Network Working Group J. Klensin, Ed.
Internet-Draft January 28, 2008
Expires: July 31, 2008
Internationalizing Domain Names for Applications (IDNA): Issues,
Explanation, and Rationale
draft-klensin-idnabis-issues-06.txt
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Abstract
A recent IAB report identified issues that have been raised with
Internationalized Domain Names (IDNs). Some of these issues require
tuning of the existing protocols and the tables on which they depend.
Based on intensive discussion by an informal design team, this
document provides an overview some of the proposals that are being
made, provides explanatory material for them and then further
explains some of the issues that have been encountered.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Context and Overview . . . . . . . . . . . . . . . . . . . 4
1.2. Discussion Forum . . . . . . . . . . . . . . . . . . . . . 4
1.3. Objectives . . . . . . . . . . . . . . . . . . . . . . . . 4
1.4. Applicability and Function of IDNA . . . . . . . . . . . . 5
1.5. Terminology . . . . . . . . . . . . . . . . . . . . . . . 6
1.5.1. Documents and Standards . . . . . . . . . . . . . . . 6
1.5.2. Terminology about Characters and Character Sets . . . 6
1.5.3. DNS-related Terminology . . . . . . . . . . . . . . . 7
1.5.4. Terminology Specific to IDNA . . . . . . . . . . . . . 8
1.5.5. Punycode is an Algorithm, not a Name . . . . . . . . . 11
1.5.6. Other Terminology Issues . . . . . . . . . . . . . . . 11
2. The Original (2003) IDNA Model . . . . . . . . . . . . . . . . 12
2.1. Proposed label . . . . . . . . . . . . . . . . . . . . . . 13
2.2. Permitted Character Identification . . . . . . . . . . . . 13
2.3. Character Mappings . . . . . . . . . . . . . . . . . . . . 13
2.4. Registry Restrictions . . . . . . . . . . . . . . . . . . 13
2.5. Punycode Conversion . . . . . . . . . . . . . . . . . . . 14
2.6. Lookup or Insertion in the Zone . . . . . . . . . . . . . 14
3. A Revised IDNA Model . . . . . . . . . . . . . . . . . . . . . 14
3.1. Localization: The Role of the Local System, Local
Preprocessing, and the User Interface . . . . . . . . . . 14
3.2. IDN Processing in the IDNA200X Model . . . . . . . . . . . 16
3.2.1. Summary of Effects . . . . . . . . . . . . . . . . . . 16
4. IDNA200X Document List . . . . . . . . . . . . . . . . . . . . 16
5. Permitted Characters: An Inclusion List . . . . . . . . . . . 17
5.1. A Tiered Model of Permitted Characters and Labels . . . . 17
5.1.1. ALWAYS . . . . . . . . . . . . . . . . . . . . . . . . 17
5.1.2. MAYBE . . . . . . . . . . . . . . . . . . . . . . . . 18
5.1.3. CONTEXTUAL RULE REQUIRED . . . . . . . . . . . . . . . 20
5.1.4. NEVER . . . . . . . . . . . . . . . . . . . . . . . . 21
5.1.5. UNASSIGNED . . . . . . . . . . . . . . . . . . . . . . 21
5.2. Category Applicability . . . . . . . . . . . . . . . . . . 21
5.2.1. Applications Using IDNs . . . . . . . . . . . . . . . 21
5.2.2. Registration Policy . . . . . . . . . . . . . . . . . 22
5.3. Layered Restrictions: Tables, Context, Registration,
Applications . . . . . . . . . . . . . . . . . . . . . . . 23
5.4. A New Character List -- History . . . . . . . . . . . . . 23
5.5. Understanding New Issues and Constraints . . . . . . . . . 24
5.6. ALWAYS, MAYBE, and Contextual Rules . . . . . . . . . . . 24
6. Issues that Constrain Possible Solutions . . . . . . . . . . . 25
6.1. Display and Network Order . . . . . . . . . . . . . . . . 25
6.2. Entry and Display in Applications . . . . . . . . . . . . 26
6.3. Linguistic Expectations: Ligatures, Digraphs, and
Alternate Character Forms . . . . . . . . . . . . . . . . 27
6.4. Case Mapping and Related Issues . . . . . . . . . . . . . 30
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6.5. Right-to-left Text . . . . . . . . . . . . . . . . . . . . 30
7. IDNs and the Robustness Principle . . . . . . . . . . . . . . 31
8. Front-end and User Interface Processing . . . . . . . . . . . 32
9. Migration and Version Synchronization . . . . . . . . . . . . 34
9.1. Design Criteria . . . . . . . . . . . . . . . . . . . . . 34
9.1.1. General IDNA Validity Criteria . . . . . . . . . . . . 34
9.1.2. Characters in Registrations . . . . . . . . . . . . . 36
9.1.3. Labels in Registration . . . . . . . . . . . . . . . . 36
9.1.4. Labels in Resolution (Lookup) . . . . . . . . . . . . 37
9.2. More Flexibility in User Agents . . . . . . . . . . . . . 38
9.3. The Question of Prefix Changes . . . . . . . . . . . . . . 39
9.3.1. Conditions Requiring a Prefix Change . . . . . . . . . 39
9.3.2. Conditions Not Requiring a Prefix Change . . . . . . . 40
9.3.3. Implications of Prefix Changes . . . . . . . . . . . . 40
9.4. Stringprep Changes and Compatibility . . . . . . . . . . . 41
9.5. The Symbol Question . . . . . . . . . . . . . . . . . . . 42
9.6. Migration Between Unicode Versions: Unassigned Code
Points . . . . . . . . . . . . . . . . . . . . . . . . . . 42
9.7. Other Compatibility Issues . . . . . . . . . . . . . . . . 43
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 44
11. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 44
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 44
12.1. IDNA Permitted Character Registry . . . . . . . . . . . . 44
12.2. IDNA Context Registry . . . . . . . . . . . . . . . . . . 45
12.3. IANA Repository of IDN Practices of TLDs . . . . . . . . . 45
13. Security Considerations . . . . . . . . . . . . . . . . . . . 45
14. Change Log . . . . . . . . . . . . . . . . . . . . . . . . . . 47
14.1. Version -01 . . . . . . . . . . . . . . . . . . . . . . . 47
14.2. Version -02 . . . . . . . . . . . . . . . . . . . . . . . 47
14.3. Version -03 . . . . . . . . . . . . . . . . . . . . . . . 47
14.4. Version -04 . . . . . . . . . . . . . . . . . . . . . . . 47
14.5. Version -05 . . . . . . . . . . . . . . . . . . . . . . . 48
14.6. Version -06 . . . . . . . . . . . . . . . . . . . . . . . 48
15. References . . . . . . . . . . . . . . . . . . . . . . . . . . 48
15.1. Normative References . . . . . . . . . . . . . . . . . . . 48
15.2. Informative References . . . . . . . . . . . . . . . . . . 50
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 51
Intellectual Property and Copyright Statements . . . . . . . . . . 52
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1. Introduction
1.1. Context and Overview
A recent IAB report [RFC4690] identified issues that have been raised
with Internationalized Domain Names (IDNs) and the associated
standards. Those standards are known as Internationalized Domain
Names in Applications (IDNA), taken from the name of the highest
level standard within that group (see Section 1.5). Based on
discussion of those issues and their impact, some of these standards
now require tuning the existing protocols and the tables on which
they depend. This document further explains, based on the results of
some intensive discussions by an informal design team, on a mailing
list, and in broader discussions, some of the issues that have been
encountered. It also provides an overview of the proposals that are
being made and explanatory material for them. Additional explanatory
material for other proposals will appear with the associated
documents.
This document begins with a discussion of the original and new IDNA
models and the general differences in strategy between the original
version of IDNA and the proposed new version. It continues with a
description of specific changes that are needed and issues that the
design must address, including some that were not explicitly
addressed in RFC 4690.
1.2. Discussion Forum
[[anchor4: RFC Editor: please remove this section.]]
This work is being discussed on the mailing list
idna-update@alvestrand.no
1.3. Objectives
The intent of the IDNA revision effort, and hence of this document
and the associated ones, is to increase the usability and
effectiveness of internationalized domain names (IDNs) while
preserving or strengthening the integrity of references that use
them. The original "hostname" (LDH) character definitions (see,
e.g., [RFC0810]) struck a balance between the creation of useful
mnemonics and the introduction of parsing problems or general
confusion in the contexts in which domain names are used. Our
objective is to preserve that balance while expanding the character
repertoire to include extended versions of Roman-derived scripts and
scripts that are not Roman in origin. No work of this sort will be
able to completely eliminate sources of visual or textual confusion:
such confusion exists even under the original rules. However, one
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can hope, through the application of different techniques at
different points (see Section 5.3), to keep problems to an acceptable
minimum. One consequence of this general objective is that the
desire of some user or marketing community to use a particular string
--whether the reason is to try to write sentences of particular
languages in the DNS, to express a facsimile of the symbol for a
brand, or for some other purpose-- is not a primary goal within the
context of applications in the domain name space.
1.4. Applicability and Function of IDNA
The IDNA standard does not require any applications to conform to it,
nor does it retroactively change those applications. An application
can elect to use IDNA in order to support IDN while maintaining
interoperability with existing infrastructure. If an application
wants to use non-ASCII characters in domain names, IDNA is the only
currently-defined option. Adding IDNA support to an existing
application entails changes to the application only, and leaves room
for flexibility in front-end processing and more specifically in the
user interface (see Section 8).
A great deal of the discussion of IDN solutions has focused on
transition issues and how IDN 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 depend on user applications,
resolvers, and DNS servers being updated in order for a user to apply
an internationalized domain name in any form or coding acceptable
under that method. While processing must be performed prior to or
after access to the DNS, no changes are needed to the DNS protocol or
any DNS servers or the resolvers on user's computers.
The IDNA specification solves the problem of extending the repertoire
of characters that can be used in domain names to include a large
subset of the Unicode repertoire.
IDNA does not extend the service offered by DNS to the applications.
Instead, the applications (and, by implication, the users) continue
to see an exact-match lookup service. Either there is a single
exactly-matching name or there is no match. This model has served
the existing applications well, but it requires, with or without
internationalized domain names, that users know the exact spelling of
the domain names that are to be typed into applications such as web
browsers and mail user agents. The introduction of the larger
repertoire of characters potentially makes the set of misspellings
larger, especially given that in some cases the same appearance, for
example on a business card, might visually match several Unicode code
points or several sequences of code points.
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IDNA allows the graceful introduction of IDNs not only by avoiding
upgrades to existing infrastructure (such as DNS servers and mail
transport agents), but also by allowing some rudimentary use of IDNs
in applications by using the ASCII representation of the non-ASCII
name labels. While such names are user-unfriendly to read and type,
and hence not optimal for user input, they allow (for instance)
replying to email and clicking on URLs even though the domain name
displayed is incomprehensible to the user. In order to allow user-
friendly input and output of the IDNs and acceptance of some
characters as equivalent to those to be processed according to the
protocol, the applications need to be modified to conform to this
specification.
IDNA uses the Unicode character repertoire, which avoids the
significant delays that would be inherent in waiting for a different
and specific character sets to be defined for IDN purposes,
presumably by some other standards developing organization.
1.5. Terminology
1.5.1. Documents and Standards
This document uses the term "IDNA2003" to refer to the set of
standards that make up and support the version of IDNA published in
2003, i.e., those commonly known as the IDNA base specification
[RFC3490], Nameprep [RFC3491], Punycode [RFC3492], and Stringprep
[RFC3454]. In this document, those names are used to refer,
conceptually, to the individual documents, with the base IDNA
specification called just "IDNA".
