Network Working Group J. Klensin
Internet-Draft January 11, 2010
Intended status: Informational
Expires: July 15, 2010
Internationalized Domain Names for Applications (IDNA): Background,
Explanation, and Rationale
draft-ietf-idnabis-rationale-17.txt
Abstract
Several years have passed since the original protocol for
Internationalized Domain Names (IDNs) was completed and deployed.
During that time, a number of issues have arisen, including the need
to update the system to deal with newer versions of Unicode. Some of
these issues require tuning of the existing protocols and the tables
on which they depend. This document provides an overview of a
revised system and provides explanatory material for its components.
Status of this Memo
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Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Context and Overview . . . . . . . . . . . . . . . . . . . 4
1.2. Discussion Forum . . . . . . . . . . . . . . . . . . . . . 5
1.3. Terminology . . . . . . . . . . . . . . . . . . . . . . . 5
1.3.1. DNS "Name" Terminology . . . . . . . . . . . . . . . . 5
1.3.2. New Terminology and Restrictions . . . . . . . . . . . 6
1.4. Objectives . . . . . . . . . . . . . . . . . . . . . . . . 6
1.5. Applicability and Function of IDNA . . . . . . . . . . . . 7
1.6. Comprehensibility of IDNA Mechanisms and Processing . . . 8
2. Processing in IDNA2008 . . . . . . . . . . . . . . . . . . . . 9
3. Permitted Characters: An Inclusion List . . . . . . . . . . . 9
3.1. A Tiered Model of Permitted Characters and Labels . . . . 10
3.1.1. PROTOCOL-VALID . . . . . . . . . . . . . . . . . . . . 10
3.1.2. CONTEXTUAL RULE REQUIRED . . . . . . . . . . . . . . . 11
3.1.2.1. Contextual Restrictions . . . . . . . . . . . . . 11
3.1.2.2. Rules and Their Application . . . . . . . . . . . 12
3.1.3. DISALLOWED . . . . . . . . . . . . . . . . . . . . . . 12
3.1.4. UNASSIGNED . . . . . . . . . . . . . . . . . . . . . . 13
3.2. Registration Policy . . . . . . . . . . . . . . . . . . . 13
3.3. Layered Restrictions: Tables, Context, Registration,
Applications . . . . . . . . . . . . . . . . . . . . . . . 14
4. Application-Related Issues . . . . . . . . . . . . . . . . . . 15
4.1. Display and Network Order . . . . . . . . . . . . . . . . 15
4.2. Entry and Display in Applications . . . . . . . . . . . . 16
4.3. Linguistic Expectations: Ligatures, Digraphs, and
Alternate Character Forms . . . . . . . . . . . . . . . . 18
4.4. Case Mapping and Related Issues . . . . . . . . . . . . . 20
4.5. Right to Left Text . . . . . . . . . . . . . . . . . . . . 21
5. IDNs and the Robustness Principle . . . . . . . . . . . . . . 21
6. Front-end and User Interface Processing for Lookup . . . . . . 22
7. Migration from IDNA2003 and Unicode Version Synchronization . 24
7.1. Design Criteria . . . . . . . . . . . . . . . . . . . . . 24
7.1.1. Summary and Discussion of IDNA Validity Criteria . . . 24
7.1.2. Labels in Registration . . . . . . . . . . . . . . . . 25
7.1.3. Labels in Lookup . . . . . . . . . . . . . . . . . . . 26
7.2. Changes in Character Interpretations . . . . . . . . . . . 28
7.2.1. Character Changes: Eszett and Final Sigma . . . . . . 28
7.2.2. Character Changes: Zero-Width Joiner and Non-Joiner . 28
7.2.3. Character Changes and the Need for Transition . . . . 28
7.2.4. Transition Strategies . . . . . . . . . . . . . . . . 29
7.3. Elimination of Character Mapping . . . . . . . . . . . . . 30
7.4. The Question of Prefix Changes . . . . . . . . . . . . . . 30
7.4.1. Conditions Requiring a Prefix Change . . . . . . . . . 31
7.4.2. Conditions Not Requiring a Prefix Change . . . . . . . 31
7.4.3. Implications of Prefix Changes . . . . . . . . . . . . 32
7.5. Stringprep Changes and Compatibility . . . . . . . . . . . 32
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7.6. The Symbol Question . . . . . . . . . . . . . . . . . . . 33
7.7. Migration Between Unicode Versions: Unassigned Code
Points . . . . . . . . . . . . . . . . . . . . . . . . . . 34
7.8. Other Compatibility Issues . . . . . . . . . . . . . . . . 36
8. Name Server Considerations . . . . . . . . . . . . . . . . . . 36
8.1. Processing Non-ASCII Strings . . . . . . . . . . . . . . . 36
8.2. Root and other DNS Server Considerations . . . . . . . . . 37
9. Internationalization Considerations . . . . . . . . . . . . . 37
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 38
10.1. IDNA Character Registry . . . . . . . . . . . . . . . . . 38
10.2. IDNA Context Registry . . . . . . . . . . . . . . . . . . 38
10.3. IANA Repository of IDN Practices of TLDs . . . . . . . . . 38
11. Security Considerations . . . . . . . . . . . . . . . . . . . 38
11.1. General Security Issues with IDNA . . . . . . . . . . . . 38
12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 39
13. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 39
14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 40
14.1. Normative References . . . . . . . . . . . . . . . . . . . 40
14.2. Informative References . . . . . . . . . . . . . . . . . . 41
Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . 43
A.1. Changes between Version -00 and Version -01 of
draft-ietf-idnabis-rationale . . . . . . . . . . . . . . . 43
A.2. Version -02 . . . . . . . . . . . . . . . . . . . . . . . 44
A.3. Version -03 . . . . . . . . . . . . . . . . . . . . . . . 44
A.4. Version -04 . . . . . . . . . . . . . . . . . . . . . . . 45
A.5. Version -05 . . . . . . . . . . . . . . . . . . . . . . . 45
A.6. Version -06 . . . . . . . . . . . . . . . . . . . . . . . 45
A.7. Version -07 . . . . . . . . . . . . . . . . . . . . . . . 46
A.8. Version -08 . . . . . . . . . . . . . . . . . . . . . . . 46
A.9. Version -09 . . . . . . . . . . . . . . . . . . . . . . . 46
A.10. Version -10 . . . . . . . . . . . . . . . . . . . . . . . 47
A.11. Version -11 . . . . . . . . . . . . . . . . . . . . . . . 47
A.12. Version -12 . . . . . . . . . . . . . . . . . . . . . . . 48
A.13. Version -13 . . . . . . . . . . . . . . . . . . . . . . . 48
A.14. Version -14 . . . . . . . . . . . . . . . . . . . . . . . 48
A.15. Version -15 . . . . . . . . . . . . . . . . . . . . . . . 49
A.16. Version -16 . . . . . . . . . . . . . . . . . . . . . . . 49
A.17. Version -17 . . . . . . . . . . . . . . . . . . . . . . . 49
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 49
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1. Introduction
1.1. Context and Overview
Internationalized Domain Names in Applications (IDNA) is a collection
of standards that allow client applications to convert some mnemonic
strings expressed in Unicode to an ASCII-compatible encoding form
("ACE") which is a valid DNS label containing only letters, digits,
and hyphens. The specific form of ACE label used by IDNA is called
an "A-label". A client can look up an exact A-label in the existing
DNS, so A-labels do not require any extensions to DNS, upgrades of
DNS servers or updates to low-level client libraries. An A-label is
recognizable from the prefix "xn--" before the characters produced by
the Punycode algorithm [RFC3492], thus a user application can
identify an A-label and convert it into Unicode (or some local coded
character set) for display.
On the registry side, IDNA allows a registry to offer
Internationalized Domain Names (IDNs) for registration as A-labels.
A registry may offer any subset of valid IDNs, and may apply any
restrictions or bundling (grouping of similar labels together in one
registration) appropriate for the context of that registry.
Registration of labels is sometimes discussed separately from lookup,
and is subject to a few specific requirements that do not apply to
lookup.
DNS clients and registries are subject to some differences in
requirements for handling IDNs. In particular, registries are urged
to register only exact, valid A-labels, while clients might do some
mapping to get from otherwise-invalid user input to a valid A-label.
The first version of IDNA was published in 2003 and is referred to
here as IDNA2003 to contrast it with the current version, which is
known as IDNA2008 (after the year in which IETF work started on it).
IDNA2003 consists of four documents: the IDNA base specification
[RFC3490], Nameprep [RFC3491], Punycode [RFC3492], and Stringprep
[RFC3454]. The current set of documents, IDNA2008, are not dependent
on any of the IDNA2003 specifications other than the one for Punycode
encoding. References to "IDNA2008", "these specifications", or
"these documents" are to the entire IDNA2008 set listed in
[IDNA2008-Defs]. 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 matched case-insensitively
[RFC1034][RFC1035]. That convention was preserved in IDNA2003 by a
case-folding operation that generally maps capital letters into
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lower-case ones. However, if case rules are enforced from one
language, another language sometimes loses the ability to treat two
characters separately. Case-insensitivity is treated slightly
differently in IDNA2008.
IDNA2003 used Unicode version 3.2 only. In order to keep up with new
characters added in new versions of UNICODE, IDNA2008 decouples its
rules from any particular version of UNICODE. Instead, the
attributes of new characters in Unicode, supplemented by a small
number of exception cases, determine how and whether the characters
can be used in IDNA labels.
This document provides informational context for IDNA2008, including
terminology, background, and policy discussions. It contains no
normative material; specifications for conformance to the IDNA2008
protocols appears entirely in the other documents in the series.
1.2. Discussion Forum
[[ RFC Editor: please remove this section. ]]
IDNA2008 is being discussed in the IETF "idnabis" Working Group and
on the mailing list idna-update@alvestrand.no
1.3. Terminology
Terminology for IDNA2008 appears in [IDNA2008-Defs]. That document
also contains a roadmap to the IDNA2008 document collection. No
attempt should be made to understand this document without the
definitions and concepts that appear there.
1.3.1. DNS "Name" Terminology
In the context of IDNs, the DNS term "name" has introduced some
confusion as people speak of DNS labels in terms of the words or
phrases of various natural languages. Historically, many of the
"names" in the DNS have been mnemonics to identify some particular
concept, object, or organization. They are typically rooted in some
language because most people think in language-based ways. But,
because they are mnemonics, they need not obey the orthographic
conventions of any language: it is not a requirement that it be
possible for them to be "words".
This distinction is important because the reasonable goal of an IDN
effort is not to be able to write the great Klingon (or language of
one's choice) novel in DNS labels but to be able to form a usefully
broad range of mnemonics in ways that are as natural as possible in a
very broad range of scripts.
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1.3.2. New Terminology and Restrictions
IDNA2008 introduces new terminology. Precise definitions are
provided (in [IDNA2008-Defs]), for the terms "U-label", "A-Label",
LDH-label (to which all valid pre-IDNA host names conformed),
Reserved-LDH-label (R-LDH-label), XN-label, Fake-A-Label, and Non-
Reserved-LDH-label (NR-LDH-label).
In addition, the term "putative label" has been adopted to refer to a
label that may appear to meet certain definitional constraints but
has not yet been sufficiently tested for validity.
