< draft-ietf-idnabis-rationale-09.txt   draft-ietf-idnabis-rationale-10.txt >
Network Working Group J. Klensin Network Working Group J. Klensin
Internet-Draft March 9, 2009 Internet-Draft June 18, 2009
Intended status: Informational Intended status: Informational
Expires: September 10, 2009 Expires: December 20, 2009
Internationalized Domain Names for Applications (IDNA): Background, Internationalized Domain Names for Applications (IDNA): Background,
Explanation, and Rationale Explanation, and Rationale
draft-ietf-idnabis-rationale-09.txt draft-ietf-idnabis-rationale-10.txt
Status of this Memo Status of this Memo
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Copyright Notice Copyright Notice
Copyright (c) 2009 IETF Trust and the persons identified as the Copyright (c) 2009 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents in effect on the date of Provisions Relating to IETF Documents in effect on the date of
publication of this document (http://trustee.ietf.org/license-info). publication of this document (http://trustee.ietf.org/license-info).
Please review these documents carefully, as they describe your rights Please review these documents carefully, as they describe your rights
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During that time, a number of issues have arisen, including the need 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 to update the system to deal with newer versions of Unicode. Some of
these issues require tuning of the existing protocols and the tables these issues require tuning of the existing protocols and the tables
on which they depend. This document provides an overview of a on which they depend. This document provides an overview of a
revised system and provides explanatory material for its components. revised system and provides explanatory material for its components.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Context and Overview . . . . . . . . . . . . . . . . . . . 4 1.1. Context and Overview . . . . . . . . . . . . . . . . . . . 4
1.2. Discussion Forum . . . . . . . . . . . . . . . . . . . . . 4 1.2. Discussion Forum . . . . . . . . . . . . . . . . . . . . . 5
1.3. Terminology . . . . . . . . . . . . . . . . . . . . . . . 5 1.3. Terminology . . . . . . . . . . . . . . . . . . . . . . . 5
1.3.1. Documents and Standards . . . . . . . . . . . . . . . 5 1.3.1. Documents and Standards . . . . . . . . . . . . . . . 5
1.3.2. DNS "Name" Terminology . . . . . . . . . . . . . . . . 5 1.3.2. DNS "Name" Terminology . . . . . . . . . . . . . . . . 5
1.3.3. New Terminology and Restrictions . . . . . . . . . . . 6 1.3.3. New Terminology and Restrictions . . . . . . . . . . . 6
1.4. Objectives . . . . . . . . . . . . . . . . . . . . . . . . 6 1.4. Objectives . . . . . . . . . . . . . . . . . . . . . . . . 7
1.5. Applicability and Function of IDNA . . . . . . . . . . . . 7 1.5. Applicability and Function of IDNA . . . . . . . . . . . . 7
1.6. Comprehensibility of IDNA Mechanisms and Processing . . . 8 1.6. Comprehensibility of IDNA Mechanisms and Processing . . . 8
2. Processing in IDNA2008 . . . . . . . . . . . . . . . . . . . . 9 2. Processing in IDNA2008 . . . . . . . . . . . . . . . . . . . . 9
3. Permitted Characters: An Inclusion List . . . . . . . . . . . 10 3. Permitted Characters: An Inclusion List . . . . . . . . . . . 9
3.1. A Tiered Model of Permitted Characters and Labels . . . . 10 3.1. A Tiered Model of Permitted Characters and Labels . . . . 10
3.1.1. PROTOCOL-VALID . . . . . . . . . . . . . . . . . . . . 11 3.1.1. PROTOCOL-VALID . . . . . . . . . . . . . . . . . . . . 10
3.1.2. Characters Valid Only in Context With Others . . . . . 11 3.1.2. CONTEXTUAL RULE REQUIRED . . . . . . . . . . . . . . . 11
3.1.2.2. Rules and Their Application . . . . . . . . . . . 12 3.1.2.2. Rules and Their Application . . . . . . . . . . . 12
3.1.3. DISALLOWED . . . . . . . . . . . . . . . . . . . . . . 12 3.1.3. DISALLOWED . . . . . . . . . . . . . . . . . . . . . . 12
3.1.4. UNASSIGNED . . . . . . . . . . . . . . . . . . . . . . 13 3.1.4. UNASSIGNED . . . . . . . . . . . . . . . . . . . . . . 13
3.2. Registration Policy . . . . . . . . . . . . . . . . . . . 14 3.2. Registration Policy . . . . . . . . . . . . . . . . . . . 13
3.3. Layered Restrictions: Tables, Context, Registration, 3.3. Layered Restrictions: Tables, Context, Registration,
Applications . . . . . . . . . . . . . . . . . . . . . . . 14 Applications . . . . . . . . . . . . . . . . . . . . . . . 14
4. Issues that Constrain Possible Solutions . . . . . . . . . . . 15 4. Issues that Constrain Possible Solutions . . . . . . . . . . . 15
4.1. Display and Network Order . . . . . . . . . . . . . . . . 15 4.1. Display and Network Order . . . . . . . . . . . . . . . . 15
4.2. Entry and Display in Applications . . . . . . . . . . . . 16 4.2. Entry and Display in Applications . . . . . . . . . . . . 16
4.3. Linguistic Expectations: Ligatures, Digraphs, and 4.3. Linguistic Expectations: Ligatures, Digraphs, and
Alternate Character Forms . . . . . . . . . . . . . . . . 17 Alternate Character Forms . . . . . . . . . . . . . . . . 17
4.4. Case Mapping and Related Issues . . . . . . . . . . . . . 20 4.4. Case Mapping and Related Issues . . . . . . . . . . . . . 18
4.5. Right to Left Text . . . . . . . . . . . . . . . . . . . . 20 4.5. Right to Left Text . . . . . . . . . . . . . . . . . . . . 19
5. IDNs and the Robustness Principle . . . . . . . . . . . . . . 21 5. IDNs and the Robustness Principle . . . . . . . . . . . . . . 20
6. Front-end and User Interface Processing for Lookup . . . . . . 22 6. Front-end and User Interface Processing for Lookup . . . . . . 20
7. Migration from IDNA2003 and Unicode Version Synchronization . 25 7. Migration from IDNA2003 and Unicode Version Synchronization . 24
7.1. Design Criteria . . . . . . . . . . . . . . . . . . . . . 25 7.1. Design Criteria . . . . . . . . . . . . . . . . . . . . . 24
7.1.1. General IDNA Validity Criteria . . . . . . . . . . . . 25 7.1.1. Summary and Discussion of IDNA Validity Criteria . . . 24
7.1.2. Labels in Registration . . . . . . . . . . . . . . . . 26 7.1.2. Labels in Registration . . . . . . . . . . . . . . . . 25
7.1.3. Labels in Lookup . . . . . . . . . . . . . . . . . . . 27 7.1.3. Labels in Lookup . . . . . . . . . . . . . . . . . . . 26
7.2. Changes in Character Interpretations . . . . . . . . . . . 28 7.2. Changes in Character Interpretations . . . . . . . . . . . 27
7.3. More Flexibility in User Agents . . . . . . . . . . . . . 30 7.3. More Flexibility in User Agents . . . . . . . . . . . . . 28
7.4. The Question of Prefix Changes . . . . . . . . . . . . . . 31 7.4. The Question of Prefix Changes . . . . . . . . . . . . . . 30
7.4.1. Conditions Requiring a Prefix Change . . . . . . . . . 32 7.4.1. Conditions Requiring a Prefix Change . . . . . . . . . 30
7.4.2. Conditions Not Requiring a Prefix Change . . . . . . . 32 7.4.2. Conditions Not Requiring a Prefix Change . . . . . . . 31
7.4.3. Implications of Prefix Changes . . . . . . . . . . . . 33 7.4.3. Implications of Prefix Changes . . . . . . . . . . . . 31
7.5. Stringprep Changes and Compatibility . . . . . . . . . . . 33 7.5. Stringprep Changes and Compatibility . . . . . . . . . . . 31
7.6. The Symbol Question . . . . . . . . . . . . . . . . . . . 34 7.6. The Symbol Question . . . . . . . . . . . . . . . . . . . 32
7.7. Migration Between Unicode Versions: Unassigned Code 7.7. Migration Between Unicode Versions: Unassigned Code
Points . . . . . . . . . . . . . . . . . . . . . . . . . . 35 Points . . . . . . . . . . . . . . . . . . . . . . . . . . 33
7.8. Other Compatibility Issues . . . . . . . . . . . . . . . . 37 7.8. Other Compatibility Issues . . . . . . . . . . . . . . . . 35
8. Name Server Considerations . . . . . . . . . . . . . . . . . . 38 8. Name Server Considerations . . . . . . . . . . . . . . . . . . 35
8.1. Processing Non-ASCII Strings . . . . . . . . . . . . . . . 38 8.1. Processing Non-ASCII Strings . . . . . . . . . . . . . . . 36
8.2. DNSSEC Authentication of IDN Domain Names . . . . . . . . 38 8.2. DNSSEC Authentication of IDN Domain Names . . . . . . . . 36
8.3. Root and other DNS Server Considerations . . . . . . . . . 39 8.3. Root and other DNS Server Considerations . . . . . . . . . 37
9. Internationalization Considerations . . . . . . . . . . . . . 39 9. Internationalization Considerations . . . . . . . . . . . . . 37
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 39 10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 37
10.1. IDNA Character Registry . . . . . . . . . . . . . . . . . 40 10.1. IDNA Character Registry . . . . . . . . . . . . . . . . . 38
10.2. IDNA Context Registry . . . . . . . . . . . . . . . . . . 40 10.2. IDNA Context Registry . . . . . . . . . . . . . . . . . . 38
10.3. IANA Repository of IDN Practices of TLDs . . . . . . . . . 40 10.3. IANA Repository of IDN Practices of TLDs . . . . . . . . . 38
11. Security Considerations . . . . . . . . . . . . . . . . . . . 40 11. Security Considerations . . . . . . . . . . . . . . . . . . . 38
11.1. General Security Issues with IDNA . . . . . . . . . . . . 40 11.1. General Security Issues with IDNA . . . . . . . . . . . . 38
12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 41 12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 39
13. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 41 13. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 39
14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 42 14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 40
14.1. Normative References . . . . . . . . . . . . . . . . . . . 42 14.1. Normative References . . . . . . . . . . . . . . . . . . . 40
14.2. Informative References . . . . . . . . . . . . . . . . . . 43 14.2. Informative References . . . . . . . . . . . . . . . . . . 41
Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . 45 Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . 43
A.1. Changes between Version -00 and Version -01 of A.1. Changes between Version -00 and Version -01 of
draft-ietf-idnabis-rationale . . . . . . . . . . . . . . . 45 draft-ietf-idnabis-rationale . . . . . . . . . . . . . . . 43
A.2. Version -02 . . . . . . . . . . . . . . . . . . . . . . . 46 A.2. Version -02 . . . . . . . . . . . . . . . . . . . . . . . 44
A.3. Version -03 . . . . . . . . . . . . . . . . . . . . . . . 46 A.3. Version -03 . . . . . . . . . . . . . . . . . . . . . . . 44
A.4. Version -04 . . . . . . . . . . . . . . . . . . . . . . . 46 A.4. Version -04 . . . . . . . . . . . . . . . . . . . . . . . 44
A.5. Version -05 . . . . . . . . . . . . . . . . . . . . . . . 47 A.5. Version -05 . . . . . . . . . . . . . . . . . . . . . . . 45
A.6. Version -06 . . . . . . . . . . . . . . . . . . . . . . . 47 A.6. Version -06 . . . . . . . . . . . . . . . . . . . . . . . 45
A.7. Version -07 . . . . . . . . . . . . . . . . . . . . . . . 48 A.7. Version -07 . . . . . . . . . . . . . . . . . . . . . . . 46
A.8. Version -08 . . . . . . . . . . . . . . . . . . . . . . . 48 A.8. Version -08 . . . . . . . . . . . . . . . . . . . . . . . 46
A.9. Version -09 . . . . . . . . . . . . . . . . . . . . . . . 48 A.9. Version -09 . . . . . . . . . . . . . . . . . . . . . . . 46
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 49 A.10. Version -10 . . . . . . . . . . . . . . . . . . . . . . . 47
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 47
1. Introduction 1. Introduction
1.1. Context and Overview 1.1. Context and Overview
The original standards for Internationalized Domain Names (IDNs) were Internationalized Domain Names in Applications (IDNA) is a collection
completed and deployed starting in 2003. Those standards are known of standards that allow client applications to convert some Unicode
as Internationalized Domain Names in Applications (IDNA), taken from mnemonics to an ASCII-compatible encoding form ("ACE") which is a
the name of the highest level standard within the group, RFC 3490 valid DNS label containing only letters, digits, and hyphens. The
[RFC3490]. After those standards were deployed, a number of issues specific form of ACE label used by IDNA is called an "A-label". A
arose that led to a call for a new version of the IDNA protocol and client can look up an exact A-label in the existing DNS, so A-labels
the associated tables, including a subset of those described in a do not require any extensions to DNS, upgrades of DNS servers or
recent IAB report [RFC4690] and the need to update the system to deal updates to low-level client libraries. An A-label is recognizable
with newer versions of Unicode. This document further explains the from the prefix "xn--" before the characters produced by the Punycode
issues that have been encountered when they are important to algorithm [RFC3492], thus a user application can identify an A-label
understanding of the revised protocols. It also provides an overview and convert it into Unicode (or some local coded character set) for
of the new IDNA model and explanatory material for it. Additional display.
explanatory material for the specific components of the proposals
appears with the associated documents.
This document and the associated ones are written from the [[anchor3: Note in draft: The above discussion, and the rest of the
perspective of an IDNA-aware user, application, or implementation. text in this section, are very informal. In particular, the term
While they may reiterate fundamental DNS rules and requirements for "A-label" is used to refer to some things that don't meet all of the
the convenience of the reader, they make no attempt to be tests for A-labels. I have tightened it somewhat from the suggested
comprehensive about DNS principles and should not be considered as a text I received, but not very much. Is the current form ok with
substitute for a thorough understanding of the DNS protocols and everyone???]]
specifications.
A good deal of the background material that appeared in RFC 3490 On the registry side, IDNA allows a registry to offer
[RFC3490] has been removed from this update. That material is either Internationalized Domain Names (IDNs) for registration as A-labels.
of historical interest only or has been covered from a more recent A registry may offer any subset of valid IDNs, and may apply any
perspective in RFC 4690 [RFC4690]. 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 is subject to a few specific requirements that do not apply to
lookup.
This document is not normative. The information it provides is DNS clients and registries are subject to some differences in
intended to make the rules, tables, and protocol easier to understand requirements for handling IDNs. In particular, registries are urged
and to provide overview information and suggestions for zone to register only exact, valid A-labels, while clients might do some
administrators and others who need to make policy, deployment, and mapping to get from otherwise-invalid user input to a valid A-label.
similar decisions about IDNs.
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. The documents that made up both versions are
listed in Section 1.3.1. The characters that are valid in A-labels
are identified from rules listed in the Tables document
[IDNA2008-Tables], but validity can be derived from the Unicode
properties of those characters with a very few exceptions.
Traditionally, DNS labels are case-insensitive [RFC1034][RFC1035].
That pattern was preserved in IDNA2003, but if case rules are
enforced from one language, another language sometimes loses the
ability to treat two characters separately. Case-sensitivity 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 determines how and whether
the characters can be used in IDNA labels.
