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'Unicode51' -- Obsolete informational reference (is this intentional?): RFC 2673 (Obsoleted by RFC 6891) -- Obsolete informational reference (is this intentional?): RFC 3454 (Obsoleted by RFC 7564) -- Obsolete informational reference (is this intentional?): RFC 3490 (Obsoleted by RFC 5890, RFC 5891) -- Obsolete informational reference (is this intentional?): RFC 3491 (Obsoleted by RFC 5891) -- Obsolete informational reference (is this intentional?): RFC 5246 (Obsoleted by RFC 8446) Summary: 1 error (**), 0 flaws (~~), 1 warning (==), 16 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group J. Klensin 3 Internet-Draft December 11, 2008 4 Obsoletes: 3490 (if approved) 5 Intended status: Standards Track 6 Expires: June 14, 2009 8 Internationalized Domain Names for Applications (IDNA): Definitions and 9 Document Framework 10 draft-ietf-idnabis-defs-05.txt 12 Status of this Memo 14 By submitting this Internet-Draft, each author represents that any 15 applicable patent or other IPR claims of which he or she is aware 16 have been or will be disclosed, and any of which he or she becomes 17 aware will be disclosed, in accordance with Section 6 of BCP 79. 19 Internet-Drafts are working documents of the Internet Engineering 20 Task Force (IETF), its areas, and its working groups. Note that 21 other groups may also distribute working documents as Internet- 22 Drafts. 24 Internet-Drafts are draft documents valid for a maximum of six months 25 and may be updated, replaced, or obsoleted by other documents at any 26 time. It is inappropriate to use Internet-Drafts as reference 27 material or to cite them other than as "work in progress." 29 The list of current Internet-Drafts can be accessed at 30 http://www.ietf.org/ietf/1id-abstracts.txt. 32 The list of Internet-Draft Shadow Directories can be accessed at 33 http://www.ietf.org/shadow.html. 35 This Internet-Draft will expire on June 14, 2009. 37 Abstract 39 This document is one of a collection that, together, describe the 40 protocol and usage context for a revision of Internationalized Domain 41 Names for Applications (IDNA), superseding the earlier version. It 42 describes the document collection and provides definitions and other 43 material that are common to the set. 45 Table of Contents 47 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 48 1.1. IDNA2008 . . . . . . . . . . . . . . . . . . . . . . . . . 3 49 1.1.1. Audiences . . . . . . . . . . . . . . . . . . . . . . 3 50 1.1.2. Normative Language . . . . . . . . . . . . . . . . . . 3 51 1.2. Discussion Forum . . . . . . . . . . . . . . . . . . . . . 3 52 1.3. Roadmap of IDNA2008 Documents . . . . . . . . . . . . . . 4 53 2. Definitions and Terminology . . . . . . . . . . . . . . . . . 4 54 2.1. Characters and Character Sets . . . . . . . . . . . . . . 4 55 2.2. DNS-related Terminology . . . . . . . . . . . . . . . . . 5 56 2.3. Terminology Specific to IDNA . . . . . . . . . . . . . . . 6 57 2.3.1. Terms for IDN Label Codings . . . . . . . . . . . . . 6 58 2.3.1.1. IDNA-valid strings, A-label, and U-label . . . . . 6 59 2.3.1.2. LDH-label and Internationalized Label . . . . . . 8 60 2.3.1.3. Internationalized Domain Name . . . . . . . . . . 8 61 2.3.1.4. Label Equivalence . . . . . . . . . . . . . . . . 8 62 2.3.1.5. ACE Prefix . . . . . . . . . . . . . . . . . . . . 9 63 2.3.1.6. Domain Name Slot . . . . . . . . . . . . . . . . . 9 64 2.3.2. Strings Proposed to be Used or Looked Up as Labels . . 11 65 2.3.3. Order of Characters in Labels . . . . . . . . . . . . 11 66 2.3.4. Punycode is an Algorithm, not a Name or Adjective . . 11 67 3. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11 68 4. Security Considerations . . . . . . . . . . . . . . . . . . . 12 69 4.1. General Issues . . . . . . . . . . . . . . . . . . . . . . 12 70 4.2. Local Character Set Issues . . . . . . . . . . . . . . . . 12 71 4.3. Visually Similar Characters . . . . . . . . . . . . . . . 12 72 4.4. IDNA Lookup, Registration, and the Base DNS 73 Specifications . . . . . . . . . . . . . . . . . . . . . . 13 74 4.5. Security Differences from IDNA2003 . . . . . . . . . . . . 13 75 4.6. Summary . . . . . . . . . . . . . . . . . . . . . . . . . 14 76 5. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 14 77 6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 15 78 6.1. Normative References . . . . . . . . . . . . . . . . . . . 15 79 6.2. Informative References . . . . . . . . . . . . . . . . . . 15 80 Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . 17 81 A.1. Version -00 . . . . . . . . . . . . . . . . . . . . . . . 17 82 A.2. Version -01 . . . . . . . . . . . . . . . . . . . . . . . 17 83 A.3. Version -02 . . . . . . . . . . . . . . . . . . . . . . . 17 84 A.4. Version -03 . . . . . . . . . . . . . . . . . . . . . . . 18 85 A.5. Version -04 . . . . . . . . . . . . . . . . . . . . . . . 18 86 A.6. Version -05 . . . . . . . . . . . . . . . . . . . . . . . 