Internet Engineering Task Force (IETF)                        J. Klensin
Request for Comments: 5894                                   August 2010
Category: Informational
ISSN: 2070-1721
        Internationalized Domain Names for Applications (IDNA):
                 Background, Explanation, and Rationale



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; see Section 2 of RFC 5741.

このドキュメントはインターネットエンジニアリングタスクフォース(IETF)の製品です。これは、IETFコミュニティの総意を表しています。これは、公開レビューを受けており、インターネットエンジニアリング運営グループ(IESG)によって公表のために承認されています。 IESGによって承認されていないすべての文書がインターネットStandardのどんなレベルの候補です。 RFC 5741のセクション2を参照してください。

Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at


Copyright Notice


Copyright (c) 2010 IETF Trust and the persons identified as the document authors. All rights reserved.

著作権(C)2010 IETF信託とドキュメントの作成者として特定の人物。全著作権所有。

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents ( 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.

この文書では、BCP 78と、この文書の発行日に有効なIETFドキュメント(に関連IETFトラストの法律の規定に従うものとします。彼らは、この文書に関してあなたの権利と制限を説明するように、慎重にこれらの文書を確認してください。コードコンポーネントは、トラスト法規定のセクションで説明4.eおよび簡体BSDライセンスで説明したように、保証なしで提供されているよう簡体BSDライセンスのテキストを含める必要があり、この文書から抽出されました。

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 Contents


   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  Context and Overview . . . . . . . . . . . . . . . . . . .  4
     1.2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  5
       1.2.1.  DNS "Name" Terminology . . . . . . . . . . . . . . . .  5
       1.2.2.  New Terminology and Restrictions . . . . . . . . . . .  6
     1.3.  Objectives . . . . . . . . . . . . . . . . . . . . . . . .  6
     1.4.  Applicability and Function of IDNA . . . . . . . . . . . .  7
     1.5.  Comprehensibility of IDNA Mechanisms and Processing  . . .  8
   2.  Processing in IDNA2008 . . . . . . . . . . . . . . . . . . . .  9
   3.  Permitted Characters: An Inclusion List  . . . . . . . . . . .  9
     3.1.  A Tiered Model of Permitted Characters and Labels  . . . . 10
       3.1.1.  PROTOCOL-VALID . . . . . . . . . . . . . . . . . . . . 10
       3.1.2.  CONTEXTUAL RULE REQUIRED . . . . . . . . . . . . . . . 11  Contextual Restrictions  . . . . . . . . . . . . . 11  Rules and Their Application  . . . . . . . . . . . 12
       3.1.3.  DISALLOWED . . . . . . . . . . . . . . . . . . . . . . 12
       3.1.4.  UNASSIGNED . . . . . . . . . . . . . . . . . . . . . . 13
     3.2.  Registration Policy  . . . . . . . . . . . . . . . . . . . 14
     3.3.  Layered Restrictions: Tables, Context, Registration, and
           Applications . . . . . . . . . . . . . . . . . . . . . . . 15
   4.  Application-Related Issues . . . . . . . . . . . . . . . . . . 15
     4.1.  Display and Network Order  . . . . . . . . . . . . . . . . 15
     4.2.  Entry and Display in Applications  . . . . . . . . . . . . 16
     4.3.  Linguistic Expectations: Ligatures, Digraphs, and
           Alternate Character Forms  . . . . . . . . . . . . . . . . 19
     4.4.  Case Mapping and Related Issues  . . . . . . . . . . . . . 20
     4.5.  Right-to-Left Text . . . . . . . . . . . . . . . . . . . . 21
   5.  IDNs and the Robustness Principle  . . . . . . . . . . . . . . 22
   6.  Front-end and User Interface Processing for Lookup . . . . . . 22
   7.  Migration from IDNA2003 and Unicode Version Synchronization  . 25
     7.1.  Design Criteria  . . . . . . . . . . . . . . . . . . . . . 25
       7.1.1.  Summary and Discussion of IDNA Validity Criteria . . . 25
       7.1.2.  Labels in Registration . . . . . . . . . . . . . . . . 26
       7.1.3.  Labels in Lookup . . . . . . . . . . . . . . . . . . . 27
     7.2.  Changes in Character Interpretations . . . . . . . . . . . 28
       7.2.1.  Character Changes: Eszett and Final Sigma  . . . . . . 28
       7.2.2.  Character Changes: Zero Width Joiner and Zero
               Width Non-Joiner . . . . . . . . . . . . . . . . . . . 29
       7.2.3.  Character Changes and the Need for Transition  . . . . 29
       7.2.4.  Transition Strategies  . . . . . . . . . . . . . . . . 30
     7.3.  Elimination of Character Mapping . . . . . . . . . . . . . 31
     7.4.  The Question of Prefix Changes . . . . . . . . . . . . . . 31
       7.4.1.  Conditions Requiring a Prefix Change . . . . . . . . . 31
       7.4.2.  Conditions Not Requiring a Prefix Change . . . . . . . 32
       7.4.3.  Implications of Prefix Changes . . . . . . . . . . . . 32
     7.5.  Stringprep Changes and Compatibility . . . . . . . . . . . 33
     7.6.  The Symbol Question  . . . . . . . . . . . . . . . . . . . 33
     7.7.  Migration between Unicode Versions: Unassigned Code
           Points . . . . . . . . . . . . . . . . . . . . . . . . . . 35
     7.8.  Other Compatibility Issues . . . . . . . . . . . . . . . . 36
   8.  Name Server Considerations . . . . . . . . . . . . . . . . . . 37
     8.1.  Processing Non-ASCII Strings . . . . . . . . . . . . . . . 37
     8.2.  Root and Other DNS Server Considerations . . . . . . . . . 37
   9.  Internationalization Considerations  . . . . . . . . . . . . . 38
   10. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 38
     10.1. IDNA Character Registry  . . . . . . . . . . . . . . . . . 38
     10.2. IDNA Context Registry  . . . . . . . . . . . . . . . . . . 39
     10.3. IANA Repository of IDN Practices of TLDs . . . . . . . . . 39
   11. Security Considerations  . . . . . . . . . . . . . . . . . . . 39
     11.1. General Security Issues with IDNA  . . . . . . . . . . . . 39
   12. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 39
   13. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 40
   14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 40
     14.1. Normative References . . . . . . . . . . . . . . . . . . . 40
     14.2. Informative References . . . . . . . . . . . . . . . . . . 41
1. Introduction
1. はじめに
1.1. Context and Overview
1.1. コンテキストと概要

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.

アプリケーションにおける国際化ドメイン名(IDNA)(参照クライアントアプリケーションのみLDHの構文を含む有効なDNSラベルであるASCII互換エンコーディング形式(「ACE」)にはUnicodeで表現いくつかのニーモニック文字列を変換できるようにする標準規格のコレクションです定義ドキュメント[RFC5890])。 IDNAによって使用されるACEラベルの特定の形態は、「ラベル」と呼ばれます。クライアントは、SO-ラベルがDNSへの拡張を必要としない、既存のDNSに低レベルのクライアントライブラリへのDNSサーバ、またはアップデートのアップグレードを正確A-ラベルを調べることができます。 A-ラベルはピュニコードアルゴリズム[RFC3492]によって生成文字の前の接頭辞「xn--」から認識可能です。従って、ユーザ・アプリケーションは、表示のためにA-ラベルを識別し、ユニコード(またはいくつかのローカルコード化文字セット)に変換することができます。

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.