This document provides informational context for IDNA2008, including
terminology, background, and policy discussions.
1.2. Discussion Forum 1.2. Discussion Forum
[[ RFC Editor: please remove this section. ]] [[ RFC Editor: please remove this section. ]]
IDNA2008 is being discussed in the IETF "idnabis" Working Group and IDNA2008 is being discussed in the IETF "idnabis" Working Group and
on the mailing list idna-update@alvestrand.no on the mailing list idna-update@alvestrand.no
1.3. Terminology 1.3. Terminology
Terminology that is critical for understanding this document and the Terminology for IDNA2008 appears in [IDNA2008-Defs]. That document
rest of the documents that make up IDNA2008, appears in also contains a roadmap to the IDNA2008 document collection. No
[IDNA2008-Defs]. That document also contains roadmap to the IDNA2008 attempt should be made to understand this document without the
document collection. No attempt should be made to understand this definitions and concepts that appear there.
document without the definitions and concepts that appear there.
1.3.1. Documents and Standards 1.3.1. Documents and Standards
This document uses the term "IDNA2003" to refer to the set of This document uses the term "IDNA2003" to refer to the set of
standards that make up and support the version of IDNA published in standards published in 2003 to define IDNA: the IDNA base
2003, i.e., those commonly known as the IDNA base specification specification [RFC3490], Nameprep [RFC3491], Punycode [RFC3492], and
[RFC3490], Nameprep [RFC3491], Punycode [RFC3492], and Stringprep Stringprep [RFC3454].
[RFC3454]. In this document, those names are used to refer,
conceptually, to the individual documents, with the base IDNA
specification called just "IDNA".
The term "IDNA2008" is used to refer to a new version of IDNA as The term "IDNA2008" is used to refer to a new version of IDNA.
described in this document and in the documents described in the IDNA2008 is not dependent on any of the IDNA2003 specifications other
document listing of [IDNA2008-Defs]. IDNA2008 is not dependent on than the one for Punycode encoding. References to "these
any of the IDNA2003 specifications other than the one for Punycode specifications" or "these documents" are to the entire IDNA2008 set
encoding. References to "these specifications" or "these documents" listed in [IDNA2008-Defs].
are to the entire IDNA2008 set.
1.3.2. DNS "Name" Terminology 1.3.2. DNS "Name" Terminology
These documents depart from historical DNS terminology and usage in In the context of IDNs, the DNS term 'name' has introduced some
one important respect. Over the years, the community has talked very confusion as people speak of DNS labels in terms of the words or
casually about "names" in the DNS, beginning with calling it "the phrases of various natural languages. Historically, many of the
domain name system". That terminology is fine in the very precise "names" in the DNS have been mnemonics to identify some particular
sense that the identifiers of the DNS do provide names for objects concept, object, or organization. They are typically rooted in some
and addresses. But, in the context of IDNs, the term has introduced
some confusion, confusion that has increased further as people have
begun to speak of DNS labels in terms of the words or phrases of
various natural languages.
Historically, many, perhaps most, of the "names" in the DNS have been
mnemonics to identify some particular concept, object, or
organization. They are typically derived from, or rooted in, some
language because most people think in language-based ways. But, language because most people think in language-based ways. But,
because they are mnemonics, they need not obey the orthographic because they are mnemonics, they need not obey the orthographic
conventions of any language: it is not a requirement that it be conventions of any language: it is not a requirement that it be
possible for them to be "words". possible for them to be "words".
This distinction is important because the reasonable goal of an IDN 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 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 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 broad range of mnemonics in ways that are as natural as possible in a
very broad range of scripts. very broad range of scripts.
skipping to change at page 6, line 43 skipping to change at page 7, line 7
o to permit future extensions that would require changing the o to permit future extensions that would require changing the
prefix, no matter how unlikely those might be (see Section 7.4); prefix, no matter how unlikely those might be (see Section 7.4);
and and
o to reduce the opportunities for attacks via the Punycode encoding o to reduce the opportunities for attacks via the Punycode encoding
algorithm itself. algorithm itself.
1.4. Objectives 1.4. Objectives
The intent of the IDNA revision effort, and hence of this document These are the main objectives in revising IDNA.
and the associated ones, is to increase the usability and
effectiveness of internationalized domain names (IDNs) while o Use a more recent version of Unicode, and allow IDNA to be
preserving or strengthening the integrity of references that use independent of Unicode versions, so that IDNA2008 need not be
them. The original "hostname" character definitions (see, e.g., update for implementations to adopt codepoints from new Unicode
[RFC0810]) struck a balance between the creation of useful mnemonics versions.
and the introduction of parsing problems or general confusion in the
contexts in which domain names are used. The objective of IDNA2008 o Fix a very small number of code-point categorizations that have
is to preserve that balance while expanding the character repertoire turned out to cause problems in the communities that use those
to include extended versions of Roman-derived scripts and scripts code-points.
that are not Roman in origin. No work of this sort is able to
completely eliminate sources of visual or textual confusion: such o Reduce the dependency on mapping, in order that the pre-mapped
confusion is possible even under the original host naming rules where forms (which are not valid IDNA labels) tend to appear less often
only ASCII characters were permitted. However, through the in various contexts, in favor of valid A-labels.
application of different techniques at different points (see
Section 3.3), it should be possible to keep problems to an acceptable o Fix some details in the bidirectional codepoint handling
minimum. One consequence of this general objective is that the algorithms.
desire of some user or marketing community to use a particular string
--whether the reason is to try to write sentences of particular
languages in the DNS, to express a facsimile of the symbol for a
brand, or for some other purpose-- is not a primary goal within the
context of applications in the domain name space.
1.5. Applicability and Function of IDNA 1.5. Applicability and Function of IDNA
The IDNA specification solves the problem of extending the repertoire The IDNA specification solves the problem of extending the repertoire
of characters that can be used in domain names to include a large of characters that can be used in domain names to include a large
subset of the Unicode repertoire. subset of the Unicode repertoire.
IDNA does not extend the service offered by DNS to the applications. IDNA does not extend DNS. Instead, the applications (and, by
Instead, the applications (and, by implication, the users) continue implication, the users) continue to see an exact-match lookup
to see an exact-match lookup service. Either there is a single service. Either there is a single exactly-matching (subject to the
exactly-matching (subject to the base DNS requirement of case- base DNS requirement of case-insensitive ASCII matching) name or
insensitive ASCII matching) name or there is no match. This model there is no match. This model has served the existing applications
has served the existing applications well, but it requires, with or well, but it requires, with or without internationalized domain
without internationalized domain names, that users know the exact names, that users know the exact spelling of the domain names that
spelling of the domain names that are to be typed into applications are to be typed into applications such as web browsers and mail user
such as web browsers and mail user agents. The introduction of the agents. The introduction of the larger repertoire of characters
larger repertoire of characters potentially makes the set of potentially makes the set of misspellings larger, especially given
misspellings larger, especially given that in some cases the same that in some cases the same appearance, for example on a business
appearance, for example on a business card, might visually match card, might visually match several Unicode code points or several
several Unicode code points or several sequences of code points. sequences of code points.
The IDNA standard does not require any applications to conform to it, The IDNA standard does not require any applications to conform to it,
nor does it retroactively change those applications. An application nor does it retroactively change those applications. An application
can elect to use IDNA in order to support IDN while maintaining can elect to use IDNA in order to support IDN while maintaining
interoperability with existing infrastructure. If an application interoperability with existing infrastructure. If an application
wants to use non-ASCII characters in domain names, IDNA is the only wants to use non-ASCII characters in domain names, IDNA is the only
currently-defined option. Adding IDNA support to an existing currently-defined option. Adding IDNA support to an existing
application entails changes to the application only, and leaves room application entails changes to the application only, and leaves room
for flexibility in front-end processing and more specifically in the for flexibility in front-end processing and more specifically in the
user interface (see Section 6). user interface (see Section 6).
skipping to change at page 8, line 29 skipping to change at page 8, line 35
In order to allow user-friendly input and output of the IDNs and In order to allow user-friendly input and output of the IDNs and
acceptance of some characters as equivalent to those to be processed acceptance of some characters as equivalent to those to be processed
according to the protocol, the applications need to be modified to according to the protocol, the applications need to be modified to
conform to this specification. conform to this specification.
This version of IDNA uses the Unicode character repertoire, for This version of IDNA uses the Unicode character repertoire, for
continuity with the original version of IDNA. continuity with the original version of IDNA.
1.6. Comprehensibility of IDNA Mechanisms and Processing 1.6. Comprehensibility of IDNA Mechanisms and Processing
One of the major goals of this work is to improve the general One goal of IDNA2008, which is aided by the main goal of reducing the
understanding of how IDNA works and what characters are permitted and dependency on mapping, is to improve the general understanding of how
what happens to them. Comprehensibility and predictability to users to users and registrants are important design goals for this effort.
and registrants are themselves important motivations and design goals End-user applications have an important role to play in increasing
for this effort. The effort includes some new terminology and a this comprehensibility.
revised and extended model, both covered in this section, and some
more specific protocol, processing, and table modifications. Details
of the latter appear in other documents (see [IDNA2008-Defs]).
Several issues are inherent in the application of IDNs and, indeed, Any system that tries to handle international characters encounters
almost any other system that tries to handle international characters some common problems. For example, a UI cannot display a character
and concepts. They range from the apparently trivial --e.g., one if no font for that character is available. In some cases,
cannot display a character for which one does not have a font internationalization enables effective localization while maintaining
available locally-- to the more complex and subtle. Many people have some global uniformity but losing some universality.
observed that internationalization is just a tool to enable effective
localization while permitting some global uniformity. Issues of
display, of exactly how various strings and characters are entered,
and so on are inherently issues about localization and user interface
design.
A protocol such as IDNA can only assume that such operations as data It is difficult to even make suggestions for end-user applications to
entry and reconciliation of differences in character forms are cope when characters and fonts are not available. Because display
possible. It may make some recommendations about how display might functions are rarely controlled by the types of applications that
work when characters and fonts are not available, but they can only would call upon IDNA, such suggestions will rarely be very effective.
be general recommendations and, because display functions are rarely
controlled by the types of applications that would call upon IDNA,
will rarely be very effective.
However, shifting responsibility for character mapping and other Converting between local character sets and normalized Unicode, if
adjustments from the protocol (where it was located in IDNA2003) to needed, is part of this set of user agent issues. This conversion
the user interface or processing before invoking IDNA raises issues introduces complexity in a system that is not Unicode-native. If a
about both what that processing should do and about compatibility for label is converted to a local character set that does not have all
references prepared in an IDNA2003 context. Those issues are the needed characters, the user agent may have to add special logic
discussed in Section 6. to avoid or reduce loss of information.
Operations for converting between local character sets and normalized The major difficulty may lie in accurately identifying the incoming
Unicode are part of this general set of user interface issues. The character set and applying the correct conversion routine. Even more
conversion is obviously not required at all in a Unicode-native difficult, the local character coding system could be based on
system that maintains all strings in Normalization Form C (NFC). conceptually different assumptions than those used by Unicode (e.g.,
(See [Unicode-UAX15] for precise definitions of NFC and NFKC if choice of font encodings used for publications in some Indic
needed.) It may, however, involve some complexity in a system that scripts). Those differences may not easily yield unambiguous
is not Unicode-native, especially if the elements of the local conversions or interpretations even if each coding system is
character set do not map exactly and unambiguously into Unicode internally consistent and adequate to represent the local language
characters or do so in a way that is not completely stable over time. and script.
Perhaps more important, if a label being converted to a local
character set contains Unicode characters that have no correspondence
in that character set, the application may have to apply special,
locally-appropriate, methods to avoid or reduce loss of information.
Depending on the system involved, the major difficulty may not lie in IDNA2008 shifts responsibility for character mapping and other
the mapping but in accurately identifying the incoming character set adjustments from the protocol (where it was located in IDNA2003) to
and then applying the correct conversion routine. If a local pre-processing before invoking IDNA. The intent is that this change
operating system uses one of the ISO 8859 character sets or an leads to greater usage of fully-valid A-Labels in display, transit
extensive national or industrial system such as GB18030 [GB18030] or and storage, which should aid comprehensibility. A careful look at
BIG5 [BIG5], one must correctly identify the character set in use pre-processing raises issues about what that pre-processing should do
before converting to Unicode even though those character coding and at what point pre-processing becomes harmful, how universally
systems are substantially or completely Unicode-compatible (i.e., all consistent pre-processing algorithms can be, and how to be compatible
of the code points in them have an exact and unique mapping to with labels prepared in a IDNA2003 context. Those issues are
Unicode code points). It may be even more difficult when the discussed in Section 6. [[anchor9: Fix section reference.]]
character coding system in local use is based on conceptually
different assumptions than those used by Unicode about, e.g., 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.
2. Processing in IDNA2008 2. Processing in IDNA2008
These specifications separate Domain Name Registration and Lookup in These specifications separate Domain Name Registration and Lookup in
the protocol specification. Doing so reflects current practice in the protocol specification. This separation reflects current
which per-registry restrictions and special processing are applied at practice in which per-registry restrictions and special processing
registration time but not during lookup. Even more important in the are applied at registration time but not during lookup. Another
longer term, it facilitates incremental addition of permitted significant benefit is that separation facilitates incremental
character groups to avoid freezing on one particular version of addition of permitted character groups to avoid freezing on one
Unicode. particular version of Unicode.
The actual registration and lookup protocols for IDNA2008 are The actual registration and lookup protocols for IDNA2008 are
specified in [IDNA2008-Protocol]. specified in [IDNA2008-Protocol].
3. Permitted Characters: An Inclusion List 3. Permitted Characters: An Inclusion List
IDNA2008 adopts the inclusion model. A code-point is assumed to be
invalid, 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 This section provides an overview of the model used to establish the
algorithm and character lists of [IDNA2008-Tables] and describes the algorithm and character lists of [IDNA2008-Tables] and describes the
names and applicability of the categories used there. Note that the names and applicability of the categories used there. Note that the
inclusion of a character in the first category group (Section 3.1.1) inclusion of a character in the first category group (Section 3.1.1)
does not imply that it can be used indiscriminately; some characters does not imply that it can be used indiscriminately; some characters
are associated with contextual rules that must be applied as well. are associated with contextual rules that must be applied as well.
The information given in this section is provided to make the rules, The information given in this section is provided to make the rules,
tables, and protocol easier to understand. The normative generating tables, and protocol easier to understand. The normative generating
rules that correspond to this informal discussion appear in rules that correspond to this informal discussion appear in
[IDNA2008-Tables] and the rules that actually determine what labels [IDNA2008-Tables] and the rules that actually determine what labels
can be registered or looked up are in [IDNA2008-Protocol]. can be registered or looked up are in [IDNA2008-Protocol].
