Network Working Group J. Klensin
Internet-Draft October 7, 2008
Intended status: Standards Track
Expires: April 10, 2009
Internationalized Domain Names for Applications (IDNA): Definitions,
Background and Rationale
draft-ietf-idnabis-rationale-03.txt
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Abstract
Several years have passed since the original protocol for
Internationalized Domain Names (IDNs) was completed and deployed.
During that time, a number of issues have arisen, including the need
to update the system to deal with newer versions of Unicode. Some of
these issues require tuning of the existing protocols and the tables
on which they depend. This document provides an overview of a
revised system and provides explanatory material for its components.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Context and Overview . . . . . . . . . . . . . . . . . . . 4
1.2. Discussion Forum . . . . . . . . . . . . . . . . . . . . . 4
1.3. Objectives . . . . . . . . . . . . . . . . . . . . . . . . 4
1.4. Applicability and Function of IDNA . . . . . . . . . . . . 5
1.5. Terminology . . . . . . . . . . . . . . . . . . . . . . . 6
1.5.1. Documents and Standards . . . . . . . . . . . . . . . 6
1.5.2. Terminology about Characters and Character Sets . . . 6
1.5.3. DNS-related Terminology . . . . . . . . . . . . . . . 7
1.5.4. Terminology Specific to IDNA . . . . . . . . . . . . . 8
1.5.5. Punycode is an Algorithm, not a Name . . . . . . . . . 12
1.5.6. Other Terminology Issues . . . . . . . . . . . . . . . 12
1.6. Comprehensibility of IDNA Mechanisms and Processing . . . 13
2. The Revised IDNA Model . . . . . . . . . . . . . . . . . . . . 14
3. Processing in IDNA2008 . . . . . . . . . . . . . . . . . . . . 14
4. IDNA2008 Document List . . . . . . . . . . . . . . . . . . . . 15
5. Permitted Characters: An Inclusion List . . . . . . . . . . . 16
5.1. A Tiered Model of Permitted Characters and Labels . . . . 16
5.1.1. PROTOCOL-VALID . . . . . . . . . . . . . . . . . . . . 17
5.1.2. DISALLOWED . . . . . . . . . . . . . . . . . . . . . . 18
5.1.3. UNASSIGNED . . . . . . . . . . . . . . . . . . . . . . 19
5.2. Registration Policy . . . . . . . . . . . . . . . . . . . 19
5.3. Layered Restrictions: Tables, Context, Registration,
Applications . . . . . . . . . . . . . . . . . . . . . . . 20
6. Issues that Constrain Possible Solutions . . . . . . . . . . . 20
6.1. Display and Network Order . . . . . . . . . . . . . . . . 20
6.2. Entry and Display in Applications . . . . . . . . . . . . 22
6.3. Linguistic Expectations: Ligatures, Digraphs, and
Alternate Character Forms . . . . . . . . . . . . . . . . 23
6.4. Case Mapping and Related Issues . . . . . . . . . . . . . 25
6.5. Right to Left Text . . . . . . . . . . . . . . . . . . . . 26
7. IDNs and the Robustness Principle . . . . . . . . . . . . . . 26
8. Front-end and User Interface Processing . . . . . . . . . . . 27
9. Relationship to IDNA2003 and Earlier Versions of Unicode . . . 30
9.1. Summary of Major Changes from IDNA2003 . . . . . . . . . . 30
9.2. Migration and Version Synchronization . . . . . . . . . . 31
9.2.1. Design Criteria . . . . . . . . . . . . . . . . . . . 31
9.2.2. More Flexibility in User Agents . . . . . . . . . . . 34
9.2.3. The Question of Prefix Changes . . . . . . . . . . . . 35
9.2.4. Stringprep Changes and Compatibility . . . . . . . . . 37
9.2.5. The Symbol Question . . . . . . . . . . . . . . . . . 38
9.2.6. Migration Between Unicode Versions: Unassigned
Code Points . . . . . . . . . . . . . . . . . . . . . 40
9.2.7. Other Compatibility Issues . . . . . . . . . . . . . . 40
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 41
11. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 42
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12. Internationalization Considerations . . . . . . . . . . . . . 42
13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 42
13.1. IDNA Character Registry . . . . . . . . . . . . . . . . . 42
13.2. IDNA Context Registry . . . . . . . . . . . . . . . . . . 43
13.3. IANA Repository of IDN Practices of TLDs . . . . . . . . . 43
14. Security Considerations . . . . . . . . . . . . . . . . . . . 43
15. Change Log . . . . . . . . . . . . . . . . . . . . . . . . . . 45
15.1. Changes between Version -00 and Version -01 of
draft-ietf-idnabis-rationale . . . . . . . . . . . . . . . 45
15.2. Version -02 . . . . . . . . . . . . . . . . . . . . . . . 45
15.3. Version -03 . . . . . . . . . . . . . . . . . . . . . . . 46
16. References . . . . . . . . . . . . . . . . . . . . . . . . . . 46
16.1. Normative References . . . . . . . . . . . . . . . . . . . 46
16.2. Informative References . . . . . . . . . . . . . . . . . . 48
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 49
Intellectual Property and Copyright Statements . . . . . . . . . . 50
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1. Introduction
[[Special note on version -03: This version has been annotated with
two different theories about what is, and is not, normative. See
Section 15.3.]]
1.1. Context and Overview
Several years have passed since the original protocol for
Internationalized Domain Names (IDNs) was completed and deployed.
During that time, a number of issues have arisen, including a subset
of those described in a recent IAB report [RFC4690] and the need to
update the system to deal with newer versions of Unicode. 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). Some tuning of the existing protocols and
the tables on which they depend is now required. Where it is
important to understanding of the revised protocols, this document
further explains the issues that have been encountered. It also
provides an overview of the new IDNA model and explanatory material
for it. Additional explanatory material for the specific components
of the proposals will appear with the associated documents.
1.2. Discussion Forum
[[anchor4: RFC Editor: please remove this section.]]
This work is being discussed in the IETF "idnabis" Working Group and
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" 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 is possible even under the original rules where only ASCII
characters were permitted. However, one can hope, through the
application of different techniques at different points (see
Section 5.3), to keep problems to an acceptable minimum. One
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consequence of this general objective is that the desire of some user
or marketing community to use a particular string --whether the
reason is to try to write sentences of particular languages in the
DNS, to express a facsimile of the symbol for a brand, or for some
other purpose-- is not a primary goal within the context of
applications in the domain name space.
1.4. Applicability and Function of IDNA
The IDNA standard does not require any applications to conform to it,
nor does it retroactively change those applications. An application
can elect to use IDNA in order to support IDN while maintaining
interoperability with existing infrastructure. If an application
wants to use non-ASCII characters in domain names, IDNA is the only
currently-defined option. Adding IDNA support to an existing
application entails changes to the application only, and leaves room
for flexibility in front-end processing and more specifically in the
user interface (see Section 8).
A great deal of the discussion of IDN solutions has focused on
transition issues and how IDNs will work in a world where not all of
the components have been updated. Proposals that were not chosen by
the original IDN Working Group would depend on user applications,
resolvers, and DNS servers being updated in order for a user to apply
an internationalized domain name in any form or coding acceptable
under that method. While processing must be performed prior to or
after access to the DNS, no changes are needed to the DNS protocol or
any DNS servers or the resolvers on user's computers.
The IDNA specification solves the problem of extending the repertoire
of characters that can be used in domain names to include a large
subset of the Unicode repertoire.
IDNA does not extend the service offered by DNS to the applications.
Instead, the applications (and, by implication, the users) continue
to see an exact-match lookup service. Either there is a single
exactly-matching name or there is no match. This model has served
the existing applications well, but it requires, with or without
internationalized domain names, that users know the exact spelling of
the domain names that are to be typed into applications such as web
browsers and mail user agents. The introduction of the larger
repertoire of characters potentially makes the set of misspellings
larger, especially given that in some cases the same appearance, for
example on a business card, might visually match several Unicode code
points or several sequences of code points.
IDNA allows the graceful introduction of IDNs not only by avoiding
upgrades to existing infrastructure (such as DNS servers and mail
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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 can be used as a last
resort to allow rudimentary IDN usage. For example, they might be
the best choice for display if it were known that relevant fonts were
not available on the user's computer. In order to allow user-
friendly input and output of the IDNs and acceptance of some
characters as equivalent to those to be processed according to the
protocol, the applications need to be modified to conform to this
specification.
IDNA uses the Unicode character repertoire, for continuity with the
original version of IDNA.
1.5. Terminology
1.5.1. Documents and Standards
[[anchor8: John Klensin believes that the definitions of "IDNA2003"
and "IDNA2008" in this subsection are normative and required to
understand discussions in Protocol, Bidi, and Tables.]]
This document uses the term "IDNA2003" to refer to the set of
standards that make up and support the version of IDNA published in
2003, i.e., those commonly known as the IDNA base specification
[RFC3490], Nameprep [RFC3491], Punycode [RFC3492], and Stringprep
[RFC3454]. In this document, those names are used to refer,
conceptually, to the individual documents, with the base IDNA
specification called just "IDNA".
The term "IDNA2008" is used to refer to a new version of IDNA as
described in this document and in the documents described in
Section 4. References to "these specifications" are to the entire
set.
1.5.2. Terminology about Characters and Character Sets
[[anchor10: Mark Davis proposes to move this section out as normative
text.]]
[[anchor11: John Klensin believes this is normative and required to
understand Protocol.]]
A code point is an integer value associated with a character in a
coded character set.
Unicode [Unicode51] is a coded character set containing almost
100,000 characters as of the current version. A single Unicode code
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point is denoted by "U+" followed by four to six hexadecimal digits,
while a range of Unicode code points is denoted by two four to six
digit hexadecimal numbers separated by "..", with no prefixes.