IDNAの最初のバージョンは2003年に出版されたと(IETF仕事がそれに始まった年後)IDNA2008として知られている現在のバージョン、とそれを対比するIDNA2003とここに呼ばれています。 IDNAベース仕様[RFC3490]、NAMEPREP [RFC3491]、ピュニコード[RFC3492]とのstringprep [RFC3454]:IDNA2003は、4つのドキュメントから成ります。文書の現在のセットは、IDNA2008は、ピュニコードエンコーディング用以外のIDNA2003仕様のいずれかに依存しません。 「IDNA2008」への参照は、「これらの仕様」、または「これらの文書は、」個別の定義ドキュメント[RFC5890]に記載されている全体IDNA2008セットにしています。・ラベルに有効な文字は、表文書[RFC5892]に記載されているルールから識別されますが、有効性は非常に少数の例外を除いて、それらの文字のUnicodeプロパティから導出することができます。

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 that 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.

この文書では、専門用語、背景、および政策議論を含めIDNA2008のための情報のコンテキストを提供します。それは何の規範的な材料が含まれていません。 IDNA2008プロトコルへの適合のための仕様は、シリーズの他の文書に完全に表示されます。

1.2. Terminology
1.2. 用語

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
1.2.1. DNS「名前」の用語

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.


1.2.2. New Terminology and Restrictions
1.2.2. 新しい用語と制限事項

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.

これらの定義はまた、定義文書の図1に示されています。 R-LDHのラベルが含まれている「 - 」のラベルの先頭から第三及び第四の文字位置に。 IDNA対応アプリケーションでは、これらの予約ラベルのサブセットのみ、すなわちA-ラベルのサブセットを使用することを許可されています。 - ラベルは大文字と小文字を区別しない文字列「xn--」で始まるR-LDHラベルのサブセットです。このプレフィックスを負担するが、ラベルには、そうでない場合は、「フェイクA-ラベル」カテゴリに有効な秋ではありません。彼らはIDNAで指定されたラベルに任意の類似点を負いませんので、予約されていないラベル(NR-LDHラベル)が暗黙的に有効です。

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 (see Section 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
1.3. 目標

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.


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
1.4. IDNAの適用性と機能

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 (see Section 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 coding 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
1.5. IDNAメカニズムと処理のわかりやすさ

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 have 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 in Section 6 and in the Mapping document [IDNA2008-Mapping].


2. Processing in IDNA2008
IDNA2008 2.処理

IDNA2008 separates Domain Name Registration and Lookup in the protocol specification (RFC 5891, Sections 4 and 5 [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.

IDNA2008はプロトコル仕様(RFC 5891、セクション4および5 [RFC5891])でドメイン名登録およびルックアップを分離します。二つのプロセスの中で最も手順は似ていますが、分離ごとのレジストリ(DNSゾーン)制限や特別な処理は、登録時にではなく、検索時に適用される現在の慣行を反映しています。もう一つの重要な利点は、分離は、Unicodeの一つの特定のバージョンに凍結を避けるために許可された文字グループの増分追加を容易にすることです。

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.


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.

このセクションでは、テーブルドキュメントのアルゴリズムと文字のリストを確立するために使用されるモデル[RFC5892]の概要を提供し、名前とそこに使用されるカテゴリの適用可能性を説明しています。 PROTOCOL-VALIDカテゴリグループ(セクション3.1.1)内の文字を含めることは、それが無差別に使用することができることを意味しないことに留意されたいです。一部の文字も同様に適用されなければならない文脈ルールに関連付けられています。

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
3.1. 許可された文字とラベルの階層モデル

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.

インクルージョンモデルに移動するとのIDNに許可されている文字の一覧については、新しい仕様を必要とします。 IDNA2003では、文字の妥当性は、文脈に依存しないと永遠に固定されている(または標準が交換されるまで)。しかし、グローバルコンテキストに依存しないルールがいくつかの文字、Unicodeで「Join_Controls」と呼ばれている、特にものは、いくつかのスクリプトの合理的な利用を行うために必要な他の人には目に見える効果を持っていないので実用的でないことが証明されています。 IDNA2003は、それらを完全に破棄することにより、文字のこれらのタイプを禁止しました。私たちは今、いくつかの条件下では、これらの「建具」の文字が合法的にいくつかの言語やスクリプトのための便利なニーモニックを許可するために必要であることを合意しています。一般に、コンテキスト依存のルールは異なる使用又は異なっ異なるスクリプトを横切って知覚される文字(そうでなければ完全に禁止される一般文字)に対応し、列が普遍処理されない場合、標準の場合に、より適切に適用されることを可能にする手助け同じ方法。

IDNA2008 divides all possible Unicode code points into four categories: PROTOCOL-VALID, CONTEXTUAL RULE REQUIRED, DISALLOWED, and UNASSIGNED.



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 the 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] and Section 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.

PROTOCOL-VALIDカテゴリに配置されている文字は、それから削除したり再分類されることはないと予想されます。理論的には文字をUnicodeから除去することができますが、このような除去は、Unicodeの安定性の原則と矛盾だろう(UTR 39参照:Unicodeのセキュリティメカニズム[Unicode52]、付録Fを)ので、発生することはありません。

3.1.2. CONTEXTUALルールが必要に

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.

一部の文字は、いくつかのスクリプトの妥当なサポートのためのIDNでの一般的な使用には適さないが、必要になることがあります。二つの最も一般的に引用された例は、ZERO WIDTH JOINERとZERO WIDTH NON-JOINER文字(ZWJ、U + 200DとZWNJ、U + 200C)であるが、Unicodeが考慮されるため、それらがそうでない場合(典型的には禁止さになるため、他の文字は特別な処置を必要とするかもしれませんそれら句読点や特殊記号)が、限られたコンテキストで許可する必要があります。彼らは誤解を招くラベルを作成したり、ラベルマッチングと解釈で容認できない曖昧さを引き起こすために使用されているの例外的な危険をもたらすので、他の文字は、この特別な治療を与えられています。 Contextual Restrictions。コンテキストの制限

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.


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.

例えば、ライティングシステムの一部としてZWJおよび/またはZWNJを必要とするインド語スクリプトを扱うレジストリは文字が目に見える効果を持っており、彼らはどこではないと登録ルールを作るためにそれに応じて場所を理解することが期待されています。これとは対照的に、ラテン語やキリル文字を主に扱ってレジストリは、文字はその文字を含む登録を受け入れることは避けるべきですので、それらのスクリプトから引き出されたラベルに埋め込むとの結果についてはるかに少ない、少なくともラベルには、存在していることを積極的に認識していない可能性がありますラテン語やキリルスクリプトから文字を使用。 Rules and Their Application。ルールとその応用

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 Sections 2 and 3 of the Tables document [RFC5892]. That document also specifies the creation of a registry for future rules.



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, see Section 7.6) simply fall outside the conventions for typical identifiers (basically letters and numbers).

