Internet Engineering Task Force (IETF)                        J. Klensin
Request for Comments: 5894                                   August 2010
Category: Informational
ISSN: 2070-1721
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.

本文件是互联网工程任务组(IETF)的产品。它代表了IETF社区的共识。它已经接受了公众审查,并已被互联网工程指导小组(IESG)批准出版。并非IESG批准的所有文件都适用于任何级别的互联网标准;见RFC 5741第2节。

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.

版权所有(c)2010 IETF信托基金和确定为文件作者的人员。版权所有。

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.

本文件受BCP 78和IETF信托有关IETF文件的法律规定的约束(自本文件出版之日起生效。请仔细阅读这些文件,因为它们描述了您对本文件的权利和限制。从本文件中提取的代码组件必须包括信托法律条款第4.e节中所述的简化BSD许可证文本,并提供简化BSD许可证中所述的无担保。

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
   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
     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. 介绍
1.1. Context and Overview
1.1. 背景和概述

Internationalized Domain Names in Applications (IDNA) is a collection of standards that allow client applications to convert some mnemonic strings expressed in Unicode to an ASCII-compatible encoding form ("ACE") that is a valid DNS label containing only LDH syntax (see the Definitions document [RFC5890]). The specific form of ACE label used by IDNA is called an "A-label". A client can look up an exact A-label in the existing DNS, so A-labels do not require any extensions to DNS, upgrades of DNS servers, or updates to low-level client libraries. An A-label is recognizable from the prefix "xn--" before the characters produced by the Punycode algorithm [RFC3492]; thus, a user application can identify an A-label and convert it into Unicode (or some local coded character set) for display.


On the registry side, IDNA allows a registry to offer Internationalized Domain Names (IDNs) for registration as A-labels. A registry may offer any subset of valid IDNs, and may apply any restrictions or bundling (grouping of similar labels together in one registration) appropriate for the context of that registry. Registration of labels is sometimes discussed separately from lookup, and it is subject to a few specific requirements that do not apply to lookup.


DNS clients and registries are subject to some differences in requirements for handling IDNs. In particular, registries are urged to register only exact, valid A-labels, while clients might do some mapping to get from otherwise-invalid user input to a valid A-label.


   The first version of IDNA was published in 2003 and is referred to
   here as IDNA2003 to contrast it with the current version, which is
   known as IDNA2008 (after the year in which IETF work started on it).
   IDNA2003 consists of four documents: the IDNA base specification
   [RFC3490], Nameprep [RFC3491], Punycode [RFC3492], and Stringprep
   [RFC3454].  The current set of documents, IDNA2008, is not dependent
   on any of the IDNA2003 specifications other than the one for Punycode
   encoding.  References to "IDNA2008", "these specifications", or
   "these documents" are to the entire IDNA2008 set listed in a separate
   Definitions document [RFC5890].  The characters that are valid in
   A-labels are identified from rules listed in the Tables document
   [RFC5892], but validity can be derived from the Unicode properties of
   those characters with a very few exceptions.
   The first version of IDNA was published in 2003 and is referred to
   here as IDNA2003 to contrast it with the current version, which is
   known as IDNA2008 (after the year in which IETF work started on it).
   IDNA2003 consists of four documents: the IDNA base specification
   [RFC3490], Nameprep [RFC3491], Punycode [RFC3492], and Stringprep
   [RFC3454].  The current set of documents, IDNA2008, is not dependent
   on any of the IDNA2003 specifications other than the one for Punycode
   encoding.  References to "IDNA2008", "these specifications", or
   "these documents" are to the entire IDNA2008 set listed in a separate
   Definitions document [RFC5890].  The characters that are valid in
   A-labels are identified from rules listed in the Tables document
   [RFC5892], but validity can be derived from the Unicode properties of
   those characters with a very few exceptions.

Traditionally, DNS labels are matched case-insensitively (as described in the DNS specifications [RFC1034][RFC1035]). That convention was preserved in IDNA2003 by a case-folding operation 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.


1.2. Terminology
1.2. 术语

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“名称”术语

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. 新术语和限制

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).

IDNA2008引入了新的术语。定义文件中提供了术语U-label、A-label、LDH label(所有有效的pre-IDNA主机名均符合该术语)、保留LDH label(R-LDH label)、XN label、伪A-label和非保留LDH label(NR-LDH label)的精确定义。

In addition, the term "putative label" has been adopted to refer to a label that may appear to meet certain definitional constraints but has not yet been sufficiently tested for validity.


These definitions are also illustrated in Figure 1 of the Definitions document. R-LDH labels contain "--" in the third and fourth character positions from the beginning of the label. In IDNA-aware applications, only a subset of these reserved labels is permitted to be used, namely the A-label subset. A-labels are a subset of the R-LDH labels that begin with the case-insensitive string "xn--". Labels that bear this prefix but that are not otherwise valid fall into the "Fake A-label" category. The Non-Reserved labels (NR-LDH labels) are implicitly valid since they do not bear any resemblance to the labels specified by IDNA.