ASCII means US-ASCII [ASCII], a coded character set containing 128
characters associated with code points in the range 0000..007F.
Unicode may be thought of as an extension of ASCII; it includes all
the ASCII characters and associates them with equivalent code points.
"Letters" are, informally, generalizations from the ASCII and common-
sense understanding of that term, i.e., characters that are used to
write text that are not digits, symbols, or punctuation. Formally,
they are characters with a Unicode General Category value starting in
"L" (see Section 4.5 of [Unicode51]).
1.5.3. DNS-related Terminology
[[anchor13: Mark Davis proposes to move this section out as normative
text.]]
[[anchor14: John Klensin believes this is normative and required to
understand Protocol.]]
When discussing the DNS, this document generally assumes the
terminology used in the DNS specifications [RFC1034] [RFC1035]. The
terms "lookup" is used to describe the combination of operations
performed by this protocol and those actually performed by a DNS
resolver. The process of placing an entry into the DNS is referred
to as "registration", similar to common contemporary usage in other
contexts. Consequently, any DNS zone administration is described as
a "registry", regardless of the actual administrative arrangements or
level in the DNS tree. A note about that relationship is included in
the text below where it seems particularly significant.
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, 0030..0039, 0041..005A, and 0061..007A. "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. Lack of clarity about
that terminology has contributed to confusion about intent in some
cases. This document generally uses the term "domain name". When it
refers to, e.g., host name syntax restrictions, it explicitly cites
the relevant defining documents. The remaining definitions in this
subsection are essentially a review.
A label is an individual component of a domain name. Labels are
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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 RFC 1123 [RFC1123],
which can be explicit as in "www.example.com." or implicit as in
"www.example.com", is not considered in this specification.) IDNA
extends the set of usable characters in labels that are treated as
text (as distinct from the binary string labels discussed in RFC 1035
and RFC 2181 [RFC2181] and the bitstring ones described in RFC 2673
[RFC2673]). For the rest of this document and in the related ones,
the term "label" is shorthand for "text label", and "every label"
means "every text label".
1.5.4. Terminology Specific to IDNA
[[anchor15: Mark Davis proposes to move this section out as normative
text.]]
[[anchor16: John Klensin believes this is normative and required to
understand Protocol and Tables.]]
This section defines some terminology to reduce dependence on terms
and definitions that have been problematic in the past.
1.5.4.1. Terms for IDN Label Codings
1.5.4.1.1. IDNA-valid strings, A-label, and U-label
To improve clarity, this document introduces three new terms in this
subsection. In the next, it defines a historical one to be slightly
more precise for IDNA contexts.
o A string is "IDNA-valid" if it meets all of the requirements of
these specifications for an IDNA label. IDNA-valid strings may
appear in either of two forms, defined immediately below. It is
expected that specific reference will be made to the form
appropriate to any context in which the distinction is important.
o An "A-label" is the ASCII-Compatible Encoding (ACE, see
Section 1.5.4.5) form of an IDNA-valid string. It must be a
complete label: IDNA is defined for labels, not for parts of them
and not for complete domain names. This means, by definition,
that every A-label will begin with the IDNA ACE prefix, "xn--",
followed by a string that is a valid output of the Punycode
algorithm and hence a maximum of 59 ASCII characters in length.
The prefix and string together must conform to all requirements
for a label that can be stored in the DNS including conformance to
the rules for the preferred form described in RFC 1034, RFC 1035,
and RFC 1123.
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o A "U-label" is an IDNA-valid string of Unicode characters,
including at least one non-ASCII character, expressed in a
standard Unicode Encoding Form -- normally UTF-8 in an Internet
transmission context -- and subject to the constraint below.
Conversions between U-labels and A-labels are performed according
to the "Punycode" specification [RFC3492], adding or removing the
ACE prefix (see Section 1.5.4.5) as needed.
To be valid, U-labels and A-labels must obey an important symmetry
constraint. While that constraint may be tested in any of several
ways, an A-label must be capable of being produced by conversion from
a U-label and a U-label must be capable of being produced by
conversion from an A-label. Among other things, this implies that
both U-labels and A-labels must be strings in Unicode NFC
[Unicode-UAX15] normalized form. These strings MUST contain only
characters specified elsewhere in this document and its companion
documents, and only in the contexts indicated as appropriate.
Any rules or conventions that apply to DNS labels in general, such as
rules about lengths of strings, apply to whichever of the U-label or
A-label would be more restrictive. For the U-label, constraints
imposed by existing protocols and their presentation forms make the
length restriction apply to the length in octets of the UTF-8 form of
those labels (which will always be greater than or equal to the
length in code points). The exception to this, of course, is that
the restriction to ASCII characters does not apply to the U-label.
A different way to look at these terms, which may be more clear to
some readers, is that U-labels, A-labels, and LDH-labels (see the
next subsection) are disjoint categories that, together, make up the
forms of legitimate strings for use in domain names that describe
hosts. Of the three, only A-labels and LDH-labels can actually
appear in DNS zone files or queries; U-labels can appear, along with
the other two, in presentation and user interface forms and in
selected protocols other than those of the DNS itself. Strings that
do not conform to the rules for one of these three categories and, in
particular, strings that contain "--" in the third and fourth
character position but are:
o not A-labels or
o cannot be processed as U-labels or A-labels as described in these
specifications,
are invalid in IDNA-conformant applications as labels in domain names
that identify Internet hosts or similar resources. This restriction
on strings containing "--" is required for three reasons:
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o to prevent confusion with pre-IDNA coding forms;
o to permit future extensions that would require changing the
prefix, no matter how unlikely those might be (see Section 9.2.3);
and
o to reduce the opportunities for attacks via the encoding system.
1.5.4.2. LDH-label and Internationalized Label
In the hope of further clarifying discussions about IDNs, these
specifications use the term "LDH-label" strictly to refer to an all-
ASCII label that obeys the preferred syntax (often known as
"hostname" (from RFC 952 [RFC0952]) or "LDH") conventions and that is
not an IDN. It should be stressed that an A-label obeys the
"hostname" rules and is sometimes described as "LDH-conformant" or in
similar language but that it is not an LDH-label as used in this
document.
1.5.4.3. Internationalized Domain Name
An "internationalized domain name" (IDN) is a domain name that may
contain any mixture of LDH-labels, A-labels, or U-labels. This
implies that every conventional domain name is an IDN (which implies
that it is possible for a domain name to be an IDN without it
containing any non-ASCII characters). 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. Because of the
diversity of characters that can be used in a U-label and the
confusion they might cause, such restrictions are mandatory for IDN
registries and zones even though the particular restrictions are not
part of these specifications. Because these restrictions, commonly
known as "registry restrictions", only affect what can be registered
and not lookup processing, they 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. See Section 5.2.
"Internationalized label" is used when a term is needed to refer to a
single label of an IDN, i.e., one that might be any of an LDH-label,
A-label, or U-label. 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.
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1.5.4.4. 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.
1.5.4.5. 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".
1.5.4.6. 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() or getaddrinfo() standard C library functions;
the part of an email address following the at-sign (@) in the
parameter to the SMTP MAIL or RCPT commands or the "From:" field of
an email message header; and the host portion of the URI in the src
attribute of an HTML ![]()
tag. General text that just happens to
contain a domain name is not a domain name slot. For example, a
domain name appearing in the plain text body of an email message is
not occupying a domain name slot.
An "IDN-aware domain name slot" is defined in this document to be a
domain name slot explicitly designated for carrying an
internationalized domain name as defined in this document. The
designation may be static (for example, in the specification of the
protocol or interface) or dynamic (for example, as a result of
negotiation in an interactive session).
An "IDN-unaware domain name slot" is defined in this document to be
any domain name slot that is not an IDN-aware domain name slot.
Obviously, this includes any domain name slot whose specification
predates IDNA.
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1.5.5. Punycode is an Algorithm, not a Name
[[anchor22: John Klensin believes it is possible that this may be
required to understand the very careful way in which the term
"Punycode" is used in Protocol. However, the way it is used there is
consistent with the way it is used in RFC 3492 so the definition here
is not required to understand Protocol and hence is not normative in
the usual IETF sense, even though it helps to understand the use of
the term "A-label".]]
There has been some confusion about whether a "Punycode string" does
or does not include the ACE prefix and about whether it is required
that such strings could have been the output of the ToASCII operation
(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
[[anchor24: John Klensin believes that, as with the subsection
immediately above, this section narrows and provides a slightly more
precise definition for some common terminology and justifies its use.
Since it does not actually define the terminology, it is explanatory,
not normative.]]
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 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
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very broad range of scripts.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
1.6. Comprehensibility of IDNA Mechanisms and Processing
One of the major goals of this work is to improve the general
understanding of how IDNA works and what characters are permitted and
what happens to them. Comprehensibility and predictability to users
and registrants are themselves important motivations and design goals
for this effort. The effort includes some new terminology and a
revised and extended model, both covered in this section, and some
more specific protocol, processing, and table modifications. Details
of the latter appear in other documents (see Section 4).
Several issues are inherent in the application of IDNs and, indeed,
almost any other system that tries to handle international characters
and concepts. They range from the apparently trivial --e.g., one
cannot display a character for which one does not have a font
available locally-- to the more complex and subtle. Many people have
observed that internationalization is just a tool to enable effective
localization while permitting some global uniformity. Issues of
display, of exactly how various strings and characters are entered,
and so on are inherently issues about localization and user interface
design.
A protocol such as IDNA can only assume that such operations as data
entry and reconciliation of differences in character forms are
possible. It may make some recommendations about how display might
work when characters and fonts are not available, but they can only
be general recommendations and, because display functions are rarely
controlled by the types of applications that would call upon IDNA,
will rarely be very effective.