一部の文字は、IDNの中に使用するためには不適切であり、これらの文字が存在しないことを確認する必要がありますので、登録とルックアップ名のルックアップを実行する(すなわち、IDNA準拠のアプリケーションの両方のために除外され、それらが存在する場合、ラベルの文字列は拒否ではなく、に変換する必要があります・ラベルと見上げた。これらの文字のいくつかは、IDNを(例えば分数スラッシュ文字、U + 2044など)、様々なHEARTシンボル、それらのいくつか(例えば、しばらく、U + 2665、U +での使用に問題があります2661、およびU + 2765、セクション7.6を参照)は、単に一般的な識別子(基本的には文字と数字)の規則から外れます。

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.



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 same 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
3.2. 登録ポリシー

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 in Section 2.4 (Specific Character Adjustments) of Unicode Identifier and Pattern Syntax [Unicode-UAX31], especially Table 4 (Candidate Characters for Exclusion from Identifiers), and Section 3.1 (General Security Profile for Identifiers) in Unicode Security Mechanisms [Unicode-UTS39].


The requirement (in Section 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.


3.3. Layered Restrictions: Tables, Context, Registration, and Applications

3.3. レイヤード制限事項:テーブル、コンテキスト、登録、およびアプリケーション

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 Issues
4.1. Display and Network Order
4.1. ディスプレイとネットワーク注文

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 or cba.fed? Applications that are in deployment today are already diverse, and one can find examples of either choice.

ドメイン名は、常にネットワーク順序(コードポイントがプロトコルで送信された順)で送信されるが、それらは異なる表示順序(コードポイントが画面または紙の上に表示される順序)を有していてもよいです。ドメイン名は、通常、右から左に書かれた文字が含まれている場合はネットワーク順ではないが、表示順序が影響を受ける可能性があります。左から右と右から左にラベルがドメイン名内の隣接している場合は、さらに複雑になります。 Webブラウザ、メールクライアント、ホストされたWebアプリケーションおよび多くを含む - - 高度に局在化することができる表示順序についての決定は、ユーザエージェントの制御下で、最終的です。ドメイン名はabc.def、両方のラベルは右から左に書かれたスクリプトで表現されている、fed.cbaとして表示またはcba.fedべきか?今日の展開であるアプリケーションは、すでに多様であり、かつ1は、いずれかの選択の例を見つけることができます。

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.

IDNは、国際化リソース識別子(IRI)[RFC3987]に表示されたときに画像が再び変化します。 IRIや国際化電子メールアドレス、ドメイン名以外の要素が含まれています。例えば、アイリスは、このような「のhttp://」などのプロトコル識別子とフィールド区切り文字の構文含まれているかを「MAILTO:」電子メールアドレスは、ドメイン名からローカル部分を分離するために「@」が含まれていながら。

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 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
4.2. アプリケーションでの入力と表示

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.

アプリケーションは、任意の文字セットまたは文字符号化方式を使用して、ドメイン名を受け入れ、表示することができます。 IDNAプロトコルは、必ずしもユーザーとアプリケーション間のインタフェースには影響を与えません。 IDNA対応アプリケーションを受け入れ、2つの形式で国際化ドメイン名を表示することができる:アプリケーションによってサポートされる国際文字セット(S)として(すなわち、Uラベルの適切なローカル表現)及びA-ラベルとして。アプリケーションは、ラベルの表示を許可するかもしれないが、おそらくデバッグのために、特別な目的のためのインタフェースとして除き、これを行うには、ディスプレイの制約に対処することは推奨されません。一般的には、彼らができるようにすべきであるが、しかし、-ラベルのユーザー入力を促しません。 -ラベルは不透明と醜いであり、それらの上に悪質なバリエーションがユーザによって容易に検出されません。可能であれば、彼らが絶対に必要なとき、彼らはこのようにのみ公開されなければなりません。 IDNラベルのいずれかにすることができるので、ラベルまたはU-ラベルとして、アプリケーションは、合理的にディスプレイの好適な方法を選択するためのオプションを有していてもよいです。 U-ラベルをレンダリングすることは、通常はデフォルトでなければなりません。

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, 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.

プロトコルおよび仕様または文字セットのネゴシエーションを処理する方法を定義文書フォーマットでは、標識は、プロトコルまたは文書フォーマットによって可能に任意の文字セットでエンコードすることができます。プロトコルまたは文書形式が一つだけの文字セットを許可する場合、ラベルは、その文字セットに与えられなければなりません。もちろん、すべてではない文字セットが適切にすべてのラベルを表すことができます。 U-ラベルはその全体を表示できない場合、(情報の損失なし)のみの選択はA-ラベルを表示することであってもよいです。

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 (see Section 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 in Section 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.


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.

多くの場合、これらの禁止事項は、ユーザーが検索プロセスへの入力として入力することができるものには影響を与えません。これは、ローカル環境に適したいくつかの文字マッピングを実行するためのユーザインタフェースをサポートするシステムのための完全に合理的です。これは通常、IDNAの実際の呼び出しに先立って行われます。少なくとも概念的に、マッピングは、上記およびプロトコルドキュメント[RFC5891]で説明したユニコード変換の一部であろう。ユーザーが明確に文字の形が等価であることを理解するであろうした環境へのローカル - しかし、それらの変更はローカルのみのものとなります。システム間の交流で使用するためには、U-ラベルとA-ラベルが情報の損失なしに前後にマッピングすることができることをはるかに重要であると思われます。

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.

この戦略の一つ、具体的な、そして非常に重要なインスタンスは、ケースの折に発生します。 ASCIIのみのDNSでは、名前が検索され、ケースに依存しない方法で一致したが、実際のケースの折りたたみは発生しません。名前はように大文字または小文字の形態(又はそれらの任意の混合物)のいずれかでDNSに配置することができ、その形状が保持され、クエリに戻り、。名前が見上げたときIDNA2003は、(DNSでのみ小文字のIDNになる)、登録時に折りたたみケースを実行することにより、非ASCII文字列のその行動を近似します。

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.


4.3. Linguistic Expectations: Ligatures, Digraphs, and Alternate Character Forms

4.3. 言語期待:合字、有向グラフ、および代替文字フォーム

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.

言語の数は、単一の音素が2つの文字を使用して書かれているアルファベットのスクリプトを使用し、「薬局」と「電話」で例えば、「pH値」、「有向グラフ」と呼ばれます。 (このような文字は、「トップハット」のように、有向グラフを形成することなく連続して表示されること。)特定の有向グラフは、異なる音素を表現するために連続的に使用される場合、それらがあろうよりも互いに接近して2つの文字を設定することにより、タイポグラフィ示すことができます。いくつかの二重字は完全に合字として参加しています。例えば、単語「百科事典」は時々U + 00E6ラテンLIGATURE AEで設定されています。結紮有向グラフの形式が指定されたスクリプトを使用するすべての言語で同じ解釈を持っている場合は、Unicode正規化の適用は、一般的な違いを解決し、一致するようになります。彼らは異なる解釈を持っている場合、マッチングはおそらく、レジストリレベルで選ばれた他の方法を利用しなければならない、またはユーザーがマッチングが発生しないことを理解することが教育を受けなければなりません。

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, unlike 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.