3.1. A Tiered Model of Permitted Characters and Labels 3.1. A Tiered Model of Permitted Characters and Labels
Moving to an inclusion model requires respecifying the list of Moving to an inclusion model involves a new specification for the
characters that are permitted in IDNs. In IDNA2003, the role and list of characters that are permitted in IDNs. In IDNA2003,
utility of characters are independent of context and fixed forever character validity is independent of context and fixed forever (or
(or until the standard is replaced). Making completely context- until the standard is replaced). However, globally context-
independent rules globally has proven impractical because some independent rules have proved to be impractical because some
characters, especially those that are called "Join_Controls" in characters, especially those that are called "Join_Controls" in
Unicode, are needed to make reasonable use of some scripts but have Unicode, are needed to make reasonable use of some scripts but have
no visible effect(s) in others. IDNA2003 prohibited those types of no visible effect in others. IDNA2003 prohibited those types of
characters entirely. But the restrictions led to a consensus that characters entirely by discarding them. We now have a consensus that
under some conditions, these "joiner" characters were legitimately under some conditions, these "joiner" characters are legitimately
needed to allow useful mnemonics for some languages and scripts. The needed to allow useful mnemonics for some languages and scripts. In
requirement to support those characters but limit their use to very general, context-dependent rules help deal with characters that are
specific contexts was reinforced by the observation that handling of used differently across different scripts, and allow the standard to
particular characters across the languages that use a script, or the be applied more appropriately in cases where a string is not
use of similar or identical-looking characters in different scripts, universally handled the same way.
is more complex than many people believed it was several years ago.
Independently of the characters chosen (see next subsection), the IDNA2008 divides all possible Unicode code-points into four
approach is to divide the characters that appear in Unicode into categories: PROTOCOL-VALID, CONTEXTUAL RULE REQUIRED, DISALLOWED and
three categories: UNASSIGNED.
3.1.1. PROTOCOL-VALID 3.1.1. PROTOCOL-VALID
Characters identified as "PROTOCOL-VALID" (often abbreviated Characters identified as "PROTOCOL-VALID" (often abbreviated
"PVALID") are, in general, permitted by IDNA for all uses in IDNs. "PVALID") are permitted in IDNs. Their use may be restricted by
Their use may be restricted by rules about the context in which they rules about the context in which they appear or by other rules that
appear or by other rules that apply to the entire label in which they apply to the entire label in which they are to be embedded. For
are to be embedded. For example, any label that contains a character example, any label that contains a character in this category that
in this category that has a "right-to-left" property must be used in has a "right-to-left" property must be used in context with the
context with the "Bidi" rules (see [IDNA2008-Bidi]). "Bidi" rules (see [IDNA2008-Bidi]).
The term "PROTOCOL-VALID" is used to stress the fact that the 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 presence of a character in this category does not imply that a given
registry need accept registrations containing any of the characters registry need accept registrations containing any of the characters
in the category. Registries are still expected to apply judgment in the category. Registries are still expected to apply judgment
about labels they will accept and to maintain rules consistent with about labels they will accept and to maintain rules consistent with
those judgments (see [IDNA2008-Protocol] and Section 3.3). those judgments (see [IDNA2008-Protocol] and Section 3.3).
Characters that are placed in the "PROTOCOL-VALID" category are Characters that are placed in the "PROTOCOL-VALID" category are
expected to never be removed from it or reclassified. While expected to never be removed from it or reclassified. While
theoretically characters could be removed from Unicode, such removal theoretically characters could be removed from Unicode, such removal
would be inconsistent with the Unicode stability principles (see would be inconsistent with the Unicode stability principles (see
[Unicode51], Appendix F) and hence should never occur. [Unicode51], Appendix F) and hence should never occur.
3.1.2. Characters Valid Only in Context With Others 3.1.2. CONTEXTUAL RULE REQUIRED
Some characters may be unsuitable for general use in IDNs but Some characters may be unsuitable for general use in IDNs but
necessary for the plausible support of some scripts. The two most necessary for the plausible support of some scripts. The two most
commonly-cited examples are the zero-width joiner and non-joiner commonly-cited examples are the zero-width joiner and non-joiner
characters (ZWJ, U+200D and ZWNJ, U+200C), but provisions for characters (ZWJ, U+200D and ZWNJ, U+200C).
unambiguous labels may require that other characters be restricted to
particular contexts. For example, the ASCII hyphen is not permitted
to start or end a label, whether that label contains non-ASCII
characters or not.
3.1.2.1. Contextual Restrictions 3.1.2.1. Contextual Restrictions
These characters must not appear in IDNs without additional Characters with contextual restrictions are identified as "CONTEXTUAL
restrictions, typically because they have no visible consequences in RULE REQUIRED" and associated with a rule. The rule defines whether
most scripts but affect format or presentation in a few others or the character is valid in a particular string, and also whether the
because they are combining characters that are safe for use only in rule itself is to be applied on lookup as well as registration.
conjunction with particular characters or scripts. In order to
permit them to be used at all, they are specially identified as A distinction is made between characters that indicate or prohibit
"CONTEXTUAL RULE REQUIRED" and, when adequately understood, joining and ones similar to them (known as "CONTEXT-JOINER" or
associated with a rule. In addition, the rule will define whether it "CONTEXTJ") and other characters requiring contextual treatment
is to be applied on lookup as well as registration. A distinction is ("CONTEXT-OTHER" or "CONTEXTO"). Only the former require full
made between characters that indicate or prohibit joining (known as testing at lookup time.
"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 It is important to note that these contextual rules cannot prevent
all uses of the relevant characters that might be confusing or all uses of the relevant characters that might be confusing or
problematic. What they are expected do is to confine applicability problematic. What they are expected do is to confine applicability
of the characters to scripts (and narrower contexts) where zone of the characters to scripts (and narrower contexts) where zone
administrators are knowledgeable enough about the use of those administrators are knowledgeable enough about the use of those
characters to be prepared to deal with them appropriately. For characters to be prepared to deal with them appropriately. For
example, a registry dealing with an Indic script that requires ZWJ example, a registry dealing with an Indic script that requires ZWJ
and/or ZWNJ as part of the writing system is expected to understand 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 where the characters have visible effect and where they do not and to
make registration rules accordingly. By contrast, a registry dealing make registration rules accordingly. By contrast, a registry dealing
with Latin or Cyrillic script might not be actively aware that the with Latin or Cyrillic script might not be actively aware that the
characters exist, much less about the consequences of embedding them characters exist, much less about the consequences of embedding them
in labels drawn from those scripts. in labels drawn from those scripts.
3.1.2.2. Rules and Their Application 3.1.2.2. Rules and Their Application
The actual rules may be DEFINED or NULL. If present, they may have Rules have descriptions such as "Must follow a character from Script
values of "True" (character may be used in any position in any XYZ", "Must occur only if the entire label is in Script ABC", or
label), "False" (character may not be used in any label), or may be a "Must occur only if the previous and subsequent characters have the
set of procedural rules that specify the context in which the DFG property". The actual rules may be DEFINED or NULL. If present,
character is permitted. 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.
Examples of descriptions of typical rules, stated informally and in Examples of descriptions of typical rules, stated informally and in
English, include "Must follow a character from Script XYZ", "Must English, include "Must follow a character from Script XYZ", "Must
occur only if the entire label is in Script ABC", "Must occur only if occur only if the entire label is in Script ABC", "Must occur only if
the previous and subsequent characters have the DFG property". the previous and subsequent characters have the DFG property".
Because it is easier to identify these characters than to know that Because it is easier to identify these characters than to know that
they are actually needed in IDNs or how to establish exactly the 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 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 version of the tables. Characters associated with null rules are not
permitted to appear in putative labels for either registration or permitted to appear in putative labels for either registration or
lookup. Of course, a later version of the tables might contain a lookup. Of course, a later version of the tables might contain a
non-null rule. non-null rule.
The description of the syntax of the rules, and the rules themselves, The actual rules and their descriptions are in [IDNA2008-Tables].
appears in [IDNA2008-Tables]. [[anchor11: ??? Section number would [[anchor12: ??? Section number would be good here.]] That document
be good here.]] also creates a registry for future rules.
3.1.3. DISALLOWED 3.1.3. DISALLOWED
Some characters are inappropriate for use in IDNs and are thus Some characters are inappropriate for use in IDNs and are thus
excluded for both registration and lookup (i.e., IDNA-conforming excluded for both registration and lookup (i.e., IDNA-conforming
applications performing name lookup should verify that these applications performing name lookup should verify that these
characters are absent; if they are present, the label strings should characters are absent; if they are present, the label strings should
be rejected rather than converted to A-labels and looked up. Some of be rejected rather than converted to A-labels and looked up. Some of
these characters are problematic for use in IDNs (such as the these characters are problematic for use in IDNs (such as the
FRACTION SLASH character, U+2044), while some of them (such as the FRACTION SLASH character, U+2044), while some of them (such as the
skipping to change at page 14, line 45 skipping to change at page 14, line 32
Identifiers" in [Unicode-Security]. Identifiers" in [Unicode-Security].
It is worth stressing that these principles of policy development and It is worth stressing that these principles of policy development and
application apply at all levels of the DNS, not only, e.g., TLD or application apply at all levels of the DNS, not only, e.g., TLD or
SLD registrations and that even a trivial, "anything permitted that SLD registrations and that even a trivial, "anything permitted that
is valid under the protocol" policy is helpful in that it helps users is valid under the protocol" policy is helpful in that it helps users
and application developers know what to expect. and application developers know what to expect.
3.3. Layered Restrictions: Tables, Context, Registration, Applications 3.3. Layered Restrictions: Tables, Context, Registration, Applications
The essence of the character rules in IDNA2008 is based on the The character rules in IDNA2008 are based on the realization that
realization that there is no single magic bullet for any of the there is no single magic bullet for any of the issues associated with
issues associated with a multiscript DNS. Instead, the IDNs. Instead, the specifications define a variety of approaches.
specifications define a variety of approaches that, together, The character tables are the first mechanism, protocol rules about
constitute multiple lines of defense against ambiguity in identifiers how those characters are applied or restricted in context are the
and loss of referential integrity. The actual character tables are second, and those two in combination constitute the limits of what
the first mechanism, protocol rules about how those characters are can be done in the protocol. As discussed in the previous section
applied or restricted in context are the second, and those two in (Section 3.2), registries are expected to restrict what they permit
combination constitute the limits of what can be done by a protocol to be registered, devising and using rules that are designed to
alone. As discussed in the previous section (Section 3.2), optimize the balance between confusion and risk on the one hand and
registries are expected to restrict what they permit to be maximum expressiveness in mnemonics on the other.
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 agents in warning In addition, there is an important role for user agents in warning
against label forms that appear problematic given their knowledge of against label forms that appear problematic given their knowledge of
local contexts and conventions. Of course, no approach based on local contexts and conventions. Of course, no approach based on
naming or identifiers alone can protect against all threats. naming or identifiers alone can protect against all threats.
4. Issues that Constrain Possible Solutions 4. Issues that Constrain Possible Solutions
4.1. Display and Network Order 4.1. Display and Network Order
The correct treatment of domain names requires a clear distinction Domain names are always transmitted in network order (the order in
between Network Order (the order in which the code points are sent in which the code points are sent in protocols), but may have a
protocols) and Display Order (the order in which the code points are different display order (the order in which the code points are
displayed on a screen or paper). The order of labels in a domain displayed on a screen or paper). When a domain name contains
name that contains characters that are normally written right to left characters that are normally written right to left, display order may
is discussed in [IDNA2008-Bidi]. In particular, there are questions be affected although network order is not. It gets even more
about the order in which labels are displayed if left to right and complicated if left to right and right to left labels are adjacent to
right to left labels are adjacent to each other, especially if there each other within a domain name. The decision about the display
are also multiple consecutive appearances of one of the types. The order is ultimately under the control of user agents --including Web
decision about the display order is ultimately under the control of browsers, mail clients, hosted Web applications and many more --
user agents --including web browsers, mail clients, and the like-- which may be highly localized. Should a domain name abc.def, in
which may be highly localized. Even when formats are specified by which both labels are represented in scripts that are written right
protocols, the full composition of an Internationalized Resource to left, be displayed as fed.cba or cba.fed? Applications that are
Identifier (IRI) [RFC3987] or Internationalized Email address in deployment today are already diverse, and one can find examples of
contains elements other than the domain name. For example, IRIs either choice.
contain protocol identifiers and field delimiter syntax such as
"http://" or "mailto:" while email addresses contain the "@" to
separate local parts from domain names. User agents are not required
to use those protocol-based forms directly but often do so. While
display, parsing, and processing within a label is specified by the
normative documents in the IDNA2008 collection, the relationship
between fully-qualified domain names and internationalized labels is
unchanged from the base DNS specifications. Comments in this
document about such full domain names are explanatory or examples of
what might be done and must not be considered normative.
Questions remain about protocol constraints implying that the overall
direction of these strings will always be left to right (or right to
left) for an IRI or email address, or if they even should conform to
such rules. These questions also have several possible answers.
Should a domain name abc.def, in which both labels are represented in The picture changes once again when an IDN appears in a
scripts that are written right to left, be displayed as fed.cba or Internationalized Resource Identifier (IRI) [RFC3987]. An IRI or
cba.fed? An IRI for clear text web access would, in network order, Internationalized Email address contains elements other than the
begin with "http://" and the characters will appear as domain name. For example, IRIs contain protocol identifiers and
"http://abc.def" -- but what does this suggest about the display field delimiter syntax such as "http://" or "mailto:" while email
order? When entering a URI to many browsers, it may be possible to addresses contain the "@" to separate local parts from domain names.
provide only the domain name and leave the "http://" to be filled in An IRI in network order begins with "http://" followed by domain
by default, assuming no tail (an approach that does not work for labels in network order, thus "http://abc.def".
protocols other than HTTP or whatever is chosen as the default). The
natural display order for the typed domain name on a right to left
system is fed.cba. Does this change if a protocol identifier, tail,
and the corresponding delimiters are specified?
While logic, precedent, and reality suggest that these are questions User agents are not required to display and allow input of IRIs
for user interface design, not IETF protocol specifications, directly but often do so. Implementors have to choose whether the
experience in the 1980s and 1990s with mixing systems in which domain overall direction of these strings will always be left to right (or
name labels were read in network order (left to right) and those in right to left) for an IRI or email address. The natural order for a
which those labels were read right to left would predict a great deal user typing a domain name on a right to left system is fed.cba.
of confusion, and heuristics that sometimes fail, if each Should the R2L user agent reverse the entire domain name each time a
implementation of each application makes its own decisions on these domain name is typed? Does this change if the user types "http://"
issues. 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.
Any version of IDNA, including the current one, must be written in If each implementation of each application makes its own decisions on
terms of the network (transmission on the wire) order of characters these issues, users will develop heuristics that will sometimes fail
in labels and for the labels in complete (fully-qualified) domain when switching applications. However, while some display order
names and must be quite precise about those relationships. While conventions, voluntarily adopted, would be desirable to reduce
some strong suggestions about display order would be desirable to confusion, such suggestions are beyond the scope of these
reduce the chances for inconsistent transcription of domain names
from printed form, such suggestions are beyond the scope of these
specifications. specifications.
4.2. Entry and Display in Applications 4.2. Entry and Display in Applications
Applications can accept domain names using any character set or sets Applications can accept and display domain names using any character
desired by the application developer, specified by the operating set or character coding system. That is, the IDNA protocol does not
system, or dictated by other constraints, and can display domain necessarily affect the interface between users and applications. An
names in any character set or character coding system. That is, the IDNA-aware application can accept and display internationalized
IDNA protocol does not affect the interface between users and
applications.