18 87 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 18 88 Intellectual Property and Copyright Statements . . . . . . . . . . 19 90 1. Introduction 92 1.1. IDNA2008 94 This document is one of a collection that, together, describe the 95 protocol and usage context for a revision of Internationalized Domain 96 Names for Applications (IDNA) that was largely completed in 2008, 97 known within the series and elsewhere as IDNA2008. The series 98 replaces an earlier version of IDNA, described in [RFC3490] and 99 [RFC3491]. It continues to use the Punycode algorithm [RFC3492] and 100 ACE (ASCII-compatible encoding) prefix from that earlier version. 101 The document collection is described in Section 1.3. As indicated 102 there, this document provides definitions and other material that are 103 common to the set. 105 1.1.1. Audiences 107 While many IETF specifications are directed exclusively to protocol 108 implementers, the character of IDNA requires that it be understood 109 and properly used by those whose responsibilities include 111 o Making decisions about what names are permitted in DNS zone files 113 o About policies related to names and naming, and 115 o About the handling of domain name strings in files and systems, 116 even with no immediate intention of looking them up. 118 This document and those concerned with the protocol definition, rules 119 for handling strings that include characters written right-to-left, 120 and the actual list of characters and categories will be of primary 121 interest to protocol implementers. This document and the one 122 containing explanatory material will be of primary interest to 123 others, although they may have to fill in some details by reference 124 to other documents in the set. 126 1.1.2. Normative Language 128 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 129 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 130 document are to be interpreted as described in RFC 2119 [RFC2119]. 132 1.2. Discussion Forum 134 [[ RFC Editor: please remove this section. ]] 136 IDNA2008 is being discussed in the IETF "idnabis" Working Group and 137 on the mailing list idna-update@alvestrand.no 139 1.3. Roadmap of IDNA2008 Documents 141 IDNA2008 consists of the following documents: 143 o This document, containing definitions and other material that are 144 needed for understanding other documents in the set. It is 145 referred to informally in other documents in the set as "Defs" or 146 "Definitions". 148 o A document [IDNA2008-Rationale] that provides an overview of the 149 protocol and associated tables together with explanatory material 150 and some rationale for the decisions that led to IDNA2008. That 151 document also contains advice for registry operations and those 152 who use internationalized domain names. It is referred to 153 informally in other documents in the set as "Rationale". It is 154 not normative. 156 o A document [IDNA2008-Protocol] that describes the core IDNA2008 157 protocol and its operations. In combination with the "Bidi" 158 document described immediately below, it explicitly updates and 159 replaces RFC 3490. It is referred to informally in other 160 documents in the set as "Protocol". 162 o A document [IDNA2008-Bidi] that specifies special rules ("Bidi") 163 for labels that contain characters that are written from right to 164 left. 166 o A specification [IDNA2008-Tables] of the categories and rules that 167 identify the code points allowed in a label written in native 168 character form (defined more specifically as a "U-label" in 169 Section 2.3.1.1 below), based on Unicode 5.1 [Unicode51] code 170 point assignments and additional rules unique to IDNA2008. The 171 Unicode-based rules are expected to be stable across Unicode 172 updates and hence independent of Unicode versions. That 173 specifications obsoletes RFC 3941 and IDN use of the tables to 174 which it refers. It is referred to informally in other documents 175 in the set as "Tables". 177 2. Definitions and Terminology 179 2.1. Characters and Character Sets 181 A code point is an integer value in the codespace of a coded 182 character set. In Unicode, these are integers from 0 to 0x10FFFF. 184 Unicode [Unicode51] is a coded character set with about 100,000 185 characters assigned to code points as of version 5.1. A single 186 Unicode code point is denoted in these documents by "U+" followed by 187 four to six hexadecimal digits, while a range of Unicode code points 188 is denoted by two four to six digit hexadecimal numbers separated by 189 "..", with no prefixes. 191 ASCII means US-ASCII [ASCII], a coded character set containing 128 192 characters associated with code points in the range 0000..007F. 193 Unicode is a superset of ASCII and may be thought of as a 194 generalization of it; it includes all the ASCII characters and 195 associates them with equivalent code points. 