問題の本質は、「AE」リガチャーは、29文字の27番目の文字はラテンアルファベットを拡張され、ノルウェー語で多くの言葉で説明することができます。これは、「AE」は、現在の正射投影基準に従って置換することができないためU + 00E4ラテン小文字A WITH分音符号、(また、29個の文字を含む)スウェーデンのアルファベットの28文字に相当します。その文字(U + 00E4)は、文字「umlauted」の北欧の言語とは異なり、2文字列「AE」は、通常、十分に許容される代替正字として扱われ、ドイツ語のアルファベットの一部です。逆しかし、真実ではない、そしてこれらの2つの文字は、必ずしも「umlauted A」に結合することができません。これはまた別のドイツ語の文字、例えば、著者「ゲーテ」の名前を書き込むために使用することはできません、「umlauted O」(U + 00F6 LATIN SMALL LETTER O WITHダイエレシス)に適用されます。また、「分音記号付き」のように、それは正しく「OE」として表現することはできませんし、それが表現されたノルウェーのアルファベット、ではなく、「ダイエレシスとO」として、スウェーデンのアルファベットの文字ですが、 "としてO」、U + 00F8を引き下げました。

Some of the ligatures that have explicit code points in Unicode were given special handling in IDNA2003 and now pose additional problems in transition. See Section 7.2.

Unicodeで明示的なコードポイントを持っている合字の中には、IDNA2003で特別な取り扱いを与え、今の移行で新たな問題を提起しました。 7.2節を参照してください。

Additional cases with alphabets written right to left are described in Section 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.

IDNAまたはDNSを使用できません情報 - マッチングと比較アルゴリズムの選択は、多くの場合、使用している言語、コンテキスト、またはその両方についての情報を必要とします。その結果、IDNA2008は、任意の特別な方法で組み合わせた文字を治療する試みません。ラベルが登録されるべき言語のコンテキストを認識しているレジストリ、ここでその言語が時々(または常に)が組み合わされた形態に相当するような2つの文字列を扱い、「変異体」モデルを適用する重大な考慮を払うべきです[RFC3743]、[RFC4290]は、または関連する文字列から生じるユーザの混乱や詐欺の機会を減少させるために、全体のいずれかの形式の登録を禁止する異なる当事者に登録されます。

4.4. Case Mapping and Related Issues
4.4. ケースマッピングと関連する問題

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- 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.

DNSサーバーは、IDNのを解析に関与し得ることはありませんので、彼らはケース非依存のマッチングを行うことはできません。このように、検索や登録に分離し、サーバ側でマッチングを行う例を維持することは、IDNAまたは任意の同様のアプローチでは不可能です。必要に応じてのみ、ケースによって異なると考えられている文字のマッチングは、それがASCII-のみDNSクライアントによって行われていなかったにもかかわらず、IDNA検索を呼び出すプログラムで、行われなければなりません。この状況は、IDNA2003に認められ、IDNA2008には何も根本的にそれを変更していないか、そうすることができます。 IDNA2003では、すべての文字は、標準化された場合、ステップでクライアントによって折り畳まれてマッピングされます。

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 in Section 7.2.

でも、一般的に小文字の区別をサポートするスクリプトでは、一部の文字は大文字のフォームを持っていません。例えば、Unicodeの場合、折り畳み動作はギリシャ最終形態シグマ(U + 03C2)は、内側フォーム(U + 03C3)にマッピングし、エスツェット(ドイツシャープS、U + 00DF)に「SS」をマッピングします。 U + 03C3の大文字は大文字のシグマ(U + 03A3)と「SS」はASCII文字列であるため、これらのマッピングは、いずれも可逆的です。 IDNA2008許可、いくつかの非互換性、ケースの折り畳みを回避し、自分自身としてこれらの文字を処理することにより、この分野では少しより多くの柔軟性のリスクが。一方向のマッピングを扱うためのアプローチは、セクション7.2で説明されています。

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. See Section 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.

IDNA2003マップ最終シグマやエスツェット他の文字に、そして逆のマッピングが可能になることはありませんので、最終的なシグマやエスツェットどちらがIDNA2003 IDNのACE形式でもそれ由来のネイティブな文字(U-ラベル)の形で表すことができます。 IDNA2008では、両方の文字がIDNに使用することができますので、これらの文字を含む任意のU-ラベルのルックアップに使用A-ラベルは今異なっています。変更するにはIDNAの接頭辞を必要とするかもしれない変化のどのような種類の議論については、セクション7.1を参照してください。拡張された議論の後、IDNABISワーキンググループは、これらの文字の変更は、プレフィックスの変更を正当化しませんでしたコンセンサスに来ました。

4.5. Right-to-Left Text
4.5. 右から左のテキスト

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 extended


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].

(順番に、アラビア文字から派生し)ターナ文字スクリプトで記述されているヘブライ語のスクリプト、およびディベヒ語(モルディブの公用語)、。 IDNA2008は、右から左へのスクリプトとその文字のための新しいルールのセットで、最終的な結合文字の制限を削除します。これらの新しいルールは、双方向のドキュメント[RFC5893]で指定されています。

5. IDNs and the Robustness Principle
5. IDNのと堅牢性の原則

The "Robustness Principle" is often stated as "Be conservative about what you send and liberal in what you accept" (see, e.g., Section 1.2.2 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.

「堅牢性の原則」は、しばしば「あなたは、あなたが受け入れるものの中に送信し、リベラルかについて保守的である」と記載されている(参照、例えば、アプリケーション層のホスト要件仕様のセクション1.2.2 [RFC1123])。この原則はIDNAに適用されます。すべての登録と便利なのIDNのソース(「送信者」)としてレジストリに原則を適用するには、レジストリは、彼らが登録し、インターネットに出すかについて保守的に責任があります。 IDNのがうまく機能するためには、ゾーン管理者(レジストリ)を持っており、登録されているかについて賢明な政策を必要としなければならない - 保守的な政策 - そしてそれらを実装して適用します。

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.

逆に、検索アプリケーションが明確にグローバル(プロトコル)の規則に違反したラベルを拒否することが期待されている(誰も真剣に受け入れられているものでリベラルであることは愚かであること必要であることを主張していません)。 1は、スクリプトまたはロケールに敏感なもので、過去、このようなグローバルなルールやお得な情報を取得しますしかし、一度、そのゴミを想定する必要があるDNSの中に置かれていない、すなわち、1つは1つが中まで見て喜んでいるかについてリベラルでなければなりませんむしろ登録することが許可されている必要があるかどうかについての推測よりもDNS。

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.


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.

システム間で送信された - - 「ワイヤー上」に置かれている任意のドメイン名のための強力な引数がありますが、ラベルのゼロ曖昧な形であることにするが、それはプロセスのドメイン名が遭遇するプログラムことは避けられませんU-ラベルまたは変異型。

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.

IDNAプロトコルを実装するアプリケーション[RFC5891]は常に任意のユーザ入力を取得し、Unicodeコードポイントの集合に変換します。そのユーザ入力が考慮されるべき異なる変換プロセスとすべて(例えば、マイクに向かって話すデジタイザのある種に手で書かれたキーボード上に入力した音声によって解釈、いくつかの異なる入力方法のいずれかによって取得されてもよいです-to-テキストエンジン、など)。任意の特定のユーザ入力を取得し、Unicodeコードポイントにそれをマッピングするプロセスは、単純なものであってもよい。ユーザが抑え「シフト」キーなどの任意の改質剤なしで、米国英語のキーボードの「」キーを打つ場合、ラテン小文字A(「A」)を描くためには、(おそらくほとんどの)多くの近代的なオペレーティングシステムの入力方法は、呼び出し元のアプリケーションに単一オクテットで符号化されたコードポイントU + 0061を生成します。

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.