An IDNA-aware application can accept and display internationalized
domain names in two formats: the internationalized character set(s) domain names in two formats: the internationalized character set(s)
supported by the application (i.e., an appropriate local supported by the application (i.e., an appropriate local
representation of a U-label), and as an A-label. Applications may representation of a U-label), and as an A-label. Applications may
allow the display of A-labels, but are encouraged to not do so except allow the display of A-labels, but are encouraged to not do so except
as an interface for special purposes, possibly for debugging, or to as an interface for special purposes, possibly for debugging, or to
cope with display limitations. In general, they should allow, but cope with display limitations. In general, they should allow, but
not encourage, user input of that label form. A-labels are opaque not encourage, user input of A-labels. A-labels are opaque and ugly
and ugly and malicious variations on them are not easily detected by and malicious variations on them are not easily detected by users.
users. Where possible, they should thus only be exposed to users and Where possible, they should thus only be exposed when they are
in contexts in which they are absolutely needed. Because IDN labels absolutely needed. Because IDN labels can be rendered either as
can be rendered either as A-labels or U-labels, the application may A-labels or U-labels, the application may reasonably have an option
reasonably have an option for the user to select the preferred method for the user to select the preferred method of display. Rendering
of display; if it does, rendering the U-label should normally be the the U-label should normally be the default.
default.
Domain names are often stored and transported in many places. For Domain names are often stored and transported in many places. For
example, they are part of documents such as mail messages and web example, they are part of documents such as mail messages and web
pages. They are transported in many parts of many protocols, such as pages. They are transported in many parts of many protocols, such as
both the control commands of SMTP and associated the message body both the control commands of SMTP and associated the message body
parts, and in the headers and the body content in HTTP. It is parts, and in the headers and the body content in HTTP. It is
important to remember that domain names appear both in domain name important to remember that domain names appear both in domain name
slots and in the content that is passed over protocols. slots and in the content that is passed over protocols.
In protocols and document formats that define how to handle In protocols and document formats that define how to handle
specification or negotiation of charsets, labels can be encoded in specification or negotiation of charsets, labels can be encoded in
any charset allowed by the protocol or document format. If a any charset allowed by the protocol or document format. If a
protocol or document format only allows one charset, the labels must protocol or document format only allows one charset, the labels must
be given in that charset. Of course, not all charsets can properly be given in that charset. Of course, not all charsets can properly
represent all labels. If a U-label cannot be displayed in its represent all labels. If a U-label cannot be displayed in its
entirety, the only choice (without loss of information) may be to entirety, the only choice (without loss of information) may be to
display the A-label. display the A-label.
In any place where a protocol or document format allows transmission Where a protocol or document format allows IDNs, labels should be in
of the characters in internationalized labels, labels should be whatever character encoding and escape mechanism the protocol or
transmitted using whatever character encoding and escape mechanism document format uses at that place. This provision is intended to
the protocol or document format uses at that place. This provision prevent situations in which, e.g., UTF-8 domain names appear embedded
is intended to prevent situations in which, e.g., UTF-8 domain names in text that is otherwise in some other character coding.
appear embedded in text that is otherwise in some other character
coding.
All protocols that use domain name slots (See Section 2.3.1.6 All protocols that use domain name slots (See Section 2.3.1.6
[[anchor14: ?? Verify this]] in [IDNA2008-Defs]) already have the [[anchor15: ?? Verify this]] in [IDNA2008-Defs]) already have the
capacity for handling domain names in the ASCII charset. Thus, capacity for handling domain names in the ASCII charset. Thus,
A-labels can inherently be handled by those protocols. A-labels can inherently be handled by those protocols.
4.3. Linguistic Expectations: Ligatures, Digraphs, and Alternate 4.3. Linguistic Expectations: Ligatures, Digraphs, and Alternate
Character Forms Character Forms
[[anchor15: There is some internal redundancy and repetition in the Users have expectations about character matching or equivalence that
material in this section. Specific suggestions about to reduce or are based on their own languages and the orthography of those
eliminate redundant text would be appreciated. If no such languages. These expectations may not always be met in a global
suggestions are received before -07 is posted, this note will be
removed.]]
Users often have expectations about character matching or equivalence
that are based on their own languages and the orthography of those
languages. These expectations may not be consistent with forms or
actions that can be naturally accommodated in a character coding
system, especially if multiple languages are written using the same system, especially if multiple languages are written using the same
script but using different conventions. A Norwegian user might script but using different conventions. Some examples:
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. A
user in German might expect a label with an o-umlaut and a label that
had "oe" substituted, but was otherwise the same, treated as
equivalent even though that substitution would be a clear error in
Swedish. 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. For that matter, an English user might expect "theater"
and "theatre" to match.
Related issues arise because there are a number of languages written o A Norwegian user might expect a label with the ae-ligature to be
with alphabetic scripts in which single phonemes are written using treated as the same label as one using the Swedish spelling with
two characters, termed a "digraph", for example, the "ph" in a-diaeresis even though applying that mapping to English would be
"pharmacy" and "telephone". (Note that characters paired in this astonishing to users.
manner can also appear consecutively without forming a digraph, as in
"tophat".) Certain digraphs are normally 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 (strictly designating setting totally
without intervening white space, although the term is sometimes
applied to close set pairs). An example of this may be seen when the
word "encyclopaedia" is set with a U+00E6 LATIN SMALL LIGATURE AE
(and some would not consider that word correctly spelled unless the
ligature form was used or the "a" was dropped entirely). When these
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, any requirements
for matching must utilize other methods, presumably at the registry
level, or users must be educated to understand that matching will not
occur.
Difficulties arise from the fact that a given ligature may be a o A user in German might expect a label with an o-umlaut and a label
completely optional typographic convenience for representing a that had "oe" substituted, but was otherwise the same, treated as
digraph in one language (as in the above example with some spelling equivalent even though that substitution would be a clear error in
conventions), while in another language it is a single character that Swedish.
may not always be correctly representable by a two-letter sequence
(as in the above example with different spelling conventions). This o A Chinese user might expect automatic matching of Simplified and
can be illustrated by many words in the Norwegian language, where the Traditional Chinese characters, but applying that matching for
"ae" ligature is the 27th letter of a 29-letter extended Latin Korean or Japanese text would create considerable confusion.
alphabet. It is equivalent to the 28th letter of the Swedish
alphabet (also containing 29 letters), U+00E4 LATIN SMALL LETTER A o An English user might expect "theater" and "theatre" to match.
WITH DIAERESIS, for which an "ae" cannot be substituted according to
current orthographic standards. 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 completely optional typographic convenience
for representing a digraph in one language (as in the above example
with some spelling conventions), while in another language it is a
single character that may not always be correctly representable by a
two-letter sequence (as in the above example with different spelling
conventions). This 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, That character (U+00E4) is also part of the German alphabet where,
unlike in the Nordic languages, the two-character sequence "ae" is unlike in the Nordic languages, the two-character sequence "ae" is
usually treated as a fully acceptable alternate orthography for the usually treated as a fully acceptable alternate orthography for the
"umlauted a" character. The inverse is however not true, and those "umlauted a" character. The inverse is however not true, and those
two characters cannot necessarily be combined into an "umlauted a". two characters cannot necessarily be combined into an "umlauted a".
This also applies to another German character, the "umlauted o" This also applies to another German character, the "umlauted o"
(U+00F6 LATIN SMALL LETTER O WITH DIAERESIS) which, for example, (U+00F6 LATIN SMALL LETTER O WITH DIAERESIS) which, for example,
cannot be used for writing the name of the author "Goethe". It is 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 also a letter in the Swedish alphabet where, like the "a with
skipping to change at page 19, line 28 skipping to change at page 18, line 28
Norwegian alphabet, where it is represented, not as "o with Norwegian alphabet, where it is represented, not as "o with
diaeresis", but as "slashed o", U+00F8. diaeresis", but as "slashed o", U+00F8.
Some of the ligatures that have explicit code points in Unicode were Some of the ligatures that have explicit code points in Unicode were
given special handling in IDNA2003 and now pose additional problems given special handling in IDNA2003 and now pose additional problems
in transition. See Section 7.2. in transition. See Section 7.2.
Additional cases with alphabets written right to left are described Additional cases with alphabets written right to left are described
in Section 4.5. in Section 4.5.
Whether ligatures and digraphs are to be treated as a sequence of Matching and comparison algorithm selection often requires
characters or as a single standalone one constitute a problem that information about the language being used, context, or both --
cannot be resolved solely by operating on scripts. They are, information that is not available to IDNA or the DNS. Consequently,
however, a key concern in the IDN context. Their satisfactory these specifications make no attempt to treat combined characters in
resolution will require support in policies set by registries, which any special way. A registry that is aware of the language context in
therefore need to be particularly mindful not just of this specific which labels are to be registered, and where that language sometimes
issue, but of all other related matters that cannot be dealt with on (or always) treats the two- character sequences as equivalent to the
an exclusively algorithmic and global basis. combined form, should give serious consideration to applying a
"variant" model [RFC3743] [RFC4290], or to prohibiting registration
Just as with the examples of different-looking characters that may be of one the forms entirely, to reduce the opportunities for user
assumed to be the same, it is in general impossible to deal with confusion and fraud that would result from the related strings being
these situations in a system such as IDNA -- or with Unicode registered to different parties.
normalization generally -- since determining what to do requires
information about the language being used, context, or both.
Consequently, these specifications make no attempt to treat these
combined characters in any special way. However, their existence
provides a prime example of a situation in which 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 the forms entirely,
to reduce the opportunities for user confusion and fraud that would
result from the related strings being registered to different
parties.
[[anchor16: Placeholder: A discussion of the Arabic digit issue [[anchor16: Placeholder: A discussion of the Arabic digit issue
should go here once it is resolved in some appropriate way.]] should go here once it is resolved in some appropriate way.]]
4.4. Case Mapping and Related Issues 4.4. Case Mapping and Related Issues
In the DNS, ASCII letters are stored with their case preserved. In the DNS, ASCII letters are stored with their case preserved.
Matching during the query process is case-independent, but none of Matching during the query process is case-independent, but none of
the information that might be represented by choices of case has been the information that might be represented by choices of case has been
lost. That model has been accidentally helpful because, as people lost. That model has been accidentally helpful because, as people
have created DNS labels by catenating words (or parts of words) to have created DNS labels by catenating words (or parts of words) to
form labels, case has often been used to distinguish among components form labels, case has often been used to distinguish among components
and make the labels more memorable. and make the labels more memorable.
The solution of keeping the characters separate but doing matching Since DNS servers do not get involved in parsing IDNs, they cannot do
independent of case is not feasible with IDNA or any IDNA-like model case-independent matching. Thus, keeping the cases separate in
because the matching would then have to be done on the server rather lookup or registration, and doing matching at the server, is not
than have characters mapped on the client. That situation was feasible with IDNA or any similar approach. Case-matching must be
recognized in IDNA2003 and nothing in these specifications done, if desired, by IDN clients even though it wasn't done by ASCII-
fundamentally changes it or could do so. In IDNA2003, all characters only DNS clients. That situation was recognized in IDNA2003 and
are case-folded and mapped. That results in upper-case characters nothing in these specifications fundamentally changes it or could do
being mapped to lower-case ones and in some other transformations of so. In IDNA2003, all characters are case-folded and mapped by
alternate forms of characters, especially those that do not have (or clients in a standardized step.
did not have) upper-case forms. For example, Greek Final Form Sigma
(U+03C2) is mapped to the medial form (U+03C3) and Eszett (German Some characters do not have upper case forms. For example the
Sharp S, U+00DF) is mapped to "ss". Neither of these mappings is Unicode case folding operation maps Greek Final Form Sigma (U+03C2)
reversible because the upper case of U+03C3 is the Upper Case Sigma to the medial form (U+03C3) and maps Eszett (German Sharp S, U+00DF)
(U+03A3) and "ss" is an ASCII string. IDNA2008 permits, at the risk to "ss". Neither of these mappings is reversible because the upper
of some incompatibility, slightly more flexibility in this area by case of U+03C3 is the Upper Case Sigma (U+03A3) and "ss" is an ASCII
avoid case folding and treating these characters as themselves. string. IDNA2008 permits, at the risk of some incompatibility,
Approaches to handling that incompatibility are discussed in slightly more flexibility in this area by avoid case folding and
Section 7.2. Although information is lost in IDNA2003's ToASCII treating these characters as themselves. Approaches to handling one-
operation so that, in some sense, neither Final Sigma nor Eszett can way mappings are discussed in Section 7.2.
be represented in an IDN at all, its guarantee of mapping when those
characters are used as input can be interpreted as violating one of Because IDNA2003 maps Final Sigma and Eszett to other characters, and
the conditions discussed in Section 7.4.1 and hence requiring a the reverse mapping is never possible, that in some sense means that
prefix change. The consensus was to not make a prefix change in neither Final Sigma nor Eszett can be represented in a IDNA2003 IDN.
spite of this issue. Of course, had a prefix change been made (at With IDNA2008, both characters can be used in an IDN and so the
the costs discussed in Section 7.4.3) there would have been several A-label used for lookup for any U-label containing those characters,
options, including, if desired, assignment of the character to the is now different. See Section 7.1 for a discussion of what kinds of
CONTEXTUAL RULE REQUIRED category and requiring that it only be used changes might require the IDNA prefix to change; this case is clearly
in carefully-selected contexts. worth discussing but the WG came to consensus not to make a prefix
change anyway.
4.5. Right to Left Text 4.5. Right to Left Text
In order to be sure that the directionality of right to left text is 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 unambiguous, IDNA2003 required that any label in which right to left
characters appear both starts and ends with them, not include any characters appear both starts and ends with them and that it not
characters with strong left to right properties (which excludes other include any characters with strong left to right properties (that
alphabetic characters but permits European digits), and rejects any excludes other alphabetic characters but permits European digits).
other string that contains a right to left character. This is one of Any other string that contains a right to left character and does not
the few places where the IDNA algorithms (both in IDNA2003 and in meet those requirements is rejected. This is one of the few places
IDAN2008) are required to examine an entire label, not just where the IDNA algorithms (both in IDNA2003 and in IDAN2008) examine
individual characters. The algorithmic model used in IDNA2003 an entire label, not just individual characters. The algorithmic
rejects the label when the final character in a right to left string model used in IDNA2003 rejects the label when the final character in
requires a combining mark in order to be correctly represented. 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 That prohibition is not acceptable for writing systems for languages
written with consonantal alphabets to which diacritical vocalic written with consonantal alphabets to which diacritical vocalic
systems are applied, and for languages with orthographies derived systems are applied, and for languages with orthographies derived
from them where the combining marks may have different functionality. from them where the combining marks may have different functionality.
In both cases the combining marks can be essential components of the In both cases the combining marks can be essential components of the
orthography. Examples of this are Yiddish, written with an extended orthography. Examples of this are Yiddish, written with an extended
Hebrew script, and Dhivehi (the official language of Maldives) which Hebrew script, and Dhivehi (the official language of Maldives) which
is written in the Thaana script (which is, in turn, derived from the is written in the Thaana script (which is, in turn, derived from the
Arabic script). IDNA2008 removes the restriction on final combining Arabic script). IDNA2008 removes the restriction on final combining
characters with a new set of rules for right to left scripts and characters with a new set of rules for right to left scripts and
their characters. Those new rules are specified in [IDNA2008-Bidi]. their characters. Those new rules are specified in [IDNA2008-Bidi].
