One document matched: draft-klensin-idnabis-issues-02.txt
Differences from draft-klensin-idnabis-issues-01.txt
Network Working Group J. Klensin, Ed.
Internet-Draft July 7, 2007
Expires: January 8, 2008
Proposed Issues and Changes for IDNA - An Overview
draft-klensin-idnabis-issues-02.txt
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Copyright Notice
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. Terminology . . . . . . . . . . . . . . . . . . . . . . . 5
1.4.1. Documents and Standards . . . . . . . . . . . . . . . 5
1.4.2. DNS-related Terminology . . . . . . . . . . . . . . . 5
1.4.3. Conformance Terminology . . . . . . . . . . . . . . . 6
2. The Original (2003) IDNA Model . . . . . . . . . . . . . . . . 6
2.1. Proposed label . . . . . . . . . . . . . . . . . . . . . . 7
2.2. Permitted Character Identification . . . . . . . . . . . . 7
2.3. Character Mappings . . . . . . . . . . . . . . . . . . . . 7
2.4. Registry Restrictions . . . . . . . . . . . . . . . . . . 7
2.5. Punycode Conversion . . . . . . . . . . . . . . . . . . . 8
2.6. Lookup or Insertion in the Zone . . . . . . . . . . . . . 8
3. A Revised IDNA Model . . . . . . . . . . . . . . . . . . . . . 8
3.1. Terminology Issues . . . . . . . . . . . . . . . . . . . . 8
3.1.1. Terms for IDN Label Codings . . . . . . . . . . . . . 8
3.1.2. Punycode as a Name, not an Algorithm . . . . . . . . . 9
3.1.3. Other Terminology Issues . . . . . . . . . . . . . . . 9
3.2. IDN Processing in the IDNA200x Model . . . . . . . . . . . 10
3.2.1. Flow Model for Registration . . . . . . . . . . . . . 10
3.2.2. Flow Model for Domain Name Resolution (Lookup) . . . . 13
3.2.3. Summary of Effects . . . . . . . . . . . . . . . . . . 15
4. IDNA200x Document List . . . . . . . . . . . . . . . . . . . . 15
5. Permitted Characters: An Inclusion List . . . . . . . . . . . 16
5.1. A Tiered Model of Permitted Characters and Labels . . . . 16
5.1.1. Always Permitted . . . . . . . . . . . . . . . . . . . 17
5.1.2. Maybe . . . . . . . . . . . . . . . . . . . . . . . . 18
5.1.3. Never . . . . . . . . . . . . . . . . . . . . . . . . 19
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. Always, Maybe, and Contextual Rules . . . . . . . . . . . 20
6. Issues that Any Solution Must Address . . . . . . . . . . . . 21
6.1. Display and Network Order . . . . . . . . . . . . . . . . 21
6.2. The Ligature and Digraph Problem . . . . . . . . . . . . . 23
6.3. Right-to-left Text . . . . . . . . . . . . . . . . . . . . 24
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 . . . . . . . . . . . . . . 30
8.3.1. Conditions requiring a prefix change . . . . . . . . . 30
8.3.2. Conditions not requiring a prefix change . . . . . . . 31
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8.4. Stringprep Changes and Compatibility . . . . . . . . . . . 31
8.5. Other Compatibility Issues . . . . . . . . . . . . . . . . 32
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 32
10. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 32
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 33
12. Security Considerations . . . . . . . . . . . . . . . . . . . 33
13. Change Log . . . . . . . . . . . . . . . . . . . . . . . . . . 33
13.1. Version -01 . . . . . . . . . . . . . . . . . . . . . . . 34
13.2. Version -02 . . . . . . . . . . . . . . . . . . . . . . . 34
14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 34
14.1. Normative References . . . . . . . . . . . . . . . . . . . 34
14.2. Informative References . . . . . . . . . . . . . . . . . . 36
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 36
Intellectual Property and Copyright Statements . . . . . . . . . . 37
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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 1.4). 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
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
can hope, through the application of different techniques at
different points (see Section 5.2), to keep problems to an acceptable
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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. Terminology
1.4.1. 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.
