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Internet Engineering Task Force (IETF)                        J. KlensinRequest for Comments: 5894                                   August 2010Category: InformationalISSN: 2070-1721Internationalized Domain Names for Applications (IDNA):Background, Explanation, and RationaleAbstract   Several years have passed since the original protocol for   Internationalized Domain Names (IDNs) was completed and deployed.   During that time, a number of issues have arisen, including the need   to update the system to deal with newer versions of Unicode.  Some of   these issues require tuning of the existing protocols and the tables   on which they depend.  This document provides an overview of a   revised system and provides explanatory material for its components.Status of This Memo   This document is not an Internet Standards Track specification; it is   published for informational purposes.   This document is a product of the Internet Engineering Task Force   (IETF).  It represents the consensus of the IETF community.  It has   received public review and has been approved for publication by the   Internet Engineering Steering Group (IESG).  Not all documents   approved by the IESG are a candidate for any level of Internet   Standard; seeSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://www.rfc-editor.org/info/rfc5894.Klensin                       Informational                     [Page 1]

RFC 5894                     IDNA Rationale                  August 2010Copyright Notice   Copyright (c) 2010 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 78 and the IETF Trust's Legal   Provisions Relating to IETF Documents   (http://trustee.ietf.org/license-info) in effect on the date of   publication of this document.  Please review these documents   carefully, as they describe your rights and restrictions with respect   to this document.  Code Components extracted from this document must   include Simplified BSD License text as described in Section 4.e of   the Trust Legal Provisions and are provided without warranty as   described in the Simplified BSD License.   This document may contain material from IETF Documents or IETF   Contributions published or made publicly available before November   10, 2008.  The person(s) controlling the copyright in some of this   material may not have granted the IETF Trust the right to allow   modifications of such material outside the IETF Standards Process.   Without obtaining an adequate license from the person(s) controlling   the copyright in such materials, this document may not be modified   outside the IETF Standards Process, and derivative works of it may   not be created outside the IETF Standards Process, except to format   it for publication as an RFC or to translate it into languages other   than English.Table of Contents1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .41.1.  Context and Overview . . . . . . . . . . . . . . . . . . .41.2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .51.2.1.  DNS "Name" Terminology . . . . . . . . . . . . . . . .51.2.2.  New Terminology and Restrictions . . . . . . . . . . .61.3.  Objectives . . . . . . . . . . . . . . . . . . . . . . . .61.4.  Applicability and Function of IDNA . . . . . . . . . . . .71.5.  Comprehensibility of IDNA Mechanisms and Processing  . . .82.  Processing in IDNA2008 . . . . . . . . . . . . . . . . . . . .93.  Permitted Characters: An Inclusion List  . . . . . . . . . . .93.1.  A Tiered Model of Permitted Characters and Labels  . . . .103.1.1.  PROTOCOL-VALID . . . . . . . . . . . . . . . . . . . .103.1.2.  CONTEXTUAL RULE REQUIRED . . . . . . . . . . . . . . .113.1.2.1.  Contextual Restrictions  . . . . . . . . . . . . .113.1.2.2.  Rules and Their Application  . . . . . . . . . . .123.1.3.  DISALLOWED . . . . . . . . . . . . . . . . . . . . . .123.1.4.  UNASSIGNED . . . . . . . . . . . . . . . . . . . . . .133.2.  Registration Policy  . . . . . . . . . . . . . . . . . . .14Klensin                       Informational                     [Page 2]

RFC 5894                     IDNA Rationale                  August 2010     3.3.  Layered Restrictions: Tables, Context, Registration, and           Applications . . . . . . . . . . . . . . . . . . . . . . .154.  Application-Related Issues . . . . . . . . . . . . . . . . . .154.1.  Display and Network Order  . . . . . . . . . . . . . . . .154.2.  Entry and Display in Applications  . . . . . . . . . . . .16     4.3.  Linguistic Expectations: Ligatures, Digraphs, and           Alternate Character Forms  . . . . . . . . . . . . . . . .194.4.  Case Mapping and Related Issues  . . . . . . . . . . . . .204.5.  Right-to-Left Text . . . . . . . . . . . . . . . . . . . .215.  IDNs and the Robustness Principle  . . . . . . . . . . . . . .226.  Front-end and User Interface Processing for Lookup . . . . . .22   7.  Migration from IDNA2003 and Unicode Version Synchronization  . 257.1.  Design Criteria  . . . . . . . . . . . . . . . . . . . . .257.1.1.  Summary and Discussion of IDNA Validity Criteria . . .257.1.2.  Labels in Registration . . . . . . . . . . . . . . . .267.1.3.  Labels in Lookup . . . . . . . . . . . . . . . . . . .277.2.  Changes in Character Interpretations . . . . . . . . . . .287.2.1.  Character Changes: Eszett and Final Sigma  . . . . . .28       7.2.2.  Character Changes: Zero Width Joiner and Zero               Width Non-Joiner . . . . . . . . . . . . . . . . . . .297.2.3.  Character Changes and the Need for Transition  . . . .297.2.4.  Transition Strategies  . . . . . . . . . . . . . . . .307.3.  Elimination of Character Mapping . . . . . . . . . . . . .317.4.  The Question of Prefix Changes . . . . . . . . . . . . . .317.4.1.  Conditions Requiring a Prefix Change . . . . . . . . .317.4.2.  Conditions Not Requiring a Prefix Change . . . . . . .327.4.3.  Implications of Prefix Changes . . . . . . . . . . . .327.5.  Stringprep Changes and Compatibility . . . . . . . . . . .337.6.  The Symbol Question  . . . . . . . . . . . . . . . . . . .33     7.7.  Migration between Unicode Versions: Unassigned Code           Points . . . . . . . . . . . . . . . . . . . . . . . . . .357.8.  Other Compatibility Issues . . . . . . . . . . . . . . . .368.  Name Server Considerations . . . . . . . . . . . . . . . . . .378.1.  Processing Non-ASCII Strings . . . . . . . . . . . . . . .378.2.  Root and Other DNS Server Considerations . . . . . . . . .379.  Internationalization Considerations  . . . . . . . . . . . . .3810. IANA Considerations  . . . . . . . . . . . . . . . . . . . . .3810.1. IDNA Character Registry  . . . . . . . . . . . . . . . . .3810.2. IDNA Context Registry  . . . . . . . . . . . . . . . . . .3910.3. IANA Repository of IDN Practices of TLDs . . . . . . . . .3911. Security Considerations  . . . . . . . . . . . . . . . . . . .3911.1. General Security Issues with IDNA  . . . . . . . . . . . .3912. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . .3913. Contributors . . . . . . . . . . . . . . . . . . . . . . . . .4014. References . . . . . . . . . . . . . . . . . . . . . . . . . .4014.1. Normative References . . . . . . . . . . . . . . . . . . .4014.2. Informative References . . . . . . . . . . . . . . . . . .41Klensin                       Informational                     [Page 3]

RFC 5894                     IDNA Rationale                  August 20101.  Introduction1.1.  Context and Overview   Internationalized Domain Names in Applications (IDNA) is a collection   of standards that allow client applications to convert some mnemonic   strings expressed in Unicode to an ASCII-compatible encoding form   ("ACE") that is a valid DNS label containing only LDH syntax (see the   Definitions document [RFC5890]).  The specific form of ACE label used   by IDNA is called an "A-label".  A client can look up an exact   A-label in the existing DNS, so A-labels do not require any   extensions to DNS, upgrades of DNS servers, or updates to low-level   client libraries.  An A-label is recognizable from the prefix "xn--"   before the characters produced by the Punycode algorithm [RFC3492];   thus, a user application can identify an A-label and convert it into   Unicode (or some local coded character set) for display.   On the registry side, IDNA allows a registry to offer   Internationalized Domain Names (IDNs) for registration as A-labels.   A registry may offer any subset of valid IDNs, and may apply any   restrictions or bundling (grouping of similar labels together in one   registration) appropriate for the context of that registry.   Registration of labels is sometimes discussed separately from lookup,   and it is subject to a few specific requirements that do not apply to   lookup.   DNS clients and registries are subject to some differences in   requirements for handling IDNs.  In particular, registries are urged   to register only exact, valid A-labels, while clients might do some   mapping to get from otherwise-invalid user input to a valid A-label.   The first version of IDNA was published in 2003 and is referred to   here as IDNA2003 to contrast it with the current version, which is   known as IDNA2008 (after the year in which IETF work started on it).   IDNA2003 consists of four documents: the IDNA base specification   [RFC3490], Nameprep [RFC3491], Punycode [RFC3492], and Stringprep   [RFC3454].  The current set of documents, IDNA2008, is not dependent   on any of the IDNA2003 specifications other than the one for Punycode   encoding.  References to "IDNA2008", "these specifications", or   "these documents" are to the entire IDNA2008 set listed in a separate   Definitions document [RFC5890].  The characters that are valid in   A-labels are identified from rules listed in the Tables document   [RFC5892], but validity can be derived from the Unicode properties of   those characters with a very few exceptions.   Traditionally, DNS labels are matched case-insensitively (as   described in the DNS specifications [RFC1034][RFC1035]).  That   convention was preserved in IDNA2003 by a case-folding operation thatKlensin                       Informational                     [Page 4]

RFC 5894                     IDNA Rationale                  August 2010   generally maps capital letters into lowercase ones.  However, if case   rules are enforced from one language, another language sometimes   loses the ability to treat two characters separately.  Case-   insensitivity is treated slightly differently in IDNA2008.   IDNA2003 used Unicode version 3.2 only.  In order to keep up with new   characters added in new versions of Unicode, IDNA2008 decouples its   rules from any particular version of Unicode.  Instead, the   attributes of new characters in Unicode, supplemented by a small   number of exception cases, determine how and whether the characters   can be used in IDNA labels.   This document provides informational context for IDNA2008, including   terminology, background, and policy discussions.  It contains no   normative material; specifications for conformance to the IDNA2008   protocols appears entirely in the other documents in the series.1.2.  Terminology   Terminology for IDNA2008 appears in the Definitions document   [RFC5890].  That document also contains a road map to the IDNA2008   document collection.  No attempt should be made to understand this   document without the definitions and concepts that appear there.1.2.1.  DNS "Name" Terminology   In the context of IDNs, the DNS term "name" has introduced some   confusion as people speak of DNS labels in terms of the words or   phrases of various natural languages.  Historically, many of the   "names" in the DNS have been mnemonics to identify some particular   concept, object, or organization.  They are typically rooted in some   language because most people think in language-based ways.  But,   because they are mnemonics, they need not obey the orthographic   conventions of any language: it is not a requirement that it be   possible for them to be "words".   This distinction is important because the reasonable goal of an IDN   effort is not to be able to write the great Klingon (or language of   one's choice) novel in DNS labels but to be able to form a usefully   broad range of mnemonics in ways that are as natural as possible in a   very broad range of scripts.Klensin                       Informational                     [Page 5]

