< draft-ietf-idnabis-rationale-03.txt   draft-ietf-idnabis-rationale-04.txt >
Network Working Group J. Klensin Network Working Group J. Klensin
Internet-Draft October 7, 2008 Internet-Draft November 2, 2008
Intended status: Standards Track Intended status: Informational
Expires: April 10, 2009 Expires: May 6, 2009
Internationalized Domain Names for Applications (IDNA): Definitions, Internationalized Domain Names for Applications (IDNA): Background,
Background and Rationale Explanation, and Rationale
draft-ietf-idnabis-rationale-03.txt draft-ietf-idnabis-rationale-04.txt
Status of this Memo Status of this Memo
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Abstract Abstract
Several years have passed since the original protocol for Several years have passed since the original protocol for
Internationalized Domain Names (IDNs) was completed and deployed. Internationalized Domain Names (IDNs) was completed and deployed.
During that time, a number of issues have arisen, including the need 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 to update the system to deal with newer versions of Unicode. Some of
these issues require tuning of the existing protocols and the tables these issues require tuning of the existing protocols and the tables
on which they depend. This document provides an overview of a on which they depend. This document provides an overview of a
revised system and provides explanatory material for its components. revised system and provides explanatory material for its components.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Context and Overview . . . . . . . . . . . . . . . . . . . 4 1.1. Context and Overview . . . . . . . . . . . . . . . . . . . 4
1.2. Discussion Forum . . . . . . . . . . . . . . . . . . . . . 4 1.2. Discussion Forum . . . . . . . . . . . . . . . . . . . . . 4
1.3. Objectives . . . . . . . . . . . . . . . . . . . . . . . . 4 1.3. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
1.4. Applicability and Function of IDNA . . . . . . . . . . . . 5 1.3.1. Documents and Standards . . . . . . . . . . . . . . . 4
1.5. Terminology . . . . . . . . . . . . . . . . . . . . . . . 6 1.3.2. DNS "Name" Terminology . . . . . . . . . . . . . . . . 5
1.5.1. Documents and Standards . . . . . . . . . . . . . . . 6 1.3.3. New Terminology and Restrictions . . . . . . . . . . . 5
1.5.2. Terminology about Characters and Character Sets . . . 6 1.4. Objectives . . . . . . . . . . . . . . . . . . . . . . . . 6
1.5.3. DNS-related Terminology . . . . . . . . . . . . . . . 7 1.5. Applicability and Function of IDNA . . . . . . . . . . . . 6
1.5.4. Terminology Specific to IDNA . . . . . . . . . . . . . 8 1.6. Comprehensibility of IDNA Mechanisms and Processing . . . 7
1.5.5. Punycode is an Algorithm, not a Name . . . . . . . . . 12 2. Processing in IDNA2008 . . . . . . . . . . . . . . . . . . . . 9
1.5.6. Other Terminology Issues . . . . . . . . . . . . . . . 12 3. Permitted Characters: An Inclusion List . . . . . . . . . . . 9
1.6. Comprehensibility of IDNA Mechanisms and Processing . . . 13 3.1. A Tiered Model of Permitted Characters and Labels . . . . 9
2. The Revised IDNA Model . . . . . . . . . . . . . . . . . . . . 14 3.1.1. PROTOCOL-VALID . . . . . . . . . . . . . . . . . . . . 10
3. Processing in IDNA2008 . . . . . . . . . . . . . . . . . . . . 14 3.1.2. DISALLOWED . . . . . . . . . . . . . . . . . . . . . . 11
4. IDNA2008 Document List . . . . . . . . . . . . . . . . . . . . 15 3.1.3. UNASSIGNED . . . . . . . . . . . . . . . . . . . . . . 12
5. Permitted Characters: An Inclusion List . . . . . . . . . . . 16 3.2. Registration Policy . . . . . . . . . . . . . . . . . . . 12
5.1. A Tiered Model of Permitted Characters and Labels . . . . 16 3.3. Layered Restrictions: Tables, Context, Registration,
5.1.1. PROTOCOL-VALID . . . . . . . . . . . . . . . . . . . . 17 Applications . . . . . . . . . . . . . . . . . . . . . . . 13
5.1.2. DISALLOWED . . . . . . . . . . . . . . . . . . . . . . 18 4. Issues that Constrain Possible Solutions . . . . . . . . . . . 13
5.1.3. UNASSIGNED . . . . . . . . . . . . . . . . . . . . . . 19 4.1. Display and Network Order . . . . . . . . . . . . . . . . 13
5.2. Registration Policy . . . . . . . . . . . . . . . . . . . 19 4.2. Entry and Display in Applications . . . . . . . . . . . . 15
5.3. Layered Restrictions: Tables, Context, Registration, 4.3. Linguistic Expectations: Ligatures, Digraphs, and
Applications . . . . . . . . . . . . . . . . . . . . . . . 20 Alternate Character Forms . . . . . . . . . . . . . . . . 16
6. Issues that Constrain Possible Solutions . . . . . . . . . . . 20 4.4. Case Mapping and Related Issues . . . . . . . . . . . . . 18
6.1. Display and Network Order . . . . . . . . . . . . . . . . 20 4.5. Right to Left Text . . . . . . . . . . . . . . . . . . . . 19
6.2. Entry and Display in Applications . . . . . . . . . . . . 22 5. IDNs and the Robustness Principle . . . . . . . . . . . . . . 19
6.3. Linguistic Expectations: Ligatures, Digraphs, and 6. Front-end and User Interface Processing . . . . . . . . . . . 20
Alternate Character Forms . . . . . . . . . . . . . . . . 23 7. Migration from IDNA2003 and Unicode Version Synchronization . 23
6.4. Case Mapping and Related Issues . . . . . . . . . . . . . 25 7.1. Design Criteria . . . . . . . . . . . . . . . . . . . . . 23
6.5. Right to Left Text . . . . . . . . . . . . . . . . . . . . 26 7.1.1. General IDNA Validity Criteria . . . . . . . . . . . . 23
7. IDNs and the Robustness Principle . . . . . . . . . . . . . . 26 7.1.2. Labels in Registration . . . . . . . . . . . . . . . . 25