197 "Letters" are, informally, generalizations from the ASCII and common- 198 sense understanding of that term, i.e., characters that are used to 199 write text that are not digits, symbols, or punctuation. Formally, 200 they are characters with a Unicode General Category value starting in 201 "L" (see Section 4.5 of [Unicode51]). 203 2.2. DNS-related Terminology 205 When discussing the DNS, this document generally assumes the 206 terminology used in the DNS specifications [RFC1034] [RFC1035]. The 207 term "lookup" is used to describe the combination of operations 208 performed by the IDNA2008 protocol and those actually performed by a 209 DNS resolver. The process of placing an entry into the DNS is 210 referred to as "registration", similar to common contemporary usage 211 in other contexts. Consequently, any DNS zone administration is 212 described as a "registry", regardless of the actual administrative 213 arrangements or level in the DNS tree. More detail about that 214 relationship is included in the "Rationale" document. 216 The term "LDH code point" is defined in this document to refer to the 217 code points associated with ASCII letters (Unicode code points 218 0041..005A and 0061..007A), digits (0030..0039), and the hyphen-minus 219 (U+002D). "LDH" is an abbreviation for "letters, digits, hyphen". 221 The base DNS specifications [RFC1034] [RFC1035] discuss "domain 222 names" and "host names", but many people use the terms 223 interchangeably, as do sections of these specifications. Lack of 224 clarity about that terminology has contributed to confusion about 225 intent in some cases. These documents generally use the term "domain 226 name". When they refer to, e.g., host name syntax restrictions, they 227 explicitly cite the relevant defining documents. The remaining 228 definitions in this subsection are essentially a review: if there is 229 any perceived difference between those definitions and the 230 definitions in the base DNS documents or those cited below, the 231 definitions in the other documents take precedence. 233 A label is an individual component of a domain name. Labels are 234 usually shown separated by dots; for example, the domain name 235 "www.example.com" is composed of three labels: "www", "example", and 236 "com". (The zero-length root label described in RFC 1123 [RFC1123], 237 which can be explicit as in "www.example.com." or implicit as in 238 "www.example.com", is not considered in this specification.) IDNA 239 extends the set of usable characters in labels that are treated as 240 text (as distinct from the binary string labels discussed in RFC 1035 241 and RFC 2181 [RFC2181] and the bitstring ones described in RFC 2673 242 [RFC2673]). For the rest of this document and in the related ones, 243 the term "label" is shorthand for "text label", and "every label" 244 means "every text label". 246 2.3. Terminology Specific to IDNA 248 This section defines some terminology to reduce dependence on terms 249 and definitions that have been problematic in the past. 251 2.3.1. Terms for IDN Label Codings 253 2.3.1.1. IDNA-valid strings, A-label, and U-label 255 To improve clarity, this subsection of the document introduces three 256 new terms. In the next subsection, it defines a historical term to 257 be slightly more precise for IDNA contexts. The relationship among 258 these terms and some others is illustrated in Figure 1. 260 o A string is "IDNA-valid" if it meets all of the requirements of 261 these specifications for an IDNA label. IDNA-valid strings may 262 appear in either of the two forms, defined immediately below, or 263 may, trivially, be ASCII strings that conform to the traditional 264 "hostname" (or "LDH") rule and that do not contain "--" as the 265 third and fourth character. These documents make specific 266 reference to the form appropriate to any context in which the 267 distinction is important. 269 o An "A-label" is the ASCII-Compatible Encoding (ACE, see 270 Section 2.3.1.5) form of an IDNA-valid string. It must be a 271 complete label: IDNA is defined for labels, not for parts of them 272 and not for complete domain names. This means, by definition, 273 that every A-label will begin with the IDNA ACE prefix, "xn--" 274 (see Section 2.3.1.5), followed by a string that is a valid output 275 of the Punycode algorithm and hence a maximum of 59 ASCII 276 characters in length. The prefix and string together must conform 277 to all requirements for a label that can be stored in the DNS 278 including conformance to the rules for the preferred form 279 described in RFC 1034, RFC 1035, and RFC 1123. A string meeting 280 that above requirements is still not an A-label unless it can be 281 decoded into a U-label. 283 o A "U-label" is an IDNA-valid string of Unicode characters, in 284 normalization form NFC and including at least one non-ASCII 285 character, expressed in a standard Unicode Encoding Form -- in an 286 Internet transmission context this will normally be UTF-8 -- and 287 subject to the constraints about permitted characters that are 288 specified in the Protocol and Tables documents as well as the 289 symmetry constraint described. Conversions between U-labels and 290 A-labels are performed according to the "Punycode" specification 291 [RFC3492], adding or removing the ACE prefix as needed. 