RFC 5894                     IDNA Rationale                  August 20101.2.2.  New Terminology and Restrictions   IDNA2008 introduces new terminology.  Precise definitions are   provided in the Definitions document for the terms U-label, A-Label,   LDH label (to which all valid pre-IDNA hostnames conformed), Reserved   LDH label (R-LDH label), XN-label, Fake A-label, and Non-Reserved LDH   label (NR-LDH label).   In addition, the term "putative label" has been adopted to refer to a   label that may appear to meet certain definitional constraints but   has not yet been sufficiently tested for validity.   These definitions are also illustrated in Figure 1 of the Definitions   document.  R-LDH labels contain "--" in the third and fourth   character positions from the beginning of the label.  In IDNA-aware   applications, only a subset of these reserved labels is permitted to   be used, namely the A-label subset.  A-labels are a subset of the   R-LDH labels that begin with the case-insensitive string "xn--".   Labels that bear this prefix but that are not otherwise valid fall   into the "Fake A-label" category.  The Non-Reserved labels (NR-LDH   labels) are implicitly valid since they do not bear any resemblance   to the labels specified by IDNA.   The creation of the Reserved-LDH category is required for three   reasons:   o  to prevent confusion with pre-IDNA coding forms;   o  to permit future extensions that would require changing the      prefix, no matter how unlikely those might be (seeSection 7.4);      and   o  to reduce the opportunities for attacks via the Punycode encoding      algorithm itself.   As with other documents in the IDNA2008 set, this document uses the   term "registry" to describe any zone in the DNS.  That term, and the   terms "zone" or "zone administration", are interchangeable.1.3.  Objectives   These are the main objectives in revising IDNA.   o  Use a more recent version of Unicode and allow IDNA to be      independent of Unicode versions, so that IDNA2008 need not be      updated for implementations to adopt code points from new Unicode      versions.Klensin                       Informational                     [Page 6]

RFC 5894                     IDNA Rationale                  August 2010   o  Fix a very small number of code point categorizations that have      turned out to cause problems in the communities that use those      code points.   o  Reduce the dependency on mapping, in favor of valid A-labels.      This will result in pre-mapped forms that are not valid IDNA      labels appearing less often in various contexts.   o  Fix some details in the bidirectional code point handling      algorithms.1.4.  Applicability and Function of IDNA   The IDNA specification solves the problem of extending the repertoire   of characters that can be used in domain names to include a large   subset of the Unicode repertoire.   IDNA does not extend DNS.  Instead, the applications (and, by   implication, the users) continue to see an exact-match lookup   service.  Either there is a single name that matches exactly (subject   to the base DNS requirement of case-insensitive ASCII matching) or   there is no match.  This model has served the existing applications   well, but it requires, with or without internationalized domain   names, that users know the exact spelling of the domain names that   are to be typed into applications such as web browsers and mail user   agents.  The introduction of the larger repertoire of characters   potentially makes the set of misspellings larger, especially given   that in some cases the same appearance, for example on a business   card, might visually match several Unicode code points or several   sequences of code points.   The IDNA standard does not require any applications to conform to it,   nor does it retroactively change those applications.  An application   can elect to use IDNA in order to support IDNs while maintaining   interoperability with existing infrastructure.  For applications that   want to use non-ASCII characters in public DNS domain names, IDNA is   the only option that is defined at the time this specification is   published.  Adding IDNA support to an existing application entails   changes to the application only, and leaves room for flexibility in   front-end processing and more specifically in the user interface (seeSection 6).   A great deal of the discussion of IDN solutions has focused on   transition issues and how IDNs will work in a world where not all of   the components have been updated.  Proposals that were not chosen by   the original IDN Working Group would have depended on updating user   applications, DNS resolvers, and DNS servers in order for a user to   apply an internationalized domain name in any form or codingKlensin                       Informational                     [Page 7]

RFC 5894                     IDNA Rationale                  August 2010   acceptable under that method.  While processing must be performed   prior to or after access to the DNS, IDNA requires no changes to the   DNS protocol, any DNS servers, or the resolvers on users' computers.   IDNA allows the graceful introduction of IDNs not only by avoiding   upgrades to existing infrastructure (such as DNS servers and mail   transport agents), but also by allowing some limited use of IDNs in   applications by using the ASCII-encoded representation of the labels   containing non-ASCII characters.  While such names are user-   unfriendly to read and type, and hence not optimal for user input,   they can be used as a last resort to allow rudimentary IDN usage.   For example, they might be the best choice for display if it were   known that relevant fonts were not available on the user's computer.   In order to allow user-friendly input and output of the IDNs and   acceptance of some characters as equivalent to those to be processed   according to the protocol, the applications need to be modified to   conform to this specification.   This version of IDNA uses the Unicode character repertoire for   continuity with the original version of IDNA.1.5.  Comprehensibility of IDNA Mechanisms and Processing   One goal of IDNA2008, which is aided by the main goal of reducing the   dependency on mapping, is to improve the general understanding of how   IDNA works and what characters are permitted and what happens to   them.  Comprehensibility and predictability to users and registrants   are important design goals for this effort.  End-user applications   have an important role to play in increasing this comprehensibility.   Any system that tries to handle international characters encounters   some common problems.  For example, a User Interface (UI) cannot   display a character if no font containing that character is   available.  In some cases, internationalization enables effective   localization while maintaining some global uniformity but losing some   universality.   It is difficult to even make suggestions as to how end-user   applications should cope when characters and fonts are not available.   Because display functions are rarely controlled by the types of   applications that would call upon IDNA, such suggestions will rarely   be very effective.   Conversion between local character sets and normalized Unicode, if   needed, is part of this set of user interface issues.  Those   conversions introduce complexity in a system that does not use   Unicode as its primary (or only) internal character coding system.   If a label is converted to a local character set that does not haveKlensin                       Informational                     [Page 8]

RFC 5894                     IDNA Rationale                  August 2010   all the needed characters, or that uses different character-coding   principles, the user interface program may have to add special logic   to avoid or reduce loss of information.   The major difficulty may lie in accurately identifying the incoming   character set and applying the correct conversion routine.  Even more   difficult, the local character coding system could be based on   conceptually different assumptions than those used by Unicode (e.g.,   choice of font encodings used for publications in some Indic   scripts).  Those differences may not easily yield unambiguous   conversions or interpretations even if each coding system is   internally consistent and adequate to represent the local language   and script.   IDNA2008 shifts responsibility for character mapping and other   adjustments from the protocol (where it was located in IDNA2003) to   pre-processing before invoking IDNA itself.  The intent is that this   change will lead to greater usage of fully-valid A-Labels or U-labels   in display, transit, and storage, which should aid comprehensibility   and predictability.  A careful look at pre-processing raises issues   about what that pre-processing should do and at what point   pre-processing becomes harmful; how universally consistent   pre-processing algorithms can be; and how to be compatible with   labels prepared in an IDNA2003 context.  Those issues are discussed   inSection 6 and in the Mapping document [IDNA2008-Mapping].2.  Processing in IDNA2008   IDNA2008 separates Domain Name Registration and Lookup in the   protocol specification (RFC 5891, Sections4 and5 [RFC5891]).   Although most steps in the two processes are similar, the separation   reflects current practice in which per-registry (DNS zone)   restrictions and special processing are applied at registration time   but not during lookup.  Another significant benefit is that   separation facilitates incremental addition of permitted character   groups to avoid freezing on one particular version of Unicode.   The actual registration and lookup protocols for IDNA2008 are   specified in the Protocol document.3.  Permitted Characters: An Inclusion List   IDNA2008 adopts the inclusion model.  A code point is assumed to be   invalid for IDN use unless it is included as part of a Unicode   property-based rule or, in rare cases, included individually by an   exception.  When an implementation moves to a new version of Unicode,   the rules may indicate new valid code points.Klensin                       Informational                     [Page 9]

RFC 5894                     IDNA Rationale                  August 2010   This section provides an overview of the model used to establish the   algorithm and character lists of the Tables document [RFC5892] and   describes the names and applicability of the categories used there.   Note that the inclusion of a character in the PROTOCOL-VALID category   group (Section 3.1.1) does not imply that it can be used   indiscriminately; some characters are associated with contextual   rules that must be applied as well.   The information given in this section is provided to make the rules,   tables, and protocol easier to understand.  The normative generating   rules that correspond to this informal discussion appear in the   Tables document, and the rules that actually determine what labels   can be registered or looked up are in the Protocol document.3.1.  A Tiered Model of Permitted Characters and Labels   Moving to an inclusion model involves a new specification for the   list of characters that are permitted in IDNs.  In IDNA2003,   character validity is independent of context and fixed forever (or   until the standard is replaced).  However, globally context-   independent rules have proved to be impractical because some   characters, especially those that are called "Join_Controls" in   Unicode, are needed to make reasonable use of some scripts but have   no visible effect in others.  IDNA2003 prohibited those types of   characters entirely by discarding them.  We now have a consensus that   under some conditions, these "joiner" characters are legitimately   needed to allow useful mnemonics for some languages and scripts.  In   general, context-dependent rules help deal with characters (generally   characters that would otherwise be prohibited entirely) that are used   differently or perceived differently across different scripts, and   allow the standard to be applied more appropriately in cases where a   string is not universally handled the same way.   IDNA2008 divides all possible Unicode code points into four   categories: PROTOCOL-VALID, CONTEXTUAL RULE REQUIRED, DISALLOWED, and   UNASSIGNED.3.1.1.  PROTOCOL-VALID   Characters identified as PROTOCOL-VALID (often abbreviated PVALID)   are permitted in IDNs.  Their use may be restricted by rules about   the context in which they appear or by other rules that apply to the   entire label in which they are to be embedded.  For example, any   label that contains a character in this category that has a   "right-to-left" property must be used in context with the Bidi rules   [RFC5893].  The term PROTOCOL-VALID is used to stress the fact that   the presence of a character in this category does not imply that a   given registry need accept registrations containing any of theKlensin                       Informational                    [Page 10]

RFC 5894                     IDNA Rationale                  August 2010   characters in the category.  Registries are still expected to apply   judgment about labels they will accept and to maintain rules   consistent with those judgments (see the Protocol document [RFC5891]   andSection 3.3).   Characters that are placed in the PROTOCOL-VALID category are   expected to never be removed from it or reclassified.  While   theoretically characters could be removed from Unicode, such removal   would be inconsistent with the Unicode stability principles (see   UTR 39: Unicode Security Mechanisms [Unicode52],Appendix F) and   hence should never occur.3.1.2.  CONTEXTUAL RULE REQUIRED   Some characters may be unsuitable for general use in IDNs but   necessary for the plausible support of some scripts.  The two most   commonly cited examples are the ZERO WIDTH JOINER and ZERO WIDTH   NON-JOINER characters (ZWJ, U+200D and ZWNJ, U+200C), but other   characters may require special treatment because they would otherwise   be DISALLOWED (typically because Unicode considers them punctuation   or special symbols) but need to be permitted in limited contexts.   Other characters are given this special treatment because they pose   exceptional danger of being used to produce misleading labels or to   cause unacceptable ambiguity in label matching and interpretation.3.1.2.1.  Contextual Restrictions   Characters with contextual restrictions are identified as CONTEXTUAL   RULE REQUIRED and are associated with a rule.  The rule defines   whether the character is valid in a particular string, and also   whether the rule itself is to be applied on lookup as well as   registration.   A distinction is made between characters that indicate or prohibit   joining and ones similar to them (known as CONTEXT-JOINER or   CONTEXTJ) and other characters requiring contextual treatment   (CONTEXT-OTHER or CONTEXTO).  Only the former require full testing at   lookup time.   It is important to note that these contextual rules cannot prevent   all uses of the relevant characters that might be confusing or   problematic.  What they are expected to do is to confine   applicability of the characters to scripts (and narrower contexts)   where zone administrators are knowledgeable enough about the use of   those characters to be prepared to deal with them appropriately.Klensin                       Informational                    [Page 11]