The term "IDNA200X" is used to refer to a new version of IDNA as
described in this document and in the documents described in
Section 4. While more common IETF usage might refer to the successor
document(s) as "IDNAbis", this document uses that term, and similar
ones, to refer to successors to the individual documents, e.g.,
"IDNAbis" is a synonym for the specific successor to RFC3490, or
"RFC3490bis". See also Section 4.
1.5.2. Terminology about Characters and Character Sets
A code point is an integer value associated with a character in a
coded character set.
Unicode [Unicode50] is a coded character set containing almost
100,000 characters as of the current version. A single Unicode code
point is denoted by "U+" followed by four to six hexadecimal digits,
while a range of Unicode code points is denoted by two four to six
digit hexadecimal numbers separated by "..", with no prefixes.
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ASCII means US-ASCII [ASCII], a coded character set containing 128
characters associated with code points in the range 0000..007F.
Unicode may be thought of as an extension of ASCII: it includes all
the ASCII characters and associates them with equivalent code points.
"Letters" are, informally, generalizations from the ASCII and common-
sense understanding of that term, i.e., characters that are used to
write text that are not symbols or punctuation. Formally, they are
characters with a Unicode General Category value starting in "L" (see
Section 4.5 of [Unicode50]).
1.5.3. DNS-related Terminology
When discussing the DNS, this document generally assumes the
terminology used in the DNS specifications [RFC1034] [RFC1035]. The
terms "lookup" and "resolution" are used interchangeably and the
process or application component that performs DNS resolution is
called a "resolver". The process of placing an entry into the DNS is
referred to as "registration" paralleling common contemporary usage
in other contexts.
The term "LDH code points" is defined in this document to mean the
code points associated with ASCII letters, digits, and the hyphen-
minus; that is, U+002D, 30..39, 41..5A, and 61..7A. "LDH" is an
abbreviation for "letters, digits, hyphen".
The base DNS specifications [RFC1034] [RFC1035] discuss "domain
names" and "host names", but many people and sections of these
specifications use the terms interchangeably. Further, because those
documents were not terribly clear, many people who are sure they know
the exact definitions of each of these terms disagree on the
definitions. In this document the term "domain name" is used in
general. This document explicitly cites those documents whenever
referring to the host name syntax restrictions defined therein. The
remaining definitions in this subsection are essentially a review.
A label is an individual part of a domain name. Labels are usually
shown separated by dots; for example, the domain name
"www.example.com" is composed of three labels: "www", "example", and
"com". (The zero-length root label described in [RFC1123], which can
be explicit as in "www.example.com." or implicit as in
"www.example.com", is not considered a label in this specification.)
IDNA extends the set of usable characters in labels that are text.
For the rest of this document, the term "label" is shorthand for
"text label", and "every label" means "every text label".
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1.5.4. Terminology Specific to IDNA
Some of the terminology used in describing IDNs in the IDNA2003
context has been a source of confusion. This section defines some
new terminology to reduce dependence on the problematic terms and
definitions that appears in RFC 3490.
1.5.4.1. Terms for IDN Label Codings
1.5.4.1.1. IDNA-valid strings, A-label, and U-label
To improve clarity, this document introduces three new terms in this
subsection. In the next, it defines a historical one to be slightly
more precise for IDNA contexts.
o A string is "IDNA-valid" if it meets all of the requirements of
these specification for an IDNA label. IDNA-valid strings may
appear in either of two forms, defined immediately below. It is
expected that specific reference will be made to the form
appropriate to any context in which the distinction is important.
o An "A-label" is the ASCII-Compatible Encoding (ACE, see
Section 1.5.4.3) form of an IDNA-valid string. It must be a
complete label: IDNA is defined for labels, not for parts of them
and, in general, not for complete domain names. This means, by
definition, that every A-label will begin with the IDNA ACE
prefix, "xn--", followed by a string that is a valid output of the
Punycode algorithm and hence a maximum of 59 ASCII characters in
length. The prefix and string together must conform to all
requirements for a label that can be stored in the DNS including
conformance to the LDH ("hostname") rule.
o A "U-label" is an IDNA-valid string of Unicode characters,
expressed in a standard Unicode Encoding Form, normally UTF-8, and
subject to the constraint below. [IDNA200X-protocol] specifies
the conversions between U-labels and A-labels.
To be valid, U-labels and A-labels must obey an important symmetry
constraint. While that constraint may be tested in any of several
ways, an A-label must be capable of being produced by conversion from
a U-label and a U-label must be capable of being produced by
conversion from an A-label. Among other things, this implies that
both U-labels and A-labels must represent strings in normalized form.
These strings MUST contain only characters specified elsewhere in
this document and its companion documents, and only in the contexts
indicated as appropriate.
Any rules or conventions that apply to DNS labels in general, such as
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rules about lengths of strings, apply to whichever of the U-label or
A-label would be more restrictive. For the U-label, constraints
imposed by existing protocols and their presentation forms make the
length restriction apply to the length in octets of the UTF-8 form of
those labels (which will always be greater than or equal to the
length in code points). The exception to this, of course, is that
the restriction to ASCII characters does not apply to the U-label.
A different way to look at these terms, which may be more clear to
some readers, is that U-labels, A-labels, and LDH-labels (see the
next subsection) are disjoint categories that, together, make up the
forms of legitimate strings for use in domain names that describe
hosts. Of the three, only A-labels and LDH-labels can actually
appear in DNS zone files or queries; U-labels can appear, along with
those two, in presentation and user interface forms and in selected
protocols other than those of the DNS itself. Strings that do not
conform to the rules for one of these three categories and, in
particular, strings that contain "-" in the third or fourth character
position but are:
o not A-labels or
o that cannot be processed as U-labels or A-labels as described in
these specifications,
are invalid as labels in domain names that identify Internet hosts or
similar resources. This restriction on strings containing "--" is
required for three reasons:
o to prevent confusion with pre-IDNA coding forms;
o to permit future extensions that would require changing the
prefix, no matter how unlikely those might be (see Section 9.3);
o and to reduce the opportunities for attacks on the encoding
system.
1.5.4.1.2. LDH-label and Internationalized Label
In the hope of further clarifying discussions about IDNs, this
document uses the term "LDH-label" strictly to refer to an all-ASCII
label that obeys the "hostname" (LDH) conventions and that is not an
IDN. In other words, the categories "U-label", "A-label", and "LDH-
label" are disjoint, with only the first two referring to IDNs. When
such a term is needed, an "internationalized label" is one that is a
member of the union of those three categories. There are some
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standardized DNS label formats, such as those for service location
(SRV) records [RFC2782] that do not fall into any of the three
categories and hence are not internationalized labels.
1.5.4.2. Equivalence
In IDNA, equivalence of labels is defined in terms of the A-labels.
If the A-labels are equal in a case-independent comparison, then the
labels are considered equivalent, no matter how they are represented.
Traditional LDH labels already have a notion of equivalence: within
that list of characters, upper case and lower case are considered
equivalent. The IDNA notion of equivalence is an extension of that
older notion. Equivalent labels in IDNA are treated as alternate
forms of the same label, just as "foo" and "Foo" are treated as
alternate forms of the same label.
1.5.4.3. ACE Prefix
The "ACE prefix" is defined in this document to be a string of ASCII
characters "xn--" that appears at the beginning of every A-label.
"ACE" stands for "ASCII-Compatible Encoding".
1.5.4.4. Domain Name Slot
A "domain name slot" is defined in this document to be a protocol
element or a function argument or a return value (and so on)
explicitly designated for carrying a domain name. Examples of domain
name slots include: the QNAME field of a DNS query; the name argument
of the gethostbyname() library function; the part of an email address
following the at-sign (@) in the From: field of an email message
header; and the host portion of the URI in the src attribute of an
HTML ![]()
tag. General text that just happens to contain a domain
name is not a domain name slot. For example, a domain name appearing
in the plain text body of an email message is not occupying a domain
name slot.
An "IDN-aware domain name slot" is defined in this document to be a
domain name slot explicitly designated for carrying an
internationalized domain name as defined in this document. The
designation may be static (for example, in the specification of the
protocol or interface) or dynamic (for example, as a result of
negotiation in an interactive session).
An "IDN-unaware domain name slot" is defined in this document to be
any domain name slot that is not an IDN-aware domain name slot.
Obviously, this includes any domain name slot whose specification
predates IDNA.
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1.5.5. Punycode is an Algorithm, not a Name
There has been some confusion about whether a "Punycode string" does
or does not include the prefix and about whether it is required that
such strings could have been the output of ToASCII (see RFC 3490,
Section 4 [RFC3490]). This specification discourages the use of the
term "Punycode" to describe anything but the encoding method and
algorithm of [RFC3492]. The terms defined above are preferred as
much more clear than terms such as "Punycode string".
1.5.6. Other Terminology Issues
The document departs from historical DNS terminology and usage in one
important respect. Over the years, the community has talked very
casually about "names" in the DNS, beginning with calling it "the
domain name system". That terminology is fine in the very precise
sense that the identifiers of the DNS do provide names for objects
and addresses. But, in the context of IDNs, the term has introduced
some confusion, confusion that has increased further as people have
begun to speak of DNS labels in terms of the words or phrases of
various natural languages.
Historically, many, perhaps most, of the "names" in the DNS have just
been mnemonics to identify some particular concept, object, or
organization. They are typically derived from, or rooted in, some
language because most people think in language-based ways. But,
because they are mnemonics, they need not obey the orthographic
conventions of any language: it is not a requirement that it be
possible for them to be "words".
This distinction is important because the reasonable goal of an IDN
effort is not to be able to write the great Klingon (or language of
one's choice) novel in DNS labels but to be able to form a usefully
broad range of mnemonics in ways that are as natural as possible in a
very broad range of scripts.
An "internationalized domain name" (IDN) is a domain name that may
contain one or more A-labels or U-labels, as appropriate, instead of
LDH labels. This implies that every conventional domain name is an
IDN (which implies that it is possible for a name to be an IDN
without it containing any non-ASCII characters). This document does
not attempt to define an "internationalized host name". Just as has
been the case with ASCII names, some DNS zone administrators may
impose restrictions, beyond those imposed by DNS or IDNA, on the
characters or strings that may be registered as labels in their
zones. Such restrictions have no effect on the syntax or semantics
of DNS protocol messages; a query for a name that matches no records
will yield the same response regardless of the reason why it is not
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in the zone. Clients issuing queries or interpreting responses
cannot be assumed to have any knowledge of zone-specific restrictions
or conventions.
2. The Original (2003) IDNA Model
IDNA is a client-side protocol, i.e., almost all of the processing is
performed by the client. The strings that appear in, and are
resolved by, the DNS conform to the traditional rules for the naming
of hosts, and consist of ASCII letters, digits, and hyphens. This
approach permits IDNA to be deployed without modifications to the DNS
itself. That, in turn, avoids both having to upgrade the entire
Internet to support IDNs and needing to incur the unknown risks to
deployed systems of DNS structural or design changes especially if
those changes need to be deployed all at the same time.
This section contains a summary of the model underlying IDNA2003. It
is approximate and is not a substitute for reading and understanding
the actual specification document [RFC3490] and the documents on
which it depends. The summary is not intended to be completely
balanced. It emphasizes some characteristics of IDNA2003 that are
particularly important to understanding the nature of the proposed
changes.
The original IDNA specifications have the logical flow in domain name
registration and resolution outlined in the balance of this section.
