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L2/09-044
Network Working Group J. Klensin
Internet-Draft December 15, 2008
Intended status: Informational
Expires: June 18, 2009
Internationalized Domain Names for Applications (IDNA): Background,
Explanation, and Rationale
draft-ietf-idnabis-rationale-06.txt
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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.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Context and Overview . . . . . . . . . . . . . . . . . . . 4
1.2. Discussion Forum . . . . . . . . . . . . . . . . . . . . . 4
1.3. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
1.3.1. Documents and Standards . . . . . . . . . . . . . . . 5
1.3.2. DNS "Name" Terminology . . . . . . . . . . . . . . . . 5
1.3.3. New Terminology and Restrictions . . . . . . . . . . . 5
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
3.1.1.1. Contextual Rules . . . . . . . . . . . . . . . . . 11
3.1.1.2. Rules and Their Application . . . . . . . . . . . 11
3.1.2. DISALLOWED . . . . . . . . . . . . . . . . . . . . . . 12
3.1.3. UNASSIGNED . . . . . . . . . . . . . . . . . . . . . . 13
3.2. Registration Policy . . . . . . . . . . . . . . . . . . . 13
3.3. Layered Restrictions: Tables, Context, Registration,
Applications . . . . . . . . . . . . . . . . . . . . . . . 13
4. Issues that Constrain Possible Solutions . . . . . . . . . . . 14
4.1. Display and Network Order . . . . . . . . . . . . . . . . 14
4.2. Entry and Display in Applications . . . . . . . . . . . . 15
4.3. Linguistic Expectations: Ligatures, Digraphs, and
Alternate Character Forms . . . . . . . . . . . . . . . . 16
4.4. Case Mapping and Related Issues . . . . . . . . . . . . . 19
4.5. Right to Left Text . . . . . . . . . . . . . . . . . . . . 20
5. IDNs and the Robustness Principle . . . . . . . . . . . . . . 20
6. Front-end and User Interface Processing . . . . . . . . . . . 21
7. Migration from IDNA2003 and Unicode Version Synchronization . 24
7.1. Design Criteria . . . . . . . . . . . . . . . . . . . . . 24
7.1.1. General IDNA Validity Criteria . . . . . . . . . . . . 24
7.1.2. Labels in Registration . . . . . . . . . . . . . . . . 26
7.1.3. Labels in Lookup . . . . . . . . . . . . . . . . . . . 27
7.2. Changes in Character Interpretations . . . . . . . . . . . 28
7.3. More Flexibility in User Agents . . . . . . . . . . . . . 29
7.4. The Question of Prefix Changes . . . . . . . . . . . . . . 31
7.4.1. Conditions Requiring a Prefix Change . . . . . . . . . 31
7.4.2. Conditions Not Requiring a Prefix Change . . . . . . . 32
7.4.3. Implications of Prefix Changes . . . . . . . . . . . . 32
7.5. Stringprep Changes and Compatibility . . . . . . . . . . . 32
7.6. The Symbol Question . . . . . . . . . . . . . . . . . . . 33
7.7. Migration Between Unicode Versions: Unassigned Code
Points . . . . . . . . . . . . . . . . . . . . . . . . . . 34
7.8. Other Compatibility Issues . . . . . . . . . . . . . . . . 35
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8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 36
9. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 37
10. Internationalization Considerations . . . . . . . . . . . . . 37
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 37
11.1. IDNA Character Registry . . . . . . . . . . . . . . . . . 37
11.2. IDNA Context Registry . . . . . . . . . . . . . . . . . . 38
11.3. IANA Repository of IDN Practices of TLDs . . . . . . . . . 38
12. Security Considerations . . . . . . . . . . . . . . . . . . . 38
12.1. General Security Issues with IDNA . . . . . . . . . . . . 38
13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 38
13.1. Normative References . . . . . . . . . . . . . . . . . . . 38
13.2. Informative References . . . . . . . . . . . . . . . . . . 40
Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . 41
A.1. Changes between Version -00 and Version -01 of
draft-ietf-idnabis-rationale . . . . . . . . . . . . . . . 41
A.2. Version -02 . . . . . . . . . . . . . . . . . . . . . . . 42
A.3. Version -03 . . . . . . . . . . . . . . . . . . . . . . . 42
A.4. Version -04 . . . . . . . . . . . . . . . . . . . . . . . 43
A.5. Version -05 . . . . . . . . . . . . . . . . . . . . . . . 43
A.6. Version -06 . . . . . . . . . . . . . . . . . . . . . . . 43
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 44
Intellectual Property and Copyright Statements . . . . . . . . . . 45
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1. Introduction
1.1. Context and Overview
The original standards for Internationalized Domain Names (IDNs) were
completed and deployed starting in 2003. Those standards are known
as Internationalized Domain Names in Applications (IDNA), taken from
the name of the highest level standard within the group, RFC 3490
[RFC3490]. After those standards were deployed, a number of issues
arose that led to a call for a new version of the IDNA protocol and
the associated tables, including a subset of those described in a
recent IAB report [RFC4690] and the need to update the system to deal
with newer versions of Unicode. This document further explains the
issues that have been encountered when they are important to
understanding of the revised protocols. It also provides an overview
of the new IDNA model and explanatory material for it. Additional
explanatory material for the specific components of the proposals
appears with the associated documents.
A good deal of the background material that appeared in RFC 3490
[RFC3490] has been removed from this update. That material is either
of historical interest only or has been covered from a more recent
perspective in RFC 4690 [RFC4690].
This document is not normative. The information it provides is
intended to make the rules, tables, and protocol easier to understand
and to provide overview information and suggestions for zone
administrators and others who need to make policy, deployment, and
similar decisions about IDNs.
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 that is critical for understanding this document and the
rest of the documents that make up IDNA2008, appears in
[IDNA2008-Defs]. That document also contains roadmap to the IDNA2008
document collection. No attempt should be made to understand this
document without the definitions and concepts that appear there.
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1.3.1. Documents and Standards
This document uses the term "IDNA2003" to refer to the set of
standards that make up and support the version of IDNA published in
2003, i.e., those commonly known as the IDNA base specification
[RFC3490], Nameprep [RFC3491], Punycode [RFC3492], and Stringprep
[RFC3454]. In this document, those names are used to refer,
conceptually, to the individual documents, with the base IDNA
specification called just "IDNA".