RFC 5894                     IDNA Rationale                  August 2010   For example, a registry dealing with an Indic script that requires   ZWJ and/or ZWNJ as part of the writing system is expected to   understand where the characters have visible effect and where they do   not and to make registration rules accordingly.  By contrast, a   registry dealing primarily with Latin or Cyrillic script might not be   actively aware that the characters exist, much less about the   consequences of embedding them in labels drawn from those scripts and   therefore should avoid accepting registrations containing those   characters, at least in labels using characters from the Latin or   Cyrillic scripts.3.1.2.2.  Rules and Their Application   Rules have descriptions such as "Must follow a character from Script   XYZ", "Must occur only if the entire label is in Script ABC", or   "Must occur only if the previous and subsequent characters have the   DFG property".  The actual rules may be DEFINED or NULL.  If present,   they may have values of "True" (character may be used in any position   in any label), "False" (character may not be used in any label), or   may be a set of procedural rules that specify the context in which   the character is permitted.   Because it is easier to identify these characters than to know that   they are actually needed in IDNs or how to establish exactly the   right rules for each one, a rule may have a null value in a given   version of the tables.  Characters associated with null rules are not   permitted to appear in putative labels for either registration or   lookup.  Of course, a later version of the tables might contain a   non-null rule.   The actual rules and their descriptions are in Sections2 and3 of   the Tables document [RFC5892].  That document also specifies the   creation of a registry for future rules.3.1.3.  DISALLOWED   Some characters are inappropriate for use in IDNs and are thus   excluded for both registration and lookup (i.e., IDNA-conforming   applications performing name lookup should verify that these   characters are absent; if they are present, the label strings should   be rejected rather than converted to A-labels and looked up.  Some of   these characters are problematic for use in IDNs (such as the   FRACTION SLASH character, U+2044), while some of them (such as the   various HEART symbols, e.g., U+2665, U+2661, and U+2765, seeSection 7.6) simply fall outside the conventions for typical   identifiers (basically letters and numbers).Klensin                       Informational                    [Page 12]

RFC 5894                     IDNA Rationale                  August 2010   Of course, this category would include code points that had been   removed entirely from Unicode should such removals ever occur.   Characters that are placed in the DISALLOWED category are expected to   never be removed from it or reclassified.  If a character is   classified as DISALLOWED in error and the error is sufficiently   problematic, the only recourse would be either to introduce a new   code point into Unicode and classify it as PROTOCOL-VALID or for the   IETF to accept the considerable costs of an incompatible change and   replace the relevant RFC with one containing appropriate exceptions.   There is provision for exception cases but, in general, characters   are placed into DISALLOWED if they fall into one or more of the   following groups:   o  The character is a compatibility equivalent for another character.      In slightly more precise Unicode terms, application of      Normalization Form KC (NFKC) to the character yields some other      character.   o  The character is an uppercase form or some other form that is      mapped to another character by Unicode case folding.   o  The character is a symbol or punctuation form or, more generally,      something that is not a letter, digit, or a mark that is used to      form a letter or digit.3.1.4.  UNASSIGNED   For convenience in processing and table-building, code points that do   not have assigned values in a given version of Unicode are treated as   belonging to a special UNASSIGNED category.  Such code points are   prohibited in labels to be registered or looked up.  The category   differs from DISALLOWED in that code points are moved out of it by   the simple expedient of being assigned in a later version of Unicode   (at which point, they are classified into one of the other categories   as appropriate).   The rationale for restricting the processing of UNASSIGNED characters   is simply that the properties of such code points cannot be   completely known until actual characters are assigned to them.  For   example, assume that an UNASSIGNED code point were included in a   label to be looked up.  Assume that the code point was later assigned   to a character that required some set of contextual rules.  With that   combination, un-updated instances of IDNA-aware software might permit   lookup of labels containing the previously unassigned characters   while updated versions of the software might restrict use of the sameKlensin                       Informational                    [Page 13]

RFC 5894                     IDNA Rationale                  August 2010   label in lookup, depending on the contextual rules.  It should be   clear that under no circumstance should an UNASSIGNED character be   permitted in a label to be registered as part of a domain name.3.2.  Registration Policy   While these recommendations cannot and should not define registry   policies, registries should develop and apply additional restrictions   as needed to reduce confusion and other problems.  For example, it is   generally believed that labels containing characters from more than   one script are a bad practice although there may be some important   exceptions to that principle.  Some registries may choose to restrict   registrations to characters drawn from a very small number of   scripts.  For many scripts, the use of variant techniques such as   those as described in the JET specification for the CJK script   [RFC3743] and its generalization [RFC4290], and illustrated for   Chinese by the tables provided by the Chinese Domain Name Consortium   [RFC4713] may be helpful in reducing problems that might be perceived   by users.   In general, users will benefit if registries only permit characters   from scripts that are well-understood by the registry or its   advisers.  If a registry decides to reduce opportunities for   confusion by constructing policies that disallow characters used in   historic writing systems or characters whose use is restricted to   specialized, highly technical contexts, some relevant information may   be found inSection 2.4 (Specific Character Adjustments) of Unicode   Identifier and Pattern Syntax [Unicode-UAX31], especially Table 4   (Candidate Characters for Exclusion from Identifiers), andSection3.1 (General Security Profile for Identifiers) in Unicode Security   Mechanisms [Unicode-UTS39].   The requirement (inSection 4.1 of the Protocol document [RFC5891])   that registration procedures use only U-labels and/or A-labels is   intended to ensure that registrants are fully aware of exactly what   is being registered as well as encouraging use of those canonical   forms.  That provision should not be interpreted as requiring that   registrants need to provide characters in a particular code sequence.   Registrant input conventions and management are part of registrant-   registrar interactions and relationships between registries and   registrars and are outside the scope of these standards.   It is worth stressing that these principles of policy development and   application apply at all levels of the DNS, not only, e.g., top level   domain (TLD) or second level domain (SLD) registrations.  Even a   trivial, "anything is permitted that is valid under the protocol"   policy is helpful in that it helps users and application developers   know what to expect.Klensin                       Informational                    [Page 14]

RFC 5894                     IDNA Rationale                  August 20103.3.  Layered Restrictions: Tables, Context, Registration, and      Applications   The character rules in IDNA2008 are based on the realization that   there is no single magic bullet for any of the security,   confusability, or other issues associated with IDNs.  Instead, the   specifications define a variety of approaches.  The character tables   are the first mechanism, protocol rules about how those characters   are applied or restricted in context are the second, and those two in   combination constitute the limits of what can be done in the   protocol.  As discussed in the previous section (Section 3.2),   registries are expected to restrict what they permit to be   registered, devising and using rules that are designed to optimize   the balance between confusion and risk on the one hand and maximum   expressiveness in mnemonics on the other.   In addition, there is an important role for user interface programs   in warning against label forms that appear problematic given their   knowledge of local contexts and conventions.  Of course, no approach   based on naming or identifiers alone can protect against all threats.4.  Application-Related Issues4.1.  Display and Network Order   Domain names are always transmitted in network order (the order in   which the code points are sent in protocols), but they may have a   different display order (the order in which the code points are   displayed on a screen or paper).  When a domain name contains   characters that are normally written right to left, display order may   be affected although network order is not.  It gets even more   complicated if left-to-right and right-to-left labels are adjacent to   each other within a domain name.  The decision about the display   order is ultimately under the control of user agents -- including Web   browsers, mail clients, hosted Web applications and many more --   which may be highly localized.  Should a domain name abc.def, in   which both labels are represented in scripts that are written right   to left, be displayed as fed.cba or cba.fed?  Applications that are   in deployment today are already diverse, and one can find examples of   either choice.   The picture changes once again when an IDN appears in an   Internationalized Resource Identifier (IRI) [RFC3987].  An IRI or   internationalized email address contains elements other than the   domain name.  For example, IRIs contain protocol identifiers and   field delimiter syntax such as "http://" or "mailto:" while email   addresses contain the "@" to separate local parts from domain names.Klensin                       Informational                    [Page 15]

RFC 5894                     IDNA Rationale                  August 2010   An IRI in network order begins with "http://" followed by domain   labels in network order, thus "http://abc.def".   User interface programs are not required to display and allow input   of IRIs directly but often do so.  Implementers have to choose   whether the overall direction of these strings will always be left to   right (or right to left) for an IRI or email address.  The natural   order for a user typing a domain name on a right-to-left system is   fed.cba.  Should the right-to-left (RTL) user interface reverse the   entire domain name each time a domain name is typed?  Does this   change if the user types "http://" right before typing a domain name,   thus implying that the user is beginning at the beginning of the   network-order IRI?  Experience in the 1980s and 1990s with mixing   systems in which domain name labels were read in network order (left   to right) and those in which those labels were read right to left   would predict a great deal of confusion.   If each implementation of each application makes its own decisions on   these issues, users will develop heuristics that will sometimes fail   when switching applications.  However, while some display order   conventions, voluntarily adopted, would be desirable to reduce   confusion, such suggestions are beyond the scope of these   specifications.4.2.  Entry and Display in Applications   Applications can accept and display domain names using any character   set or character coding system.  The IDNA protocol does not   necessarily affect the interface between users and applications.  An   IDNA-aware application can accept and display internationalized   domain names in two formats: as the internationalized character   set(s) supported by the application (i.e., an appropriate local   representation of a U-label) and as an A-label.  Applications may   allow the display of A-labels, but are encouraged not to do so except   as an interface for special purposes, possibly for debugging, or to   cope with display limitations.  In general, they should allow, but   not encourage, user input of A-labels.  A-labels are opaque and ugly,   and malicious variations on them are not easily detected by users.   Where possible, they should thus only be exposed when they are   absolutely needed.  Because IDN labels can be rendered either as   A-labels or U-labels, the application may reasonably have an option   for the user to select the preferred method of display.  Rendering   the U-label should normally be the default.   Domain names are often stored and transported in many places.  For   example, they are part of documents such as mail messages and web   pages.  They are transported in many parts of many protocols, such as   both the control commands of SMTP and associated message body parts,Klensin                       Informational                    [Page 16]