The creation of the Reserved-LDH category is required for three reasons:


o to prevent confusion with pre-IDNA coding forms;

o 防止与IDNA前编码形式混淆;

o to permit future extensions that would require changing the prefix, no matter how unlikely those might be (see Section 7.4); and

o 允许将来需要更改前缀的扩展,无论这种情况多么不可能(见第7.4节);和

o to reduce the opportunities for attacks via the Punycode encoding algorithm itself.

o 通过Punycode编码算法本身减少攻击机会。

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. 目标

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 使用最新版本的Unicode,并允许IDNA独立于Unicode版本,这样就不需要更新IDNA2008来实现采用新Unicode版本的代码点。

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 修复极少量的代码点分类,这些分类结果导致使用这些代码点的社区出现问题。

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 减少对映射的依赖,有利于有效的A标签。这将导致在各种上下文中出现的预映射表单不是有效的IDNA标签。

o Fix some details in the bidirectional code point handling algorithms.

o 修复双向代码点处理算法中的一些细节。

1.4. Applicability and Function of IDNA
1.4. IDNA的适用性和功能

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机制和处理的可理解性

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
2. IDNA2008中的处理

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在协议规范中分离域名注册和查找(RFC 5891,第4节和第5节[RFC5891])。虽然这两个过程中的大多数步骤是相似的,但分离反映了当前的做法,即每个注册表(DNS区域)限制和特殊处理在注册时应用,而不是在查找期间应用。另一个显著的好处是,分离有助于增量添加允许的字符组,以避免在特定版本的Unicode上冻结。

The actual registration and lookup protocols for IDNA2008 are specified in the Protocol document.


3. Permitted Characters: An Inclusion List
3. 允许的字符:包含列表

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.


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. 允许的字符和标签的分层模型

Moving to an inclusion model involves a new specification for the list of characters that are permitted in IDNs. In IDNA2003, character validity is independent of context and fixed forever (or until the standard is replaced). However, globally context-independent rules have proved to be impractical because some characters, especially those that are called "Join_Controls" in Unicode, are needed to make reasonable use of some scripts but have no visible effect in others. IDNA2003 prohibited those types of characters entirely by discarding them. We now have a consensus that under some conditions, these "joiner" characters are legitimately needed to allow useful mnemonics for some languages and scripts. In general, context-dependent rules help deal with characters (generally characters that would otherwise be prohibited entirely) that are used differently or perceived differently across different scripts, and allow the standard to be applied more appropriately in cases where a string is not universally handled the same way.


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


3.1.1. 协议有效

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.

放置在PROTOCOL-VALID类别中的字符永远不会从中删除或重新分类。虽然理论上可以从Unicode中删除字符,但这种删除不符合Unicode稳定性原则(参见UTR 39:Unicode安全机制[Unicode52],附录F),因此不应发生。

3.1.2. 需要上下文规则

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.

某些字符可能不适合在IDN中通用,但对于某些脚本的合理支持是必需的。最常引用的两个示例是零宽度连接符和零宽度非连接符字符(ZWJ,U+200D和ZWNJ,U+200C),但其他字符可能需要特殊处理,因为它们在其他情况下是不允许的(通常是因为Unicode认为它们是标点符号或特殊符号),但在有限的上下文中需要允许。其他字符被给予这种特殊处理,因为它们极有可能被用来制作误导性标签或在标签匹配和解释中造成不可接受的歧义。 Contextual Restrictions 上下文限制

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.

例如,处理需要ZWJ和/或ZWNJ作为书写系统一部分的INDI脚本的注册表应了解字符在何处具有可见效果,在何处不具有可见效果,并相应地制定注册规则。相比之下,主要处理拉丁或西里尔文字的注册中心可能不会主动意识到这些字符的存在,更不用说将它们嵌入从这些文字中提取的标签的后果了,因此应该避免接受包含这些字符的注册,至少在标签中使用来自拉丁或西里尔文字的字符。 Rules and Their Application 规则及其应用

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.


3.1.3. 不允许

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).


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 该字符是另一个字符的兼容等效项。在更精确的Unicode术语中,对字符应用规范化形式KC(NFKC)会产生其他字符。

o The character is an uppercase form or some other form that is mapped to another character by Unicode case folding.

o 字符是大写形式或通过Unicode大小写折叠映射到另一个字符的其他形式。

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.

o 字符是一种符号或标点符号形式,或者更一般地说,它不是字母、数字或用于形成字母或数字的标记。

3.1.4. 未分配

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. 注册政策

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. 分层限制:表、上下文、注册和应用程序

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. 与应用程序相关的问题
4.1. Display and Network Order
4.1. 显示和网络顺序

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.


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.


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. 应用程序中的输入和显示

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.


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.


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.


