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IETF RFC 5894
Internationalized Domain Names for Applications (IDNA): Background, Explanation, and Rationale
Last modified on Wednesday, August 4th, 2010
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Internet Engineering Task Force (IETF) J. Klensin
Request for Comments: 5894 August 2010
Category: Informational
ISSN: 2070-1721
Internationalized Domain Names for Applications (IDNA):
Background, Explanation, and Rationale
 Abstract
Several years have passed since the original protocol for
Internationalized Domain Names (IDNs) was completed and deployed.
During that time, a number of issues have arisen, including the need
to update the system to deal with newer versions of Unicode. Some of
these issues require tuning of the existing protocols and the tables
on which they depend. This document provides an overview of a
revised system and provides explanatory material for its components.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are a candidate for any level of Internet
Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/RFC 5894.
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Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
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include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
This document may contain material from IETF Documents or IETF
Contributions published or made publicly available before November
10, 2008. The person(s) controlling the copyright in some of this
material may not have granted the IETF Trust the right to allow
modifications of such material outside the IETF Standards Process.
Without obtaining an adequate license from the person(s) controlling
the copyright in such materials, this document may not be modified
outside the IETF Standards Process, and derivative works of it may
not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other
than English.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Context and Overview . . . . . . . . . . . . . . . . . . . 4
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 5
1.2.1. DNS "Name" Terminology . . . . . . . . . . . . . . . . 5
1.2.2. New Terminology and Restrictions . . . . . . . . . . . 6
1.3. Objectives . . . . . . . . . . . . . . . . . . . . . . . . 6
1.4. Applicability and Function of IDNA . . . . . . . . . . . . 7
1.5. Comprehensibility of IDNA Mechanisms and Processing . . . 8
2. Processing in IDNA2008 . . . . . . . . . . . . . . . . . . . . 9
3. Permitted Characters: An Inclusion List . . . . . . . . . . . 9
3.1. A Tiered Model of Permitted Characters and Labels . . . . 10
3.1.1. PROTOCOL-VALID . . . . . . . . . . . . . . . . . . . . 10
3.1.2. CONTEXTUAL RULE REQUIRED . . . . . . . . . . . . . . . 11
3.1.2.1. Contextual Restrictions . . . . . . . . . . . . . 11
3.1.2.2. Rules and Their Application . . . . . . . . . . . 12
3.1.3. DISALLOWED . . . . . . . . . . . . . . . . . . . . . . 12
3.1.4. UNASSIGNED . . . . . . . . . . . . . . . . . . . . . . 13
3.2. Registration Policy . . . . . . . . . . . . . . . . . . . 14
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3.3. Layered Restrictions: Tables, Context, Registration, and
Applications . . . . . . . . . . . . . . . . . . . . . . . 15
4. Application-Related Issues . . . . . . . . . . . . . . . . . . 15
4.1. Display and Network Order . . . . . . . . . . . . . . . . 15
4.2. Entry and Display in Applications . . . . . . . . . . . . 16
4.3. Linguistic Expectations: Ligatures, Digraphs, and
Alternate Character Forms . . . . . . . . . . . . . . . . 19
4.4. Case Mapping and Related Issues . . . . . . . . . . . . . 20
4.5. Right-to-Left Text . . . . . . . . . . . . . . . . . . . . 21
5. IDNs and the Robustness Principle . . . . . . . . . . . . . . 22
6. Front-end and User Interface Processing for Lookup . . . . . . 22
7. Migration from IDNA2003 and Unicode Version Synchronization . 25
7.1. Design Criteria . . . . . . . . . . . . . . . . . . . . . 25
7.1.1. Summary and Discussion of IDNA Validity Criteria . . . 25
7.1.2. Labels in Registration . . . . . . . . . . . . . . . . 26
7.1.3. Labels in Lookup . . . . . . . . . . . . . . . . . . . 27
7.2. Changes in Character Interpretations . . . . . . . . . . . 28
7.2.1. Character Changes: Eszett and Final Sigma . . . . . . 28
7.2.2. Character Changes: Zero Width Joiner and Zero
Width Non-Joiner . . . . . . . . . . . . . . . . . . . 29
7.2.3. Character Changes and the Need for Transition . . . . 29
7.2.4. Transition Strategies . . . . . . . . . . . . . . . . 30
7.3. Elimination of Character Mapping . . . . . . . . . . . . . 31
7.4. The Question of Prefix Changes . . . . . . . . . . . . . . 31
7.4.1. Conditions Requiring a Prefix Change . . . . . . . . . 31
7.4.2. Conditions Not Requiring a Prefix Change . . . . . . . 32
7.4.3. Implications of Prefix Changes . . . . . . . . . . . . 32
7.5. Stringprep Changes and Compatibility . . . . . . . . . . . 33
7.6. The Symbol Question . . . . . . . . . . . . . . . . . . . 33
7.7. Migration between Unicode Versions: Unassigned Code
Points . . . . . . . . . . . . . . . . . . . . . . . . . . 35
7.8. Other Compatibility Issues . . . . . . . . . . . . . . . . 36
8. Name Server Considerations . . . . . . . . . . . . . . . . . . 37
8.1. Processing Non-ASCII Strings . . . . . . . . . . . . . . . 37
8.2. Root and Other DNS Server Considerations . . . . . . . . . 37
9. Internationalization Considerations . . . . . . . . . . . . . 38
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 38
10.1. IDNA Character Registry . . . . . . . . . . . . . . . . . 38
10.2. IDNA Context Registry . . . . . . . . . . . . . . . . . . 39
10.3. IANA Repository of IDN Practices of TLDs . . . . . . . . . 39
11. Security Considerations . . . . . . . . . . . . . . . . . . . 39
11.1. General Security Issues with IDNA . . . . . . . . . . . . 39
12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 39
13. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 40
14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 40
14.1. Normative References . . . . . . . . . . . . . . . . . . . 40
14.2. Informative References . . . . . . . . . . . . . . . . . . 41
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1. Introduction
1.1. Context and Overview
Internationalized Domain Names in Applications (IDNA) is a collection
of standards that allow client applications to convert some mnemonic
strings expressed in Unicode to an ASCII-compatible encoding form
("ACE") that is a valid DNS label containing only LDH syntax (see the
Definitions document [RFC 5890]). The specific form of ACE label used
by IDNA is called an "A-label". A client can look up an exact
A-label in the existing DNS, so A-labels do not require any
extensions to DNS, upgrades of DNS servers, or updates to low-level
client libraries. An A-label is recognizable from the prefix "xn--"
before the characters produced by the Punycode algorithm [RFC 3492];
thus, a user application can identify an A-label and convert it into
Unicode (or some local coded character set) for display.
On the registry side, IDNA allows a registry to offer
Internationalized Domain Names (IDNs) for registration as A-labels.
A registry may offer any subset of valid IDNs, and may apply any
restrictions or bundling (grouping of similar labels together in one
registration) appropriate for the context of that registry.
Registration of labels is sometimes discussed separately from lookup,
and it is subject to a few specific requirements that do not apply to
lookup.
DNS clients and registries are subject to some differences in
requirements for handling IDNs. In particular, registries are urged
to register only exact, valid A-labels, while clients might do some
mapping to get from otherwise-invalid user input to a valid A-label.
The first version of IDNA was published in 2003 and is referred to
here as IDNA2003 to contrast it with the current version, which is
known as IDNA2008 (after the year in which IETF work started on it).
IDNA2003 consists of four documents: the IDNA base specification
[RFC 3490], Nameprep [RFC 3491], Punycode [RFC 3492], and Stringprep
[RFC 3454]. The current set of documents, IDNA2008, is not dependent
on any of the IDNA2003 specifications other than the one for Punycode
encoding. References to "IDNA2008", "these specifications", or
"these documents" are to the entire IDNA2008 set listed in a separate
Definitions document [RFC 5890]. The characters that are valid in
A-labels are identified from rules listed in the Tables document
[RFC 5892], but validity can be derived from the Unicode properties of
those characters with a very few exceptions.
Traditionally, DNS labels are matched case-insensitively (as
described in the DNS specifications [RFC 1034][RFC 1035]). That
convention was preserved in IDNA2003 by a case-folding operation that
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generally maps capital letters into lowercase ones. However, if case
rules are enforced from one language, another language sometimes
loses the ability to treat two characters separately. Case-
insensitivity is treated slightly differently in IDNA2008.
IDNA2003 used Unicode version 3.2 only. In order to keep up with new
characters added in new versions of Unicode, IDNA2008 decouples its
rules from any particular version of Unicode. Instead, the
attributes of new characters in Unicode, supplemented by a small
number of exception cases, determine how and whether the characters
can be used in IDNA labels.
This document provides informational context for IDNA2008, including
terminology, background, and policy discussions. It contains no
normative material; specifications for conformance to the IDNA2008
protocols appears entirely in the other documents in the series.
1.2. Terminology
Terminology for IDNA2008 appears in the Definitions document
[RFC 5890]. That document also contains a road map to the IDNA2008
document collection. No attempt should be made to understand this
document without the definitions and concepts that appear there.
1.2.1. DNS "Name" Terminology
In the context of IDNs, the DNS term "name" has introduced some
confusion as people speak of DNS labels in terms of the words or
phrases of various natural languages. Historically, many of the
"names" in the DNS have been mnemonics to identify some particular
concept, object, or organization. They are typically rooted in some
language because most people think in language-based ways. But,
because they are mnemonics, they need not obey the orthographic
conventions of any language: it is not a requirement that it be
possible for them to be "words".
This distinction is important because the reasonable goal of an IDN
effort is not to be able to write the great Klingon (or language of
one's choice) novel in DNS labels but to be able to form a usefully
broad range of mnemonics in ways that are as natural as possible in a
very broad range of scripts.
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1.2.2. New Terminology and Restrictions
IDNA2008 introduces new terminology. Precise definitions are
provided in the Definitions document for the terms U-label, A-Label,
LDH label (to which all valid pre-IDNA hostnames conformed), Reserved
LDH label (R-LDH label), XN-label, Fake A-label, and Non-Reserved LDH
label (NR-LDH label).