The term "IDNA2008" is used to refer to a new version of IDNA as
described in this document and in the documents described in the
document listing of [IDNA2008-Defs]. IDNA2008 is not dependent on
any of the IDNA2003 specifications other than the one for Punycode
encoding. References to "these specifications" or "these documents"
are to the entire IDNA2008 set.
1.3.2. DNS "Name" Terminology
These documents depart from historical DNS terminology and usage in
one important respect. Over the years, the community has talked very
casually about "names" in the DNS, beginning with calling it "the
domain name system". That terminology is fine in the very precise
sense that the identifiers of the DNS do provide names for objects
and addresses. But, in the context of IDNs, the term has introduced
some confusion, confusion that has increased further as people have
begun to speak of DNS labels in terms of the words or phrases of
various natural languages.
Historically, many, perhaps most, of the "names" in the DNS have been
mnemonics to identify some particular concept, object, or
organization. They are typically derived from, or rooted in, some
language because most people think in language-based ways. But,
because they are mnemonics, they need not obey the orthographic
conventions of any language: it is not a requirement that it be
possible for them to be "words".
This distinction is important because the reasonable goal of an IDN
effort is not to be able to write the great Klingon (or language of
one's choice) novel in DNS labels but to be able to form a usefully
broad range of mnemonics in ways that are as natural as possible in a
very broad range of scripts.
1.3.3. New Terminology and Restrictions
These documents introduce new terminology, and precise definitions,
for the terms "U-labels", "A-labels", labels that are "IDNA-valid",
and an "LDH-label" (differing from an LDH-conformant label or fully-
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qualified domain name). They also introduce a restriction, for IDNA-
conformant applications and DNS zones in which IDNA is used, on
strings used as labels that contain "--" in the third and fourth
positions, essentially requiring that such strings be IDNA-valid.
This restriction on strings containing "--" 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.
Figure 1 of the Definitions Document [IDNA2008-Defs] illustrates the
terminology used by IDNA for various types of labels and strings and
their relationship.
1.4. Objectives
The intent of the IDNA revision effort, and hence of this document
and the associated ones, is to increase the usability and
effectiveness of internationalized domain names (IDNs) while
preserving or strengthening the integrity of references that use
them. The original "hostname" character definitions (see, e.g.,
[RFC0810]) struck a balance between the creation of useful mnemonics
and the introduction of parsing problems or general confusion in the
contexts in which domain names are used. The objective of IDNA2008
is to preserve that balance while expanding the character repertoire
to include extended versions of Roman-derived scripts and scripts
that are not Roman in origin. No work of this sort is able to
completely eliminate sources of visual or textual confusion: such
confusion is possible even under the original host naming rules where
only ASCII characters were permitted. However, through the
application of different techniques at different points (see
Section 3.3), it should be possible to keep problems to an acceptable
minimum. One consequence of this general objective is that the
desire of some user or marketing community to use a particular string
--whether the reason is to try to write sentences of particular
languages in the DNS, to express a facsimile of the symbol for a
brand, or for some other purpose-- is not a primary goal within the
context of applications in the domain name space.
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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 the service offered by DNS to the applications.
Instead, the applications (and, by implication, the users) continue
to see an exact-match lookup service. Either there is a single
exactly-matching name 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 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
prior to or after access to the DNS, IDNA requires no changes to the
DNS protocol or any DNS servers or the resolvers on user's 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 rudimentary 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.
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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.
IDNA uses the Unicode character repertoire, for continuity with the
original version of IDNA.
1.6. Comprehensibility of IDNA Mechanisms and Processing
One of the major goals of this work 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 themselves important motivations and design goals
for this effort. The effort includes some new terminology and a
revised and extended model, both covered in this section, and some
more specific protocol, processing, and table modifications. Details
of the latter appear in other documents (see [IDNA2008-Defs]).
Several issues are inherent in the application of IDNs and, indeed,
almost any other system that tries to handle international characters
and concepts. They range from the apparently trivial --e.g., one
cannot display a character for which one does not have a font
available locally-- to the more complex and subtle. Many people have
observed that internationalization is just a tool to enable effective
localization while permitting some global uniformity. Issues of
display, of exactly how various strings and characters are entered,
and so on are inherently issues about localization and user interface
design.
A protocol such as IDNA can only assume that such operations as data
entry and reconciliation of differences in character forms are
possible. It may make some recommendations about how display might
work when characters and fonts are not available, but they can only
be general recommendations and, because display functions are rarely
controlled by the types of applications that would call upon IDNA,
will rarely be very effective.
However, shifting responsibility for character mapping and other
adjustments from the protocol (where it was located in IDNA2003) to
the user interface or processing before invoking IDNA raises issues
about both what that processing should do and about compatibility for
references prepared in an IDNA2003 context. Those issues are
discussed in Section 6.
Operations for converting between local character sets and normalized
Unicode are part of this general set of user interface issues. The
conversion is obviously not required at all in a Unicode-native
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system that maintains all strings in Normalization Form C (NFC).
(See [Unicode-UAX15] for precise definitions of NFC and NFKC if
needed.) It may, however, involve some complexity in a system that
is not Unicode-native, especially if the elements of the local
character set do not map exactly and unambiguously into Unicode
characters or do so in a way that is not completely stable over time.
Perhaps more important, if a label being converted to a local
character set contains Unicode characters that have no correspondence
in that character set, the application may have to apply special,
locally-appropriate, methods to avoid or reduce loss of information.
Depending on the system involved, the major difficulty may not lie in
the mapping but in accurately identifying the incoming character set
and then applying the correct conversion routine. If a local
operating system uses one of the ISO 8859 character sets or an
extensive national or industrial system such as GB18030 [GB18030] or
BIG5 [BIG5], one must correctly identify the character set in use
before converting to Unicode even though those character coding
systems are substantially or completely Unicode-compatible (i.e., all
of the code points in them have an exact and unique mapping to
Unicode code points). It may be even more difficult when the
character coding system in local use is based on conceptually
different assumptions than those used by Unicode about, e.g., about
font encodings used for publications in some Indic scripts. Those
differences may not easily yield unambiguous conversions or
interpretations even if each coding system is internally consistent
and adequate to represent the local language and script.