One specific, and very important, instance of this strategy arises with case folding. In the ASCII-only DNS, names are looked up and matched in a case-independent way, but no actual case folding occurs. Names can be placed in the DNS in either uppercase or lowercase form (or any mixture of them) and that form is preserved, returned in queries, and so on. IDNA2003 approximated that behavior for non-ASCII strings by performing case folding at registration time (resulting in only lowercase IDNs in the DNS) and when names were looked up.


As suggested earlier in this section, it appears to be desirable to do as little character mapping as possible as long as Unicode works correctly (e.g., Normalization Form C (NFC) mapping to resolve different codings for the same character is still necessary although the specifications require that it be performed prior to invoking the protocol) in order to make the mapping between A-labels and U-labels idempotent. Case mapping is not an exception to this principle. If only lowercase characters can be registered in the DNS (i.e., be present in a U-label), then IDNA2008 should prohibit uppercase characters as input even though user interfaces to applications should probably map those characters. Some other considerations reinforce this conclusion. For example, in ASCII case mapping for individual characters, uppercase(character) is always equal to uppercase(lowercase(character)). That may not be true with IDNs. In some scripts that use case distinctions, there are a few characters that do not have counterparts in one case or the other. The relationship between uppercase and lowercase may even be language-dependent, with different languages (or even the same language in different areas) expecting different mappings. User interface programs can meet the expectations of users who are accustomed to the case-insensitive DNS environment by performing case folding prior to IDNA processing, but the IDNA procedures themselves should neither require such mapping nor expect them when they are not natural to the localized environment.


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

4.3. 语言期望:连字、有向图和交替字符形式

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 一个挪威用户可能期望一个带有ae连字的标签与一个使用瑞典语拼写和A-diaeresis的标签被视为相同的标签,即使将这种映射应用到英语会让用户感到惊讶。

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 德国用户可能会期望使用o-umlaut的标签和替换了“oe”但在其他方面相同的标签被视为等效标签,即使替换在瑞典语中是一个明显的错误。

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 中国用户可能希望自动匹配简体和繁体汉字,但将这种匹配应用于韩文或日文文本会造成相当大的混乱。

o An English user might expect "theater" and "theatre" to match.

o 英国用户可能希望“剧院”和“剧院”匹配。

A number of languages use alphabetic scripts in which single phonemes are written using two characters, termed a "digraph", for example, the "ph" in "pharmacy" and "telephone". (Such characters can also appear consecutively without forming a digraph, as in "tophat".) Certain digraphs may be indicated typographically by setting the two characters closer together than they would be if used consecutively to represent different phonemes. Some digraphs are fully joined as ligatures. For example, the word "encyclopaedia" is sometimes set with a U+00E6 LATIN SMALL LIGATURE AE. When ligature and digraph forms have the same interpretation across all languages that use a given script, application of Unicode normalization generally resolves the differences and causes them to match. When they have different interpretations, matching must utilize other methods, presumably chosen at the registry level, or users must be educated to understand that matching will not occur.


The nature of the problem can be illustrated by many words in the Norwegian language, where the "ae" ligature is the 27th letter of a 29-letter extended Latin alphabet. It is equivalent to the 28th letter of the Swedish alphabet (also containing 29 letters), U+00E4 LATIN SMALL LETTER A WITH DIAERESIS, for which an "ae" cannot be substituted according to current orthographic standards. That character (U+00E4) is also part of the German alphabet where, 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.

在北欧语言中,两个字符序列“ae”通常被视为“umlauted a”字符完全可接受的替代正字法。然而,相反的情况并非如此,这两个字符不一定能组合成一个“umlauted a”。这也适用于另一个德语字符“umlauted o”(U+00F6拉丁文小写字母o,带分音符),例如,它不能用于书写作者的名字“歌德”。它也是瑞典字母表中的一个字母,与“带分音符的a”一样,不能正确表示为“oe”,而在挪威字母表中,它表示为“带分音符的o”,而不是“带分音符的o”,U+00F8。

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


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.


4.4. Case Mapping and Related Issues
4.4. 案例映射和相关问题

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.


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.


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.

由于IDNA2003将最终Sigma和Eszett映射到其他字符,并且反向映射是不可能的,因此最终Sigma和Eszett既不能以IDNA2003 IDN的ACE形式表示,也不能以其派生的本机字符(U标签)形式表示。使用IDNA2008,这两个字符都可以在IDN中使用,因此用于查找包含这些字符的任何U标签的A标签现在不同了。请参阅第7.1节,了解哪些类型的更改可能需要更改IDNA前缀;经过长时间的讨论,IDNABIS工作组达成共识,即这些字符的更改并不能证明前缀更改是合理的。

4.5. Right-to-Left Text
4.5. 从右到左文本

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].


5. IDNs and the Robustness Principle
5. IDNs及其鲁棒性原理

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.