The term "Unicode" in this document refers to Unicode 3.2 [Unicode32]
when it is used in the context of IDNA2003 and to Unicode 5.0
[Unicode50] in the context of IDNA200x. For most of the purposes of
this document -- i.e., general explanation and issues that do not
address specific code points, blocks, scripts, or properties --
Unicode 3.2, Unicode 4.0 [Unicode40], and Unicode 5.0 are essentially
equivalent.
1.4.2. 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.
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1.4.3. Conformance Terminology
This document is an intermediate working form of what will eventually
be split into a protocol specification that replaces IDNA2003 and
some explanatory material. The use of conformance-related terms in
discussions of the protocol conforms to the provisions of [RFC2119].
2. The Original (2003) IDNA Model
IDNA is a client-side protocol, i.e., almost all of the processing is
performed by the client. The strings that appear in and are resolved
by the DNS conform to the traditional rules for the naming of hosts,
and consist of ASCII letters, digits, and hyphens. This approach
permits IDNA to be deployed without modifications to the DNS itself.
That, in turn, avoids both having to upgrade the entire Internet to
support IDNs and needing to incur the unknown risks to deployed
systems of DNS structural or design changes especially if those
changes need to be deployed all at the same time.
This section contains a summary of the model underlying IDNA2003. It
is approximate and is not a substitute for reading and understanding
the actual specification document [RFC3490] and the documents on
which it depends. The summary is not intended to be completely
balanced: it emphasizes some characteristics of IDNA2003 that are
particularly important to understanding the nature of the proposed
changes.
The original IDNA specifications have the logical flow in domain name
registration and resolution outlined in the balance of this section.
They are not defined this way; instead, the steps are presented here
for convenience in comparison to what is being proposed in this
document and the associated ones. In particular, IDNA2003 does not
make as strong a distinction between procedures for registration and
those for resolution as the ones suggested in Section 3 and
Section 5.1.
The IDNA2003 specification explicitly includes the equivalents of the
steps in Section 2.2, Section 2.3, and Section 2.5 below. While the
other steps are present --either inside the protocol or presumed to
be performed before or after it-- they are not discussed explicitly.
That omission has been a source of confusion. Another source has
been definition of IDNA2003 as an algorithm, expressed partially in
prose and partially in pseudo code and tables. The steps below
follow the more traditional IETF practice: the functions are
specified, rather than the algorithms. The breakdown into steps is
for clarity of explanation; any implementation that produces the same
result with the same inputs is conforming.
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2.1. Proposed label
The registrant submits a request for an IDN or the user attempts to
look up an IDN. The registrant or user typically produces the
request string by keyboard entry of a character sequence. That
sequence is validated only on the basis of its displayed appearance,
without knowledge of the character coding used for its internal
representation or other local details of the way the operating system
processes it. This string is converted to Unicode if necessary.
IDNA2003 assumes that the conversion is straightforward enough not to
be considered by the protocol.
2.2. Permitted Character Identification
The Unicode string is examined to prohibit characters that IDNA does
not permit in input. The list of excluded characters is quite
limited because IDNA2003 permits almost all Unicode characters to be
used as input, with many of them mapped into others.
2.3. Character Mappings
The label string is processed through the Nameprep [RFC3491] profile
of the Stringprep [RFC3454] tables and procedure. Among other
things, these procedures apply the Unicode normalization procedure
NFKC [Unicode-UAX15] which converts compatibility characters to their
base forms and resolves the different ways in which some characters
can be represented in Unicode into a canonical form. In IDNA2003,
one-way case mapping was also performed, partially simulating the
query-time folding operation that the DNS provides for ASCII strings.
2.4. Registry Restrictions
Registries at all levels of the DNS, not just the top level, are
expected to establish policies about the labels that may be
registered and for the processes associated with that action (see the
discussion of guidelines and statements in [RFC4690]). Such
restrictions have always existed in the DNS and have always been
applied at registration time, with the most notable example being
enforcement of the hostname (LDH) convention itself. For IDNs, the
restrictions to be applied are not an IETF matter except insofar as
they derive from restrictions imposed by application protocols (e.g.,
email has always required a more restricted syntax for domain names
than the restrictions of the DNS itself). Because these are
restrictions on what can be registered, it is not generally necessary
that they be global. If a name is not found on resolution, it is not
relevant whether it could have been registered; only that it was not
registered. Registry restrictions might include prohibition of
mixed-script labels or restrictions on labels permitted in a zone if
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certain other labels are already present. The "variant" systems
discussed in [RFC3743] and [RFC4290] are examples of fairly
sophisticated registry restriction models. The various sets of ICANN
IDN Guidelines [ICANN-Guidelines] also suggest restrictions that
might sensibly be imposed.
