Network Working Group J. Klensin, Ed.
Internet-Draft November 17, 2007
Expires: May 20, 2008
Internationalizing Domain Names for Applications (IDNA): Issues and
Rationale
draft-klensin-idnabis-issues-04.txt
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Copyright (C) The IETF Trust (2007).
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 . . . . . . . . . . . . . 7
1.5.5. Punycode is an Algorithm, not a Name . . . . . . . . . 10
1.5.6. Other Terminology Issues . . . . . . . . . . . . . . . 10
2. The Original (2003) IDNA Model . . . . . . . . . . . . . . . . 11
2.1. Proposed label . . . . . . . . . . . . . . . . . . . . . . 12
2.2. Permitted Character Identification . . . . . . . . . . . . 12
2.3. Character Mappings . . . . . . . . . . . . . . . . . . . . 12
2.4. Registry Restrictions . . . . . . . . . . . . . . . . . . 12
2.5. Punycode Conversion . . . . . . . . . . . . . . . . . . . 13
2.6. Lookup or Insertion in the Zone . . . . . . . . . . . . . 13
3. A Revised IDNA Model . . . . . . . . . . . . . . . . . . . . . 13
3.1. Localization: The Role of the Local System and User
Interface . . . . . . . . . . . . . . . . . . . . . . . . 13
3.2. IDN Processing in the IDNA200x Model . . . . . . . . . . . 14
3.2.1. Summary of Effects . . . . . . . . . . . . . . . . . . 14
4. IDNA200x Document List . . . . . . . . . . . . . . . . . . . . 15
5. Permitted Characters: An Inclusion List . . . . . . . . . . . 15
5.1. A Tiered Model of Permitted Characters and Labels . . . . 15
5.1.1. YES (Always Permitted) . . . . . . . . . . . . . . . . 16
5.1.2. MAYBE . . . . . . . . . . . . . . . . . . . . . . . . 17
5.1.3. CONTEXTUAL RULE REQUIRED . . . . . . . . . . . . . . . 18
5.1.4. NEVER . . . . . . . . . . . . . . . . . . . . . . . . 18
5.2. Layered Restrictions: Tables, Context, Registration,
Applications . . . . . . . . . . . . . . . . . . . . . . . 19
5.3. A New Character List -- History . . . . . . . . . . . . . 19
5.4. Understanding New Issues and Constraints . . . . . . . . . 20
5.5. YES, MAYBE, and Contextual Rules . . . . . . . . . . . . . 20
6. Issues that Any Solution Must Address . . . . . . . . . . . . 21
6.1. Display and Network Order . . . . . . . . . . . . . . . . 21
6.2. Entry and Display in Applications . . . . . . . . . . . . 22
6.3. The Ligature and Digraph Problem . . . . . . . . . . . . . 23
6.4. Right-to-left Text . . . . . . . . . . . . . . . . . . . . 25
7. IDNs and the Robustness Principle . . . . . . . . . . . . . . 25
8. Migration and Version Synchronization . . . . . . . . . . . . 26
8.1. Design Criteria . . . . . . . . . . . . . . . . . . . . . 26
8.2. More Flexibility in User Agents . . . . . . . . . . . . . 29
8.3. The Question of Prefix Changes . . . . . . . . . . . . . . 31
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8.3.1. Conditions requiring a prefix change . . . . . . . . . 31
8.3.2. Conditions not requiring a prefix change . . . . . . . 31
8.4. Stringprep Changes and Compatibility . . . . . . . . . . . 32
8.5. The Symbol Question . . . . . . . . . . . . . . . . . . . 33
8.6. Other Compatibility Issues . . . . . . . . . . . . . . . . 33
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 34
10. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 34
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 34
11.1. IDNA Permitted Character Registry . . . . . . . . . . . . 34
11.2. IDNA Context Registry . . . . . . . . . . . . . . . . . . 34
11.3. IANA Repository of TLD IDN Practices . . . . . . . . . . . 35
12. Security Considerations . . . . . . . . . . . . . . . . . . . 35
13. Change Log . . . . . . . . . . . . . . . . . . . . . . . . . . 36
13.1. Version -01 . . . . . . . . . . . . . . . . . . . . . . . 36
13.2. Version -02 . . . . . . . . . . . . . . . . . . . . . . . 36
13.3. Version -03 . . . . . . . . . . . . . . . . . . . . . . . 37
13.4. Version -04 . . . . . . . . . . . . . . . . . . . . . . . 37
14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 37
14.1. Normative References . . . . . . . . . . . . . . . . . . . 37
14.2. Informative References . . . . . . . . . . . . . . . . . . 39
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 40
Intellectual Property and Copyright Statements . . . . . . . . . . 41
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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 5.2), 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 or even a
particularly important one.