293 To be valid, U-labels and A-labels must obey an important symmetry 294 constraint. While that constraint may be tested in any of several 295 ways, an A-label must be capable of being produced by conversion from 296 a U-label and a U-label must be capable of being produced by 297 conversion from an A-label. Among other things, this implies that 298 both U-labels and A-labels must be strings in Unicode NFC 299 [Unicode-UAX15] normalized form. These strings MUST contain only 300 characters specified elsewhere in this document series, and only in 301 the contexts indicated as appropriate. 303 Any rules or conventions that apply to DNS labels in general, such as 304 rules about lengths of strings, apply to whichever of the U-label or 305 A-label would be more restrictive. For the U-label, constraints 306 imposed by existing protocols and their presentation forms make the 307 length restriction apply to the length in octets of the UTF-8 form of 308 those labels (which will always be greater than or equal to the 309 length in code points). The exception to this, of course, is that 310 the restriction to ASCII characters does not apply to the U-label. 312 A different way to look at these terms, which may be more clear to 313 some readers, is that U-labels, A-labels, and LDH-labels (see the 314 next subsection) are disjoint categories that, together, make up the 315 forms of legitimate strings for use in domain names that describe 316 hosts. Of the three, only A-labels and LDH-labels can actually 317 appear in DNS zone files or queries; U-labels can appear, along with 318 the other two, in presentation and user interface forms and in 319 selected protocols other than those of the DNS itself. Strings that 320 do not conform to the rules for one of these three categories and, in 321 particular, strings that contain "--" in the third and fourth 322 character position but are: 324 o not A-labels or 326 o cannot be processed as U-labels or A-labels as described in these 327 specifications, 329 are invalid in IDNA-conformant applications as labels in domain names 330 that identify Internet hosts or similar resources. 332 2.3.1.2. LDH-label and Internationalized Label 334 These specifications use the term "LDH-label" strictly to refer to an 335 all-ASCII label that obeys the preferred syntax (often known as 336 "hostname" (from RFC 952 [RFC0952]) or "LDH") conventions and that is 337 not an IDN. It should be stressed that an A-label obeys the 338 "hostname" rules and is sometimes described as "LDH-conformant", or 339 in similar language, but it is not an LDH-label as that term is 340 defined in these specifications. 342 2.3.1.3. Internationalized Domain Name 344 An "internationalized domain name" (IDN) is a domain name that may 345 contain any mixture of LDH-labels, A-labels, or U-labels. This 346 implies that every conventional domain name is an IDN (which implies 347 that it is possible for a domain name to be an IDN without it 348 containing any non-ASCII characters). Just as has been the case with 349 ASCII names, some DNS zone administrators may impose restrictions, 350 beyond those imposed by DNS or IDNA, on the characters or strings 351 that may be registered as labels in their zones. Because of the 352 diversity of characters that can be used in a U-label and the 353 confusion they might cause, such restrictions are mandatory for IDN 354 registries and zones even though the particular restrictions are not 355 part of these specifications. Because these restrictions, commonly 356 known as "registry restrictions", only affect what can be registered 357 and not lookup processing, they have no effect on the syntax or 358 semantics of DNS protocol messages; a query for a name that matches 359 no records will yield the same response regardless of the reason why 360 it is not in the zone. Clients issuing queries or interpreting 361 responses cannot be assumed to have any knowledge of zone-specific 362 restrictions or conventions. See the section on registration policy 363 in [IDNA2008-Rationale] for additional discussion. 365 "Internationalized label" is used when a term is needed to refer to a 366 single label of an IDN, i.e., one that might be any of an LDH-label, 367 A-label, or U-label. There are some standardized DNS label formats, 368 such as those for service location (SRV) records [RFC2782], that do 369 not fall into any of the three categories and hence are not 370 internationalized labels. 372 2.3.1.4. Label Equivalence 374 In IDNA, equivalence of labels is defined in terms of the A-labels. 375 If the A-labels are equal in a case-independent comparison, then the 376 labels are considered equivalent, no matter how they are represented. 