The string produced by the above steps is checked and processed as
appropriate to local registry restrictions. Application of those
registry restrictions may result in the rejection of some labels or
the application of special restrictions to others.
2.5. Punycode Conversion
The resulting label (in Unicode code point character form) is
processed with the Punycode algorithm [RFC3492] and converted to a
form suitable for storage in the DNS (the "xn--..." form).
2.6. Lookup or Insertion in the Zone
For registration, the Punycode-encoded label is then placed in the
DNS by insertion into a zone. For lookup, that label is processed
according to normal DNS query procedures [RFC1035].
3. A Revised IDNA Model
One of the major goals of this work is to improve the general
understanding of how IDNA works and what characters are permitted and
what happens to them. Comprehensibility and predictability to users
and registrants are themselves important motivations and design goals
for this effort. The effort includes some new terminology and a
revised and extended model, both covered in this section, and some
more specific protocol, processing, and table modifications. Details
of the latter appear in other documents (see Section 4).
3.1. Terminology Issues
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.
3.1.1. Terms for IDN Label Codings
3.1.1.1. 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
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"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 (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.
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 most restrictive. The exception to this, of course,
is that the restriction to ASCII characters does not apply to the
U-label.
3.1.1.2. LDH-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.
There are some standardized DNS label formats, such as those for
service location (SRV) records [RFC2782] that do not fall into any of
these categories.
3.1.2. Punycode as a Name, not an Algorithm
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".
3.1.3. 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
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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.
3.2. IDN Processing in the IDNA200x Model
3.2.1. Flow Model for Registration
3.2.1.1. Proposed label
The registrant submits a request for an IDN. The user typically
produces the request string by the keyboard entry of a character
sequence, as above (Section 2.1).
3.2.1.2. Conversion to Unicode
Some system routine, or a localized front-end to the IDNA process,
ensures that the proposed label is a Unicode string. This is
obviously trivial 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. 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
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and adequate to represent the local language and script. For
consistency with other steps in IDNA processing, this step must also
convert any surrogate forms that might be present into full-character
representation.
3.2.1.3. Permitted Character Identification
The Unicode string is examined to prohibit characters that IDNA does
not permit in input. IDNA200x uses an inclusion-based approach,
i.e., a list of characters that are permitted, rather than the
exclusion-based approach of IDNA2003. Consequently, it does much
less mapping than the earlier version.
Under the proposed IDNA200x, the string in Unicode form will be
rejected if it contains characters that are not on the list of
characters acceptable as IDNA input for registration. While there
are certain groups of characters that will never be accepted, the
ones that are will gradually expand from a list of "IDNA-possible"
characters (the "Always" and both "Maybe" categories of Section 5.1).
Characters or sequences that are unassigned in Unicode MUST NOT be
part of labels registered in the DNS. See Section 5 for an extended
discussion of the IDNA200x character table and its applicability and
Section 8 for a discussion of Unicode versioning and related issues.
For example, Unicode contains several blocks of "Mathematical"
characters that are visually identical to ASCII ones except for font
and style distinctions. IDNA2003 permits these characters as input,
then maps them (using NFKC) into their ASCII equivalents. They
cannot be recovered from the A-label once the mappings are performed.
These mappings, and similar ones, are prohibited as input into
IDNA200x: they may be accepted by a user interface, but must be
converted (in whatever way the user interface designer considers
appropriate) before being passed into IDNA itself.
3.2.1.4. Nameprep Mappings
In the model of IDNA200x, IDN-specific operations, corresponding to
Nameprep2003 and the corresponding version of Stringprep, will be
specified as needed to depend on Unicode properties, rather than on
explicit character lists that are in turn dependent on a specific
version of Unicode. This change in definition does not change the
functional model of IDNA processing but conceptually turns it into
the clear set of steps described here and localizes dependencies on
Unicode definitions and properties. The key operation is fairly
minimal use of Unicode normalization, as described below.