In addition, the term "putative label" has been adopted to refer to a
label that may appear to meet certain definitional constraints but
has not yet been sufficiently tested for validity.
These definitions are also illustrated in Figure 1 of the Definitions
document. R-LDH labels contain "--" in the third and fourth
character positions from the beginning of the label. In IDNA-aware
applications, only a subset of these reserved labels is permitted to
be used, namely the A-label subset. A-labels are a subset of the
R-LDH labels that begin with the case-insensitive string "xn--".
Labels that bear this prefix but that are not otherwise valid fall
into the "Fake A-label" category. The Non-Reserved labels (NR-LDH
labels) are implicitly valid since they do not bear any resemblance
to the labels specified by IDNA.
The creation of the Reserved-LDH category is required for three
reasons:
o to prevent confusion with pre-IDNA coding forms;
o to permit future extensions that would require changing the
prefix, no matter how unlikely those might be (see Section 7.4);
and
o to reduce the opportunities for attacks via the Punycode encoding
algorithm itself.
As with other documents in the IDNA2008 set, this document uses the
term "registry" to describe any zone in the DNS. That term, and the
terms "zone" or "zone administration", are interchangeable.
1.3. Objectives
These are the main objectives in revising IDNA.
o Use a more recent version of Unicode and allow IDNA to be
independent of Unicode versions, so that IDNA2008 need not be
updated for implementations to adopt code points from new Unicode
versions.
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o Fix a very small number of code point categorizations that have
turned out to cause problems in the communities that use those
code points.
o Reduce the dependency on mapping, in favor of valid A-labels.
This will result in pre-mapped forms that are not valid IDNA
labels appearing less often in various contexts.
o Fix some details in the bidirectional code point handling
algorithms.
1.4. Applicability and Function of IDNA
The IDNA specification solves the problem of extending the repertoire
of characters that can be used in domain names to include a large
subset of the Unicode repertoire.
IDNA does not extend DNS. Instead, the applications (and, by
implication, the users) continue to see an exact-match lookup
service. Either there is a single name that matches exactly (subject
to the base DNS requirement of case-insensitive ASCII matching) or
there is no match. This model has served the existing applications
well, but it requires, with or without internationalized domain
names, that users know the exact spelling of the domain names that
are to be typed into applications such as web browsers and mail user
agents. The introduction of the larger repertoire of characters
potentially makes the set of misspellings larger, especially given
that in some cases the same appearance, for example on a business
card, might visually match several Unicode code points or several
sequences of code points.
The IDNA standard does not require any applications to conform to it,
nor does it retroactively change those applications. An application
can elect to use IDNA in order to support IDNs while maintaining
interoperability with existing infrastructure. For applications that
want to use non-ASCII characters in public DNS domain names, IDNA is
the only option that is defined at the time this specification is
published. Adding IDNA support to an existing application entails
changes to the application only, and leaves room for flexibility in
front-end processing and more specifically in the user interface (see
Section 6).
A great deal of the discussion of IDN solutions has focused on
transition issues and how IDNs will work in a world where not all of
the components have been updated. Proposals that were not chosen by
the original IDN Working Group would have depended on updating user
applications, DNS resolvers, and DNS servers in order for a user to
apply an internationalized domain name in any form or coding
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acceptable under that method. While processing must be performed
prior to or after access to the DNS, IDNA requires no changes to the
DNS protocol, any DNS servers, or the resolvers on users' computers.
IDNA allows the graceful introduction of IDNs not only by avoiding
upgrades to existing infrastructure (such as DNS servers and mail
transport agents), but also by allowing some limited use of IDNs in
applications by using the ASCII-encoded representation of the labels
containing non-ASCII characters. While such names are user-
unfriendly to read and type, and hence not optimal for user input,
they can be used as a last resort to allow rudimentary IDN usage.
For example, they might be the best choice for display if it were
known that relevant fonts were not available on the user's computer.
In order to allow user-friendly input and output of the IDNs and
acceptance of some characters as equivalent to those to be processed
according to the protocol, the applications need to be modified to
conform to this specification.
This version of IDNA uses the Unicode character repertoire for
continuity with the original version of IDNA.
1.5. Comprehensibility of IDNA Mechanisms and Processing
One goal of IDNA2008, which is aided by the main goal of reducing the
dependency on mapping, is to improve the general understanding of how
IDNA works and what characters are permitted and what happens to
them. Comprehensibility and predictability to users and registrants
are important design goals for this effort. End-user applications
have an important role to play in increasing this comprehensibility.
Any system that tries to handle international characters encounters
some common problems. For example, a User Interface (UI) cannot
display a character if no font containing that character is
available. In some cases, internationalization enables effective
localization while maintaining some global uniformity but losing some
universality.
It is difficult to even make suggestions as to how end-user
applications should cope when characters and fonts are not available.
Because display functions are rarely controlled by the types of
applications that would call upon IDNA, such suggestions will rarely
be very effective.
Conversion between local character sets and normalized Unicode, if
needed, is part of this set of user interface issues. Those
conversions introduce complexity in a system that does not use
Unicode as its primary (or only) internal character coding system.
If a label is converted to a local character set that does not have
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all the needed characters, or that uses different character-coding
principles, the user interface program may have to add special logic
to avoid or reduce loss of information.
The major difficulty may lie in accurately identifying the incoming
character set and applying the correct conversion routine. Even more
difficult, the local character coding system could be based on
conceptually different assumptions than those used by Unicode (e.g.,
choice of font encodings used for publications in some Indic
scripts). Those differences may not easily yield unambiguous
conversions or interpretations even if each coding system is
internally consistent and adequate to represent the local language
and script.
IDNA2008 shifts responsibility for character mapping and other
adjustments from the protocol (where it was located in IDNA2003) to
pre-processing before invoking IDNA itself. The intent is that this
change will lead to greater usage of fully-valid A-Labels or U-labels
in display, transit, and storage, which should aid comprehensibility
and predictability. A careful look at pre-processing raises issues
about what that pre-processing should do and at what point
pre-processing becomes harmful; how universally consistent
pre-processing algorithms can be; and how to be compatible with
labels prepared in an IDNA2003 context. Those issues are discussed
in Section 6 and in the Mapping document [IDNA2008-Mapping].
2. Processing in IDNA2008
IDNA2008 separates Domain Name Registration and Lookup in the
protocol specification (RFC 5891, Sections 4 and 5 [RFC 5891]).
Although most steps in the two processes are similar, the separation
reflects current practice in which per-registry (DNS zone)
restrictions and special processing are applied at registration time
but not during lookup. Another significant benefit is that
separation facilitates incremental addition of permitted character
groups to avoid freezing on one particular version of Unicode.
The actual registration and lookup protocols for IDNA2008 are
specified in the Protocol document.
3. Permitted Characters: An Inclusion List
IDNA2008 adopts the inclusion model. A code point is assumed to be
invalid for IDN use unless it is included as part of a Unicode
property-based rule or, in rare cases, included individually by an
exception. When an implementation moves to a new version of Unicode,
the rules may indicate new valid code points.
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This section provides an overview of the model used to establish the
algorithm and character lists of the Tables document [RFC 5892] and
describes the names and applicability of the categories used there.
Note that the inclusion of a character in the PROTOCOL-VALID category
group (Section 3.1.1) does not imply that it can be used
indiscriminately; some characters are associated with contextual
rules that must be applied as well.
The information given in this section is provided to make the rules,
tables, and protocol easier to understand. The normative generating
rules that correspond to this informal discussion appear in the
Tables document, and the rules that actually determine what labels
can be registered or looked up are in the Protocol document.
3.1. A Tiered Model of Permitted Characters and Labels
Moving to an inclusion model involves a new specification for the
list of characters that are permitted in IDNs. In IDNA2003,
character validity is independent of context and fixed forever (or
until the standard is replaced). However, globally context-
independent rules have proved to be impractical because some
characters, especially those that are called "Join_Controls" in
Unicode, are needed to make reasonable use of some scripts but have
no visible effect in others. IDNA2003 prohibited those types of
characters entirely by discarding them. We now have a consensus that
under some conditions, these "joiner" characters are legitimately
needed to allow useful mnemonics for some languages and scripts. In
general, context-dependent rules help deal with characters (generally
characters that would otherwise be prohibited entirely) that are used
differently or perceived differently across different scripts, and
allow the standard to be applied more appropriately in cases where a
string is not universally handled the same way.
IDNA2008 divides all possible Unicode code points into four
categories: PROTOCOL-VALID, CONTEXTUAL RULE REQUIRED, DISALLOWED, and
UNASSIGNED.
3.1.1. PROTOCOL-VALID
Characters identified as PROTOCOL-VALID (often abbreviated PVALID)
are permitted in IDNs. Their use may be restricted by rules about
the context in which they appear or by other rules that apply to the
entire label in which they are to be embedded. For example, any
label that contains a character in this category that has a
"right-to-left" property must be used in context with the Bidi rules
[RFC 5893]. 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
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characters in the category. Registries are still expected to apply
judgment about labels they will accept and to maintain rules
consistent with those judgments (see the Protocol document [RFC 5891]
and Section 3.3).
Characters that are placed in the PROTOCOL-VALID category are
expected to never be removed from it or reclassified. While
theoretically characters could be removed from Unicode, such removal
would be inconsistent with the Unicode stability principles (see
UTR 39: Unicode Security Mechanisms [Unicode52], Appendix F) and
hence should never occur.
3.1.2. CONTEXTUAL RULE REQUIRED
Some characters may be unsuitable for general use in IDNs but
necessary for the plausible support of some scripts. The two most
commonly cited examples are the ZERO WIDTH JOINER and ZERO WIDTH
NON-JOINER characters (ZWJ, U+200D and ZWNJ, U+200C), but other
characters may require special treatment because they would otherwise
be DISALLOWED (typically because Unicode considers them punctuation
or special symbols) but need to be permitted in limited contexts.
Other characters are given this special treatment because they pose
exceptional danger of being used to produce misleading labels or to
cause unacceptable ambiguity in label matching and interpretation.
3.1.2.1. Contextual Restrictions
Characters with contextual restrictions are identified as CONTEXTUAL
RULE REQUIRED and are associated with a rule. The rule defines
whether the character is valid in a particular string, and also
whether the rule itself is to be applied on lookup as well as
registration.