5. IDNs and the Robustness Principle 5. IDNs and the Robustness Principle
The model of IDNs described in this document can be seen as a The "Robustness Principle" is often stated as "Be conservative about
particular instance of the "Robustness Principle" that has been so what you send and liberal in what you accept" (See, e.g., Section
important to other aspects of Internet protocol design. This 1.2.2 of the applications-layer Host Requirements specification
principle is often stated as "Be conservative about what you send and [RFC1123]) This principle applies to IDNA. In applying the principle
liberal in what you accept" (See, e.g., Section 1.2.2 of the to registries as the source ("sender") of all registered and useful
applications-layer Host Requirements specification [RFC1123]). For IDNs, registries are responsible for being conservative about what
IDNs to work well, not only must the protocol be carefully designed they register and put out in the Internet. For IDNs to work well,
and implemented, but zone administrators (registries) must have and zone administrators (registries) must have and require sensible
require sensible policies about what is registered -- conservative policies about what is registered -- conservative policies -- and
policies -- and implement and enforce them. implement and enforce them.
Conversely, lookup applications are expected to reject labels that Conversely, lookup applications are expected to reject labels that
clearly violate global (protocol) rules (no one has ever seriously clearly violate global (protocol) rules (no one has ever seriously
claimed that being liberal in what is accepted requires being claimed that being liberal in what is accepted requires being
stupid). However, once one gets past such global rules and deals stupid). However, once one gets past such global rules and deals
with anything sensitive to script or locale, it is necessary to 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 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 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 than guessing about whether it should have been permitted to be
registered. registered.
As mentioned elsewhere, if a string cannot be successfully found in If a string cannot be successfully found in the DNS after the lookup
the DNS after the lookup processing described here, it makes no processing described here, it makes no difference whether it simply
difference whether it simply wasn't registered or was prohibited by wasn't registered or was prohibited by some rule at the registry.
some rule at the registry. Applications should, however, be Application implementors should be aware that where DNS wildcards are
sensitive to the fact that, because of the possibility of DNS used, the ability to successfully resolve a name does not guarantee
wildcards, the ability to successfully resolve a name does not that it was actually registered.
guarantee that it was actually registered.
If lookup applications, as a user interface (UI) or other local
matter, decide to warn about some strings that are valid under the
global rules but that they perceive as dangerous, that is their
prerogative and we can only hope that the market (and maybe
regulators) will reinforce the good choices and discourage the poor
ones. In this context, a lookup application that decides a string
that is valid under the protocol is dangerous and refuses to look it
up is in violation of the protocols; one that is willing to look
something up, but warns against it, is exercising a local choice.
6. Front-end and User Interface Processing for Lookup 6. Front-end and User Interface Processing for Lookup
[[anchor18: Note in Draft: While this section has been revised in
version -10 to improve clarity, a significant revision is expected
once the discussions of mapping stabilize.]]
Domain names may be identified and processed in many contexts. They Domain names may be identified and processed in many contexts. They
may be typed in by users either by themselves or embedded in an may be typed in by users either by themselves or embedded in an
identifier structured for a particular protocol or class of protocols identifier such as email addresses, URIs, or IRIs. They may occur in
such a email addresses, URIs, or IRIs. They may occur in running running text or be processed by one system after being provided in
text or be processed by one system after being provided in another. another. Systems may try to normalize URLs to determine (or guess)
Systems may wish to try to normalize URLs so as to determine (or whether a reference is valid or two references point to the same
guess) whether a reference is valid or two references point to the object without actually looking the objects up (comparison without
same object without actually looking the objects up and comparing lookup is necessary for URI types that are not intended to be
them (that is necessary, not just a choice, for URI types that are resolved). Some of these goals may be more easily and reliably
not intended to be resolved). Some of these goals may be more easily satisfied than others. While there are strong arguments for any
and reliably satisfied than others. While there are strong arguments domain name that is placed "on the wire" -- transmitted between
for any domain name that is placed "on the wire" -- transmitted systems -- to be in the zero-ambiguity forms of A-labels, it is
between systems -- to be in the zero-ambiguity forms of A-labels, it inevitable that programs that process domain names will encounter
is inevitable that programs that process domain names will encounter
U-labels or variant forms. U-labels or variant forms.
One source of such forms will be labels created under IDNA2003 This section discusses these mapping and transformation issues among
because that protocol allowed labels that were transformed from names, contrasting IDNA2003 and IDNA2008 behavior. The discussion
native-character format by mapping some characters into others before applies only in operations that look up names or interpret files.
conversion into ACE ("xn--...") format. One consequence of the There are several reasons why registration activities should require
transformations was that, when the ToUnicode and ToASCII operations final names and verification of those names by the would-be
of IDNA2003 were applied, ToUnicode(ToASCII(original-label)) often registrant.
did not produce the original label. IDNA2008 explicitly defines
A-labels and U-labels as different forms of the same abstract label,
forms that are stable when conversions are performed between them
(without mappings). A different way of explaining this is that there
are, today, domain names in files on the Internet that use characters
that cannot be represented directly in, or recovered from, (A-label)
domain names but for which interpretations are provided by IDNA2003.
There are two major categories of such characters, those that are One source of label forms that are neither A-labels nor U-labels will
removed by NFKC normalization and those upper-case characters that be labels created under IDNA2003. That protocol allowed labels that
are mapped to lower-case (there are also a few characters that are were transformed from native-character format by mapping some
given special-case mapping treatment in Stringprep, including lower- characters into others before conversion into A-label format. One
case characters that are case-folded into other lower-case characters consequence of the transformations was that conversion from the
or strings). A-label format back to native characters often did not produce the
original label. IDNA2008 explicitly defines A-labels and U-labels as
different forms of the same abstract label, forms that are stable
when conversions are performed between them (without mappings).
A different way of explaining this is that there are, today, domain
names in files on the Internet that use characters that cannot be
represented directly in, or recovered from, (A-label) domain names
but for which interpretations were provided by IDNA2003). There are
two major categories of characters irreversibly remapped by
Stringprep, those that are removed by NFKC normalization and those
upper-case characters that are mapped to lower-case (there are also a
few characters that are given special-case mapping treatment,
including lower-case characters that are case-folded into other
lower-case characters or strings and those that are simply
eliminated).
Other issues in domain name identification and processing arise Other issues in domain name identification and processing arise
because IDNA2003 specified that several other characters be treated because IDNA2003 specified that several other characters be treated
as equivalent to the ASCII period (dot, full stop) character used as as equivalent to the ASCII period (dot, full stop) character used as
a label separator. If a string that might be a domain name appears a label separator. If a string that might be a domain name appears
in an arbitrary context (such as running text), it is difficult, even in an arbitrary context (such as running text), it is difficult, even
with only ASCII characters, to know whether an actual domain name (or with only ASCII characters, to know whether an actual domain name (or
a protocol parameter like a URI) is present and where it starts and a protocol parameter like a URI) is present and where it starts and
ends. When using Unicode, this gets even more difficult if treatment ends. When using Unicode, this gets even more difficult if treatment
of certain special characters (like the dot that separates labels in of certain special characters (like the dot that separates labels in
a domain name) depends on context (e.g., prior knowledge of whether a domain name) depends on context (e.g., prior knowledge of whether
the string represents a domain name or not). That knowledge is not the string represents a domain name or not). That knowledge is not
available if the primary heuristic for identifying the presence of available if the primary heuristic for identifying the presence of
domain names in strings depends on the presence of dots separating domain names in strings depends on the presence of dots separating
groups of characters with no intervening spaces. groups of characters with no intervening spaces.
As discussed elsewhere in this document, the IDNA2008 model removes [[anchor19: Placeholder: In serial efforts to move the mapping model
all of these mappings and interpretations, including the equivalence out of the protocol and leave it unspecified here, this paragraph has
of different forms of dots, from the protocol, discouraging such become a complete botch. Rewrite when the mapping plan stabilizes.]]
mappings and leaving them, when necessary, to local processing. This The IDNA2008 model removes all of these mappings and interpretations,
should not be taken to imply that local processing is optional or can including the equivalence of different forms of dots, from the
be avoided entirely, even if doing so might have been desirable in a protocol, discouraging such mappings and leaving them, when
world without IDNA2003 IDNs in files and archives. Instead, unless necessary, to local processing. This should not be taken to imply
the program context is such that it is known that any IDNs that that local processing is optional or can be avoided entirely, even if
appear will contain either U-label or A-label forms, or that other doing so might have been desirable in a world without IDNA2003 IDNs
forms can safely be rejected, some local processing of apparent in files and archives. Instead, unless the program context is such
domain name strings will be required, both to maintain compatibility that it is known that any IDNs that appear will contain either
with IDNA2003 and to prevent user astonishment. Such local U-label or A-label forms, or that other forms can safely be rejected,
processing, while not specified in this document or the associated some local processing of apparent domain name strings will be
ones, will generally take one of two forms: required, both to maintain compatibility with IDNA2003 and to prevent
user astonishment. Such local processing, while not specified in
this document or the associated ones, will generally take one of two
forms:
o Generic Preprocessing. o Generic Preprocessing.
When the context in which the program or system that processes When the context in which the program or system that processes
domain names operates is global, a reasonable balance must be domain names operates is global, a reasonable balance must be
found that is sensitive to the broad range of local needs and found that is sensitive to the broad range of local needs and
assumptions while, at the same time, not sacrificing the needs of assumptions while, at the same time, not sacrificing the needs of
one language, script, or user population to those of another. one language, script, or user population to those of another.
For this case, the best practice will usually be to apply NFKC and For this case, the best practice will usually be to apply NFKC and
case-mapping (or, perhaps better yet, Stringprep itself), plus case-mapping (or, perhaps better yet, Stringprep itself), plus
skipping to change at page 25, line 9 skipping to change at page 23, line 49
User interfaces involving Latin-based scripts should take special User interfaces involving Latin-based scripts should take special
care when considering how to handle case mapping because small care when considering how to handle case mapping because small
differences in label strings may cause behavior that is astonishing differences in label strings may cause behavior that is astonishing
to users. Because case-insensitive comparison is done for ASCII to users. Because case-insensitive comparison is done for ASCII
strings by DNS-servers, an all-ASCII label is treated as case- strings by DNS-servers, an all-ASCII label is treated as case-
insensitive. However, if even one of the characters of that string insensitive. However, if even one of the characters of that string
is replaced by one that requires the label to be given IDN treatment is replaced by one that requires the label to be given IDN treatment
(e.g., by adding a diacritical mark), then the label effectively (e.g., by adding a diacritical mark), then the label effectively
becomes case-sensitive because only lower-case characters are becomes case-sensitive because only lower-case characters are
permitted in IDNs. This suggests that case mapping for Latin-based permitted in IDNs. Preprocessing in applications to handle case
scripts (and possibly other scripts with case distinctions) as a mapping for Latin-based scripts (and possibly other scripts with case
preprocessing matter in applications may be wise to prevent user distinctions) may be wise to prevent user astonishment. However, all
astonishment, but, since all applications may not do this and applications may not do this and ambiguity in transport is not
ambiguity in transport is not desirable, the that case-dependent desirable. Consequently the case-dependent forms should not be
forms should not be stored in files. stored in files.
The comments above apply only in operations that look up names or
interpret files. There are several reasons why registration
activities should require final names and verification of those names
by the would-be registrant.
7. Migration from IDNA2003 and Unicode Version Synchronization 7. Migration from IDNA2003 and Unicode Version Synchronization
7.1. Design Criteria 7.1. Design Criteria
As mentioned above and in RFC 4690, two key goals of the IDNA2008 As mentioned above and in RFC 4690, two key goals of the IDNA2008
design are to enable applications to be agnostic about whether they design are
are being run in environments supporting any Unicode version from 3.2
onward and to permit incrementally adding new characters, character
groups, scripts, and other character collections as they are
incorporated into Unicode, without disruption and, in the long term,
without "heavy" processes such as those involving IETF consensus.
(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.) The
mechanisms that support this are outlined above and elsewhere in the
IDNA2008 document set, but this section reviews them in a context
that may be more helpful to those who need to understand the approach
and make plans for it.
7.1.1. General IDNA Validity Criteria o to enable applications to be agnostic about whether they are being
run in environments supporting any Unicode version from 3.2
onward,
The general criteria for a putative label, and the collection of o to permit incrementally adding new characters, character groups,
characters that make it up, to be considered IDNA-valid are (the scripts, and other character collections as they are incorporated
into Unicode, doing so without disruption and, in the long term,
without "heavy" processes (an IETF consensus process is required
by the IDNA2008 specifications and is expected to be required and
used until significant experience accumulates with IDNA operations
and new versions of Unicode).
7.1.1. Summary and Discussion of IDNA Validity Criteria
The general criteria for a label to be considered IDNA-valid are (the
actual rules are rigorously defined in the "Protocol" and "Tables" actual rules are rigorously defined in the "Protocol" and "Tables"
documents): documents):
o The characters are "letters", marks needed to form letters, o The characters are "letters", marks needed to form letters,
numerals, or other code points used to write words in some numerals, or other code points used to write words in some
language. Symbols, drawing characters, and various notational language. Symbols, drawing characters, and various notational
characters are intended to be permanently excluded -- some because characters are intended to be permanently excluded. There is no
they are harmful in URI, IRI, or similar contexts (e.g., evidence that they are important enough to Internet operations or
characters that appear to be slashes or other reserved URI internationalization to justify expansion of domain names beyond
punctuation) and others because there is no evidence that they are the general principle of "letters, digits, and hyphen".
important enough to Internet operations or internationalization to (Additional discussion and rationale for the symbol decision
justify expansion of domain names beyond the general principle of appears in Section 7.6).
"letters, digits, and hyphen" and the complexities that would come
with it (additional discussion and rationale for the symbol
decision appears in Section 7.6).
o Other than in very exceptional cases, e.g., where they are needed o Other than in very exceptional cases, e.g., where they are needed
to write substantially any word of a given language, punctuation to write substantially any word of a given language, punctuation
characters are excluded as well. The fact that a word exists is characters are excluded. The fact that a word exists is not proof
not proof that it should be usable in a DNS label and DNS labels that it should be usable in a DNS label and DNS labels are not
are not expected to be usable for multiple-word phrases (although expected to be usable for multiple-word phrases (although they are
they are certainly not prohibited if the conventions and certainly not prohibited if the conventions and orthography of a
orthography of a particular language cause that to be possible). particular language cause that to be possible).
Even for English, very common constructions -- contractions like
"don't" or "it's", names that are written with apostrophes such as
"O'Reilly", or characters for which apostrophes are common
substitutes cannot be represented in DNS labels. Words in English
whose usually-preferred spellings include diacritical marks cannot
be represented under the original hostname rules, but most can be
represented if treated as IDNs.
o Characters that are unassigned (have no character assignment at o Characters that are unassigned (have no character assignment at
all) in the version of Unicode being used by the registry or all) in the version of Unicode being used by the registry or
application are not permitted, even on lookup. The issues application are not permitted, even on lookup. The issues
involved in this decision are discussed in Section 7.7. involved in this decision are discussed in Section 7.7.
o Any character that is mapped to another character by a current o Any character that is mapped to another character by a current
version of NFKC is prohibited as input to IDNA (for either version of NFKC is prohibited as input to IDNA (for either
registration or lookup). With a few exceptions, this principle registration or lookup). With a few exceptions, this principle
excludes any character mapped to another by Nameprep [RFC3491]. excludes any character mapped to another by Nameprep [RFC3491].