時には、このプロセスはやや複雑です:ユーザーは、それ以上の長音記号付きラテン小文字Aを描画するために、「A」キーを打つが続く組み合わせ長音記号を表現するために、キーの特定のセットを打つかもしれません。オペレーティング・システムは、ユーザによって選択された入力方法、およびアプリケーションは、入力された方法で通信するにもパラメータに応じて、結果は、U + 0101コード・ポイントであるかもしれない(UTF-8で2つのオクテットとして符号化またはUTF- 16、UTF-32で4つのオクテット、等)、コード・ポイントU + 0061のコードポイントU + 0304(再び、使用されるエンコードに依存し、三個の以上のオクテットで符号化された)、あるいはコード・ポイントU +続いFF41コードポイントU + 0304に続く(および何らかの形で符号化されました)。これらの例は、その文字セットのUnicodeコードポイントを使用していないオペレーティングシステムや入力方法の問題を脇に残して。

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.

IDNA2003は、Unicodeコードポイントのセットに(方法のメカニズムが使用されたどのような入力を介して)入力をユーザから採取されたことにより、モデル、マッピングされたを使用し、その後さらにNAMEPREPプロファイル[RFC3491]を使用して、Unicodeコードポイントの集合にマッピング。この手順では、二つの別々のマッピングステップがある:最初に、マッピングは、(オペレーティングシステム、アプリケーション、またはいくつかの組み合わせによって制御されるかもしれない)入力方法によって行われ、その後、IDNAのNAMEPREP部によって実行される第2のマッピングプロトコル。 NAMEPREPで行わマッピングが他の文字、特定の正規の一部の文字を再マップする特定のマッピングテーブル、および禁止文字のセットを含みます。

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.


7. Migration from IDNA2003 and Unicode Version Synchronization
7.1. Design Criteria
7.1. 設計基準

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
7.1.1. IDNAの妥当性基準の概要と考察

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 in Section 7.6.)

文字O「文字」、文字、数字、またはいくつかの言語の単語を書くために使用される他のコードポイントを形成するのに必要なマークです。記号は、文字を描画し、様々な表記文字が恒久的に除外されることが意図されています。彼らは「文字、数字、およびハイフン」の一般的な原則を越えたドメイン名の拡大を正当化するには、インターネット事業や国際化に十分に重要であるという証拠はありません。 (シンボル決定のための追加の議論と論拠は、7.6節に表示されます。)

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 in Section 7.7.


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].

O NFKCの現在のバージョンによって別の文字にマッピングされている任意の文字(登録またはルックアップのいずれかのために)IDNAへの入力として禁止されています。いくつかの例外を除いて、この原則はNAMEPREP [RFC3491]で別にマッピングされた任意の文字を除外します。

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
7.1.2. 登録のラベル

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 in Section 4 of the Protocol document [RFC5891]):

DNSゾーンに登録された任意のラベルを検証する必要があります - すなわち、そのラベルの基準が満たされなければならない - 意図したとおりに動作するアプリケーションのためのために。この原則は新しいものではありません。 DNSが最初に配備されたので、例えば、ゾーン管理者は、名前がこれらの要件を期待するアプリケーションによって課される「ホスト名」要件[RFC0952]に合致することを検証することが期待されています。このような特殊なサービス・ロケーション・フォーマット[RFC2782]の後添加のような他のアプリケーションコンテキストは、ゾーン管理者に新たな要件を課しました。 IDNのが含まれていますゾーンの場合、Unicodeバージョン-自立のための支援は、ゾーンに配置されたすべての文字列の制限が必要です。特に、そのようなゾーンの(正確な規則は、プロトコルドキュメント[RFC5891]のセクション4で表示されます)。

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 in Section 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 or resolving DNS labels, especially IDN DNS labels, must be able to assume that applicable registration rules were followed for names entered into the DNS.

このモデルでは、レジストリテーブルは、新しいスクリプトや新キャラクターの他のセットを有効にするために(ユニコード関連テーブルと許可IDN文字のテーブルの両方)を更新する必要があります。それはそれらをサポートすることを希望するまでとしない限り、レジストリは、ユニコードの新しいバージョン、または新しく認可文字によって影響されることはありません。ラベルを検索するために必要とされるよりも小切手のより広範なセット - ゾーン管理者はIDNAだけでなく、そのローカルポリシーの有効性を検証する責任があります。システム見上げまたはDNSラベル、特にIDNのDNSラベルを解決するには、名前がDNSに入力するために適用される登録規則が守られたと仮定することができなければなりません。

7.1.3. Labels in Lookup
7.1.3. ルックアップでラベル

Any application processing a label through IDNA so it can be looked up in a DNS zone is required to (the exact rules appear in Section 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".


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
7.2. 文字解釈の変更

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
7.2.1. 文字の変更:エスツェットと最終シグマ

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 (see Section 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).

小文字の区別を行い、これらのスクリプトでは、明白かつユニークな大文字小文字は歴史的に1、またはその逆に合わせて利用できなかったために、いくつかの文字があります。これらの文字は、IDNA2003ためのstringprepテーブルを構築する際に使用されるマッピングは、異なる文字または文字のセットを生成、(Unicode標準[Unicode52]のセクション5.18を参照)ユニコードtoCaseFold操作を使用して行きました。これらの動作は可逆的ではなく、伝統的な大文字または小文字の変換よりもさらに多くの情報を失うが、比較のために、これらの変換よりも有用です。このタイプの2つの注目すべき文字は、ドイツ語の文字エスツェット(シャープS、U + 00DF)とギリシャ語の最終形態シグマ(U + 03C2)です。前者は、ASCII文字列「SS」、中間(小文字)は、Sigma(U + 03C3)に後者に折り畳まれた場合です。

7.2.2. Character Changes: Zero Width Joiner and Zero Width Non-Joiner
7.2.2. 文字の変更:ゼロ幅ジョイナーとゼロ幅非ジョイナー

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 in Section 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.

IDNA2003は、効果的に、彼らが現れた任意のラベルからこれらの文字をドロップし、その文字列と同じようにそれらを含む文字列を処理する、何にもZERO WIDTH JOINER(ZWJ、U + 200D)とZERO WIDTH NON-JOINER(ZWNJ、U + 200C)の両方をマッピングししませんでした。上記3.1.2項で述べたように、これらの文字は、特定のスクリプトのために多くの合理的なニーモニックを書くために不可欠です。しかし、文脈上の制約で、IDNA2008で有効なものとして、それらを処理し、エスツェットファイナルシグマに存在しているとほぼ同じ問題を提起:IDNA2003の下で有効であった文字列は、下の同じ文字列よりも、ラベル、および異なるA-ラベルなどの異なる解釈を持っていますこの新しいバージョン。

7.2.3. Character Changes and the Need for Transition
7.2.3. 文字の変更と移行の必要性

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.