377 Because of the isomorphism of A-labels and U-labels in IDNA2008, it 378 is possible to compare U-labels directly; see [IDNA2008-Protocol] for 379 details. Traditional LDH labels already have a notion of 380 equivalence: within that list of characters, upper case and lower 381 case are considered equivalent. The IDNA notion of equivalence is an 382 extension of that older notion. Equivalent labels in IDNA are 383 treated as alternate forms of the same label, just as "foo" and "Foo" 384 are treated as alternate forms of the same label. 386 2.3.1.5. ACE Prefix 388 The "ACE prefix" is defined in this document to be a string of ASCII 389 characters "xn--" that appears at the beginning of every A-label. 390 "ACE" stands for "ASCII-Compatible Encoding". 392 2.3.1.6. Domain Name Slot 394 A "domain name slot" is defined in this document to be a protocol 395 element or a function argument or a return value (and so on) 396 explicitly designated for carrying a domain name. Examples of domain 397 name slots include: the QNAME field of a DNS query; the name argument 398 of the gethostbyname() or getaddrinfo() standard C library functions; 399 the part of an email address following the at-sign (@) in the 400 parameter to the SMTP MAIL or RCPT commands or the "From:" field of 401 an email message header; and the host portion of the URI in the src 402 attribute of an HTML tag. A string that has the syntax of a 403 domain name but that appears in general text is not in a domain name 404 slot. For example, a domain name appearing in the plain text body of 405 an email message is not occupying a domain name slot. 407 An "IDN-aware domain name slot" is defined for this set of documents 408 to be a domain name slot explicitly designated for carrying an 409 internationalized domain name as defined in this document. The 410 designation may be static (for example, in the specification of the 411 protocol or interface) or dynamic (for example, as a result of 412 negotiation in an interactive session). 414 An "IDN-unaware domain name slot" is defined for this set of 415 documents to be any domain name slot that is not an IDN-aware domain 416 name slot. Obviously, this includes any domain name slot whose 417 specification predates IDNA. 419 The figure on this page illustrates the relationships among some of 420 the terms defined above. The parenthesized numbers refer to the 421 notes below the figure. 423 _______________________ _______________________ 424 | ASCII Labels | | Non-ASCII | 425 | | | | 426 | ___________________| | __________________| 427 | |LDH-conforming (1)| | | U-label (2) | 428 | | | | |_________________| 429 | | ________________| | | | 430 | | | LDH-label | | | Binary Label | 431 | | |_______________| | | (including | 432 | | | A-label | | | high bit on) | 433 | | |_______________| | |_________________| 434 | | | | | | | 435 | | | Broken IDN | | | Bit String | 436 | | | e.g., xn--?,| | | Label | 437 | | | abc--def | | |_________________| 438 | | |_______________| |______________________| 439 | |__________________| 440 | ___________________| 441 | |Not-LDH-Conforming| 442 | | | 443 | | ________________| 444 | | |SRV & SRV-like | 445 | | | e.g., _tcp | 446 | | |_______________| 447 | | | Leading or | 448 | | | trailing | 449 | | | hyphens | 450 | | |_______________| 451 | | | Other non-LDH | 452 | | | ASCII chars | 453 | | | e.g., #$%&_ | 454 | | |_______________| 455 | |__________________| 456 |_____________________| 458 (1) These subtypes are indistinguishable to IDNA-unaware 459 applications. 460 (2) To IDNA-unaware applications, U-labels are 461 indistinguishable from Binary ones. 463 Figure 1: IDNA and Related DNS Terminology Space 465 2.3.2. Strings Proposed to be Used or Looked Up as Labels 467 Strings are encountered at many places in these specifications that 468 are expected to be processed as labels of particular types but that 469 are not yet fully validated to conform to the requirements for the 470 particular type of label in question. If XYZ is a type of label 471 (e.g., "A" for A-label or "U" for a U-label), then the term "putative 472 XYZ-label" is used to refer to such a string before it is fully 473 validated or tested. 475 Similarly, terms similar to "a string in the form of an XYZ-label" 476 are used to refer to a string that appears to obey the syntax for an 477 XYZ-label on superficial examination. Specifically, a string that 478 would comply with the LDH syntax except that some characters are non- 479 ASCII is considered to be in the form of a U-label and one that 480 starts in "xn--" and is otherwise all-ASCII is considered to be in 481 the form of an A-label. 