Tables used to identify the characters that are IDNA-valid are The principles above drive the design of rules that are specified
expected to be driven by the principles above, principles that are exactly in [IDNA2008-Tables]. Those rules identify the characters
specified exactly in [IDNA2008-Tables]). The rules given there are that are IDNA-valid. The rules themselves are normative, and the
normative, rather than being just an interpretation of the tables. tables are derived from them, rather than vice versa.
7.1.2. Labels in Registration 7.1.2. Labels in Registration
Anyone entering a label into a DNS zone must properly validate that Any label registered in a DNS zone must be validated -- i.e., the
label -- i.e., be sure that the criteria for that label are met -- in criteria for that label must be met -- in order for applications to
order for applications to work as intended. This principle is not work as intended. This principle is not new. For example, since the
new. For example, since the DNS was first deployed, zone DNS was first deployed, zone administrators have been expected to
administrators have been expected to verify that names meet verify that names meet "hostname" requirements [RFC0952] where those
"hostname" [RFC0952] where necessary for the expected applications. requirements are imposed by the expected applications. Other
Later addition of special service location formats [RFC2782] imposed applications contexts, such as the later addition of special service
new requirements on zone administrators for the use of labels that location formats [RFC2782] imposed new requirements on zone
conform to the requirements of those formats. For zones that will administrators. For zones that will contain IDNs, support for
contain IDNs, support for Unicode version-independence requires Unicode version-independence requires restrictions on all strings
restrictions on all strings placed in the zone. In particular, for placed in the zone. In particular, for such zones:
such zones:
o Any label that appears to be an A-label, i.e., any label that o Any label that appears to be an A-label, i.e., any label that
starts in "xn--", must be IDNA-valid, i.e., they must be valid starts in "xn--", must be IDNA-valid, i.e., they must be valid
A-labels, as discussed in Section 2 above. A-labels, as discussed in Section 2 above.
o The Unicode tables (i.e., tables of code points, character o The Unicode tables (i.e., tables of code points, character
classes, and properties) and IDNA tables (i.e., tables of classes, and properties) and IDNA tables (i.e., tables of
contextual rules such as those that appear in the Tables contextual rules such as those that appear in the Tables
document), must be consistent on the systems performing or document), must be consistent on the systems performing or
validating labels to be registered. Note that this does not validating labels to be registered. Note that this does not
require that tables reflect the latest version of Unicode, only require that tables reflect the latest version of Unicode, only
that all tables used on a given system are consistent with each that all tables used on a given system are consistent with each
other. other.
Under this model, a registry (or entity communicating with a registry Under this model, registry tables will need to be updated (both the
to accomplish name registrations) will need to update its tables -- Unicode-associated tables and the tables of permitted IDN characters)
both the Unicode-associated tables and the tables of permitted IDN to enable a new script or other set of new characters. The registry
characters -- to enable a new script or other set of new characters. will not be affected by newer versions of Unicode, or newly-
It will not be affected by newer versions of Unicode, or newly- authorized characters, until and unless it wishes to support them.
authorized characters, until and unless it wishes to make those The zone administrator is responsible for verifying IDNA-validity as
registrations. The zone administrator is also responsible -- under well as its local policies -- a more extensive set of checks than are
the protocol and to registrants and users -- for both checking as required for looking up the labels. Systems looking up or resolving
required by the protocol and verification that whatever policies it DNS labels, especially IDN DNS labels, must be able to assume that
develops are complied with, whether those policies are for minimizing applicable registration rules were followed for names entered into
risks due to confusable characters and sequences, for preserving the DNS.
language or script integrity, or for other purposes. Those checking
and verification procedures are more extensive than those that are is
expected of applications systems that look names up.
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.
7.1.3. Labels in Lookup 7.1.3. Labels in Lookup
Anyone looking up a label in a DNS zone is required to Anyone looking up a label in a DNS zone is required to
o Maintain a consistent set of tables, as discussed above. As with o Maintain IDNA and Unicode tables that are consistent with regard
registration, the tables need not reflect the latest version of to versions, i.e., unless the application actually executes the
Unicode but they must be consistent. classification rules in [IDNA2008-Tables], its IDNA tables must be
derived from the version of Unicode that is supported more
generally on the system. As with registration, the tables need
not reflect the latest version of Unicode but they must be
consistent.
o Validate the characters in labels to be looked up only to the o Validate the characters in labels to be looked up only to the
extent of determining that the U-label does not contain either extent of determining that the U-label does not contain
code points prohibited by IDNA (categorized as "DISALLOWED") or "DISALLOWED" code points or code points that are unassigned in its
code points that are unassigned in its version of Unicode. version of Unicode.
o Validate the label itself for conformance with a small number of o Validate the label itself for conformance with a small number of
whole-label rules, notably verifying that there are no leading whole-label rules. In particular, it must verify that
combining marks, that the "bidi" conditions are met if right to
left characters appear, that any required contextual rules are
available and that, if such rules are associated with Joiner
Controls, they are tested.
o Avoid validating other contextual rules about characters, * there are no leading combining marks,
including mixed-script label prohibitions, although such rules may
be used to influence presentation decisions in the user interface.
[[anchor20: Check this, and all similar statements, against
Protocol when that is finished.]]
By avoiding applying its own interpretation of which labels are valid * the "bidi" conditions are met if right to left characters
as a means of rejecting lookup attempts, the lookup application appear,
becomes less sensitive to version incompatibilities with the
particular zone registry associated with the domain name. * any required contextual rules are available, and
* any contextual rules that are associated with Joiner Controls
are tested.
o Do not reject labels based on other contextual rules about
characters, including mixed-script label prohibitions. Such rules
may be used to influence presentation decisions in the user
interface, but not to avoid looking up domain names.
Lookup applications that following 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 An application or client that processes names according to this
protocol and then resolves them in the DNS will be able to locate any protocol and then resolves them in the DNS will be able to locate any
name that is validly registered, as long as its version of the name that is registered, as long as those registrations are IDNA-
Unicode-associated tables is sufficiently up-to-date to interpret all value and its version of the IDNA tables is sufficiently up-to-date
of the characters in the label. Messages to users should distinguish to interpret all of the characters in the label. Messages to users
between "label contains an unallocated code point" and other types of should distinguish between "label contains an unallocated code point"
lookup failures. A failure on the basis of an old version of Unicode and other types of lookup failures. A failure on the basis of an old
may lead the user to a desire to upgrade to a newer version, but will version of Unicode may lead the user to a desire to upgrade to a
have no other ill effects (this is consistent with behavior in the newer version, but will have no other ill effects (this is consistent
transition to the DNS when some hosts could not yet handle some forms with behavior in the transition to the DNS when some hosts could not
of names or record types). yet handle some forms of names or record types).
7.2. Changes in Character Interpretations 7.2. Changes in Character Interpretations
[[anchor21: Note in Draft: This subsection is completely new in [[anchor22: This subsection will need to be rewritten when the
version -04 and has been further tuned in -05 and -06 of this mapping decisions stabilize.]]
document. It could almost certainly use improvement, although this
note will be removed if there are not significant suggestions about
the -06 version. It also contains some material that is redundant
with material in other sections. I have not tried to remove that
material and will not do so until the WG concludes that this section
is relatively stable, but would appreciate help in identifying what
should be removed or how this might be enhanced to contain more of
that other material. --JcK]]
In those scripts that make case distinctions, there are a few In those scripts that make case distinctions, there are a few
characters for which an obvious and unique upper case character has characters for which an obvious and unique upper case character has
not historically been available to match a lower case one or vice not historically been available to match a lower case one or vice
versa. For those characters, the mappings used in constructing the versa. For those characters, the mappings used in constructing the
Stringprep tables for IDNA2003, performed using the Unicode CaseFold Stringprep tables for IDNA2003, performed using the Unicode CaseFold
operation (See Section 5.8 of the Unicode Standard [Unicode51]), operation (See Section 5.8 of the Unicode Standard [Unicode51]),
generate different characters or sets of characters. Those generate different characters or sets of characters. Those
operations are not reversible and lose even more information than operations are not reversible and lose even more information than
traditional upper case or lower case transformations, but are more traditional upper case or lower case transformations, but are more
skipping to change at page 29, line 22 skipping to change at page 27, line 40
notable characters of this type are the German character Eszett notable characters of this type are the German character Eszett
(Sharp S, U+00DF) and the Greek Final Form Sigma (U+03C2). The (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 former is case-folded to the ASCII string "ss", the latter to a
medial (Lower Case) Sigma (U+03C3). medial (Lower Case) Sigma (U+03C3).
The decision to eliminate mappings, including case folding, from the The decision to eliminate mappings, including case folding, from the
IDNA2008 protocol in order to make A-labels and U-labels idempotent IDNA2008 protocol in order to make A-labels and U-labels idempotent
made these characters problematic. If they were to be disallowed, made these characters problematic. If they were to be disallowed,
important words and mnemonics could not be written in important words and mnemonics could not be written in
orthographically reasonable ways. If they were to be permitted as orthographically reasonable ways. If they were to be permitted as
characters distinct from the forms produced by case folding, there distinct characters, there would be no information loss and
would be no information loss and registries would have maximum registries would have more flexibility, but IDNA2003 and IDNA2008
flexibility, but labels using those characters that were looked up lookups might result in different A-labels.
according to IDNA2003 rules would be transformed into A-labels using
their case-mapped variations while lookup according to IDNA2008 rules
would be based on different A-labels that represented the actual
characters.
With the understanding that there would be incompatibility either way With the understanding that there would be incompatibility either way
but a judgment that the incompatibility was not significant enough to but a judgment that the incompatibility was not significant enough to
justify a prefix change, the WG concluded that Eszett and Final Form justify a prefix change, the WG concluded that Eszett and Final Form
Sigma should be treated as distinct and Protocol-Valid characters. Sigma should be treated as distinct and Protocol-Valid characters.
The decision faces registries, especially registries maintaining Registries, especially those maintaining zones for third parties,
zones for third parties, with a variation on what has become a must decide how to introduce a new service in a way that does not
familiar problem: how to introduce a new service in a way that does create confusion or significantly weaken or invalidate existing
not create confusion or significantly weaken or invalidate existing identifiers. This is not a new problem; registries were faced with
identifiers. similar issues when IDNs were introduced and when other new forms of
strings have been permitted as labels.
There have traditionally been several approaches to problems of this There are several approaches to problems of this type. Without any
type. Without any preference or claim to completeness, these are: preference or claim to completeness, some of these, all of which have
been used by registries in the past for similar transitions, are:
o Do not permit use of the newly-available character at the registry o Do not permit use of the newly-available character at the registry
level. This might cause lookup failures if a domain name were to level. This might cause lookup failures if a domain name were to
be written with the expectation of the IDNA2003 mapping behavior, be written with the expectation of the IDNA2003 mapping behavior,
but would eliminate any possibility of false matches. but would eliminate any possibility of false matches.
o Hold a "sunrise"-like arrangement in which holders of labels that o Hold a "sunrise"-like arrangement in which holders of labels
might have resulted from previous mapping (labels containing "ss" containing "ss" in the Eszett case or Lower Case Sigma are given
in the Eszett case or ones containing Lower Case Sigma in the priority (and perhaps other benefits) for registering the
Final Sigma case) are given priority (and perhaps other benefits) corresponding string containing Eszett or Final Sigma
for registering the corresponding string containing the newly- respectively.
available characters.
o Adopt some sort of "variant" approach in which registrants either o Adopt some sort of "variant" approach in which registrants obtain
obtained labels with both character forms or one of them was labels with both character forms.
blocked from registration by anyone but the registrant of the
other form.
In principle, lookup applications could also compensate for the o Adopt a different form of "variant" approach in which registration
difference in interpretation by looking up the string according to of additional names is either not permitted at all or permitted
the interpretation specified in these documents and then, if that only by the registrant who already has one of the names.
failed, doing the lookup with the mapping, simulating the IDNA2003
interpretation. The risk of false positives is such that this is
generally to be discouraged unless the application is able to engage
in a "is this what you meant" dialogue with the end user.
7.3. More Flexibility in User Agents 7.3. More Flexibility in User Agents
[[anchor23: Note in Draft: This section is mapping-related and may
need to be revised after that issue settles down.]] Also, it is
closely related to Section 4.2 and may need to be cross-referenced
from it or consolidated into it.
These documents do not specify mappings between one character or code These documents do not specify mappings between one character or code
point and others for any reason. Instead, they prohibit the point and others. Instead, IDNA2008 prohibits characters that would
characters that would be mapped to others by normalization, upper be mapped to others by normalization, upper case to lower case
case to lower case changes, or other rules. As examples, while changes, or other rules. As examples, while mathematical characters
mathematical characters based on Latin ones are accepted as input to based on Latin ones are accepted as input to IDNA2003, they are
IDNA2003, they are prohibited in IDNA2008. Similarly, double-width prohibited in IDNA2008. Similarly, double-width characters and other
characters and other variations are prohibited as IDNA input. variations are prohibited as IDNA input.
Since the rules in [IDNA2008-Tables] have the effect that only Since the rules in [IDNA2008-Tables] have the effect that only
strings that are not transformed by NFKC are valid, if an application strings that are not transformed by NFKC are valid, if an application
chooses to perform NFKC normalization before lookup, that operation chooses to perform NFKC normalization before lookup, that operation
is safe since this will never make the application unable to look up is safe since this will never make the application unable to look up
any valid string. However, as discussed above, the application any valid string. However, as discussed above, the application
cannot guarantee that any other application will perform that cannot guarantee that any other application will perform that
mapping, so it should be used only with caution and for informed mapping, so it should be used only with caution and for informed
users. users.
skipping to change at page 31, line 10 skipping to change at page 29, line 24
clearly understand that the character forms are equivalent. For use clearly understand that the character forms are equivalent. For use
in interchange among systems, it appears to be much more important in interchange among systems, it appears to be much more important
that U-labels and A-labels can be mapped back and forth without loss that U-labels and A-labels can be mapped back and forth without loss
of information. of information.
One specific, and very important, instance of this strategy arises One specific, and very important, instance of this strategy arises
with case-folding. In the ASCII-only DNS, names are looked up and 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. matched in a case-independent way, but no actual case-folding occurs.
Names can be placed in the DNS in either upper or lower case form (or Names can be placed in the DNS in either upper or lower case form (or
any mixture of them) and that form is preserved, returned in queries, any mixture of them) and that form is preserved, returned in queries,
and so on. IDNA2003 simulated that behavior for non-ASCII strings by and so on. IDNA2003 approximated that behavior for non-ASCII strings
performing case-folding at registration time (resulting in only by performing case-folding at registration time (resulting in only
lower-case IDNs in the DNS) and when names were looked up. lower-case IDNs in the DNS) and when names were looked up.