Because IDNA (specifically Nameprep) profiles Stringprep differently
than do other protocols, any changes that are required in the
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Nameprep-Stringprep relationship will be specified in a way that will
not have any effect on those other protocols (see Section 8.4 and
Section 12).
Filtering is specified prior to Nameprep in case IDNA-specific
processing rules are required for specific characters or code points
for which normalization would lose information. This early filtering
step also rejects proposed labels containing compatibility characters
other than those for which special exceptions are made. NFKC mapping
would otherwise quietly transform those characters into other ones.
The filtered string is then normalized to make string comparison
possible, compensating for the possibility of representing some
strings in several different ways in Unicode. Because many of the
characters permitted and then mapped to others in IDNA2003 are not
permitted by IDNA200x (since most characters that would be mapped to
others by compatibility equivalences are prohibited), the
normalization operation is less extensive. Unlike IDNA2003, IDNA200x
does no case mapping in either registration or lookup (see
Section 8.2).
3.2.1.5. Post-Nameprep Character String Checking and Processing
All characters produced as output of the preceding step are then
verified for permissibility by IDNA. Conceptually, these tests are,
in order:
1. Each code point is verified to be assigned in the version of
Unicode in use (See Section 8).
2. Each code point is checked for its presence as "Always permitted"
in the table of included characters for registration or, if
appropriate for the specific registry, as "Maybe permitted" (see
Section 5).
3. Labels that contain code points that require a specific context,
such as occurring only adjacent to certain other characters or
only in labels with specific types of other characters, are
tested to be sure that context is present and correct.
4. Additional special tests for right-to-left strings are applied.
Strings that have been produced by the steps above, and whose
contents pass the above tests, are U-labels.
To summarize, tests are made here for invalid combinations of
characters, and for labels that are invalid even if the individual
characters they contain are all valid. For example, labels
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containing invisible ("zero-width") characters may be permitted in
context with characters whose presentation forms are significantly
changed by the presence or absence of the zero-width characters,
while other labels in which zero-width characters appear may be
rejected. Additional transformations that do not occur as the result
of the steps above may be specified at this point by IDNA200x. As
the list of characters permitted to be registered expands, new rules,
similar to those suggested for zero-width characters, may accompany
them.
3.2.1.6. 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. As
discussed above (Section 2.4), such restrictions have always existed
in the DNS.
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.
3.2.1.7. Punycode Conversion
The resulting U-label is converted to an A-label (i.e., the encoding
of that label according to the Punycode algorithm with the prefix
included, the "xn--..." form). The definition of the Punycode method
itself is not affected by IDNA200x.
3.2.1.8. Insertion in the Zone
The A-label is then registered in the DNS by insertion into a zone.
3.2.2. Flow Model for Domain Name Resolution (Lookup)
Resolution is conceptually different from registration and different
tests are applied on the client. The resolution-side tests are more
permissive and rely heavily on the assumption that names that are
present in the DNS are valid. Among other things, this distinction
facilitates expansion of the permitted character lists to include new
scripts and accommodate new version of Unicode. As with other parts
of the IDN effort, there are some trade offs in these decisions.
Banning characters that are generally problematic so that they can be
rejected in the parsing process prior to actual lookup may improve
the overall health and safety of the Internet and improve
interoperability by, for example, avoiding parsing ambiguities when
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IDNs appear in context rather than as isolated domain names.
3.2.2.1. User input
The user supplies a string in the local character set, typically by
typing it or clicking on, or cutting and pasting, a resource
identifier, e.g., a URI [RFC3986] or IRI [RFC3987]. Processing in
this step and the next two are local matters, to be accomplished
prior to actual invocation of IDNAbis, but at least this one and the
next one must be accomplished in some way.
3.2.2.2. Conversion to Unicode
The local character set, character coding conventions, and, as
necessary, display and presentation conventions, are converted to
Unicode (without surrogates), paralleling the process above
(Section 3.2.1.2).
3.2.2.3. User Interface Character Changes
The Unicode string MAY then be processed, in a way specific to the
local environment, to make the result of the IDNA processing match
user expectations. For instance, at this step, it would be
reasonable to case-fold all upper case characters to lower case, if
this makes sense in the user's environment. The principles
underlying this step are discussed in Section 8.2.