Conversely, lookup applications are expected to reject labels that clearly violate global (protocol) rules (no one has ever seriously claimed that being liberal in what is accepted requires being stupid). However, once one gets past such global rules and deals with anything sensitive to script or locale, it is necessary to assume that garbage has not been placed into the DNS, i.e., one must be liberal about what one is willing to look up in the DNS rather than guessing about whether it should have been permitted to be registered.


If a string cannot be successfully found in the DNS after the lookup processing described here, it makes no difference whether it simply wasn't registered or was prohibited by some rule at the registry. Application implementers should be aware that where DNS wildcards are used, the ability to successfully resolve a name does not guarantee that it was actually registered.


6. Front-end and User Interface Processing for Lookup
6. 查找的前端和用户界面处理

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.


An application that implements the IDNA protocol [RFC5891] will always take any user input and convert it to a set of Unicode code points. That user input may be acquired by any of several different input methods, all with differing conversion processes to be taken into consideration (e.g., typed on a keyboard, written by hand onto some sort of digitizer, spoken into a microphone and interpreted by a speech-to-text engine, etc.). The process of taking any particular user input and mapping it into a Unicode code point may be a simple one: if a user strikes the "A" key on a US English keyboard, without any modifiers such as the "Shift" key held down, in order to draw a Latin small letter A ("a"), many (perhaps most) modern operating system input methods will produce to the calling application the code point U+0061, encoded in a single octet.


Sometimes the process is somewhat more complicated: a user might strike a particular set of keys to represent a combining macron followed by striking the "A" key in order to draw a Latin small letter A with a macron above it. Depending on the operating system, the input method chosen by the user, and even the parameters with which the application communicates with the input method, the result might be the code point U+0101 (encoded as two octets in UTF-8 or UTF-16, four octets in UTF-32, etc.), the code point U+0061 followed by the code point U+0304 (again, encoded in three or more octets, depending upon the encoding used) or even the code point U+FF41 followed by the code point U+0304 (and encoded in some form). These examples leave aside the issue of operating systems and input methods that do not use Unicode code points for their character set.


In every case, applications (with the help of the operating systems on which they run and the input methods used) need to perform a mapping from user input into Unicode code points.


IDNA2003 used a model whereby input was taken from the user, mapped (via whatever input method mechanisms were used) to a set of Unicode code points, and then further mapped to a set of Unicode code points using the Nameprep profile [RFC3491]. In this procedure, there are two separate mapping steps: first, a mapping done by the input method (which might be controlled by the operating system, the application, or some combination) and then a second mapping performed by the Nameprep portion of the IDNA protocol. The mapping done in Nameprep includes a particular mapping table to re-map some characters to other characters, a particular normalization, and a set of prohibited characters.


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. 从IDNA2003和Unicode版本同步迁移
7.1. Design Criteria
7.1. 设计标准

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 使应用程序不知道它们是否在支持3.2以后的任何Unicode版本的环境中运行。

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).

o 允许在将新字符、字符组、脚本和其他字符集合合并到Unicode中时以增量方式添加新字符、字符组、脚本和其他字符集合,这样做不会中断,从长远来看,也不会出现“繁重”的过程(IDNA2008规范要求IETF协商一致过程,预计在IDNA操作和Unicode新版本积累大量经验之前,IETF协商一致过程将被要求和使用)。

7.1.1. Summary and Discussion of IDNA Validity Criteria
7.1.1. IDNA有效性标准的总结与讨论

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.)

o 这些字符是“字母”,是构成字母、数字或其他用于在某种语言中书写单词的代码点所需的标记。符号、图形字符和各种符号字符将被永久排除在外。没有证据表明它们对互联网运营或国际化的重要性足以证明域名的扩展超出了“字母、数字和连字符”的一般原则。(第7.6节对符号决定的其他讨论和理由进行了说明。)

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 除了在非常特殊的情况下,例如,需要他们书写给定语言的任何单词,标点符号除外。一个单词存在的事实并不能证明它应该在DNS标签中可用,并且DNS标签也不能用于多个单词短语(尽管如果特定语言的约定和正字法导致这一点成为可能,那么DNS标签肯定不会被禁止)。

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 注册表或应用程序使用的Unicode版本中不允许未分配(根本没有字符分配)的字符,即使在查找时也是如此。第7.7节讨论了本决定涉及的问题。

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 当前版本的NFKC映射到另一个字符的任何字符都禁止作为IDNA的输入(用于注册或查找)。除少数例外情况外,此原则不包括通过Nameprep[RFC3491]映射到另一个字符的任何字符。

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. 注册中的标签

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]):


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 任何看似A标签的标签,即任何以“xn-”开头的标签,必须在IDNA下有效,即它们必须是有效的A标签,如上文第2节所述。

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.

o Unicode表(即代码点、字符类和属性的表)和IDNA表(即上下文规则的表,如表文档中出现的那些),在执行或验证要注册的标签的系统上必须一致。请注意,这并不要求表反映最新版本的Unicode,只要求给定系统上使用的所有表彼此一致。

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.