RFC 5894                     IDNA Rationale                  August 2010   and in the headers and the body content in HTTP.  It is important to   remember that domain names appear both in domain name slots and in   the content that is passed over protocols, and it would be helpful if   protocols explicitly define what their domain name slots are.   In protocols and document formats that define how to handle   specification or negotiation of charsets, labels can be encoded in   any charset allowed by the protocol or document format.  If a   protocol or document format only allows one charset, the labels must   be given in that charset.  Of course, not all charsets can properly   represent all labels.  If a U-label cannot be displayed in its   entirety, the only choice (without loss of information) may be to   display the A-label.   Where a protocol or document format allows IDNs, labels should be in   whatever character encoding and escape mechanism the protocol or   document format uses in the local environment.  This provision is   intended to prevent situations in which, e.g., UTF-8 domain names   appear embedded in text that is otherwise in some other character   coding.   All protocols that use domain name slots (seeSection 2.3.2.6 in the   Definitions document [RFC5890]) already have the capacity for   handling domain names in the ASCII charset.  Thus, A-labels can   inherently be handled by those protocols.   IDNA2008 does not specify required mappings between one character or   code point and others.  An extended discussion of mapping issues   appears inSection 6 and specific recommendations appear in the   Mapping document [IDNA2008-Mapping].  In general, IDNA2008 prohibits   characters that would be mapped to others by normalization or other   rules.  As examples, while mathematical characters based on Latin   ones are accepted as input to IDNA2003, they are prohibited in   IDNA2008.  Similarly, uppercase characters, double-width characters,   and other variations are prohibited as IDNA input although mapping   them as needed in user interfaces is strongly encouraged.   Since the rules in the Tables document [RFC5892] have the effect that   only strings that are not transformed by NFKC are valid, if an   application chooses to perform NFKC normalization before lookup, that   operation is safe since this will never make the application unable   to look up any valid string.  However, as discussed above, the   application cannot guarantee that any other application will perform   that mapping, so it should be used only with caution and for informed   users.Klensin                       Informational                    [Page 17]

RFC 5894                     IDNA Rationale                  August 2010   In many cases, these prohibitions should have no effect on what the   user can type as input to the lookup process.  It is perfectly   reasonable for systems that support user interfaces to perform some   character mapping that is appropriate to the local environment.  This   would normally be done prior to actual invocation of IDNA.  At least   conceptually, the mapping would be part of the Unicode conversions   discussed above and in the Protocol document [RFC5891].  However,   those changes will be local ones only -- local to environments in   which users will clearly understand that the character forms are   equivalent.  For use in interchanges among systems, it appears to be   much more important that U-labels and A-labels can be mapped back and   forth without loss of information.   One specific, and very important, instance of this strategy arises   with case folding.  In the ASCII-only DNS, names are looked up and   matched in a case-independent way, but no actual case folding occurs.   Names can be placed in the DNS in either uppercase or lowercase form   (or any mixture of them) and that form is preserved, returned in   queries, and so on.  IDNA2003 approximated that behavior for   non-ASCII strings by performing case folding at registration time   (resulting in only lowercase IDNs in the DNS) and when names were   looked up.   As suggested earlier in this section, it appears to be desirable to   do as little character mapping as possible as long as Unicode works   correctly (e.g., Normalization Form C (NFC) mapping to resolve   different codings for the same character is still necessary although   the specifications require that it be performed prior to invoking the   protocol) in order to make the mapping between A-labels and U-labels   idempotent.  Case mapping is not an exception to this principle.  If   only lowercase characters can be registered in the DNS (i.e., be   present in a U-label), then IDNA2008 should prohibit uppercase   characters as input even though user interfaces to applications   should probably map those characters.  Some other considerations   reinforce this conclusion.  For example, in ASCII case mapping for   individual characters, uppercase(character) is always equal to   uppercase(lowercase(character)).  That may not be true with IDNs.  In   some scripts that use case distinctions, there are a few characters   that do not have counterparts in one case or the other.  The   relationship between uppercase and lowercase may even be language-   dependent, with different languages (or even the same language in   different areas) expecting different mappings.  User interface   programs can meet the expectations of users who are accustomed to the   case-insensitive DNS environment by performing case folding prior to   IDNA processing, but the IDNA procedures themselves should neither   require such mapping nor expect them when they are not natural to the   localized environment.Klensin                       Informational                    [Page 18]

RFC 5894                     IDNA Rationale                  August 20104.3.  Linguistic Expectations: Ligatures, Digraphs, and Alternate      Character Forms   Users have expectations about character matching or equivalence that   are based on their own languages and the orthography of those   languages.  These expectations may not always be met in a global   system, especially if multiple languages are written using the same   script but using different conventions.  Some examples:   o  A Norwegian user might expect a label with the ae-ligature to be      treated as the same label as one using the Swedish spelling with      a-diaeresis even though applying that mapping to English would be      astonishing to users.   o  A German user might expect a label with an o-umlaut and a label      that had "oe" substituted, but was otherwise the same, to be      treated as equivalent even though that substitution would be a      clear error in Swedish.   o  A Chinese user might expect automatic matching of Simplified and      Traditional Chinese characters, but applying that matching for      Korean or Japanese text would create considerable confusion.   o  An English user might expect "theater" and "theatre" to match.   A number of languages use alphabetic scripts in which single phonemes   are written using two characters, termed a "digraph", for example,   the "ph" in "pharmacy" and "telephone".  (Such characters can also   appear consecutively without forming a digraph, as in "tophat".)   Certain digraphs may be indicated typographically by setting the two   characters closer together than they would be if used consecutively   to represent different phonemes.  Some digraphs are fully joined as   ligatures.  For example, the word "encyclopaedia" is sometimes set   with a U+00E6 LATIN SMALL LIGATURE AE.  When ligature and digraph   forms have the same interpretation across all languages that use a   given script, application of Unicode normalization generally resolves   the differences and causes them to match.  When they have different   interpretations, matching must utilize other methods, presumably   chosen at the registry level, or users must be educated to understand   that matching will not occur.   The nature of the problem can be illustrated by many words in the   Norwegian language, where the "ae" ligature is the 27th letter of a   29-letter extended Latin alphabet.  It is equivalent to the 28th   letter of the Swedish alphabet (also containing 29 letters),   U+00E4 LATIN SMALL LETTER A WITH DIAERESIS, for which an "ae" cannot   be substituted according to current orthographic standards.  That   character (U+00E4) is also part of the German alphabet where, unlikeKlensin                       Informational                    [Page 19]

RFC 5894                     IDNA Rationale                  August 2010   in the Nordic languages, the two-character sequence "ae" is usually   treated as a fully acceptable alternate orthography for the "umlauted   a" character.  The inverse is however not true, and those two   characters cannot necessarily be combined into an "umlauted a".  This   also applies to another German character, the "umlauted o"   (U+00F6 LATIN SMALL LETTER O WITH DIAERESIS) which, for example,   cannot be used for writing the name of the author "Goethe".  It is   also a letter in the Swedish alphabet where, like the "a with   diaeresis", it cannot be correctly represented as "oe" and in the   Norwegian alphabet, where it is represented, not as "o with   diaeresis", but as "slashed o", U+00F8.   Some of the ligatures that have explicit code points in Unicode were   given special handling in IDNA2003 and now pose additional problems   in transition.  SeeSection 7.2.   Additional cases with alphabets written right to left are described   inSection 4.5.   Matching and comparison algorithm selection often requires   information about the language being used, context, or both --   information that is not available to IDNA or the DNS.  Consequently,   IDNA2008 makes no attempt to treat combined characters in any special   way.  A registry that is aware of the language context in which   labels are to be registered, and where that language sometimes (or   always) treats the two-character sequences as equivalent to the   combined form, should give serious consideration to applying a   "variant" model [RFC3743][RFC4290] or to prohibiting registration of   one of the forms entirely, to reduce the opportunities for user   confusion and fraud that would result from the related strings being   registered to different parties.4.4.  Case Mapping and Related Issues   In the DNS, ASCII letters are stored with their case preserved.   Matching during the query process is case-independent, but none of   the information that might be represented by choices of case has been   lost.  That model has been accidentally helpful because, as people   have created DNS labels by catenating words (or parts of words) to   form labels, case has often been used to distinguish among components   and make the labels more memorable.   Since DNS servers do not get involved in parsing IDNs, they cannot do   case-independent matching.  Thus, keeping the cases separate in   lookup or registration, and doing matching at the server, is not   feasible with IDNA or any similar approach.  Matching of characters   that are considered to differ only by case must be done, if desired,   by programs invoking IDNA lookup even though it wasn't done by ASCII-Klensin                       Informational                    [Page 20]

RFC 5894                     IDNA Rationale                  August 2010   only DNS clients.  That situation was recognized in IDNA2003 and   nothing in IDNA2008 fundamentally changes it or could do so.  In   IDNA2003, all characters are case folded and mapped by clients in a   standardized step.   Even in scripts that generally support case distinctions, some   characters do not have uppercase forms.  For example, the Unicode   case-folding operation maps Greek Final Form Sigma (U+03C2) to the   medial form (U+03C3) and maps Eszett (German Sharp S, U+00DF) to   "ss".  Neither of these mappings is reversible because the uppercase   of U+03C3 is the uppercase Sigma (U+03A3) and "ss" is an ASCII   string.  IDNA2008 permits, at the risk of some incompatibility,   slightly more flexibility in this area by avoiding case folding and   treating these characters as themselves.  Approaches to handling one-   way mappings are discussed inSection 7.2.   Because IDNA2003 maps Final Sigma and Eszett to other characters, and   the reverse mapping is never possible, neither Final Sigma nor Eszett   can be represented in the ACE form of IDNA2003 IDN nor in the native   character (U-label) form derived from it.  With IDNA2008, both   characters can be used in an IDN and so the A-label used for lookup   for any U-label containing those characters is now different.  SeeSection 7.1 for a discussion of what kinds of changes might require   the IDNA prefix to change; after extended discussions, the IDNABIS   Working Group came to consensus that the change for these characters   did not justify a prefix change.4.5.  Right-to-Left Text   In order to be sure that the directionality of right-to-left text is   unambiguous, IDNA2003 required that any label in which right-to-left   characters appear both starts and ends with them and that it does not   include any characters with strong left-to-right properties (that   excludes other alphabetic characters but permits European digits).   Any other string that contains a right-to-left character and does not   meet those requirements is rejected.  This is one of the few places   where the IDNA algorithms (both in IDNA2003 and in IDNA2008) examine   an entire label, not just individual characters.  The algorithmic   model used in IDNA2003 rejects the label when the final character in   a right-to-left string requires a combining mark in order to be   correctly represented.   That prohibition is not acceptable for writing systems for languages   written with consonantal alphabets to which diacritical vocalic   systems are applied, and for languages with orthographies derived   from them where the combining marks may have different functionality.   In both cases, the combining marks can be essential components of the   orthography.  Examples of this are Yiddish, written with an extendedKlensin                       Informational                    [Page 21]