Other examples of processing for localization that might be applied,
if appropriate, at this point include interpreting the KANA MIDDLE
DOT to separate domain name components from each other, standardizing
different "width" forms of the same character, or giving special
treatment to characters whose presentation forms are dependent only
on placement in the label.
Because these transformations are local, it is important that domain
names that might be passed between systems (e.g., in IRIs) be
U-labels or A-labels and not forms that might be accepted locally as
a consequence of this step. This step is not standardized, and not
specified further here.
3.2.2.4. Pre-Nameprep Validation and Character List Testing
Again in parallel to the registration procedure, the Unicode string
is checked to verify that all characters that appear in it are valid
for IDNA resolution input. As discussed in Section 5, the resolution
check is more liberal than that of Section 3.2.1.4: characters that
fall into the "Maybe" (see Section 5.1) categories in the inclusion
tables do not lead to label rejection on resolution although
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unassigned and prohibited ("Never") code points MUST BE rejected.
Instead, except for the "Never" category, the resolver MUST rely on
the presence or absence of labels in the DNS to determine their
validity and the validity of the characters they contain: if they are
registered, they are presumed to be valid; if they are not, their
possible validity is not relevant.
3.2.2.5. Nameprep Processing
As for registration, the validated Unicode string is normalized
(using NFKC); no case-mapping is performed. If a code point
unassigned in the current version of Unicode is actually assigned in
a later version, the resolver and the application containing it and
calling it should be upgraded when possible; the protocol cannot
automatically provide that upgrade. See Section 8 for more
discussion on this issue.
3.2.2.6. Post-Nameprep Processing
Any necessary processing or filtering is applied to the normalized
output string from the above. In the cases we can anticipate, this
step will be null. It is included in the model in case, e.g., full-
label checks are needed on lookup.
3.2.2.7. Punycode Conversion
The validated string, a U-label, is converted to an A-label.
3.2.2.8. DNS Name Resolution
The A-label is looked up in the DNS, using normal DNS procedures.
3.2.3. Summary of Effects
Separating Domain Name Registration and Resolution in the protocol
specification has one substantive impact. With IDNA2003, the tests
and steps made in these two parts of the protocol are essentially
identical. Separating them reflects current practice in which per-
registry restrictions and special processing are applied at
registration time but not on resolution. Even more important in the
longer term, it allows incremental addition of permitted character
groups to avoid freezing on one particular version of Unicode.
4. IDNA200x Document List
[[anchor19: This section will need to be extensively revised or
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removed before publication.]]
The following documents are expected to be 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.
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
[[anchor20: *** Still needs work. ??? In particular, the version -02
restructuring may not suffice. If it does not, version -03 should
explicitly address such topics as "Principles" and "Update Procedure"
***]]
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
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
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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.
Independent of the characters chosen (see next subsection), the
theory is to divide the characters that appear in Unicode into four
categories:
5.1.1. Always Permitted
Characters identified as "Always" 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 contextual 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 Section 3.2.1.6 and
Section 5.2).
Characters that are placed in the "always" category are never removed
from it unless the code points themselves are removed from Unicode (a
condition that may never occur).
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5.1.2. Maybe
Characters that are used to write the languages of the world and that
are thought of broadly as "letters" rather than, e.g., symbols or
punctuation, and that have not been placed in the "always" or "never"
categories (see Section 5.1.3 for the latter) belong to the "Maybe"
category. As implied above, the collection of scripts and characters
in "Maybe" have 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 "Always" category. However, if a
registry is permitting registrations only in a small number of
scripts with whose use it is very familiar -- familiar enough to
develop rules that are safe in its own environment -- it may be
entirely appropriate for it permit registrations that use characters
from one or both of the "Maybe" categories as well as the "Always"
one.
Applications are expected to not treat "Always" and "Maybe"
differently with regard to name resolution ("lookup"). They may
choose to provide warnings to users when labels or fully-qualified
names containing characters in the "Maybe" categories are to be
presented to users.