2. Processing in IDNA2008
These specifications separate Domain Name Registration and Lookup in
the protocol specification. Doing so reflects current practice in
which per-registry restrictions and special processing are applied at
registration time but not during lookup. Even more important in the
longer term, it 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
This section provides an overview of the model used to establish the
algorithm and character lists of [IDNA2008-Tables] and describes the
names and applicability of the categories used there. Note that the
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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 requires respecifying the list of
characters that are permitted in IDNs. In IDNA2003, the role and
utility of characters are independent of context and fixed forever
(or until the standard is replaced). Making completely context-
independent rules globally has proven 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(s) in others. IDNA2003 prohibited those types of
characters entirely. But the restrictions were much too severe to
permit an adequate range of mnemonics for identifiers based on some
languages. The requirement to support those characters but limit
their use to very specific contexts was reinforced by the observation
that handling of particular characters across the languages that use
a script, or the use of similar or identical-looking characters in
different scripts, is less well understood than many people believed
it was several years ago.
Independently of the characters chosen (see next subsection), the
approach is to divide the characters that appear in Unicode into
three categories:
3.1.1. PROTOCOL-VALID
Characters identified as "PROTOCOL-VALID" (often abbreviated
"PVALID") are, in general, permitted by IDNA for all uses 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
about labels they will accept and to maintain rules consistent with
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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.
3.1.1.1. Contextual Rules
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 provisions for
unambiguous labels may require that other characters be restricted to
particular contexts. For example, the ASCII hyphen is not permitted
to start or end a label, whether that label contains non-ASCII
characters or not.
These characters must not appear in IDNs without additional
restrictions, typically because they have no visible consequences in
most scripts but affect format or presentation in a few others or
because they are combining characters that are safe for use only in
conjunction with particular characters or scripts. In order to
permit them to be used at all, they are specially identified as
"CONTEXTUAL RULE REQUIRED" and, when adequately understood,
associated with a rule. In addition, the rule will define whether it
is to be applied on lookup as well as registration. A distinction is
made between characters that indicate or prohibit joining (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.
3.1.1.2. Rules and Their Application
The actual rules may be present or absent. 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.
Examples of descriptions of typical rules, stated informally and in
English, include "Must follow a character from Script XYZ", "Must
occur only if the entire label is in Script ABC", "Must occur only if
the previous and subsequent characters have the DFG property".
Because it is easier to identify these characters than to know that
they are actually needed in IDNs or how to establish exactly the
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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 description of the syntax of the rules, and the rules themselves,
appears in [IDNA2008-Tables].
3.1.2. 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
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
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form a letter or digit.
3.1.3. 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).
3.2. Registration Policy
While these recommendations cannot and should not define registry
policies, registries should develop and apply additional restrictions
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 3843 [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].
It is worth stressing that these principles of policy development and
application apply at all levels of the DNS, not only, e.g., TLD
registrations and that even a trivial, "anything 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 essence of the character rules in IDNA2008 is based on the
realization that there is no single magic bullet for any of the
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issues associated with a multiscript DNS. Instead, the
specifications define a variety of approaches that, together,
constitute multiple lines of defense against ambiguity in identifiers
and loss of referential integrity. The actual 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 by a protocol
alone. As discussed in the previous section (Section 3.2),
registries are expected to restrict what they permit to be
registered, devising and using rules that are designed to optimize
the balance between confusion and risk on the one hand and maximum
expressiveness in mnemonics on the other.
In addition, there is an important role for user agents in warning
against label forms that appear unreasonable given their knowledge of
local contexts and conventions. Of course, no approach based on
naming or identifiers alone can protect against all threats.
4. Issues that Constrain Possible Solutions
4.1. Display and Network Order
The correct treatment of domain names requires a clear distinction
between Network Order (the order in which the code points are sent in
protocols) and Display Order (the order in which the code points are
displayed on a screen or paper). The order of labels in a domain
name that contains characters that are normally written right to left
is discussed in [IDNA2008-Bidi]. In particular, there are questions
about the order in which labels are displayed if left to right and
right to left labels are adjacent to each other, especially if there
are also multiple consecutive appearances of one of the types. The
decision about the display order is ultimately under the control of
user agents --including web browsers, mail clients, and the like--
which may be highly localized. Even when formats are specified by
protocols, the full composition of an Internationalized Resource
Identifier (IRI) [RFC3987] 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. User agents are not required
to use those protocol-based forms directly but often do so. While
display, parsing, and processing within a label is specified by the
normative documents in the IDNA2008 collection, the relationship
between fully-qualified domain names and internationalized labels is
unchanged from the base DNS specifications. Comments in this
document about such full domain names are explanatory or examples of
what might be done and must not be considered normative.
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Questions remain about protocol constraints implying that the overall
direction of these strings will always be left to right (or right to
left) for an IRI or email address, or if they even should conform to
such rules. These questions also have several possible answers.
Should a domain name abc.def, in which both labels are represented in
scripts that are written right to left, be displayed as fed.cba or
cba.fed? An IRI for clear text web access would, in network order,
begin with "http://" and the characters will appear as
"http://abc.def" -- but what does this suggest about the display
order? When entering a URI to many browsers, it may be possible to
provide only the domain name and leave the "http://" to be filled in
by default, assuming no tail (an approach that does not work for
protocols other than HTTP or whatever is chosen as the default). The
natural display order for the typed domain name on a right to left
system is fed.cba. Does this change if a protocol identifier, tail,
and the corresponding delimiters are specified?
While logic, precedent, and reality suggest that these are questions
for user interface design, not IETF protocol specifications,
experience in the 1980s and 1990s with mixing systems in which domain
name labels were read in network order (left to right) and those in
which those labels were read right to left would predict a great deal
of confusion, and heuristics that sometimes fail, if each
implementation of each application makes its own decisions on these
issues.
Any version of IDNA, including the current one, must be written in
terms of the network (transmission on the wire) order of characters
in labels and for the labels in complete (fully-qualified) domain
names and must be quite precise about those relationships. While
some strong suggestions about display order would be desirable to
reduce the chances for inconsistent transcription of domain names
from printed form, such suggestions are beyond the scope of these
specifications.