They are not defined this way; instead, the steps are presented here
for convenience in comparison to what is being proposed in this
document and the associated ones. In particular, IDNA2003 does not
make as strong a distinction between procedures for registration and
those for resolution as the ones suggested in Section 3 and
Section 5.1.
The IDNA2003 specification explicitly includes the equivalents of the
steps in Section 2.2, Section 2.3, and Section 2.5 below. While the
other steps are present --either inside the protocol or presumed to
be performed before or after it-- they are not discussed explicitly.
That omission has been a source of confusion. Another source has
been the definition of IDNA2003 as an algorithm, expressed partially
in prose and partially in pseudo code and tables. The steps below
follow the more traditional IETF practice: the functions are
specified, rather than the algorithms. The breakdown into steps is
for clarity of explanation; any implementation that produces the same
result with the same inputs is conforming.
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2.1. Proposed label
The registrant submits a request for an IDN or the user attempts to
look up an IDN. The registrant or user typically produces the
request string by keyboard entry of a character sequence. That
sequence is validated only on the basis of its displayed appearance,
without knowledge of the character coding used for its internal
representation or other local details of the way the operating system
processes it. This string is converted to Unicode if necessary.
IDNA2003 assumes that the conversion is straightforward enough not to
be considered by the protocol.
2.2. Permitted Character Identification
The Unicode string is examined to prohibit characters that IDNA does
not permit in input. The list of excluded characters is quite
limited because IDNA2003 permits almost all Unicode characters to be
used as input, with many of them mapped into others.
2.3. Character Mappings
The label string is processed through the Nameprep [RFC3491] profile
of the Stringprep [RFC3454] tables and procedure. Among other
things, these procedures apply the Unicode normalization procedure
NFKC [Unicode-UAX15] which converts compatibility characters to their
base forms and resolves the different ways in which some characters
can be represented in Unicode into a canonical form. In IDNA2003,
one-way case mapping was also performed, partially simulating the
query-time folding operation that the DNS provides for ASCII strings.
2.4. Registry Restrictions
Registries at all levels of the DNS, not just the top level, are
expected to establish policies about the labels that may be
registered and for the processes associated with that action (see the
discussion of guidelines and statements in [RFC4690]). Such
restrictions have always existed in the DNS and have always been
applied at registration time, with the most notable example being
enforcement of the hostname (LDH) convention itself. For IDNs, the
restrictions to be applied are not an IETF matter except insofar as
they derive from restrictions imposed by application protocols (e.g.,
email has always required a more restricted syntax for domain names
than the restrictions of the DNS itself). Because these are
restrictions on what can be registered, it is not generally necessary
that they be global. If a name is not found on resolution, it is not
relevant whether it could have been registered; only that it was not
registered. Registry restrictions might include prohibition of
mixed-script labels or restrictions on labels permitted in a zone if
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certain other labels are already present. The "variant" systems
discussed in [RFC3743] and [RFC4290] are examples of fairly
sophisticated registry restriction models. The various sets of ICANN
IDN Guidelines [ICANN-Guidelines] also suggest restrictions that
might sensibly be imposed.
The string produced by the above steps is checked and processed as
appropriate to local registry restrictions. Application of those
registry restrictions may result in the rejection of some labels or
the application of special restrictions to others.
2.5. Punycode Conversion
The resulting label (in Unicode code point character form) is
processed with the Punycode algorithm [RFC3492] and converted to a
form suitable for storage in the DNS (the "xn--..." form).
2.6. Lookup or Insertion in the Zone
For registration, the Punycode-encoded label is then placed in the
DNS by insertion into a zone. For lookup, that label is processed
according to normal DNS query procedures [RFC1035].
3. A Revised IDNA Model
One of the major goals of this work is to improve the general
understanding of how IDNA works and what characters are permitted and
what happens to them. Comprehensibility and predictability to users
and registrants are themselves important motivations and design goals
for this effort. The effort includes some new terminology and a
revised and extended model, both covered in this section, and some
more specific protocol, processing, and table modifications. Details
of the latter appear in other documents (see Section 4).
3.1. Localization: The Role of the Local System, Local Preprocessing,
and the User Interface
Several issues are inherent in the application of IDNs and, indeed,
almost any other system that tries to handle international characters
and concepts. They range from the apparently trivial --e.g., one
cannot display a character for which one does not have a font
available locally-- to the more complex and subtle. Many people have
observed that internationalization is just a tool to permit effective
localization while permitting some global uniformity. Issues of
display, of exactly how various strings and characters are entered,
and so on are inherently issues about localization and user interface
design.
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A protocol such as IDNA can only assume that such operations as data
entry and reconciliation of differences in character forms are
possible. It may make some recommendations about how display might
work when characters and fonts are not available, but they can only
be general recommendations and, because display functions are rarely
controlled by the types of applications that would call upon IDNA,
will rarely be very useful.
However, shifting responsibility for character mapping and other
adjustments from the protocol (where it was located in IDNA2003) to
the user interface or processing before invoking IDNA raises issues
about both what that processing should do and about compatibility for
references prepared in an IDNA2003 context. Those issues are
discussed in Section 8.
Operations for converting between local character sets and Unicode
are part of this general set of user interface issues. The
conversion is obviously not required at all in a Unicode-native
system where no conversion is required. It may, however, involve
some complexity in one that is not, especially if the elements of the
local character set do not map exactly and unambiguously into Unicode
characters and do so in a way that is completely stable over time.
Perhaps more important, if a label being converted to a local
character set contains Unicode characters that have no correspondence
in that character set, the application may have to apply special,
locally-appropriate, methods to avoid or reduce loss of information.
Depending on the system involved, the major difficulty may not lie in
the mapping but in accurately identifying the incoming character set
and then applying the correct conversion routine. If a local
operating system uses one of the ISO 8859 character sets or an
extensive national or industrial system such as GB18030 [GB18030] or
BIG5 [BIG5] one must correctly identify the character set in use
before converting to Unicode even though those character coding
systems are substantially or completely Unicode-compatible (i.e., all
of the code points in them have an exact and unique mapping to
Unicode code points). It may be even more difficult when the
character coding system in local use is based on conceptually
different assumptions than those used by Unicode about, e.g., how
different presentation or combining forms are handled, such as
proposals now being developed for Tamil. 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.
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3.2. IDN Processing in the IDNA200X Model
3.2.1. Summary of Effects
Separating Domain Name Registration and Resolution in the protocol
specification has one substantive impact. With IDNA2003, the tests
and steps made in these two parts of the protocol are essentially
identical. Separating them reflects current practice in which per-
registry restrictions and special processing are applied at
registration time but not on resolution. Even more important in the
longer term, it allows incremental addition of permitted character
groups to avoid freezing on one particular version of Unicode.
4. IDNA200X Document List
[[anchor20: This section will need to be extensively revised or
removed before publication.]]
The following documents are being produced as part of the IDNA200X
effort.
o A revised version of this document, containing an overview,
rationale, and conformance conditions.
o A separate document, drawn from material in early versions of this
one, that explicitly updates and replaces RFC 3490 but which has
most rationale material from that document moved to this one
[IDNA200X-protocol].
o A document describing the "Bidi problem" with Stringprep and
proposing a solution [IDNA200X-Bidi].
o A list of code points allowed in a U-label, based on Unicode 5.0
code assignments. See Section 5.
o One or more documents containing guidance and suggestions for
registries (in this context, those responsible for establishing
policies for any zone file in the DNS, not only those at the top
or second level). The documents in this category may not all be
IETF products and may be prepared and completed asynchronously
with those described above.
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5. Permitted Characters: An Inclusion List
This section describes the model used to establish the algorithm and
character lists of [IDNA200X-Tables] and describes the names and
applicability of the categories used there. Note that the inclusion
of a character in one of the first three categories does not imply
that it can be used indiscriminately; some characters are associated
with contextual rules that must be applied as well.
5.1. A Tiered Model of Permitted Characters and Labels
Moving to an inclusion model requires a new list of characters that
are permitted in IDNs. In IDNA2003, the role and utility of
characters are independent of context and fixed forever. Making
those rules globally has proven impractical, partially because
handling of particular characters across the languages that use a
script, or the use of similar or identical-looking characters in
different scripts, are less well understood than many people believed
several years ago. Conversely, IDNA2003 prohibited some characters
entirely to avoid dealing with some of the issues discussed here --
restrictions that were much too severe for mnemonics based on some
languages.
Independently of the characters chosen (see next subsection), the
theory is to divide the characters that appear in Unicode into four
categories:
5.1.1. ALWAYS
Characters identified as "ALWAYS" are permitted for all uses in IDNs,
but may be associated with other restrictions (for example, any
character in this group that has a "right to left" property must be
used in context with the "Bidi" rules). The presence of a character
in this category implies that it has been examined and determined to
be appropriate for IDN use, and that it is well-understood that
contextual protocol restrictions, such as rules about the use of
given characters in the presence or absence of others, are not
required (see Section 5.1.3 below). That, in turn, indicates that
the community of users, and the communities whose written languages
share the script in which the character appears, have agreed that the
script and its components are sufficiently well understood for use
for IDN purposes. Those communities are informally referred to as
"script communities" below, but with the understanding that there is
rarely such a thing as a single coherent community of users of a
script. Instead, there are one or more languages communities, with
writing systems based on a the same script, but varying in ways that
can be of significance in the development of IDN support for it in a
manner that is equally useful to all. Because the DNS does not
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convey language information, some mechanism must be arrived at for
determining usage norms for the script that are compatible with the
usage of all of the language communities that share it. This
subsection discusses characters, rather than scripts, because it is
explicitly understood that various segments of the community of users
of a script may decide to include some of its characters and not
others.
A typical example of this occurs with CJK characters, where the
community, after careful study, has concluded that some characters of
the script are suitable for IDN use while many others may not be.
The characters considered generally suitable would be placed in
ALWAYS; other characters would be left in a MAYBE category (probably
MAYBE NO) until and unless the community decided to expand the list
of generally-acceptable character. See [RFC3743] and [RFC4713] for
examples of discussions of some of the issues involved).
Because of this condition, which requires evaluation by individual
script communities of the characters suitable for use in IDNs (not
just, e.g., the general stability of the scripts in which those
characters are embedded) it is not feasible to define the boundary
point between this category and the next one by general properties of
the characters, such as the Unicode property lists.
Despite use of the term "ALWAYS", the presence of a character on this
list does not imply that a given registry need accept registrations
containing any of the characters in the category. Registries are
still expected to apply judgment about labels they will accept and to
maintain rules consistent with those judgments (see
[IDNA200X-protocol] and Section 5.3).
Characters that are placed in the "ALWAYS" category are never removed
from it unless the code points themselves are removed from Unicode
(such removal would be inconsistent with the Unicode stability
principles (see [Unicode50], Appendix F) and hence should never
occur).
5.1.2. MAYBE
Characters that are used to write the languages of the world and that
are thought of broadly as "letters" rather than, e.g., symbols or
punctuation, and that have not been placed in the "ALWAYS" or "NEVER"
categories (see Section 5.1.4 for the latter) belong to the "MAYBE"
category. As implied above, any script (and, in some cases,
individual characters within the script) will be classified as
"MAYBE" if it has not yet been reviewed and finally approved by the
script community. In particular, inclusion in "MAYBE" gives no
guarantee that characters that require special contextual rules
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(tests on the entire label or on adjacent characters, see
Section 5.1.3) are identified and rules specified for them.
In general and for maximum safety, registries SHOULD confine
themselves to characters from the "ALWAYS" category. However, if a
registry is permitting registrations that only use the characters of
a small number of scripts, and it is very familiar with the usage of
those characters in Unicode and in domain names, it may be
appropriate for it to develop its own rules for inclusion of
characters in "MAYBE" categories in labels it registers.