A distinction is made between characters that indicate or prohibit
joining and ones similar to them (known as CONTEXT-JOINER or
CONTEXTJ) and other characters requiring contextual treatment
(CONTEXT-OTHER or CONTEXTO). Only the former require full testing at
lookup time.
It is important to note that these contextual rules cannot prevent
all uses of the relevant characters that might be confusing or
problematic. What they are expected to do is to confine
applicability of the characters to scripts (and narrower contexts)
where zone administrators are knowledgeable enough about the use of
those characters to be prepared to deal with them appropriately.
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For example, a registry dealing with an Indic script that requires
ZWJ and/or ZWNJ as part of the writing system is expected to
understand where the characters have visible effect and where they do
not and to make registration rules accordingly. By contrast, a
registry dealing primarily with Latin or Cyrillic script might not be
actively aware that the characters exist, much less about the
consequences of embedding them in labels drawn from those scripts and
therefore should avoid accepting registrations containing those
characters, at least in labels using characters from the Latin or
Cyrillic scripts.
3.1.2.2. Rules and Their Application
Rules have descriptions such as "Must follow a character from Script
XYZ", "Must occur only if the entire label is in Script ABC", or
"Must occur only if the previous and subsequent characters have the
DFG property". The actual rules may be DEFINED or NULL. If present,
they may have values of "True" (character may be used in any position
in any label), "False" (character may not be used in any label), or
may be a set of procedural rules that specify the context in which
the character is permitted.
Because it is easier to identify these characters than to know that
they are actually needed in IDNs or how to establish exactly the
right rules for each one, a rule may have a null value in a given
version of the tables. Characters associated with null rules are not
permitted to appear in putative labels for either registration or
lookup. Of course, a later version of the tables might contain a
non-null rule.
The actual rules and their descriptions are in Sections 2 and 3 of
the Tables document [RFC 5892]. That document also specifies the
creation of a registry for future rules.
3.1.3. DISALLOWED
Some characters are inappropriate for use in IDNs and are thus
excluded for both registration and lookup (i.e., IDNA-conforming
applications performing name lookup should verify that these
characters are absent; if they are present, the label strings should
be rejected rather than converted to A-labels and looked up. Some of
these characters are problematic for use in IDNs (such as the
FRACTION SLASH character, U+2044), while some of them (such as the
various HEART symbols, e.g., U+2665, U+2661, and U+2765, see
Section 7.6) simply fall outside the conventions for typical
identifiers (basically letters and numbers).
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Of course, this category would include code points that had been
removed entirely from Unicode should such removals ever occur.
Characters that are placed in the DISALLOWED category are expected to
never be removed from it or reclassified. If a character is
classified as DISALLOWED in error and the error is sufficiently
problematic, the only recourse would be either to introduce a new
code point into Unicode and classify it as PROTOCOL-VALID or for the
IETF to accept the considerable costs of an incompatible change and
replace the relevant RFC with one containing appropriate exceptions.
There is provision for exception cases but, in general, characters
are placed into DISALLOWED if they fall into one or more of the
following groups:
o The character is a compatibility equivalent for another character.
In slightly more precise Unicode terms, application of
Normalization Form KC (NFKC) to the character yields some other
character.
o The character is an uppercase form or some other form that is
mapped to another character by Unicode case folding.
o The character is a symbol or punctuation form or, more generally,
something that is not a letter, digit, or a mark that is used to
form a letter or digit.
3.1.4. UNASSIGNED
For convenience in processing and table-building, code points that do
not have assigned values in a given version of Unicode are treated as
belonging to a special UNASSIGNED category. Such code points are
prohibited in labels to be registered or looked up. The category
differs from DISALLOWED in that code points are moved out of it by
the simple expedient of being assigned in a later version of Unicode
(at which point, they are classified into one of the other categories
as appropriate).
The rationale for restricting the processing of UNASSIGNED characters
is simply that the properties of such code points cannot be
completely known until actual characters are assigned to them. For
example, assume that an UNASSIGNED code point were included in a
label to be looked up. Assume that the code point was later assigned
to a character that required some set of contextual rules. With that
combination, un-updated instances of IDNA-aware software might permit
lookup of labels containing the previously unassigned characters
while updated versions of the software might restrict use of the same
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label in lookup, depending on the contextual rules. It should be
clear that under no circumstance should an UNASSIGNED character be
permitted in a label to be registered as part of a domain name.
3.2. Registration Policy
While these recommendations cannot and should not define registry
policies, registries should develop and apply additional restrictions
as needed to reduce confusion and other problems. For example, it is
generally believed that labels containing characters from more than
one script are a bad practice although there may be some important
exceptions to that principle. Some registries may choose to restrict
registrations to characters drawn from a very small number of
scripts. For many scripts, the use of variant techniques such as
those as described in the JET specification for the CJK script
[RFC 3743] and its generalization [RFC 4290], and illustrated for
Chinese by the tables provided by the Chinese Domain Name Consortium
[RFC 4713] may be helpful in reducing problems that might be perceived
by users.
In general, users will benefit if registries only permit characters
from scripts that are well-understood by the registry or its
advisers. If a registry decides to reduce opportunities for
confusion by constructing policies that disallow characters used in
historic writing systems or characters whose use is restricted to
specialized, highly technical contexts, some relevant information may
be found in Section 2.4 (Specific Character Adjustments) of Unicode
Identifier and Pattern Syntax [Unicode-UAX31], especially Table 4
(Candidate Characters for Exclusion from Identifiers), and Section
3.1 (General Security Profile for Identifiers) in Unicode Security
Mechanisms [Unicode-UTS39].
The requirement (in Section 4.1 of the Protocol document [RFC 5891])
that registration procedures use only U-labels and/or A-labels is
intended to ensure that registrants are fully aware of exactly what
is being registered as well as encouraging use of those canonical
forms. That provision should not be interpreted as requiring that
registrants need to provide characters in a particular code sequence.
Registrant input conventions and management are part of registrant-
registrar interactions and relationships between registries and
registrars and are outside the scope of these standards.
It is worth stressing that these principles of policy development and
application apply at all levels of the DNS, not only, e.g., top level
domain (TLD) or second level domain (SLD) registrations. Even a
trivial, "anything is permitted that is valid under the protocol"
policy is helpful in that it helps users and application developers
know what to expect.
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3.3. Layered Restrictions: Tables, Context, Registration, and
Applications
The character rules in IDNA2008 are based on the realization that
there is no single magic bullet for any of the security,
confusability, or other issues associated with IDNs. Instead, the
specifications define a variety of approaches. The character tables
are the first mechanism, protocol rules about how those characters
are applied or restricted in context are the second, and those two in
combination constitute the limits of what can be done in the
protocol. As discussed in the previous section (Section 3.2),
registries are expected to restrict what they permit to be
registered, devising and using rules that are designed to optimize
the balance between confusion and risk on the one hand and maximum
expressiveness in mnemonics on the other.
In addition, there is an important role for user interface programs
in warning against label forms that appear problematic given their
knowledge of local contexts and conventions. Of course, no approach
based on naming or identifiers alone can protect against all threats.
4. Application-Related Issues
4.1. Display and Network Order
Domain names are always transmitted in network order (the order in
which the code points are sent in protocols), but they may have a
different display order (the order in which the code points are
displayed on a screen or paper). When a domain name contains
characters that are normally written right to left, display order may
be affected although network order is not. It gets even more
complicated if left-to-right and right-to-left labels are adjacent to
each other within a domain name. The decision about the display
order is ultimately under the control of user agents -- including Web
browsers, mail clients, hosted Web applications and many more --
which may be highly localized. Should a domain name abc.def, in
which both labels are represented in scripts that are written right
to left, be displayed as fed.cba or cba.fed? Applications that are
in deployment today are already diverse, and one can find examples of
either choice.
The picture changes once again when an IDN appears in an
Internationalized Resource Identifier (IRI) [RFC 3987]. An IRI or
internationalized email address contains elements other than the
domain name. For example, IRIs contain protocol identifiers and
field delimiter syntax such as "http://" or "mailto:" while email
addresses contain the "@" to separate local parts from domain names.
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An IRI in network order begins with "http://" followed by domain
labels in network order, thus "http://abc.def".
User interface programs are not required to display and allow input
of IRIs directly but often do so. Implementers have to choose
whether the overall direction of these strings will always be left to
right (or right to left) for an IRI or email address. The natural
order for a user typing a domain name on a right-to-left system is
fed.cba. Should the right-to-left (RTL) user interface reverse the
entire domain name each time a domain name is typed? Does this
change if the user types "http://" right before typing a domain name,
thus implying that the user is beginning at the beginning of the
network-order IRI? Experience in the 1980s and 1990s with mixing
systems in which domain name labels were read in network order (left
to right) and those in which those labels were read right to left
would predict a great deal of confusion.
If each implementation of each application makes its own decisions on
these issues, users will develop heuristics that will sometimes fail
when switching applications. However, while some display order
conventions, voluntarily adopted, would be desirable to reduce
confusion, such suggestions are beyond the scope of these
specifications.
4.2. Entry and Display in Applications
Applications can accept and display domain names using any character
set or character coding system. The IDNA protocol does not
necessarily affect the interface between users and applications. An
IDNA-aware application can accept and display internationalized
domain names in two formats: as the internationalized character
set(s) supported by the application (i.e., an appropriate local
representation of a U-label) and as an A-label. Applications may
allow the display of A-labels, but are encouraged not to do so except
as an interface for special purposes, possibly for debugging, or to
cope with display limitations. In general, they should allow, but
not encourage, user input of A-labels. A-labels are opaque and ugly,
and malicious variations on them are not easily detected by users.
Where possible, they should thus only be exposed when they are
absolutely needed. Because IDN labels can be rendered either as
A-labels or U-labels, the application may reasonably have an option
for the user to select the preferred method of display. Rendering
the U-label should normally be the default.