4.2. Entry and Display in Applications
Applications can accept domain names using any character set or sets
desired by the application developer, specified by the operating
system, or dictated by other constraints, and can display domain
names in any character set or character coding system. That is, the
IDNA protocol does not affect the interface between users and
applications.
An IDNA-aware application can accept and display internationalized
domain names in two formats: the internationalized character set(s)
supported by the application (i.e., an appropriate local
representation of a U-label), and as an A-label. Applications may
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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 that label form. A-labels are opaque
and ugly and malicious variations on them are not easily detected by
users. Where possible, they should thus only be exposed to users and
in contexts in which 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; if it does, 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 the 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.
In protocols and document formats that define how to handle
specification or negotiation of charsets, labels can be encoded in
any charset allowed by the protocol or document format. If a
protocol or document format only allows one charset, the labels must
be given in that charset. Of course, not all charsets can properly
represent all labels. If a U-label cannot be displayed in its
entirety, the only choice (without loss of information) may be to
display the A-label.
In any place where a protocol or document format allows transmission
of the characters in internationalized labels, labels should be
transmitted using 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 already have the capacity
for handling domain names in the ASCII charset. Thus, A-labels can
inherently be handled by those protocols.
4.3. Linguistic Expectations: Ligatures, Digraphs, and Alternate
Character Forms
[[anchor13: There is some internal redundancy and repetition in the
material in this section. Specific suggestions about to reduce or
eliminate redundant text would be appreciated. If no such
suggestions are received before -07 is posted, this not will be
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removed.]]
Users often have expectations about character matching or equivalence
that are based on their own languages and the orthography of those
languages. These expectations may not be consistent with forms or
actions that can be naturally accommodated in a character coding
system, especially if multiple languages are written using the same
script but using different conventions. 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. A
user in German might expect a label with an o-umlaut and a label that
had "oe" substituted, but was otherwise the same, treated as
equivalent even though that substitution would be a clear error in
Swedish. A Chinese user might expect automatic matching of
Simplified and Traditional Chinese characters, but applying that
matching for Korean or Japanese text would create considerable
confusion. For that matter, an English user might expect "theater"
and "theatre" to match.
Related issues arise because there are a number of languages written
with alphabetic scripts in which single phonemes are written using
two characters, termed a "digraph", for example, the "ph" in
"pharmacy" and "telephone". (Note that characters paired in this
manner can also appear consecutively without forming a digraph, as in
"tophat".) Certain digraphs are normally indicated typographically
by setting the two characters closer together than they would be if
used consecutively to represent different phonemes. Some digraphs
are fully joined as ligatures (strictly designating setting totally
without intervening white space, although the term is sometimes
applied to close set pairs). An example of this may be seen when the
word "encyclopaedia" is set with a U+00E6 LATIN SMALL LIGATURE AE
(and some would not consider that word correctly spelled unless the
ligature form was used or the "a" was dropped entirely). When these
ligature and digraph forms have the same interpretation across all
languages that use a given script, application of Unicode
normalization generally resolves the differences and causes them to
match. When they have different interpretations, any requirements
for matching must utilize other methods, presumably at the registry
level, or users must be educated to understand that matching will not
occur.
Difficulties arise from the fact that a given ligature may be a
completely optional typographic convenience for representing a
digraph in one language (as in the above example with some spelling
conventions), while in another language it is a single character that
may not always be correctly representable by a two-letter sequence
(as in the above example with different spelling conventions). This
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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
in Section 4.5.
Whether ligatures and digraphs are to be treated as a sequence of
characters or as a single standalone one constitute a problem that
cannot be resolved solely by operating on scripts. They are,
however, a key concern in the IDN context. Their satisfactory
resolution will require support in policies set by registries, which
therefore need to be particularly mindful not just of this specific
issue, but of all other related matters that cannot be dealt with on
an exclusively algorithmic and global basis.
Just as with the examples of different-looking characters that may be
assumed to be the same, it is in general impossible to deal with
these situations in a system such as IDNA -- or with Unicode
normalization generally -- since determining what to do requires
information about the language being used, context, or both.
Consequently, these specifications make no attempt to treat these
combined characters in any special way. However, their existence
provides a prime example of a situation in which a registry that is
aware of the language context in which labels are to be registered,
and where that language sometimes (or always) treats the two-
character sequences as equivalent to the combined form, should give
serious consideration to applying a "variant" model [RFC3743]
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[RFC4290], or to prohibiting registration of one the forms entirely,
to reduce the opportunities for user confusion and fraud that would
result from the related strings being registered to different
parties.
[[anchor14: Placeholder: A discussion of the Arabic digit issue
shoudl go here once it is resolved in some appropriate way.]]
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.
The solution of keeping the characters separate but doing matching
independent of case is not feasible with IDNA or any IDNA-like model
because the matching would then have to be done on the server rather
than have characters mapped on the client. That situation was
recognized in IDNA2003 and nothing in these specifications
fundamentally changes it or could do so. In IDNA2003, all characters
are case-folded and mapped. That results in upper-case characters
being mapped to lower-case ones and in some other transformations of
alternate forms of characters, especially those that do not have (or
did not have) upper-case forms. For example, Greek Final Form Sigma
(U+03C2) is mapped to the medial form (U+03C3) and Eszett (German
Sharp S, U+00DF) is mapped 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
avoid case folding and treating these characters as themselves.
Approaches to handling that incompatibility are discussed in
Section 7.2. Although information is lost in IDNA2003's ToASCII
operation so that, in some sense, neither Final Sigma nor Eszett can
be represented in an IDN at all, its guarantee of mapping when those
characters are used as input can be interpreted as violating one of
the conditions discussed in Section 7.4.1 and hence requiring a
prefix change. The consensus was to not make a prefix change in
spite of this issue. Of course, had a prefix change been made (at
the costs discussed in Section 7.4.3) there would have been several
options, including, if desired, assignment of the character to the
CONTEXTUAL RULE REQUIRED category and requiring that it only be used
in carefully-selected contexts.