Applications are expected to not treat "ALWAYS" and "MAYBE"
differently with regard to name resolution ("lookup"). They may
choose to provide warnings to users when labels or fully-qualified
names containing characters in the "MAYBE" categories are to be
presented to users.
There are actually two subcategories of MAYBE. The assignment of a
character to one or the other represents an estimate of whether the
character will eventually be treated as "ALWAYS" or "NEVER" (some
characters may, however, remain in the "MAYBE" categories
indefinitely). Since the differences between the "MAYBE"
subcategories do not affect the protocol, characters may be moved
back and forth between them as information and knowledge accumulates.
5.1.2.1. Subcategory MAYBE YES
These are letter, digit, or letter-like characters that are generally
presumed to be appropriate in DNS labels, but for which either:
o no specific in-depth evaluation of script or character use in IDNs
has yet been performed by the communities of users of the relevant
languages, or
o those communities have evaluated the character and concluded that,
while it may be suitable for use in some applications or in
combination with specific registry rules, it is not appropriate
for general use. An example of this would arise with characters
that are expected to be processed as JET variants (see [RFC3743]
but that are not preferred variants.
The risk with characters in the "MAYBE YES" category is that it may
later be discovered that contextual rules are required for their safe
use with labels that otherwise contain characters from arbitrary
scripts or that the characters themselves may be problematic.
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5.1.2.2. Subcategory MAYBE NO
In general, these are characters that are not letter-like, but are
not excluded completely by some other rule. Given the general ban on
characters other than letters and digits, it is likely that they will
be moved to "NEVER" when their contexts are fully understood by the
relevant community. However, since characters once moved to "NEVER"
cannot be moved back out, conservatism about making that
classification is in order.
Characters from scripts that are not believed to be used for any
actively-used language, and characters that the communities of users
of languages that share a script have concluded are inappropriate for
IDN use, will also generally be classified as "MAYBE NO" unless there
are substantive reasons to ban those characters entirely (i.e.,
classify them as "NEVER", see Section 5.1.4).
5.1.3. CONTEXTUAL RULE REQUIRED
There are several characters in Unicode that are unsuitable for
general use in IDNs but necessary for the plausible support of some
scripts. The two most commonly-cited examples are the zero-width
joiner and non-joiner characters (ZWNJ, U+200C, and ZWJ, U+200D), but
provisions for unambiguous labels may require that other characters
be restricted to particular contexts.
These characters must not appear in IDNs without additional
restrictions, typically because they are invisible in most scripts
but affect format or presentation in a few others or because they are
combining characters that are safe for use only in conjunction with
particular characters or scripts. In order to permit them to be used
at all, these characters are assigned to the category "CONTEXTUAL
RULE REQUIRED" and, when adequately understood, associated with a
rule. Examples of typical rules include "Must follow a character
from Script XYZ", "MAY occur only if the entire label is in Script
ABC", "MAY occur only if the previous and subsequent characters have
the DEF property".
Because it is easier to identify these characters than to know that
they are actually needed in IDNs or how to establish exactly the
right rules for each one, a character in the CONTEXTUAL RULE REQUIRED
category may have a null (missing) rule set in a given version of the
tables. Such characters MUST NOT appear in putative labels for
either registration or lookup. Of course, a later version of the
tables might contain a non-null rule.
Once a code point is assigned to this category, it is never removed.
If the assignment was somehow made in error, a contextual rule can be
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introduced to specify "can be used without further checking" or
"never permitted, treat a NEVER". The latter is, of course,
operationally equivalent to a null rule but conceptually a little bit
different.
If there is a rule, it MUST be evaluated and tested on registration
and SHOULD be evaluated and tested on lookup. If the test fails, the
label should not be processed for registration or lookup in the DNS.
5.1.4. NEVER
Some characters are sufficiently problematic for use in IDNs that
they should be excluded for both registration and lookup (i.e.,
conforming applications performing name resolution 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.
Of course, this category includes code points that have been removed
entirely from Unicode should such characters ever occur.
Characters that are placed in the "NEVER" category are never removed
from it or reclassified. If a character is classified as "NEVER" in
error and the error is sufficiently problematic, the only recourse is
to introduce a new code point into Unicode and classify it as "MAYBE"
or "ALWAYS" as appropriate.
5.1.5. 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 MUST
NOT appear in labels to be registered or looked up. The category
differs from NEVER 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.
5.2. Category Applicability
While it is discussed in other ways elsewhere in this document (see
Section 5.6, it is useful to stress here that these categories are
combined into much smaller groups for each of the contexts in which
they are used. Specifically:
5.2.1. Applications Using IDNs
For an application using IDNs --looking them up or otherwise
examining and organizing them-- ALWAYS, CONTEXTUAL RULE REQUIRED with
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an associated rule, MAYBE YES, and MAYBE NO are all equivalent. Any
character appearing in a putative label that falls into any of those
categories MUST be treated as valid and looked up or otherwise
processed. Of course, if the character falls into CONTEXTUAL RULE
REQUIRED, the rule must be evaluated and followed.
Similarly, NEVER, UNASSIGNED, and CONTEXTUAL RULE REQUIRED with no
rule specified are equivalent. Putative labels containing characters
in any of them MUST NOT be looked up.
5.2.2. Registration Policy
For an entity making policies for what may or may not be registered,
NEVER, UNASSIGNED, and CONTEXTUAL RULE REQUIRED with no rule
specified still make up one group and label strings with any
character in those categories MUST NOT be registered. If a character
is in CONTEXTUAL RULE REQUIRED but application of the rule fails,
that putative label also MUST NOT be registered.
For the other categories, these specifications only offer
recommendations. Registries MAY, and usually SHOULD, impose
additional restrictions, that make sense in their environments. The
recommendations are that, in making registration rules and
restrictions, characters in ALWAYS and characters with contextual
rules that can be successfully applied can generally be viewed as
safe for use in labels.
Characters in MAYBE YES are probably safe but should be permitted by
registries only with caution because they may not have not been fully
evaluated and there is a slight possibility that some of them may be
reclassified into CONTEXTUAL RULE REQUIRED or, less likely, into
NEVER. Consequently a registry should permit these characters only
if it has a sufficient understanding of them to evaluate the risks of
subsequent changes that might render labels unresolvable in the
future. In general, careful use of characters in MAYBE YES should
not be problematic for domains serving communities in which the
relevant script is heavily used and thoroughly understood but should
be discouraged for global domains and domains for which that
condition does not apply.
MAYBE NO implies a stronger warning about possibly registration
policies than MAYBE YES. The category includes characters from
archaic scripts, sets of characters that the relevant script
communities have concluded are probably inappropriate in domain
names, and characters whose relationship to others argues very
strongly for using variant techniques to ensure that different
entities not be able to register labels that differ only by
substitutions within character pairs. The specific variants
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discussed for Chinese characters in [RFC4713] are good examples of
this but character from other scripts may deserve similar treatment.
These recommendations do not address, but registries are encouraged
to apply, addition restrictions to reduce confusion and other
problems. For example, it is generally believed that labels
containing characters from more than one script are a bad practice
although 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 [RFC3743] and
[RFC4290] may be helpful in reducing problems that might be perceived
by users.
5.3. Layered Restrictions: Tables, Context, Registration, Applications
The essence of the character rules in IDNAbis is that there is no
magic bullet for any of the issues associated with a multiscript DNS.
Instead, we have defined a variety of approaches that, together,
constitute multiple lines of defense against ambiguity in identifiers
and loss of referential integrity. The actual character tables are
the first mechanism, protocol rules about how those characters are
applied or restricted in context are the second, and those two in
combination constitute the limits of what can be done by a protocol
alone. 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.
5.4. A New Character List -- History
[[anchor29: RFC Editor: please delete this subsection.]]
A preliminary version of a character list that reflects the above
categories has been was developed by the contributors to this
document [IDNA200X-Tables]. An earlier, initial, version was
developed by going through Unicode 5.0 one block and one character
class at a time and determining which characters, classes, and blocks
were clearly acceptable for IDNs, which one were clearly unacceptable
(e.g., all blocks consisting entirely of compatibility characters and
non-language symbols were excluded as were a number of character
classes), and which blocks and classes were in need of further study
or input from the relevant language communities. That effort was
successful, but not at the level of producing a directly-useful
character table. Additional iterations on the mailing list and with
UTC participation largely dropped the use of Unicode blocks and
focused on character classes, scripts, and properties together with
understandings gained from other Unicode Consortium efforts. Those
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iterations have been more successful. The iterative process has led
to the conclusion that the best strategy is likely to be a mixed one
consisting of (i) classification into "ALWAYS" and "MAYBE YES" versus
"MAYBE NO" and "NEVER" based on Unicode properties and a few
exceptions and (ii) discrimination between "ALWAYS" and "MAYBE YES"
and between "MAYBE NO" and "NEVER" based on script community criteria
about IDN appropriateness will be needed. An alternative would
involve an entirely new property specifically associated with
appropriateness for IDN use, but it is not clear that is either
necessary or desirable.
5.5. Understanding New Issues and Constraints
The discussion in [IDNA200X-Bidi] illustrates some areas in which
more work and input is needed. Other issues are raised by the
Unicode "presentation form" model and, in particular, by the need for
zero-width characters in some limited cases to correctly designate
those forms and by some other issues with combining characters in
different contexts. It is expected that, once expert and materially-
concerned parties are identified to supply contextual rules, such
problems will be resolved quickly and the questioned collections of
characters either added to the list of permitted characters or
permanently excluded.
5.6. ALWAYS, MAYBE, and Contextual Rules
As discussed above, characters will be associated with the "ALWAYS"
or "MAYBE YES" properties if they can plausibly be used in an IDN.
They are classified as "MAYBE NO" if it appears unlikely that they
should be used in IDNs but there is uncertainty on that point. Non-
language characters and other character codes that can be identified
as globally inappropriate for IDNs, such as conventional spaces and
punctuation, will be assigned to "NEVER" (i.e., will never be
permitted in IDNs). A character associated with "CONTEXTUAL RULE
REQUIRED" is acceptable in a label if it is associated with the
identifier of a contextual rule set and the test implied by the rule
set is successful. If no such identifier is present in the version
of the tables in use, the character is treated as roughly equivalent
to "NEVER", i.e., it MUST NOT be used in either registration or
lookup with that version of the tables. Because a rule set
identifier may be installed in a later table version, this status is
obviously not permanent. This general approach could, obviously, be
implemented in several ways, not just by the exact arrangements
suggested above.
The property and rule sets are used as follows:
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o Systems supporting domain name resolution SHOULD attempt to
resolve any label consisting entirely of characters that are in
the "ALWAYS" or "MAYBE" categories, including those that have not
been permanently excluded but that have not been classified with
regard to whether additional restrictions are needed, i.e., they
are categorized as "MAYBE YES" or "MAYBE NO". They MUST NOT
attempt to resolve label strings that contain unassigned character
positions (i.e., are part of the special UNASSIGNED category
discussed in Section 5.1.5 or those that contain "NEVER"
characters.
o Systems providing domain name registration functions MUST NOT
register any label that contains characters classified as "NEVER"
nor code points that are unassigned in the version of Unicode they
are using. If a character in a label has associated contextual
rules, they MUST NOT register the label unless the conditions
required by those rules are satisfied. They SHOULD NOT register
labels that contain a character assigned to a "MAYBE" category.