RFC 5894                     IDNA Rationale                  August 2010   Hebrew script, and Dhivehi (the official language of Maldives), which   is written in the Thaana script (which is, in turn, derived from the   Arabic script).  IDNA2008 removes the restriction on final combining   characters with a new set of rules for right-to-left scripts and   their characters.  Those new rules are specified in the Bidi document   [RFC5893].5.  IDNs and the Robustness Principle   The "Robustness Principle" is often stated as "Be conservative about   what you send and liberal in what you accept" (see, e.g.,Section1.2.2 of the applications-layer Host Requirements specification   [RFC1123]).  This principle applies to IDNA.  In applying the   principle to registries as the source ("sender") of all registered   and useful IDNs, registries are responsible for being conservative   about what they register and put out in the Internet.  For IDNs to   work well, zone administrators (registries) must have and require   sensible policies about what is registered -- conservative policies   -- and implement and enforce them.   Conversely, lookup applications are expected to reject labels that   clearly violate global (protocol) rules (no one has ever seriously   claimed that being liberal in what is accepted requires being   stupid).  However, once one gets past such global rules and deals   with anything sensitive to script or locale, it is necessary to   assume that garbage has not been placed into the DNS, i.e., one must   be liberal about what one is willing to look up in the DNS rather   than guessing about whether it should have been permitted to be   registered.   If a string cannot be successfully found in the DNS after the lookup   processing described here, it makes no difference whether it simply   wasn't registered or was prohibited by some rule at the registry.   Application implementers should be aware that where DNS wildcards are   used, the ability to successfully resolve a name does not guarantee   that it was actually registered.6.  Front-end and User Interface Processing for Lookup   Domain names may be identified and processed in many contexts.  They   may be typed in by users themselves or embedded in an identifier such   as an email address, URI, or IRI.  They may occur in running text or   be processed by one system after being provided in another.  Systems   may try to normalize URLs to determine (or guess) whether a reference   is valid or if two references point to the same object without   actually looking the objects up (comparison without lookup is   necessary for URI types that are not intended to be resolved).  Some   of these goals may be more easily and reliably satisfied than others.Klensin                       Informational                    [Page 22]

RFC 5894                     IDNA Rationale                  August 2010   While there are strong arguments for any domain name that is placed   "on the wire" -- transmitted between systems -- to be in the zero-   ambiguity forms of A-labels, it is inevitable that programs that   process domain names will encounter U-labels or variant forms.   An application that implements the IDNA protocol [RFC5891] will   always take any user input and convert it to a set of Unicode code   points.  That user input may be acquired by any of several different   input methods, all with differing conversion processes to be taken   into consideration (e.g., typed on a keyboard, written by hand onto   some sort of digitizer, spoken into a microphone and interpreted by a   speech-to-text engine, etc.).  The process of taking any particular   user input and mapping it into a Unicode code point may be a simple   one: if a user strikes the "A" key on a US English keyboard, without   any modifiers such as the "Shift" key held down, in order to draw a   Latin small letter A ("a"), many (perhaps most) modern operating   system input methods will produce to the calling application the code   point U+0061, encoded in a single octet.   Sometimes the process is somewhat more complicated: a user might   strike a particular set of keys to represent a combining macron   followed by striking the "A" key in order to draw a Latin small   letter A with a macron above it.  Depending on the operating system,   the input method chosen by the user, and even the parameters with   which the application communicates with the input method, the result   might be the code point U+0101 (encoded as two octets in UTF-8 or   UTF-16, four octets in UTF-32, etc.), the code point U+0061 followed   by the code point U+0304 (again, encoded in three or more octets,   depending upon the encoding used) or even the code point U+FF41   followed by the code point U+0304 (and encoded in some form).  These   examples leave aside the issue of operating systems and input methods   that do not use Unicode code points for their character set.   In every case, applications (with the help of the operating systems   on which they run and the input methods used) need to perform a   mapping from user input into Unicode code points.   IDNA2003 used a model whereby input was taken from the user, mapped   (via whatever input method mechanisms were used) to a set of Unicode   code points, and then further mapped to a set of Unicode code points   using the Nameprep profile [RFC3491].  In this procedure, there are   two separate mapping steps: first, a mapping done by the input method   (which might be controlled by the operating system, the application,   or some combination) and then a second mapping performed by the   Nameprep portion of the IDNA protocol.  The mapping done in Nameprep   includes a particular mapping table to re-map some characters to   other characters, a particular normalization, and a set of prohibited   characters.Klensin                       Informational                    [Page 23]

RFC 5894                     IDNA Rationale                  August 2010   Note that the result of the two-step mapping process means that the   mapping chosen by the operating system or application in the first   step might differ significantly from the mapping supplied by the   Nameprep profile in the second step.  This has advantages and   disadvantages.  Of course, the second mapping regularizes what gets   looked up in the DNS, making for better interoperability between   implementations that use the Nameprep mapping.  However, the   application or operating system may choose mappings in their input   methods, which when passed through the second (Nameprep) mapping   result in characters that are "surprising" to the end user.   The other important feature of IDNA2003 is that, with very few   exceptions, it assumes that any set of Unicode code points provided   to the Nameprep mapping can be mapped into a string of Unicode code   points that are "sensible", even if that means mapping some code   points to nothing (that is, removing the code points from the   string).  This allowed maximum flexibility in input strings.   The present version of IDNA (IDNA2008) differs significantly in   approach from the original version.  First and foremost, it does not   provide explicit mapping instructions.  Instead, it assumes that the   application (perhaps via an operating system input method) will do   whatever mapping it requires to convert input into Unicode code   points.  This has the advantage of giving flexibility to the   application to choose a mapping that is suitable for its user given   specific user requirements, and avoids the two-step mapping of the   original protocol.  Instead of a mapping, IDNA2008 provides a set of   categories that can be used to specify the valid code points allowed   in a domain name.   In principle, an application ought to take user input of a domain   name and convert it to the set of Unicode code points that represent   the domain name the user intends.  As a practical matter, of course,   determining user intent is a tricky business, so an application needs   to choose a reasonable mapping from user input.  That may differ   based on the particular circumstances of a user, depending on locale,   language, type of input method, etc.  It is up to the application to   make a reasonable choice.Klensin                       Informational                    [Page 24]

RFC 5894                     IDNA Rationale                  August 20107.  Migration from IDNA2003 and Unicode Version Synchronization7.1.  Design Criteria   As mentioned above and in the IAB review and recommendations for IDNs   [RFC4690], two key goals of the IDNA2008 design are:   o  to enable applications to be agnostic about whether they are being      run in environments supporting any Unicode version from 3.2      onward.   o  to permit incrementally adding new characters, character groups,      scripts, and other character collections as they are incorporated      into Unicode, doing so without disruption and, in the long term,      without "heavy" processes (an IETF consensus process is required      by the IDNA2008 specifications and is expected to be required and      used until significant experience accumulates with IDNA operations      and new versions of Unicode).7.1.1.  Summary and Discussion of IDNA Validity Criteria   The general criteria for a label to be considered valid under IDNA   are (the actual rules are rigorously defined in the Protocol   [RFC5891] and Tables [RFC5892] documents):   o  The characters are "letters", marks needed to form letters,      numerals, or other code points used to write words in some      language.  Symbols, drawing characters, and various notational      characters are intended to be permanently excluded.  There is no      evidence that they are important enough to Internet operations or      internationalization to justify expansion of domain names beyond      the general principle of "letters, digits, and hyphen".      (Additional discussion and rationale for the symbol decision      appears inSection 7.6.)   o  Other than in very exceptional cases, e.g., where they are needed      to write substantially any word of a given language, punctuation      characters are excluded.  The fact that a word exists is not proof      that it should be usable in a DNS label, and DNS labels are not      expected to be usable for multiple-word phrases (although they are      certainly not prohibited if the conventions and orthography of a      particular language cause that to be possible).   o  Characters that are unassigned (have no character assignment at      all) in the version of Unicode being used by the registry or      application are not permitted, even on lookup.  The issues      involved in this decision are discussed inSection 7.7.Klensin                       Informational                    [Page 25]

RFC 5894                     IDNA Rationale                  August 2010   o  Any character that is mapped to another character by a current      version of NFKC is prohibited as input to IDNA (for either      registration or lookup).  With a few exceptions, this principle      excludes any character mapped to another by Nameprep [RFC3491].   The principles above drive the design of rules that are specified   exactly in the Tables document.  Those rules identify the characters   that are valid under IDNA.  The rules themselves are normative, and   the tables are derived from them, rather than vice versa.7.1.2.  Labels in Registration   Any label registered in a DNS zone must be validated -- i.e., the   criteria for that label must be met -- in order for applications to   work as intended.  This principle is not new.  For example, since the   DNS was first deployed, zone administrators have been expected to   verify that names meet "hostname" requirements [RFC0952] where those   requirements are imposed by the expected applications.  Other   applications contexts, such as the later addition of special service   location formats [RFC2782] imposed new requirements on zone   administrators.  For zones that will contain IDNs, support for   Unicode version-independence requires restrictions on all strings   placed in the zone.  In particular, for such zones (the exact rules   appear inSection 4 of the Protocol document [RFC5891]):   o  Any label that appears to be an A-label, i.e., any label that      starts in "xn--", must be valid under IDNA, i.e., they must be      valid A-labels, as discussed inSection 2 above.   o  The Unicode tables (i.e., tables of code points, character      classes, and properties) and IDNA tables (i.e., tables of      contextual rules such as those that appear in the Tables      document), must be consistent on the systems performing or      validating labels to be registered.  Note that this does not      require that tables reflect the latest version of Unicode, only      that all tables used on a given system are consistent with each      other.   Under this model, registry tables will need to be updated (both the   Unicode-associated tables and the tables of permitted IDN characters)   to enable a new script or other set of new characters.  The registry   will not be affected by newer versions of Unicode, or newly   authorized characters, until and unless it wishes to support them.   The zone administrator is responsible for verifying validity for IDNA   as well as its local policies -- a more extensive set of checks than   are required for looking up the labels.  Systems looking up orKlensin                       Informational                    [Page 26]