解析DNS标签,尤其是IDN DNS标签,必须能够假设输入DNS的名称遵循了适用的注册规则。

7.1.3. Labels in Lookup
7.1.3. 查找中的标签

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 维护版本一致的IDNA和Unicode表格,即,除非应用程序实际执行表格文档[RFC5892]中的分类规则,否则其IDNA表格必须派生自系统上更普遍支持的Unicode版本。与注册一样,表不需要反映最新版本的Unicode,但它们必须一致。

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 仅在确定U型标签不包含“不允许的”代码点或Unicode版本中未指定的代码点的范围内,验证要查找的标签中的字符。

o Validate the label itself for conformance with a small number of whole-label rules. In particular, it must verify that:

o 验证标签本身是否符合少量完整标签规则。特别是,它必须核实:

* there are no leading combining marks,

* 没有前导组合标记,

* the Bidi conditions are met if right-to-left characters appear,

* 如果出现从右向左的字符,则满足Bidi条件,

* any required contextual rules are available, and

* 任何必要的上下文规则都可用,并且

* any contextual rules that are associated with joiner characters (and CONTEXTJ characters more generally) are tested.

* 测试与joiner字符(以及更一般的CONTEXTJ字符)关联的任何上下文规则。

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.

o 不要拒绝基于其他有关字符的上下文规则的标签,包括混合脚本标签禁止。这些规则可用于影响用户界面中的表示决策,但不能避免查找域名。

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. 人物解读的变化

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. 特征变化:Eszett和最终西格玛

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).

在那些区分大小写的脚本中,有几个字符的明显且唯一的大写字符在历史上从未与小写字符匹配,反之亦然。对于这些字符,使用Unicode toCaseFold操作(参见Unicode标准[Unicode52]第5.18节)为IDNA2003构建Stringprep表时使用的映射生成不同的字符或字符集。这些操作是不可逆的,与传统的大写或小写转换相比,它们丢失的信息甚至更多,但在比较方面比那些转换更有用。这种类型的两个显著字符是德语字符Eszett(夏普S,U+00DF)和希腊语的最后形式Sigma(U+03C2)。前者大小写折叠为ASCII字符串“ss”,后者为中间(小写)Sigma(U+03C3)。

7.2.2. Character Changes: Zero Width Joiner and Zero Width Non-Joiner
7.2.2. 字符更改:零宽度连接符和零宽度非连接符

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.


7.2.3. Character Changes and the Need for Transition
7.2.3. 性格变化和过渡的必要性

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. 过渡战略

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:


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.

1. 不允许在注册表级别使用新可用的字符。如果域名是按照IDNA2003映射行为编写的,这可能会导致查找失败,但会消除任何错误匹配的可能性。

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.

2. 保持一种类似“日出”的安排,在这种安排中,在Eszett情况下包含“ss”、在该情况下包含小写Sigma、或在上下文中可能包含ZWJ或ZWNJ的标签持有者,在注册包含Eszett、最终Sigma的对应字符串时,被给予优先权(以及可能的其他好处),或相应的零宽度字符。

3. Adopt some sort of "variant" approach in which registrants obtain labels with both character forms.

3. 采用某种“变体”方法,即注册人获得两种字符形式的标签。

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. 采用不同形式的“变体”方法,即如果根据IDNA2003进行解释,则不允许注册产生相同a标签的附加字符串,或者只允许已经拥有其中一个名称的注册人注册。

5. Ignore the issue and assume that the marketplace or other mechanisms will sort things out.

5. 忽略这个问题,假设市场或其他机制会解决问题。

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. 消除字符映射

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].


7.4. The Question of Prefix Changes
7.4. 前缀更改问题

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.

社区非常关注需要更改IDNA ACE前缀(“xn--”,在IDNA2003中使用)的条件。如果算法的修改方式会在随后的注册过渡期间产生严重的歧义,那么前缀更改显然是必要的。本节总结了工作组关于在何种条件下有必要更改前缀以及这种更改的影响的结论。

7.4.1. Conditions Requiring a Prefix Change
7.4.1. 需要更改前缀的条件

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.

1. 将A标签转换为Unicode(即U标签)将在IDNA2003下生成一个字符串,在IDNA2008下生成另一个字符串。

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).

2. 在很多情况下,一个在IDNA2003和IDNA2008下都有效的输入字符串会产生两个不同版本的a标签。这种情况被认为基本上等同于上述情况,除了极少数边缘情况无法证明前缀更改的合理性(见第7.2节)。

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.

3. 对将插入DNS中的字符串的语义进行了根本性更改,例如,如果决定尝试在编码中除了字符串本身之外还包括语言或脚本信息。

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.