These definitions are also illustrated in Figure 1 of the Definitions
Document [IDNA2008-Defs]. R-LDH-labels contain "--" in the third and
fourth character from the beginning of the label. In IDNA-aware
applications, only a subset of these reserved labels is permitted to
be used, namely the A-label subset. A-labels are a subset of the
R-LDH-labels that begin with the case-insensitive string "xn--".
Labels that bear this prefix but which are not otherwise valid fall
into the "Fake A-label" category. The non-reserved labels (NR-LDH-
labels) are implicitly valid since they do not bear any resemblance
to the labels specified by IDNA.
The creation of the Reserved-LDH category is required for three
reasons:
o to prevent confusion with pre-IDNA coding forms;
o to permit future extensions that would require changing the
prefix, no matter how unlikely those might be (see Section 7.4);
and
o to reduce the opportunities for attacks via the Punycode encoding
algorithm itself.
As with other documents in the IDNA2008 set, this document uses the
term "registry" to describe any zone in the DNS. That term, and the
terms "zone" or "zone administration", are interchangeable.
1.4. Objectives
These are the main objectives in revising IDNA.
o Use a more recent version of Unicode, and allow IDNA to be
independent of Unicode versions, so that IDNA2008 need not be
updated for implementations to adopt code points from new Unicode
versions.
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o Fix a very small number of code point categorizations that have
turned out to cause problems in the communities that use those
code points.
o Reduce the dependency on mapping, in order that the pre-mapped
forms (which are not valid IDNA labels) tend to appear less often
in various contexts, in favor of valid A-labels.
o Fix some details in the bidirectional code point handling
algorithms.
1.5. Applicability and Function of IDNA
The IDNA specification solves the problem of extending the repertoire
of characters that can be used in domain names to include a large
subset of the Unicode repertoire.
IDNA does not extend DNS. Instead, the applications (and, by
implication, the users) continue to see an exact-match lookup
service. Either there is a single exactly-matching name (subject to
the base DNS requirement of case-insensitive ASCII matching) or there
is no match. This model has served the existing applications well,
but it requires, with or without internationalized domain names, that
users know the exact spelling of the domain names that are to be
typed into applications such as web browsers and mail user agents.
The introduction of the larger repertoire of characters potentially
makes the set of misspellings larger, especially given that in some
cases the same appearance, for example on a business card, might
visually match several Unicode code points or several sequences of
code points.
The IDNA standard does not require any applications to conform to it,
nor does it retroactively change those applications. An application
can elect to use IDNA in order to support IDN while maintaining
interoperability with existing infrastructure. If an application
wants to use non-ASCII characters in public DNS domain names, IDNA is
the only currently-defined option. Adding IDNA support to an
existing application entails changes to the application only, and
leaves room for flexibility in front-end processing and more
specifically in the user interface (see Section 6).
A great deal of the discussion of IDN solutions has focused on
transition issues and how IDNs will work in a world where not all of
the components have been updated. Proposals that were not chosen by
the original IDN Working Group would have depended on updating of
user applications, DNS resolvers, and DNS servers in order for a user
to apply an internationalized domain name in any form or coding
acceptable under that method. While processing must be performed
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prior to or after access to the DNS, IDNA requires no changes to the
DNS protocol, any DNS servers, or the resolvers on users' computers.
IDNA allows the graceful introduction of IDNs not only by avoiding
upgrades to existing infrastructure (such as DNS servers and mail
transport agents), but also by allowing some limited use of IDNs in
applications by using the ASCII-encoded representation of the labels
containing non-ASCII characters. While such names are user-
unfriendly to read and type, and hence not optimal for user input,
they can be used as a last resort to allow rudimentary IDN usage.
For example, they might be the best choice for display if it were
known that relevant fonts were not available on the user's computer.
In order to allow user-friendly input and output of the IDNs and
acceptance of some characters as equivalent to those to be processed
according to the protocol, the applications need to be modified to
conform to this specification.
This version of IDNA uses the Unicode character repertoire, for
continuity with the original version of IDNA.
1.6. Comprehensibility of IDNA Mechanisms and Processing
One goal of IDNA2008, which is aided by the main goal of reducing the
dependency on mapping, is to improve the general understanding of how
IDNA works and what characters are permitted and what happens to
them. Comprehensibility and predictability to users and registrants
are important design goals for this effort. End-user applications
have an important role to play in increasing this comprehensibility.
Any system that tries to handle international characters encounters
some common problems. For example, a UI cannot display a character
if no font for that character is available. In some cases,
internationalization enables effective localization while maintaining
some global uniformity but losing some universality.
It is difficult to even make suggestions for end-user applications to
cope when characters and fonts are not available. Because display
functions are rarely controlled by the types of applications that
would call upon IDNA, such suggestions will rarely be very effective.
Converting between local character sets and normalized Unicode, if
needed, is part of this set of user agent issues. This conversion
introduces complexity in a system that is not Unicode-native. If a
label is converted to a local character set that does not have all
the needed characters, or that uses different character-coding
principles, the user agent may have to add special logic to avoid or
reduce loss of information.
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The major difficulty may lie in accurately identifying the incoming
character set and applying the correct conversion routine. Even more
difficult, the local character coding system could be based on
conceptually different assumptions than those used by Unicode (e.g.,
choice of font encodings used for publications in some Indic
scripts). Those differences may not easily yield unambiguous
conversions or interpretations even if each coding system is
internally consistent and adequate to represent the local language
and script.
IDNA2008 shifts responsibility for character mapping and other
adjustments from the protocol (where it was located in IDNA2003) to
pre-processing before invoking IDNA itself. The intent is that this
change will lead to greater usage of fully-valid A-Labels or U-labels
in display, transit and storage, which should aid comprehensibility
and predictability. A careful look at pre-processing raises issues
about what that pre-processing should do and at what point pre-
processing becomes harmful, how universally consistent pre-processing
algorithms can be, and how to be compatible with labels prepared in a
IDNA2003 context. Those issues are discussed in Section 6 and in the
separate document [IDNA2008-Mapping].
2. Processing in IDNA2008
IDNA2008 separates Domain Name Registration and Lookup in the
protocol specification. Although most steps in the two processes are
similar, the separation reflects current practice in which per-
registry (DNS zone) restrictions and special processing are applied
at registration time but not during lookup. Another significant
benefit is that separation facilitates incremental addition of
permitted character groups to avoid freezing on one particular
version of Unicode.
The actual registration and lookup protocols for IDNA2008 are
specified in [IDNA2008-Protocol].
3. Permitted Characters: An Inclusion List
IDNA2008 adopts the inclusion model. A code point is assumed to be
invalid for IDN use unless it is included as part of a Unicode
property-based rule or, in rare cases, included individually by an
exception. When an implementation moves to a new version of Unicode,
the rules may indicate new valid code points.
This section provides an overview of the model used to establish the
algorithm and character lists of [IDNA2008-Tables] and describes the
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names and applicability of the categories used there. Note that the
inclusion of a character in the first category group (Section 3.1.1)
does not imply that it can be used indiscriminately; some characters
are associated with contextual rules that must be applied as well.
The information given in this section is provided to make the rules,
tables, and protocol easier to understand. The normative generating
rules that correspond to this informal discussion appear in
[IDNA2008-Tables] and the rules that actually determine what labels
can be registered or looked up are in [IDNA2008-Protocol].
3.1. A Tiered Model of Permitted Characters and Labels
Moving to an inclusion model involves a new specification for the
list of characters that are permitted in IDNs. In IDNA2003,
character validity is independent of context and fixed forever (or
until the standard is replaced). However, globally context-
independent rules have proved to be impractical because some
characters, especially those that are called "Join_Controls" in
Unicode, are needed to make reasonable use of some scripts but have
no visible effect in others. IDNA2003 prohibited those types of
characters entirely by discarding them. We now have a consensus that
under some conditions, these "joiner" characters are legitimately
needed to allow useful mnemonics for some languages and scripts. In
general, context-dependent rules help deal with characters (generally
characters that would otherwise be prohibited entirely) that are used
differently or perceived differently across different scripts, and
allow the standard to be applied more appropriately in cases where a
string is not universally handled the same way.
IDNA2008 divides all possible Unicode code points into four
categories: PROTOCOL-VALID, CONTEXTUAL RULE REQUIRED, DISALLOWED and
UNASSIGNED.
3.1.1. PROTOCOL-VALID
Characters identified as "PROTOCOL-VALID" (often abbreviated
"PVALID") are permitted in IDNs. Their use may be restricted by
rules about the context in which they appear or by other rules that
apply to the entire label in which they are to be embedded. For
example, any label that contains a character in this category that
has a "right-to-left" property must be used in context with the
"Bidi" rules (see [IDNA2008-Bidi]).
The term "PROTOCOL-VALID" is used to stress the fact that the
presence of a character in this category does not imply that a given
registry need accept registrations containing any of the characters
in the category. Registries are still expected to apply judgment
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about labels they will accept and to maintain rules consistent with
those judgments (see [IDNA2008-Protocol] and Section 3.3).
Characters that are placed in the "PROTOCOL-VALID" category are
expected to never be removed from it or reclassified. While
theoretically characters could be removed from Unicode, such removal
would be inconsistent with the Unicode stability principles (see
[Unicode51], Appendix F) and hence should never occur.
3.1.2. CONTEXTUAL RULE REQUIRED
Some characters may be unsuitable for general use in IDNs but
necessary for the plausible support of some scripts. The two most
commonly-cited examples are the zero-width joiner and non-joiner
characters (ZWJ, U+200D and ZWNJ, U+200C) but other characters may
require special treatment because they would otherwise be DISALLOWED
(typically because Unicode considers them punctuation or special
symbols) but need to be permitted in limited contexts. Other
characters are given this special treatment because they pose
exceptional danger of being used to produce misleading labels or to
cause unacceptable ambiguity in label matching and interpretation.
3.1.2.1. Contextual Restrictions
Characters with contextual restrictions are identified as "CONTEXTUAL
RULE REQUIRED" and associated with a rule. The rule defines whether
the character is valid in a particular string, and also whether the
rule itself is to be applied on lookup as well as registration.
A distinction is made between characters that indicate or prohibit
joining and ones similar to them (known as "CONTEXT-JOINER" or
"CONTEXTJ") and other characters requiring contextual treatment
("CONTEXT-OTHER" or "CONTEXTO"). Only the former require full
testing at lookup time.
It is important to note that these contextual rules cannot prevent
all uses of the relevant characters that might be confusing or
problematic. What they are expected to do is to confine
applicability of the characters to scripts (and narrower contexts)
where zone administrators are knowledgeable enough about the use of
those characters to be prepared to deal with them appropriately.
For example, a registry dealing with an Indic script that requires
ZWJ and/or ZWNJ as part of the writing system is expected to
understand where the characters have visible effect and where they do
not and to make registration rules accordingly. By contrast, a
registry dealing primarily with Latin or Cyrillic script might not be
actively aware that the characters exist, much less about the
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consequences of embedding them in labels drawn from those scripts and
therefore should avoid accepting registrations containing those
characters, at least in Latin or Cyrillic-script labels.
3.1.2.2. Rules and Their Application
Rules have descriptions such as "Must follow a character from Script
XYZ", "Must occur only if the entire label is in Script ABC", or
"Must occur only if the previous and subsequent characters have the
DFG property". The actual rules may be DEFINED or NULL. If present,
they may have values of "True" (character may be used in any position
in any label), "False" (character may not be used in any label), or
may be a set of procedural rules that specify the context in which
the character is permitted.