483 2.3.3. Order of Characters in Labels 485 Because IDN labels may contain characters that are read, and 486 preferentially displayed, from right to left, there is a potential 487 ambiguity about which character in a label is "first". For the 488 purposes of these specifications, labels are considered, and 489 characters numbered, strictly in the order in which they appear "on 490 the wire". That order is equivalent to the leftmost character being 491 treated as first in a label that is read left-to-right and to the 492 righmost character being first in a label that is read right-to-left. 493 The "Bidi" specification contains additional discussion of the 494 conditions that influence reading order. 496 2.3.4. Punycode is an Algorithm, not a Name or Adjective 498 There has been some confusion about whether a "Punycode string" does 499 or does not include the ACE prefix and about whether it is required 500 that such strings could have been the output of the ToASCII operation 501 (see RFC 3490, Section 4 [RFC3490]). This specification discourages 502 the use of the term "Punycode" to describe anything but the encoding 503 method and algorithm of [RFC3492]. The terms defined above are 504 preferred as much more clear than the term "Punycode string". 506 3. IANA Considerations 508 Actions for IANA are specified in other documents in this series 509 [IDNA2008-Protocol] [IDNA2008-Tables]. An overview of the 510 relationships among the various IANA registries appears in 511 [IDNA2008-Rationale]. This document does not specify any actions for 512 IANA. 514 4. Security Considerations 516 4.1. General Issues 518 Security on the Internet partly relies on the DNS. Thus, any change 519 to the characteristics of the DNS can change the security of much of 520 the Internet. 522 Domain names are used by users to identify and connect to Internet 523 servers. The security of the Internet is compromised if a user 524 entering a single internationalized name is connected to different 525 servers based on different interpretations of the internationalized 526 domain name. In addition to characters that are permitted by 527 IDNA2003 and its mapping conventions, the current specification 528 changes the interpretation of a few characters that were mapped to 529 others in the earlier version; zone administrators should be aware of 530 the problems that might raise and take appropriate measures. The 531 context for this issue is discussed in more detail in 532 [IDNA2008-Rationale]). 534 In addition to the Security Considerations material that appears in 535 this document, [IDNA2008-Bidi] contains a discussion of security 536 issues specific to labels containing characters from scripts that are 537 normally written right to left. 539 4.2. Local Character Set Issues 541 When systems use local character sets other than ASCII and Unicode, 542 these specifications leave the problem of converting between the 543 local character set and Unicode up to the application or local 544 system. If different applications (or different versions of one 545 application) implement different rules for conversions among coded 546 character sets, they could interpret the same name differently and 547 contact different servers. This problem is not solved by security 548 protocols, such as Transport Layer Security (TLS) [RFC5246], that do 549 not take local character sets into account. 551 4.3. Visually Similar Characters 553 To help prevent confusion between characters that are visually 554 similar, it is suggested that implementations provide visual 555 indications where a domain name contains multiple scripts (or what 556 are considered multiple scripts in a local environment in which some 557 mixed-script use is normal). Such mechanisms can also be used to 558 show when a name contains a mixture of simplified and traditional 559 Chinese characters, or to distinguish zero and one from upper-case 560 "O" and lower-case "L". DNS zone administrators may impose 561 restrictions (subject to the limitations identified elsewhere in 562 these documents) that try to minimize characters that have similar 563 appearance or similar interpretations. It is worth noting that there 564 are no comprehensive technical solutions to the problems of 565 confusable characters. One can reduce the extent of the problems in 566 various ways, but probably never eliminate it. Some specific 567 suggestions about identification and handling of confusable 568 characters appear in a Unicode Consortium publication 569 [Unicode-UTR36]. 571 4.4. IDNA Lookup, Registration, and the Base DNS Specifications 573 The Protocol specification [IDNA2008-Protocol] describes procedures 574 for registering and looking up labels that are not compatible with 575 the preferred syntax described in the base DNS specifications (STD13 576 [RFC1034] [RFC1035] and Host Requirements [RFC1123]) because they 577 contain non-ASCII characters. These procedures depend on the use of 578 a special ASCII-compatible encoding form that contains only 579 characters permitted in host names by those earlier specifications. 580 The encoding used is Punycode [RFC3492]. No security issues such as 581 string length increases or new allowed values are introduced by the 582 encoding process or the use of these encoded values, apart from those 583 introduced by the ACE encoding itself. 