A procedure for assigning rules to characters with the "MAYBE YES" or
"MAYBE NO" property, and for assigning (or not) the property to
characters assigned in future version of Unicode, is outlined under
Section 12. A key part of that procedure will be specifications that
make it possible to add new characters and blocks without long delays
in implementation. The procedure will result in an update to
existing IANA-maintained registries.
6. Issues that Constrain Possible Solutions
6.1. Display and Network Order
The correct treatment of domain names requires a clear distinction
between Network Order (the order in which the code points are sent in
protocols) and Display Order (the order in which the code points are
displayed on a screen or paper). The order of labels in a domain
name is discussed in [IDNA200X-Bidi]. There are, however, also
questions about the order in which labels are displayed if left-to-
right and right-to-left labels are adjacent to each other, especially
if there are also multiple consecutive appearances of one of the
types. The decision about the display order is ultimately under the
control of user agents --including web browsers, mail clients, and
the like-- which may be highly localized. Even when formats are
specified by protocols, the full composition of an Internationalized
Resource Identifier (IRI) [RFC3987] or Internationalized Email
address contains elements other than the domain name. For example,
IRIs contain protocol identifiers and field delimiter syntax such as
"http://" or "mailto:" while email addresses contain the "@" to
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separate local parts from domain names. User agents are not required
to use those protocol-based forms directly but often do so. While
display, parsing, and processing within a label is specified by the
IDNA protocol and the associated documents, the relationship between
fully-qualified domain names and internationalized labels is
unchanged from the base DNS specifications. Comments here about such
full domain names are explanatory or examples of what might be done
and must not be considered normative.
Questions remain about protocol constraints implying that the overall
direction of these strings will always be left-to-right (or right-to-
left) for an IRI or email address, or if they even should conform to
such rules. These questions also have several possible answers.
Should a domain name abc.def, in which both labels are represented in
scripts that are written right-to-left, be displayed as fed.cba or
cba.fed? An IRI for clear text web access would, in network order,
begin with "http://" and the characters will appear as
"http://abc.def" -- but what does this suggest about the display
order? When entering a URI to many browsers, it may be possible to
provide only the domain name and leave the "http://" to be filled in
by default, assuming no tail (an approach that does not work for
other protocols). The natural display order for the typed domain
name on a right-to-left system is fed.cba. Does this change if a
protocol identifier, tail, and the corresponding delimiters are
specified?
While logic, precedent, and reality suggest that these are questions
for user interface design, not IETF protocol specifications,
experience in the 1980s and 1990s with mixing systems in which domain
name labels were read in network order (left-to-right) and those in
which those labels were read right-to-left would predict a great deal
of confusion, and heuristics that sometimes fail, if each
implementation of each application makes its own decisions on these
issues.
It should be obvious that any revision of IDNA must be more clear
about the distinction between network and display order for complete
(fully-qualified) domain names, as well as simply for individual
labels, than the original specification was. It is likely that some
strong suggestions should be made about display order as well.
6.2. Entry and Display in Applications
Applications can accept domain names using any character set or sets
desired by the application developer, and can display domain names in
any charset. That is, the IDNA protocol does not affect the
interface between users and applications.
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An IDNA-aware application can accept and display internationalized
domain names in two formats: the internationalized character set(s)
supported by the application (i.e., an appropriate local
representation of a U-label), and as an A-label. Applications MAY
allow the display and user input of A-labels, but are not encouraged
to do so except as an interface for special purposes, possibly for
debugging, or to cope with display limitations. A-labels are opaque
and ugly, and, where possible, should thus only be exposed to users
who absolutely need them. Because IDN labels can be rendered either
as the A-labels or U-labels, the application may reasonably have an
option for the user to select the preferred method of display; if it
does, rendering the U-label should normally be the default.
Domain names are often stored and transported in many places. For
example, they are part of documents such as mail messages and web
pages. They are transported in many parts of many protocols, such as
both the control commands and the RFC 2822 body parts of SMTP, and
the headers and the body content in HTTP. It is important to
remember that domain names appear both in domain name slots and in
the content that is passed over protocols.
In protocols and document formats that define how to handle
specification or negotiation of charsets, labels can be encoded in
any charset allowed by the protocol or document format. If a
protocol or document format only allows one charset, the labels MUST
be given in that charset. Of course, not all charsets can properly
represent all labels. If a U-label cannot be displayed in its
entirety, the only choice (without loss of information) may be to
display the A-label.
In any place where a protocol or document format allows transmission
of the characters in internationalized labels, labels SHOULD be
transmitted using whatever character encoding and escape mechanism
the protocol or document format uses at that place.
All protocols that use domain name slots already have the capacity
for handling domain names in the ASCII charset. Thus, A-labels can
inherently be handled by those protocols.
6.3. Linguistic Expectations: Ligatures, Digraphs, and Alternate
Character Forms
Users often have expectations about character matching or equivalence
that are based on their languages, and the orthography of those
languages, rather than anything that can be naturally accommodated in
a character coding system, especially if multiple languages are
written using the same script but using different conventions. A
Norwegian user might expect a label with the ae-ligature to be
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treated as the same label as one using the Swedish spelling with
a-umlaut even though applying that mapping to English would be
astonishing to users. A user in German might expect a label with an
o-umlaut and a label that had "oe" substituted, but was otherwise the
same, treated as equivalent even though that substitution would be a
clear error in Swedish. A Chinese user might expect automatic
matching of Simplified and Traditional Chinese characters, but
applying that matching for Korean or Japanese text would create
considerable confusion. For that matter, an English user might
expect "theater" and "theatre" to match.
Related issues arise because there are a number of languages written
with alphabetic scripts in which single phonemes are written using
two characters, termed a "digraph", for example, the "ph" in
"pharmacy" and "telephone". (Note that characters paired in this
manner can also appear consecutively without forming a digraph, as in
"tophat".) Certain digraphs are normally indicated typographically
by setting the two characters closer together than they would be if
used consecutively to represent different phonemes. Some digraphs
are fully joined as ligatures (strictly designating setting totally
without intervening white space, although the term is sometimes
applied to close set pairs). An example of this may be seen when the
word "encyclopaedia" is set with a U+00E6 LATIN SMALL LIGATURE AE
(and some would not consider that word correctly spelled unless the
ligature form was used or the "a" was dropped entirely). When these
ligature and digraph forms have the same interpretation across all
languages that use a given script, application of Unicode
normalization generally resolves the differences and causes them to
match. When they have different interpretations, any requirements
for matching must utilize other methods or users must be educated to
understand that matching will not occur.
Difficulties arise from the fact that a given ligature may be a
completely optional typographic convenience for representing a
digraph in one language (as in the above example with some spelling
conventions), while in another language it is a single character that
may not always be correctly representable by a two-letter sequence
(as in the above example with different spelling conventions). This
can be illustrated by many words in the Norwegian language, where the
"ae" ligature is the 27th letter of a 29-letter extended Latin
alphabet. It is equivalent to the 28th letter of the Swedish
alphabet (also containing 29 letters), U+00E4 LATIN SMALL LETTER A
WITH DIAERESIS, for which an "ae" cannot be substituted according to
current orthographic standards.
That character (U+00E4) is also part of the German alphabet where,
unlike in the Nordic languages, the two-character sequence "ae" is
usually treated as a fully acceptable alternate orthography. The
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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, in parallel to the "umlauted a", it cannot be
correctly represented as "oe" and in the Norwegian alphabet, where it
is represented, not as "umlauted o", 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.
For example, the German character Eszett (Sharp S, U+00DF) is
retained as itself by NFKC but mapped by Stringprep to "ss", but the
closely-related, but less frequently seen, character "Long S T"
(U+FB05) is a compatibility character that is mapped out by NFKC.
Unless exceptions are made, both will be treated as NEVER by
IDNA200X. But there is significant interest in an exception,
especially for Eszett. Depending on what the exception was, making
it would either raise some backward compatibility problems with
IDNA2003 or create an unusual special case that would highlight
differences in preferred orthography between German as written in
Germany and German as written in some other countries, notably
Switzerland. Additional discussion of issues with Eszett appear in
Section 9.7.
Additional cases with alphabets written right-to-left are described
in Section 6.5.
These issues with whether ligatures and digraphs are to be treated as
a sequence of characters or as a single standalone one constitute a
problem that cannot be resolved solely by operating on scripts. They
are, however, a key concern in the IDN context. Their satisfactory
resolution will require support in policies set by registries, which
therefore need to be particularly mindful not just of this specific
issue, but of all other related matters that cannot be dealt with on
an exclusively algorithmic basis.
Just as with the examples of different-looking characters that may be
assumed to be the same, it is in general impossible to deal with
these situations in a system such as IDNA -- or with Unicode
normalization generally -- since determining what to do requires
information about the language being used, context, or both.
Consequently, these specifications make no attempt to treat these
combined characters in any special way. However, their existence
provides a prime example of a situation in which a registry that is
aware of the language context in which labels are to be registered,
and where that language sometimes (or always) treats the two-
character sequences as equivalent to the combined form, should give
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serious consideration to applying a "variant" model [RFC3743]
[RFC4290] to reduce the opportunities for user confusion and fraud
that would result from the related strings being registered to
different parties.
6.4. Case Mapping and Related Issues
Traditionally in the DNS, ASCII letters have been stored with their
case preserved. Matching during the query process has been case-
independent, but none of the information that might be represented by
choices of case has been lost. That model has been accidentally
helpful because, as people have created DNS labels by catenating
words (or parts of words) to form labels, case has often been used to
distinguish among components and make the labels more memorable.
The solution of keeping the characters separate but doing matching
independent of case is not feasible with an IDNA-like model because
the matching would then have to be done on the server rather than
have characters mapped on the client. That situation was recognized
in IDNA2003 and nothing in IDNA200X fundamentally changes it or could
do so. In IDNA, all upper-case characters are mapped to lower-case
ones and, in general, all code points that represent alternate forms
of the same character are mapped to that character (including mapping
Greek final form sigma to the lower case sigma character). IDNA200X
permits, at the risk of some incompatibility, slightly more
flexibility in this area. That additional flexibility still does not
solve the problem with final form sigma and other characters that
Unicode treats as completely separate characters that match only
under casemapping if at all. Many people now believe these should be
handled as separate characters so information about them can be
preserved in the transformations to A-labels and back. However
making a change to permit that behavior would create a situation in
which the same string, valid in both protocols, would be interpreted
differently by IDNA2003 and IDNA200X. That would violate one of the
conditions discussed in Section 9.3.1 and hence require a prefix
change. Of course, if a prefix change were made (at the costs
discussed in Section 9.3.3) there would be several options,
including, if desired, assigning the characer to the CONTEXTUAL RULE
REQUIRED category and requiring that it only be used in carefully-
selected contexts.
6.5. 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, may not include any
characters with strong left-to-right properties (which excludes other
alphabetic characters but permits European digits), and rejects any
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other string that contains a right-to-left character. This is one of
the few places where the IDNA algorithms (both old and new) are
required to look at an entire label, not just at individual
characters. Unfortunately, the algorithmic model used in IDNA2003
fails when the final character in a right-to-left string requires a
combining mark in order to be correctly represented. The mark will
be the final code point in the string but is not identified with the
right-to-left character attribute and Stringprep therefore rejects
the string.
This problem manifests itself in languages written with consonantal
alphabets to which diacritical vocalic systems are applied, and in
languages with orthographies derived from them where the combining
marks may have different functionality. In both cases the combining
marks can be essential components of the orthography. Examples of
this are Yiddish, written with an extended Hebrew script, and Dhivehi
(the official language of Maldives) which is written in the Thaana
script (which is, in turn, derived from the Arabic script). Other
languages are still being investigated, but the new rules for right
to left scripts are described in [IDNA200X-Bidi].