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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, not include any
characters with strong left to right properties (which excludes other
alphabetic characters but permits European digits), and rejects any
other string that contains a right to left character. This is one of
the few places where the IDNA algorithms (both in IDNA2003 and in
IDAN2008) are required to 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 model of IDNs described in this document can be seen as a
particular instance of the "Robustness Principle" that has been so
important to other aspects of Internet protocol design. This
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]). For
IDNs to work well, not only must the protocol be carefully designed
and implemented, but zone administrators (registries) must have and
require sensible policies about what is registered -- conservative
policies -- and implement and enforce them.
Conversely, lookup applications are expected to reject labels that
clearly violate global (protocol) rules (no one has ever seriously
claimed that being liberal in what is accepted requires being
stupid). However, once one gets past such global rules and deals
with anything sensitive to script or locale, it is necessary to
assume that garbage has not been placed into the DNS, i.e., one must
be liberal about what one is willing to look up in the DNS rather
than guessing about whether it should have been permitted to be
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registered.
As mentioned elsewhere, 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. Applications should, however, be
sensitive to the fact that, because of the possibility of DNS
wildcards, the ability to successfully resolve a name does not
guarantee that it was actually registered.
If lookup applications, as a user interface (UI) or other local
matter, decide to warn about some strings that are valid under the
global rules but that they perceive as dangerous, that is their
prerogative and we can only hope that the market (and maybe
regulators) will reinforce the good choices and discourage the poor
ones. In this context, a lookup application that decides a string
that is valid under the protocol is dangerous and refuses to look it
up is in violation of the protocols; one that is willing to look
something up, but warns against it, is exercising a local choice.
6. Front-end and User Interface Processing
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 structured for a particular protocol or class of protocols
such a email addresses, URIs, or IRIs. They may occur in running
text or be processed by one system after being provided in another.
Systems may wish to try to normalize URLs so as to determine (or
guess) whether a reference is valid or two references point to the
same object without actually looking the objects up and comparing
them (that is necessary, not just a choice, 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 minimum-ambiguity forms of A-labels,
U-labels, or LDH-labels, it is inevitable that programs that process
domain names will encounter variant forms.
One source of such forms will be labels created under IDNA2003
because that protocol allowed labels that were transformed before
they were turned from native-character into ACE ("xn--...") format by
mapping some characters into other. One consequence of the
transformations was that, when the ToUnicode and ToASCII operations
of IDNA2003 were applied, ToUnicode(ToASCII(original-label)) often
did not produce the original label. IDNA2008 explicitly defines
A-labels and U-labels as different forms of the same abstract label,
forms that are stable when conversions are performed between them,
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without mappings. A different way of explaining this is that there
are, today, domain names in files on the Internet that use characters
that cannot be represented directly in, or recovered from, (A-label)
domain names but for which interpretations are provided by IDNA2003.
There are two major categories of such characters, those that are
removed by NFKC normalization and those upper-case characters that
are mapped to lower-case (there are also a few characters that are
given special-case mapping treatment in Stringprep including lower-
case characters that are case-folded into other lower-case characters
or strings).
Other issues in domain name identification and processing arise
because IDNA2003 specified that several other characters be treated
as equivalent to the ASCII period (dot, full stop) character used as
a label separator. If a string that might be a domain name appears
in an arbitrary context (such as running text), it is difficult, even
with only ASCII characters, to know whether an actual domain name (or
a protocol parameter like a URI) is present and where it starts and
ends. When using Unicode, this gets even more difficult if treatment
of certain special characters (like the dot that separates labels in
a domain name) depends on context (e.g., prior knowledge of whether
the string represents a domain name or not). That knowledge is not
available if the primary heuristic for identifying the presence of
domain names in strings depends on the presence of dots separating
groups of characters with no intervening spaces.
As discussed elsewhere in this document, the IDNA2008 model removes
all of these mappings and interpretations, including the equivalence
of different forms of dots, from the protocol, discouraging such
mappings and leaving them, when necessary, to local processing. This
should not be taken to imply that local processing is optional or can
be avoided entirely, even if doing so might have been desirable in a
world without IDNA2003 IDNs in files and archives. Instead, unless
the program context is such that it is known that any IDNs that
appear will contain either U-label or A-label forms, or that other
forms can safely be rejected, some local processing of apparent
domain name strings will be required, both to maintain compatibility
with IDNA2003 and to prevent user astonishment. Such local
processing, while not specified in this document or the associated
ones, will generally take one of two forms:
o Generic Preprocessing.
When the context in which the program or system that processes
domain names operates is global, a reasonable balance must be
found that is sensitive to the broad range of local needs and
assumptions while, at the same time, not sacrificing the needs of
one language, script, or user population to those of another.
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For this case, the best practice will usually be to apply NFKC and
case-mapping (or, perhaps better yet, Stringprep itself), plus
dot-mapping where appropriate, to the domain name string prior to
applying IDNA. That practice will not only yield a reasonable
compromise of user experience with protocol requirements but will
be almost completely compatible with the various forms permitted
by IDNA2003.
o Highly Localized Preprocessing.
Unlike the case above, there will be some situations in which
software will be highly localized for a particular environment and
carefully adapted to the expectations of users in that
environment. The many discussions about using the Internet to
preserve and support local cultures suggest that these cases may
be more common in the future than they have been so far.
In these cases, we should avoid trying to tell implementers what
they should do, if only because they are quite likely (and for
good reason) to ignore us. We would assume that they would map
characters that the intuitions of their users would suggest be
mapped and would hope that they would do that mapping as early as
possible, storing A-label or U-label forms in files and
transporting only those forms between systems. One can imagine
switches about whether some sorts of mappings occur, warnings
before applying them or, in a slightly more extreme version of the
approach taken in Internet Explorer version 7 (IE7), systems that
utterly refuse to handle "strange" characters at all if they
appear in U-label form. None of those local decisions are a
threat to interoperability as long as (i) only U-labels and
A-labels are used in interchange with systems outside the local
environment, (ii) no character that would be valid in a U-label as
itself is mapped to something else, (iii) any local mappings are
applied as a preprocessing step (or, for conversions from U-labels
or A-labels to presentation forms, postprocessing), not as part of
IDNA processing proper, and (iv) appropriate consideration is
given to labels that might have entered the environment in
conformance to IDNA2003.