4. 在Unicode中添加了足够多的字符,以便块偏移的Punycode机制不再引用编号较高的平面和块。这种情况即使在长期内也不太可能出现,而且在未来几年内肯定不会出现。

7.4.2. Conditions Not Requiring a Prefix Change
7.4.2. 不需要更改前缀的条件

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.

1. 禁止某些字符作为IDNA的输入。这种禁止可能会使以前注册的名称无法访问,但不会更改这些名称。

2. Adjustments in IDNA tables or actions, including normalization definitions, that affected characters that were already invalid under IDNA2003.

2. IDNA表或操作中的调整,包括规范化定义,影响了IDNA2003下已经无效的字符。

3. Changes in the style of the IDNA definition that did not alter the actions performed by IDNA.

3. IDNA定义样式的更改,不会改变IDNA执行的操作。

7.4.3. Implications of Prefix Changes
7.4.3. 前缀更改的影响

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. Stringprep更改和兼容性

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.


It is particularly important to keep IDNA processing separate from processing for various security protocols because some of the constraints that are necessary for smooth and comprehensible use of IDNs may be unwanted or undesirable in other contexts. For example, the criteria for good passwords or passphrases are very different from those for desirable IDNs: passwords should be hard to guess, while domain names should normally be easily memorable. Similarly, internationalized Small Computer System Interface (SCSI) identifiers and other protocol components are likely to have different requirements than IDNs.


7.6. The Symbol Question
7.6. 符号问题

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

1. 正如其他地方所讨论的,最初的IDNA规范假设在IDN中应该直接或通过映射到其他字符来允许尽可能多的Unicode字符。这

specification operates on an inclusion model, extrapolating from the original "hostname" rules (LDH, see the Definitions document [RFC5890]) -- which have served the Internet very well -- to a Unicode base rather than an ASCII base.


2. Symbol names are more problematic than letters because there may be no general agreement on whether a particular glyph matches a symbol; there are no uniform conventions for naming; variations such as outline, solid, and shaded forms may or may not exist; and so on. As just one example, consider a "heart" symbol as it might appear in a logo that might be read as "I love...". While the user might read such a logo as "I love..." or "I heart...", considerable knowledge of the coding distinctions made in Unicode is needed to know that there is more than one "heart" character (e.g., U+2665, U+2661, and U+2765) and how to describe it. These issues are of particular importance if strings are expected to be understood or transcribed by the listener after being read out loud.

2. 符号名称的问题比字母更大,因为对于某个符号是否与某个符号匹配,可能没有普遍的一致意见;命名没有统一的约定;轮廓、实体和阴影形式等变化可能存在,也可能不存在;等等作为一个例子,考虑一个“心”的符号,因为它可能出现在一个标志中,可以被称为“我爱…”。虽然用户可能会阅读诸如“我爱…”或“我的心…”之类的徽标,但需要相当多的Unicode编码区别知识才能知道有多个“心”字符(例如U+2665、U+2661和U+2765)以及如何描述它。如果希望听者在大声朗读后理解或转录字符串,则这些问题尤为重要。

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.

3. 盲人互联网用户使用的屏幕阅读器的设计变得相当复杂,他们必须听IDN域名的渲染,并可能在键盘上重现这些渲染。当熟悉该语言的人看不到字符的名称时,屏幕阅读器的设计就变得相当复杂。

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. 作为一个简化的例子,假设一个人想要在标签中使用“心”或“星”符号。这是有问题的,因为这些名称在Unicode命名系统中是不明确的(实际的Unicode名称需要更多的限定)。如果不仔细研究代码表,用户或潜在注册者无法知道代码表是否含糊不清(例如,有多个“心脏”字符)。相反,看到假想标签的用户不知道是否要阅读它——试着通过语音将其传送给同事——作为“心”、“爱”、“黑心”或下面的任何其他示例。

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. 实际情况甚至比这更糟。对于一个普通、随意的用户来说,如果不知道如何寻找区别,就不可能分辨出U+2665和U+2765的心脏与U+2606和U+2729的星星之间的区别。我们有一颗白心(U+2661)和几个黑心。因此,将一个标签描述为包含一颗心是毫无疑问的模棱两可:我们只能知道它包含几个看起来像心或名称中有“心”的字符之一。在城市中,“正方形”是位置名称的一个流行部分,人们可能也希望在标签中使用正方形符号,而且Unicode中的各种风格的正方形远多于红心或星星。

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. Unicode版本之间的迁移:未分配的代码点

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 需要进行涉及字符上下文的测试(例如,某些字符仅允许与其他特定类型的字符相邻)以及完整标签的完整性测试。不允许使用未分配的代码点,因为在向特定代码点分配字符并完全理解这些字符的属性之前,无法确定特定代码点是否需要上下文规则(以及这些规则应该是什么)。

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 无法预先知道新分配的代码点是否与表文档[RFC5892]中的规则不允许的字符(例如兼容字符)关联,并且具有足够的可靠性。在IDNA2003中,由于没有对NFKC的直接依赖(Stringprep表中的许多条目都基于NFKC,但IDNA2003仅依赖于Stringprep),因此兼容性字符的分配可能会产生一些奇怪的情况,但这不会是问题。在IDNA2008中,除非出现特定于字符的例外情况,否则不允许使用兼容字符,允许查找包含未分配字符的字符串将违反不允许使用的字符不被查找的原则。

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

o Unicode标准规定未分配的代码点将自身规范化(以及在相关情况下,大小写折叠)。如果稍后将代码点指定给字符,尤其是如果新指定的代码点具有以下组合类:

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).