RFC 5894                     IDNA Rationale                  August 2010   resolving DNS labels, especially IDN DNS labels, must be able to   assume that applicable registration rules were followed for names   entered into the DNS.7.1.3.  Labels in Lookup   Any application processing a label through IDNA so it can be looked   up in a DNS zone is required to (the exact rules appear inSection 5   of the Protocol document [RFC5891]):   o  Maintain IDNA and Unicode tables that are consistent with regard      to versions, i.e., unless the application actually executes the      classification rules in the Tables document [RFC5892], its IDNA      tables must be derived from the version of Unicode that is      supported more generally on the system.  As with registration, the      tables need not reflect the latest version of Unicode, but they      must be consistent.   o  Validate the characters in labels to be looked up only to the      extent of determining that the U-label does not contain      "DISALLOWED" code points or code points that are unassigned in its      version of Unicode.   o  Validate the label itself for conformance with a small number of      whole-label rules.  In particular, it must verify that:      *  there are no leading combining marks,      *  the Bidi conditions are met if right-to-left characters appear,      *  any required contextual rules are available, and      *  any contextual rules that are associated with joiner characters         (and CONTEXTJ characters more generally) are tested.   o  Do not reject labels based on other contextual rules about      characters, including mixed-script label prohibitions.  Such rules      may be used to influence presentation decisions in the user      interface, but not to avoid looking up domain names.   To further clarify the rules about handling characters that require   contextual rules, note that one can have a context-required character   (i.e., one that requires a rule), but no rule.  In that case, the   character is treated the same way DISALLOWED characters are treated,   until and unless a rule is supplied.  That state is more or less   equivalent to "the idea of permitting this character is accepted in   principle, but it won't be permitted in practice until consensus is   reached on a safe way to use it".Klensin                       Informational                    [Page 27]

RFC 5894                     IDNA Rationale                  August 2010   The ability to add a rule more or less exempts these characters from   the prohibition against reclassifying characters from DISALLOWED to   PVALID.   And, obviously, "no rule" is different from "have a rule, but the   test either succeeds or fails".   Lookup applications that follow these rules, rather than having their   own criteria for rejecting lookup attempts, are not sensitive to   version incompatibilities with the particular zone registry   associated with the domain name except for labels containing   characters recently added to Unicode.   An application or client that processes names according to this   protocol and then resolves them in the DNS will be able to locate any   name that is registered, as long as those registrations are valid   under IDNA and its version of the IDNA tables is sufficiently up to   date to interpret all of the characters in the label.  Messages to   users should distinguish between "label contains an unallocated code   point" and other types of lookup failures.  A failure on the basis of   an old version of Unicode may lead the user to a desire to upgrade to   a newer version, but will have no other ill effects (this is   consistent with behavior in the transition to the DNS when some hosts   could not yet handle some forms of names or record types).7.2.  Changes in Character Interpretations   As a consequence of the elimination of mapping, the current version   of IDNA changes the interpretation of a few characters relative to   its predecessors.  This subsection outlines the issues and discusses   possible transition strategies.7.2.1.  Character Changes: Eszett and Final Sigma   In those scripts that make case distinctions, there are a few   characters for which an obvious and unique uppercase character has   not historically been available to match a lowercase one, or vice   versa.  For those characters, the mappings used in constructing the   Stringprep tables for IDNA2003, performed using the Unicode   toCaseFold operation (seeSection 5.18 of the Unicode Standard   [Unicode52]), generate different characters or sets of characters.   Those operations are not reversible and lose even more information   than traditional uppercase or lowercase transformations, but are more   useful than those transformations for comparison purposes.  Two   notable characters of this type are the German character Eszett   (Sharp S, U+00DF) and the Greek Final Form Sigma (U+03C2).  The   former is case folded to the ASCII string "ss", the latter to a   medial (lowercase) Sigma (U+03C3).Klensin                       Informational                    [Page 28]

RFC 5894                     IDNA Rationale                  August 20107.2.2.  Character Changes: Zero Width Joiner and Zero Width Non-Joiner   IDNA2003 mapped both ZERO WIDTH JOINER (ZWJ, U+200D) and ZERO WIDTH   NON-JOINER (ZWNJ, U+200C) to nothing, effectively dropping these   characters from any label in which they appeared and treating strings   containing them as identical to strings that did not.  As discussed   inSection 3.1.2 above, those characters are essential for writing   many reasonable mnemonics for certain scripts.  However, treating   them as valid in IDNA2008, even with contextual restrictions, raises   approximately the same problem as exists with Eszett and Final Sigma:   strings that were valid under IDNA2003 have different interpretations   as labels, and different A-labels, than the same strings under this   newer version.7.2.3.  Character Changes and the Need for Transition   The decision to eliminate mandatory and standardized mappings,   including case folding, from the IDNA2008 protocol in order to make   A-labels and U-labels idempotent made these characters problematic.   If they were to be disallowed, important words and mnemonics could   not be written in orthographically reasonable ways.  If they were to   be permitted as distinct characters, there would be no information   loss and registries would have more flexibility, but IDNA2003 and   IDNA2008 lookups might result in different A-labels.   With the understanding that there would be incompatibility either way   but a judgment that the incompatibility was not significant enough to   justify a prefix change, the Working Group concluded that Eszett and   Final Form Sigma should be treated as distinct and Protocol-Valid   characters.   Since these characters are interpreted in different ways under the   older and newer versions of IDNA, transition strategies and policies   will be necessary.  Some actions can reasonably be taken by   applications' client programs (those that perform lookup operations   or cause them to be performed), but because of the diversity of   situations and uses of the DNS, much of the responsibility will need   to fall on registries.   Registries, especially those maintaining zones for third parties,   must decide how to introduce a new service in a way that does not   create confusion or significantly weaken or invalidate existing   identifiers.  This is not a new problem; registries were faced with   similar issues when IDNs were introduced (potentially, and especially   for Latin-based scripts, in conflict with existing labels that had   been rendered in ASCII characters by applying more or less   standardized conventions) and when other new forms of strings have   been permitted as labels.Klensin                       Informational                    [Page 29]

RFC 5894                     IDNA Rationale                  August 20107.2.4.  Transition Strategies   There are several approaches to the introduction of new characters or   changes in interpretation of existing characters from their mapped   forms in the earlier version of IDNA.  The transition issue is   complicated because the forms of these labels after the   ToUnicode(ToASCII()) translation in IDNA2003 not only remain valid   but do not provide strong indications of what the registrant   intended: a string containing "ss" could have simply been intended to   be that string or could have been intended to contain an Eszett; a   string containing lowercase Sigma could have been intended to contain   Final Sigma (one might make heuristic guesses based on position in a   string, but the long tradition of forming labels by concatenating   words makes such heuristics unreliable), and strings that do not   contain ZWJ or ZWNJ might have been intended to contain them.   Without any preference or claim to completeness, some of these, all   of which have been used by registries in the past for similar   transitions, are:   1.  Do not permit use of the newly available character at the       registry level.  This might cause lookup failures if a domain       name were to be written with the expectation of the IDNA2003       mapping behavior, but would eliminate any possibility of false       matches.   2.  Hold a "sunrise"-like arrangement in which holders of labels       containing "ss" in the Eszett case, lowercase Sigma in that case,       or that might have contained ZWJ or ZWNJ in context, are given       priority (and perhaps other benefits) for registering the       corresponding string containing Eszett, Final Sigma, or the       appropriate zero-width character respectively.   3.  Adopt some sort of "variant" approach in which registrants obtain       labels with both character forms.   4.  Adopt a different form of "variant" approach in which       registration of additional strings that would produce the same       A-label if interpreted according to IDNA2003 is either not       permitted at all or permitted only by the registrant who already       has one of the names.   5.  Ignore the issue and assume that the marketplace or other       mechanisms will sort things out.   In any event, a registry (at any level of the DNS tree) that chooses   to permit labels to be registered that contains these characters, or   considers doing so, will have to address the relationship with   existing, possibly conflicting, labels in some way, just asKlensin                       Informational                    [Page 30]

RFC 5894                     IDNA Rationale                  August 2010   registries that already had a considerable number of labels did when   IDNs were first introduced.7.3.  Elimination of Character Mapping   As discussed at length inSection 6, IDNA2003, via Nameprep (seeSection 7.5), mapped many characters into related ones.  Those   mappings no longer exist as requirements in IDNA2008.  These   specifications strongly prefer that only A-labels or U-labels be used   in protocol contexts and as much as practical more generally.   IDNA2008 does anticipate situations in which some mapping at the time   of user input into lookup applications is appropriate and desirable.   The issues are discussed inSection 6 and specific recommendations   are made in the Mapping document [IDNA2008-Mapping].7.4.  The Question of Prefix Changes   The conditions that would have required a change in the IDNA ACE   prefix ("xn--", used in IDNA2003) were of great concern to the   community.  A prefix change would have clearly been necessary if the   algorithms were modified in a manner that would have created serious   ambiguities during subsequent transition in registrations.  This   section summarizes the working group's conclusions about the   conditions under which a change in the prefix would have been   necessary and the implications of such a change.7.4.1.  Conditions Requiring a Prefix Change   An IDN prefix change would have been needed if a given string would   be looked up or otherwise interpreted differently depending on the   version of the protocol or tables being used.  This IDNA upgrade   would have required a prefix change if, and only if, one of the   following four conditions were met:   1.  The conversion of an A-label to Unicode (i.e., a U-label) would       have yielded one string under IDNA2003 and a different string       under IDNA2008.   2.  In a significant number of cases, an input string that was valid       under IDNA2003 and also valid under IDNA2008 would have yielded       two different A-labels with the different versions.  This       condition is believed to be essentially equivalent to the one       above except for a very small number of edge cases that were not       found to justify a prefix change (seeSection 7.2).       Note that if the input string was valid under one version and not       valid under the other, this condition would not apply.  See the       first item inSection 7.4.2, below.Klensin                       Informational                    [Page 31]

RFC 5894                     IDNA Rationale                  August 2010   3.  A fundamental change was made to the semantics of the string that       would be inserted in the DNS, e.g., if a decision were made to       try to include language or script information in the encoding in       addition to the string itself.   4.  A sufficiently large number of characters were added to Unicode       so that the Punycode mechanism for block offsets would no longer       reference the higher-numbered planes and blocks.  This condition       is unlikely even in the long term and certain not to arise in the       next several years.7.4.2.  Conditions Not Requiring a Prefix Change   As a result of the principles described above, none of the following   changes required a new prefix:   1.  Prohibition of some characters as input to IDNA.  Such a       prohibition might make names that were previously registered       inaccessible, but did not change those names.   2.  Adjustments in IDNA tables or actions, including normalization       definitions, that affected characters that were already invalid       under IDNA2003.   3.  Changes in the style of the IDNA definition that did not alter       the actions performed by IDNA.7.4.3.  Implications of Prefix Changes   While it might have been possible to make a prefix change, the costs   of such a change are considerable.  Registries could not have   converted all IDNA2003 ("xn--") registrations to a new form at the   same time and synchronize that change with applications supporting   lookup.  Unless all existing registrations were simply to be declared   invalid (and perhaps even then), systems that needed to support both   labels with old prefixes and labels with new ones would be required   to first process a putative label under the IDNA2008 rules and try to   look it up and then, if it were not found, would be required to   process the label under IDNA2003 rules and look it up again.  That   process would probably have significantly slowed down all processing   that involved IDNs in the DNS, especially since a fully-qualified   name might contain a mixture of labels that were registered with the   old and new prefixes.  That would have made DNS caching very   difficult.  In addition, looking up the same input string as two   separate A-labels would have created some potential for confusion and   attacks, since the labels could map to different targets and then   resolve to different entries in the DNS.Klensin                       Informational                    [Page 32]