7.2.4. Transition Strategies
7.2.4. 移行戦略

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:

新しいキャラクターやIDNAの以前のバージョンでは、そのマッピングされたフォームから既存の文字の解釈の変更の導入にはいくつかのアプローチがあります。 「SS」を含む文字列は、単純に意図されている可能性が:IDNA2003でのToUnicode(もしToASCII())の翻訳だけでなく、後にこれらのラベルの形式が有効なままが、登録者が意図したものの強い指示を提供しないため、移行の問題は複雑ですその文字列であるか、またはエスツェットを含むように意図されている可能性が。小文字のシグマを含む文字列は、最終シグマ(1、文字列内の位置に基づいてヒューリスティック推測をするかもしれませんが、単語を連結してラベルを形成する長い伝統が、このような経験則が不安定になり)、およびZWJかを含まない文字列を含むように意図されている可能性がZWNJは、それらを含むように意図されている場合があります。同様の遷移のために、過去にレジストリによって使用されているすべてが完全性任意の好みや主張、これらのいくつか、しないと、以下のとおりです。

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.

4. IDNA2003に従って解釈場合、同じA-ラベルを生成する追加の文字列の登録が全く許可のみ既に名前の1つを有する登録者によって許可されていないかれる「変異体」アプローチの異なる形態を採用。

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 as registries that already had a considerable number of labels did when IDNs were first introduced.


7.3. Elimination of Character Mapping
7.3. 文字マッピングの排除

As discussed at length in Section 6, IDNA2003, via Nameprep (see Section 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 in Section 6 and specific recommendations are made in the Mapping document [IDNA2008-Mapping].

NAMEPREPを介して、第6、IDNA2003の長さで論じたように、関連したものに多くの文字をマッピングし(セクション7.5参照)。これらのマッピングは、もはやIDNA2008における要件として存在しません。これらの仕様は強くのみ、ラベルやU-ラベルはプロトコルの文脈で使用され、より一般的に実用的な限りすることを好みます。 IDNA2008は、ルックアップ・アプリケーションにユーザ入力時にいくつかのマッピングが適切で所望される状況を予想しません。問題はセクション6に記載されている、特定の推奨事項は、マッピング・ドキュメント[IDNA2008マッピング]で作られます。

7.4. The Question of Prefix Changes
7.4. プレフィックスの変更の質問

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.

(IDNA2003で使用される「xn--」、)IDNA ACEプレフィックスの変更を必要とした条件は、地域社会への大きな懸念でした。アルゴリズムは、登録の後続の移行の間に深刻なあいまいさを作成していた方法で変更された場合は、プレフィックスの変更は明らかに必要であったであろう。このセクションでは、プレフィックスの変更が必要であったと思われる条件とその変更の影響についてワーキンググループの結論をまとめました。

7.4.1. Conditions Requiring a Prefix Change
7.4.1. プレフィックスの変更を必要とする状態

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 (see Section 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 in Section 7.4.2, below.

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.

ブロックオフセットのピュニコード機構がもはや高い番号の面とのブロックを参照しないであろうように、文字の4 A十分に大きな数は、Unicodeに添加しました。この条件は、今後数年間に発生していなくても、長期的かつ一定ではほとんどありません。

7.4.2. Conditions Not Requiring a Prefix Change
7.4.2. プレフィックスの変更を必要としない条件

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
7.4.3. プレフィックスの変更の意味合い

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.


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
7.5. 文字列前の変更との互換性

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.

NAMEPREP仕様[RFC3491]、IDNA2003の重要な部分は、のstringprep [RFC3454]のプロファイルです。 NAMEPREPがIDNAに特定のstringprepプロファイルであるが、のstringprepは、他のプロトコルの数によって使用されます。文字列準備がIDNA2008によって変更されたとした、のIDNの取り扱いを改善するために、これらの変更は、彼らが識別および認証プロトコルに影響を与えた最も顕著な場合には、非DNSの使用のために問題を引き起こす可能性があります。 IDNA2008のいくつかの要素がIDNA2003が許可IDNA2003または禁止する文字列で禁止文字列に解釈を与えます。に記載されているようにこれらの要素は、表文書[RFC5892]に新しいインクルージョン情報、登録または参照(セクション3)のための入力として使用できる文字の数の減少、および右から左への文字列の処理にも変更を含みます双方向ドキュメント[RFC5893]。 IDNA2008は全くNAMEPREPや文字列準備を使用していないので、他のプロトコルへの副作用の変更はありません。

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.

IDNの円滑かつ理解使用するために必要な制約のいくつかは、他の状況で望ましくないまたは望ましくないかもしれないので、様々なセキュリティプロトコルの処理とは別のIDNA処理を維持することが特に重要です。例えば、良いパスワードまたはパスフレーズのための基準が望ましいIDNのためのものとは非常に異なっている:ドメイン名は、通常、簡単に思い出に残るべきである一方、パスワードは、推測するのは難しいはずです。同様に、国際化のSCSI(Small Computer System Interface)識別子と他のプロトコルコンポーネントは、IDNのとは異なる要件を持っている可能性があります。

7.6. The Symbol Question
7.6. シンボルの質問

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. This 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.

1.他の場所で議論したように、元のIDNA仕様は、できるだけ多くのUnicode文字のIDNに、直接、またはマッピングを介して他の文字に、許可されるべきであると仮定しました。非常によく、インターネットを務めてきた - - ユニコードベースではなく、ASCIIベースにこの仕様はオリジナルの「ホスト名」のルール(LDH、定義文書[RFC5890]を参照)から外挿する、包含モデルで動作します。

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.

特定のグリフ、シンボルに一致するかどうかには一般的な合意が存在しない可能性があるため、2シンボル名は文字よりも問題が多いです。命名のための統一規則はありません。そのような輪郭のような変形は、固体、斜線形態が、または存在しなくてもよいです。等々。それは「私が愛して...」と読まれるかもしれないロゴに表示される可能性がありますようほんの一例として、「心」のシンボルを考えます。 「私は...大好き」またはユーザーは、このようなロゴを読むかもしれませんが、「私は心...」、Unicodeで作られたコーディングの区別のかなりの知識が複数の「心」の文字があることを知っておく必要がある(たとえば、 、U + 2665、U + 2661、およびU + 2765)とどのようにそれを記述します。文字列を読み上げた後に理解またはリスナーによって転写されることが予想されている場合、これらの問題が特に重要です。

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.

4本の簡単な例として、1がラベルに「心」や「星」記号を使用していたと仮定します。それらの名前は、(実際のUnicode名ははるかに資格が必要です)命名のユニコードシステムではあいまいであるので、これは問題があります。ユーザーまたは登録-だろうが知る方法がない - コード表の不在慎重に検討を - それは曖昧であるかどうか(例えば、複数の「心」の文字がある場合)またはありません。逆に、仮想のラベルを見て、ユーザはそれを読むかどうかわからない - 「心」として、「愛」として、「黒ハート」として、またはのいずれかと - 音声で同僚に送信してみてください下記の他の例。

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.

5.実際の状況はこれよりさらに悪くなります。何とか区別を探すために知らなくても、U + 2665とU + 2765の心とU + 2606とU + 2729の星を区別するために、通常、カジュアル、利用者のための可能な方法はありません。私たちは、白いハート(U + 2661)と、いくつかの黒の心を持っています。その結果、心臓を含むものとしてラベルを記述することは絶望的にあいまいです:私たちは、それが心のように見えるか、その名前に「心」を持っているいくつかの文字のいずれかが含まれていることを知ることができます。 「スクエア」は、場所の名前の人気の一部である都市では、1は、だけでなく、ほかのラベルに正方形の記号を使用することもできますし、ハートや星があるよりも、Unicodeで様々な味のはるかに多くの正方形があります。

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
7.7. 未割り当てコードポイント:Unicodeのバージョン間の移行

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新たに割り当てられたコード・ポイントは、表文書のルールによって禁止される文字[RFC5892](例えば、互換文字など)に関連するかどうか、および十分な信頼性を有する、事前に知ることができません。 IDNA2003では、以来、NFKCに直接依存がない(文字列準備のテーブルのエントリの多くはNFKCに基づいていますが、IDNA2003は、文字列準備にのみ依存)、互換性の文字の割り当ては、いくつかの奇妙な状況が生じる可能性がありますが、それは問題ではないでしょう。キャラクター固有の例外をしないと、互換性の文字が許可されないIDNA2008では、見上げたことが割り当てられていない文字を含む文字列を許可することは禁止さで文字が見上げていないことを原則に違反することになります。

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 that 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.