However, shifting responsibility for character mapping and other
adjustments from the protocol (where it was located in IDNA2003) to
the user interface or processing before invoking IDNA raises issues
about both what that processing should do and about compatibility for
references prepared in an IDNA2003 context. Those issues are
discussed in Section 8.
Operations for converting between local character sets and normalized
Unicode are part of this general set of user interface issues. The
conversion is obviously not required at all in a Unicode-native
system that maintains all strings in Normalization Form C (NFC). It
may, however, involve some complexity in a system that is not
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Unicode-native, especially if the elements of the local character set
do not map exactly and unambiguously into Unicode characters or do so
in a way that is not completely stable over time. Perhaps more
important, if a label being converted to a local character set
contains Unicode characters that have no correspondence in that
character set, the application may have to apply special, locally-
appropriate, methods to avoid or reduce loss of information.
Depending on the system involved, the major difficulty may not lie in
the mapping but in accurately identifying the incoming character set
and then applying the correct conversion routine. If a local
operating system uses one of the ISO 8859 character sets or an
extensive national or industrial system such as GB18030 [GB18030] or
BIG5 [BIG5], one must correctly identify the character set in use
before converting to Unicode even though those character coding
systems are substantially or completely Unicode-compatible (i.e., all
of the code points in them have an exact and unique mapping to
Unicode code points). It may be even more difficult when the
character coding system in local use is based on conceptually
different assumptions than those used by Unicode about, e.g., about
font encodings used for publications in some Indic scripts. Those
differences may not easily yield unambiguous conversions or
interpretations even if each coding system is internally consistent
and adequate to represent the local language and script.
2. The Revised 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.
[[anchor26: This paragraph is somewhat redundant with material
above.It will be dropped in -03 if there are not strong arguments for
keeping it here.]]
3. Processing in IDNA2008
These specifications separate Domain Name Registration and Lookup in
the protocol specification. Doing so reflects current practice in
which per-registry restrictions and special processing are applied at
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registration time but not during lookup. Even more important in the
longer term, it facilitates incremental addition of permitted
character groups to avoid freezing on one particular version of
Unicode.
The actual registration and lookup protocols for IDNA2008 are
specified in [IDNA2008-Protocol].
4. IDNA2008 Document List
[[anchor28: Mark Davis proposes to move this section out as normative
text.]]
[[anchor29: John Klensin believes this is a roadmap and that it is
not normative in the sense that the IETF ordinarily uses the term.
It might, however, be considered part of the definition of IDNA2008
(see above).]]
[[anchor30: This section will need to be extensively revised or
removed before publication.]]
The following documents are being produced as part of the IDNA2008
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
[IDNA2008-Protocol].
o A document describing the "Bidi problem" with Stringprep and
proposing a solution [IDNA2008-Bidi].
o A specification of the categories and rules that identify the code
points allowed in a U-label, based on Unicode 5.0 code
assignments. See Section 5 and [IDNA2008-Tables].
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 be IETF
products and may be prepared and completed asynchronously with
those described above.
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5. Permitted Characters: An Inclusion List
[[anchor31: Mark Davis proposes to move this section out as normative
text.]]
[[anchor32: John Klensin believes this is definitely not normative.
It is a description of the IDNA2008 model, but the actual normative
definitions of "PROTOCOL-VALID", "DISALLOWED", etc. are the rule and
category definitions in Tables.]]
This section provides an overview of the model used to establish the
algorithm and character lists of [IDNA2008-Tables] and describes the
names and applicability of the categories used there. Note that the
inclusion of a character in the first category group does not imply
that it can be used indiscriminately; some characters are associated
with contextual rules that must be applied as well.
The information given in this section is provided to make the rules,
tables, and protocol easier to understand. It is not normative. The
normative generating rules appear in [IDNA2008-Tables] and the rules
that actually determine what labels can be registered or looked up
are in [IDNA2008-Protocol].
5.1. A Tiered Model of Permitted Characters and Labels
[[anchor33: Mark Davis proposes to move this section out as normative
text.]]
[[anchor34: John Klensin believes this is not normative. See comment
at the beginning of Section 5, above.]]
Moving to an inclusion model requires respecifying the list of
characters that are permitted in IDNs. In IDNA2003, the role and
utility of characters are independent of context and fixed forever
(or until the standard is replaced). Making completely context-
independent rules globally has proven impractical because some
characters, especially those that are called "Join_Controls" in
Unicode, are needed to make reasonable use of some scripts but have
no visible effect(s) in others. Of necessity, IDNA2003 prohibited
those types of characters entirely. But the restrictions were much
too severe to permit an adequate range of mnemonics for terminology
based on some languages. The requirement to support those characters
but limit their use to very specific contexts was reinforced by the
observation that handling of particular characters across the
languages that use a script, or the use of similar or identical-
looking characters in different scripts, is less well understood than
many people believed it was several years ago.
Independently of the characters chosen (see next subsection), the
theory is to divide the characters that appear in Unicode into three
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categories:
5.1.1. PROTOCOL-VALID
Characters identified as "PROTOCOL-VALID" (often abbreviated
"PVALID") are, in general, permitted by IDNA for all uses in IDNs.
Their use may be restricted by rules about the context in which they
appear or by other rules that apply to the entire label in which they
are to be embedded. For example, any label that contains a character
in this category that has a "right-to-left" property must be used in
context with the "Bidi" rules (see [IDNA2008-Bidi]).
The term "PROTOCOL-VALID" is used to stress the fact that the
presence of a character in this category does not imply that a given
registry need accept registrations containing any of the characters
in the category. Registries are still expected to apply judgment
about labels they will accept and to maintain rules consistent with
those judgments (see [IDNA2008-Protocol] and Section 5.3).
Characters that are placed in the "PROTOCOL-VALID" category are never
removed from it unless the code points themselves are removed from
Unicode (such removal would be inconsistent with the Unicode
stability principles (see [Unicode51], Appendix F) and hence should
never occur).
[[anchor36: Placeholder: Does this topic or comment need additional
discussion or explanation?]]
5.1.1.1. Contextual Rules
Some characters may be unsuitable for general use in IDNs but
necessary for the plausible support of some scripts. The two most
commonly-cited examples are the zero-width joiner and non-joiner
characters (ZWJ, U+200D and ZWNJ, U+200C), but provisions for
unambiguous labels may require that other characters be restricted to
particular contexts. For example, the ASCII hyphen is not permitted
to start or end a label, whether that label contains non-ASCII
characters or not.
These characters must not appear in IDNs without additional
restrictions, typically because they have no visible consequences in
most scripts but affect format or presentation in a few others or
because they are combining characters that are safe for use only in
conjunction with particular characters or scripts. In order to
permit them to be used at all, they are specially identified as
"CONTEXTUAL RULE REQUIRED" and, when adequately understood,
associated with a rule. In addition, the rule will define whether it
is to be applied on lookup as well as registration. A distinction is
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made between characters that indicate or prohibit joining (known as
"CONTEXT-JOINER" or "CONTEXTJ") and other characters requiring
contextual treatment ("CONTEXT-OTHER" or "CONTEXTO"). Only the
former are fully tested at lookup time.
5.1.1.2. Rules and Their Application
The actual rules may be present or absent. If present, they may have
values of "True" (character may be used in any position in any
label), "False" (character may not be used in any label), or may be a
set of procedural rules that specify the context in which the
character is permitted.
Examples of descriptions of typical rules, stated informally and in
English, include "Must follow a character from Script XYZ", "MUST
occur only if the entire label is in Script ABC", "MUST occur only if
the previous and subsequent characters have the DFG property".
Because it is easier to identify these characters than to know that
they are actually needed in IDNs or how to establish exactly the
right rules for each one, a rule may have a null value in a given
version of the tables. Characters associated with null rules 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.
The description of the syntax of the rules, and the rules themselves,
appears in [IDNA2008-Tables].
5.1.2. DISALLOWED
Some characters are sufficiently problematic for use in IDNs that
they should be excluded for both registration and lookup (i.e., IDNA-
conforming applications performing name lookup should verify that
these characters are absent; if they are present, the label strings
should be rejected rather than converted to A-labels and looked up.
Of course, this category would include code points that had been
removed entirely from Unicode should such removals ever occur.
Characters that are placed in the "DISALLOWED" category are expected
to never be removed from it or reclassified. If a character is
classified as "DISALLOWED" in error and the error is sufficiently
problematic, the only recourse would be either to introduce a new
code point into Unicode and classify it as "PROTOCOL-VALID" or for
the IETF to accept the considerable costs of an incompatible change
and replace the relevant RFC with one containing appropriate
exceptions.
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[[anchor38: Note in Draft: the permanence of DISALLOWED was still
under discussion in the WG when this draft was posted. The text
above reflects the editor's opinion about the emerging consensus but
is subject to change as the discussion continues.]]
There is provision for exception cases but, in general, characters
are placed into "DISALLOWED" if they fall into one or more of the
following groups:
o The character is a compatibility equivalent for another character.
In slightly more precise Unicode terms, application of
normalization method NFKC to the character yields some other
character.
o The character is an upper-case form or some other form that is
mapped to another character by Unicode casefolding.
o The character is a symbol or punctuation form or, more generally,
something that is not a letter, digit, or a mark that is used to
form a letter or digit.
5.1.3. UNASSIGNED
For convenience in processing and table-building, code points that do
not have assigned values in a given version of Unicode are treated as
belonging to a special UNASSIGNED category. Such code points MUST
NOT appear in labels to be registered or looked up. The category
differs from DISALLOWED in that code points are moved out of it by
the simple expedient of being assigned in a later version of Unicode
(at which point, they are classified into one of the other categories
as appropriate).
5.2. Registration Policy
[[anchor39: Mark Davis proposes to move this section out as normative
text.]]
[[anchor40: John Klensin believes this cannot be normative, since it
does not contain any definitions or rules: it is simply explanatory
of the role of registry policy activities. There is a requirement in
Protocol to have such policies, but it stands alone, without this
subsection.]]