Because it is easier to identify these characters than to know that
they are actually needed in IDNs or how to establish exactly the
right rules for each one, a rule may have a null value in a given
version of the tables. Characters associated with null rules are not
permitted to appear in putative labels for either registration or
lookup. Of course, a later version of the tables might contain a
non-null rule.
The actual rules and their descriptions are in Sections 2 and 3 of
[IDNA2008-Tables]. That document also specifies the creation of a
registry for future rules.
3.1.3. DISALLOWED
Some characters are inappropriate for use in IDNs and are thus
excluded for both registration and lookup (i.e., IDNA-conforming
applications performing name lookup should verify that these
characters are absent; if they are present, the label strings should
be rejected rather than converted to A-labels and looked up. Some of
these characters are problematic for use in IDNs (such as the
FRACTION SLASH character, U+2044), while some of them (such as the
various HEART symbols, e.g., U+2665, U+2661, and U+2765, see
Section 7.6) simply fall outside the conventions for typical
identifiers (basically letters and numbers).
Of course, this category would include code points that had been
removed entirely from Unicode should such removals ever occur.
Characters that are placed in the "DISALLOWED" category are expected
to never be removed from it or reclassified. If a character is
classified as "DISALLOWED" in error and the error is sufficiently
problematic, the only recourse would be either to introduce a new
code point into Unicode and classify it as "PROTOCOL-VALID" or for
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the IETF to accept the considerable costs of an incompatible change
and replace the relevant RFC with one containing appropriate
exceptions.
There is provision for exception cases but, in general, characters
are placed into "DISALLOWED" if they fall into one or more of the
following groups:
o The character is a compatibility equivalent for another character.
In slightly more precise Unicode terms, application of
normalization method NFKC to the character yields some other
character.
o The character is an upper-case form or some other form that is
mapped to another character by Unicode casefolding.
o The character is a symbol or punctuation form or, more generally,
something that is not a letter, digit, or a mark that is used to
form a letter or digit.
3.1.4. UNASSIGNED
For convenience in processing and table-building, code points that do
not have assigned values in a given version of Unicode are treated as
belonging to a special UNASSIGNED category. Such code points are
prohibited in labels to be registered or looked up. The category
differs from DISALLOWED in that code points are moved out of it by
the simple expedient of being assigned in a later version of Unicode
(at which point, they are classified into one of the other categories
as appropriate).
The rationale for restricting the processing of UNASSIGNED characters
is simply that the properties of such code points cannot be
completely known until actual characters are assigned to them. For
example, assume that an UNASSIGNED code point were included in a
label to be looked up. Assume that the code point was later assigned
to a character that required some set of contextual rules. With that
combination, un-updated instances of IDNA-aware software might permit
lookup of labels containing the previously-unassigned characters
while updated versions of the software might restrict use of the same
label in lookup, depending on the contextual rules. It should be
clear that under no circumstance should an UNASSIGNED character be
permitted in a label to be registered as part of a domain name.
3.2. Registration Policy
While these recommendations cannot and should not define registry
policies, registries should develop and apply additional restrictions
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as needed to reduce confusion and other problems. For example, it is
generally believed that labels containing characters from more than
one script are a bad practice although there may be some important
exceptions to that principle. Some registries may choose to restrict
registrations to characters drawn from a very small number of
scripts. For many scripts, the use of variant techniques such as
those as described in RFC 3743 [RFC3743] and RFC 4290 [RFC4290], and
illustrated for Chinese by the tables described in RFC 4713 [RFC4713]
may be helpful in reducing problems that might be perceived by users.
In general, users will benefit if registries only permit characters
from scripts that are well-understood by the registry or its
advisers. If a registry decides to reduce opportunities for
confusion by constructing policies that disallow characters used in
historic writing systems or characters whose use is restricted to
specialized, highly technical contexts, some relevant information may
be found in Section 2.4 "Specific Character Adjustments", Table 4
"Candidate Characters for Exclusion from Identifiers" of
[Unicode-UAX31] and Section 3.1. "General Security Profile for
Identifiers" in [Unicode-Security].
The requirement (in Section 4.1 of [IDNA2008-Protocol]) that
registration procedures use only U-labels and/or A-labels is intended
to ensure that registrants are fully aware of exactly what is being
registered as well as encouraging use of those canonical forms. That
provision should not be interpreted as requiring that registrants
need to provide characters in a particular code sequence. Registrant
input conventions and management are part of registrant-registrar
interactions and relationships between registries and registrars and
are outside the scope of these standards.
It is worth stressing that these principles of policy development and
application apply at all levels of the DNS, not only, e.g., TLD or
SLD registrations. Even a trivial, "anything is permitted that is
valid under the protocol" policy is helpful in that it helps users
and application developers know what to expect.
3.3. Layered Restrictions: Tables, Context, Registration, Applications
The character rules in IDNA2008 are based on the realization that
there is no single magic bullet for any of the security,
confusability, or other issues associated with IDNs. Instead, the
specifications define a variety of approaches. The character tables
are the first mechanism, protocol rules about how those characters
are applied or restricted in context are the second, and those two in
combination constitute the limits of what can be done in the
protocol. As discussed in the previous section (Section 3.2),
registries are expected to restrict what they permit to be
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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
against label forms that appear problematic given their knowledge of
local contexts and conventions. Of course, no approach based on
naming or identifiers alone can protect against all threats.
4. Application-Related Issues
4.1. Display and Network Order
Domain names are always transmitted in network order (the order in
which the code points are sent in protocols), but may have a
different display order (the order in which the code points are
displayed on a screen or paper). When a domain name contains
characters that are normally written right to left, display order may
be affected although network order is not. It gets even more
complicated if left to right and right to left labels are adjacent to
each other within a domain name. The decision about the display
order is ultimately under the control of user agents --including Web
browsers, mail clients, hosted Web applications and many more --
which may be highly localized. Should a domain name abc.def, in
which both labels are represented in scripts that are written right
to left, be displayed as fed.cba or cba.fed? Applications that are
in deployment today are already diverse, and one can find examples of
either choice.
The picture changes once again when an IDN appears in a
Internationalized Resource Identifier (IRI) [RFC3987]. An IRI or
Internationalized Email address contains elements other than the
domain name. For example, IRIs contain protocol identifiers and
field delimiter syntax such as "http://" or "mailto:" while email
addresses contain the "@" to separate local parts from domain names.
An IRI in network order begins with "http://" followed by domain
labels in network order, thus "http://abc.def".
User agents are not required to display and allow input of IRIs
directly but often do so. Implementors have to choose whether the
overall direction of these strings will always be left to right (or
right to left) for an IRI or email address. The natural order for a
user typing a domain name on a right to left system is fed.cba.
Should the R2L user agent reverse the entire domain name each time a
domain name is typed? Does this change if the user types "http://"
right before typing a domain name, thus implying that the user is
beginning at the beginning of the network order IRI? Experience in
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the 1980s and 1990s with mixing systems in which domain name labels
were read in network order (left to right) and those in which those
labels were read right to left would predict a great deal of
confusion.
If each implementation of each application makes its own decisions on
these issues, users will develop heuristics that will sometimes fail
when switching applications. However, while some display order
conventions, voluntarily adopted, would be desirable to reduce
confusion, such suggestions are beyond the scope of these
specifications.
4.2. Entry and Display in Applications
Applications can accept and display domain names using any character
set or character coding system. The IDNA protocol does not
necessarily affect the interface between users and applications. An
IDNA-aware application can accept and display internationalized
domain names in two formats: the internationalized character set(s)
supported by the application (i.e., an appropriate local
representation of a U-label), and as an A-label. Applications may
allow the display of A-labels, but are encouraged to not do so except
as an interface for special purposes, possibly for debugging, or to
cope with display limitations. In general, they should allow, but
not encourage, user input of A-labels. A-labels are opaque, ugly,
and malicious variations on them are not easily detected by users.
Where possible, they should thus only be exposed when they are
absolutely needed. Because IDN labels can be rendered either as
A-labels or U-labels, the application may reasonably have an option
for the user to select the preferred method of display. Rendering
the U-label should normally be the default.
Domain names are often stored and transported in many places. For
example, they are part of documents such as mail messages and web
pages. They are transported in many parts of many protocols, such as
both the control commands of SMTP and associated message body parts,
and in the headers and the body content in HTTP. It is important to
remember that domain names appear both in domain name slots and in
the content that is passed over protocols and it would be helpful if
protocols explicitly define what their domain name slots are.
In protocols and document formats that define how to handle
specification or negotiation of charsets, labels can be encoded in
any charset allowed by the protocol or document format. If a
protocol or document format only allows one charset, the labels must
be given in that charset. Of course, not all charsets can properly
represent all labels. If a U-label cannot be displayed in its
entirety, the only choice (without loss of information) may be to
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display the A-label.
Where a protocol or document format allows IDNs, labels should be in
whatever character encoding and escape mechanism the protocol or
document format uses at that place. This provision is intended to
prevent situations in which, e.g., UTF-8 domain names appear embedded
in text that is otherwise in some other character coding.
All protocols that use domain name slots (See Section 2.3.1.6 in
[IDNA2008-Defs]) already have the capacity for handling domain names
in the ASCII charset. Thus, A-labels can inherently be handled by
those protocols.
IDNA2008 does not specify required mappings between one character or
code point and others. An extended discussion of mapping issues
occurs in Section 6 and specific recommendations appear in
[IDNA2008-Mapping]. In general, IDNA2008 prohibits characters that
would be mapped to others by normalization or other rules. As
examples, while mathematical characters based on Latin ones are
accepted as input to IDNA2003, they are prohibited in IDNA2008.
Similarly, upper-case characters, double-width characters, and other
variations are prohibited as IDNA input although mapping them as
needed in user interfaces is strongly encouraged.
Since the rules in [IDNA2008-Tables] have the effect that only
strings that are not transformed by NFKC are valid, if an application
chooses to perform NFKC normalization before lookup, that operation
is safe since this will never make the application unable to look up
any valid string. However, as discussed above, the application
cannot guarantee that any other application will perform that
mapping, so it should be used only with caution and for informed
users.
In many cases these prohibitions should have no effect on what the
user can type as input to the lookup process. It is perfectly
reasonable for systems that support user interfaces to perform some
character mapping that is appropriate to the local environment. This
would normally be done prior to actual invocation of IDNA. At least
conceptually, the mapping would be part of the Unicode conversions
discussed above and in [IDNA2008-Protocol]. However, those changes
will be local ones only -- local to environments in which users will
clearly understand that the character forms are equivalent. For use
in interchange among systems, it appears to be much more important
that U-labels and A-labels can be mapped back and forth without loss
of information.
One specific, and very important, instance of this strategy arises
with case-folding. In the ASCII-only DNS, names are looked up and
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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
any mixture of them) and that form is preserved, returned in queries,
and so on. IDNA2003 approximated that behavior for non-ASCII strings
by performing case-folding at registration time (resulting in only
lower-case IDNs in the DNS) and when names were looked up.