上記に特定の技術的な移行の問題とは無関係に、簡単に使用するか、より良い特定の言語に対応できるようにする既存のスクリプト文字の任意の添加は、遷移の問題につながる可能性があることを観察することができます。そのような添加は、特定の文字列、必ずしも新しい文字が存在しない可能性がユニコードの以前のバージョンとは異なるであろうキーボード遷移モジュールにおいて、例えば、反射された変更を書き込むための好ましい形態を変更してもよいです。ラベルにアクセスしようとする試みは、古い慣習をラベルに使用されたかどうか、レジストリのアクションを必要とする、古いか新しい規則のいずれかを使用することができますので、これは本来の移行の問題を作成します。移行メカニズムを検討する必要性がより良い書き込みシステムに対応するために、ユニコードの進化に固有のものであるとのIDNがDNSまたはどのようにそれらのメカニズムのバージョン間の遷移が発生して表現されているかとは無関係です。このタイプの遷移のための要件は、Unicode 5.1.0にマラヤーラムChilluを添加することによって示されています。

7.8. Other Compatibility Issues
7.8. その他の互換性の問題

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 not 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.

2003 IDNAモデルは、それが開発されたコンテキストのいくつかの奇妙なアーティファクトを含んでいます。これらの、多くは、すべてではないが、登録プロセスを登録する「ソース」名(IDNAとNAMEPREPを通って処理されていない名前)を許可する場合は特に、エクスプロイトの潜在的な道です。一例として、ドイツ語で使用される文字エスツェット、以来、シーケンス「SS」にIDNA2003でマッピングされているのではなく、それ自体として保持するか禁止されて、その文字を含む文字列が、それはASCIIでそうで、本当にIDNではありません(U-ラベルの意味において、上記で定義)。 NAMEPREPはエスツェットをマッピングした後、その結果は、ASCII文字列であるので、それはxn--プレフィックスを得ることはありませんが、ユーザーに表示できる文字列は、IDNのように見えます。 IDNA2008は、このアーティファクトを排除します。文字は、それ自体として許可されるか、またはそれが禁止されています。のみ、特定の言語や文化的な文脈で意味をなす特別な場合には、適切な場合には、ローカリゼーションの問題として扱うことができます。

8. Name Server Considerations
8.1. Processing Non-ASCII Strings
8.1. 非ASCII文字列を処理

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 in RFC 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.

既存のDNSサーバは、のIDNの非ASCII形式を処理するためのIDNAルールを知っているので、それらから保護する必要はありません。彼らはIDNAに先行するので名前はDNSサーバのデータベース(例えば、マスターファイル(RFC 1034で説明したように)と、DNS更新メッセージ[RFC2136])を入力することができ、それを通してすべての既存のチャネルはIDNAを認識することができませんでした。このドキュメントの他のセクションでは、このようなチャネルを通じてDNSサーバのデータベースに入る国際化ドメイン名がすでに同等のASCII A-ラベル形式に変換されていることを確実にすることによって、必要なシールドを提供します。

Because of the distinction made between the algorithms for Registration and Lookup in Sections 4 and 5 (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
8.2. ルートおよびその他のDNSサーバーの考慮事項

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, 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.

DNSラベルと完全修飾ドメイン名を識別し、インターネット上のリソースを参照するのに役立つニーモニックを提供しています。 IDNのは、西ヨーロッパとローマ由来のもの以外の言語と文字に基づいてそれらのセットが含まれるように、これらのニーモニックの範囲を拡大します。しかし、ドメイン「名前」には、一般的には、任意の言語の単語ではありません。文字セットと言語でIETFポリシー(BCP 18 [RFC2277])の推奨は、言語識別は、言語固有のコンテキストを提供するために使用される状況に適用可能です。 DNSは、対照的に、グローバルおよび国際的で、最終的に言語とは何の関係もありません。一般のIDNに言語(または類似のコンテキスト)を添加すること、または特にDNSマッチングに、DNSプロトコル自体に非常に重要な変更されるであろう、DNSに文脈依存マッチングを意味するであろう。また、ユーザーがそのラベルを検索するために、特定のラベルに関連付けられている言語を識別するために必要となることを意味します。多くのラベルがどの言語の単語ではなく、一部が複数の単語かもしれないので、その知識は、一般的に利用できません。

10. IANA Considerations
10. IANAの考慮事項

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
10.1. IDNAキャラクターレジストリ

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.


10.2. IDNA Context Registry
10.2. IDNAコンテキストレジストリ

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 in Section 3.1.2). The details for those rules appear in the Tables document.


10.3. IANA Repository of IDN Practices of TLDs
10.3. TLDのIDNプラクティスのIANAリポジトリ

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 Considerations
11.1. General Security Issues with IDNA
11.1. IDNAとセキュリティ問題

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 Ben

エディタと貢献は重要な初期(事前ワーキンググループ)時には、テキスト、ポール・ホフマン、サイモンJosefsson氏、そしてサム・ワイラーを伴うレビューコメントを、貢献した人たちに感謝の意を表したいと思います。また、いくつかの具体的なアイデアを提案、テキスト、またはヴィントン・サーフ、フランクEllerman、マイケル・エバーソン、Asmusフライターク、エリックのファンデPoel、ミシェルSuignard、およびケンウィスラーでは不明供給されたセクションについてのコメントから導入しました。おかげで、この文書の内容にかなりの改善にとベンを含むいくつかの他の人につながった会話のためのヴィントン・サーフ、リサDusseault、デビーGarside、およびJefsey Morfinにも起因しています

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. References
14.1. Normative References
14.1. 引用規格

[RFC3490] Faltstrom, P., Hoffman, P., and A. Costello, "Internationalizing Domain Names in Applications (IDNA)", RFC 3490, March 2003.

[RFC3490] Faltstrom、P.、ホフマン、P.、およびA.コステロ、 "アプリケーションにおける国際化ドメイン名(IDNA)"、RFC 3490、2003年3月。

[RFC3492] Costello, A., "Punycode: A Bootstring encoding of Unicode for Internationalized Domain Names in Applications (IDNA)", RFC 3492, March 2003.

[RFC3492]コステロ、A.、 "ピュニコード:アプリケーションにおける国際化ドメイン名のUnicodeのブートストリングのエンコード(IDNA)"、RFC 3492、2003年3月。

[RFC5890] Klensin, J., "Internationalized Domain Names for Applications (IDNA): Definitions and Document Framework", RFC 5890, August 2010.