In either case, it is vital that user interface designs and, where
the interfaces are not sufficient, users, be aware that the only
forms of domain names that this protocol anticipates will resolve
globally or compare equal when crude methods (i.e., those not
conforming to the strict definition of label equivalence given in
[IDNA2008-Defs]) are used are those in which all native-script labels
are in U-label form. Forms that assume mapping will occur,
especially forms that were not valid under IDNA2003, may or may not
function in predictable ways across all implementations.
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User interfaces involving Latin-based scripts should take special
care when considering how to handle case mapping because small
differences in label strings may cause behavior that is astonishing
to users. Because case-insensitive mapping is done for ASCII strings
by DNS-servers, an all-ASCII label is treated as case-insensitive.
However, if even one of the characters of that string is replaced by
one that requires the label to be given IDN treatment (e.g., by
adding a diacritical mark), then the label immediately becomes case-
sensitive. This suggests that case mapping for Latin-based scripts
(and possibly other scripts with case distinctions) as a
preprocessing matter in applications may be wise to prevent user
astonishment, but, since all applications may not do this and
ambiguity in transport is not desirable, the that case-dependent
forms should not be stored in files.
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 to enable applications to be agnostic about whether they
are being run in environments supporting any Unicode version from 3.2
onward and to permit incrementally adding new characters, character
groups, scripts, and other character collections as they are
incorporated into Unicode, without disruption and, in the long term,
without "heavy" processes such as those involving IETF consensus.
(An IETF consensus process is required by the IDNA2008 specifications
and is expected to be required and used until significant experience
accumulates with IDNA operations and new versions of Unicode.) The
mechanisms that support this are outlined above and elsewhere in the
IDNA2008 document set, but this section reviews them in a context
that may be more helpful to those who need to understand the approach
and make plans for it.
7.1.1. General IDNA Validity Criteria
The general criteria for a putative label, and the collection of
characters that make it up, to be considered IDNA-valid are (the
actual rules are rigorously defined in the "Protocol" and "Tables"
documents):
o The characters are "letters", marks needed to form letters,
numerals, or other code points used to write words in some
language. Symbols, drawing characters, and various notational
characters are permanently excluded -- some because they are
actively dangerous in URI, IRI, or similar contexts and others
because there is no evidence that they are important enough to
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Internet operations or internationalization to justify expansion
of domain names beyond the general principle of "letters, digits,
and hyphen" and the complexities that would come with it
(additional discussion and rationale for the symbol decision
appears in Section 7.6).
o Other than in very exceptional cases, e.g., where they are needed
to write substantially any word of a given language, punctuation
characters are excluded as well. 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).
Even for English, very common constructions -- contractions like
"don't" or "it's", names that are written with apostrophes such as
"O'Reilly", or characters for which apostrophes are common
substitutes cannot be represented in DNS labels. Words in English
whose usually-preferred spellings include diacritical marks cannot
be represented under the original hostname rules, but most can be
represented if treated as IDNs.
o Characters that are unassigned (have no character assignment at
all) in the version of Unicode being used by the registry or
application are not permitted, even on lookup. There are at least
two reasons for this.
* Tests involving the context of characters (e.g., some
characters being permitted only adjacent to ones of specific
types but otherwise invisible or very problematic for other
reasons) and integrity tests on complete labels are needed.
Unassigned code points cannot be permitted because one cannot
determine whether particular code points will require
contextual rules (and what those rules should be) before
characters are assigned to them and the properties of those
characters fully understood.
* Unicode specifies that an unassigned code point normalizes (and
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, creating confusion and/or violating
other rules listed here.
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
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excludes any character mapped to another by Nameprep [RFC3491].
Tables used to identify the characters that are IDNA-valid are
expected to be driven by the principles above, principles that are
specified exactly in [IDNA2008-Tables]). The rules given there are
normative, rather than being just an interpretation of the tables.
7.1.2. Labels in Registration
Anyone entering a label into a DNS zone must properly validate that
label -- i.e., be sure that the criteria for that label are 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" [RFC0952] where necessary for the expected applications.
Later addition of special service location formats [RFC2782] imposed
new requirements on zone administrators for the use of labels that
conform to the requirements of those formats. 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:
o Any label that appears to be an A-label, i.e., any label that
starts in "xn--", must be IDNA-valid, i.e., they must be valid
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, a registry (or entity communicating with a registry
to accomplish name registrations) will need to update its tables --
both the Unicode-associated tables and the tables of permitted IDN
characters -- to enable a new script or other set of new characters.
It will not be affected by newer versions of Unicode, or newly-
authorized characters, until and unless it wishes to make those
registrations. The zone administrator is also responsible -- under
the protocol and to registrants and users -- for both checking as
required by the protocol and verification that whatever policies it
develops are complied with, whether those policies are for minimizing
risks due to confusable characters and sequences, for preserving
language or script integrity, or for other purposes. Those checking
and verification procedures are more extensive than those that are is
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expected of applications systems that look names up.
Systems looking up or resolving DNS labels, especially IDN DNS
labels, must be able to assume that applicable registration rules
were followed for names entered into the DNS.
7.1.3. Labels in Lookup
Anyone looking up a label in a DNS zone is required to
o Maintain a consistent set of tables, as discussed above. 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 either
code points prohibited by IDNA (categorized as "DISALLOWED") 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, notably verifying that there are no leading
combining marks, that the "bidi" conditions are met if right to
left characters appear, that any required contextual rules are
available and that, if such rules are associated with Joiner
Controls, they are tested.
o Avoid validating other contextual rules about characters,
including mixed-script label prohibitions, although such rules may
be used to influence presentation decisions in the user interface.
[[anchor18: Check this, and all similar statements, against
Protocol when that is finished.]]
By avoiding applying its own interpretation of which labels are valid
as a means of rejecting lookup attempts, the lookup application
becomes less sensitive to version incompatibilities with the
particular zone registry associated with the domain name.