Domain names are often stored and transported in many places. For
example, they are part of documents such as mail messages and web
pages. They are transported in many parts of many protocols, such as
both the control commands of SMTP and associated message body parts,
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and in the headers and the body content in HTTP. It is important to
remember that domain names appear both in domain name slots and in
the content that is passed over protocols, and it would be helpful if
protocols explicitly define what their domain name slots are.
In protocols and document formats that define how to handle
specification or negotiation of charsets, labels can be encoded in
any charset allowed by the protocol or document format. If a
protocol or document format only allows one charset, the labels must
be given in that charset. Of course, not all charsets can properly
represent all labels. If a U-label cannot be displayed in its
entirety, the only choice (without loss of information) may be to
display the A-label.
Where a protocol or document format allows IDNs, labels should be in
whatever character encoding and escape mechanism the protocol or
document format uses in the local environment. This provision is
intended to prevent situations in which, e.g., UTF-8 domain names
appear embedded in text that is otherwise in some other character
coding.
All protocols that use domain name slots (see Section 2.3.2.6 in the
Definitions document [RFC 5890]) already have the capacity for
handling domain names in the ASCII charset. Thus, A-labels can
inherently be handled by those protocols.
IDNA2008 does not specify required mappings between one character or
code point and others. An extended discussion of mapping issues
appears in Section 6 and specific recommendations appear in the
Mapping document [IDNA2008-Mapping]. In general, IDNA2008 prohibits
characters that would be mapped to others by normalization or other
rules. As examples, while mathematical characters based on Latin
ones are accepted as input to IDNA2003, they are prohibited in
IDNA2008. Similarly, uppercase characters, double-width characters,
and other variations are prohibited as IDNA input although mapping
them as needed in user interfaces is strongly encouraged.
Since the rules in the Tables document [RFC 5892] have the effect that
only strings that are not transformed by NFKC are valid, if an
application chooses to perform NFKC normalization before lookup, that
operation is safe since this will never make the application unable
to look up any valid string. However, as discussed above, the
application cannot guarantee that any other application will perform
that mapping, so it should be used only with caution and for informed
users.
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In many cases, these prohibitions should have no effect on what the
user can type as input to the lookup process. It is perfectly
reasonable for systems that support user interfaces to perform some
character mapping that is appropriate to the local environment. This
would normally be done prior to actual invocation of IDNA. At least
conceptually, the mapping would be part of the Unicode conversions
discussed above and in the Protocol document [RFC 5891]. However,
those changes will be local ones only -- local to environments in
which users will clearly understand that the character forms are
equivalent. For use in interchanges among systems, it appears to be
much more important that U-labels and A-labels can be mapped back and
forth without loss of information.
One specific, and very important, instance of this strategy arises
with case folding. In the ASCII-only DNS, names are looked up and
matched in a case-independent way, but no actual case folding occurs.
Names can be placed in the DNS in either uppercase or lowercase form
(or any mixture of them) and that form is preserved, returned in
queries, and so on. IDNA2003 approximated that behavior for
non-ASCII strings by performing case folding at registration time
(resulting in only lowercase IDNs in the DNS) and when names were
looked up.
As suggested earlier in this section, it appears to be desirable to
do as little character mapping as possible as long as Unicode works
correctly (e.g., Normalization Form C (NFC) mapping to resolve
different codings for the same character is still necessary although
the specifications require that it be performed prior to invoking the
protocol) in order to make the mapping between A-labels and U-labels
idempotent. Case mapping is not an exception to this principle. If
only lowercase characters can be registered in the DNS (i.e., be
present in a U-label), then IDNA2008 should prohibit uppercase
characters as input even though user interfaces to applications
should probably map those characters. Some other considerations
reinforce this conclusion. For example, in ASCII case mapping for
individual characters, uppercase(character) is always equal to
uppercase(lowercase(character)). That may not be true with IDNs. In
some scripts that use case distinctions, there are a few characters
that do not have counterparts in one case or the other. The
relationship between uppercase and lowercase may even be language-
dependent, with different languages (or even the same language in
different areas) expecting different mappings. User interface
programs can meet the expectations of users who are accustomed to the
case-insensitive DNS environment by performing case folding prior to
IDNA processing, but the IDNA procedures themselves should neither
require such mapping nor expect them when they are not natural to the
localized environment.
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4.3. Linguistic Expectations: Ligatures, Digraphs, and Alternate
Character Forms
Users have expectations about character matching or equivalence that
are based on their own languages and the orthography of those
languages. These expectations may not always be met in a global
system, especially if multiple languages are written using the same
script but using different conventions. Some examples:
o A Norwegian user might expect a label with the ae-ligature to be
treated as the same label as one using the Swedish spelling with
a-diaeresis even though applying that mapping to English would be
astonishing to users.
o A German user might expect a label with an o-umlaut and a label
that had "oe" substituted, but was otherwise the same, to be
treated as equivalent even though that substitution would be a
clear error in Swedish.
o A Chinese user might expect automatic matching of Simplified and
Traditional Chinese characters, but applying that matching for
Korean or Japanese text would create considerable confusion.
o An English user might expect "theater" and "theatre" to match.
A number of languages use alphabetic scripts in which single phonemes
are written using two characters, termed a "digraph", for example,
the "ph" in "pharmacy" and "telephone". (Such characters can also
appear consecutively without forming a digraph, as in "tophat".)
Certain digraphs may be indicated typographically by setting the two
characters closer together than they would be if used consecutively
to represent different phonemes. Some digraphs are fully joined as
ligatures. For example, the word "encyclopaedia" is sometimes set
with a U+00E6 LATIN SMALL LIGATURE AE. When ligature and digraph
forms have the same interpretation across all languages that use a
given script, application of Unicode normalization generally resolves
the differences and causes them to match. When they have different
interpretations, matching must utilize other methods, presumably
chosen at the registry level, or users must be educated to understand
that matching will not occur.
The nature of the problem can be illustrated by many words in the
Norwegian language, where the "ae" ligature is the 27th letter of a
29-letter extended Latin alphabet. It is equivalent to the 28th
letter of the Swedish alphabet (also containing 29 letters),
U+00E4 LATIN SMALL LETTER A WITH DIAERESIS, for which an "ae" cannot
be substituted according to current orthographic standards. That
character (U+00E4) is also part of the German alphabet where, unlike
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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.
Matching and comparison algorithm selection often requires
information about the language being used, context, or both --
information that is not available to IDNA or the DNS. Consequently,
IDNA2008 makes no attempt to treat combined characters in any special
way. A registry that is aware of the language context in which
labels are to be registered, and where that language sometimes (or
always) treats the two-character sequences as equivalent to the
combined form, should give serious consideration to applying a
"variant" model [RFC 3743][RFC 4290] or to prohibiting registration of
one of the forms entirely, to reduce the opportunities for user
confusion and fraud that would result from the related strings being
registered to different parties.
4.4. Case Mapping and Related Issues
In the DNS, ASCII letters are stored with their case preserved.
Matching during the query process is case-independent, but none of
the information that might be represented by choices of case has been
lost. That model has been accidentally helpful because, as people
have created DNS labels by catenating words (or parts of words) to
form labels, case has often been used to distinguish among components
and make the labels more memorable.
Since DNS servers do not get involved in parsing IDNs, they cannot do
case-independent matching. Thus, keeping the cases separate in
lookup or registration, and doing matching at the server, is not
feasible with IDNA or any similar approach. Matching of characters
that are considered to differ only by case must be done, if desired,
by programs invoking IDNA lookup even though it wasn't done by ASCII-
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only DNS clients. That situation was recognized in IDNA2003 and
nothing in IDNA2008 fundamentally changes it or could do so. In
IDNA2003, all characters are case folded and mapped by clients in a
standardized step.
Even in scripts that generally support case distinctions, some
characters do not have uppercase forms. For example, the Unicode
case-folding operation maps Greek Final Form Sigma (U+03C2) to the
medial form (U+03C3) and maps Eszett (German Sharp S, U+00DF) to
"ss". Neither of these mappings is reversible because the uppercase
of U+03C3 is the uppercase Sigma (U+03A3) and "ss" is an ASCII
string. IDNA2008 permits, at the risk of some incompatibility,
slightly more flexibility in this area by avoiding case folding and
treating these characters as themselves. Approaches to handling one-
way mappings are discussed in Section 7.2.
Because IDNA2003 maps Final Sigma and Eszett to other characters, and
the reverse mapping is never possible, neither Final Sigma nor Eszett
can be represented in the ACE form of IDNA2003 IDN nor in the native
character (U-label) form derived from it. With IDNA2008, both
characters can be used in an IDN and so the A-label used for lookup
for any U-label containing those characters is now different. See
Section 7.1 for a discussion of what kinds of changes might require
the IDNA prefix to change; after extended discussions, the IDNABIS
Working Group came to consensus that the change for these characters
did not justify a prefix change.
4.5. Right-to-Left Text
In order to be sure that the directionality of right-to-left text is
unambiguous, IDNA2003 required that any label in which right-to-left
characters appear both starts and ends with them and that it does not
include any characters with strong left-to-right properties (that
excludes other alphabetic characters but permits European digits).
Any other string that contains a right-to-left character and does not
meet those requirements is rejected. This is one of the few places
where the IDNA algorithms (both in IDNA2003 and in IDNA2008) examine
an entire label, not just individual characters. The algorithmic
model used in IDNA2003 rejects the label when the final character in
a right-to-left string requires a combining mark in order to be
correctly represented.
That prohibition is not acceptable for writing systems for languages
written with consonantal alphabets to which diacritical vocalic
systems are applied, and for languages with orthographies derived
from them where the combining marks may have different functionality.
In both cases, the combining marks can be essential components of the
orthography. Examples of this are Yiddish, written with an extended
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Hebrew script, and Dhivehi (the official language of Maldives), which
is written in the Thaana script (which is, in turn, derived from the
Arabic script). IDNA2008 removes the restriction on final combining
characters with a new set of rules for right-to-left scripts and
their characters. Those new rules are specified in the Bidi document
[RFC 5893].
5. IDNs and the Robustness Principle
The "Robustness Principle" is often stated as "Be conservative about
what you send and liberal in what you accept" (see, e.g., Section
1.2.2 of the applications-layer Host Requirements specification
[RFC 1123]). This principle applies to IDNA. In applying the
principle to registries as the source ("sender") of all registered
and useful IDNs, registries are responsible for being conservative
about what they register and put out in the Internet. For IDNs to
work well, zone administrators (registries) must have and require
sensible policies about what is registered -- conservative policies
-- and implement and enforce them.