[RFC5890] Klensin、J.、 "アプリケーション(IDNA)のための国際化ドメイン名:定義とドキュメントフレームワーク"、RFC 5890、2010年8月。

[RFC5891] Klensin, J., "Internationalized Domain Names in Applications (IDNA): Protocol", RFC 5891, August 2010.

[RFC5891] Klensin、J.、 "アプリケーション(IDNA)で国際化ドメイン名:プロトコル"、RFC 5891、2010年8月。

[RFC5892] Faltstrom, P., "The Unicode Code Points and Internationalized Domain Names for Applications (IDNA)", RFC 5892, August 2010.

[RFC5892] Faltstrom、P.、 "Unicodeのコードポイントとアプリケーションのための国際化ドメイン名(IDNA)"、RFC 5892、2010年8月。

[RFC5893] Alvestrand, H. and C. Karp, "Right-to-Left Scripts for Internationalized Domain Names for Applications (IDNA)", RFC 5893, August 2010.

[RFC5893] Alvestrand、H.およびC.カープ、 "アプリケーションのための国際化ドメイン名のために右から左へのスクリプト(IDNA)"、RFC 5893、2010年8月。

[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). <>.

【Unicode52]ユニコードコンソーシアム。 "Unicode標準、バージョン5.2.0"、(カリフォルニア州マウンテンビュー:ユニコードコンソーシアム、2009年ISBN 978-1-936213-00-9)によって定義されたUnicode標準、バージョン5.2.0、。 <>。

14.2. Informative References
14.2. 参考文献

[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.

[RFC0952] Harrenstien、K.、スタール、M.、およびE. Feinler、 "DoDのインターネットホストテーブル仕様"、RFC 952、1985年10月。

[RFC1034] Mockapetris, P., "Domain names - concepts and facilities", STD 13, RFC 1034, November 1987.

[RFC1034] Mockapetris、P.、 "ドメイン名 - 概念と設備"、STD 13、RFC 1034、1987年11月。

[RFC1035] Mockapetris, P., "Domain names - implementation and specification", STD 13, RFC 1035, November 1987.

[RFC1035] Mockapetris、P.、 "ドメイン名 - 実装及び仕様"、STD 13、RFC 1035、1987年11月。

[RFC1123] Braden, R., "Requirements for Internet Hosts - Application and Support", STD 3, RFC 1123, October 1989.

[RFC1123]ブレーデン、R.、 "インターネットホストのための要件 - 、アプリケーションとサポート"、STD 3、RFC 1123、1989年10月。

[RFC2136] Vixie, P., Thomson, S., Rekhter, Y., and J. Bound, "Dynamic Updates in the Domain Name System (DNS UPDATE)", RFC 2136, April 1997.

[RFC2136]いるVixie、P.、トムソン、S.、Rekhter、Y.、およびJ.バウンド、 "ドメインネームシステムにおける動的更新(DNS更新)"、RFC 2136、1997年4月。

[RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS Specification", RFC 2181, July 1997.

"DNS仕様の明確化" [RFC2181]エルツ、R.とR.ブッシュ、RFC 2181、1997年7月。

[RFC2277] Alvestrand, H., "IETF Policy on Character Sets and Languages", BCP 18, RFC 2277, January 1998.

[RFC2277] Alvestrand、H.、 "文字セットと言語のIETF方針"、BCP 18、RFC 2277、1998年1月。

[RFC2671] Vixie, P., "Extension Mechanisms for DNS (EDNS0)", RFC 2671, August 1999.

[RFC2671]いるVixie、P.、 "DNS用拡張メカニズム(EDNS0)"、RFC 2671、1999年8月。

[RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for specifying the location of services (DNS SRV)", RFC 2782, February 2000.

[RFC2782] Gulbrandsenの、A.、いるVixie、P.、およびL. Esibov、 "サービスの場所を特定するためのDNS RR(DNSのSRV)"、RFC 2782、2000年2月。

[RFC3454] Hoffman, P. and M. Blanchet, "Preparation of Internationalized Strings ("stringprep")", RFC 3454, December 2002.

[RFC3454]ホフマン、P.及びM.ブランシェ、 "国際化された文字列の調製(" 文字列準備 ")"、RFC 3454、2002年12月。

[RFC3491] Hoffman, P. and M. Blanchet, "Nameprep: A Stringprep Profile for Internationalized Domain Names (IDN)", RFC 3491, March 2003.

[RFC3491]ホフマン、P.とM.ブランシェ、 "NAMEPREP:国際化ドメイン名のためのstringprepプロフィール(IDN)"、RFC 3491、2003年3月。

[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.

[RFC3743]小西、K.、黄、K.、銭、H.、およびY.コ、 "国際化ドメイン名のための共同エンジニアリングチーム(JET)ガイドライン中国語、日本語、韓国語用(IDN)登録と管理"、 RFC 3743、2004年4月。

[RFC3987] Duerst, M. and M. Suignard, "Internationalized Resource Identifiers (IRIs)", RFC 3987, January 2005.

[RFC3987] Duerst、M.およびM. Suignard、 "国際化リソース識別Fiers(IRI)"、RFC 3987、2005年1月。

[RFC4290] Klensin, J., "Suggested Practices for Registration of Internationalized Domain Names (IDN)", RFC 4290, December 2005.

[RFC4290] Klensin、J.、 "国際化ドメイン名(IDN)の登録のための推奨プラクティス"、RFC 4290、2005年12月。

[RFC4343] Eastlake, D., "Domain Name System (DNS) Case Insensitivity Clarification", RFC 4343, January 2006.

[RFC4343]イーストレイク、D.、 "ドメインネームシステム(DNS)大文字小文字の明確化"、RFC 4343、2006年1月。

[RFC4690] Klensin, J., Faltstrom, P., Karp, C., and IAB, "Review and Recommendations for Internationalized Domain Names (IDNs)", RFC 4690, September 2006.

[RFC4690] Klensin、J.、Faltstrom、P.、カープ、C.、およびIAB、RFC 4690 "国際化ドメイン名(IDNの)のレビューと提言"、2006年9月。

[RFC4713] Lee, X., Mao, W., Chen, E., Hsu, N., and J. Klensin, "Registration and Administration Recommendations for Chinese Domain Names", RFC 4713, October 2006.

[RFC4713]リー、X.、真央、W.、チェン、E.、スー、N.、およびJ. Klensin、RFC 4713、2006年10月 "中国のドメイン名の登録と管理勧告"。

[Unicode-UAX31] The Unicode Consortium, "Unicode Standard Annex #31: Unicode Identifier and Pattern Syntax, Revision 11", September 2009, <>.

[ユニコード-UAX31]のUnicodeコンソーシアム、 "Unicode規格附属書#31:Unicodeの識別子およびパターンの構文、リビジョン11"、2009年9月、<> 。

[Unicode-UTS39] The Unicode Consortium, "Unicode Technical Standard #39: Unicode Security Mechanisms, Revision 2", August 2006, <>.

[ユニコード-UTS39]のUnicodeコンソーシアム、 "Unicodeの技術標準#39:Unicodeのセキュリティメカニズム、リビジョン2"、2006年8月、<>。

Author's Address


John C Klensin 1770 Massachusetts Ave, Ste 322 Cambridge, MA 02140 USA

ジョン・S Clensin 1770マサチューセッツアベニュー、隣接する322ケンブリッジ、MA 02140彼

Phone: +1 617 245 1457 EMail:

電話:+1 617 245 1457 Eメール