As suggested earlier in this section, it appears to be desirable to As suggested earlier in this section, it appears to be desirable to
do as little character mapping as possible consistent with having do as little character mapping as possible as long as Unicode works
Unicode work correctly (e.g., NFC mapping to resolve different correctly (e.g., NFC mapping to resolve different codings for the
codings for the same character is still necessary although the same character is still necessary although the specifications require
specifications require that it be performed prior to invoking the that it be performed prior to invoking the protocol) in order to make
protocol) and to make the mapping between A-labels and U-labels the mapping between A-labels and U-labels idempotent. Case-mapping
idempotent. Case-mapping is not an exception to this principle. If is not an exception to this principle. If only lower case characters
only lower case characters can be registered in the DNS (i.e., be can be registered in the DNS (i.e., be present in a U-label), then
present in a U-label), then IDNA2008 should prohibit upper-case IDNA2008 should prohibit upper-case characters as input. Some other
characters as input (and therefore does so). Some other considerations reinforce this conclusion. For example, in ASCII
considerations reinforce this conclusion. For example, an essential case-mapping for individual characters, uppercase(character) must be
element of the ASCII case-mapping functions is that, for individual equal to uppercase(lowercase(character)). That may not be true with
characters, uppercase(character) must be equal to IDNs. In some scripts that use case distinctions, there are a few
uppercase(lowercase(character)). That requirement may not be characters that do not have counterparts in one case or the other.
satisfied with IDNs. For example, there are some characters in The relationship between upper case and lower case may even be
scripts that use case distinction that do not have counterparts in language-dependent, with different languages (or even the same
one case or the other. The relationship between upper case and lower language in different areas) expecting different mappings. User
case may even be language-dependent, with different languages (or agents can meet the expectations of users who are accustomed to the
even the same language in different areas) expecting different case-insensitive DNS environment by performing case folding prior to
mappings. Of course, the expectations of users who are accustomed to IDNA processing, but the IDNA procedures themselves should neither
a case-insensitive DNS environment will probably be well-served if require such mapping nor expect them when they are not natural to the
user agents perform case folding prior to IDNA processing, but the localized environment.
IDNA procedures themselves should neither require such mapping nor
expect them when they are not natural to the localized environment.
7.4. The Question of Prefix Changes 7.4. The Question of Prefix Changes
The conditions that would require a change in the IDNA ACE prefix The conditions that would require a change in the IDNA ACE prefix
("xn--" for the version of IDNA specified in [RFC3490]) have been a ("xn--" for the version of IDNA specified in [RFC3490]) have been a
great concern to the community. A prefix change would clearly be great concern to the community. A prefix change would clearly be
necessary if the algorithms were modified in a manner that would necessary if the algorithms were modified in a manner that would
create serious ambiguities during subsequent transition in create serious ambiguities during subsequent transition in
registrations. This section summarizes our conclusions about the registrations. This section summarizes our conclusions about the
conditions under which changes in prefix would be necessary and the conditions under which changes in prefix would be necessary and the
implications of such a change. implications of such a change.
7.4.1. Conditions Requiring a Prefix Change 7.4.1. Conditions Requiring a Prefix Change
An IDN prefix change is needed if a given string would be looked up An IDN prefix change is needed if a given string would be looked up
or otherwise interpreted differently depending on the version of the or otherwise interpreted differently depending on the version of the
protocol or tables being used. Consequently, work to update IDNs protocol or tables being used. An IDNA upgrade would require a
would require a prefix change if, and only if, one of the following prefix change if, and only if, one of the following four conditions
four conditions were met: were met:
1. The conversion of an A-label to Unicode (i.e., a U-label) yields 1. The conversion of an A-label to Unicode (i.e., a U-label) yields
one string under IDNA2003 (RFC3490) and a different string under one string under IDNA2003 (RFC3490) and a different string under
IDNA2008. IDNA2008.
2. An input string that is valid under IDNA2003 and also valid under 2. In a significant number of cases, an input string that is valid
IDNA2008 yields two different A-labels with the different under IDNA2003 and also valid under IDNA2008 yields two different
versions of IDNA. This condition is believed to be essentially A-labels with the different versions. This condition is believed
equivalent to the one above except for a very small number of to be essentially equivalent to the one above except for a very
edge cases which may not, pragmatically, justify a prefix change small number of edge cases which may not justify a prefix change
(See Section 7.2). (See Section 7.2).
Note, however, that if the input string is valid under one Note that if the input string is valid under one version and not
version and not valid under the other, this condition does not valid under the other, this condition does not apply. See the
apply. See the first item in Section 7.4.2, below. first item in Section 7.4.2, below.
3. A fundamental change is made to the semantics of the string that 3. A fundamental change is made to the semantics of the string that
is inserted in the DNS, e.g., if a decision were made to try to is inserted in the DNS, e.g., if a decision were made to try to
include language or specific script information in that string, include language or script information in the encoding in
rather than having it be just a string of characters. addition to the string itself.
4. A sufficiently large number of characters is added to Unicode so 4. A sufficiently large number of characters is added to Unicode so
that the Punycode mechanism for block offsets no longer has that the Punycode mechanism for block offsets can no longer
enough capacity to reference the higher-numbered planes and reference the higher-numbered planes and blocks. This condition
blocks. This condition is unlikely even in the long term and is unlikely even in the long term and certain not to arise in the
certain not to arise in the next few years. next several years.
7.4.2. Conditions Not Requiring a Prefix Change 7.4.2. Conditions Not Requiring a Prefix Change
In particular, as a result of the principles described above, none of As a result of the principles described above, none of the following
the following changes require a new prefix: changes require a new prefix:
1. Prohibition of some characters as input to IDNA. This may make 1. Prohibition of some characters as input to IDNA. This may make
names that are now registered inaccessible, but does not require names that are now registered inaccessible, but does not change
a prefix change. those names.
2. Adjustments in IDNA tables or actions, including normalization 2. Adjustments in IDNA tables or actions, including normalization
definitions, that affect characters that were already invalid definitions, that affect characters that were already invalid
under IDNA2003. under IDNA2003.
3. Changes in the style of the IDNA definition that does not alter 3. Changes in the style of the IDNA definition that does not alter
the actions performed by IDNA. the actions performed by IDNA.
7.4.3. Implications of Prefix Changes 7.4.3. Implications of Prefix Changes
While it might be possible to make a prefix change, the costs of such While it might be possible to make a prefix change, the costs of such
a change are considerable. Even if they wanted to do so, registries a change are considerable. Registries could not convert all IDNA2003
could not convert all IDNA2003 ("xn--") registrations to a new form ("xn--") registrations to a new form at the same time and synchronize
at the same time and synchronize that change with applications that change with applications supporting lookup. Unless all existing
supporting lookup. Unless all existing registrations were simply to registrations were simply to be declared invalid (and perhaps even
be declared invalid (and perhaps even then) systems that needed to then) systems that needed to support both labels with old prefixes
support both labels with old prefixes and labels with new ones would and labels with new ones would first process a putative label under
first process a putative label under the IDNA2008 rules and try to the IDNA2008 rules and try to look it up and then, if it were not
look it up and then, if it were not found, would process the label found, would process the label under IDNA2003 rules and look it up
under IDNA2003 rules and look it up again. That process could again. That process could significantly slow down all processing
significantly slow down all processing that involved IDNs in the DNS that involved IDNs in the DNS especially since a fully-qualified name
especially since, in principle, a fully-qualified name could contain might contain a mixture of labels that were registered with the old
a mixture of labels that were registered with the old and new and new prefixes. That would make DNS caching very difficult. In
prefixes, a situation that would make the use of DNS caching very addition, looking up the same input string as two separate A-labels
difficult. In addition, looking up the same input string as two creates some potential for confusion and attacks, since the labels
separate A-labels would create some potential for confusion and could map to different targets and then resolve to different entries
attacks, since they could, in principle, map to different targets and in the DNS.
then resolve to different entries in the DNS.
Consequently, a prefix change is to be avoided if at all possible, Consequently, a prefix change is to be avoided if at all possible,
even if it means accepting some IDNA2003 decisions about character even if it means accepting some IDNA2003 decisions about character
distinctions as irreversible and/or giving special treatment to edge distinctions as irreversible and/or giving special treatment to edge
cases. cases.
7.5. Stringprep Changes and Compatibility 7.5. Stringprep Changes and Compatibility
The Nameprep [RFC3491] specification, a key part of IDNA2003, is a The Nameprep [RFC3491] specification, a key part of IDNA2003, is a
profile of Stringprep [RFC3454]. While Nameprep is a Stringprep profile of Stringprep [RFC3454]. While Nameprep is a Stringprep
profile specific to IDNA, Stringprep is used by a number of other profile specific to IDNA, Stringprep is used by a number of other
protocols. Concerns have been expressed about problems for non-DNS protocols. Were Stringprep to be modified by IDNA2008, those changes
uses of Stringprep being caused by changes to the specification to improve the handling of IDNs could cause problems for non-DNS
intended to improve the handling of IDNs, most notably as this might uses, most notably if they affected identification and authentication
affect identification and authentication protocols. The proposed new protocols. Several elements of IDNA2008 give interpretations to
inclusion tables [IDNA2008-Tables], the reduction in the number of strings prohibited under IDNA2003 or prohibit strings that IDNA2003
characters permitted as input for registration or lookup (Section 3), permitted. Those elements include the proposed new inclusion tables
and even the proposed changes in handling of right to left strings [IDNA2008-Tables], the reduction in the number of characters
[IDNA2008-Bidi] either give interpretations to strings prohibited permitted as input for registration or lookup (Section 3), and even
under IDNA2003 or prohibit strings that IDNA2003 permitted. The the proposed changes in handling of right to left strings
IDNA2008 protocol does not use either Nameprep or Stringprep at all, [IDNA2008-Bidi]. IDNA2008 does not use Nameprep or Stringprep at
so there are no side-effect changes to other protocols. all, so there are no side-effect changes to other protocols.
It is particularly important to keep IDNA processing separate from It is particularly important to keep IDNA processing separate from
processing for various security protocols because some of the processing for various security protocols because some of the
constraints that are necessary for smooth and comprehensible use of constraints that are necessary for smooth and comprehensible use of
IDNs may be unwanted or undesirable in other contexts. For example, IDNs may be unwanted or undesirable in other contexts. For example,
the criteria for good passwords or passphrases are very different the criteria for good passwords or passphrases are very different
from those for desirable IDNs: passwords should be hard to guess, from those for desirable IDNs: passwords should be hard to guess,
while domain names should normally be easily memorable. Similarly, while domain names should normally be easily memorable. Similarly,
internationalized SCSI identifiers and other protocol components are internationalized SCSI identifiers and other protocol components are
likely to have different requirements than IDNs. likely to have different requirements than IDNs.
skipping to change at page 34, line 23 skipping to change at page 32, line 35
One of the major differences between this specification and the One of the major differences between this specification and the
original version of IDNA is that the original version permitted non- original version of IDNA is that the original version permitted non-
letter symbols of various sorts, including punctuation and line- letter symbols of various sorts, including punctuation and line-
drawing symbols, in the protocol. They were always discouraged in drawing symbols, in the protocol. They were always discouraged in
practice. In particular, both the "IESG Statement" about IDNA and practice. In particular, both the "IESG Statement" about IDNA and
all versions of the ICANN Guidelines specify that only language all versions of the ICANN Guidelines specify that only language
characters be used in labels. This specification disallows symbols characters be used in labels. This specification disallows symbols
entirely. There are several reasons for this, which include: entirely. There are several reasons for this, which include:
o As discussed elsewhere, the original IDNA specification assumed 1. As discussed elsewhere, the original IDNA specification assumed
that as many Unicode characters as possible should be permitted, that as many Unicode characters as possible should be permitted,
directly or via mapping to other characters, in IDNs. This directly or via mapping to other characters, in IDNs. This
specification operates on an inclusion model, extrapolating from specification operates on an inclusion model, extrapolating from
the LDH rules -- which have served the Internet very well -- to a the original "hostname" rules (LDH, see [IDNA2008-Defs]) -- which
Unicode base rather than an ASCII base. have served the Internet very well -- to a Unicode base rather
than an ASCII base.
o Most Unicode names for letters are, in most cases, fairly 2. Symbol names are more problematic than letters because there may
intuitive, unambiguous and recognizable to users of the relevant be no general agreement on whether a particular glyph matches a
script. Symbol names are more problematic because there may be no symbol; there are no uniform conventions for naming; variations
general agreement on whether a particular glyph matches a symbol; such as outline, solid, and shaded forms may or may not exist;
there are no uniform conventions for naming; variations such as and so on. As just one example, consider a "heart" symbol as it
outline, solid, and shaded forms may or may not exist; and so on. might appear in a logo that might be read as "I love...". While
As just one example, consider a "heart" symbol as it might appear the user might read such a logo as "I love..." or "I heart...",
in a logo that might be read as "I love...". While the user might considerable knowledge of the coding distinctions made in Unicode
read such a logo as "I love..." or "I heart...", considerable is needed to know that there more than one "heart" character
knowledge of the coding distinctions made in Unicode is needed to (e.g., U+2665, U+2661, and U+2765) and how to describe it. These
know that there more than one "heart" character (e.g., U+2665, issues are of particular importance if strings are expected to be
U+2661, and U+2765) and how to describe it. These issues are of understood or transcribed by the listener after being read out
particular importance if strings are expected to be understood or loud.
transcribed by the listener after being read out loud. [[anchor24: The above paragraph remains controversial as to
[[anchor22: The above paragraph remains controversial as to whether it is valid. The WG will need to make a decision if this
whether it is valid. The WG will need to make a decision if this section is not dropped entirely.]]
section is not dropped entirely.]]
o Consider the case of a screen reader used by blind Internet users 3. Consider the case of a screen reader used by blind Internet users
who must listen to renderings of IDN domain names and possibly who must listen to renderings of IDN domain names and possibly
reproduce them on the keyboard. reproduce them on the keyboard.
o As a simplified example of this, assume one wanted to use a 4. As a simplified example of this, assume one wanted to use a
"heart" or "star" symbol in a label. This is problematic because "heart" or "star" symbol in a label. This is problematic because
those names are ambiguous in the Unicode system of naming (the those names are ambiguous in the Unicode system of naming (the
actual Unicode names require far more qualification). A user or actual Unicode names require far more qualification). A user or
would-be registrant has no way to know -- absent careful study of would-be registrant has no way to know -- absent careful study of
the code tables -- whether it is ambiguous (e.g., where there are the code tables -- whether it is ambiguous (e.g., where there are
multiple "heart" characters) or not. Conversely, the user seeing multiple "heart" characters) or not. Conversely, the user seeing
the hypothetical label doesn't know whether to read it -- try to the hypothetical label doesn't know whether to read it -- try to
transmit it to a colleague by voice -- as "heart", as "love", as transmit it to a colleague by voice -- as "heart", as "love", as
"black heart", or as any of the other examples below. "black heart", or as any of the other examples below.
o The actual situation is even worse than this. There is no 5. The actual situation is even worse than this. There is no
possible way for a normal, casual, user to tell the difference 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 between the hearts of U+2665 and U+2765 and the stars of U+2606
and U+2729 or the without somehow knowing to look for a and U+2729 or the without somehow knowing to look for a
distinction. We have a white heart (U+2661) and few black hearts. distinction. We have a white heart (U+2661) and few black
Consequently, describing a label as containing a heart hopelessly hearts. Consequently, describing a label as containing a heart
ambiguous: we can only know that it contains one of several hopelessly ambiguous: we can only know that it contains one of
characters that look like hearts or have "heart" in their names. several characters that look like hearts or have "heart" in their
In cities where "Square" is a popular part of a location name, one names. In cities where "Square" is a popular part of a location
might well want to use a square symbol in a label as well and name, one might well want to use a square symbol in a label as
there are far more squares of various flavors in Unicode than well and there are far more squares of various flavors in Unicode
there are hearts or stars. than there are hearts or stars.
o The consequence of these ambiguities of description and The consequence of these ambiguities is that symbols are a very poor
dependencies on distinctions that were, or were not, made in basis for reliable communication. Consistent with this conclusion,
Unicode codings is that symbols are a very poor basis for reliable the Unicode standard recommends that strings used in identifiers not
communication. Consistent with this conclusion, the Unicode contain symbols or punctuation [Unicode-UAX31]. Of course, these
standard recommends that strings used in identifiers not contain difficulties with symbols do not arise with actual pictographic
symbols or punctuation [Unicode-UAX31]. Of course, these languages and scripts which would be treated like any other language
difficulties with symbols do not arise with actual pictographic characters; the two should not be confused.