While these recommendations cannot and should not define registry
policies, registries SHOULD develop and apply additional restrictions
to reduce confusion and other problems. For example, it is generally
believed that labels containing characters from more than one script
are a bad practice although there may be some important exceptions to
that principle. Some registries may choose to restrict registrations
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to characters drawn from a very small number of scripts. For many
scripts, the use of variant techniques such as those as described in
[RFC3743] and [RFC4290], and illustrated for Chinese by the tables
described in RFC 4713 [RFC4713] may be helpful in reducing problems
that might be perceived by users. It is worth stressing that these
principles of policy development and application apply at all levels
of the DNS, not only, e.g., TLD registrations and that even a
trivial, "anything permitted that is valid under the protocol" policy
is helpful in that it helps users and application developers know
what to expect.
5.3. Layered Restrictions: Tables, Context, Registration, Applications
The essence of the character rules in IDNA2008 is based on the
realization that there is no magic bullet for any of the issues
associated with a multiscript DNS. Instead, the specifications
define a variety of approaches that, together, constitute multiple
lines of defense against ambiguity in identifiers and loss of
referential integrity. The actual character tables are the first
mechanism, protocol rules about how those characters are applied or
restricted in context are the second, and those two in combination
constitute the limits of what can be done by a protocol alone. As
discussed in the previous section (Section 5.2), registries are
expected to restrict what they permit to be registered, devising and
using rules that are designed to optimize the balance between
confusion and risk on the one hand and maximum expressiveness in
mnemonics on the other.
In addition, there is an important role for user agents in warning
against label forms that appear unreasonable given their knowledge of
local contexts and conventions. Of course, no approach based on
naming or identifiers alone can protect against all threats.
6. Issues that Constrain Possible Solutions
6.1. Display and Network Order
[[anchor43: John Klensin asks whether the material in Bidi that
depends on a clear distinction between Network Order and Display
order is sufficiently described to be self-sufficient. If it is not,
then either more comprehensive definitions must be added there or at
least part of this subsection is normative for Bidi.]]
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
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name that contains characters that are normally written right to left
is discussed in [IDNA2008-Bidi]. In particular, there are questions
about the order in which labels are displayed if left to right and
right to left labels are adjacent to each other, especially if there
are also multiple consecutive appearances of one of the types. The
decision about the display order is ultimately under the control of
user agents --including web browsers, mail clients, and the like--
which may be highly localized. Even when formats are specified by
protocols, the full composition of an Internationalized Resource
Identifier (IRI) [RFC3987] or Internationalized Email address
contains elements other than the domain name. For example, IRIs
contain protocol identifiers and field delimiter syntax such as
"http://" or "mailto:" while email addresses contain the "@" to
separate local parts from domain names. User agents are not required
to use those protocol-based forms directly but often do so. While
display, parsing, and processing within a label is specified by the
IDNA protocol and the associated documents, the relationship between
fully-qualified domain names and internationalized labels is
unchanged from the base DNS specifications. Comments here about such
full domain names are explanatory or examples of what might be done
and must not be considered normative.
Questions remain about protocol constraints implying that the overall
direction of these strings will always be left to right (or right to
left) for an IRI or email address, or if they even should conform to
such rules. These questions also have several possible answers.
Should a domain name abc.def, in which both labels are represented in
scripts that are written right to left, be displayed as fed.cba or
cba.fed? An IRI for clear text web access would, in network order,
begin with "http://" and the characters will appear as
"http://abc.def" -- but what does this suggest about the display
order? When entering a URI to many browsers, it may be possible to
provide only the domain name and leave the "http://" to be filled in
by default, assuming no tail (an approach that does not work for
other protocols). The natural display order for the typed domain
name on a right to left system is fed.cba. Does this change if a
protocol identifier, tail, and the corresponding delimiters are
specified?
While logic, precedent, and reality suggest that these are questions
for user interface design, not IETF protocol specifications,
experience in the 1980s and 1990s with mixing systems in which domain
name labels were read in network order (left to right) and those in
which those labels were read right to left would predict a great deal
of confusion, and heuristics that sometimes fail, if each
implementation of each application makes its own decisions on these
issues.
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It should be obvious that any revision of IDNA, including the current
one, must be clear about the network (transmission on the wire) order
of characters in labels and for the labels in complete (fully-
qualified) domain names. In order to prevent user confusion and, in
particular, to reduce the chances for inconsistent transcription of
domain names from printed form, 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 or specified by the operating
system, 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 of A-labels, but are encouraged to not do so except
as an interface for special purposes, possibly for debugging, or to
cope with display limitations. In general, they SHOULD allow, but
not encourage, user input of that label form. A-labels are opaque
and ugly, and, where possible, should thus only be exposed to users
and in contexts in which they are absolutely needed. Because IDN
labels can be rendered either as A-labels or U-labels, the
application may reasonably have an option for the user to select the
preferred method of display; if it does, rendering the U-label should
normally be the default.
Domain names are often stored and transported in many places. For
example, they are part of documents such as mail messages and web
pages. They are transported in many parts of many protocols, such as
both the control commands 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.
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In any place where a protocol or document format allows transmission
of the characters in internationalized labels, labels SHOULD be
transmitted using whatever character encoding and escape mechanism
the protocol or document format uses at that place. This provision
is intended to prevent situations in which, e.g., UTF-8 domain names
appear embedded in text that is otherwise in some other character
coding.
All protocols that use domain name slots already have the capacity
for handling domain names in the ASCII charset. Thus, A-labels can
inherently be handled by those protocols.
6.3. Linguistic Expectations: Ligatures, Digraphs, and Alternate
Character Forms
Users often have expectations about character matching or equivalence
that are based on their languages and the orthography of those
languages. These expectations may not be consistent with forms or
actions that can be naturally accommodated in a character coding
system, especially if multiple languages are written using the same
script but using different conventions. A Norwegian user might
expect a label with the ae-ligature to be treated as the same label
as one using the Swedish spelling with a-umlaut even though applying
that mapping to English would be astonishing to users. A user in
German might expect a label with an o-umlaut and a label that had
"oe" substituted, but was otherwise the same, treated as equivalent
even though that substitution would be a clear error in Swedish. A
Chinese user might expect automatic matching of Simplified and
Traditional Chinese characters, but applying that matching for Korean
or Japanese text would create considerable confusion. For that
matter, an English user might expect "theater" and "theatre" to
match.
Related issues arise because there are a number of languages written
with alphabetic scripts in which single phonemes are written using
two characters, termed a "digraph", for example, the "ph" in
"pharmacy" and "telephone". (Note that characters paired in this
manner can also appear consecutively without forming a digraph, as in
"tophat".) Certain digraphs are normally indicated typographically
by setting the two characters closer together than they would be if
used consecutively to represent different phonemes. Some digraphs
are fully joined as ligatures (strictly designating setting totally
without intervening white space, although the term is sometimes
applied to close set pairs). An example of this may be seen when the
word "encyclopaedia" is set with a U+00E6 LATIN SMALL LIGATURE AE
(and some would not consider that word correctly spelled unless the
ligature form was used or the "a" was dropped entirely). When these
ligature and digraph forms have the same interpretation across all
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languages that use a given script, application of Unicode
normalization generally resolves the differences and causes them to
match. When they have different interpretations, any requirements
for matching must utilize other methods or users must be educated to
understand that matching will not occur.
Difficulties arise from the fact that a given ligature may be a
completely optional typographic convenience for representing a
digraph in one language (as in the above example with some spelling
conventions), while in another language it is a single character that
may not always be correctly representable by a two-letter sequence
(as in the above example with different spelling conventions). This
can be illustrated by many words in the Norwegian language, where the
"ae" ligature is the 27th letter of a 29-letter extended Latin
alphabet. It is equivalent to the 28th letter of the Swedish
alphabet (also containing 29 letters), U+00E4 LATIN SMALL LETTER A
WITH DIAERESIS, for which an "ae" cannot be substituted according to
current orthographic standards.
That character (U+00E4) is also part of the German alphabet where,
unlike in the Nordic languages, the two-character sequence "ae" is
usually treated as a fully acceptable alternate orthography for the
"umlauted a" character. The inverse is however not true, and those
two characters cannot necessarily be combined into an "umlauted a".
This also applies to another German character, the "umlauted o"
(U+00F6 LATIN SMALL LETTER O WITH DIAERESIS) which, for example,
cannot be used for writing the name of the author "Goethe". It is
also a letter in the Swedish alphabet where, like the "umlauted a",
it cannot be correctly represented as "oe" and in the Norwegian
alphabet, where it is represented, not as "umlauted o", but as
"slashed o", U+00F8.
Some of the ligatures that have explicit code points in Unicode were
given special handling in IDNA2003 and now pose additional problems
as people argue that they should have been treated differently to
preserve important information. For example, the German character
Eszett (Sharp S, U+00DF) is retained as itself by NFKC but case-
folded by Stringprep to "ss", but the closely-related, but less
frequently seen, character "Long S T" (U+FB05) is a compatibility
character that is mapped out by NFKC. Unless exceptions are made,
both will be treated as DISALLOWED by IDNA2008. But there is
significant interest in an exception, especially for Eszett.
Depending on what the exception was, making it would either raise
some backward compatibility problems with IDNA2003 or create an
unusual special case that would highlight differences in preferred
orthography between German as written in Germany and German as
written in some other countries, notably Switzerland. Additional
discussion of issues with Eszett appear in Section 9.2.7.
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Additional cases with alphabets written right to left are described
in Section 6.5.
Whether ligatures and digraphs are to be treated as a sequence of
characters or as a single standalone one constitute a problem that
cannot be resolved solely by operating on scripts. They are,
however, a key concern in the IDN context. Their satisfactory
resolution will require support in policies set by registries, which
therefore need to be particularly mindful not just of this specific
issue, but of all other related matters that cannot be dealt with on
an exclusively algorithmic 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.
