label and transforms it into a sequence of code points in the 277 ASCII range (0..7F). If ToASCII succeeds, the original sequence and the 278 resulting sequence are equivalent labels. 280 It is important to note that the ToASCII operation can fail. If the 281 ToASCII operation fails on any label in a domain name, that domain name 282 MUST NOT be used as an internationalized domain name. The application 283 needs to have some method of dealing with this failure. 285 The inputs to ToASCII are a sequence of code points; the AllowUnassigned 286 flag; and the UseSTD3ASCIIRules flag. The output of ToASCII is either a 287 sequence of ASCII code points or a failure condition. 289 ToASCII never alters a sequence of code points that are all in the ASCII 290 range to begin with (although it could fail). Applying the ToASCII 291 operation multiple times has exactly the same effect as applying it just 292 once. 294 ToASCII consists of the following steps: 296 1. If all code points in the sequence are in the ASCII range (0..7F) 297 then skip to step 3. 299 2. Perform the steps specified in [NAMEPREP] and fail if there is 300 an error. The AllowUnassigned flag is used in [NAMEPREP]. 302 3. If the UseSTD3ASCIIRules flag is set, then perform these checks: 304 (a) Verify the absence of non-LDH ASCII code points; that is, 305 the absence of 0..2C, 2E..2F, 3A..40, 5B..60, and 7B..7F. 307 (b) Verify the absence of leading and trailing hyphen-minus; 308 that is, the absence of U+002D at the beginning and end of 309 the sequence. 311 4. If all code points in the sequence are in the ASCII range 312 (0..7F), then skip to step 8. 314 5. Verify that the sequence does NOT begin with the ACE prefix. 316 6. Encode the sequence using the encoding algorithm in [PUNYCODE] 317 and fail if there is an error. 319 7. Prepend the ACE prefix. 321 8. Verify that the number of code points is in the range 1 to 63 322 inclusive. 324 4.2 ToUnicode 326 The ToUnicode operation takes a sequence of Unicode code points that 327 make up one label and returns a sequence of Unicode code points. If the 328 input sequence is a label in ACE form, then the result is an equivalent 329 internationalized label that is not in ACE form, otherwise the original 330 sequence is returned unaltered. 332 ToUnicode never fails. If any step fails, then the original input 333 sequence is returned immediately in that step. 335 The inputs to ToUnicode are a sequence of code points; the 336 AllowUnassigned flag; and the UseSTD3ASCIIRules flag. The output of 337 ToUnicode is always a sequence of Unicode code points. 339 1. If all code points in the sequence are in the ASCII range (0..7F) 340 then skip to step 3. 342 2. Perform the steps specified in [NAMEPREP] and fail if there is an 343 error. (If step 3 of ToASCII is also performed here, it will not 344 affect the overall behavior of ToUnicode, but it is not 345 necessary.) The AllowUnassigned flag is used in [NAMEPREP]. 347 3. Verify that the sequence begins with the ACE prefix, and save a 348 copy of the sequence. 350 4. Remove the ACE prefix. 352 5. Decode the sequence using the decoding algorithm in [PUNYCODE] 353 and fail if there is an error. Save a copy of the result of 354 this step. 356 6. Apply ToASCII. 358 7. Verify that the result of step 6 matches the saved copy from 359 step 3, using a case-insensitive ASCII comparison. 361 8. Return the saved copy from step 5. 363 5. ACE prefix 365 [[ Note to the IESG and Internet Draft readers: The two uses of the 366 string "IESG--" below are to be changed at time of publication to a 367 prefix which fulfills the requirements in the first paragraph. IANA will 368 assign this value. ]] 370 The ACE prefix, used in the conversion operations (section 4), is two 371 alphanumeric ASCII characters followed by two hyphen-minuses. It cannot 372 be any of the prefixes already used in earlier documents, which includes 373 the following: "bl--", "bq--", "dq--", "lq--", "mq--", "ra--", "wq--" 374 and "zq--". The ToASCII and ToUnicode operations MUST recognize the ACE 375 prefix in a case-insensitive manner. 377 The ACE prefix for IDNA is "IESG--". 379 This means that an ACE label might be "IESG--de-jg4avhby1noc0d", where 380 "de-jg4avhby1noc0d" is the part of the ACE label that is generated by 381 the encoding steps in [PUNYCODE]. 383 While all ACE labels begin with the ACE prefix, not all labels beginning 384 with the ACE prefix are necessarily ACE labels. Non-ACE labels that 385 begin with the ACE prefix will confuse users and SHOULD NOT be allowed 386 in DNS zones. 388 6. Implications for typical applications using DNS 390 In IDNA, applications perform the processing needed to input 391 internationalized domain names from users, display internationalized 392 domain names to users, and process the inputs and outputs from DNS and 393 other protocols that carry domain names. 