585 Domain names (or portions of them) are sometimes compared against a 586 set of domains to be given special treatment if a match occurs, e.g., 587 treated as more privileged than others or blocked in some way. In 588 such situations, it is especially important that the comparisons be 589 done properly, as specified in the Requirements section of 590 [IDNA2008-Protocol]. For labels already in ASCII form (i.e., are 591 LDH-labels or A-labels), the proper comparison reduces to the same 592 case-insensitive ASCII comparison that has always been used for ASCII 593 labels. 595 The introduction of IDNA means that any existing labels that start 596 with the ACE prefix would be construed as A-labels, at least until 597 they failed one of the relevant tests, whether or not that was the 598 intent of the zone administrator or registrant. There is no evidence 599 that this has caused any practical problems since RFC 3490 was 600 adopted, but the risk still exists in principle. 602 4.5. Security Differences from IDNA2003 604 The registration and lookup models described in this set of documents 605 change the mechanisms available for lookup applications to determine 606 the validity of labels they encounter. In some respects, the ability 607 to test is strengthened. For example, putative labels that contain 608 unassigned code points will now be rejected, while IDNA2003 permitted 609 them (something that is now recognized as a considerable source of 610 risk). On the other hand, the protocol specification no longer 611 assumes that the application that looks up a name will be able to 612 determine, and apply, information about the protocol version used in 613 registration. In theory, that may increase risk since the 614 application will be able to do less pre-lookup validation. In 615 practice, the protection afforded by that test has been largely 616 illusory for reasons explained in RFC 4690 [RFC4690] and elsewhere in 617 these documents. 619 Any change to the Stringprep [RFC3454] procedure that is profiled and 620 used in IDNA2003, or, more broadly, the IETF's model of the use of 621 internationalized character strings in different protocols, creates 622 some risk of inadvertent changes to those protocols, invalidating 623 deployed applications or databases, and so on. But these 624 specifications do not change Stringprep at all; they merely bypass 625 it. Because these documents do not depend on Stringprep, the 626 question of upgrading other protocols that do have that dependency 627 can be left to experts on those protocols: the IDNA changes and 628 possible upgrades to security protocols or conventions are 629 independent issues. 631 4.6. Summary 633 No mechanism involving names or identifiers alone can protect against 634 a wide variety of security threats and attacks that are largely 635 independent of the naming or identification system. These attacks 636 include spoofed pages, DNS query trapping and diversion, and so on. 638 5. Acknowledgments 640 The initial version of this document was created largely by 641 extracting text from the "rationale" document [IDNA2008-Rationale]. 642 See the section of this name, and the one entitled "Contributors", in 643 it. 645 Specific textual suggestions after the extraction process came from 646 Vint Cerf and Bill McQuillan. 648 6. References 649 6.1. Normative References 651 [ASCII] American National Standards Institute (formerly United 652 States of America Standards Institute), "USA Code for 653 Information Interchange", ANSI X3.4-1968, 1968. 655 ANSI X3.4-1968 has been replaced by newer versions with 656 slight modifications, but the 1968 version remains 657 definitive for the Internet. 659 [RFC1034] Mockapetris, P., "Domain names - concepts and facilities", 660 STD 13, RFC 1034, November 1987. 662 [RFC1035] Mockapetris, P., "Domain names - implementation and 663 specification", STD 13, RFC 1035, November 1987. 665 [RFC1123] Braden, R., "Requirements for Internet Hosts - Application 666 and Support", STD 3, RFC 1123, October 1989. 668 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 669 Requirement Levels", BCP 14, RFC 2119, March 1997. 671 [Unicode-UAX15] 672 The Unicode Consortium, "Unicode Standard Annex #15: 673 Unicode Normalization Forms", March 2008, 674 . 676 [Unicode51] 677 The Unicode Consortium, "The Unicode Standard, Version 678 5.1.0", 2008. 680 defined by: The Unicode Standard, Version 5.0, Boston, MA, 681 Addison-Wesley, 2007, ISBN 0-321-48091-0, as amended by 682 Unicode 5.1.0 683 (http://www.unicode.org/versions/Unicode5.1.0/). 685 6.2. Informative References 687 [IDNA2008-Bidi] 688 Alvestrand, H. and C. Karp, "An updated IDNA criterion for 689 right to left scripts", July 2008, . 692 [IDNA2008-Protocol] 693 Klensin, J., "Internationalized Domain Names in 694 Applications (IDNA): Protocol", November 2008, . 