As suggested earlier in this section, it appears to be desirable to
do as little character mapping as possible as long as Unicode works
correctly (e.g., NFC mapping to resolve different codings for the
same character is still necessary although the specifications require
that it be performed prior to invoking the protocol) in order to make
the mapping between A-labels and U-labels idempotent. Case-mapping
is not an exception to this principle. If only lower case characters
can be registered in the DNS (i.e., be present in a U-label), then
IDNA2008 should prohibit upper-case characters as input even though
user interfaces to applications should probably map those characters.
Some other considerations reinforce this conclusion. For example, in
ASCII case-mapping for individual characters, uppercase(character)
must be equal to uppercase(lowercase(character)). That may not be
true with IDNs. In some scripts that use case distinctions, there
are a few characters that do not have counterparts in one case or the
other. The relationship between upper case and lower case may even
be language-dependent, with different languages (or even the same
language in different areas) expecting different mappings. User
agents can meet the expectations of users who are accustomed to the
case-insensitive DNS environment by performing case folding prior to
IDNA processing, but the IDNA procedures themselves should neither
require such mapping nor expect them when they are not natural to the
localized environment.
4.3. Linguistic Expectations: Ligatures, Digraphs, and Alternate
Character Forms
Users have expectations about character matching or equivalence that
are based on their own languages and the orthography of those
languages. These expectations may not always be met in a global
system, especially if multiple languages are written using the same
script but using different conventions. Some examples:
o A Norwegian user might expect a label with the ae-ligature to be
treated as the same label as one using the Swedish spelling with
a-diaeresis even though applying that mapping to English would be
astonishing to users.
o A 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
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Swedish.
o A Chinese user might expect automatic matching of Simplified and
Traditional Chinese characters, but applying that matching for
Korean or Japanese text would create considerable confusion.
o An English user might expect "theater" and "theatre" to match.
A number of languages use alphabetic scripts in which single phonemes
are written using two characters, termed a "digraph", for example,
the "ph" in "pharmacy" and "telephone". (Such characters can also
appear consecutively without forming a digraph, as in "tophat".)
Certain digraphs may be indicated typographically by setting the two
characters closer together than they would be if used consecutively
to represent different phonemes. Some digraphs are fully joined as
ligatures. For example, the word "encyclopaedia" is sometimes set
with a U+00E6 LATIN SMALL LIGATURE AE. When ligature and digraph
forms have the same interpretation across all languages that use a
given script, application of Unicode normalization generally resolves
the differences and causes them to match. When they have different
interpretations, matching must utilize other methods, presumably
chosen at the registry level, or users must be educated to understand
that matching will not occur.
The nature of the problem can be illustrated by many words in the
Norwegian language, where the "ae" ligature is the 27th letter of a
29-letter extended Latin alphabet. It is equivalent to the 28th
letter of the Swedish alphabet (also containing 29 letters), U+00E4
LATIN SMALL LETTER A WITH DIAERESIS, for which an "ae" cannot be
substituted according to current orthographic standards. That
character (U+00E4) is also part of the German alphabet where, unlike
in the Nordic languages, the two-character sequence "ae" is usually
treated as a fully acceptable alternate orthography for the "umlauted
a" character. The inverse is however not true, and those two
characters cannot necessarily be combined into an "umlauted a". This
also applies to another German character, the "umlauted o" (U+00F6
LATIN SMALL LETTER O WITH DIAERESIS) which, for example, cannot be
used for writing the name of the author "Goethe". It is also a
letter in the Swedish alphabet where, like the "a with diaeresis", it
cannot be correctly represented as "oe" and in the Norwegian
alphabet, where it is represented, not as "o with diaeresis", but as
"slashed o", U+00F8.
Some of the ligatures that have explicit code points in Unicode were
given special handling in IDNA2003 and now pose additional problems
in transition. See Section 7.2.
Additional cases with alphabets written right to left are described
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in Section 4.5.
Matching and comparison algorithm selection often requires
information about the language being used, context, or both --
information that is not available to IDNA or the DNS. Consequently,
IDNA2008 makes no attempt to treat combined characters in any special
way. A registry that is aware of the language context in which
labels are to be registered, and where that language sometimes (or
always) treats the two- character sequences as equivalent to the
combined form, should give serious consideration to applying a
"variant" model [RFC3743][RFC4290], or to prohibiting registration of
one of the forms entirely, to reduce the opportunities for user
confusion and fraud that would result from the related strings being
registered to different parties.
4.4. Case Mapping and Related Issues
In the DNS, ASCII letters are stored with their case preserved.
Matching during the query process is case-independent, but none of
the information that might be represented by choices of case has been
lost. That model has been accidentally helpful because, as people
have created DNS labels by catenating words (or parts of words) to
form labels, case has often been used to distinguish among components
and make the labels more memorable.
Since DNS servers do not get involved in parsing IDNs, they cannot do
case-independent matching. Thus, keeping the cases separate in
lookup or registration, and doing matching at the server, is not
feasible with IDNA or any similar approach. Case-matching must be
done, if desired, by IDN clients even though it wasn't done by ASCII-
only DNS clients. That situation was recognized in IDNA2003 and
nothing in IDNA2008 fundamentally changes it or could do so. In
IDNA2003, all characters are case-folded and mapped by clients in a
standardized step.
Some characters do not have upper case forms. For example the
Unicode case folding operation maps Greek Final Form Sigma (U+03C2)
to the medial form (U+03C3) and maps Eszett (German Sharp S, U+00DF)
to "ss". Neither of these mappings is reversible because the upper
case of U+03C3 is the Upper Case Sigma (U+03A3) and "ss" is an ASCII
string. IDNA2008 permits, at the risk of some incompatibility,
slightly more flexibility in this area by avoiding case folding and
treating these characters as themselves. Approaches to handling one-
way mappings are discussed in Section 7.2.
Because IDNA2003 maps Final Sigma and Eszett to other characters, and
the reverse mapping is never possible, neither Final Sigma nor Eszett
can be represented in the ACE form of IDNA2003 IDN nor in the native
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character (U-label) form derived from it. With IDNA2008, both
characters can be used in an IDN and so the A-label used for lookup
for any U-label containing those characters, is now different. See
Section 7.1 for a discussion of what kinds of changes might require
the IDNA prefix to change; after extended discussions, the WG came to
consensus that the change for these characters did not justify a
prefix change.
4.5. Right to Left Text
In order to be sure that the directionality of right to left text is
unambiguous, IDNA2003 required that any label in which right to left
characters appear both starts and ends with them and that it not
include any characters with strong left to right properties (that
excludes other alphabetic characters but permits European digits).
Any other string that contains a right to left character and does not
meet those requirements is rejected. This is one of the few places
where the IDNA algorithms (both in IDNA2003 and in IDAN2008) examine
an entire label, not just individual characters. The algorithmic
model used in IDNA2003 rejects the label when the final character in
a right to left string requires a combining mark in order to be
correctly represented.
That prohibition is not acceptable for writing systems for languages
written with consonantal alphabets to which diacritical vocalic
systems are applied, and for languages with orthographies derived
from them where the combining marks may have different functionality.
In both cases the combining marks can be essential components of the
orthography. Examples of this are Yiddish, written with an extended
Hebrew script, and Dhivehi (the official language of Maldives) which
is written in the Thaana script (which is, in turn, derived from the
Arabic script). IDNA2008 removes the restriction on final combining
characters with a new set of rules for right to left scripts and
their characters. Those new rules are specified in [IDNA2008-Bidi].
5. IDNs and the Robustness Principle
The "Robustness Principle" is often stated as "Be conservative about
what you send and liberal in what you accept" (See, e.g., Section
1.2.2 of the applications-layer Host Requirements specification
[RFC1123]) This principle applies to IDNA. In applying the principle
to registries as the source ("sender") of all registered and useful
IDNs, registries are responsible for being conservative about what
they register and put out in the Internet. For IDNs to work well,
zone administrators (registries) must have and require sensible
policies about what is registered -- conservative policies -- and
implement and enforce them.
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Conversely, lookup applications are expected to reject labels that
clearly violate global (protocol) rules (no one has ever seriously
claimed that being liberal in what is accepted requires being
stupid). However, once one gets past such global rules and deals
with anything sensitive to script or locale, it is necessary to
assume that garbage has not been placed into the DNS, i.e., one must
be liberal about what one is willing to look up in the DNS rather
than guessing about whether it should have been permitted to be
registered.
If a string cannot be successfully found in the DNS after the lookup
processing described here, it makes no difference whether it simply
wasn't registered or was prohibited by some rule at the registry.
Application implementors should be aware that where DNS wildcards are
used, the ability to successfully resolve a name does not guarantee
that it was actually registered.
6. Front-end and User Interface Processing for Lookup
Domain names may be identified and processed in many contexts. They
may be typed in by users either by themselves or embedded in an
identifier such as email addresses, URIs, or IRIs. They may occur in
running text or be processed by one system after being provided in
another. Systems may try to normalize URLs to determine (or guess)
whether a reference is valid or two references point to the same
object without actually looking the objects up (comparison without
lookup is necessary for URI types that are not intended to be
resolved). Some of these goals may be more easily and reliably
satisfied than others. While there are strong arguments for any
domain name that is placed "on the wire" -- transmitted between
systems -- to be in the zero-ambiguity forms of A-labels, it is
inevitable that programs that process domain names will encounter
U-labels or variant forms.
An application that implements the IDNA protocol [IDNA2008-Protocol]
will always take any user input and convert it to a set of Unicode
code points. That user input may be acquired by any of several
different input methods, all with differing conversion processes to
be taken into consideration (e.g., typed on a keyboard, written by
hand onto some sort of digitizer, spoken into a microphone and
interpreted by a speech-to-text engine, etc.). The process of taking
any particular user input and mapping it into a Unicode code point
may be a simple one: If a user strikes the "A" key on a US English
keyboard, without any modifiers such as the "Shift" key held down, in
order to draw a Latin small letter A ("a"), many (perhaps most)
modern operating system input methods will produce to the calling
application the code point U+0061, encoded in a single octet.
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Sometimes the process is somewhat more complicated: a user might
strike a particular set of keys to represent a combining macron
followed by striking the "A" key in order to draw a Latin small
letter A with a macron above it. Depending on the operating system,
the input method chosen by the user, and even the parameters with
which the application communicates with the input method, the result
might be the code point U+0101 (encoded as two octets in UTF-8 or
UTF-16, four octets in UTF-32, etc.), the code point U+0061 followed
by the code point U+0304 (again, encoded in three or more octets,
depending upon the encoding used) or even the code point U+FF41
followed by the code point U+0304 (and encoded in some form). And
these examples leave aside the issue of operating systems and input
methods that do not use Unicode code points for their character set.
In every case, applications (with the help of the operating systems
on which they run and the input methods used) need to perform a
mapping from user input into Unicode code points.
The original version of the IDNA protocol [RFC3490] used a model
whereby input was taken from the user, mapped (via whatever input
method mechanisms were used) to a set of Unicode code points, and
then further mapped to a set of Unicode code points using the
Nameprep profile specified in [RFC3491]. In this procedure, there
are two separate mapping steps: First, a mapping done by the input
method (which might be controlled by the operating system, the
application, or some combination) and then a second mapping performed
by the Nameprep portion of the IDNA protocol. The mapping done in
Nameprep includes a particular mapping table to re-map some
characters to other characters, a particular normalization, and a set
of prohibited characters.