7. IDNs and the Robustness Principle
The model of IDNs described in this document can be seen as a
particular instance of the "Robustness Principle" that has been so
important to other aspects of Internet protocol design. This
principle is often stated as "Be conservative about what you send and
liberal in what you accept" (See, e.g., RFC 1123, Section 1.2.2
[RFC1123]). For IDNs to work well, registries must have or require
sensible policies about what is registered -- conservative policies
-- and implement and enforce them. Registries, registrars, or other
actors who do not do so, or who get too liberal, too greedy, or too
weird may deserve punishment that will primarily be meted out in the
marketplace or by consumer protection rules and legislation. One can
debate whether or not "punishment by browser vendor" is an effective
marketplace tool, but it falls into the general category of
approaches being discussed here. In any event, the Protocol Police
(an important, although mythical, Internet mechanism for enforcing
protocol conformance) are going to be worth about as much here as
they usually are -- i.e., very little -- simply because, unlike the
marketplace and legal and regulatory mechanisms, they have no
enforcement power.
Conversely, resolvers can (and SHOULD or maybe MUST) 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
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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.
As mentioned above, if a string doesn't resolve, it makes no
difference whether it simply wasn't registered or was prohibited by
some rule.
If resolvers, as a user interface (UI) or other local matter, decide
to warn about some strings that are valid under the global rules but
that they perceive as dangerous, that is their prerogative and we can
only hope that the market (and maybe regulators) will reward the good
choices and punish the bad ones. In this context, a resolver that
decides a string that is valid under the protocol is dangerous and
refuses to look it up is in violation of the protocols; one that is
willing to look something up, but warns against it, is exercising a
local choice.
8. Front-end and User Interface Processing
Domain names may be identified and processed in many contexts. They
may be typed in by users either by themselves or as part of URIs or
IRIs. They may occur in running text or be processed by one system
after being provided in another. They may wish to try to normalize
URLs so as to determine (or guess) whether a reference is valid or
two references point to the same object without actually looking the
objects up and comparing them. Some of these goals may be more
easily and reliably satisfied than others. While there are strong
arguments for any domain name that is placed "on the wire" --
transmitted between systems -- to be in the minimum-ambiguity forms
of A-labels, U-labels, or LDH-labels, it is inevitable that programs
that process domain names will encounter variant forms. One source
of such forms will be labels created under IDNA2003. Because of the
way that protocol was specified, there are a significant number of
domain names in files on the Internet that use characters that cannot
be represented directly in domain names but for which interpretations
are provided. There are two major categories of such characters,
those that are removed by NFKC normalization and those upper-case
characters that are mapped to lower-case (there are also a few
characters that are given special-case mapping treatment in
Stringprep).
Other issues in domain name identification and processing arise
because IDNA2003 specified that several other characters be treated
as equivalent to the ASCII period (dot, full stop) character used as
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a label separator. If a domain name appears in an arbitrary context
(such as running text), one may be faced with the requirement to know
that a string is a domain name in order to adjust for the different
forms of dots but also to have traditional dots to recognize that a
string is a domain name -- an obvious contradiction.
As discussed elsewhere in this document, the IDNA200X model is to
remove all of these mappings and interpretations, including the
equivalence of different forms of dots, from the protocol, leaving
such mappings to local processing. This should not be taken to imply
that local processing is optional or can be avoided entirely.
Instead, unless the program context is such that it is known that any
IDNs that appear will be either U-labels or A-labels, some local
processing of apparent domain name strings will be required, both to
maintain compatibility with IDNA2003 and to prevent user
astonishment. Such local processing, while not specified in this
document or the associated ones, will generally take one of two
forms:
o Generic Preprocessing.
When the context in which the program or system that processes
domain names operates is global, a reasonable balance must be
found that is sensitive to the broad range of local needs and
assumptions while, at the same time, not sacrificing the needs of
one language, script, or user population to those of another.
For this case, the best practice will usually be to apply NFKC and
case-mapping (or, perhaps better yet, Stringprep itself), plus
dot-mapping where appropriate, to the domain name string prior to
applying IDNA. That practice will not only yield a reasonable
compromise of user experience with protocol requirements but will
be almost completely compatible with the various forms permitted
by IDNA2003.
o Highly Localized Preprocessing.
Unlike the case above, there will be some situations in which
software will be highly localized for a particular environment and
carefully adapted to the expectations of users in that
environment. The many recent discussions about using the Internet
to preserve and support local cultures suggest that these cases
may be more common in the future than they have been so far.
In these cases, we should avoid trying to tell implementers what
they should do, if only because they are quite likely (and for
good reason) to ignore us. We would assume that they would map
characters that the intuitions of their users would suggest be
mapped. One can imagine switches about whether some sorts of
mappings occur, warnings before applying them or, in a slightly
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more extreme version of the approach taken in Internet Explorer
version 7 (IE7), utterly refuse to handle "strange" characters at
all if they appear in U-label form. None of those local decisions
are a threat to interoperability as long as (i) only U-labels and
A-labels are used in interchange with systems outside the local
environment, (ii) no character that would be valid in a U-label as
itself is mapped to something else, (iii) any local mappings are
applied as a preprocessing step (or, for conversions from U-labels
or A-labels to presentation forms, postprocessing), not as part of
IDNA processing proper, and (iv) appropriate consideration is
given to labels that might have entered the environment in
conformance to IDNA2003.
9. Migration and Version Synchronization
9.1. Design Criteria
As mentioned above and in RFC 4690, two key goals of this work are to
enable applications to be agnostic about whether they are being run
in environments supporting any Unicode version from 3.2 onward and to
permit incrementally adding permitted scripts and other character
collections without disruption. The mechanisms that support this are
outlined above, but this section reviews them in a context that may
be more helpful to those who need to understand the approach and make
plans for it.
9.1.1. General IDNA Validity Criteria
The general criteria for a putative label, and the collection of
characters that make it up, to be considered IDNA-valid are:
o The characters are "letters", numerals, or otherwise used to write
words in some language. Symbols, drawing characters, and various
notational characters are permanently excluded -- some because
they are actively dangerous in URI, IRI, or similar contexts and
others because there is no evidence that they are important enough
to Internet operations or internationalization to justify
inclusion and the complexities that would come with it (additional
discussion and rationale for the symbol decision appears in
Section 9.5).
If strings are read out loud, rather than seen on paper, there are
opportunities for considerable confusion between the name of a
symbol (and a single symbol may have multiple names) and the
symbol itself.
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o As a simplified example of this, assume one wanted to use a
"heart" or "star" symbol in a label. This is problematic because
the those names are ambiguous in the Unicode system of naming (the
actual Unicode names require far more qualification). A user or
would-be registrant has no way to know --absent careful study of
the code tables-- whether it is ambiguous (e.g., where there are
multiple "heart" characters) or not. Conversely, the user seeing
the hypothetical label doesn't know whether to read it --try to
transmit it to a colleague by voice-- as "heart", as "love", as
"black heart", or as any of the other examples below.
o The actual situation is even worse than this. There is no
possible way for a normal, casual, user to tell the difference
between the hearts of U+2665 and U+2765 and the stars of U+2606
and U+2729 or the without somehow knowing to look for a
distinction. We have a white heart (U+2661) and few black hearts
and describing a label containing a heart symbol is hopelessly
ambiguous. In cities where "Square" is a popular part of a
location name, one might well want to use a square symbol in a
label as well and there are far more squares of various flavors in
Unicode than there are hearts or stars.
o Unlike font and style variations in language (and "mathematical")
characters, identification of compatibility encodings and the
application of NFKC is of no help here. All of these symbols (and
many other pairs and triples) are treated as valid, independent,
non-reducible, code points.
o Other than in very exceptional cases, e.g., where they are needed
to write substantially any word of a given language, punctuation
characters are excluded as well. The fact that a word exists is
not proof that it should be usable in a DNS label and DNS labels
are not expected to be usable for multiple-word phrases (although
they are not prohibited if the conventions and orthography of a
particular language cause that to be possible).
o Characters that are unassigned in the version of Unicode being
used by the registry or application are not permitted, even on
resolution (lookup). There are at least two reasons for this.
First, unlike the conditions contemplated in IDNA2003 (except for
right-to-left text), we now understand that tests involving the
context of characters (e.g., some characters being permitted only
adjacent to other ones of specific types) and integrity tests on
complete labels will be needed. Unassigned code points cannot be
permitted because one cannot determine the contextual rules that
particular code points will require before characters are assigned
to them and the properties of those characters fully understood.
Second, Unicode specifies that an unassigned code point normalizes
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to itself. If the code point is later assigned to a character,
and particularly if the newly-assigned code point has a combining
class that determines its placement relative to other combining
characters, it could normalize to some other code point or
sequence, creating confusion and/or violating other rules listed
here.
o Any character that is mapped to another character by Nameprep2003
or by a current version of NFKC is prohibited as input to IDNA
(for either registration or resolution). Implementers of user
interfaces to applications are free to make those conversions when
they consider them suitable for their operating system
environments, context, or users.
Tables used to identify the characters that are IDNA-valid are
expected to be driven by the principles above. The principles are
not just an interpretation of the tables.
9.1.2. Characters in Registrations
For registration purposes, the collection of IDNA-valid characters
will be a growing list. The conditions for entry to the list for a
set of characters are (i) that they meet the conditions for IDNA-
valid characters discussed immediately above and (ii) that consensus
can be reached about usage and contextual rules. Because it is
likely that such consensus cannot be reached immediately about the
correct contextual rules for some characters -- e.g., the use of
invisible ("zero-width") characters to modify presentation forms --
some sets of characters may be deferred from the IDNA-valid set even
if they appear in a current version of Unicode. Of course,
characters first assigned code points in later versions of Unicode
would need to be introduced into IDNA only after those code points
are assigned.
9.1.3. Labels in Registration
Anyone entering a label into a DNS zone must properly validate that
label -- i.e., be sure that the criteria for an A-label are met -- in
order for Unicode version-independence to be possible. In
particular:
o Any label that contains hyphens as its third and fourth characters
MUST be IDNA-valid. This implies that, (i) if the third and
fourth characters are hyphens, the first and second ones MUST be
"xn" until and unless this specification is updated to permit
other prefixes and (ii) labels starting in "xn--" MUST be valid
A-labels, as discussed in Section 3 above.
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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 described above), 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.
Systems looking up or resolving DNS labels MUST be able to assume
that those rules were followed.
9.1.4. Labels in Resolution (Lookup)
Anyone looking up a label in a DNS zone MUST
o Maintain a consistent set of tables, as discussed above. As with
registration, the tables need not reflect the latest version of
Unicode but they MUST be consistent.
o Validate labels to be looked up only to the extent of determining
that the U-label does not contain either code points prohibited by
IDNA (categorized as "NEVER") or code points that are unassigned
in its version of Unicode. No attempt should be made to validate
contextual rules about characters, including mixed-script label
prohibitions, although such rules MAY be used to influence
presentation decisions in the user interface.
By avoiding applying its own interpretation of which labels are valid
as a means of rejecting lookup attempts, the resolver application
becomes less sensitive to version incompatibilities with the
particular zone registry associated with the domain name.
Under this model, a registry (or entity communicating with a registry
to accomplish name registrations) will need to update its tables --
both the Unicode-associated tables and the tables of permitted IDN
characters -- to enable a new script or other set of new characters.