697 [IDNA2008-Rationale] 698 Klensin, J., "Internationalized Domain Names for 699 Applications (IDNA): Background, Explanation, and 700 Rationale", November 2008, . 703 [IDNA2008-Tables] 704 Faltstrom, P., "The Unicode Code Points and IDNA", 705 July 2008, . 708 A version of this document is available in HTML format at 709 http://stupid.domain.name/idnabis/ 710 draft-ietf-idnabis-tables-02.html 712 [RFC0952] Harrenstien, K., Stahl, M., and E. Feinler, "DoD Internet 713 host table specification", RFC 952, October 1985. 715 [RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS 716 Specification", RFC 2181, July 1997. 718 [RFC2673] Crawford, M., "Binary Labels in the Domain Name System", 719 RFC 2673, August 1999. 721 [RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for 722 specifying the location of services (DNS SRV)", RFC 2782, 723 February 2000. 725 [RFC3454] Hoffman, P. and M. Blanchet, "Preparation of 726 Internationalized Strings ("stringprep")", RFC 3454, 727 December 2002. 729 [RFC3490] Faltstrom, P., Hoffman, P., and A. Costello, 730 "Internationalizing Domain Names in Applications (IDNA)", 731 RFC 3490, March 2003. 733 [RFC3491] Hoffman, P. and M. Blanchet, "Nameprep: A Stringprep 734 Profile for Internationalized Domain Names (IDN)", 735 RFC 3491, March 2003. 737 [RFC3492] Costello, A., "Punycode: A Bootstring encoding of Unicode 738 for Internationalized Domain Names in Applications 739 (IDNA)", RFC 3492, March 2003. 741 [RFC4690] Klensin, J., Faltstrom, P., Karp, C., and IAB, "Review and 742 Recommendations for Internationalized Domain Names 743 (IDNs)", RFC 4690, September 2006. 745 [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security 746 (TLS) Protocol Version 1.2", RFC 5246, August 2008. 748 [Unicode-UTR36] 749 The Unicode Consortium, "Unicode Technical Report #36: 750 Unicode Security Considerations", July 2008, 751 . 753 Appendix A. Change Log 755 [[RFC Editor: Please remove this appendix]] 757 A.1. Version -00 759 This document was created by pulling selected material out of 760 draft-ietf-idnabis-rationale-03 ("Rationale") after a WG consensus 761 call indicated that the rearrangement was appropriate. Mark Davis 762 made the major contribution of getting the process started by 763 identifying particular sections to be moved, even though this draft 764 does not completely reflect his list. 766 For Version -00 only, each section is identified with the associated 767 former section of Rationale-03. Those sections were edited after 768 incorporation into this document, so "Formerly" should be interpreted 769 very loosely. 771 A.2. Version -01 773 o Typographical errors corrected and some sections slightly renamed 774 for clarity. 776 o Other adjustments made to synchronize with current versions of 777 "Rationale" and "Protocol". 779 A.3. Version -02 781 o All back pointers to section numbers in Rationale have been 782 removed. 784 o Some definitions clarified. Added one about string order. 786 o Usual small editorial tuning. 788 A.4. Version -03 790 o Additional fine tuning based on discussions during and immediately 791 before IETF 72. 793 A.5. Version -04 795 o Corrections of text and improvement of definitions based on 796 discussions after -03 was released. 798 o Discussion of label comparisons tightened and made more consistent 799 with Protocol. 801 o Definitions of categories of labels supplemented with a picture. 803 o Explicit text added (Section 2.3.2) to define strings that look 804 like A-labels or U-labels but are not. 806 A.6. Version -05 808 o Consolidated Security Considerations sections, moving material 809 from Protocol and Rationale here. 811 Author's Address 813 John C Klensin 814 1770 Massachusetts Ave, Ste 322 815 Cambridge, MA 02140 816 USA 818 Phone: +1 617 245 1457 819 Email: john+ietf@jck.com 821 Full Copyright Statement 823 Copyright (C) The IETF Trust (2008). 825 This document is subject to the rights, licenses and restrictions 826 contained in BCP 78, and except as set forth therein, the authors 827 retain all their rights. 829 This document and the information contained herein are provided on an 830 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS 831 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND 832 THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS 833 OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF 834 THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED 835 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. 837 Intellectual Property 839 The IETF takes no position regarding the validity or scope of any 840 Intellectual Property Rights or other rights that might be claimed to 841 pertain to the implementation or use of the technology described in 842 this document or the extent to which any license under such rights 843 might or might not be available; nor does it represent that it has 844 made any independent effort to identify any such rights. 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