Note that the result of the two step mapping process means that the
mapping chosen by the operating system or application in the first
step might differ significantly from the mapping supplied by the
Nameprep profile in the second step. This has advantages and
disadvantages. Of course, the second mapping regularizes what gets
looked up in the DNS, making for better interoperability between
implementations which use the Nameprep mapping. However, the
application or operating system may choose mappings in their input
methods, which when passed through the second (Nameprep) mapping
result in characters that are "surprising" to the end user.
The other important feature of the original version of the IDNA
protocol is that, with very few exceptions, it assumes that any set
of Unicode code points provided to the Nameprep mapping can be mapped
into a string of Unicode code points that are "sensible", even if
that means mapping some code points to nothing (that is, removing the
code points from the string). This allowed maximum flexibility in
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input strings.
The present version of IDNA differs significantly in approach from
the original version. First and foremost, it does not provide
explicit mapping instructions. Instead, it assumes that the
application (perhaps via an operating system input method) will do
whatever mapping it requires to convert input into Unicode code
points. This has the advantage of giving flexibility to the
application to choose a mapping that is suitable for its user given
specific user requirements, and avoids the two-step mapping of the
original protocol. Instead of a mapping, the current version of IDNA
provides a set of categories that can be used to specify the valid
code points allowed in a domain name.
In principle, an application ought to take user input of a domain
name and convert it to the set of Unicode code points that represent
the domain name the user intends. As a practical matter, of course,
determining user intent is a tricky business, so an application needs
to choose a reasonable mapping from user input. That may differ
based on the particular circumstances of a user, depending on locale,
language, type of input method, etc. It is up to the application to
make a reasonable choice.
7. Migration from IDNA2003 and Unicode Version Synchronization
7.1. Design Criteria
As mentioned above and in RFC 4690, two key goals of the IDNA2008
design are
o to enable applications to be agnostic about whether they are being
run in environments supporting any Unicode version from 3.2
onward,
o to permit incrementally adding new characters, character groups,
scripts, and other character collections as they are incorporated
into Unicode, doing so without disruption and, in the long term,
without "heavy" processes (an IETF consensus process is required
by the IDNA2008 specifications and is expected to be required and
used until significant experience accumulates with IDNA operations
and new versions of Unicode).
7.1.1. Summary and Discussion of IDNA Validity Criteria
The general criteria for a label to be considered valid under IDNA
are (the actual rules are rigorously defined in [IDNA2008-Protocol]
and [IDNA2008-Tables]):
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o The characters are "letters", marks needed to form letters,
numerals, or other code points used to write words in some
language. Symbols, drawing characters, and various notational
characters are intended to be permanently excluded. There is no
evidence that they are important enough to Internet operations or
internationalization to justify expansion of domain names beyond
the general principle of "letters, digits, and hyphen".
(Additional discussion and rationale for the symbol decision
appears in Section 7.6).
o Other than in very exceptional cases, e.g., where they are needed
to write substantially any word of a given language, punctuation
characters are excluded. The fact that a word exists is not proof
that it should be usable in a DNS label and DNS labels are not
expected to be usable for multiple-word phrases (although they are
certainly not prohibited if the conventions and orthography of a
particular language cause that to be possible).
o Characters that are unassigned (have no character assignment at
all) in the version of Unicode being used by the registry or
application are not permitted, even on lookup. The issues
involved in this decision are discussed in Section 7.7.
o Any character that is mapped to another character by a current
version of NFKC is prohibited as input to IDNA (for either
registration or lookup). With a few exceptions, this principle
excludes any character mapped to another by Nameprep [RFC3491].
The principles above drive the design of rules that are specified
exactly in [IDNA2008-Tables]. Those rules identify the characters
that are valid under IDNA. The rules themselves are normative, and
the tables are derived from them, rather than vice versa.
7.1.2. Labels in Registration
Any label registered in a DNS zone must be validated -- i.e., the
criteria for that label must be met -- in order for applications to
work as intended. This principle is not new. For example, since the
DNS was first deployed, zone administrators have been expected to
verify that names meet "hostname" requirements [RFC0952] where those
requirements are imposed by the expected applications. Other
applications contexts, such as the later addition of special service
location formats [RFC2782] imposed new requirements on zone
administrators. For zones that will contain IDNs, support for
Unicode version-independence requires restrictions on all strings
placed in the zone. In particular, for such zones (the exact rules
appear in the Protocol Document, Section 4 [IDNA2008-Protocol]):
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o Any label that appears to be an A-label, i.e., any label that
starts in "xn--", must be valid under IDNA, i.e., they must be
valid A-labels, as discussed in Section 2 above.
o The Unicode tables (i.e., tables of code points, character
classes, and properties) and IDNA tables (i.e., tables of
contextual rules such as those that appear in the Tables
document), must be consistent on the systems performing or
validating labels to be registered. Note that this does not
require that tables reflect the latest version of Unicode, only
that all tables used on a given system are consistent with each
other.
Under this model, registry tables will need to be updated (both the
Unicode-associated tables and the tables of permitted IDN characters)
to enable a new script or other set of new characters. The registry
will not be affected by newer versions of Unicode, or newly-
authorized characters, until and unless it wishes to support them.
The zone administrator is responsible for verifying validity for IDNA
as well as its local policies -- a more extensive set of checks than
are required for looking up the labels. Systems looking up or
resolving DNS labels, especially IDN DNS labels, must be able to
assume that applicable registration rules were followed for names
entered into the DNS.
7.1.3. Labels in Lookup
Any application processing a label through IDNA so it can be looked
up in a DNS zone is required to (the exact rules appear in the
Protocol Document, Section 5 [IDNA2008-Protocol])
o Maintain IDNA and Unicode tables that are consistent with regard
to versions, i.e., unless the application actually executes the
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
extent of determining that the U-label does not contain
"DISALLOWED" code points or code points that are unassigned in its
version of Unicode.
o Validate the label itself for conformance with a small number of
whole-label rules. In particular, it must verify that
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* there are no leading combining marks,
* the "bidi" conditions are met if right to left characters
appear,
* any required contextual rules are available, and
* any contextual rules that are associated with Joiner Controls
(and "CONTEXTJ" characters more generally) are tested.
o Do not reject labels based on other contextual rules about
characters, including mixed-script label prohibitions. Such rules
may be used to influence presentation decisions in the user
interface, but not to avoid looking up domain names.
To further clarify the rules about handling characters that require
contextual rules, note that one can have a context-required character
(i.e., one that requires a rule), but no rule. In that case, the
character is treated the same way DISALLOWED characters are treated,
until and unless a rule is supplied. That state is more or less
equivalent to "the idea of permitting this character is accepted in
principle, but it won't be permitted in practice until consensus is
reached on a safe way to use it".
The ability to add a rule more or less exempts these characters from
the prohibition against reclassifying characters from DISALLOWED to
PVALID.
And, obviously, "no rule" is different from "have a rule, but the
test either succeeds or fails".
Lookup applications that follow these rules, rather than having their
own criteria for rejecting lookup attempts, are not sensitive to
version incompatibilities with the particular zone registry
associated with the domain name except for labels containing
characters recently added to Unicode.
An application or client that processes names according to this
protocol and then resolves them in the DNS will be able to locate any
name that is registered, as long as those registrations are valid
under IDNA and its version of the IDNA tables is sufficiently up-to-
date to interpret all of the characters in the label. Messages to
users should distinguish between "label contains an unallocated code
point" and other types of lookup failures. A failure on the basis of
an old version of Unicode may lead the user to a desire to upgrade to
a newer version, but will have no other ill effects (this is
consistent with behavior in the transition to the DNS when some hosts
could not yet handle some forms of names or record types).
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7.2. Changes in Character Interpretations
As a consequence of the elimination of mapping, the current version
of IDNA changes the interpretation of a few characters relative to
its predecessors. This subsection outlines the issues and discusses
possible transition strategies.
7.2.1. Character Changes: Eszett and Final Sigma
In those scripts that make case distinctions, there are a few
characters for which an obvious and unique upper case character has
not historically been available to match a lower case one or vice
versa. For those characters, the mappings used in constructing the
Stringprep tables for IDNA2003, performed using the Unicode CaseFold
operation (See Section 5.8 of the Unicode Standard [Unicode51]),
generate different characters or sets of characters. Those
operations are not reversible and lose even more information than
traditional upper case or lower case transformations, but are more
useful than those transformations for comparison purposes. Two
notable characters of this type are the German character Eszett
(Sharp S, U+00DF) and the Greek Final Form Sigma (U+03C2). The
former is case-folded to the ASCII string "ss", the latter to a
medial (Lower Case) Sigma (U+03C3).
7.2.2. Character Changes: Zero-Width Joiner and Non-Joiner
IDNA2003 mapped both Zero-Width Joiner (ZWJ, U+200D) and Zero-Width
Non-Joiner (ZWNJ, U+200C) to nothing, effectively dropping these
characters from any label in which they appeared and treating strings
containing them as identical to strings that did not. As discussed
in Section 3.1.2 above, those characters are essential for writing
many reasonable mnemonics for certain scripts. However, treating
them as valid in the current version of IDNA, even with contextual
restrictions, raises approximately the same problem as exists with
Eszett and Final Sigma: strings that were valid under IDNA2003 have
different interpretations as labels, and different A-labels, than the
same strings under this newer version.
7.2.3. Character Changes and the Need for Transition
The decision to eliminate mandatory and standardized mappings,
including case folding, from the IDNA2008 protocol in order to make
A-labels and U-labels idempotent made these characters problematic.
If they were to be disallowed, important words and mnemonics could
not be written in orthographically reasonable ways. If they were to
be permitted as distinct characters, there would be no information
loss and registries would have more flexibility, but IDNA2003 and
IDNA2008 lookups might result in different A-labels.
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With the understanding that there would be incompatibility either way
but a judgment that the incompatibility was not significant enough to
justify a prefix change, the WG concluded that Eszett and Final Form
Sigma should be treated as distinct and Protocol-Valid characters.
Since these characters are interpreted in different ways under the
older and newer versions of IDNA, transition strategies and policies
will be necessary. Some actions can reasonably be taken by
applications client programs (those that perform lookup operations or
cause them to be performed) but, because of the diversity of
situations and uses of the DNS, much of the responsibility will need
to fall on registries.
Registries, especially those maintaining zones for third parties,
must decide how to introduce a new service in a way that does not
create confusion or significantly weaken or invalidate existing
identifiers. This is not a new problem; registries were faced with
similar issues when IDNs were introduced (potentially, and especially
for Latin-based scripts, in conflict with existing labels that had
been rendered in ASCII character by applying more or less
standardized conventions) and when other new forms of strings have
been permitted as labels.
7.2.4. Transition Strategies
There are several approaches to the introduction of new characters or
changes in interpretation of existing characters from their mapped
forms in the earlier version of IDNA. The transition issue is
complicated because the forms of these labels after
ToUnicode(ToASCII()) translation in IDNA2003 not only remain valid
but do not provide strong indications of what the registrant
intended: a string containing "ss" could have simply been intended to
be that string or could have been intended to contain an Eszett, a
string containing lower-case Sigma could have been intended to
contain Final Sigma (one might make heuristic guesses based on
position in a string, but the long tradition of forming labels by
concatenating words makes such heuristics unreliable), and strings
that do not contain ZWJ or ZWNJ might have been intended to contain
them. Without any preference or claim to completeness, some of
these, all of which have been used by registries in the past for
similar transitions, are:
1. Do not permit use of the newly-available character at the
registry level. This might cause lookup failures if a domain
name were to be written with the expectation of the IDNA2003
mapping behavior, but would eliminate any possibility of false
matches.