Conversely, lookup applications are expected to reject labels that
clearly violate global (protocol) rules (no one has ever seriously
claimed that being liberal in what is accepted requires being
stupid). However, once one gets past such global rules and deals
with anything sensitive to script or locale, it is necessary to
assume that garbage has not been placed into the DNS, i.e., one must
be liberal about what one is willing to look up in the DNS rather
than guessing about whether it should have been permitted to be
registered.
If a string cannot be successfully found in the DNS after the lookup
processing described here, it makes no difference whether it simply
wasn't registered or was prohibited by some rule at the registry.
Application implementers should be aware that where DNS wildcards are
used, the ability to successfully resolve a name does not guarantee
that it was actually registered.
6. Front-end and User Interface Processing for Lookup
Domain names may be identified and processed in many contexts. They
may be typed in by users themselves or embedded in an identifier such
as an email address, URI, or IRI. They may occur in running text or
be processed by one system after being provided in another. Systems
may try to normalize URLs to determine (or guess) whether a reference
is valid or if two references point to the same object without
actually looking the objects up (comparison without lookup is
necessary for URI types that are not intended to be resolved). Some
of these goals may be more easily and reliably satisfied than others.
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While there are strong arguments for any domain name that is placed
"on the wire" -- transmitted between systems -- to be in the zero-
ambiguity forms of A-labels, it is inevitable that programs that
process domain names will encounter U-labels or variant forms.
An application that implements the IDNA protocol [RFC 5891] will
always take any user input and convert it to a set of Unicode code
points. That user input may be acquired by any of several different
input methods, all with differing conversion processes to be taken
into consideration (e.g., typed on a keyboard, written by hand onto
some sort of digitizer, spoken into a microphone and interpreted by a
speech-to-text engine, etc.). The process of taking any particular
user input and mapping it into a Unicode code point may be a simple
one: if a user strikes the "A" key on a US English keyboard, without
any modifiers such as the "Shift" key held down, in order to draw a
Latin small letter A ("a"), many (perhaps most) modern operating
system input methods will produce to the calling application the code
point U+0061, encoded in a single octet.
Sometimes the process is somewhat more complicated: a user might
strike a particular set of keys to represent a combining macron
followed by striking the "A" key in order to draw a Latin small
letter A with a macron above it. Depending on the operating system,
the input method chosen by the user, and even the parameters with
which the application communicates with the input method, the result
might be the code point U+0101 (encoded as two octets in UTF-8 or
UTF-16, four octets in UTF-32, etc.), the code point U+0061 followed
by the code point U+0304 (again, encoded in three or more octets,
depending upon the encoding used) or even the code point U+FF41
followed by the code point U+0304 (and encoded in some form). These
examples leave aside the issue of operating systems and input methods
that do not use Unicode code points for their character set.
In every case, applications (with the help of the operating systems
on which they run and the input methods used) need to perform a
mapping from user input into Unicode code points.
IDNA2003 used a model whereby input was taken from the user, mapped
(via whatever input method mechanisms were used) to a set of Unicode
code points, and then further mapped to a set of Unicode code points
using the Nameprep profile [RFC 3491]. In this procedure, there are
two separate mapping steps: first, a mapping done by the input method
(which might be controlled by the operating system, the application,
or some combination) and then a second mapping performed by the
Nameprep portion of the IDNA protocol. The mapping done in Nameprep
includes a particular mapping table to re-map some characters to
other characters, a particular normalization, and a set of prohibited
characters.
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Note that the result of the two-step mapping process means that the
mapping chosen by the operating system or application in the first
step might differ significantly from the mapping supplied by the
Nameprep profile in the second step. This has advantages and
disadvantages. Of course, the second mapping regularizes what gets
looked up in the DNS, making for better interoperability between
implementations that use the Nameprep mapping. However, the
application or operating system may choose mappings in their input
methods, which when passed through the second (Nameprep) mapping
result in characters that are "surprising" to the end user.
The other important feature of IDNA2003 is that, with very few
exceptions, it assumes that any set of Unicode code points provided
to the Nameprep mapping can be mapped into a string of Unicode code
points that are "sensible", even if that means mapping some code
points to nothing (that is, removing the code points from the
string). This allowed maximum flexibility in input strings.
The present version of IDNA (IDNA2008) differs significantly in
approach from the original version. First and foremost, it does not
provide explicit mapping instructions. Instead, it assumes that the
application (perhaps via an operating system input method) will do
whatever mapping it requires to convert input into Unicode code
points. This has the advantage of giving flexibility to the
application to choose a mapping that is suitable for its user given
specific user requirements, and avoids the two-step mapping of the
original protocol. Instead of a mapping, IDNA2008 provides a set of
categories that can be used to specify the valid code points allowed
in a domain name.
In principle, an application ought to take user input of a domain
name and convert it to the set of Unicode code points that represent
the domain name the user intends. As a practical matter, of course,
determining user intent is a tricky business, so an application needs
to choose a reasonable mapping from user input. That may differ
based on the particular circumstances of a user, depending on locale,
language, type of input method, etc. It is up to the application to
make a reasonable choice.
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7. Migration from IDNA2003 and Unicode Version Synchronization
7.1. Design Criteria
As mentioned above and in the IAB review and recommendations for IDNs
[RFC 4690], two key goals of the IDNA2008 design are:
o to enable applications to be agnostic about whether they are being
run in environments supporting any Unicode version from 3.2
onward.
o to permit incrementally adding new characters, character groups,
scripts, and other character collections as they are incorporated
into Unicode, doing so without disruption and, in the long term,
without "heavy" processes (an IETF consensus process is required
by the IDNA2008 specifications and is expected to be required and
used until significant experience accumulates with IDNA operations
and new versions of Unicode).
7.1.1. Summary and Discussion of IDNA Validity Criteria
The general criteria for a label to be considered valid under IDNA
are (the actual rules are rigorously defined in the Protocol
[RFC 5891] and Tables [RFC 5892] documents):
o The characters are "letters", marks needed to form letters,
numerals, or other code points used to write words in some
language. Symbols, drawing characters, and various notational
characters are intended to be permanently excluded. There is no
evidence that they are important enough to Internet operations or
internationalization to justify expansion of domain names beyond
the general principle of "letters, digits, and hyphen".
(Additional discussion and rationale for the symbol decision
appears in Section 7.6.)
o Other than in very exceptional cases, e.g., where they are needed
to write substantially any word of a given language, punctuation
characters are excluded. The fact that a word exists is not proof
that it should be usable in a DNS label, and DNS labels are not
expected to be usable for multiple-word phrases (although they are
certainly not prohibited if the conventions and orthography of a
particular language cause that to be possible).
o Characters that are unassigned (have no character assignment at
all) in the version of Unicode being used by the registry or
application are not permitted, even on lookup. The issues
involved in this decision are discussed in Section 7.7.
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o Any character that is mapped to another character by a current
version of NFKC is prohibited as input to IDNA (for either
registration or lookup). With a few exceptions, this principle
excludes any character mapped to another by Nameprep [RFC 3491].
The principles above drive the design of rules that are specified
exactly in the Tables document. Those rules identify the characters
that are valid under IDNA. The rules themselves are normative, and
the tables are derived from them, rather than vice versa.
7.1.2. Labels in Registration
Any label registered in a DNS zone must be validated -- i.e., the
criteria for that label must be met -- in order for applications to
work as intended. This principle is not new. For example, since the
DNS was first deployed, zone administrators have been expected to
verify that names meet "hostname" requirements [RFC 952] where those
requirements are imposed by the expected applications. Other
applications contexts, such as the later addition of special service
location formats [RFC 2782] imposed new requirements on zone
administrators. For zones that will contain IDNs, support for
Unicode version-independence requires restrictions on all strings
placed in the zone. In particular, for such zones (the exact rules
appear in Section 4 of the Protocol document [RFC 5891]):
o Any label that appears to be an A-label, i.e., any label that
starts in "xn--", must be valid under IDNA, i.e., they must be
valid A-labels, as discussed in Section 2 above.
o The Unicode tables (i.e., tables of code points, character
classes, and properties) and IDNA tables (i.e., tables of
contextual rules such as those that appear in the Tables
document), must be consistent on the systems performing or
validating labels to be registered. Note that this does not
require that tables reflect the latest version of Unicode, only
that all tables used on a given system are consistent with each
other.
Under this model, registry tables will need to be updated (both the
Unicode-associated tables and the tables of permitted IDN characters)
to enable a new script or other set of new characters. The registry
will not be affected by newer versions of Unicode, or newly
authorized characters, until and unless it wishes to support them.
The zone administrator is responsible for verifying validity for IDNA
as well as its local policies -- a more extensive set of checks than
are required for looking up the labels. Systems looking up or
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resolving DNS labels, especially IDN DNS labels, must be able to
assume that applicable registration rules were followed for names
entered into the DNS.
7.1.3. Labels in Lookup
Any application processing a label through IDNA so it can be looked
up in a DNS zone is required to (the exact rules appear in Section 5
of the Protocol document [RFC 5891]):
o Maintain IDNA and Unicode tables that are consistent with regard
to versions, i.e., unless the application actually executes the
classification rules in the Tables document [RFC 5892], its IDNA
tables must be derived from the version of Unicode that is
supported more generally on the system. As with registration, the
tables need not reflect the latest version of Unicode, but they
must be consistent.
o Validate the characters in labels to be looked up only to the
extent of determining that the U-label does not contain
"DISALLOWED" code points or code points that are unassigned in its
version of Unicode.
o Validate the label itself for conformance with a small number of
whole-label rules. In particular, it must verify that:
* there are no leading combining marks,
* the Bidi conditions are met if right-to-left characters appear,
* any required contextual rules are available, and
* any contextual rules that are associated with joiner characters
(and CONTEXTJ characters more generally) are tested.
o Do not reject labels based on other contextual rules about
characters, including mixed-script label prohibitions. Such rules
may be used to influence presentation decisions in the user
interface, but not to avoid looking up domain names.