RFC 5894                     IDNA Rationale                  August 2010   Consequently, a prefix change should have been, and was, avoided if   at all possible, even if it means accepting some IDNA2003 decisions   about character distinctions as irreversible and/or giving special   treatment to edge cases.7.5.  Stringprep Changes and Compatibility   The Nameprep specification [RFC3491], a key part of IDNA2003, is a   profile of Stringprep [RFC3454].  While Nameprep is a Stringprep   profile specific to IDNA, Stringprep is used by a number of other   protocols.  Were Stringprep to have been modified by IDNA2008, those   changes to improve the handling of IDNs could cause problems for   non-DNS uses, most notably if they affected identification and   authentication protocols.  Several elements of IDNA2008 give   interpretations to strings prohibited under IDNA2003 or prohibit   strings that IDNA2003 permitted.  Those elements include the new   inclusion information in the Tables document [RFC5892], the reduction   in the number of characters permitted as input for registration or   lookup (Section 3), and even the changes in handling of right-to-left   strings as described in the Bidi document [RFC5893].  IDNA2008 does   not use Nameprep or Stringprep at all, so there are no side-effect   changes to other protocols.   It is particularly important to keep IDNA processing separate from   processing for various security protocols because some of the   constraints that are necessary for smooth and comprehensible use of   IDNs may be unwanted or undesirable in other contexts.  For example,   the criteria for good passwords or passphrases are very different   from those for desirable IDNs: passwords should be hard to guess,   while domain names should normally be easily memorable.  Similarly,   internationalized Small Computer System Interface (SCSI) identifiers   and other protocol components are likely to have different   requirements than IDNs.7.6.  The Symbol Question   One of the major differences between this specification and the   original version of IDNA is that IDNA2003 permitted non-letter   symbols of various sorts, including punctuation and line-drawing   symbols, in the protocol.  They were always discouraged in practice.   In particular, both the "IESG Statement" about IDNA and all versions   of the ICANN Guidelines specify that only language characters be used   in labels.  This specification disallows symbols entirely.  There are   several reasons for this, which include:   1.  As discussed elsewhere, the original IDNA specification assumed       that as many Unicode characters as possible should be permitted,       directly or via mapping to other characters, in IDNs.  ThisKlensin                       Informational                    [Page 33]

RFC 5894                     IDNA Rationale                  August 2010       specification operates on an inclusion model, extrapolating from       the original "hostname" rules (LDH, see the Definitions document       [RFC5890]) -- which have served the Internet very well -- to a       Unicode base rather than an ASCII base.   2.  Symbol names are more problematic than letters because there may       be no general agreement on whether a particular glyph matches a       symbol; there are no uniform conventions for naming; variations       such as outline, solid, and shaded forms may or may not exist;       and so on.  As just one example, consider a "heart" symbol as it       might appear in a logo that might be read as "I love...".  While       the user might read such a logo as "I love..." or "I heart...",       considerable knowledge of the coding distinctions made in Unicode       is needed to know that there is more than one "heart" character       (e.g., U+2665, U+2661, and U+2765) and how to describe it.  These       issues are of particular importance if strings are expected to be       understood or transcribed by the listener after being read out       loud.   3.  Design of a screen reader used by blind Internet users who must       listen to renderings of IDN domain names and possibly reproduce       them on the keyboard becomes considerably more complicated when       the names of characters are not obvious and intuitive to anyone       familiar with the language in question.   4.  As a simplified example of this, assume one wanted to use a       "heart" or "star" symbol in a label.  This is problematic because       those names are ambiguous in the Unicode system of naming (the       actual Unicode names require far more qualification).  A user or       would-be registrant has no way to know -- absent careful study of       the code tables -- whether it is ambiguous (e.g., where there are       multiple "heart" characters) or not.  Conversely, the user seeing       the hypothetical label doesn't know whether to read it -- try to       transmit it to a colleague by voice -- as "heart", as "love", as       "black heart", or as any of the other examples below.   5.  The actual situation is even worse than this.  There is no       possible way for a normal, casual, user to tell the difference       between the hearts of U+2665 and U+2765 and the stars of U+2606       and U+2729 without somehow knowing to look for a distinction.  We       have a white heart (U+2661) and few black hearts.  Consequently,       describing a label as containing a heart is hopelessly ambiguous:       we can only know that it contains one of several characters that       look like hearts or have "heart" in their names.  In cities where       "Square" is a popular part of a location name, one might well       want to use a square symbol in a label as well and there are far       more squares of various flavors in Unicode than there are hearts       or stars.Klensin                       Informational                    [Page 34]

RFC 5894                     IDNA Rationale                  August 2010   The consequence of these ambiguities is that symbols are a very poor   basis for reliable communication.  Consistent with this conclusion,   the Unicode standard recommends that strings used in identifiers not   contain symbols or punctuation [Unicode-UAX31].  Of course, these   difficulties with symbols do not arise with actual pictographic   languages and scripts which would be treated like any other language   characters; the two should not be confused.7.7.  Migration between Unicode Versions: Unassigned Code Points   In IDNA2003, labels containing unassigned code points are looked up   on the assumption that, if they appear in labels and can be mapped   and then resolved, the relevant standards must have changed and the   registry has properly allocated only assigned values.   In the IDNA2008 protocol, strings containing unassigned code points   must not be either looked up or registered.  In summary, the status   of an unassigned character with regard to the DISALLOWED,   PROTOCOL-VALID, and CONTEXTUAL RULE REQUIRED categories cannot be   evaluated until a character is actually assigned and known.  There   are several reasons for this, with the most important ones being:   o  Tests involving the context of characters (e.g., some characters      being permitted only adjacent to others of specific types) and      integrity tests on complete labels are needed.  Unassigned code      points cannot be permitted because one cannot determine whether      particular code points will require contextual rules (and what      those rules should be) before characters are assigned to them and      the properties of those characters fully understood.   o  It cannot be known in advance, and with sufficient reliability,      whether a newly assigned code point will be associated with a      character that would be disallowed by the rules in the Tables      document [RFC5892] (such as a compatibility character).  In      IDNA2003, since there is no direct dependency on NFKC (many of the      entries in Stringprep's tables are based on NFKC, but IDNA2003      depends only on Stringprep), allocation of a compatibility      character might produce some odd situations, but it would not be a      problem.  In IDNA2008, where compatibility characters are      DISALLOWED unless character-specific exceptions are made,      permitting strings containing unassigned characters to be looked      up would violate the principle that characters in DISALLOWED are      not looked up.   o  The Unicode Standard specifies that an unassigned code point      normalizes (and, where relevant, case folds) to itself.  If the      code point is later assigned to a character, and particularly if      the newly assigned code point has a combining class thatKlensin                       Informational                    [Page 35]

RFC 5894                     IDNA Rationale                  August 2010      determines its placement relative to other combining characters,      it could normalize to some other code point or sequence.   It is possible to argue that the issues above are not important and   that, as a consequence, it is better to retain the principle of   looking up labels even if they contain unassigned characters because   all of the important scripts and characters have been coded as of   Unicode 5.2 (or even earlier), and hence unassigned code points will   be assigned only to obscure characters or archaic scripts.   Unfortunately, that does not appear to be a safe assumption for at   least two reasons.  First, much the same claim of completeness has   been made for earlier versions of Unicode.  The reality is that a   script that is obscure to much of the world may still be very   important to those who use it.  Cultural and linguistic preservation   principles make it inappropriate to declare the script of no   importance in IDNs.  Second, we already have counterexamples, e.g.,   in the relationships associated with new Han characters being added   (whether in the BMP or in Unicode Plane 2).   Independent of the technical transition issues identified above, it   can be observed that any addition of characters to an existing script   to make it easier to use or to better accommodate particular   languages may lead to transition issues.  Such additions may change   the preferred form for writing a particular string, changes that may   be reflected, e.g., in keyboard transition modules that would   necessarily be different from those for earlier versions of Unicode   where the newer characters may not exist.  This creates an inherent   transition problem because attempts to access labels may use either   the old or the new conventions, requiring registry action whether or   not the older conventions were used in labels.  The need to consider   transition mechanisms is inherent to evolution of Unicode to better   accommodate writing systems and is independent of how IDNs are   represented in the DNS or how transitions among versions of those   mechanisms occur.  The requirement for transitions of this type is   illustrated by the addition of Malayalam Chillu in Unicode 5.1.0.7.8.  Other Compatibility Issues   The 2003 IDNA model includes several odd artifacts of the context in   which it was developed.  Many, if not all, of these are potential   avenues for exploits, especially if the registration process permits   "source" names (names that have not been processed through IDNA and   Nameprep) to be registered.  As one example, since the character   Eszett, used in German, is mapped by IDNA2003 into the sequence "ss"   rather than being retained as itself or prohibited, a string   containing that character, but that is otherwise in ASCII, is not   really an IDN (in the U-label sense defined above).  After Nameprep   maps out the Eszett, the result is an ASCII string and so it does notKlensin                       Informational                    [Page 36]

RFC 5894                     IDNA Rationale                  August 2010   get an xn-- prefix, but the string that can be displayed to a user   appears to be an IDN.  IDNA2008 eliminates this artifact.  A   character is either permitted as itself or it is prohibited; special   cases that make sense only in a particular linguistic or cultural   context can be dealt with as localization matters where appropriate.8.  Name Server Considerations8.1.  Processing Non-ASCII Strings   Existing DNS servers do not know the IDNA rules for handling   non-ASCII forms of IDNs, and therefore need to be shielded from them.   All existing channels through which names can enter a DNS server   database (for example, master files (as described inRFC 1034) and   DNS update messages [RFC2136]) could not be IDNA-aware because they   predate IDNA.  Other sections of this document provide the needed   shielding by ensuring that internationalized domain names entering   DNS server databases through such channels have already been   converted to their equivalent ASCII A-label forms.   Because of the distinction made between the algorithms for   Registration and Lookup in Sections4 and5 (respectively) of the   Protocol document [RFC5891] (a domain name containing only ASCII code   points cannot be converted to an A-label), there cannot be more than   one A-label form for any given U-label.   As specified in clarifications to the DNS specification [RFC2181],   the DNS protocol explicitly allows domain labels to contain octets   beyond the ASCII range (0000..007F), and this document does not   change that.  However, although the interpretation of octets   0080..00FF is well-defined in the DNS, many application protocols   support only ASCII labels and there is no defined interpretation of   these non-ASCII octets as characters and, in particular, no   interpretation of case-independent matching for them (e.g., see the   clarification on DNS case insensitivity [RFC4343]).  If labels   containing these octets are returned to applications, unpredictable   behavior could result.  The A-label form, which cannot contain those   characters, is the only standard representation for internationalized   labels in the DNS protocol.8.2.  Root and Other DNS Server Considerations   IDNs in A-label form will generally be somewhat longer than current   domain names, so the bandwidth needed by the root servers is likely   to go up by a small amount.  Also, queries and responses for IDNs   will probably be somewhat longer than typical queries historically,Klensin                       Informational                    [Page 37]