395 The components and interfaces between them can be represented 396 pictorially as: 398 +------+ 399 | User | 400 +------+ 401 ^ 402 | Input and display: local interface methods 403 | (pen, keyboard, glowing phosphorus, ...) 404 +-------------------|-------------------------------+ 405 | v | 406 | +-----------------------------+ | 407 | | Application | | 408 | | (ToASCII and ToUnicode | | 409 | | operations may be | | 410 | | called here) | | 411 | +-----------------------------+ | 412 | ^ ^ | End system 413 | | | | 414 | Call to resolver: | | Application-specific | 415 | ACE | | protocol: | 416 | v | ACE unless the | 417 | +----------+ | protocol is updated | 418 | | Resolver | | to handle other | 419 | +----------+ | encodings | 420 | ^ | | 421 +-----------------|----------|----------------------+ 422 DNS protocol: | | 423 ACE | | 424 v v 425 +-------------+ +---------------------+ 426 | DNS servers | | Application servers | 427 +-------------+ +---------------------+ 429 The box labeled "Application" is where the application splits a host 430 name into labels, sets the appropriate flags, and performs the ToASCII 431 and ToUnicode operations. This is described in section 4. 433 6.1 Entry and display in applications 435 Applications can accept domain names using any character set or sets 436 desired by the application developer, and can display domain names in 437 any charset. That is, the IDNA protocol does not affect the interface 438 between users and applications. 440 An IDNA-aware application can accept and display internationalized 441 domain names in two formats: the internationalized character set(s) 442 supported by the application, and as an ACE label. ACE labels that are 443 displayed or input MUST always include the ACE prefix. Applications MAY 444 allow input and display of ACE labels, but are not encouraged to do so 445 except as an interface for special purposes, possibly for debugging. ACE 446 encoding is opaque and ugly, and should thus only be exposed to users 447 who absolutely need it. The optional use, especially during a transition 448 period, of ACE encodings in the user interface is described in section 449 6.4. Because name labels encoded as ACE name labels can be rendered 450 either as the encoded ASCII characters or the proper decoded characters, 451 the application MAY have an option for the user to select the preferred 452 method of display; if it does, rendering the ACE SHOULD NOT be the 453 default. 455 Domain names are often stored and transported in many places. For 456 example, they are part of documents such as mail messages and web pages. 457 They are transported in many parts of many protocols, such as both the 458 control commands and the RFC 2822 body parts of SMTP, and the headers 459 and the body content in HTTP. It is important to remember that domain 460 names appear both in domain name slots and in the content that is passed 461 over protocols. 463 In protocols and document formats that define how to handle 464 specification or negotiation of charsets, labels can be encoded in any 465 charset allowed by the protocol or document format. If a protocol or 466 document format only allows one charset, the labels MUST be given in 467 that charset. 469 In any place where a protocol or document format allows transmission of 470 the characters in internationalized labels, internationalized labels 471 SHOULD be transmitted using whatever character encoding and escape 472 mechanism that the protocol or document format uses at that place. 474 All protocols that use domain name slots already have the capacity for 475 handling domain names in the ASCII charset. Thus, ACE labels 476 (internationalized labels that have been processed with the ToASCII 477 operation) can inherently be handled by those protocols. 479 6.2 Applications and resolver libraries 481 Applications normally use functions in the operating system when they 482 resolve DNS queries. Those functions in the operating system are often 483 called "the resolver library", and the applications communicate with the 484 resolver libraries through a programming interface (API). 486 Because these resolver libraries today expect only domain names in 487 ASCII, applications MUST prepare labels that are passed to the resolver 488 library using the ToASCII operation. Labels received from the resolver 489 library contain only ASCII characters; internationalized labels that 490 cannot be represented directly in ASCII use the ACE form. ACE labels 491 always include the ACE prefix. 493 IDNA-aware applications MUST be able to work with both 494 non-internationalized labels (those that conform to [STD13] and [STD3]) 495 and internationalized labels. 