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2. Hold a "sunrise"-like arrangement in which holders of labels
containing "ss" in the Eszett case, Lower Case Sigma in that
case, or that might have contained ZWJ or ZWNJ in context, are
given priority (and perhaps other benefits) for registering the
corresponding string containing Eszett, Final Sigma, or the
appropriate Zero-width character respectively.
3. Adopt some sort of "variant" approach in which registrants obtain
labels with both character forms.
4. Adopt a different form of "variant" approach in which
registration of additional strings that would produce the same
A-label if interpreted according to IDNA2003 is either not
permitted at all or permitted only by the registrant who already
has one of the names.
5. Ignore the issue and assume that the marketplace or other
mechanisms will sort things out.
In any event, a registry (at any level of the DNS tree) that chooses
to permit labels to be registered that contains these characters, or
considers doing so, will have to address the relationship with
existing, possibly-conflicting, labels in some way, just as
registries that already had a considerable number of labels did when
IDNs were first introduced.
7.3. Elimination of Character Mapping
As discussed at length in Section 6, IDNA2003, via Nameprep (see
Section 7.5), mapped many characters into related ones. Those
mappings no longer exist as requirements in IDNA2008. These
specifications strongly prefer that only A-labels or U-labels be used
in protocol contexts and as much as practical more generally.
IDNA2008 does anticipate situations in which some mapping at the time
of user input into lookup applications is appropriate and desirable.
The issues are discussed in Section 6 and specific recommendations
are made in [IDNA2008-Mapping].
7.4. The Question of Prefix Changes
The conditions that would have required a change in the IDNA ACE
prefix ("xn--" for the version of IDNA specified in [RFC3490]) were
of great concern to the community. A prefix change would have
clearly been necessary if the algorithms were modified in a manner
that would have created serious ambiguities during subsequent
transition in registrations. This section summarizes the working
group's conclusions about the conditions under which a change in the
prefix would have been necessary and the implications of such a
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change.
7.4.1. Conditions Requiring a Prefix Change
An IDN prefix change would have been needed if a given string would
be looked up or otherwise interpreted differently depending on the
version of the protocol or tables being used. This IDNA upgrade
would have required a prefix change if, and only if, one of the
following four conditions were met:
1. The conversion of an A-label to Unicode (i.e., a U-label) would
have yielded one string under IDNA2003 (RFC3490) and a different
string under IDNA2008.
2. In a significant number of cases, an input string that was valid
under IDNA2003 and also valid under IDNA2008 would have yielded
two different A-labels with the different versions. This
condition is believed to be essentially equivalent to the one
above except for a very small number of edge cases that were not
found to justify a prefix change (See Section 7.2).
Note that if the input string was valid under one version and not
valid under the other, this condition would not apply. See the
first item in Section 7.4.2, below.
3. A fundamental change was made to the semantics of the string that
would be inserted in the DNS, e.g., if a decision were made to
try to include language or script information in the encoding in
addition to the string itself.
4. A sufficiently large number of characters were added to Unicode
so that the Punycode mechanism for block offsets would no longer
reference the higher-numbered planes and blocks. This condition
is unlikely even in the long term and certain not to arise in the
next several years.
7.4.2. Conditions Not Requiring a Prefix Change
As a result of the principles described above, none of the following
changes required a new prefix:
1. Prohibition of some characters as input to IDNA. Such a
prohibition might make names that were previously registered
inaccessible, but did not change those names.
2. Adjustments in IDNA tables or actions, including normalization
definitions, that affected characters that were already invalid
under IDNA2003.
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3. Changes in the style of the IDNA definition that did not alter
the actions performed by IDNA.
7.4.3. Implications of Prefix Changes
While it might have been possible to make a prefix change, the costs
of such a change are considerable. Registries could not have
converted all IDNA2003 ("xn--") registrations to a new form at the
same time and synchronize that change with applications supporting
lookup. Unless all existing registrations were simply to be declared
invalid (and perhaps even then) systems that needed to support both
labels with old prefixes and labels with new ones would be required
to first process a putative label under the IDNA2008 rules and try to
look it up and then, if it were not found, would be required to
process the label under IDNA2003 rules and look it up again. That
process would probably have significantly slowed down all processing
that involved IDNs in the DNS especially since a fully-qualified name
might contain a mixture of labels that were registered with the old
and new prefixes. That would have made DNS caching very difficult.
In addition, looking up the same input string as two separate
A-labels would have created some potential for confusion and attacks,
since the labels could map to different targets and then resolve to
different entries in the DNS.
Consequently, a prefix change should have been, and was, avoided if
at all possible, even if it means accepting some IDNA2003 decisions
about character distinctions as irreversible and/or giving special
treatment to edge cases.
7.5. Stringprep Changes and Compatibility
The Nameprep [RFC3491] specification, a key part of IDNA2003, is a
profile of Stringprep [RFC3454]. While Nameprep is a Stringprep
profile specific to IDNA, Stringprep is used by a number of other
protocols. Were Stringprep to have been modified by IDNA2008, those
changes to improve the handling of IDNs could cause problems for non-
DNS uses, most notably if they affected identification and
authentication protocols. Several elements of IDNA2008 give
interpretations to strings prohibited under IDNA2003 or prohibit
strings that IDNA2003 permitted. Those elements include the proposed
new inclusion tables [IDNA2008-Tables], the reduction in the number
of characters permitted as input for registration or lookup
(Section 3), and even the proposed changes in handling of right to
left strings [IDNA2008-Bidi]. IDNA2008 does not use Nameprep or
Stringprep at all, so there are no side-effect changes to other
protocols.
It is particularly important to keep IDNA processing separate from
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processing for various security protocols because some of the
constraints that are necessary for smooth and comprehensible use of
IDNs may be unwanted or undesirable in other contexts. For example,
the criteria for good passwords or passphrases are very different
from those for desirable IDNs: passwords should be hard to guess,
while domain names should normally be easily memorable. Similarly,
internationalized SCSI identifiers and other protocol components are
likely to have different requirements than IDNs.
7.6. The Symbol Question
One of the major differences between this specification and the
original version of IDNA is that the original version permitted non-
letter symbols of various sorts, including punctuation and line-
drawing symbols, in the protocol. They were always discouraged in
practice. In particular, both the "IESG Statement" about IDNA and
all versions of the ICANN Guidelines specify that only language
characters be used in labels. This specification disallows symbols
entirely. There are several reasons for this, which include:
1. As discussed elsewhere, the original IDNA specification assumed
that as many Unicode characters as possible should be permitted,
directly or via mapping to other characters, in IDNs. This
specification operates on an inclusion model, extrapolating from
the original "hostname" rules (LDH, see [IDNA2008-Defs]) -- which
have served the Internet very well -- to a Unicode base rather
than an ASCII base.
2. Symbol names are more problematic than letters because there may
be no general agreement on whether a particular glyph matches a
symbol; there are no uniform conventions for naming; variations
such as outline, solid, and shaded forms may or may not exist;
and so on. As just one example, consider a "heart" symbol as it
might appear in a logo that might be read as "I love...". While
the user might read such a logo as "I love..." or "I heart...",
considerable knowledge of the coding distinctions made in Unicode
is needed to know that there is more than one "heart" character
(e.g., U+2665, U+2661, and U+2765) and how to describe it. These
issues are of particular importance if strings are expected to be
understood or transcribed by the listener after being read out
loud.
3. Design of a screen reader used by blind Internet users who must
listen to renderings of IDN domain names and possibly reproduce
them on the keyboard becomes considerably more complicated when
the names of characters are not obvious and intuitive to anyone
familiar with the language in question.
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4. As a simplified example of this, assume one wanted to use a
"heart" or "star" symbol in a label. This is problematic because
those names are ambiguous in the Unicode system of naming (the
actual Unicode names require far more qualification). A user or
would-be registrant has no way to know -- absent careful study of
the code tables -- whether it is ambiguous (e.g., where there are
multiple "heart" characters) or not. Conversely, the user seeing
the hypothetical label doesn't know whether to read it -- try to
transmit it to a colleague by voice -- as "heart", as "love", as
"black heart", or as any of the other examples below.
5. The actual situation is even worse than this. There is no
possible way for a normal, casual, user to tell the difference
between the hearts of U+2665 and U+2765 and the stars of U+2606
and U+2729 without somehow knowing to look for a distinction. We
have a white heart (U+2661) and few black hearts. Consequently,
describing a label as containing a heart is hopelessly ambiguous:
we can only know that it contains one of several characters that
look like hearts or have "heart" in their names. In cities where
"Square" is a popular part of a location name, one might well
want to use a square symbol in a label as well and there are far
more squares of various flavors in Unicode than there are hearts
or stars.
The consequence of these ambiguities is that symbols are a very poor
basis for reliable communication. Consistent with this conclusion,
the Unicode standard recommends that strings used in identifiers not
contain symbols or punctuation [Unicode-UAX31]. Of course, these
difficulties with symbols do not arise with actual pictographic
languages and scripts which would be treated like any other language
characters; the two should not be confused.
7.7. Migration Between Unicode Versions: Unassigned Code Points
In IDNA2003, labels containing unassigned code points are looked up
on the assumption that, if they appear in labels and can be mapped
and then resolved, the relevant standards must have changed and the
registry has properly allocated only assigned values.
In the IDNA2008 protocol, strings containing unassigned code points
must not be either looked up or registered. In summary, the status
of an unassigned character with regard to the DISALLOWED, PROTOCOL-
VALID, and CONTEXTUAL RULE REQUIRED categories cannot be evaluated
until a character is actually assigned and known. There are several
reasons for this, with the most important ones being:
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o Tests involving the context of characters (e.g., some characters
being permitted only adjacent to others of specific types) and
integrity tests on complete labels are needed. Unassigned code
points cannot be permitted because one cannot determine whether
particular code points will require contextual rules (and what
those rules should be) before characters are assigned to them and
the properties of those characters fully understood.
o It cannot be known in advance, and with sufficient reliability,
whether a newly-assigned code point will be associated with a
character that would be disallowed by the rules in
[IDNA2008-Tables] (such as a compatibility character). In
IDNA2003, since there is no direct dependency on NFKC (many of the
entries in Stringprep's tables are based on NFKC, but IDNA2003
depends only on Stringprep), allocation of a compatibility
character might produce some odd situations, but it would not be a
problem. In IDNA2008, where compatibility characters are
DISALLOWED unless character-specific exceptions are made,
permitting strings containing unassigned characters to be looked
up would violate the principle that characters in DISALLOWED are
not looked up.
o The Unicode Standard specifies that an unassigned code point
normalizes (and, where relevant, case folds) to itself. If the
code point is later assigned to a character, and particularly if
the newly-assigned code point has a combining class that
determines its placement relative to other combining characters,
it could normalize to some other code point or sequence.