有可能认为上述问题并不重要,因此,最好保留查找标签的原则,即使标签包含未分配的字符,因为所有重要的脚本和字符都是在Unicode 5.2(或更早)时编码的,因此,未分配的代码点将只分配给晦涩的字符或古老的脚本。不幸的是,这似乎不是一个安全的假设,至少有两个原因。首先,对于Unicode的早期版本,也有很多相同的完整性声明。现实情况是,一个对世界上很多人来说都不太清楚的脚本对那些使用它的人来说可能仍然非常重要。文化和语言保护原则使得在IDN中声明脚本不重要是不合适的。其次,我们已经有了反例,例如,在与正在添加的新汉字相关联的关系中(无论是在BMP中还是在Unicode平面2中)。

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.

与上面确定的技术转换问题无关,可以观察到,在现有脚本中添加字符以使其更易于使用或更好地适应特定语言可能会导致转换问题。这样的添加可能会改变写入特定字符串的首选形式,这些变化可能会反映在键盘转换模块中,例如,键盘转换模块必然不同于早期版本的Unicode(其中可能不存在较新的字符)。这会产生固有的转换问题,因为访问标签的尝试可能使用旧的或新的约定,无论标签中是否使用旧的约定,都需要注册表操作。需要考虑转换机制是Unicode的演进所固有的,以便更好地适应写入系统,并且独立于DNS在DNS中如何表示,或者如何发生这些机制的版本之间的转换。通过在Unicode 5.1.0中添加Malayalam Chillu来说明这种类型转换的要求。

7.8. Other Compatibility Issues
7.8. 其他兼容性问题

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.


8. Name Server Considerations
8. 名称服务器注意事项
8.1. Processing Non-ASCII Strings
8.1. 处理非ASCII字符串

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.

现有DNS服务器不知道处理非ASCII形式IDN的IDNA规则,因此需要屏蔽这些规则。名称可以通过其进入DNS服务器数据库的所有现有通道(例如,主文件(如RFC 1034中所述)和DNS更新消息[RFC2136])都不能识别IDNA,因为它们早于IDNA。本文档的其他部分通过确保通过此类通道进入DNS服务器数据库的国际化域名已转换为其等效的ASCII A标签形式,提供了所需的屏蔽。

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. 根目录和其他DNS服务器注意事项

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
9. 国际化考虑

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标签和完全限定的域名提供帮助识别和引用Internet上资源的助记符。IDN扩展了这些助记符的范围,包括那些基于语言和字符集的助记符,而不是西欧和罗马派生的助记符。但域名通常不是任何语言中的单词。IETF字符集和语言政策(BCP 18[RFC2277])的建议适用于使用语言标识提供特定语言上下文的情况。相比之下,DNS是全球性和国际性的,最终与语言无关。将语言(或类似上下文)添加到IDN中,或者特别添加到DNS匹配中,将意味着DNS中的上下文相关匹配,这将是对DNS协议本身的一个非常重要的更改。这还意味着用户需要识别与特定标签相关联的语言,以便查找该标签。这种知识通常是不可用的,因为许多标签不是任何语言中的单词,有些可能是多个语言中的单词。

10. IANA Considerations
10. IANA考虑

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字符注册表

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上下文注册表

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. IANA TLD IDN实践知识库

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.

该注册中心历史上被称为“IANA语言字符集注册中心”或“IANA脚本注册中心”(这两个术语都有点误导性),由IANA应ICANN的要求进行维护。它用于提供顶级域(TLD)注册机构自愿参与的IDN策略的中央文档存储库,并与ICANN IDN使用指南结合使用。

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. 安全考虑
11.1. General Security Issues with IDNA
11.1. IDNA的一般安全问题

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
12. 致谢

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

编辑和撰稿人要向那些提供重要早期(工作组前期)评论意见的人表示感谢,这些评论有时还附有文本、保罗·霍夫曼(Paul Hoffman)、西蒙·约瑟夫森(Simon Josefsson)和山姆·韦勒(Sam Weiler)。此外,从文特·瑟夫、弗兰克·埃勒曼、迈克尔·埃弗森、阿斯穆斯·弗雷塔格、埃里克·范德波尔、米歇尔·苏伊格纳德和肯·惠斯勒提供的关于不明确章节的建议、文本或评论中纳入了一些具体想法。感谢Vint Cerf、Lisa Dusseault、Debbie Garside和Jefsey Morfin的对话,这些对话使本文档的内容有了很大的改进,也感谢其他人,包括Ben

Campbell, Martin Duerst, Subramanian Moonesamy, Peter Saint-Andre, and Dan Winship, for catching specific errors and recommending corrections.