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 validly registered, as long as its version of the
Unicode-associated 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
[[anchor19: Note in Draft: This subsection is completely new in
version -04 and has been further tuned in -05 and -06 of this
document. It could almost certainly use improvement, although this
note will be removed if there are not significant suggestions about
the -06 version. It also contains some material that is redundant
with material in other sections. I have not tried to remove that
material and will not do so until the WG concludes that this section
is relatively stable, but would appreciate help in identifying what
should be removed or how this might be enhanced to contain more of
that other material. --JcK]]
In those scripts that make case distinctions, there are a few
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).
The decision to eliminate 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
characters distinct from the forms produced by case folding, there
would be no information loss and registries would have maximum
flexibility, but labels using those characters that were looked up
according to IDNA2003 rules would be transformed into A-labels using
their case-mapped variations while lookup according to IDNA2008 rules
would be based on different A-labels that represented the actual
characters.
With the understanding that there would be incompatibility either way
but a judgment that the incompatibility was not significant enough to
just a prefix change, the WG concluded that Eszett and Final Form
Sigma should be treated as distinct and Protocol-Valid characters.
The decision faces registries, especially registries maintaining
zones for third parties, with a variation on what has become a
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familiar problem: how to introduce a new service in a way that does
not create confusion or significantly weaken or invalidate existing
identifiers.
There have traditionally been several approaches to problems of this
type. Without any preference or claim to completeness, these are:
o Do not permit use of the newly-available character at the registry
level. This might cause lookup failures if a domain name were to
be written with the expectation of the IDNA2003 mapping behavior,
but would eliminate any possibility of false matches.
o Hold a "sunrise"-like arrangement in which holders of the
previously-mapped labels (labels containing "ss" in the Eszett
case or ones containing Lower Case Sigma in the Final Sigma case)
are given priority (and perhaps other benefits) for registering
the corresponding string containing the newly-available
characters.
o Adopt some sort of "variant" approach in which registrants either
obtained labels with both character forms or one of them was
blocked from registration by anyone but the registrant of the
other form.
In principle, lookup applications could also compensate for the
difference in interpretation by looking up the string according to
the interpretation specified in these documents and then, if that
failed, doing the lookup with the mapping, simulating the IDNA2003
interpretation. The risk of false positives is such that this is
generally to be discouraged unless the application is able to engage
in a "is this what you meant" dialogue with the end user.
7.3. More Flexibility in User Agents
These specifications do not include mappings between one character or
code point and others for any reason. Instead, they prohibit the
characters that would be mapped to others by normalization, upper
case to lower case changes, or other rules. As examples, while
mathematical characters based on Latin ones are accepted as input to
IDNA2003, they are prohibited in IDNA2008. Similarly, double-width
characters and other variations are prohibited as IDNA input.
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
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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
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 simulated 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 consistent with having
Unicode work 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) and 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 (and therefore does so). Some other
considerations reinforce this conclusion. For example, an essential
element of the ASCII case-mapping functions is that, for individual
characters, uppercase(character) must be equal to
uppercase(lowercase(character)). That requirement may not be
satisfied with IDNs. For example, there are some characters in
scripts that use case distinction 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. Of course, the expectations of users who are accustomed to
a case-insensitive DNS environment will probably be well-served if
user agents perform case folding prior to IDNA processing, but the
IDNA procedures themselves should neither require such mapping nor
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expect them when they are not natural to the localized environment.
7.4. The Question of Prefix Changes
The conditions that would require a change in the IDNA ACE prefix
("xn--" for the version of IDNA specified in [RFC3490]) have been a
great concern to the community. A prefix change would clearly be
necessary if the algorithms were modified in a manner that would
create serious ambiguities during subsequent transition in
registrations. This section summarizes our conclusions about the
conditions under which changes in prefix would be necessary and the
implications of such a change.
7.4.1. Conditions Requiring a Prefix Change
An IDN prefix change is needed if a given string would be looked up
or otherwise interpreted differently depending on the version of the
protocol or tables being used. Consequently, work to update IDNs
would require 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) yields
one string under IDNA2003 (RFC3490) and a different string under
IDNA2008.
2. An input string that is valid under IDNA2003 and also valid under
IDNA2008 yields two different A-labels with the different
versions of IDNA. This condition is believed to be essentially
equivalent to the one above except for a very small number of
edge cases which may not, pragmatically, justify a prefix change
(See Section 7.2).
Note, however, that if the input string is valid under one
version and not valid under the other, this condition does not
apply. See the first item in Section 7.4.2, below.
3. A fundamental change is made to the semantics of the string that
is inserted in the DNS, e.g., if a decision were made to try to
include language or specific script information in that string,
rather than having it be just a string of characters.
4. A sufficiently large number of characters is added to Unicode so
that the Punycode mechanism for block offsets no longer has
enough capacity to reference the higher-numbered planes and
blocks. This condition is unlikely even in the long term and
certain not to arise in the next few years.
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7.4.2. Conditions Not Requiring a Prefix Change
In particular, as a result of the principles described above, none of
the following changes require a new prefix:
1. Prohibition of some characters as input to IDNA. This may make
names that are now registered inaccessible, but does not require
a prefix change.
2. Adjustments in IDNA tables or actions, including normalization
definitions, that affect characters that were already invalid
under IDNA2003.
3. Changes in the style of the IDNA definition that does not alter
the actions performed by IDNA.
7.4.3. Implications of Prefix Changes
While it might be possible to make a prefix change, the costs of such
a change are considerable. Even if they wanted to do so, all
registries could not convert 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 first process a putative label under the IDNA2008
rules and try to look it up and then, if it were not found, would
process the label under IDNA2003 rules and look it up again. That
process could significantly slow down all processing that involved
IDNs in the DNS especially since, in principle, a fully-qualified
name could contain a mixture of labels that were registered with the
old and new prefixes, a situation that would make the use of DNS
caching very difficult. In addition, looking up the same input
string as two separate A-labels would create some potential for
confusion and attacks, since they could, in principle, map to
different targets and then resolve to different entries in the DNS.
Consequently, a prefix change is to be 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. Concerns have been expressed about problems for non-DNS
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uses of Stringprep being caused by changes to the specification
intended to improve the handling of IDNs, most notably as this might
affect identification and authentication protocols. 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] either give interpretations to strings prohibited
under IDNA2003 or prohibit strings that IDNA2003 permitted. The
IDNA2008 protocol does not use either Nameprep or Stringprep at all,
so there are no side-effect changes to other protocols.