It is possible to argue that the issues above are not important and
that, as a consequence, it is better to retain the principle of
looking up labels even if they contain unassigned characters because
all of the important scripts and characters have been coded as of
Unicode 5.1 and hence unassigned code points will be assigned only to
obscure characters or archaic scripts. Unfortunately, that does not
appear to be a safe assumption for at least two reasons. First, much
the same claim of completeness has been made for earlier versions of
Unicode. The reality is that a script that is obscure to much of the
world may still be very important to those who use it. Cultural and
linguistic preservation principles make it inappropriate to declare
the script of no importance in IDNs. Second, we already have
counterexamples in, e.g., the relationships associated with new Han
characters being added (whether in the BMP or in Unicode Plane 2).
Independent of the technical transition issues identified above, it
can be observed that any addition of characters to an existing script
to make it easier to use or to better accommodate particular
languages may lead to transition issues. Such additions may change
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the preferred form for writing a particular string, changes that may
be reflected, e.g., in keyboard transition modules that would
necessarily be different from those for earlier versions of Unicode
where the newer characters may not exist. This creates an inherent
transition problem because attempts to access labels may use either
the old or the new conventions, requiring registry action whether the
older conventions were used in labels or not. The need to consider
transition mechanisms is inherent to evolution of Unicode to better
accommodate writing systems and is independent of how IDNs are
represented in the DNS or how transitions among versions of those
mechanisms occur. The requirement for transitions of this type is
illustrated by the addition of Malayalam Chillu in Unicode 5.1.0.
7.8. Other Compatibility Issues
The 2003 IDNA model includes several odd artifacts of the context in
which it was developed. Many, if not all, of these are potential
avenues for exploits, especially if the registration process permits
"source" names (names that have not been processed through IDNA and
Nameprep) to be registered. As one example, since the character
Eszett, used in German, is mapped by IDNA2003 into the sequence "ss"
rather than being retained as itself or prohibited, a string
containing that character but that is otherwise in ASCII is not
really an IDN (in the U-label sense defined above) at all. After
Nameprep maps the Eszett out, the result is an ASCII string and so
does not get an xn-- prefix, but the string that can be displayed to
a user appears to be an IDN. The newer version of the protocol
eliminates this artifact. A character is either permitted as itself
or it is prohibited; special cases that make sense only in a
particular linguistic or cultural context can be dealt with as
localization matters where appropriate.
8. Name Server Considerations
8.1. Processing Non-ASCII Strings
Existing DNS servers do not know the IDNA rules for handling non-
ASCII forms of IDNs, and therefore need to be shielded from them.
All existing channels through which names can enter a DNS server
database (for example, master files (as described in RFC 1034) and
DNS update messages [RFC2136]) are IDN-unaware because they predate
IDNA. Other sections of this document provide the needed shielding
by ensuring that internationalized domain names entering DNS server
databases through such channels have already been converted to their
equivalent ASCII A-label forms.
Because of the distinction made between the algorithms for
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Registration and Lookup in [IDNA2008-Protocol] (a domain name
containing only ASCII code points cannot be converted to an A-label),
there cannot be more than one A-label form for any given U-label.
As specified in RFC 2181 [RFC2181], the DNS protocol explicitly
allows domain labels to contain octets beyond the ASCII range
(0000..007F), and this document does not change that. However,
although the interpretation of octets 0080..00FF is well-defined in
the DNS, many application protocols support only ASCII labels and
there is no defined interpretation of these non-ASCII octets as
characters and, in particular, no interpretation of case-independent
matching for them (see, e.g., [RFC4343]). If labels containing these
octets are returned to applications, unpredictable behavior could
result. The A-label form, which cannot contain those characters, is
the only standard representation for internationalized labels in the
DNS protocol.
8.2. Root and other DNS Server Considerations
IDNs in A-label form will generally be somewhat longer than current
domain names, so the bandwidth needed by the root servers is likely
to go up by a small amount. Also, queries and responses for IDNs
will probably be somewhat longer than typical queries historically,
so EDNS0 [RFC2671] support may be more important (otherwise, queries
and responses may be forced to go to TCP instead of UDP).
9. Internationalization Considerations
DNS labels and fully-qualified domain names provide mnemonics that
assist in identifying and referring to resources on the Internet.
IDNs expand the range of those mnemonics to include those based on
languages and character sets other than Western European and Roman-
derived ones. But domain "names" are not, in general, words in any
language. The recommendations of the IETF policy on character sets
and languages, (BCP 18 [RFC2277]) are applicable to situations in
which language identification is used to provide language-specific
contexts. The DNS is, by contrast, global and international and
ultimately has nothing to do with languages. Adding languages (or
similar context) to IDNs generally, or to DNS matching in particular,
would imply context dependent matching in DNS, which would be a very
significant change to the DNS protocol itself. It would also imply
that users would need to identify the language associated with a
particular label in order to look that label up. That knowledge is
generally not available because many labels are not words in any
language and some may be words in more than one.
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10. IANA Considerations
This section gives an overview of IANA registries required for IDNA.
The actual definitions of, and specifications for, the first two,
which 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
The distinction among the major categories "UNASSIGNED",
"DISALLOWED", "PROTOCOL-VALID", and "CONTEXTUAL RULE REQUIRED" is
made by special categories and rules that are integral elements of
[IDNA2008-Tables]. While not normative, an IANA registry of
characters and scripts and their categories, updated for each new
version of Unicode and the characters it contains, will be convenient
for programming and validation purposes. The details of this
registry are specified in [IDNA2008-Tables].
10.2. IDNA Context Registry
IANA will create and maintain a list of approved contextual rules for
characters that are defined in the IDNA Character Registry list as
requiring a Contextual Rule (i.e., the types of rule described in
Section 3.1.2). The details for those rules appear in
[IDNA2008-Tables].
10.3. IANA Repository of IDN Practices of TLDs
This registry, historically described as the "IANA Language Character
Set Registry" or "IANA Script Registry" (both somewhat misleading
terms) is maintained by IANA at the request of ICANN. It is used to
provide a central documentation repository of the IDN policies used
by top level domain (TLD) registries who volunteer to contribute to
it and is used in conjunction with ICANN Guidelines for IDN use.
It is not an IETF-managed registry and, while the protocol changes
specified here may call for some revisions to the tables, IDNA2008
has no direct effect on that registry and no IANA action is required
as a result.
11. Security Considerations
11.1. General Security Issues with IDNA
This document is purely explanatory and informational and
consequently introduces no new security issues. It would, of course,
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be a poor idea for someone to try to implement from it; such an
attempt would almost certainly lead to interoperability problems and
might lead to security ones. A discussion of security issues with
IDNA, including some relevant history, appears in [IDNA2008-Defs].
12. Acknowledgments
The editor and contributors would like to express their thanks to
those who contributed significant early (pre-WG) review comments,
sometimes accompanied by text, Paul Hoffman, Simon Josefsson, and Sam
Weiler. In addition, some specific ideas were incorporated from
suggestions, text, or comments about sections that were unclear
supplied by Vint Cerf, Frank Ellerman, Michael Everson, Asmus
Freytag, Erik van der Poel, Michel Suignard, and Ken Whistler.
Thanks are also due to Vint Cerf, Lisa Dusseault, Debbie Garside, and
Jefsey Morfin for conversations that led to considerable improvements
in the content of this document and to several others, including Ben
Campbell, Martin Duerst, Subramanian Moonesamy, Peter Saint-Andre,
and Dan Winship, for catching specific errors and recommending
corrections.
A meeting was held on 30 January 2008 to attempt to reconcile
differences in perspective and terminology about this set of
specifications between the design team and members of the Unicode
Technical Consortium. The discussions at and subsequent to that
meeting were very helpful in focusing the issues and in refining the
specifications. The active participants at that meeting were (in
alphabetic order as usual) Harald Alvestrand, Vint Cerf, Tina Dam,
Mark Davis, Lisa Dusseault, Patrik Faltstrom (by telephone), Cary
Karp, John Klensin, Warren Kumari, Lisa Moore, Erik van der Poel,
Michel Suignard, and Ken Whistler. We express our thanks to Google
for support of that meeting and to the participants for their
contributions.
Useful comments and text on the WG versions of the draft were
received from many participants in the IETF "IDNABIS" WG and a number
of document changes resulted from mailing list discussions made by
that group. Marcos Sanz provided specific analysis and suggestions
that were exceptionally helpful in refining the text, as did Vint
Cerf, Martin Duerst, Andrew Sullivan, and Ken Whistler. Lisa
Dusseault provided extensive editorial suggestions during the spring
of 2009, most of which were incorporated.
13. Contributors
While the listed editor held the pen, the core of this document and
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the initial WG version represents the joint work and conclusions of
an ad hoc design team consisting of the editor and, in alphabetic
order, Harald Alvestrand, Tina Dam, Patrik Faltstrom, and Cary Karp.
Considerable material describing mapping principles has been
incorporated from a draft of [IDNA2008-Mapping] by Pete Resnick and
Paul Hoffman. In addition, there were many specific contributions
and helpful comments from those listed in the Acknowledgments section
and others who have contributed to the development and use of the
IDNA protocols.
14. References
14.1. Normative References
[ASCII] American National Standards Institute (formerly United
States of America Standards Institute), "USA Code for
Information Interchange", ANSI X3.4-1968, 1968.
ANSI X3.4-1968 has been replaced by newer versions with
slight modifications, but the 1968 version remains
definitive for the Internet.
[IDNA2008-Bidi]
Alvestrand, H. and C. Karp, "An updated IDNA criterion for
right to left scripts", August 2009, .
[IDNA2008-Defs]
Klensin, J., "Internationalized Domain Names for
Applications (IDNA): Definitions and Document Framework",
August 2009, .
[IDNA2008-Protocol]
Klensin, J., "Internationalized Domain Names in
Applications (IDNA): Protocol", August 2009, .
[IDNA2008-Tables]
Faltstrom, P., "The Unicode Code Points and IDNA",
August 2009, .
A version of this document is available in HTML format at
http://stupid.domain.name/idnabis/
draft-ietf-idnabis-tables-06.html
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[RFC3490] Faltstrom, P., Hoffman, P., and A. Costello,
"Internationalizing Domain Names in Applications (IDNA)",
RFC 3490, March 2003.
[RFC3492] Costello, A., "Punycode: A Bootstring encoding of Unicode
for Internationalized Domain Names in Applications
(IDNA)", RFC 3492, March 2003.
[Unicode-UAX15]
The Unicode Consortium, "Unicode Standard Annex #15:
Unicode Normalization Forms", March 2008,
.
[Unicode51]
The Unicode Consortium, "The Unicode Standard, Version
5.1.0", 2008.
defined by: The Unicode Standard, Version 5.0, Boston, MA,
Addison-Wesley, 2007, ISBN 0-321-48091-0, as amended by
Unicode 5.1.0
(http://www.unicode.org/versions/Unicode5.1.0/).
14.2. Informative References
[BIG5] Institute for Information Industry of Taiwan, "Computer
Chinese Glyph and Character Code Mapping Table, Technical
Report C-26", 1984.
There are several forms and variations and a closely-
related standard, CNS 11643. See the discussion in
Chapter 3 of Lunde, K., CJKV Information Processing,
O'Reilly & Associates, 1999
[GB18030] "Chinese National Standard GB 18030-2000: Information
Technology -- Chinese ideograms coded character set for
information interchange -- Extension for the basic set.",
2000.