497 It is expected that new versions of the resolver libraries in the future 498 will be able to accept domain names in other formats than ASCII, and 499 application developers might one day pass not only domain names in 500 Unicode, but also in local script to a new API for the resolver 501 libraries in the operating system. Thus the ToASCII and ToUnicode 502 operations might be performed inside these new versions of the resolver 503 libraries. 505 Domain names stored in zones follow the rules for "stored strings" from 506 [STRINGPREP]. DNS requests follow the rules for "queries" from 507 [STRINGPREP]. 509 6.3 DNS servers 511 An operating system might have a set of libraries for performing the 512 ToASCII operation. The input to such a library might be in one or more 513 charsets that are used in applications (UTF-8 and UTF-16 are likely 514 candidates for almost any operating system, and script-specific charsets 515 are likely for localized operating systems). 517 For internationalized labels that cannot be represented directly in 518 ASCII, DNS servers MUST use the ACE form produced by the ToASCII 519 operation. All IDNs served by DNS servers MUST contain only ASCII 520 characters. 522 If a signaling system which makes negotiation possible between old and 523 new DNS clients and servers is standardized in the future, the encoding 524 of the query in the DNS protocol itself can be changed from ACE to 525 something else, such as UTF-8. The question whether or not this should 526 be used is, however, a separate problem and is not discussed in this 527 memo. 529 6.4 Avoiding exposing users to the raw ACE encoding 531 All applications that might show the user a domain name obtained from a 532 domain name slot, such as from gethostbyaddr or part of a mail header, 533 SHOULD be updated as soon as possible in order to prevent users from 534 seeing the ACE. 536 If an application decodes an ACE name using ToUnicode but cannot show 537 all of the characters in the decoded name, such as if the name contains 538 characters that the output system cannot display, the application SHOULD 539 show the name in ACE format (which always includes the ACE prefix) 540 instead of displaying the name with the replacement character (U+FFFD). 541 This is to make it easier for the user to transfer the name correctly to 542 other programs. Programs that by default show the ACE form when they 543 cannot show all the characters in a name label SHOULD also have a 544 mechanism to show the name that is produced by the ToUnicode operation 545 with as many characters as possible and replacement characters in the 546 positions where characters cannot be displayed. 548 The ToUnicode operation does not alter labels that are not valid ACE 549 labels, even if they begin with the ACE prefix. After ToUnicode has been 550 applied, if a label still begins with the ACE prefix, then it is not a 551 valid ACE label, and is not equivalent to any of the intermediate 552 Unicode strings constructed by ToUnicode. 554 6.5 Bidirectional text in domain names 556 The display of domain names that contain bidirectional text is not 557 covered in this document. It may be covered in a future version of this 558 document, or may be covered in a different document. 560 For developers interested in displaying domain names that have 561 bidirectional text, the Unicode standard has an extensive discussion of 562 how to deal with reorder glyphs for display when dealing with 563 bidirectional text such as Arabic or Hebrew. See [UAX9] for more 564 information. In particular, all Unicode text is stored in logical order. 566 6.6 DNSSEC authentication of IDN domain names 568 DNS Security [DNSSEC] is a method for supplying cryptographic 569 verification information along with DNS messages. Public Key 570 Cryptography is used in conjunction with digital signatures to provide a 571 means for a requester of domain information to authenticate the source 572 of the data. This ensures that it can be traced back to a trusted 573 source, either directly, or via a chain of trust linking the source of 574 the information to the top of the DNS hierarchy. 576 IDNA specifies that all internationalized domain names served by DNS 577 servers that cannot be represented directly in ASCII must use the ACE 578 form produced by the ToASCII operation. This operation must be performed 579 prior to a zone being signed by the private key for that zone. Because 580 of this ordering, it is important to recognize that DNSSEC authenticates 581 the ASCII domain name, not the Unicode form or the mapping between the 582 Unicode form and the ASCII form. In other words, the output of ToASCII 583 is the canonical name. In the presence of DNSSEC, this is the name that 584 MUST be signed in the zone and MUST be validated against. 