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. Migration Between Unicode Versions: Unassigned Code Points
In IDNA2003, labels containing unassigned code points are looked up In IDNA2003, labels containing unassigned code points are looked up
on the assumption that, if they appear in labels and can be mapped on the assumption that, if they appear in labels and can be mapped
and then resolved, the relevant standards must have changed and the and then resolved, the relevant standards must have changed and the
registry has properly allocated only assigned values. registry has properly allocated only assigned values.
In the protocol as described in these documents, strings containing In the protocol described in these documents, strings containing
unassigned code points must not be either looked up or registered. 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 There are several reasons for this, with the most important ones
being: being:
o Tests involving the context of characters (e.g., some characters
being permitted only adjacent to others of specific types) and
integrity tests on complete labels are needed. Unassigned code
points cannot be permitted because one cannot determine whether
particular code points will require contextual rules (and what
those rules should be) before characters are assigned to them and
the properties of those characters fully understood.
o It cannot be known in advance, and with sufficient reliability, o It cannot be known in advance, and with sufficient reliability,
that a code point that was not previously assigned will not be that a no newly-assigned code point will associated with a
assigned to a compatibility character or one that would be character that would be disallowed by the rules in
otherwise disallowed by the rules in [IDNA2008-Tables]. In [IDNA2008-Tables] (such as a compatibility character). In
IDNA2003, since there is no direct dependency on NFKC (many of the IDNA2003, since there is no direct dependency on NFKC (many of the
entries in Stringprep's tables are based on NFKC, but IDNA2003 entries in Stringprep's tables are based on NFKC, but IDNA2003
depends only on Stringprep), allocation of a compatibility depends only on Stringprep), allocation of a compatibility
character might produce some odd situations, but it would not be a character might produce some odd situations, but it would not be a
problem. In IDNA2008, where compatibility characters are assigned problem. In IDNA2008, where compatibility characters are
to DISALLOWED unless character-specific exceptions are made, DISALLOWED unless character-specific exceptions are made,
permitting strings containing unassigned characters to be looked permitting strings containing unassigned characters to be looked
up would permit violating the principle that characters in up would violate the principle that characters in DISALLOWED are
DISALLOWED are not looked up. not looked up.
o The Unicode Standard specifies that an unassigned code point o The Unicode Standard specifies that an unassigned code point
normalizes (and, where relevant, case folds) to itself. If the normalizes (and, where relevant, case folds) to itself. If the
code point is later assigned to a character, and particularly if code point is later assigned to a character, and particularly if
the newly-assigned code point has a combining class that the newly-assigned code point has a combining class that
determines its placement relative to other combining characters, determines its placement relative to other combining characters,
it could normalize to some other code point or sequence, creating it could normalize to some other code point or sequence.
confusion and/or violating other rules listed here.
o Tests involving the context of characters (e.g., some characters
being permitted only adjacent to ones of specific types but
otherwise invisible or very problematic for other reasons) 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 More generally, the status of an unassigned character with regard
to the DISALLOWED and PROTOCOL-VALID categories, and whether
contextual rules are required with the latter, cannot be evaluated
until a character is actually assigned and known. By contrast,
characters that are actually DISALLOWED are placed in that
category only as a consequence of rules applied to known
properties or per-character evaluation.
Another way to look at this is that permitting an unassigned
character to be looked up is nearly equivalent to reclassifying a
character from DISALLOWED to PROTOCOL-VALID since different systems
will interpret the character in different ways.
It is possible to argue that the issues above are not important and 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 that, as a consequence, it is better to retain the principle of
looking up labels even if they contain unassigned characters because looking up labels even if they contain unassigned characters because
all of the important scripts and characters have been coded as of all of the important scripts and characters have been coded as of
Unicode 5.1 and hence unassigned code points will be assigned only to Unicode 5.1 and hence unassigned code points will be assigned only to
obscure characters or archaic scripts. Unfortunately, that does not obscure characters or archaic scripts. Unfortunately, that does not
appear to be a safe assumption for at least two reasons. First, much 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 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 Unicode. The reality is that a script that is obscure to much of the
skipping to change at page 39, line 31 skipping to change at page 37, line 29
and responses may be forced to go to TCP instead of UDP). and responses may be forced to go to TCP instead of UDP).
9. Internationalization Considerations 9. Internationalization Considerations
DNS labels and fully-qualified domain names provide mnemonics that DNS labels and fully-qualified domain names provide mnemonics that
assist in identifying and referring to resources on the Internet. assist in identifying and referring to resources on the Internet.
IDNs expand the range of those mnemonics to include those based on IDNs expand the range of those mnemonics to include those based on
languages and character sets other than Western European and Roman- languages and character sets other than Western European and Roman-
derived ones. But domain "names" are not, in general, words in any derived ones. But domain "names" are not, in general, words in any
language. The recommendations of the IETF policy on character sets language. The recommendations of the IETF policy on character sets
and languages, BCP 18 [RFC2277] are applicable to situations in which and languages, (BCP 18 [RFC2277]) are applicable to situations in
language identification is used to provide language-specific which language identification is used to provide language-specific
contexts. The DNS is, by contrast, global and international and contexts. The DNS is, by contrast, global and international and
ultimately has nothing to do with languages. Adding languages (or ultimately has nothing to do with languages. Adding languages (or
similar context) to IDNs generally, or to DNS matching in particular, similar context) to IDNs generally, or to DNS matching in particular,
would imply context dependent matching in DNS, which would be a very would imply context dependent matching in DNS, which would be a very
significant change to the DNS protocol itself. It would also imply significant change to the DNS protocol itself. It would also imply
that users would need to identify the language associated with a that users would need to identify the language associated with a
particular label in order to look that label up, a decision that particular label in order to look that label up. That knowledge is
would be impossible in many or most cases. generally not available because many labels are not words in any
language and some may be words in more than one.
10. IANA Considerations 10. IANA Considerations
This section gives an overview of registries required for IDNA. The This section gives an overview of IANA registries required for IDNA.
actual definitions of the first two appear in [IDNA2008-Tables]. The actual definitions of, and specifications for, the first two,
which must be newly-created for IDNA2008, appear in
[IDNA2008-Tables]. This document describes the registries but does
not specify any IANA actions.
10.1. IDNA Character Registry 10.1. IDNA Character Registry
The distinction among the three major categories "UNASSIGNED", The distinction among the major categories "UNASSIGNED",
"DISALLOWED", and "PROTOCOL-VALID" is made by special categories and "DISALLOWED", "PROTOCOL-VALID", and "CONTEXTUAL RULE REQUIRED" is
rules that are integral elements of [IDNA2008-Tables]. Convenience made by special categories and rules that are integral elements of
in programming and validation requires a registry of characters and [IDNA2008-Tables]. While not normative, an IANA registry of
scripts and their categories, updated for each new version of Unicode characters and scripts and their categories, updated for each new
and the characters it contains. The details of this registry are version of Unicode and the characters it contains, will be convenient
specified in [IDNA2008-Tables]. for programming and validation purposes. The details of this
registry are specified in [IDNA2008-Tables].
10.2. IDNA Context Registry 10.2. IDNA Context Registry
For characters that are defined in the IDNA Character Registry list IANA will create and maintain a list of approved contextual rules for
as PROTOCOL-VALID but requiring a contextual rule (i.e., the types of characters that are defined in the IDNA Character Registry list as
rule described in Section 3.1.2), IANA will create and maintain a requiring a Contextual Rule (i.e., the types of rule described in
list of approved contextual rules. The details for those rules Section 3.1.2). The details for those rules appear in
appear in [IDNA2008-Tables]. [IDNA2008-Tables].
10.3. IANA Repository of IDN Practices of TLDs 10.3. IANA Repository of IDN Practices of TLDs
This registry, historically described as the "IANA Language Character This registry, historically described as the "IANA Language Character
Set Registry" or "IANA Script Registry" (both somewhat misleading Set Registry" or "IANA Script Registry" (both somewhat misleading
terms) is maintained by IANA at the request of ICANN. It is used to terms) is maintained by IANA at the request of ICANN. It is used to
provide a central documentation repository of the IDN policies used provide a central documentation repository of the IDN policies used
by top level domain (TLD) registries who volunteer to contribute to by top level domain (TLD) registries who volunteer to contribute to
it and is used in conjunction with ICANN Guidelines for IDN use. it and is used in conjunction with ICANN Guidelines for IDN use.
It is not an IETF-managed registry and, while the protocol changes It is not an IETF-managed registry and, while the protocol changes
specified here may call for some revisions to the tables, these specified here may call for some revisions to the tables, these
specifications have no direct effect on that registry and no IANA specifications have no direct effect on that registry and no IANA
action is required as a result. action is required as a result.
11. Security Considerations 11. Security Considerations
11.1. General Security Issues with IDNA 11.1. General Security Issues with IDNA
This document in the IDNA2008 series is purely explanatory and This document is purely explanatory and informational and
informational and consequently introduces no new security issues. It consequently introduces no new security issues. It would, of course,
would, of course, be a poor idea for someone to try to implement from be a poor idea for someone to try to implement from it; such an
it; such an attempt would almost certainly lead to interoperability attempt would almost certainly lead to interoperability problems and
problems and might lead to security ones. A discussion of security might lead to security ones. A discussion of security issues with
issues with IDNA, including some relevant history, appears in IDNA, including some relevant history, appears in [IDNA2008-Defs].
[IDNA2008-Defs].
12. Acknowledgments 12. Acknowledgments
The editor and contributors would like to express their thanks to The editor and contributors would like to express their thanks to
those who contributed significant early (pre-WG) review comments, those who contributed significant early (pre-WG) review comments,
sometimes accompanied by text, especially Mark Davis, Paul Hoffman, sometimes accompanied by text, especially Mark Davis, Paul Hoffman,
Simon Josefsson, and Sam Weiler. In addition, some specific ideas Simon Josefsson, and Sam Weiler. In addition, some specific ideas
were incorporated from suggestions, text, or comments about sections were incorporated from suggestions, text, or comments about sections
that were unclear supplied by Vint Cerf, Frank Ellerman, Michael that were unclear supplied by Vint Cerf, Frank Ellerman, Michael
Everson, Asmus Freytag, Erik van der Poel, Michel Suignard, and Ken Everson, Asmus Freytag, Erik van der Poel, Michel Suignard, and Ken
Whistler, although, as usual, they bear little or no responsibility Whistler, although, as usual, they bear little or no responsibility
for the conclusions the editor and contributors reached after for the conclusions the editor and contributors reached after
receiving their suggestions. Thanks are also due to Vint Cerf, receiving their suggestions. Thanks are also due to Vint Cerf, Lisa
Debbie Garside, and Jefsey Morfin for conversations that led to Dusseault, Debbie Garside, and Jefsey Morfin for conversations that
considerable improvements in the content of this document. led to considerable improvements in the content of this document.
A meeting was held on 30 January 2008 to attempt to reconcile A meeting was held on 30 January 2008 to attempt to reconcile
differences in perspective and terminology about this set of differences in perspective and terminology about this set of
specifications between the design team and members of the Unicode specifications between the design team and members of the Unicode
Technical Consortium. The discussions at and subsequent to that Technical Consortium. The discussions at and subsequent to that
meeting were very helpful in focusing the issues and in refining the meeting were very helpful in focusing the issues and in refining the
specifications. The active participants at that meeting were (in specifications. The active participants at that meeting were (in
alphabetic order as usual) Harald Alvestrand, Vint Cerf, Tina Dam, alphabetic order as usual) Harald Alvestrand, Vint Cerf, Tina Dam,
Mark Davis, Lisa Dusseault, Patrik Faltstrom (by telephone), Cary Mark Davis, Lisa Dusseault, Patrik Faltstrom (by telephone), Cary
Karp, John Klensin, Warren Kumari, Lisa Moore, Erik van der Poel, Karp, John Klensin, Warren Kumari, Lisa Moore, Erik van der Poel,
Michel Suignard, and Ken Whistler. We express our thanks to Google Michel Suignard, and Ken Whistler. We express our thanks to Google
for support of that meeting and to the participants for their for support of that meeting and to the participants for their
contributions. contributions.
Useful comments and text on the WG versions of the draft were Useful comments and text on the WG versions of the draft were
received from many participants in the IETF "IDNABIS" WG and a number received from many participants in the IETF "IDNABIS" WG and a number
of document changes resulted from mailing list discussions made by of document changes resulted from mailing list discussions made by
that group. Marcos Sanz provided specific analysis and suggestions that group. Marcos Sanz provided specific analysis and suggestions
that were exceptionally helpful in refining the text, as did Vint that were exceptionally helpful in refining the text, as did Vint
Cerf, Mark Davis, Martin Duerst, Andrew Sullivan, and Ken Whistler. Cerf, Mark Davis, Martin Duerst, Andrew Sullivan, and Ken Whistler.
Lisa Dusseault provided extensive editorial suggestions during the
spring of 2009, most of which were incorporated.
13. Contributors 13. Contributors
While the listed editor held the pen, the core of this document and While the listed editor held the pen, the core of this document and
the initial WG version represents the joint work and conclusions of the initial WG version represents the joint work and conclusions of
an ad hoc design team consisting of the editor and, in alphabetic an ad hoc design team consisting of the editor and, in alphabetic
order, Harald Alvestrand, Tina Dam, Patrik Faltstrom, and Cary Karp. order, Harald Alvestrand, Tina Dam, Patrik Faltstrom, and Cary Karp.
In addition, there were many specific contributions and helpful In addition, there were many specific contributions and helpful
comments from those listed in the Acknowledgments section and others comments from those listed in the Acknowledgments section and others
who have contributed to the development and use of the IDNA who have contributed to the development and use of the IDNA
skipping to change at page 49, line 5 skipping to change at page 47, line 5
o Added discussion of adding characters to an existing script to the o Added discussion of adding characters to an existing script to the
discussion of unassigned code point transitions in Section 7.7. discussion of unassigned code point transitions in Section 7.7.
o Tightened up the discussion of non-ASCII string processing o Tightened up the discussion of non-ASCII string processing
(Section 8.1) slightly. (Section 8.1) slightly.
o Removed some placeholders and comments that have been around long o Removed some placeholders and comments that have been around long
enough to be considered acceptable or that no longer seem enough to be considered acceptable or that no longer seem
necessary for other reasons. necessary for other reasons.
A.10. Version -10
o Extensive editorial improvements, mostly due to suggestions from
Lisa Dusseault.
o Changes required for the new "mapping" approach and document have,
in general, not been incorporated despite several suggestions.
The editor intends to wait until the mapping model is stable, or
at least until -11 of this document, before trying to incorporate
those suggestions.
Author's Address Author's Address
John C Klensin John C Klensin
1770 Massachusetts Ave, Ste 322 1770 Massachusetts Ave, Ste 322
Cambridge, MA 02140 Cambridge, MA 02140
USA USA
Phone: +1 617 245 1457 Phone: +1 617 245 1457
Email: john+ietf@jck.com Email: john+ietf@jck.com
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