6.4. Case Mapping and Related Issues
Traditionally in the DNS, ASCII letters have been stored with their
case preserved. Matching during the query process has been case-
independent, but none of the information that might be represented by
choices of case has been lost. That model has been accidentally
helpful because, as people have created DNS labels by catenating
words (or parts of words) to form labels, case has often been used to
distinguish among components and make the labels more memorable.
The solution of keeping the characters separate but doing matching
independent of case is not feasible with an IDNA-like model because
the matching would then have to be done on the server rather than
have characters mapped on the client. That situation was recognized
in IDNA2003 and nothing in IDNA2008 fundamentally changes it or could
do so. In IDNA2003, all upper-case characters are mapped to lower-
case ones and, in general, all code points that represent alternate
forms of the same character are mapped to that character (including
mapping Greek final form sigma to the medial form). IDNA2008
permits, at the risk of some incompatibility, slightly more
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flexibility in this area. That additional flexibility still does not
solve the problem with final form sigma and other characters that
Unicode treats as completely separate characters that match only
under casemapping if at all. Many people now believe these should be
handled as separate characters so information about them can be
preserved in the transformations to A-labels and back. However
making a change to permit that behavior would create a situation in
which the same string, valid in both protocols, would be interpreted
differently by IDNA2003 and IDNA2008. In principle, that would
violate one of the conditions discussed in Section 9.2.3.1 and hence
require a prefix change. Of course, if a prefix change were made (at
the costs discussed in Section 9.2.3.3) there would be several
options, including, if desired, assigning the characer to the
CONTEXTUAL RULE REQUIRED category and requiring that it only be used
in carefully-selected contexts.
6.5. Right to Left Text
In order to be sure that the directionality of right to left text is
unambiguous, IDNA2003 required that any label in which right to left
characters appear both starts and ends with them, may not include any
characters with strong left to right properties (which excludes other
alphabetic characters but permits European digits), and rejects any
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. The algorithmic model used in IDNA2003 rejects the label
when the final character in a right to left string requires a
combining mark in order to be correctly represented.
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). The new
rules for right to left scripts are described in [IDNA2008-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
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[RFC1123]). For IDNs to work well, not only must the protocol be
carefully designed and implemented, but zone administrators
(registries) must have and require sensible policies about what is
registered -- conservative policies -- and implement and enforce
them.
Conversely, lookup applications 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 elsewhere, if a string cannot be successfully found in
the DNS after the lookup processing described here, it makes no
difference whether it simply wasn't registered or was prohibited by
some rule.
If lookup applications, as a user interface (UI) or other local
matter, decide to warn about some strings that are valid under the
global rules but that they perceive as dangerous, that is their
prerogative and we can only hope that the market (and maybe
regulators) will reinforce the good choices and discourage the poor
ones. In this context, a lookup application that decides a string
that is valid under the protocol is dangerous and refuses to look it
up is in violation of the protocols; one that is willing to look
something up, but warns against it, is exercising a local choice.
8. Front-end and User Interface Processing
Domain names may be identified and processed in many contexts. They
may be typed in by users either by themselves or as part of URIs or
IRIs. They may occur in running text or be processed by one system
after being provided in another. Systems may wish to try to
normalize URLs so as to determine (or guess) whether a reference is
valid or two references point to the same object without actually
looking the objects up and comparing them (that is necessary, not
just a choice, for URI types that are not intended to be resolved).
Some of these goals may be more easily and reliably satisfied than
others. While there are strong arguments for any domain name that is
placed "on the wire" -- transmitted between systems -- to be in the
minimum-ambiguity forms of A-labels, U-labels, or LDH-labels, it is
inevitable that programs that process domain names will encounter
variant forms.
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One source of such forms will be labels created under IDNA2003
because that protocol allowed labels that were transformed before
they were turned from native-character into ACE ("xn--...") format.
One consequence of the transformations was that, when the ToUnicode
and ToASCII operations of IDNA2003 were applied,
ToUnicode(ToASCII(original-label)) often did not produce the
original-label. IDNA2008 explicitly defines A-labels and U-labels as
different forms of the same abstract label, forms that are stable
when conversions are performed between them, without mappings. A
different way of explaining this is that there are, today, domain
names in files on the Internet that use characters that cannot be
represented directly in, or recovered from, (A-label) domain names
but for which interpretations are provided by IDNA2003. There are
two major categories of such characters, those that are removed by
NFKC normalization and those upper-case characters that are mapped to
lower-case (there are also a few characters that are given special-
case mapping treatment in Stringprep).
Other issues in domain name identification and processing arise
because IDNA2003 specified that several other characters be treated
as equivalent to the ASCII period (dot, full stop) character used as
a label separator. If a string that might be a domain name appears
in an arbitrary context (such as running text), it is difficult, even
with only ASCII characters, to know whether an actual domain name (or
a protocol parameter like a URI) is present and where it starts and
ends. When using Unicode, this gets even more difficult if treatment
of certain special characters (like the dot that separates labels in
a domain name) depends on context (e.g., prior knowledge of whether
the string represents a domain name or not). That knowledge is not
available if the primary heuristic for identifying the presence of
domain names in strings depends on the presence of dots separating
groups of characters with no intervening spaces.
[[anchor45: Above text is a substitute for an earlier (pre -01)
version and is hoped to be more clear. Comments and improvements
welcome.]]
As discussed elsewhere in this document, the IDNA2008 model removes
all of these mappings and interpretations, including the equivalence
of different forms of dots, from the protocol, discouraging such
mappings and leaving them, when necessary, to local processing. This
should not be taken to imply that local processing is optional or can
be avoided entirely. Instead, unless the program context is such
that it is known that any IDNs that appear will be either U-labels or
A-labels, or that other forms can safely be rejected, some local
processing of apparent domain name strings will be required, both to
maintain compatibility with IDNA2003 and to prevent user
astonishment. Such local processing, while not specified in this
document or the associated ones, will generally take one of two
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forms:
o Generic Preprocessing.
When the context in which the program or system that processes
domain names operates is global, a reasonable balance must be
found that is sensitive to the broad range of local needs and
assumptions while, at the same time, not sacrificing the needs of
one language, script, or user population to those of another.
For this case, the best practice will usually be to apply NFKC and
case-mapping (or, perhaps better yet, Stringprep itself), plus
dot-mapping where appropriate, to the domain name string prior to
applying IDNA. That practice will not only yield a reasonable
compromise of user experience with protocol requirements but will
be almost completely compatible with the various forms permitted
by IDNA2003.
o Highly Localized Preprocessing.
Unlike the case above, there will be some situations in which
software will be highly localized for a particular environment and
carefully adapted to the expectations of users in that
environment. The many discussions about using the Internet to
preserve and support local cultures suggest that these cases may
be more common in the future than they have been so far.
In these cases, we should avoid trying to tell implementers what
they should do, if only because they are quite likely (and for
good reason) to ignore us. We would assume that they would map
characters that the intuitions of their users would suggest be
mapped and would hope that they would do that mapping as early as
possible, storing A-label or U-label forms in files and
transporting only those forms between systems. One can imagine
switches about whether some sorts of mappings occur, warnings
before applying them or, in a slightly more extreme version of the
approach taken in Internet Explorer version 7 (IE7), systems that
utterly refuse to handle "strange" characters at all if they
appear in U-label form. None of those local decisions are a
threat to interoperability as long as (i) only U-labels and
A-labels are used in interchange with systems outside the local
environment, (ii) no character that would be valid in a U-label as
itself is mapped to something else, (iii) any local mappings are
applied as a preprocessing step (or, for conversions from U-labels
or A-labels to presentation forms, postprocessing), not as part of
IDNA processing proper, and (iv) appropriate consideration is
given to labels that might have entered the environment in
conformance to IDNA2003. [[anchor46: Placeholder: there have been
suggestions that this text be removed entirely. Comments (or
improved text) welcome.]]
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In either case, it is vital that user interface designs and, where
the interfaces are not sufficient, users, be aware that the only
forms of domain names that this protocol anticipates will resolve
globally or compare equal when crude methods (i.e., those not
conforming to Section 1.5.4.4) are used are those in which all
native-script labels are in U-label form. Forms that assume mapping
will occur, especially forms that were not valid under IDNA2003, may
or may not function in predictable ways across all implementations.
9. Relationship to IDNA2003 and Earlier Versions of Unicode
9.1. Summary of Major Changes from IDNA2003
[[anchor48: Mark Davis proposes to move this section out as normative
text.]]
[[anchor49: John Klensin claims it is not normative and must not be
normative lest it be taken as an alternate definition for IDNA2008.]]
1. Update base character set from Unicode 3.2 to Unicode version-
agnostic.
2. Separate the definitions for the "registration" and "lookup"
activities.
3. Disallow symbol and punctuation characters except where special
exceptions are necessary.
4. Remove the mapping and normalization steps from the protocol and
have them instead done by the applications themselves, possibly
in a local fashion, before invoking the protocol.
5. Change the way that the protocol specifies which characters are
allowed in labels from "humans decide what the table of
codepoints contains" to "decision about codepoints are based on
Unicode properties plus a small exclusion list created by
humans".
6. Introduce the new concept of characters that can be used only in
specific contexts.
7. Allow typical words and names in languages such as Dhivehi and
Yiddish to be expressed.
8. Make bidirectional domain names (delimited strings of labels,
not just labels standing on their own) display in a non-
surprising fashion whether they appear in obvious domain name
contexts or as part of running text in paragraphs.
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9. Remove the dot separator from the mandatory part of the
protocol.
10. Make some currently-valid labels that are not actually IDNA
labels invalid.