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 the user interface.
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 use
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.
IDNA allows the graceful introduction of IDNs not only by avoiding
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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, 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 set 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 possible new version of
IDNA without specifying which particular documents would be affected.
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 tens of
thousands of characters. 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 hexadecimal numbers separated
by "..", with no prefixes.
ASCII means US-ASCII [ASCII], a coded character set containing 128
characters associated with code points in the range 00..7F. Unicode
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may be thought of as an extension of ASCII: it includes all the ASCII
characters and associates them with equivalent code points.
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".
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.
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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. A
string is "IDNA-valid" if it meets all of the requirements of this
specification for an IDNA label. It may be either an "A-label" or a
"U-label", and it is expected that specific reference will be made to
the form appropriate to any context in which the distinction is
important. An "A-label" is the ASCII-Compatible Encoding (ACE) form
of an IDNA-valid string. It must be a complete label and valid as
the output of ToASCII, regardless of how it is actually produced.
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 rule. A "U-label" is an IDNA-valid string of
Unicode-coded characters that is a valid output of performing
ToUnicode on an A-label, again regardless of how the label is
actually produced. A Unicode string that cannot be generated by
decoding a valid A-label is not a valid U-label. [IDNA200X-protocol]
specifies the conversions between U-labels and A-labels.
Any rules or conventions that apply to DNS labels in general, such as
rules about lengths of strings, apply to whichever of the U-label or
A-label would be more restrictive. 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 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 the DNS
ones themselves. 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.
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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
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
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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.
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
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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
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
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been 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.
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.,
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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
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 and 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
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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.
A protocol such as IDNA can only assume that such operations as data
entry 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.
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. It may be
especially 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. 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.
3.2. IDN Processing in the IDNA200x Model
[[anchor20: Placeholder ??? Do we need a summary of the two parts
here???]]
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
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longer term, it allows incremental addition of permitted character
groups to avoid freezing on one particular version of Unicode.
4. IDNA200x Document List
[[anchor22: 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.
5. Permitted Characters: An Inclusion List
This section describes the model used to establish the algorithm and
character lists of [IDNA200X-Permitted] 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
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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. YES (Always Permitted)
Characters identified as "YES" are permitted for all uses in IDNs,
but may be associated with contextual 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 in addition to those already
specified, such as rules about the use of given characters, are not
required. That, in turn, indicates that the script community
relevant to that character, reflecting appropriate authorities for
all of the known languages that use that script, has agreed that the
script and its components are sufficiently well understood. This
subsection discusses characters, rather than scripts, because it is
explicitly understood that a script community may decide to include
some characters of the script and not others.
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 its name, 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 5.2).
Characters that are placed in the "YES" category are never removed
from it unless the code points themselves are removed from Unicode (a
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condition that may 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 "YES" or "NEVER"
categories (see Section 5.1.4 for the latter) belong to the "MAYBE"
category. As implied above, the collection of scripts and characters
in "MAYBE" has not yet been reviewed and finally approved by the
script community. It is possible that they may be appropriate for
general use only when special contextual rules (tests on the entire
label or on adjacent characters) are identified and specified.
In general and for maximum safety, registries SHOULD confine
themselves to characters from the "YES" category. However, if a
registry is permitting registrations only in a small number of
scripts the usage of which it is familiar with to develop rules that
are safe in its own environment -- it may be entirely appropriate for
it permit registrations that use characters from the "MAYBE"
categories as well as the "YES" one.
Applications are expected to not treat "YES" 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 "YES" 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, for which no specific in-
depth script or character evaluation has been performed. 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
These are characters that are not letter-like, but are not excluded
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.
5.1.3. CONTEXTUAL RULE REQUIRED
These characters are unsafe for general use in IDNs, 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.
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
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to introduce a new code point into Unicode and classify it as "MAYBE"
or "YES" as appropriate.
5.2. 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 need to have a variety of approaches that, together,
constitute multiple lines of defense. 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 in a protocol
context. Registrars 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.
5.3. 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-Permitted]. 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
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 "YES" and "MAYBE YES" versus
"MAYBE NO" and "NEVER" based on Unicode properties and a few
exceptions and (ii) discrimination between "YES" 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.
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5.4. 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.
5.5. YES, MAYBE, and Contextual Rules
As discussed above, characters will be associated with the "YES" 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:
o Systems supporting domain name resolution SHOULD attempt to
resolve any label consisting entirely of characters that are in
the "YES" 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 or those that contain "NEVER" characters.
o Systems providing domain name registration functions MUST NOT
register any label that contains characters classified as "NEVER"
OR code point positions that are unassigned in the version of
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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 11. 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 Any Solution Must Address
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
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.
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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.
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.