It is particularly important to keep IDNA processing separate from
processing for various security protocols because some of the
constraints that are necessary for smooth and comprehensible use of
IDNs may be unwanted or undesirable in other contexts. For example,
the criteria for good passwords or passphrases are very different
from those for desirable IDNs: passwords should be hard to guess,
while domain names should normally be easily memorable. Similarly,
internationalized 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:
o 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 LDH rules -- which have served the Internet very well -- to a
Unicode base rather than an ASCII base.
o Most Unicode names for letters are, in most cases, fairly
intuitive, unambiguous and recognizable to users of the relevant
script. Symbol names are more problematic 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
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knowledge of the coding distinctions made in Unicode is needed to
know that there 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.
[[anchor20: The above paragraph remains controversial as to
whether it is valid. The WG will need to make a decision if this
section is not dropped entirely.]]
o 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.
o 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 or the 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 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.
o The consequence of these ambiguities of description and
dependencies on distinctions that were, or were not, made in
Unicode codings 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
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registry has properly allocated only assigned values.
In the protocol as described in these documents, strings containing
unassigned code points must not be either looked up or registered.
There are several reasons for this, with the most important ones
being:
o It cannot be known with sufficient reliability in advance that a
code point that was not previously assigned will not be assigned
to a compatibility character or one that would be otherwise
disallowed by the rules in [IDNA2008-Tables]. In IDNA2003, since
there is no direct dependency on NFKC (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 generally assigned to DISALLOWED,
permitting strings containing unassigned characters to be looked
up would permit violating the principle that characters in
DISALLOWED are not looked up.
o More generally, the status of an unassigned character with regard
to the DISALLOWED and PROTOCOL-VALID categories, and whether
contextual rules are required with the latter, cannot be evaluated
until a character is actually assigned and known. By contrast,
characters that are actually DISALLOWED are placed in that
category only as a consequence of rules applied to known
properties or per-character evaluation.
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).
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
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"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. 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, especially Mark Davis, 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 Frank Ellerman, Michael Everson, Asmus
Freytag, Erik van der Poel, Michel Suignard, and Ken Whistler,
although, as usual, they bear little or no responsibility for the
conclusions the editor and contributors reached after receiving their
suggestions. Thanks are also due to Vint Cerf, Debbie Garside, and
Jefsey Morphin for conversations that led to considerable
improvements in the content of this document.
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
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Cerf, Mark Davis, Martin Duerst, Ken Whistler, and Andrew Sullivan.
9. Contributors
While the listed editor held the pen, this core of this document and
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.
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.
10. 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, a decision that
would be impossible in many or most cases.
11. IANA Considerations
This section gives an overview of registries required for IDNA. The
actual definitions of the first two appear in [IDNA2008-Tables].
11.1. IDNA Character Registry
The distinction among the three major categories "UNASSIGNED",
"DISALLOWED", and "PROTOCOL-VALID" is made by special categories and
rules that are integral elements of [IDNA2008-Tables]. Convenience
in programming and validation requires a registry of characters and
scripts and their categories, updated for each new version of Unicode
and the characters it contains. The details of this registry are
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specified in [IDNA2008-Tables].
11.2. IDNA Context Registry
For characters that are defined in the IDNA Character Registry list
as PROTOCOL-VALID but requiring a contextual rule (i.e., the types of
rule described in Section 3.1.1.1), IANA will create and maintain a
list of approved contextual rules. The details for those rules
appear in [IDNA2008-Tables].
11.3. 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, these
specifications have no direct effect on that registry and no IANA
action is required as a result.
12. Security Considerations
12.1. General Security Issues with IDNA
This document in the IDNA2008 series is purely explanatory and
informational and consequently introduces no new security issues. It
would, of course, be a poor idea for someone to try to implement from
it; such an attempt would almost certainly lead to interoperability
problems and might lead to security ones. A discussion of security
issues with IDNA, including some relevant history, appears in
[IDNA2008-Defs].
13. References
13.1. 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
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definitive for the Internet.
[IDNA2008-Bidi]
Alvestrand, H. and C. Karp, "An updated IDNA criterion for
right to left scripts", July 2008, <https://
datatracker.ietf.org/drafts/draft-ietf-idnabis-bidi/>.
[IDNA2008-Defs]
Klensin, J., "Internationalized Domain Names for
Applications (IDNA): Definitions and Document Framework",
November 2008, <https://datatracker.ietf.org/drafts/
draft-ietf-idnabis-defs/>.
[IDNA2008-Protocol]
Klensin, J., "Internationalized Domain Names in
Applications (IDNA): Protocol", November 2008, <https://
datatracker.ietf.org/drafts/draft-ietf-idnabis-protocol/>.
[IDNA2008-Tables]
Faltstrom, P., "The Unicode Code Points and IDNA",
July 2008, <https://datatracker.ietf.org/drafts/
draft-ietf-idnabis-tables/>.
A version of this document is available in HTML format at
http://stupid.domain.name/idnabis/
draft-ietf-idnabis-tables-02.html
[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,
<http://www.unicode.org/reports/tr15/>.
[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/).
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Internet-Draft IDNA Rationale December 2008
13.2. 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.
[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.
[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.
[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.
[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.
Klensin Expires June 18, 2009 [Page 40]
Internet-Draft IDNA Rationale December 2008
[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.
[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,
<http://www.unicode.org/reports/tr39/>.
[Unicode-UAX31]
The Unicode Consortium, "Unicode Standard Annex #31:
Unicode Identifier and Pattern Syntax", March 2008,
<http://www.unicode.org/reports/tr31/>.
[Unicode-UTR36]
The Unicode Consortium, "Unicode Technical Report #36:
Unicode Security Considerations", July 2008,
<http://www.unicode.org/reports/tr36/>.
Appendix A. Change Log
[[ RFC Editor: Please remove this appendix. ]]
A.1. Changes between Version -00 and Version -01 of
draft-ietf-idnabis-rationale
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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.
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.
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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.
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.
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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.
o More editorial fine-tuning.
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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