9.2. Migration and Version Synchronization
9.2.1. 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 or, subsequent to this version,
"heavy" processes such as formation of an IETF WG. The mechanisms
that support this are outlined above, but this section reviews them
in a context that may be more helpful to those who need to understand
the approach and make plans for it.
9.2.1.1. General IDNA Validity Criteria
The general criteria for a putative label, and the collection of
characters that make it up, to be considered IDNA-valid are:
o The characters are "letters", marks needed to form letters,
numerals, or other code points used to write words in some
language. Symbols, drawing characters, and various notational
characters are permanently excluded -- some because they are
actively dangerous in URI, IRI, or similar contexts and others
because there is no evidence that they are important enough to
Internet operations or internationalization to justify inclusion
and the complexities that would come with it (additional
discussion and rationale for the symbol decision appears in
Section 9.2.5).
o Other than in very exceptional cases, e.g., where they are needed
to write substantially any word of a given language, punctuation
characters are excluded as well. The fact that a word exists is
not proof that it should be usable in a DNS label and DNS labels
are not expected to be usable for multiple-word phrases (although
they are certainly not prohibited if the conventions and
orthography of a particular language cause that to be possible).
Even for English, very common constructions -- contractions like
"don't" or "it's", names that are written with apostrophes such as
"O'Reilly" or characters for which apostrophes are common
substitutes, and words whose usually-preferred spellings retain
diacritical marks from earlier forms -- cannot be represented in
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DNS labels.
o Characters that are unassigned (have no character assignment at
all) in the version of Unicode being used by the registry or
application are not permitted, even on lookup. There are at least
two reasons for this. Tests involving the context of characters
(e.g., some characters being permitted only adjacent to ones of
specific types but otherwise invisible or very problematic for
other reasons) and integrity tests on complete labels are needed.
Unassigned code points cannot be permitted because one cannot
determine whether particular code points will require contextual
rules (and what those rules should be) before characters are
assigned to them and the properties of those characters fully
understood. Second, Unicode specifies that an unassigned code
point normalizes and case folds to itself. If the code point is
later assigned to a character, and particularly if the newly-
assigned code point has a combining class that determines its
placement relative to other combining characters, it could
normalize to some other code point or sequence, creating confusion
and/or violating other rules listed here.
o Any character that is mapped to another character by Nameprep2003
or by a current version of NFKC is prohibited as input to IDNA
(for either registration or lookup). 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 (described in more
precise form in [IDNA2008-Tables]). The principles are not just an
interpretation of the tables.
9.2.1.2. Labels in Registration
Anyone entering a label into a DNS zone must properly validate that
label -- i.e., be sure that the criteria for that label are met -- in
order for applications to work as intended. This principle is not
new: for example, zone administrators are expected to verify that
names meet "hostname" [RFC0952] or special service location formats
[RFC2782] where necessary for the expected applications. For zones
that will contain IDNs, support for Unicode version-independence
requires restrictions on all strings placed in the zone. In
particular, for such zones:
o Any label that appears to be an A-label, i.e., any label that
starts in "xn--", MUST be IDNA-valid, i.e., that they MUST be
valid A-labels, as discussed in Section 2 above.
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o The Unicode tables (i.e., tables of code points, character
classes, and properties) and IDNA tables (i.e., tables of
contextual rules such as those described above), MUST be
consistent on the systems performing or validating labels to be
registered. Note that this does not require that tables reflect
the latest version of Unicode, only that all tables used on a
given system are consistent with each other.
[[anchor51: Note in draft: the above text was changed significantly
between -00 and -01 to clearly restrict its scope to zones supporting
IDNA and to eliminate comments about labels containing "--" in the
third and forth positions but with different prefixes. There appears
to be consensus that more extensive rules belong in a "best
practices" document about appropriate DNS labels, but that document
is not in-scope for the IDNABIS WG.]]
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.
Systems looking up or resolving DNS labels, especially IDN DNS
labels, MUST be able to assume that applicable registration rules
were followed for names entered into the DNS.
9.2.1.3. Labels in Lookup
[[anchor53: John Klensin believes that this subsection is normative.
It is not merely explanatory because it contains RFC 2119 conformity
statements and provisions that are not present in Protocol. In spite
of not being algorithmic, perhaps it should be moved into Protocol.
However, it is an explanation that is important to the non-
implementer audience.]]
Anyone looking up a label in a DNS zone
o 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.
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o MUST validate the characters in labels to be looked up only to the
extent of determining that the U-label does not contain either
code points prohibited by IDNA (categorized as "DISALLOWED") or
code points that are unassigned in its version of Unicode.
o MUST validate the label itself for conformance with a small number
of whole-label rules, notably verifying that there are no leading
combining marks, that the "bidi" conditions are met if right to
left characters appear, that any required contextual rules are
available and that, if such rules are associated with Joiner
Controls, they are tested.
o MUST NOT validate other 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 lookup application
becomes less sensitive to version incompatibilities with the
particular zone registry associated with the domain name.
An application or client that looks processes names according to this
protocol and then resolves them in the DNS will be able to locate any
name that is validly registered, as long as its version of the
Unicode-associated tables is sufficiently up-to-date to interpret all
of the characters in the label. It SHOULD distinguish, in its
messages to users, between "label contains an unallocated code point"
and other types of lookup failures. A failure on the basis of an old
version of Unicode may lead the user to a desire to upgrade to a
newer version, but will have no other ill effects (this is consistent
with behavior in the transition to the DNS when some hosts could not
yet handle some forms of names or record types).
9.2.2. More Flexibility in User Agents
These specifications do not perform mappings between one character or
code point and others for any reason. Instead, they prohibits the
characters that would be mapped to others by normalization, case
folding, or other rules. As examples, while mathematical characters
based on Latin ones are accepted as input to IDNA2003, they are
prohibited in IDNA2008. Similarly, double-width characters and other
variations are prohibited as IDNA input.
Since the rules in [IDNA2008-Tables] provide that only strings that
are stable under NFKC are valid, if it is convenient for an
application to perform NFKC normalization before lookup, that
operation is safe since this will never make the application unable
to look up any valid string.
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In many cases these prohibitions should have no effect on what the
user can type as input to the lookup process. It is perfectly
reasonable for systems that support user interfaces to perform some
character mapping that is appropriate to the local environment. This
would normally be done prior to actual invocation of IDNA. At least
conceptually, the mapping would be part of the Unicode conversions
discussed above and in [IDNA2008-Protocol]. However, those changes
will be local ones only -- local to environments in which users will
clearly understand that the character forms are equivalent. For use
in interchange among systems, it appears to be much more important
that U-labels and A-labels can be mapped back and forth without loss
of information.
One specific, and very important, instance of this strategy arises
with case-folding. In the ASCII-only DNS, names are looked up and
matched in a case-independent way, but no actual case-folding occurs.
Names can be placed in the DNS in either upper or lower case form (or
any mixture of them) and that form is preserved, returned in queries,
and so on. IDNA2003 simulated that behavior 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 although the
specifications require that it be performed prior to invoking the
protocol) and to make the mapping between A-labels and U-labels
idempotent. Case-mapping is not an exception to this principle. If
only lower case characters can be registered in the DNS (i.e., be
present in a U-label), then IDNA2008 should prohibit upper-case
characters as input. 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) expecting different
mappings. Of course, the expectations of users who are accustomed to
a case-insensitive DNS environment will probably be well-served if
user agents perform case mapping prior to IDNA processing, but the
IDNA procedures themselves should neither require such mapping nor
expect them when they are not natural to the localized environment.
9.2.3. 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
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great concern to the community. A prefix change would clearly be
necessary if the algorithms were modified in a manner that would
create serious ambiguities during subsequent transition in
registrations. This section summarizes our conclusions about the
conditions under which changes in prefix would be necessary and the
implications of such a change.
9.2.3.1. Conditions Requiring a Prefix Change
An IDN prefix change is needed if a given string would be looked up
or otherwise interpreted differently depending on the version of the
protocol or tables being used. Consequently, work to update IDNs
would require a prefix change if, and only if, one of the following
four conditions were met:
1. The conversion of an A-label to Unicode (i.e., a U-label) yields
one string under IDNA2003 (RFC3490) and a different string under
IDNA2008.
2. An input string that is valid under IDNA2003 and also valid under
IDNA2008 yields two different A-labels with the different
versions of IDNA. This condition is believed to be essentially
equivalent to the one above.
Note, however, that if the input string is valid under one
version and not valid under the other, this condition does not
apply. See the first item in Section 9.2.3.2, below.
3. A fundamental change is made to the semantics of the string that
is inserted in the DNS, e.g., if a decision were made to try to
include language or specific script information in that string,
rather than having it be just a string of characters.
4. A sufficiently large number of characters is added to Unicode so
that the Punycode mechanism for block offsets no longer has
enough capacity to reference the higher-numbered planes and
blocks. This condition is unlikely even in the long term and
certain not to arise in the next few years.
9.2.3.2. 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.
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2. Adjustments in Stringprep tables or IDNA actions, including
normalization definitions, that affect characters that were
already invalid under IDNA2003.
3. Changes in the style of definitions of Stringprep or Nameprep
that do not alter the actions performed by them.
Of course, because these specifications do not involve changes to
Stringprep or Nameprep, the third condition above and part of the
second are moot.
9.2.3.3. Implications of Prefix Changes
While it might be possible to make a prefix change, the costs of such
a change are considerable. Even if they wanted to do so, all
registries could not convert all IDNA2003 ("xn--") registrations to a
new form at the same time and synchronize that change with
applications supporting lookup. Unless all existing registrations
were simply to be declared invalid, and perhaps even then, systems
that needed to support both labels with old prefixes and labels with
new ones would first process a putative label under the IDNA2008
rules and try to look it up and then, if it were not found, would
process the label under IDNA2003 rules and look it up again. That
process could significantly slow down all processing that involved
IDNs in the DNS especially since, in principle, a fully-qualified
name could contain a mixture of labels that were registered with the
old and new prefixes, a situation that would make the use of DNS
caching very difficult. In addition, looking up the same input
string as two separate A-labels would create some potential for
confusion and attacks, since they could, in principle, map to
different targets and then resolve to different DNS label nodes.