Campbell、Martin Duerst、Subramanian Moonesay、Peter Saint Andre和Dan Winship,用于捕捉特定错误并建议更正。

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.

IETF“IDNABIS”工作组的许多参与者对工作组版本的工作草案提出了有用的意见和文本,该工作组的邮件列表讨论导致了一些文件更改。马科斯·桑兹(Marcos Sanz)提供了特别有用的具体分析和建议,文特·瑟夫(Vint Cerf)、马丁·杜尔斯特(Martin Duerst)、安德鲁·沙利文(Andrew Sullivan)和肯·惠斯勒(Ken Whistler)也提供了这些分析和建议。Lisa Dusseault在2009年春季提供了广泛的编辑建议,其中大部分被合并。

13. Contributors
13. 贡献者

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.

虽然列出的编辑手握笔,但本文件的核心和工作组的初始版本代表了一个特设设计团队的共同工作和结论,该团队由编辑以及按字母顺序排列的Harald Alvestrand、Tina Dam、Patrik Faltstrom和Cary Karp组成。彼得·雷斯尼克(Pete Resnick)和保罗·霍夫曼(Paul Hoffman)的测绘文件[IDNA2008测绘]草稿中包含了大量描述测绘原则的材料。此外,感谢部分所列人员以及其他对IDNA协议的开发和使用做出贡献的人员也做出了许多具体贡献和有益评论。

14. References
14. 工具书类
14.1. Normative References
14.1. 规范性引用文件

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

[RFC3490]Faltstrom,P.,Hoffman,P.,和A.Costello,“应用程序中的域名国际化(IDNA)”,RFC 34902003年3月。

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

[RFC3492]Costello,A.,“Punycode:应用程序中国际化域名的Unicode引导字符串编码(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.,“应用程序的国际化域名(IDNA):定义和文档框架”,RFC 58902010年8月。

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

[RFC5891]Klensin,J.,“应用程序中的国际化域名(IDNA):协议”,RFC 58912010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代码点和应用程序的国际化域名(IDNA)”,RFC 58922010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.和C.Karp,“应用程序国际化域名(IDNA)的从右到左脚本”,RFC 58932010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). <>.

[Unicode 52]Unicode联盟。Unicode标准,版本5.2.0,定义为:“Unicode标准,版本5.2.0”(加利福尼亚州山景城:Unicode联盟,2009年。ISBN 978-1-936213-00-9)<>.

14.2. Informative References
14.2. 资料性引用

[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.,Stahl,M.和E.Feinler,“国防部互联网主机表规范”,RFC 952,1985年10月。

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

[RFC1034]Mockapetris,P.,“域名-概念和设施”,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.,“域名-实现和规范”,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]Braden,R.,“互联网主机的要求-应用和支持”,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]Vixie,P.,Thomson,S.,Rekhter,Y.,和J.Bound,“域名系统中的动态更新(DNS更新)”,RFC 21361997年4月。

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

[RFC2181]Elz,R.和R.Bush,“DNS规范的澄清”,RFC 21811997年7月。

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

[RFC2277]Alvestrand,H.,“IETF字符集和语言政策”,BCP 18,RFC 2277,1998年1月。

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

[RFC2671]Vixie,P.,“DNS的扩展机制(EDNS0)”,RFC 26711999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,A.,Vixie,P.和L.Esibov,“用于指定服务位置(DNS SRV)的DNS RR”,RFC 2782,2000年2月。

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

[RFC3454]Hoffman,P.和M.Blanchet,“国际化弦的准备(“stringprep”)”,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]Hoffman,P.和M.Blanchet,“Nameprep:国际化域名(IDN)的Stringprep配置文件”,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]Konishi,K.,Huang,K.,Qian,H.,和Y.Ko,“中国,日本和韩国的国际域名(IDN)注册和管理联合工程团队(JET)指南”,RFC 37432004年4月。

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

[RFC3987]Duerst,M.和M.Suignard,“国际化资源标识符(IRIs)”,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.,“国际域名(IDN)注册的建议做法”,RFC 42902005年12月。

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

[RFC4343]Eastlake,D.,“域名系统(DNS)案例不敏感澄清”,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.,Karp,C.,和IAB,“国际化域名(IDN)的审查和建议”,RFC 46902006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]Lee,X.,Mao,W.,Chen,E.,Hsu,N.,和J.Klensin,“中文域名的注册和管理建议”,RFC 4713,2006年10月。

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


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


Author's Address


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


   Phone: +1 617 245 1457
   Phone: +1 617 245 1457