RFC 5894                     IDNA Rationale                  August 2010   so Extension Mechanisms for DNS (EDNS0) [RFC2671] support may be more   important (otherwise, queries and responses may be forced to go to   TCP instead of UDP).9.  Internationalization Considerations   DNS labels and fully-qualified domain names provide mnemonics that   assist in identifying and referring to resources on the Internet.   IDNs expand the range of those mnemonics to include those based on   languages and character sets other than Western European and Roman-   derived ones.  But domain "names" are not, in general, words in any   language.  The recommendations of the IETF policy on character sets   and languages (BCP 18 [RFC2277]) are applicable to situations in   which language identification is used to provide language-specific   contexts.  The DNS is, by contrast, global and international and   ultimately has nothing to do with languages.  Adding languages (or   similar context) to IDNs generally, or to DNS matching in particular,   would imply context-dependent matching in DNS, which would be a very   significant change to the DNS protocol itself.  It would also imply   that users would need to identify the language associated with a   particular label in order to look that label up.  That knowledge is   generally not available because many labels are not words in any   language and some may be words in more than one.10.  IANA Considerations   This section gives an overview of IANA registries required for IDNA.   The actual definitions of, and specifications for, the first two,   which have been newly created for IDNA2008, appear in the Tables   document [RFC5892].  This document describes the registries, but it   does not specify any IANA actions.10.1.  IDNA Character Registry   The distinction among the major categories "UNASSIGNED",   "DISALLOWED", "PROTOCOL-VALID", and "CONTEXTUAL RULE REQUIRED" is   made by special categories and rules that are integral elements of   the Tables document.  While not normative, an IANA registry of   characters and scripts and their categories, updated for each new   version of Unicode and the characters it contains, are convenient for   programming and validation purposes.  The details of this registry   are specified in the Tables document.Klensin                       Informational                    [Page 38]

RFC 5894                     IDNA Rationale                  August 201010.2.  IDNA Context Registry   IANA has created and now maintains a list of approved contextual   rules for characters that are defined in the IDNA Character Registry   list as requiring a Contextual Rule (i.e., the types of rules   described inSection 3.1.2).  The details for those rules appear in   the Tables document.10.3.  IANA Repository of IDN Practices of TLDs   This registry, historically described as the "IANA Language Character   Set Registry" or "IANA Script Registry" (both somewhat misleading   terms), is maintained by IANA at the request of ICANN.  It is used to   provide a central documentation repository of the IDN policies used   by top level domain (TLD) registries who volunteer to contribute to   it and is used in conjunction with ICANN Guidelines for IDN use.   It is not an IETF-managed registry and, while the protocol changes   specified here may call for some revisions to the tables, IDNA2008   has no direct effect on that registry and no IANA action is required   as a result.11.  Security Considerations11.1.  General Security Issues with IDNA   This document is purely explanatory and informational and   consequently introduces no new security issues.  It would, of course,   be a poor idea for someone to try to implement from it; such an   attempt would almost certainly lead to interoperability problems and   might lead to security ones.  A discussion of security issues with   IDNA, including some relevant history, appears in the Definitions   document [RFC5890].12.  Acknowledgments   The editor and contributors would like to express their thanks to   those who contributed significant early (pre-working group) review   comments, sometimes accompanied by text, Paul Hoffman, Simon   Josefsson, and Sam Weiler.  In addition, some specific ideas were   incorporated from suggestions, text, or comments about sections that   were unclear supplied by Vint Cerf, Frank Ellerman, Michael Everson,   Asmus Freytag, Erik van der Poel, Michel Suignard, and Ken Whistler.   Thanks are also due to Vint Cerf, Lisa Dusseault, Debbie Garside, and   Jefsey Morfin for conversations that led to considerable improvements   in the content of this document and to several others, including BenKlensin                       Informational                    [Page 39]

RFC 5894                     IDNA Rationale                  August 2010   Campbell, Martin Duerst, Subramanian Moonesamy, Peter Saint-Andre,   and Dan Winship, for catching specific errors and recommending   corrections.   A meeting was held on 30 January 2008 to attempt to reconcile   differences in perspective and terminology about this set of   specifications between the design team and members of the Unicode   Technical Consortium.  The discussions at and subsequent to that   meeting were very helpful in focusing the issues and in refining the   specifications.  The active participants at that meeting were (in   alphabetic order, as usual) Harald Alvestrand, Vint Cerf, Tina Dam,   Mark Davis, Lisa Dusseault, Patrik Faltstrom (by telephone), Cary   Karp, John Klensin, Warren Kumari, Lisa Moore, Erik van der Poel,   Michel Suignard, and Ken Whistler.  We express our thanks to Google   for support of that meeting and to the participants for their   contributions.   Useful comments and text on the working group versions of the working   draft were received from many participants in the IETF "IDNABIS"   working group and a number of document changes resulted from mailing   list discussions made by that group.  Marcos Sanz provided specific   analysis and suggestions that were exceptionally helpful in refining   the text, as did Vint Cerf, Martin Duerst, Andrew Sullivan, and Ken   Whistler.  Lisa Dusseault provided extensive editorial suggestions   during the spring of 2009, most of which were incorporated.13.  Contributors   While the listed editor held the pen, the core of this document and   the initial working group version represents the joint work and   conclusions of an ad hoc design team consisting of the editor and, in   alphabetic order, Harald Alvestrand, Tina Dam, Patrik Faltstrom, and   Cary Karp.  Considerable material describing mapping principles has   been incorporated from a draft of the Mapping document   [IDNA2008-Mapping] by Pete Resnick and Paul Hoffman.  In addition,   there were many specific contributions and helpful comments from   those listed in the Acknowledgments section and others who have   contributed to the development and use of the IDNA protocols.14.  References14.1.  Normative References   [RFC3490]    Faltstrom, P., Hoffman, P., and A. Costello,                "Internationalizing Domain Names in Applications                (IDNA)",RFC 3490, March 2003.Klensin                       Informational                    [Page 40]

RFC 5894                     IDNA Rationale                  August 2010   [RFC3492]    Costello, A., "Punycode: A Bootstring encoding of                Unicode for Internationalized Domain Names in                Applications (IDNA)",RFC 3492, March 2003.   [RFC5890]    Klensin, J., "Internationalized Domain Names for                Applications (IDNA): Definitions and Document                Framework",RFC 5890, August 2010.   [RFC5891]    Klensin, J., "Internationalized Domain Names in                Applications (IDNA): Protocol",RFC 5891, August 2010.   [RFC5892]    Faltstrom, P., "The Unicode Code Points and                Internationalized Domain Names for Applications (IDNA)",RFC 5892, August 2010.   [RFC5893]    Alvestrand, H. and C. Karp, "Right-to-Left Scripts for                Internationalized Domain Names for Applications (IDNA)",RFC 5893, August 2010.   [Unicode52]  The Unicode Consortium.  The Unicode Standard, Version                5.2.0, defined by: "The Unicode Standard, Version                5.2.0", (Mountain View, CA: The Unicode Consortium,                2009. ISBN 978-1-936213-00-9).                <http://www.unicode.org/versions/Unicode5.2.0/>.14.2.  Informative References   [IDNA2008-Mapping]                Resnick, P. and P. Hoffman, "Mapping Characters in                Internationalized Domain Names for Applications (IDNA)",                Work in Progress, April 2010.   [RFC0952]    Harrenstien, K., Stahl, M., and E. Feinler, "DoD                Internet host table specification",RFC 952,                October 1985.   [RFC1034]    Mockapetris, P., "Domain names - concepts and                facilities", STD 13,RFC 1034, November 1987.   [RFC1035]    Mockapetris, P., "Domain names - implementation and                specification", STD 13,RFC 1035, November 1987.   [RFC1123]    Braden, R., "Requirements for Internet Hosts -                Application and Support", STD 3,RFC 1123, October 1989.   [RFC2136]    Vixie, P., Thomson, S., Rekhter, Y., and J.  Bound,                "Dynamic Updates in the Domain Name System (DNS                UPDATE)",RFC 2136, April 1997.Klensin                       Informational                    [Page 41]

RFC 5894                     IDNA Rationale                  August 2010   [RFC2181]    Elz, R. and R. Bush, "Clarifications to the DNS                Specification",RFC 2181, July 1997.   [RFC2277]    Alvestrand, H., "IETF Policy on Character Sets and                Languages",BCP 18,RFC 2277, January 1998.   [RFC2671]    Vixie, P., "Extension Mechanisms for DNS (EDNS0)",RFC 2671, August 1999.   [RFC2782]    Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for                specifying the location of services (DNS SRV)",RFC 2782, February 2000.   [RFC3454]    Hoffman, P. and M. Blanchet, "Preparation of                Internationalized Strings ("stringprep")",RFC 3454,                December 2002.   [RFC3491]    Hoffman, P. and M. Blanchet, "Nameprep: A Stringprep                Profile for Internationalized Domain Names (IDN)",RFC 3491, March 2003.   [RFC3743]    Konishi, K., Huang, K., Qian, H., and Y. Ko, "Joint                Engineering Team (JET) Guidelines for Internationalized                Domain Names (IDN) Registration and Administration for                Chinese, Japanese, and Korean",RFC 3743, April 2004.   [RFC3987]    Duerst, M. and M. Suignard, "Internationalized Resource                Identifiers (IRIs)",RFC 3987, January 2005.   [RFC4290]    Klensin, J., "Suggested Practices for Registration of                Internationalized Domain Names (IDN)",RFC 4290,                December 2005.   [RFC4343]    Eastlake, D., "Domain Name System (DNS) Case                Insensitivity Clarification",RFC 4343, January 2006.   [RFC4690]    Klensin, J., Faltstrom, P., Karp, C., and IAB, "Review                and Recommendations for Internationalized Domain Names                (IDNs)",RFC 4690, September 2006.   [RFC4713]    Lee, X., Mao, W., Chen, E., Hsu, N., and J.  Klensin,                "Registration and Administration Recommendations for                Chinese Domain Names",RFC 4713, October 2006.Klensin                       Informational                    [Page 42]

RFC 5894                     IDNA Rationale                  August 2010   [Unicode-UAX31]                The Unicode Consortium, "Unicode Standard Annex #31:                Unicode Identifier and Pattern Syntax, Revision 11",                September 2009,                <http://www.unicode.org/reports/tr31/tr31-11.html>.   [Unicode-UTS39]                The Unicode Consortium, "Unicode Technical Standard #39:                Unicode Security Mechanisms, Revision 2", August 2006,                <http://www.unicode.org/reports/tr39/tr39-2.html>.Author's Address   John C Klensin   1770 Massachusetts Ave, Ste 322   Cambridge, MA  02140   USA   Phone: +1 617 245 1457   EMail: john+ietf@jck.comKlensin                       Informational                    [Page 43]