Consequently, a prefix change is to be avoided if at all possible,
even if it means accepting some IDNA2003 decisions about character
distinctions as irreversible.
9.2.4. Stringprep Changes and Compatibility
Concerns have been expressed about problems for non-DNS uses of
Stringprep being caused by changes to the specification intended to
improve the handling of IDNs, most notably as this might affect
identification and authentication protocols. Section 9.2.3, above,
essentially also applies in this context. The proposed new inclusion
tables [IDNA2008-Tables], the reduction in the number of characters
permitted as input for registration or lookup (Section 5), and even
the proposed changes in handling of right to left strings
[IDNA2008-Bidi] either give interpretations to strings prohibited
under IDNA2003 or prohibit strings that IDNA2003 permitted. Strings
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that are valid under both IDNA2003 and IDNA2008, 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 [IDNA2008-Tables] are unlikely to have any effect at all,
especially not on registries and registrations that follow rules
already in existence when this work started.
9.2.5. The Symbol Question
One of the major differences between this specification and the
original version of IDNA is that the original version permitted non-
letter symbols of various sorts, including punctuation and line-
drawing symbols, in the protocol. They were always discouraged in
practice. In particular, both the "IESG Statement" about IDNA and
all versions of the ICANN Guidelines specify that only language
characters be used in labels. This specification disallows symbols
entirely. There are several reasons for this, which include:
o As discussed elsewhere, the original IDNA specification assumed
that as many Unicode characters as possible should be permitted,
directly or via mapping to other characters, in IDNs. This
specification operates on an inclusion model, extrapolating from
the LDH rules --which have served the Internet very well-- to a
Unicode base rather than an ASCII base.
o Most Unicode names for letters are, in most cases, fairly
intuitive, unambiguous and recognizable to users of the relevant
script. Symbol names are more problematic because there may be no
general agreement on whether a particular glyph matches a symbol;
there are no uniform conventions for naming; variations such as
outline, solid, and shaded forms may or may not exist; and so on.
As just one example, consider a "heart" symbol as it might appear
in a logo that might be read as "I love...". While the user might
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read such a logo as "I love..." or "I heart...", considerable
knowledge of the coding distinctions made in Unicode is needed to
know that there more than one "heart" character (e.g., U+2665,
U+2661, and U+2765) and how to describe it. These issues are of
particular importance if strings are expected to be understood or
transcribed by the listener after being read out loud.
[[anchor54: The above paragraph remains controversial as to
whether it is valid. The WG will need to make a decision if this
section is not dropped entirely.]]
o As a simplified example of this, assume one wanted to use a
"heart" or "star" symbol in a label. This is problematic because
the those names are ambiguous in the Unicode system of naming (the
actual Unicode names require far more qualification). A user or
would-be registrant has no way to know --absent careful study of
the code tables-- whether it is ambiguous (e.g., where there are
multiple "heart" characters) or not. Conversely, the user seeing
the hypothetical label doesn't know whether to read it --try to
transmit it to a colleague by voice-- as "heart", as "love", as
"black heart", or as any of the other examples below.
o The actual situation is even worse than this. There is no
possible way for a normal, casual, user to tell the difference
between the hearts of U+2665 and U+2765 and the stars of U+2606
and U+2729 or the without somehow knowing to look for a
distinction. We have a white heart (U+2661) and few black hearts
and describing a label containing a heart symbol is hopelessly
ambiguous. In cities where "Square" is a popular part of a
location name, one might well want to use a square symbol in a
label as well and there are far more squares of various flavors in
Unicode than there are hearts or stars.
o The consequence of these ambiguities of description and
dependencies on distinctions that were, or were not, made in
Unicode codings, is that symbols are a very poor basis for
reliable communication. Consistent with this conclusion, the
Unicode standard recommends that strings used in identifiers not
contain symbols or punctuation [Unicode-UAX31]. Of course, these
difficulties with symbols do not arise with actual pictographic
languages and scripts which would be treated like any other
language characters; the two should not be confused.
[[anchor55: Note in Draft: Should the above section be significantly
trimmed or eliminated?]]
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9.2.6. Migration Between Unicode Versions: Unassigned Code Points
In IDNA2003, labels containing unassigned code points are looked up
on the theory that, if they appear in labels and can be mapped and
then resolved, the relevant standards must have changed and the
registry has properly allocated only assigned values.
In this specification, strings containing unassigned code points MUST
NOT be either looked up or registered. There are several reasons for
this, with the most important ones being:
o It cannot be known with sufficient reliability in advance that a
code point that was not previously assigned will not be assigned
to a compatibility character. In IDNA2003, since there is no
direct dependency on NFKC (Stringprep's tables are based on NFKC,
but IDNA2003 depends only on Stringprep), allocation of a
compatibility character might produce some odd situations, but it
would not be a problem. In IDNA2008, where compatibility
characters are generally assigned to DISALLOWED, permitting
strings containing unassigned characters to be looked up would
permit violating the principle that characters in DISALLOWED are
not looked up.
o More generally, the status of an unassigned character with regard
to the DISALLOWED and PROTOCOL-VALID categories, and whether
contextual rules are required with the latter, cannot be evaluated
until a character is actually assigned and known.
It is possible to argue that the issues above are not important and
that, as a consequence, it is better to retain the principle of
looking up labels even if they contain unassigned characters because
all of the important scripts and characters have been coded as of
Unicode 5.1 and hence unassigned code points will be assigned only to
obscure characters or archaic scripts. Unfortunately, that does not
appear to be a safe assumption for at least two reasons. First, much
the same claim of completeness has been made for earlier versions of
Unicode. The reality is that a script that is obscure to much of the
world may still be very important to those who use it. Cultural and
linguistic preservation principles make it inappropriate to declare
the script of no importance in IDNs. Second, we already have
counterexamples in, e.g., the relationships associated with new Han
characters being added (whether in the BMP or in Unicode Plane 2).
9.2.7. Other Compatibility Issues
The existing (2003) IDNA model includes several odd artifacts of the
context in which it was developed. Many, if not all, of these are
potential avenues for exploits, especially if the registration
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process permits "source" names (names that have not been processed
through IDNA and nameprep) to be registered. As one example, since
the character Eszett, used in German, is mapped by IDNA2003 into the
sequence "ss" rather than being retained as itself or prohibited, a
string containing that character but that is otherwise in ASCII is
not really an IDN (in the U-label sense defined above) 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 IDNA2008 eliminates this
artifact. A character is either permitted as itself or it is
prohibited; special cases that make sense only in a particular
linguistic or cultural context can be dealt with as localization
matters where appropriate.
10. Acknowledgments
The editor and contributors would like to express their thanks to
those who contributed significant early (pre-WG) review comments,
sometimes accompanied by text, especially Mark Davis, Paul Hoffman,
Simon Josefsson, and Sam Weiler. In addition, some specific ideas
were incorporated from suggestions, text, or comments about sections
that were unclear supplied by Frank Ellerman, Michael Everson, Asmus
Freytag, Erik van der Poel, Michel Suignard, and Ken Whistler,
although, as usual, they bear little or no responsibility for the
conclusions the editor and contributors reached after receiving their
suggestions. Thanks are also due to Vint Cerf, Debbie Garside, and
Jefsey Morphin for conversations that led to considerable
improvements in the content of this document.
A meeting was held on 30 January 2008 to attempt to reconcile
differences in perspective and terminology about this set of
specifications between the design team and members of the Unicode
Technical Consortium. The discussions at and subsequent to that
meeting were very helpful in focusing the issues and in refining the
specifications. The active participants at that meeting were (in
alphabetic order as usual) Harald Alvestrand, Vint Cerf, Tina Dam,
Mark Davis, Lisa Dusseault, Patrik Faltstrom (by telephone), Cary
Karp, John Klensin, Warren Kumari, Lisa Moore, Erik van der Poel,
Michel Suignard, and Ken Whistler. We express our thanks to Google
for support of that meeting and to the participants for their
contributions.
Special thanks are due to Paul Hoffman for permission to extract
material from his Internet-Draft to form the basis for Section 9.1.
Useful comments and text on the WG versions of the draft were
received from many participants in the IETF "IDNABIS" WG and a number
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of document changes resulted from mailing list discussions made by
that group. Marcos Sanz provided specific analysis and suggestions
that were exceptionally helpful in refining the text.
11. Contributors
While the listed editor held the pen, this core of this document and
the initial WG version represents the joint work and conclusions of
an ad hoc design team consisting of the editor and, in alphabetic
order, Harald Alvestrand, Tina Dam, Patrik Faltstrom, and Cary Karp.
In addition, there were many specific contributions and helpful
comments from those listed in the Acknowledgments section and others
who have contributed to the development and use of the IDNA
protocols.
12. Internationalization Considerations
DNS labels and fully-qualified domain names provide mnemonics that
assist in identifying and referring to resources on the Internet.
IDNs expand the range of those mnemonics to include those based on
languages and character sets other than Western European and Roman-
derived ones. But domain "names" are not, in general, words in any
language. The recommendations of the IETF policy on character sets
and languages, BCP 18 [RFC2277] are applicable to situations in which
language identification is used to provide language-specific
contexts. The DNS is, by contrast, global and international and
ultimately has nothing to do with languages. Adding languages (or
similar context) to IDNs generally, or to DNS matching in particular,
would imply context dependent matching in DNS, which would be a very
significant change to the DNS protocol itself. It would also imply
that users would need to identify the language associated with a
particular label in order to look that label up, a decision that
would be impossible in many or most cases.