There are two "Maybe" subcategories. The assignment of a character
to one or the other represents an estimate of whether the character
will eventually be treated as "Always" or "Never" (some characters
may, however, remain in the "Maybe" categories indefinitely). Since
the difference 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. Maybe yes
These are letter, digit, or letter-like characters that are generally
presumed to be appropriate in DNS labels, by 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. Maybe no
This a characters that are not letter-like, but that 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. Never
Some characters are sufficiently problematic for use in IDNs that
they should be excluded for both registration and lookup (i.e.,
conforming applications performing name resolution should verify that
these characters are absent; if they are present, the label strings
should be rejected rather than converted to A-labels and looked up.
Of course, this category includes code points that have been removed
entirely from Unicode should such characters ever occur..
Characters that are placed in the "never" category are never removed
from it or reclassified. If a character is classified as "never" in
error and the error is sufficiently problematic, the only recourse is
to introduce a new code point into Unicode and classify it as "Maybe"
or "Always" as appropriate.
5.2. 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 from 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
[[anchor26: 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
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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, but, as of the time this draft
was posted, appear to be leading to the conclusion that, at minimum,
a mixed strategy consisting of classification into "always" and
"maybe yes" versus "maybe no" and "never" based on Unicode properties
and a few exceptions and discrimination between "always" and "maybe
yes" and between "maybe no" and "never" based of script community
criteria about IDN appropriateness will be needed. An alternative
would involve an entirely new property specifically associated with
appropriateness for IDN use, but it is not clear that is either
necessary or desirable.
5.4. Understanding New Issues and Constraints
[[anchor28: Note in draft: This section should probably be either
dropped in -03 or replaced by a more extensive discussion of
contextual rules, including rules for ZWJ and ZWNJ.]]
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. Always, Maybe, and Contextual Rules
As discussed above, characters will be associated with the "always"
or "maybe yes" properties if they can plausibly be used in an IDN.
They are classified as "maybe no" if it appears unlikely that they
should be used in IDNs but there is uncertainty on that point. Non-
language characters and other character codes that can be identified
as globally inappropriate for IDNs, such as conventional spaces and
punctuation, will be assigned to "never" (i.e., will never be
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permitted in IDNs). Any character associated with "Always" or
"Maybe" may also be associated with the identifier of a contextual
rule set. Rule sets provide information about the context of
permitted uses of a character and will have values such as "permitted
only when all characters in the label are in a particular script",
"permitted only following or preceeding particular characters", and
so on. Conceptually, any character not explicitly associated with a
rule set is used with the implied rule "permitted globally". 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 "Always" or "Maybe" categories, including those that have not
been permanently excluded but that have not been classified with
regard to whether additional restrictions are needed, i.e., they
are categorized as "Maybe yes" or "Maybe no". They MUST NOT
attempt to resolve label strings that contain unassigned character
positions 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
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, will be developed
as part of this work. A key part of that procedure will be
specifications that make it possible to add new characters and blocks
without long delays in implementation.
[[anchor30: That procedure is an important issue and this is a
placeholder.]]
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
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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.
Questions remain about protocol constraints implying that the overall
direction of these strings will always be left-to-right (or right-to-
left) for an IRI or email address, or if they even should conform to
such rules. These questions also have several possible answers.
Should a domain name abc.def, in which both labels are represented in
scripts that are written right-to-left, be displayed as fed.cba or
cba.fed? An IRI for clear text web access would, in network order,
begin with "http://" and the characters will appear as
"http://abc.def" -- but what does this suggest about the display
order? When entering a URI to many browsers, it may be possible to
provide only the domain name and leave the "http://" to be filled in
by default, assuming no tail (an approach that does not work for
other protocols). The natural display order for the typed domain
name on a right-to-left system is fed.cba. Does this change if a
protocol identifier, tail, and the corresponding delimiters are
specified?
While logic, precedent, and reality suggest that these are questions
for user interface design, not IETF protocol specifications,
experience in the 1980s and 1990s with mixing systems in which domain
name labels were read in network order (left-to-right) and those in
which those labels were read right-to-left would predict a great deal
of confusion, and heuristics that sometimes fail, if each
implementation of each application makes its own decisions on these
issues.
It should be obvious that any revision of IDNA must be more clear
about the distinction between network and display order for complete
(fully-qualified) domain names, as well as simply for individual
labels, than the original specification was. It is likely that some
strong suggestions should be made about display order as well.