[IDNA2008-Mapping]
Resnick, P., "Mapping Characters in IDNA", August 2009, .
[RFC0810] Feinler, E., Harrenstien, K., Su, Z., and V. White, "DoD
Internet host table specification", RFC 810, March 1982.
[RFC0952] Harrenstien, K., Stahl, M., and E. Feinler, "DoD Internet
host table specification", RFC 952, October 1985.
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[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, November 1987.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[RFC1123] Braden, R., "Requirements for Internet Hosts - Application
and Support", STD 3, RFC 1123, October 1989.
[RFC2136] Vixie, P., Thomson, S., Rekhter, Y., and J. Bound,
"Dynamic Updates in the Domain Name System (DNS UPDATE)",
RFC 2136, April 1997.
[RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS
Specification", RFC 2181, July 1997.
[RFC2277] Alvestrand, H., "IETF Policy on Character Sets and
Languages", BCP 18, RFC 2277, January 1998.
[RFC2671] Vixie, P., "Extension Mechanisms for DNS (EDNS0)",
RFC 2671, August 1999.
[RFC2673] Crawford, M., "Binary Labels in the Domain Name System",
RFC 2673, August 1999.
[RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
specifying the location of services (DNS SRV)", RFC 2782,
February 2000.
[RFC3454] Hoffman, P. and M. Blanchet, "Preparation of
Internationalized Strings ("stringprep")", RFC 3454,
December 2002.
[RFC3491] Hoffman, P. and M. Blanchet, "Nameprep: A Stringprep
Profile for Internationalized Domain Names (IDN)",
RFC 3491, March 2003.
[RFC3743] Konishi, K., Huang, K., Qian, H., and Y. Ko, "Joint
Engineering Team (JET) Guidelines for Internationalized
Domain Names (IDN) Registration and Administration for
Chinese, Japanese, and Korean", RFC 3743, April 2004.
[RFC3987] Duerst, M. and M. Suignard, "Internationalized Resource
Identifiers (IRIs)", RFC 3987, January 2005.
[RFC4290] Klensin, J., "Suggested Practices for Registration of
Internationalized Domain Names (IDN)", RFC 4290,
December 2005.
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[RFC4343] Eastlake, D., "Domain Name System (DNS) Case Insensitivity
Clarification", RFC 4343, January 2006.
[RFC4690] Klensin, J., Faltstrom, P., Karp, C., and IAB, "Review and
Recommendations for Internationalized Domain Names
(IDNs)", RFC 4690, September 2006.
[RFC4713] Lee, X., Mao, W., Chen, E., Hsu, N., and J. Klensin,
"Registration and Administration Recommendations for
Chinese Domain Names", RFC 4713, October 2006.
[Unicode-Security]
The Unicode Consortium, "Unicode Technical Standard #39:
Unicode Security Mechanisms", August 2008,
.
[Unicode-UAX31]
The Unicode Consortium, "Unicode Standard Annex #31:
Unicode Identifier and Pattern Syntax", March 2008,
.
[Unicode-UTR36]
The Unicode Consortium, "Unicode Technical Report #36:
Unicode Security Considerations", July 2008,
.
Appendix A. Change Log
[[ RFC Editor: Please remove this appendix. ]]
A.1. Changes between Version -00 and Version -01 of
draft-ietf-idnabis-rationale
o Clarified the U-label definition to note that U-labels must
contain at least one non-ASCII character. Also clarified the
relationship among label types.
o Rewrote the discussion of Labels in Registration (Section 7.1.2)
and related text about IDNA-validity (in the "Defs" document as of
-04 of this one) to narrow its focus and remove more general
restrictions. Added a temporary note in line to explain the
situation.
o Changed the "IDNA uses Unicode" statement to focus on
compatibility with IDNA2003 and avoid more general or
controversial assertions.
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o Added a discussion of examples to Section 7.1
o Made a number of other small editorial changes and corrections
suggested by Mark Davis.
o Added several more discussion anchors and notes and expanded or
updated some existing ones.
A.2. Version -02
o Trimmed change log, removing information about pre-WG drafts.
o Adjusted discussion of Contextual Rules to match the new location
of the tables and some conceptual material.
o Rewrote the material on preprocessing somewhat.
o Moved the material about relationships with IDNA2003 to be part of
a single section on transitions.
o Removed several placeholders and made editorial changes in
accordance with decisions made at IETF 72 in Dublin and not
disputed on the mailing list.
A.3. Version -03
This special update to the Rationale document is intended to try to
get the discussion of what is normative or not under control. While
the IETF does not normally annotate individual sections of documents
with whether they are normative or not, concerns that we don't know
which is which, claims that some material is normative that would be
problematic if so classified, etc., argue that we should at least be
able to have a clear discussion on the subject.
Two annotations have been applied to sections that might reasonably
be considered normative. One annotation is based on the list of
sections in Mark Davis's note of 29 September (http://
www.alvestrand.no/pipermail/idna-update/2008-September/002667.html).
The other is based on an elaboration of John Klensin's response on 7
October (http://www.alvestrand.no/pipermail/idna-update/2008-October/
002691.html). These should just be considered two suggestions to
illuminate and, one hopes, advance the Working Group's discussions.
Some additional editorial changes have been made, but they are
basically trivial. In the editor's judgment, it is not possible to
make significantly more progress with this document until the matter
of document organization is settled.
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A.4. Version -04
o Definitional and other normative material moved to new document
(draft-ietf-idnabis-defs). Version -03 annotations removed.
o Material on differences between IDNA2003 and IDNA2008 moved to an
appendix in Protocol.
o Material left over from the origins of this document as a
preliminary proposal has been removed or rewritten.
o Changes made to reflect consensus call results, including removing
several placeholder notes for discussion.
o Added more material, including discussion of historic scripts, to
Section 3.2 on registration policies.
o Added a new section (Section 7.2) to contain specific discussion
of handling of characters that are interpreted differently in
input to IDNA2003 and 2008.
o Some material, including this section/appendix, rearranged.
A.5. Version -05
o Many small editorial changes, including changes to eliminate the
last vestiges of what appeared to be 2119 language (upper-case
MUST, SHOULD, or MAY) and small adjustments to terminology.
A.6. Version -06
o Removed Security Considerations material and pointed to Defs,
where it now appears as of version 05.
o Started changing uses of "IDNA2008" in running text to "in these
specifications" or the equivalent. These documents are titled
simply "IDNA"; once they are standardized, "the current version"
may be a more appropriate reference than one containing a year.
As discussed on the mailing list, we can and should discuss how to
refer to these documents at an appropriate time (e.g., when we
know when we will be finished) but, in the interim, it seems
appropriate to simply start getting rid of the version-specific
terminology where it can naturally be removed.
o Additional discussion of mappings, etc., especially for case-
sensitivity.
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o Clarified relationship to base DNS specifications.
o Consolidated discussion of lookup of unassigned characters.
o More editorial fine-tuning.
A.7. Version -07
o Revised terminology by adding terms: NR-LDH-label, Invalid-A-label
(or False-A-label), R-LDH-label, valid IDNA-label in
Section 1.3.2.
o Moved the "name server considerations" material to this document
from Protocol because it is non-normative and not part of the
protocol itself.
o To improve clarity, redid discussion of the reasons why looking up
unassigned code points is prohibited.
o Editorial and other non-substantive corrections to reflect earlier
errors as well as new definitions and terminology.
A.8. Version -08
o Slight revision to "contextual" discussion (Section 3.1.2) and
moving it to a separate subsection, rather than under "PVALID",
for better parallelism with Tables. Also reflected Mark's
comments about the limitations of the approach.
o Added placeholder notes as reminders of where references to the
other documents need Section numbers. More of these will be added
as needed (feel free to identify relevant places), but the actual
section numbers will not be inserted until the documents are
completely stable, i.e., on their way to the RFC Editor.
A.9. Version -09
o Small editorial changes to clarify transition possibilities.
o Small clarification to the description of DNS "exact match".
o Added discussion of adding characters to an existing script to the
discussion of unassigned code point transitions in Section 7.7.
o Tightened up the discussion of non-ASCII string processing
(Section 8.1) slightly.
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o Removed some placeholders and comments that have been around long
enough to be considered acceptable or that no longer seem
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.
A.11. Version -11
o Several placeholders for additional material or editing have been
removed since no comments have been received.
o Updated references.
o Corrected an apparent patching error in Section 1.6 and another
one in Section 4.3.
o Adjusted several sections that had not properly reflected removal
of the material that is now in the Definitions document and
removed an unnecessary one.
o New material added to Section 3.2 about registration policy issues
to reflect discussions on the mailing list.
o Incorporated mapping material from the former "Architectural
Principles" of version -01 of the Mapping draft into Section 6 and
removed most of the prior mapping material and explanations.
o Eliminated the former Section 7.3 ("More Flexibility in User
Agents"), moving its material into Section 4.2. The replacement
section is basically a placeholder to retain the mapping issues as
one of the migration topics. Note that this item and the previous
one involve considerable text, so people should check things
carefully.
o Corrected several typographical and editorial errors that don't
fall into any of the above categories.
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A.12. Version -12
o Got rid of the term "IDNA-valid". It no longer appears in
Definitions and we didn't really need the extra term. Where the
concept was needed, the text now says "valid under IDNA" or
equivalent.
o Adjusted Acknowledgments to remove Mark Davis's name, per his
request and advice from IETF Trust Counsel.
o Incorporated other changes from WG Last Call.
o Small typographical and editorial corrections.
A.13. Version -13
o Substituted in Section numbers to references to other IDNA2008
documents.
A.14. Version -14
This is the version of the document produced to reflect comments on
IETF Last Call. For the convenience of those who made comments and
of the IESG in evaluating them, this section therefore identifies
non-editorial changes made in response to Last Call comments in
somewhat more detail than may be usual.
o Removed the discussion of DNSSEC after extensive discussion on the
IETF and IDNABIS lists.
o Modified the discussion of prefix changes to make it clear that
the decisions have been made, rather than still representing open
issues. (Dan Winship review, 20091013)
o Suggested explicit identification of domain name slots in
protocols that use IDNA. Peter Saint-Andre, 20091019.
o Several other clarifications as suggested by Peter Saint-Andre,
20091019.
o Several minor editorial corrections per suggestions in Ben
Campbell's Gen-ART review 20091013.
o Typo corrections.
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A.15. Version -15
o Rewrote and expanded the "transition" material of Section 7.2.
A.16. Version -16
This version contains changes made at IESG request during their
review. Some additional comments were logged during or immediately
before the 7 January 2010 teleconference, so this is not the final
I-D version.
o Altered use of "these documents" and "these specifications" back
to "IDNA2008", undoing the change made in Appendix A.6. The
convolutions became ambiguous in places.
o Added a sentence to the Introduction to make the non-normative
status of this document even more clear and added references to
7.1.2 and 7.1.3 to point to the more formal definitions.
A.17. Version -17
o Final IESG comments picked up and included. A few more editorial/
typographic errors caught and fixed.
o Section 4 title adjusted to better match its content.
Author's Address
John C Klensin
1770 Massachusetts Ave, Ste 322
Cambridge, MA 02140
USA
Phone: +1 617 245 1457
Email: john+ietf@jck.com
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