To further clarify the rules about handling characters that require
contextual rules, note that one can have a context-required character
(i.e., one that requires a rule), but no rule. In that case, the
character is treated the same way DISALLOWED characters are treated,
until and unless a rule is supplied. That state is more or less
equivalent to "the idea of permitting this character is accepted in
principle, but it won't be permitted in practice until consensus is
reached on a safe way to use it".
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The ability to add a rule more or less exempts these characters from
the prohibition against reclassifying characters from DISALLOWED to
PVALID.
And, obviously, "no rule" is different from "have a rule, but the
test either succeeds or fails".
Lookup applications that follow these rules, rather than having their
own criteria for rejecting lookup attempts, are not sensitive to
version incompatibilities with the particular zone registry
associated with the domain name except for labels containing
characters recently added to Unicode.
An application or client that processes names according to this
protocol and then resolves them in the DNS will be able to locate any
name that is registered, as long as those registrations are valid
under IDNA and its version of the IDNA tables is sufficiently up to
date to interpret all of the characters in the label. Messages to
users should distinguish between "label contains an unallocated code
point" and other types of lookup failures. A failure on the basis of
an old version of Unicode may lead the user to a desire to upgrade to
a newer version, but will have no other ill effects (this is
consistent with behavior in the transition to the DNS when some hosts
could not yet handle some forms of names or record types).
7.2. Changes in Character Interpretations
As a consequence of the elimination of mapping, the current version
of IDNA changes the interpretation of a few characters relative to
its predecessors. This subsection outlines the issues and discusses
possible transition strategies.
7.2.1. Character Changes: Eszett and Final Sigma
In those scripts that make case distinctions, there are a few
characters for which an obvious and unique uppercase character has
not historically been available to match a lowercase one, or vice
versa. For those characters, the mappings used in constructing the
Stringprep tables for IDNA2003, performed using the Unicode
toCaseFold operation (see Section 5.18 of the Unicode Standard
[Unicode52]), generate different characters or sets of characters.
Those operations are not reversible and lose even more information
than traditional uppercase or lowercase transformations, but are more
useful than those transformations for comparison purposes. Two
notable characters of this type are the German character Eszett
(Sharp S, U+00DF) and the Greek Final Form Sigma (U+03C2). The
former is case folded to the ASCII string "ss", the latter to a
medial (lowercase) Sigma (U+03C3).
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7.2.2. Character Changes: Zero Width Joiner and Zero Width Non-Joiner
IDNA2003 mapped both ZERO WIDTH JOINER (ZWJ, U+200D) and ZERO WIDTH
NON-JOINER (ZWNJ, U+200C) to nothing, effectively dropping these
characters from any label in which they appeared and treating strings
containing them as identical to strings that did not. As discussed
in Section 3.1.2 above, those characters are essential for writing
many reasonable mnemonics for certain scripts. However, treating
them as valid in IDNA2008, even with contextual restrictions, raises
approximately the same problem as exists with Eszett and Final Sigma:
strings that were valid under IDNA2003 have different interpretations
as labels, and different A-labels, than the same strings under this
newer version.
7.2.3. Character Changes and the Need for Transition
The decision to eliminate mandatory and standardized mappings,
including case folding, from the IDNA2008 protocol in order to make
A-labels and U-labels idempotent made these characters problematic.
If they were to be disallowed, important words and mnemonics could
not be written in orthographically reasonable ways. If they were to
be permitted as distinct characters, there would be no information
loss and registries would have more flexibility, but IDNA2003 and
IDNA2008 lookups might result in different A-labels.
With the understanding that there would be incompatibility either way
but a judgment that the incompatibility was not significant enough to
justify a prefix change, the Working Group concluded that Eszett and
Final Form Sigma should be treated as distinct and Protocol-Valid
characters.
Since these characters are interpreted in different ways under the
older and newer versions of IDNA, transition strategies and policies
will be necessary. Some actions can reasonably be taken by
applications' client programs (those that perform lookup operations
or cause them to be performed), but because of the diversity of
situations and uses of the DNS, much of the responsibility will need
to fall on registries.
Registries, especially those maintaining zones for third parties,
must decide how to introduce a new service in a way that does not
create confusion or significantly weaken or invalidate existing
identifiers. This is not a new problem; registries were faced with
similar issues when IDNs were introduced (potentially, and especially
for Latin-based scripts, in conflict with existing labels that had
been rendered in ASCII characters by applying more or less
standardized conventions) and when other new forms of strings have
been permitted as labels.
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7.2.4. Transition Strategies
There are several approaches to the introduction of new characters or
changes in interpretation of existing characters from their mapped
forms in the earlier version of IDNA. The transition issue is
complicated because the forms of these labels after the
ToUnicode(ToASCII()) translation in IDNA2003 not only remain valid
but do not provide strong indications of what the registrant
intended: a string containing "ss" could have simply been intended to
be that string or could have been intended to contain an Eszett; a
string containing lowercase Sigma could have been intended to contain
Final Sigma (one might make heuristic guesses based on position in a
string, but the long tradition of forming labels by concatenating
words makes such heuristics unreliable), and strings that do not
contain ZWJ or ZWNJ might have been intended to contain them.
Without any preference or claim to completeness, some of these, all
of which have been used by registries in the past for similar
transitions, are:
1. Do not permit use of the newly available character at the
registry level. This might cause lookup failures if a domain
name were to be written with the expectation of the IDNA2003
mapping behavior, but would eliminate any possibility of false
matches.
2. Hold a "sunrise"-like arrangement in which holders of labels
containing "ss" in the Eszett case, lowercase Sigma in that case,
or that might have contained ZWJ or ZWNJ in context, are given
priority (and perhaps other benefits) for registering the
corresponding string containing Eszett, Final Sigma, or the
appropriate zero-width character respectively.
3. Adopt some sort of "variant" approach in which registrants obtain
labels with both character forms.
4. Adopt a different form of "variant" approach in which
registration of additional strings that would produce the same
A-label if interpreted according to IDNA2003 is either not
permitted at all or permitted only by the registrant who already
has one of the names.
5. Ignore the issue and assume that the marketplace or other
mechanisms will sort things out.
In any event, a registry (at any level of the DNS tree) that chooses
to permit labels to be registered that contains these characters, or
considers doing so, will have to address the relationship with
existing, possibly conflicting, labels in some way, just as
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registries that already had a considerable number of labels did when
IDNs were first introduced.
7.3. Elimination of Character Mapping
As discussed at length in Section 6, IDNA2003, via Nameprep (see
Section 7.5), mapped many characters into related ones. Those
mappings no longer exist as requirements in IDNA2008. These
specifications strongly prefer that only A-labels or U-labels be used
in protocol contexts and as much as practical more generally.
IDNA2008 does anticipate situations in which some mapping at the time
of user input into lookup applications is appropriate and desirable.
The issues are discussed in Section 6 and specific recommendations
are made in the Mapping document [IDNA2008-Mapping].
7.4. The Question of Prefix Changes
The conditions that would have required a change in the IDNA ACE
prefix ("xn--", used in IDNA2003) were of great concern to the
community. A prefix change would have clearly been necessary if the
algorithms were modified in a manner that would have created serious
ambiguities during subsequent transition in registrations. This
section summarizes the working group's conclusions about the
conditions under which a change in the prefix would have been
necessary and the implications of such a change.
7.4.1. Conditions Requiring a Prefix Change
An IDN prefix change would have been needed if a given string would
be looked up or otherwise interpreted differently depending on the
version of the protocol or tables being used. This IDNA upgrade
would have required a prefix change if, and only if, one of the
following four conditions were met:
1. The conversion of an A-label to Unicode (i.e., a U-label) would
have yielded one string under IDNA2003 and a different string
under IDNA2008.
2. In a significant number of cases, an input string that was valid
under IDNA2003 and also valid under IDNA2008 would have yielded
two different A-labels with the different versions. This
condition is believed to be essentially equivalent to the one
above except for a very small number of edge cases that were not
found to justify a prefix change (see Section 7.2).
Note that if the input string was valid under one version and not
valid under the other, this condition would not apply. See the
first item in Section 7.4.2, below.
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3. A fundamental change was made to the semantics of the string that
would be inserted in the DNS, e.g., if a decision were made to
try to include language or script information in the encoding in
addition to the string itself.
4. A sufficiently large number of characters were added to Unicode
so that the Punycode mechanism for block offsets would no longer
reference the higher-numbered planes and blocks. This condition
is unlikely even in the long term and certain not to arise in the
next several years.
7.4.2. Conditions Not Requiring a Prefix Change
As a result of the principles described above, none of the following
changes required a new prefix:
1. Prohibition of some characters as input to IDNA. Such a
prohibition might make names that were previously registered
inaccessible, but did not change those names.
2. Adjustments in IDNA tables or actions, including normalization
definitions, that affected characters that were already invalid
under IDNA2003.
3. Changes in the style of the IDNA definition that did not alter
the actions performed by IDNA.
7.4.3. Implications of Prefix Changes
While it might have been possible to make a prefix change, the costs
of such a change are considerable. Registries could not have
converted all IDNA2003 ("xn--") registrations to a new form at the
same time and synchronize that change with applications supporting
lookup. Unless all existing registrations were simply to be declared
invalid (and perhaps even then), systems that needed to support both
labels with old prefixes and labels with new ones would be required
to first process a putative label under the IDNA2008 rules and try to
look it up and then, if it were not found, would be required to
process the label under IDNA2003 rules and look it up again. That
process would probably have significantly slowed down all processing
that involved IDNs in the DNS, especially since a fully-qualified
name might contain a mixture of labels that were registered with the
old and new prefixes. That would have made DNS caching very
difficult. In addition, looking up the same input string as two
separate A-labels would have created some potential for confusion and
attacks, since the labels could map to different targets and then
resolve to different entries in the DNS.
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Consequently, a prefix change should have been, and was, avoided if
at all possible, even if it means accepting some IDNA2003 decisions
about character distinctions as irreversible and/or giving special
treatment to edge cases.