It will not be affected by newer versions of Unicode, or newly-
authorized characters, until and unless it wishes to make those
registrations. The registration side is also responsible --under the
protocol and to registrants and users-- for much more careful
checking than is expected of applications systems that look names up,
both checking as required by the protocol and checking required by
whatever policies it develops for minimizing risks due to confusable
characters and sequences and preserving language or script integrity.
An application or client that looks names up in the DNS will be able
to resolve any name that is registered, as long as its version of the
Unicode-associated tables is sufficiently up-to-date to interpret all
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of the characters in the label. It SHOULD distinguish, in its
messages to users, 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).
9.2. More Flexibility in User Agents
One key philosophical difference between IDNA2003 and this proposal
is that the former provided mappings for many characters into others.
These mappings were not reversible: the original string could not be
recovered from the form stored in the DNS and, probably as a
consequence, users became confused about what characters were valid
for IDNs and which ones were not. Too many times, the answer to the
question "can this character be used in an IDN" was "it depends on
exactly what you mean by 'used'".
IDNA200X does not perform these mappings but, instead, prohibits the
characters that would be mapped to others. As examples, while
mathematical characters based on Latin ones are accepted as input to
IDNA2003, they are prohibited in IDNA200X. Similarly, double-width
characters and other variations are prohibited as IDNA input.
Since the rules in [IDNA200X-Tables] provide that only strings that
are stable under NFKC are valid, if it is convenient for an
application to perform NFKC normalization before lookup, that
operation is safe since this will never make the application unable
to look up any valid string.
In many cases these prohibitions should have no effect on what the
user can type at resolution time: it is perfectly reasonable for
systems that support user interfaces at lookup time, to perform some
character mapping that is appropriate to the local environment prior
to actual invocation of IDNA as part of the Unicode conversions of
[IDNA200X-protocol] above. However, those changes will be local ones
only -- local to environments in which users will clearly understand
that the character forms are equivalent. For use in interchange
among systems, it appears to be much more important that U-labels and
A-labels can be mapped back and forth without loss of information.
One specific, and very important, instance of this change in strategy
arises with case-folding. In the ASCII-only DNS, names are looked up
and matched in a case-independent way, but no actual case-folding
occurs. Names can be placed in the DNS in either upper or lower case
form (or any mixture of them) and that form is preserved, returned in
queries, and so on. IDNA2003 attempted to simulate that behavior by
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performing case-mapping at registration time (resulting in only
lower-case IDNs in the DNS) and when names were looked up.
As suggested earlier in this section, it appears to be desirable to
do as little character mapping as possible consistent with having
Unicode work correctly (e.g., NFC mapping to resolve different
codings for the same character is still necessary) and to make the
mapping between A-labels and U-labels idempotent. Case-mapping is
not an exception to this principle. If only lower case characters
can be registered in the DNS (i.e., present in a U-label), then
IDNA200X should prohibit upper-case characters as input. Some other
considerations reinforce this conclusion. For example, an essential
element of the ASCII case-mapping functions is that
uppercase(character) must be equal to
uppercase(lowercase(character)). That requirement may not be
satisfied with IDNs. The relationship between upper case and lower
case may even be language-dependent, with different languages (or
even the same language in different areas) using different mappings.
Of course, the expectations of users who are accustomed to a case-
insensitive DNS environment will probably be well-served if user
agents perform case mapping prior to IDNA processing, but the IDNA
procedures themselves should neither require such mapping nor expect
it when it isn't natural to the localized environment.
9.3. The Question of Prefix Changes
The conditions that would require a change in the IDNA "prefix"
("xn--" for the version of IDNA specified in [RFC3490]) have been a
great concern to the community. A prefix change would clearly be
necessary if the algorithms were modified in a manner that would
create serious ambiguities during subsequent transition in
registrations. This section summarizes our conclusions about the
conditions under which changes in prefix would be necessary and the
implications of such a change.
9.3.1. Conditions Requiring a Prefix Change
An IDN prefix change is needed if a given string would resolve or
otherwise be interpreted differently depending on the version of the
protocol or tables being used. Consequently, work to update IDNs
would require a prefix change if, and only if, one of the following
four conditions were met:
1. The conversion of an A-label to Unicode (i.e., a U-label) yields
one string under IDNA2003 (RFC3490) and a different string under
IDNA200X.
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2. An input string that is valid under IDNA2003 and also valid under
IDNA200X yields two different A-labels with the different
versions of IDNA. This condition is believed to be essentially
equivalent to the one above.
Note, however, that if the input string is valid under one
version and not valid under the other, this condition does not
apply. See the first item in Section 9.3.2, below.
3. A fundamental change is made to the semantics of the string that
is inserted in the DNS, e.g., if a decision were made to try to
include language or specific script information in that string,
rather than having it be just a string of characters.
4. A sufficiently large number of characters is added to Unicode so
that the Punycode mechanism for block offsets no longer has
enough capacity to reference the higher-numbered planes and
blocks. This condition is unlikely even in the long term and
certain not to arise in the next few years.
9.3.2. Conditions Not Requiring a Prefix Change
In particular, as a result of the principles described above, none of
the following changes require a new prefix:
1. Prohibition of some characters as input to IDNA. This may make
names that are now registered inaccessible, but does not require
a prefix change.
2. Adjustments in Stringprep tables or IDNA actions, including
normalization definitions, that affect characters that were
already invalid under IDNA2003.
3. Changes in the style of definitions of Stringprep or Nameprep
that do not alter the actions performed by them.
9.3.3. Implications of Prefix Changes
While it might be possible to make a prefix change, the costs of such
a change are considerable. Even if they wanted to do so, all
registries could not convert all IDNA2003 ("xn--") registrations to a
new form at the same time, and the costs would be considerable.
Unless all existing registrations were simply to be declared invalid,
and perhaps even then, systems that needed to support both labels
with old prefixes and labels with new ones would first process a
putative label under the IDNA200X rules and try to look it up and
then, if it were not found, would process the label under IDNA2003
rules and look it up again. That process could significantly slow
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down all processing that involved IDNs in the DNS especially since,
in principle, a fully-qualified name could contain a mixture of
labels that were registered with the old and new prefixes, a
situation that would make the use of DNS caching very difficult. In
addition, looking up the same input string as two separate A-labels
would create some potential for confusion and attacks, since they
could, in principle, resolve to different targets.
Consequently, a prefix change is to be avoided if at all possible,
even if it means accepting some IDNA2003 decisions about character
distinctions as irreversible.
9.4. Stringprep Changes and Compatibility
Concerns have been expressed about problems for non-DNS uses of
Stringprep being caused by changes to the specification intended to
improve the handling of IDNs, most notably as this might affect
identification and authentication protocols. Section 9.3, above,
essentially also applies in this context. The proposed new inclusion
tables [IDNA200X-Tables], the reduction in the number of characters
permitted as input for registration or resolution (Section 5), and
even the proposed changes in handling of right-to-left strings
[IDNA200X-Bidi] either give interpretations to strings prohibited
under IDNA2003 or prohibit strings that IDNA2003 permitted. Strings
that are valid under both IDNA2003 and IDNA200X, and the
corresponding versions of Stringprep, are not changed in
interpretation. This protocol does not use either Nameprep or
Stringprep as specified in IDNA2003.
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. Similarly, internationalized SCSI
identifiers and other protocol components are likely to have
different requirements than IDNs.
Perhaps even more important in practice, since most other known uses
of Stringprep encode or process characters that are already in
normalized form and expect the use of only those characters that can
be used in writing words of languages, the changes proposed here and
in [IDNA200X-Tables] are unlikely to have any effect at all,
especially not on registries and registrations that follow rules
already in existence when this work started.
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9.5. The Symbol Question
One of the major differences between this specification and the
original version of IDNA is that the original version permitted non-
letter symbols of various sorts, including punctuation and line-
drawing symbols, in the protocol. They were always discouraged in
practice. In particular, both the "IESG Statement" about IDNA and
all versions of the ICANN Guidelines specify that only language
characters be used in labels. This specification bans the symbols
entirely. There are several reasons for this, which include:
o As discussed elsewhere, the original IDNA specification assumed
that as many Unicode characters as possible should be permitted,
directly or via mapping to other characters, in IDNs. This
specification operates on an inclusion model, extrapolating from
the LDH rules --which have served the Internet very well-- to a
Unicode base rather than an ASCII base.
o Most Unicode names for letters are, in most cases, fairly
intuitive, unambiguous and recognizable to users of the relevant
script. Symbol names are more problematic because there may be no
general agreement on whether a particular glyph matches a symbol,
there are no uniform conventions for naming, variations such as
outline, solid, and shaded forms may or may not exist, and so on.
As just one example, consider a "heart" symbol as it might appear
in a logo that might be read as "I love...". While the user might
read such a logo as "I love..." or "I heart...", considerable
knowledge of the coding distinctions made in Unicode is needed to
know that there more than one "heart" character (e.g., U+2665,
U+2661, and U+2765) and how to describe it.
o The consequence of these ambiguities of description and
dependencies on distinctions that were, or were not, made in
Unicode codings, is that symbols are a very poor basis for
reliable communication. 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.
9.6. Migration Between Unicode Versions: Unassigned Code Points
In IDNA2003, labels containing unassigned code points are resolved on
the theory that, if they appear in labels and can be resolved, the
relevant standards much have changed and the registry has properly
allocated only assigned values. This is the one main provision in
IDNA2003 for migration to new versions of Unicode.
In this specification, strings containing unassigned code points MUST
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NOT be either looked up or registered. There are several reasons for
this, with the most important ones being:
o It cannot be known with sufficient reliability in advance that a
code point that was not previously assigned will not be assigned
to a compatibility character. In IDNA2003, since there is no
direct dependency on NFKC (Stringprep's tables are based on NFKC,
but IDNA2003 depends only on Stringprep), allocation of a
compatibility character might produce some odd situations, but it
would not be a problem. In IDNA200X, where compatibility
characters are generally assigned to NEVER, permitting strings
containing unassigned characters to be looked up would permit
violating the principle that characters in NEVER are not looked
up.
o More generally, the status of an unassigned character with regard
to the ALWAYS, MAYBE, and CONTEXTUAL RULE REQUIRED (with the last
being the most important) cannot be evaluated until a character is
actually assigned and known.
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 will have been coded by
Unicode 5.1 and hence unassigned code points will be assigned only to
obscure characters or archaic scripts. Unfortunately, that does not
appear to be a safe assumption for at least two reasons. First, much
the same claim of completeness was made for Unicode 4.0 and 5.0. The
reality is that a script that is obscure to much of the rest of the
world may still be very important to those who use it and cultural
preservation principles will make it inappropriate to declare the
script of no importance in IDNs. Second, we already have
counterexamples in, e.g., the relationships associated with new Han
characters being added (whether in the BMP or in Unicode Plane 2.
9.7. Other Compatibility Issues
The existing (2003) IDNA model has several odd artifacts which occur
largely by accident. 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 otherwise in ASCII is not really an IDN
(in the U-label sense defined above) at all. After Nameprep maps the
Eszett out, the result is an ASCII string and so does not get an xn--
prefix, but the string that can be displayed to a user appears to be
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an IDN. The proposed IDNA200X 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.
10. Acknowledgments
The editor and contributors would like to express their thanks to
those who contributed significant early review comments, sometimes
accompanied by text, especially Mark Davis, Paul Hoffman, Simon
Josefsson, and Sam Weiler. In addition, some specific ideas were
incorporated from suggestions, text, or comments about sections that
were unclear supplied by Frank Ellerman, Michael Everson, Asmus
Freytag, Michel Suignard, and Ken Whistler, although, as usual, they
bear little or no responsibility for the conclusions the editor and
contributors reached after receiving their suggestions. Thanks are
also due to Vint Cerf, Debbie Garside, and Jefsey Morphin for
conversations that led to considerable improvements in the content of
this document.