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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. The Ligature and Digraph Problem
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).
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
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(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
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.
Additional cases with alphabets written right-to-left are described
in Section 6.4. This constitutes a problem that cannot be resolved
solely by operating on scripts. It is, however, a key concern in the
IDN context. Its 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
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.
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6.4. 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
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
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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
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) 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 (if they are properly
defined); one that is willing to look something up, but warns against
it, is exercising a UI choice.
8. Migration and Version Synchronization
8.1. 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.
1. The general criteria for a putative label, and the collection of
characters that make it up, to be considered IDNA-valid are:
* 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
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similar contexts and others because there is no evidence that
they are important enough to Internet operations or
internationalization to justify large numbers of special cases
and character-specific handling (additional discussion and
rationale for the symbol decision appears in Section 8.5). 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. 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).
* Characters that are unassigned in the version of Unicode being
used by the registry or application are not permitted, even on
resolution (lookup). This is because, 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.
* 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.
2. 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
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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.
3. 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:
* 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.
* 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.
4. Anyone looking up a label in a DNS zone MUST
* 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.
* 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
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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
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).
8.2. 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-Permitted] provide that only strings
that are stable under NFKC are valid, if it is convenient for an
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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
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.
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8.3. 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.
8.3.1. 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.
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 8.3.2, 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.
8.3.2. 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:
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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 do not affect characters that
have already been invalid under IDNA2003.
3. Changes in the style of definitions of Stringprep or Nameprep
that do not alter the actions performed by them.
8.4. 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 8.3, above,
essentially also applies in this context. The proposed new inclusion
tables [IDNA200X-Permitted], 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-Permitted] 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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8.5. The Symbol Question
[[anchor37: Move this material and integrate with the Symbol
discussion above???]]
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 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 Unicode names for letters are fairly intuitive, recognizable to
uses of the relevant script, and unambiguous. 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 as result,
symbols are a very poor basis for reliable communications. 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.
8.6. 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
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.
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9. 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.
10. 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.
11. IANA Considerations
11.1. IDNA Permitted Character Registry
The distinction between "MAYBE" code points and those classified into
"YES" 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.
11.2. IDNA Context Registry
For characters that are defined in the permitted character as
requiring a contextual rule, 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
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machine processing and publish the location of that version.
11.3. IANA Repository of TLD IDN Practices
This registry is maintained by IANA at the request of ICANN, 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
effect on that registry and no IANA action is required as a result.
12. 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 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
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 suggestion
about identification and handling of confusable characters appear in
a Unicode Consortium publication [???]
The registration and resolution models described above and in
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[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 8.3.2) 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
synchronization dependency between IDNA changes and possible upgrades
to security protocols or conventions.
13. Change Log
[[anchor44: RFC Editor: Please remove this section.]]
13.1. 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.
13.2. Version -02
o Corrected several editorial errors including an accidentally-
introduced misstatement about NFKC.
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o Extensively revised the document to synchronize its terminology
with version 03 of [IDNA200X-Permitted] 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.
13.3. 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.
13.4. Version -04
o Moved more text from "protocol" and further reorganized material.
o Provided new material on "Contextual Rule Required.
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. References
14.1. Normative References
[ASCII] American National Standards Institute (formerly United
States of America Standards Institute), "USA Code for
Information Interchange", ANSI X3.4-1968, 1968.
ANSI X3.4-1968 has been replaced by newer versions with
slight modifications, but the 1968 version remains
definitive for the Internet.
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[IDNA200X-Bidi]
Alvestrand, H. and C. Karp, "An IDNA problem in right-to-
left scripts", October 2006, .
[IDNA200X-Permitted]
Faltstrom, P., "The Unicode Codepoints and IDN",
February 2007, .
A version of this document, is available in HTML format at
http://stupid.domain.name/idnabis/
draft-faltstrom-idnabis-tables-02.txt
[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.
[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.
[RFC4290] Klensin, J., "Suggested Practices for Registration of
Internationalized Domain Names (IDN)", RFC 4290,
December 2005.
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[Unicode-UAX15]
The Unicode Consortium, "Unicode Standard Annex #15:
Unicode Normalization Forms", 2006,
.
[Unicode32]
The Unicode Consortium, "The Unicode Standard, Version
3.0", 2000.
(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
14.2. Informative References
[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.
[RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
specifying the location of services (DNS SRV)", RFC 2782,
February 2000.
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[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.
[RFC4690] Klensin, J., Faltstrom, P., Karp, C., and IAB, "Review and
Recommendations for Internationalized Domain Names
(IDNs)", RFC 4690, September 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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Full Copyright Statement
Copyright (C) The IETF Trust (2007).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
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Klensin Expires May 20, 2008 [Page 41]