7.5. Stringprep Changes and Compatibility
The Nameprep specification [RFC 3491], a key part of IDNA2003, is a
profile of Stringprep [RFC 3454]. While Nameprep is a Stringprep
profile specific to IDNA, Stringprep is used by a number of other
protocols. Were Stringprep to have been modified by IDNA2008, those
changes to improve the handling of IDNs could cause problems for
non-DNS uses, most notably if they affected identification and
authentication protocols. Several elements of IDNA2008 give
interpretations to strings prohibited under IDNA2003 or prohibit
strings that IDNA2003 permitted. Those elements include the new
inclusion information in the Tables document [RFC 5892], the reduction
in the number of characters permitted as input for registration or
lookup (Section 3), and even the changes in handling of right-to-left
strings as described in the Bidi document [RFC 5893]. IDNA2008 does
not use Nameprep or Stringprep at all, so there are no side-effect
changes to other protocols.
It is particularly important to keep IDNA processing separate from
processing for various security protocols because some of the
constraints that are necessary for smooth and comprehensible use of
IDNs may be unwanted or undesirable in other contexts. For example,
the criteria for good passwords or passphrases are very different
from those for desirable IDNs: passwords should be hard to guess,
while domain names should normally be easily memorable. Similarly,
internationalized Small Computer System Interface (SCSI) identifiers
and other protocol components are likely to have different
requirements than IDNs.
7.6. The Symbol Question
One of the major differences between this specification and the
original version of IDNA is that IDNA2003 permitted non-letter
symbols of various sorts, including punctuation and line-drawing
symbols, in the protocol. They were always discouraged in practice.
In particular, both the "IESG Statement" about IDNA and all versions
of the ICANN Guidelines specify that only language characters be used
in labels. This specification disallows symbols entirely. There are
several reasons for this, which include:
1. As discussed elsewhere, the original IDNA specification assumed
that as many Unicode characters as possible should be permitted,
directly or via mapping to other characters, in IDNs. This
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specification operates on an inclusion model, extrapolating from
the original "hostname" rules (LDH, see the Definitions document
[RFC 5890]) -- which have served the Internet very well -- to a
Unicode base rather than an ASCII base.
2. Symbol names are more problematic than letters because there may
be no general agreement on whether a particular glyph matches a
symbol; there are no uniform conventions for naming; variations
such as outline, solid, and shaded forms may or may not exist;
and so on. As just one example, consider a "heart" symbol as it
might appear in a logo that might be read as "I love...". While
the user might read such a logo as "I love..." or "I heart...",
considerable knowledge of the coding distinctions made in Unicode
is needed to know that there is more than one "heart" character
(e.g., U+2665, U+2661, and U+2765) and how to describe it. These
issues are of particular importance if strings are expected to be
understood or transcribed by the listener after being read out
loud.
3. Design of a screen reader used by blind Internet users who must
listen to renderings of IDN domain names and possibly reproduce
them on the keyboard becomes considerably more complicated when
the names of characters are not obvious and intuitive to anyone
familiar with the language in question.
4. As a simplified example of this, assume one wanted to use a
"heart" or "star" symbol in a label. This is problematic because
those names are ambiguous in the Unicode system of naming (the
actual Unicode names require far more qualification). A user or
would-be registrant has no way to know -- absent careful study of
the code tables -- whether it is ambiguous (e.g., where there are
multiple "heart" characters) or not. Conversely, the user seeing
the hypothetical label doesn't know whether to read it -- try to
transmit it to a colleague by voice -- as "heart", as "love", as
"black heart", or as any of the other examples below.
5. The actual situation is even worse than this. There is no
possible way for a normal, casual, user to tell the difference
between the hearts of U+2665 and U+2765 and the stars of U+2606
and U+2729 without somehow knowing to look for a distinction. We
have a white heart (U+2661) and few black hearts. Consequently,
describing a label as containing a heart is hopelessly ambiguous:
we can only know that it contains one of several characters that
look like hearts or have "heart" in their names. In cities where
"Square" is a popular part of a location name, one might well
want to use a square symbol in a label as well and there are far
more squares of various flavors in Unicode than there are hearts
or stars.
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The consequence of these ambiguities is that symbols are a very poor
basis for reliable communication. Consistent with this conclusion,
the Unicode standard recommends that strings used in identifiers not
contain symbols or punctuation [Unicode-UAX31]. Of course, these
difficulties with symbols do not arise with actual pictographic
languages and scripts which would be treated like any other language
characters; the two should not be confused.
7.7. Migration between Unicode Versions: Unassigned Code Points
In IDNA2003, labels containing unassigned code points are looked up
on the assumption that, if they appear in labels and can be mapped
and then resolved, the relevant standards must have changed and the
registry has properly allocated only assigned values.
In the IDNA2008 protocol, strings containing unassigned code points
must not be either looked up or registered. In summary, the status
of an unassigned character with regard to the DISALLOWED,
PROTOCOL-VALID, and CONTEXTUAL RULE REQUIRED categories cannot be
evaluated until a character is actually assigned and known. There
are several reasons for this, with the most important ones being:
o Tests involving the context of characters (e.g., some characters
being permitted only adjacent to others of specific types) and
integrity tests on complete labels are needed. Unassigned code
points cannot be permitted because one cannot determine whether
particular code points will require contextual rules (and what
those rules should be) before characters are assigned to them and
the properties of those characters fully understood.
o It cannot be known in advance, and with sufficient reliability,
whether a newly assigned code point will be associated with a
character that would be disallowed by the rules in the Tables
document [RFC 5892] (such as a compatibility character). In
IDNA2003, since there is no direct dependency on NFKC (many of the
entries in Stringprep's tables are based on NFKC, but IDNA2003
depends only on Stringprep), allocation of a compatibility
character might produce some odd situations, but it would not be a
problem. In IDNA2008, where compatibility characters are
DISALLOWED unless character-specific exceptions are made,
permitting strings containing unassigned characters to be looked
up would violate the principle that characters in DISALLOWED are
not looked up.
o The Unicode Standard specifies that an unassigned code point
normalizes (and, where relevant, case folds) to itself. If the
code point is later assigned to a character, and particularly if
the newly assigned code point has a combining class that
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determines its placement relative to other combining characters,
it could normalize to some other code point or sequence.
It is possible to argue that the issues above are not important and
that, as a consequence, it is better to retain the principle of
looking up labels even if they contain unassigned characters because
all of the important scripts and characters have been coded as of
Unicode 5.2 (or even earlier), and hence unassigned code points will
be assigned only to obscure characters or archaic scripts.
Unfortunately, that does not appear to be a safe assumption for at
least two reasons. First, much the same claim of completeness has
been made for earlier versions of Unicode. The reality is that a
script that is obscure to much of the world may still be very
important to those who use it. Cultural and linguistic preservation
principles make it inappropriate to declare the script of no
importance in IDNs. Second, we already have counterexamples, e.g.,
in the relationships associated with new Han characters being added
(whether in the BMP or in Unicode Plane 2).
Independent of the technical transition issues identified above, it
can be observed that any addition of characters to an existing script
to make it easier to use or to better accommodate particular
languages may lead to transition issues. Such additions may change
the preferred form for writing a particular string, changes that may
be reflected, e.g., in keyboard transition modules that would
necessarily be different from those for earlier versions of Unicode
where the newer characters may not exist. This creates an inherent
transition problem because attempts to access labels may use either
the old or the new conventions, requiring registry action whether or
not the older conventions were used in labels. The need to consider
transition mechanisms is inherent to evolution of Unicode to better
accommodate writing systems and is independent of how IDNs are
represented in the DNS or how transitions among versions of those
mechanisms occur. The requirement for transitions of this type is
illustrated by the addition of Malayalam Chillu in Unicode 5.1.0.
7.8. Other Compatibility Issues
The 2003 IDNA model includes several odd artifacts of the context in
which it was developed. Many, if not all, of these are potential
avenues for exploits, especially if the registration process permits
"source" names (names that have not been processed through IDNA and
Nameprep) to be registered. As one example, since the character
Eszett, used in German, is mapped by IDNA2003 into the sequence "ss"
rather than being retained as itself or prohibited, a string
containing that character, but that is otherwise in ASCII, is not
really an IDN (in the U-label sense defined above). After Nameprep
maps out the Eszett, the result is an ASCII string and so it does not
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get an xn-- prefix, but the string that can be displayed to a user
appears to be an IDN. IDNA2008 eliminates this artifact. A
character is either permitted as itself or it is prohibited; special
cases that make sense only in a particular linguistic or cultural
context can be dealt with as localization matters where appropriate.
8. Name Server Considerations
8.1. Processing Non-ASCII Strings
Existing DNS servers do not know the IDNA rules for handling
non-ASCII forms of IDNs, and therefore need to be shielded from them.
All existing channels through which names can enter a DNS server
database (for example, master files (as described in RFC 1034) and
DNS update messages [RFC 2136]) could not be IDNA-aware because they
predate IDNA. Other sections of this document provide the needed
shielding by ensuring that internationalized domain names entering
DNS server databases through such channels have already been
converted to their equivalent ASCII A-label forms.
Because of the distinction made between the algorithms for
Registration and Lookup in Sections 4 and 5 (respectively) of the
Protocol document [RFC 5891] (a domain name containing only ASCII code
points cannot be converted to an A-label), there cannot be more than
one A-label form for any given U-label.
As specified in clarifications to the DNS specification [RFC 2181],
the DNS protocol explicitly allows domain labels to contain octets
beyond the ASCII range (0000..007F), and this document does not
change that. However, although the interpretation of octets
0080..00FF is well-defined in the DNS, many application protocols
support only ASCII labels and there is no defined interpretation of
these non-ASCII octets as characters and, in particular, no
interpretation of case-independent matching for them (e.g., see the
clarification on DNS case insensitivity [RFC 4343]). If labels
containing these octets are returned to applications, unpredictable
behavior could result. The A-label form, which cannot contain those
characters, is the only standard representation for internationalized
labels in the DNS protocol.