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6.2. 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
(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.
This 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 [IDNA200X-BIDI] and Section 6.3. 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
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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, as discussed in Section 2.2.6 of [RFC4690],
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, IDNAbis makes 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.
6.3. Right-to-left Text
In order to be sure that the directionality of right-to-left text is
unambiguous, Stringprep requires 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 essentially
look at an entire label, not just at individual characters.
Unfortunately, the algorithmic model, as defined in Stringprep, 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 200x equivalent to
Nameprep processing must be adjusted accordingly.
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7. IDNs and the Robustness Principle
The model of IDNs described in this document can be seen as a
particular instance of the "Robustness Principle" that has been so
important to other aspects of Internet protocol design. This
principle is often stated as "Be conservative about what you send and
liberal in what you accept" (See, e.g., RFC 1123, Section 1.2.2
[RFC1123]). For IDNs to work well, registries must have or require
sensible policies about what is registered -- conservative policies
-- and implement and enforce them. Registries, registrars, or other
actors who do not do so, or who get too liberal, too greedy, or too
weird may deserve punishment that will primarily be meted out in the
marketplace or by consumer protection rules and legislation. One can
debate whether or not "punishment by browser vendor" is an effective
marketplace tool, but it falls into the general category of
approaches being discussed here. In any event, the Protocol Police
(an important, although mythical, Internet mechanism for enforcing
protocol conformance) are going to be worth about as much here as
they usually are -- i.e., very little -- simply because, unlike the
marketplace and legal and regulatory mechanisms, they have no
enforcement power.
Conversely, resolvers can (and SHOULD or maybe MUST) reject labels
that clearly violate global (protocol) rules (no one has ever
seriously claimed that being liberal in what is accepted requires
being stupid). However, once one gets past such global rules and
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 with other things, if something 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.
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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
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. If strings are read out
loud, rather than seen on paper, there are opportunities for
considerable confusion between with the DNS entry is the name
of a symbol (and a single symbol may have multiple names) and
the symbol itself. names are 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.
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* 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
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 in particular
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
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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
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).
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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.
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
Section 3.2.1.2 and Section 3.2.2.2 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, that
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uppercase(character) = uppercase(lowercase(character)), may not be
satisfied with IDNs: the relationship may even be language-dependent.
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.
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
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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:
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 other 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 to Nameprep on 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. If Nameprep changes are needed by these revised
protocols, the changes will be made either by creating a new
specification that will not modify Stringprep2003 or a new version of
Stringprep that contains additional tables without any effect on the
older ones.
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
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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.
8.5. 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.
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 and text supplied by 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
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Acknowledgments section and others who have contributed to the
development and use of the IDNA protocols.
11. IANA Considerations
While this document does not contain specific actions for IANA, it
anticipates the creation of a registry of Unicode blocks and
characters permitted in IDNs and a mechanism for expanding that
registry. See Section 5.
12. Security Considerations
The registration and resolution models described above 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
[[anchor39: RFC Editor: Please remove this section.]]
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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
Corrected several editorial errors including an accidentally-
introduced misstatement about NFKC.
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.
14. References
14.1. Normative References
[IDNA200X-BIDI]
Alvestrand, H. and C. Karp, "An IDNA problem in right-to-
left scripts", October 2006, <http://www.ietf.org/
internet-drafts/draft-alvestrand-idna-bidi-00.txt>.
[IDNA200X-Permitted]
Faltstrom, P., "The Unicode Codepoints and IDN",
February 2007, <http://stupid.domain.name/idnabis/
draft-faltstrom-idnabis-tables-02.txt>.
A version of this document, is available in HTML format at
http://stupid.domain.name/idnabis/
draft-faltstrom-idnabis-tables-02.txt
[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)",
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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.
[Unicode-UAX15]
The Unicode Consortium, "Unicode Standard Annex #15:
Unicode Normalization Forms", 2006,
<http://www.unicode.org/reports/tr15/>.
[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
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14.2. Informative References
[ICANN-Guidelines]
ICANN, "IDN Implementation Guidelines", 2006,
<http://www.icann.org/topics/idn/>.
[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.
[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
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