13. IANA Considerations
This section gives an overview of registries required for IDNA. The
actual definitions of the first two appear in [IDNA2008-Tables].
13.1. IDNA Character Registry
The distinction among the three major categories "UNASSIGNED",
"DISALLOWED", and "PROTOCOL-VALID" is made by special categories and
rules that are integral elements of [IDNA2008-Tables]. Convenience
in programming and validation requires a registry of characters and
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scripts and their categories, updated for each new version of Unicode
and the characters it contains. The details of this registry are
specified in [IDNA2008-Tables].
13.2. IDNA Context Registry
For characters that are defined in the IDNA Character Registry list
as PROTOCOL-VALID but requiring a contextual rule (i.e., the types of
rule described in Section 5.1.1.1), IANA will create and maintain a
list of approved contextual rules. The details for those rules
appear in [IDNA2008-Tables].
13.3. IANA Repository of IDN Practices of TLDs
This registry, historically described as the "IANA Language Character
Set Registry" or "IANA Script Registry" (both somewhat misleading
terms) is maintained by IANA at the request of ICANN. It is used to
provide a central documentation repository of the IDN policies used
by top level domain (TLD) registries who volunteer to contribute to
it and is used in conjunction with ICANN Guidelines for IDN use.
It is not an IETF-managed registry and, while the protocol changes
specified here may call for some revisions to the tables, these
specifications have no direct effect on that registry and no IANA
action is required as a result.
14. Security Considerations
[[anchor62: John Klensin claims that Security Considerations sections
are usually considered normative in the IETF.]]
Security on the Internet partly relies on the DNS. Thus, any change
to the characteristics of the DNS can change the security of much of
the Internet.
Domain names are used by users to identify and connect to Internet
servers. The security of the Internet is compromised if a user
entering a single internationalized name is connected to different
servers based on different interpretations of the internationalized
domain name.
When systems use local character sets other than ASCII and Unicode,
this specification leaves the problem of transcoding between the
local character set and Unicode up to the application or local
system. If different applications (or different versions of one
application) implement different transcoding rules, they could
interpret the same name differently and contact different servers.
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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 administrators may impose
restrictions (subject to the limitations identified elsewhere in this
document) that try to minimize characters that have similar
appearance or similar interpretations. It is worth noting that there
are no comprehensive technical solutions to the problems of
confusable characters. One can reduce the extent of the problems in
various ways, but probably never eliminate it. Some specific
suggestions about identification and handling of confusable
characters appear in a Unicode Consortium publication
[Unicode-UTR36].
The registration and lookup models described above and in
[IDNA2008-Protocol] change the mechanisms available for lookup
applications to determine the validity of labels they encounter. In
some respects, the ability to test is strengthened. For example,
putative labels that contain unassigned code points will now be
rejected, while IDNA2003 permitted them (something that is now
recognized as a considerable source of risk). On the other hand, the
protocol specification no longer assumes that the application that
looks up a name will be able to determine, and apply, information
about the protocol version used in registration. In theory, that may
increase risk since the application will be able to do less pre-
lookup validation. In practice, the protection afforded by that test
has been largely illusory for reasons explained in RFC 4690 and
above.
Any change to Stringprep or, more broadly, the IETF's model of the
use of internationalized character strings in different protocols,
creates some risk of inadvertent changes to those protocols,
invalidating deployed applications or databases, and so on. Our
current hypothesis is that the same considerations that would require
changing the IDN prefix (see Section 9.2.3.2) 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
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upgrades to security protocols or conventions.
No mechanism involving names or identifiers alone can protect a wide
variety of security threats and attacks that are largely independent
of them including spoofed pages, DNS query trapping and diversion,
and so on.
15. Change Log
[[anchor64: RFC Editor: Please remove this section.]]
15.1. Changes between Version -00 and Version -01 of
draft-ietf-idnabis-rationale
o Clarified the U-label definition to note that U-labels must
contain at least one non-ASCII character. Also clarified the
relationship among label types.
o Rewrote the discussion of Labels in Registration (Section 9.2.1.2)
and related text in Section 1.5.4.1.1 to narrow its focus and
remove more general restrictions. Added a temporary note in line
to explain the situation.
o Changed the "IDNA uses Unicode" statement to focus on
compatibility with IDNA2003 and avoid more general or
controversial assertions.
o Added a discussion of examples to Section 9.2.1
o Made a number of other small editorial changes and corrections
suggested by Mark Davis.
o Added several more discussion anchors and notes and expanded or
updated some existing ones.
15.2. Version -02
o Trimmed change log, removing information about pre-WG drafts.
o Adjusted discussion of Contextual Rules to match the new location
of the tables and some conceptual material.
o Rewrote the material on preprocessing somewhat.
o Moved the material about relationships with IDNA2003 to be part of
a single section on transitions.
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o Removed several placeholders and made editorial changes in
accordance with decisions made at IETF 72 in Dublin and not
disputed on the mailing list.
15.3. Version -03
This special update to the Rationale document is intended to try to
get the discussion of what is normative or not under control. While
the IETF does not normally annotate individual sections of documents
with whether they are normative or not, concerns that we don't know
which is which, claims that some material is normative that would be
problematic if so classified, etc., argue that we should at least be
able to have a clear discussion on the subject.
Two annotations have been applied to sections that might reasonably
be considered normative. One annotation is based on the list of
sections in Mark Davis's note of 29 September (http://
www.alvestrand.no/pipermail/idna-update/2008-September/002667.html).
The other is based on an elaboration of John Klensin's response on 7
October (http://www.alvestrand.no/pipermail/idna-update/2008-October/
002691.html). These should just be considered two suggestions to
illuminate and, one hopes, advance the Working Group's discussions.
Some additional editorial changes have been made, but they are
basically trivial. In the editor's judgment, it is not possible to
make significantly more progress with this document until the matter
of document organization is settled.
16. References
16.1. 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.
[IDNA2008-Bidi]
Alvestrand, H. and C. Karp, "An updated IDNA criterion for
right to left scripts", July 2008, .
[IDNA2008-Protocol]
Klensin, J., "Internationalized Domain Names in
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Applications (IDNA): Protocol", September 2008, .
[IDNA2008-Tables]
Faltstrom, P., "The Unicode Code Points and IDNA",
July 2008, .
A version of this document is available in HTML format at
http://stupid.domain.name/idnabis/
draft-ietf-idnabis-tables-02.html
[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.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.
[RulesInit]
Faltstrom, P., Ed., "The Unicode Codepoints and IDNA:
Appendix A Contextual Rules Table", July 2008, .
[Unicode-UAX15]
The Unicode Consortium, "Unicode Standard Annex #15:
Unicode Normalization Forms", March 2008,
.
[Unicode51]
The Unicode Consortium, "The Unicode Standard, Version
5.1.0", 2008.
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defined by: The Unicode Standard, Version 5.0, Boston, MA,
Addison-Wesley, 2007, ISBN 0-321-48091-0, as amended by
Unicode 5.1.0
(http://www.unicode.org/versions/Unicode5.1.0/).
16.2. Informative References
[BIG5] Institute for Information Industry of Taiwan, "Computer
Chinese Glyph and Character Code Mapping Table, Technical
Report C-26", 1984.
There are several forms and variations and a closely-
related standard, CNS 11643. See the discussion in
Chapter 3 of Lunde, K., CJKV Information Processing,
O'Reilly & Associates, 1999
[GB18030] "Chinese National Standard GB 18030-2000: Information
Technology -- Chinese ideograms coded character set for
information interchange -- Extension for the basic set.",
2000.
[RFC0810] Feinler, E., Harrenstien, K., Su, Z., and V. White, "DoD
Internet host table specification", RFC 810, March 1982.
[RFC0952] Harrenstien, K., Stahl, M., and E. Feinler, "DoD Internet
host table specification", RFC 952, October 1985.
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, November 1987.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[RFC1123] Braden, R., "Requirements for Internet Hosts - Application
and Support", STD 3, RFC 1123, October 1989.
[RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS
Specification", RFC 2181, July 1997.
[RFC2277] Alvestrand, H., "IETF Policy on Character Sets and
Languages", BCP 18, RFC 2277, January 1998.
[RFC2673] Crawford, M., "Binary Labels in the Domain Name System",
RFC 2673, August 1999.
[RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
specifying the location of services (DNS SRV)", RFC 2782,
February 2000.
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[RFC3743] Konishi, K., Huang, K., Qian, H., and Y. Ko, "Joint
Engineering Team (JET) Guidelines for Internationalized
Domain Names (IDN) Registration and Administration for
Chinese, Japanese, and Korean", RFC 3743, April 2004.
[RFC3987] Duerst, M. and M. Suignard, "Internationalized Resource
Identifiers (IRIs)", RFC 3987, January 2005.
[RFC4290] Klensin, J., "Suggested Practices for Registration of
Internationalized Domain Names (IDN)", RFC 4290,
December 2005.
[RFC4690] Klensin, J., Faltstrom, P., Karp, C., and IAB, "Review and
Recommendations for Internationalized Domain Names
(IDNs)", RFC 4690, September 2006.
[RFC4713] Lee, X., Mao, W., Chen, E., Hsu, N., and J. Klensin,
"Registration and Administration Recommendations for
Chinese Domain Names", RFC 4713, October 2006.
[Unicode-UAX31]
The Unicode Consortium, "Unicode Standard Annex #31:
Unicode Identifier and Pattern Syntax", March 2008,
.
[Unicode-UTR36]
The Unicode Consortium, "Unicode Technical Report #36:
Unicode Security Considerations", July 2008,
.
Author's Address
John C Klensin
1770 Massachusetts Ave, Ste 322
Cambridge, MA 02140
USA
Phone: +1 617 245 1457
Email: john+ietf@jck.com
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