8.2. Root and Other DNS Server Considerations
IDNs in A-label form will generally be somewhat longer than current
domain names, so the bandwidth needed by the root servers is likely
to go up by a small amount. Also, queries and responses for IDNs
will probably be somewhat longer than typical queries historically,
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so Extension Mechanisms for DNS (EDNS0) [RFC 2671] support may be more
important (otherwise, queries and responses may be forced to go to
TCP instead of UDP).
9. Internationalization Considerations
DNS labels and fully-qualified domain names provide mnemonics that
assist in identifying and referring to resources on the Internet.
IDNs expand the range of those mnemonics to include those based on
languages and character sets other than Western European and Roman-
derived ones. But domain "names" are not, in general, words in any
language. The recommendations of the IETF policy on character sets
and languages (BCP 18 [RFC 2277]) are applicable to situations in
which language identification is used to provide language-specific
contexts. The DNS is, by contrast, global and international and
ultimately has nothing to do with languages. Adding languages (or
similar context) to IDNs generally, or to DNS matching in particular,
would imply context-dependent matching in DNS, which would be a very
significant change to the DNS protocol itself. It would also imply
that users would need to identify the language associated with a
particular label in order to look that label up. That knowledge is
generally not available because many labels are not words in any
language and some may be words in more than one.
10. IANA Considerations
This section gives an overview of IANA registries required for IDNA.
The actual definitions of, and specifications for, the first two,
which have been newly created for IDNA2008, appear in the Tables
document [RFC 5892]. This document describes the registries, but it
does not specify any IANA actions.
10.1. IDNA Character Registry
The distinction among the major categories "UNASSIGNED",
"DISALLOWED", "PROTOCOL-VALID", and "CONTEXTUAL RULE REQUIRED" is
made by special categories and rules that are integral elements of
the Tables document. While not normative, an IANA registry of
characters and scripts and their categories, updated for each new
version of Unicode and the characters it contains, are convenient for
programming and validation purposes. The details of this registry
are specified in the Tables document.
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10.2. IDNA Context Registry
IANA has created and now maintains a list of approved contextual
rules for characters that are defined in the IDNA Character Registry
list as requiring a Contextual Rule (i.e., the types of rules
described in Section 3.1.2). The details for those rules appear in
the Tables document.
10.3. IANA Repository of IDN Practices of TLDs
This registry, historically described as the "IANA Language Character
Set Registry" or "IANA Script Registry" (both somewhat misleading
terms), is maintained by IANA at the request of ICANN. It is used to
provide a central documentation repository of the IDN policies used
by top level domain (TLD) registries who volunteer to contribute to
it and is used in conjunction with ICANN Guidelines for IDN use.
It is not an IETF-managed registry and, while the protocol changes
specified here may call for some revisions to the tables, IDNA2008
has no direct effect on that registry and no IANA action is required
as a result.
11. Security Considerations
11.1. General Security Issues with IDNA
This document is purely explanatory and informational and
consequently introduces no new security issues. It would, of course,
be a poor idea for someone to try to implement from it; such an
attempt would almost certainly lead to interoperability problems and
might lead to security ones. A discussion of security issues with
IDNA, including some relevant history, appears in the Definitions
document [RFC 5890].
12. Acknowledgments
The editor and contributors would like to express their thanks to
those who contributed significant early (pre-working group) review
comments, sometimes accompanied by text, Paul Hoffman, Simon
Josefsson, and Sam Weiler. In addition, some specific ideas were
incorporated from suggestions, text, or comments about sections that
were unclear supplied by Vint Cerf, Frank Ellerman, Michael Everson,
Asmus Freytag, Erik van der Poel, Michel Suignard, and Ken Whistler.
Thanks are also due to Vint Cerf, Lisa Dusseault, Debbie Garside, and
Jefsey Morfin for conversations that led to considerable improvements
in the content of this document and to several others, including Ben
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RFC 5894 IDNA Rationale August 2010
Campbell, Martin Duerst, Subramanian Moonesamy, Peter Saint-Andre,
and Dan Winship, for catching specific errors and recommending
corrections.
A meeting was held on 30 January 2008 to attempt to reconcile
differences in perspective and terminology about this set of
specifications between the design team and members of the Unicode
Technical Consortium. The discussions at and subsequent to that
meeting were very helpful in focusing the issues and in refining the
specifications. The active participants at that meeting were (in
alphabetic order, as usual) Harald Alvestrand, Vint Cerf, Tina Dam,
Mark Davis, Lisa Dusseault, Patrik Faltstrom (by telephone), Cary
Karp, John Klensin, Warren Kumari, Lisa Moore, Erik van der Poel,
Michel Suignard, and Ken Whistler. We express our thanks to Google
for support of that meeting and to the participants for their
contributions.
Useful comments and text on the working group versions of the working
draft were received from many participants in the IETF "IDNABIS"
working group and a number of document changes resulted from mailing
list discussions made by that group. Marcos Sanz provided specific
analysis and suggestions that were exceptionally helpful in refining
the text, as did Vint Cerf, Martin Duerst, Andrew Sullivan, and Ken
Whistler. Lisa Dusseault provided extensive editorial suggestions
during the spring of 2009, most of which were incorporated.
13. Contributors
While the listed editor held the pen, the core of this document and
the initial working group version represents the joint work and
conclusions of an ad hoc design team consisting of the editor and, in
alphabetic order, Harald Alvestrand, Tina Dam, Patrik Faltstrom, and
Cary Karp. Considerable material describing mapping principles has
been incorporated from a draft of the Mapping document
[IDNA2008-Mapping] by Pete Resnick and Paul Hoffman. In addition,
there were many specific contributions and helpful comments from
those listed in the Acknowledgments section and others who have
contributed to the development and use of the IDNA protocols.
14. References
14.1. Normative References
[RFC 3490] Faltstrom, P., Hoffman, P., and A. Costello,
"Internationalizing Domain Names in Applications
(IDNA)", RFC 3490, March 2003.
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RFC 5894 IDNA Rationale August 2010
[RFC 3492] Costello, A., "Punycode: A Bootstring encoding of
Unicode for Internationalized Domain Names in
Applications (IDNA)", RFC 3492, March 2003.
[RFC 5890] Klensin, J., "Internationalized Domain Names for
Applications (IDNA): Definitions and Document
Framework", RFC 5890, August 2010.
[RFC 5891] Klensin, J., "Internationalized Domain Names in
Applications (IDNA): Protocol", RFC 5891, August 2010.
[RFC 5892] Faltstrom, P., "The Unicode Code Points and
Internationalized Domain Names for Applications (IDNA)",
RFC 5892, August 2010.
[RFC 5893] Alvestrand, H. and C. Karp, "Right-to-Left Scripts for
Internationalized Domain Names for Applications (IDNA)",
RFC 5893, August 2010.
[Unicode52] The Unicode Consortium. The Unicode Standard, Version
5.2.0, defined by: "The Unicode Standard, Version
5.2.0", (Mountain View, CA: The Unicode Consortium,
2009. ISBN 978-1-936213-00-9).
<http://www.unicode.org/versions/Unicode5.2.0/>.
14.2. Informative References
[IDNA2008-Mapping]
Resnick, P. and P. Hoffman, "Mapping Characters in
Internationalized Domain Names for Applications (IDNA)",
Work in Progress, April 2010.
[RFC 952] Harrenstien, K., Stahl, M., and E. Feinler, "DoD
Internet host table specification", RFC 952,
October 1985.
[RFC 1034] Mockapetris, P., "Domain names - concepts and
facilities", STD 13, RFC 1034, November 1987.
[RFC 1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[RFC 1123] Braden, R., "Requirements for Internet Hosts -
Application and Support", STD 3, RFC 1123, October 1989.
[RFC 2136] Vixie, P., Thomson, S., Rekhter, Y., and J. Bound,
"Dynamic Updates in the Domain Name System (DNS
UPDATE)", RFC 2136, April 1997.
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RFC 5894 IDNA Rationale August 2010
[RFC 2181] Elz, R. and R. Bush, "Clarifications to the DNS
Specification", RFC 2181, July 1997.
[RFC 2277] Alvestrand, H., "IETF Policy on Character Sets and
Languages", BCP 18, RFC 2277, January 1998.
[RFC 2671] Vixie, P., "Extension Mechanisms for DNS (EDNS0)",
RFC 2671, August 1999.
[RFC 2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
specifying the location of services (DNS SRV)",
RFC 2782, February 2000.
[RFC 3454] Hoffman, P. and M. Blanchet, "Preparation of
Internationalized Strings ("stringprep")", RFC 3454,
December 2002.
[RFC 3491] Hoffman, P. and M. Blanchet, "Nameprep: A Stringprep
Profile for Internationalized Domain Names (IDN)",
RFC 3491, March 2003.
[RFC 3743] 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.
[RFC 3987] Duerst, M. and M. Suignard, "Internationalized Resource
Identifiers (IRIs)", RFC 3987, January 2005.
[RFC 4290] Klensin, J., "Suggested Practices for Registration of
Internationalized Domain Names (IDN)", RFC 4290,
December 2005.
[RFC 4343] Eastlake, D., "Domain Name System (DNS) Case
Insensitivity Clarification", RFC 4343, January 2006.
[RFC 4690] Klensin, J., Faltstrom, P., Karp, C., and IAB, "Review
and Recommendations for Internationalized Domain Names
(IDNs)", RFC 4690, September 2006.
[RFC 4713] Lee, X., Mao, W., Chen, E., Hsu, N., and J. Klensin,
"Registration and Administration Recommendations for
Chinese Domain Names", RFC 4713, October 2006.
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RFC 5894 IDNA Rationale August 2010
[Unicode-UAX31]
The Unicode Consortium, "Unicode Standard Annex #31:
Unicode Identifier and Pattern Syntax, Revision 11",
September 2009,
<http://www.unicode.org/reports/tr31/tr31-11.html>.
[Unicode-UTS39]
The Unicode Consortium, "Unicode Technical Standard #39:
Unicode Security Mechanisms, Revision 2", August 2006,
<http://www.unicode.org/reports/tr39/tr39-2.html>.
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
Klensin Informational PAGE 43
Internationalized Domain Names for Applications (IDNA): Background, Explanation, and Rationale
RFC TOTAL SIZE: 115174 bytes
PUBLICATION DATE: Wednesday, August 4th, 2010
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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