11. Contributors
While the listed editor held the pen, this document represents the
joint work and conclusions of an ad hoc design team consisting of the
editor and, in alphabetic order, Harald Alvestrand, Tina Dam, Patrik
Faltstrom, and Cary Karp. In addition, there were many specific
contributions and helpful comments from those listed in the
Acknowledgments section and others who have contributed to the
development and use of the IDNA protocols.
12. IANA Considerations
12.1. IDNA Permitted Character Registry
The distinction between "MAYBE" code points and those classified into
"ALWAYS" and "NEVER" (see Section 5) requires a registry of
characters and scripts and their categories. IANA is requested to
establish that registry, using the "expert reviewer" model. Unlike
usual practice, we recommend that the "expert reviewer" be a
committee that reflects expertise on the relevant scripts, and
encourage IANA, the IESG, and IAB to establish liaisons and work
together with other relevant standards bodies to populate that
committee and its procedures over the long term.
Updates to this registry will typically require changes, either to
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properties to which classification rules can be applied or to
explicit tables (such as those in [IDNA200X-Tables]) in those
implementations that wish to reflect the most up-to-date rules. In
particular, implementations that continue to use older tables will be
unable to take advantage of, or look up labels using, newly-
introduced Unicode characters.
12.2. IDNA Context Registry
For characters that are defined in the permitted character list as
requiring a contextual rule (i.e., the types of rule described in
Section 5.1.3, IANA will create and maintain a list of approved
contextual rules, using the registration methods described above.
IANA should develop a format for that registry, or a copy of it
maintained in parallel, that is convenient for retrieval and machine
processing and publish the location of that version.
12.3. IANA Repository of IDN Practices of TLDs
This registry, often described as the "IANA Language Character Set
Registry" or "IANA Script Registry" (both somewhat misleading terms)
is maintained by IANA at the request of ICANN. It is used to provide
a central documentation repository of the IDN policies used by top
level domain (TLD) registries who volunteer to contribute to it and
is used in conjunction with ICANN Guidelines for IDN use.
It is not an IETF-managed registry and, while the protocol changes
specified here may call for some revisions to the tables, these
specifications have no direct effect on that registry and no IANA
action is required as a result.
13. Security Considerations
Security on the Internet partly relies on the DNS. Thus, any change
to the characteristics of the DNS can change the security of much of
the Internet.
Domain names are used by users to identify and connect to Internet
servers. The security of the Internet is compromised if a user
entering a single internationalized name is connected to different
servers based on different interpretations of the internationalized
domain name.
When systems use local character sets other than ASCII and Unicode,
this specification leaves the problem of transcoding between the
local character set and Unicode up to the application or local
system. If different applications (or different versions of one
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application) implement different transcoding rules, they could
interpret the same name differently and contact different servers.
This problem is not solved by security protocols like TLS that do not
take local character sets into account.
To help prevent confusion between characters that are visually
similar, it is suggested that implementations provide visual
indications where a domain name contains multiple scripts. Such
mechanisms can also be used to show when a name contains a mixture of
simplified and traditional Chinese characters, or to distinguish zero
and one from O and l. DNS zone adminstrators may impose restrictions
(subject to the limitations identified elsewhere in this document)
that try to minimize characters that have similar appearance or
similar interpretations. It is worth noting that there are no
comprehensive technical solutions to the problems of confusable
characters. One can reduce the extent of the problems in various
ways, but probably never eliminate it. Some specific suggestions
about identification and handling of confusable characters appear in
a Unicode Consortium publication [Unicode-UTR36].
The registration and resolution models described above and in
[IDNA200X-protocol] change the mechanisms available for applications
and resolvers to determine the validity of labels they encounter. In
some respects, the ability to test is strengthened. For example,
putative labels that contain unassigned code points will now be
rejected, while IDNA2003 permitted them (something that is now
recognized as a considerable source of risk). On the other hand, the
protocol specification no longer assumes that the application that
looks up a name will be able to determine, and apply, information
about the protocol version used in registration. In theory, that may
increase risk since the application will be able to do less pre-
lookup validation. In practice, the protection afforded by that test
has been largely illusory for reasons explained in RFC 4690 and
above.
Any change to Stringprep or, more broadly, the IETF's model of the
use of internationalized character strings in different protocols,
creates some risk of inadvertent changes to those protocols,
invalidating deployed applications or databases, and so on. Our
current hypothesis is that the same considerations that would require
changing the IDN prefix (see Section 9.3.2) are the ones that would,
e.g., invalidate certificates or hashes that depend on Stringprep,
but those cases require careful consideration and evaluation. More
important, it is not necessary to change Stringprep2003 at all in
order to make the IDNA changes contemplated here. It is far
preferable to create a separate document, or separate profile
components, for IDN work, leaving the question of upgrading to other
protocols to experts on them and eliminating any possible
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synchronization dependency between IDNA changes and possible upgrades
to security protocols or conventions.
14. Change Log
[[anchor46: RFC Editor: Please remove this section.]]
14.1. Version -01
Version -01 of this document is a considerable rewrite from -00.
Many sections have been clarified or extended and several new
sections have been added to reflect discussions in a number of
contexts since -00 was issued.
14.2. Version -02
o Corrected several editorial errors including an accidentally-
introduced misstatement about NFKC.
o Extensively revised the document to synchronize its terminology
with version 03 of [IDNA200X-Tables] and to provide a better
conceptual framework for its categories and how they are used.
Added new material to clarify terminology and relationships with
other efforts. More subtle changes in this version lay the
groundwork for separating the document into a conceptual overview
and a protocol specification for version 03.
14.3. Version -03
o Removed protocol materials to a separate document and incorporated
rationale and explanation materials from the original
specification in RFC 3960 into this document. Cleaned up earlier
text to reflect a more mature specification and restructured
several sections and added additional rationale material.
o Strengthened and clarified the A-label / U-label/ LDH-label
definition.
o Retitled the document to reflect its evolving role.
14.4. Version -04
o Moved more text from "protocol" and further reorganized material.
o Provided new material on "Contextual Rule Required.
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o Improved consistency of terminology, both internally and with the
"tables" document.
o Improved the IANA Considerations section and discussed the
existing IDNA-related registry.
o More small changes to increase consistency.
14.5. Version -05
Changed "YES" category back to "ALWAYS" to re-synch with the tables
document and provide clearer terminology.
14.6. Version -06
o Clarified the prohibitions on strings that look like A-labels but
are not and on unassigned code points.
o Clarified length restrictions on IDN labels.
o Revised the terminology definitions to remove the impression of
circularity and removed invocations of ToASCII and ToUnicode,
which do not exist in IDNA200X.
o Added a new section on front-end processing.
o Added a new section to discuss case-mapping.
o Extended the discussion of prefix changes to identify the
implications of making one.
o Several more editorial improvements, corrected references, and
similar adjustments.
15. References
15.1. Normative References
[ASCII] American National Standards Institute (formerly United
States of America Standards Institute), "USA Code for
Information Interchange", ANSI X3.4-1968, 1968.
ANSI X3.4-1968 has been replaced by newer versions with
slight modifications, but the 1968 version remains
definitive for the Internet.
[IDNA200X-Bidi]
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Alvestrand, H. and C. Karp, "An updated IDNA criterion for
right-to-left scripts", January 2008, .
[IDNA200X-Tables]
Faltstrom, P., "The Unicode Code Points and IDN",
November 2007, .
A version of this document is available in HTML format at
http://stupid.domain.name/idnabis/
draft-faltstrom-idnabis-tables-03.html
[IDNA200X-protocol]
Klensin, J., "Internationalizing Domain Names in
Applications (IDNA): Protocol", November 2007, .
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3454] Hoffman, P. and M. Blanchet, "Preparation of
Internationalized Strings ("stringprep")", RFC 3454,
December 2002.
[RFC3490] Faltstrom, P., Hoffman, P., and A. Costello,
"Internationalizing Domain Names in Applications (IDNA)",
RFC 3490, March 2003.
[RFC3491] Hoffman, P. and M. Blanchet, "Nameprep: A Stringprep
Profile for Internationalized Domain Names (IDN)",
RFC 3491, March 2003.
[RFC3492] Costello, A., "Punycode: A Bootstring encoding of Unicode
for Internationalized Domain Names in Applications
(IDNA)", RFC 3492, March 2003.
[Unicode-UAX15]
The Unicode Consortium, "Unicode Standard Annex #15:
Unicode Normalization Forms", 2006,
.
[Unicode32]
The Unicode Consortium, "The Unicode Standard, Version
3.0", 2000.
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(Reading, MA, Addison-Wesley, 2000. ISBN 0-201-61633-5).
Version 3.2 consists of the definition in that book as
amended by the Unicode Standard Annex #27: Unicode 3.1
(http://www.unicode.org/reports/tr27/) and by the Unicode
Standard Annex #28: Unicode 3.2
(http://www.unicode.org/reports/tr28/).
[Unicode40]
The Unicode Consortium, "The Unicode Standard, Version
4.0", 2003.
[Unicode50]
The Unicode Consortium, "The Unicode Standard, Version
5.0", 2007.
Boston, MA, USA: Addison-Wesley. ISBN 0-321-48091-0
15.2. Informative References
[BIG5] Institute for Information Industry of Taiwan, "Computer
Chinese Glyph and Character Code Mapping Table, Technical
Report C-26", 1984.
There are several forms and variations and a closely-
related standard, CNS 11643. See the discussion in
Chapter 3 of Lunde, K., CJKV Information Processing,
O'Reilly & Associates, 1999
[GB18030] "Chinese National Standard GB 18030-2000: Information
Technology -- Chinese ideograms coded character set for
information interchange -- Extension for the basic set.",
2000.
[ICANN-Guidelines]
ICANN, "IDN Implementation Guidelines", 2006,
.
[RFC0810] Feinler, E., Harrenstien, K., Su, Z., and V. White, "DoD
Internet host table specification", RFC 810, March 1982.
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, November 1987.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[RFC1123] Braden, R., "Requirements for Internet Hosts - Application
and Support", STD 3, RFC 1123, October 1989.
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[RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
specifying the location of services (DNS SRV)", RFC 2782,
February 2000.
[RFC3743] Konishi, K., Huang, K., Qian, H., and Y. Ko, "Joint
Engineering Team (JET) Guidelines for Internationalized
Domain Names (IDN) Registration and Administration for
Chinese, Japanese, and Korean", RFC 3743, April 2004.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, January 2005.
[RFC3987] Duerst, M. and M. Suignard, "Internationalized Resource
Identifiers (IRIs)", RFC 3987, January 2005.
[RFC4290] Klensin, J., "Suggested Practices for Registration of
Internationalized Domain Names (IDN)", RFC 4290,
December 2005.
[RFC4690] Klensin, J., Faltstrom, P., Karp, C., and IAB, "Review and
Recommendations for Internationalized Domain Names
(IDNs)", RFC 4690, September 2006.
[RFC4713] Lee, X., Mao, W., Chen, E., Hsu, N., and J. Klensin,
"Registration and Administration Recommendations for
Chinese Domain Names", RFC 4713, October 2006.
[Unicode-UTR36]
The Unicode Consortium, "Unicode Technical Report #36:
Unicode Security Considerations", August 2006,
.
Author's Address
John C Klensin (editor)
1770 Massachusetts Ave, Ste 322
Cambridge, MA 02140
USA
Phone: +1 617 245 1457
Fax:
Email: john+ietf@jck.com
URI:
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