586 One consequence of this for sites deploying IDNA in the presence of 587 DNSSEC is that any special purpose proxies or forwarders used to 588 transform user input into IDNs must be earlier in the resolution flow 589 than DNSSEC authenticating nameservers for DNSSEC to work. 591 6.7 Limitations of IDNA 593 The IDNA protocol does not solve all linguistic issues with users 594 inputting names in different scripts. Many important language-based and 595 script-based mappings are not covered in IDNA and must be handled 596 outside the protocol. For example, names that are entered in a mix of 597 traditional and simplified Chinese characters will not be mapped to a 598 single canonical name. Another example is Scandinavian names that are 599 entered with U+00F6 (LATIN SMALL LETTER O WITH DIAERESIS) will not be 600 mapped to U+00F8 (LATIN SMALL LETTER O WITH STROKE). 602 7. Name Server Considerations 604 Internationalized domain name data in zone files (as specified by 605 section 5 of RFC 1035) MUST be processed with ToASCII before it is 606 entered in the zone files. 608 It is imperative that there be only one ASCII encoding for a particular 609 domain name. Thus, a primary master name server MUST NOT contain an 610 ACE-encoded label that decodes to an ASCII label. The ToASCII operation 611 assures that no such names are ever output from the operation. 613 Name servers MUST NOT serve records with domain names that contain 614 non-ASCII characters; such names MUST be converted to ACE form by the 615 ToASCII operation in order to be served. If names that are not processed 616 by ToASCII are passed to an application, it will result in unpredictable 617 behavior. Note that [STRINGPREP] describes how to handle versioning of 618 unallocated codepoints. 620 8. Root Server Considerations 622 IDNs are likely to be somewhat longer than current host names, so the 623 bandwidth needed by the root servers should go up by a small amount. 624 Also, queries and responses for IDNs will probably be somewhat longer 625 than typical queries today, so more queries and responses may be forced 626 to go to TCP instead of UDP. 628 9. References 630 9.1 Normative references 632 [PUNYCODE] Adam Costello, "Punycode: An encoding of Unicode for use with 633 IDNA", draft-ietf-idn-punycode. 635 [NAMEPREP] Paul Hoffman and Marc Blanchet, "Nameprep: A Stringprep 636 Profile for Internationalized Domain Names", draft-ietf-idn-nameprep. 638 [STD3] Bob Braden, "Requirements for Internet Hosts -- Communication 639 Layers" (RFC 1122) and "Requirements for Internet Hosts -- Application 640 and Support" (RFC 1123), STD 3, October 1989. 642 [STD13] Paul Mockapetris, "Domain names - concepts and facilities" (RFC 643 1034) and "Domain names - implementation and specification" (RFC 1035), 644 STD 13, November 1987. 646 [STRINGPREP] Paul Hoffman and Marc Blanchet, "Preparation of 647 Internationalized Strings ("stringprep")", draft-hoffman-stringprep, 648 work in progress 650 9.2 Informative references 652 [DNSSEC] Don Eastlake, "Domain Name System Security Extensions", RFC 653 2535, March 1999. 655 [RFC2119] Scott Bradner, "Key words for use in RFCs to Indicate 656 Requirement Levels", March 1997, RFC 2119. 658 [UAX9] Unicode Standard Annex #9, The Bidirectional Algorithm, 659 . 661 [UNICODE] The Unicode Standard, Version 3.1.0: The Unicode Consortium. 662 The Unicode Standard, Version 3.0. Reading, MA, Addison-Wesley 663 Developers Press, 2000. ISBN 0-201-61633-5, as amended by: Unicode 664 Standard Annex #27: Unicode 3.1, 665 . 667 [USASCII] Vint Cerf, "ASCII format for Network Interchange", October 668 1969, RFC 20. 670 10. Security Considerations 672 Security on the Internet partly relies on the DNS. Thus, any change to 673 the characteristics of the DNS can change the security of much of the 674 Internet. 676 This memo describes an algorithm which encodes characters that are not 677 valid according to STD3 and STD13 into octet values that are valid. No 678 security issues such as string length increases or new allowed values 679 are introduced by the encoding process or the use of these encoded 680 values, apart from those introduced by the ACE encoding itself. 682 Domain names are used by users to connect to Internet servers. The 683 security of the Internet would be compromised if a user entering a 684 single internationalized name could be connected to different servers 685 based on different interpretations of the internationalized domain name. 687 Because this document normatively refers to [NAMEPREP], it includes the 688 security considerations from that document as well. 690 11. Authors' Addresses 692 Patrik Faltstrom 693 Cisco Systems 694 Arstaangsvagen 31 J 695 S-117 43 Stockholm Sweden 696 paf@cisco.com 698 Paul Hoffman 699 Internet Mail Consortium and VPN Consortium 700 127 Segre Place 701 Santa Cruz, CA 95060 USA 702 phoffman@imc.org 704 Adam M. Costello 705 University of California, Berkeley 706 idna-spec.amc @ nicemice.net