8. Front-end and User Interface Processing . . . . . . . . . . . 27 7.1.3. Labels in Lookup . . . . . . . . . . . . . . . . . . . 26
9. Relationship to IDNA2003 and Earlier Versions of Unicode . . . 30 7.2. Changes in Character Interpretations . . . . . . . . . . . 27
9.1. Summary of Major Changes from IDNA2003 . . . . . . . . . . 30 7.3. More Flexibility in User Agents . . . . . . . . . . . . . 28
9.2. Migration and Version Synchronization . . . . . . . . . . 31 7.4. The Question of Prefix Changes . . . . . . . . . . . . . . 30
9.2.1. Design Criteria . . . . . . . . . . . . . . . . . . . 31 7.4.1. Conditions Requiring a Prefix Change . . . . . . . . . 30
9.2.2. More Flexibility in User Agents . . . . . . . . . . . 34 7.4.2. Conditions Not Requiring a Prefix Change . . . . . . . 31
9.2.3. The Question of Prefix Changes . . . . . . . . . . . . 35 7.4.3. Implications of Prefix Changes . . . . . . . . . . . . 31
9.2.4. Stringprep Changes and Compatibility . . . . . . . . . 37 7.5. Stringprep Changes and Compatibility . . . . . . . . . . . 31
9.2.5. The Symbol Question . . . . . . . . . . . . . . . . . 38 7.6. The Symbol Question . . . . . . . . . . . . . . . . . . . 32
9.2.6. Migration Between Unicode Versions: Unassigned 7.7. Migration Between Unicode Versions: Unassigned Code
Code Points . . . . . . . . . . . . . . . . . . . . . 40 Points . . . . . . . . . . . . . . . . . . . . . . . . . . 33
9.2.7. Other Compatibility Issues . . . . . . . . . . . . . . 40 7.8. Other Compatibility Issues . . . . . . . . . . . . . . . . 34
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 41 8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 35
11. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 42 9. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 36
12. Internationalization Considerations . . . . . . . . . . . . . 42 10. Internationalization Considerations . . . . . . . . . . . . . 36
13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 42 11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 36
13.1. IDNA Character Registry . . . . . . . . . . . . . . . . . 42 11.1. IDNA Character Registry . . . . . . . . . . . . . . . . . 36
13.2. IDNA Context Registry . . . . . . . . . . . . . . . . . . 43 11.2. IDNA Context Registry . . . . . . . . . . . . . . . . . . 37
13.3. IANA Repository of IDN Practices of TLDs . . . . . . . . . 43 11.3. IANA Repository of IDN Practices of TLDs . . . . . . . . . 37
14. Security Considerations . . . . . . . . . . . . . . . . . . . 43 12. Security Considerations . . . . . . . . . . . . . . . . . . . 37
15. Change Log . . . . . . . . . . . . . . . . . . . . . . . . . . 45 12.1. General Security Issues with IDNA . . . . . . . . . . . . 37
15.1. Changes between Version -00 and Version -01 of 12.2. Security Differences from IDNA2003 . . . . . . . . . . . . 37
draft-ietf-idnabis-rationale . . . . . . . . . . . . . . . 45 13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 38
15.2. Version -02 . . . . . . . . . . . . . . . . . . . . . . . 45 13.1. Normative References . . . . . . . . . . . . . . . . . . . 38
15.3. Version -03 . . . . . . . . . . . . . . . . . . . . . . . 46 13.2. Informative References . . . . . . . . . . . . . . . . . . 39
16. References . . . . . . . . . . . . . . . . . . . . . . . . . . 46 Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . 41
16.1. Normative References . . . . . . . . . . . . . . . . . . . 46 A.1. Changes between Version -00 and Version -01 of
16.2. Informative References . . . . . . . . . . . . . . . . . . 48 draft-ietf-idnabis-rationale . . . . . . . . . . . . . . . 41
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 49 A.2. Version -02 . . . . . . . . . . . . . . . . . . . . . . . 42
Intellectual Property and Copyright Statements . . . . . . . . . . 50 A.3. Version -03 . . . . . . . . . . . . . . . . . . . . . . . 42
A.4. Version -04 . . . . . . . . . . . . . . . . . . . . . . . 42
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 43
Intellectual Property and Copyright Statements . . . . . . . . . . 44
1. Introduction 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 1.1. Context and Overview
Several years have passed since the original protocol for The original standards for Internationalized Domain Names (IDNs) were
Internationalized Domain Names (IDNs) was completed and deployed. completed and deployed starting in 2003. Those standards are known
During that time, a number of issues have arisen, including a subset as Internationalized Domain Names in Applications (IDNA), taken from
of those described in a recent IAB report [RFC4690] and the need to the name of the highest level standard within the group, RFC 3490
update the system to deal with newer versions of Unicode. Those [RFC3490]. After those standards were deployed, a number of issues
standards are known as Internationalized Domain Names in Applications arose that called for a new version of the IDNA protocol and the
(IDNA), taken from the name of the highest level standard within that associated tables, including a subset of those described in a recent
group (see Section 1.5). Some tuning of the existing protocols and IAB report [RFC4690] and the need to update the system to deal with
the tables on which they depend is now required. Where it is newer versions of Unicode. This document further explains the issues
important to understanding of the revised protocols, this document that have been encountered when they are important to understanding
further explains the issues that have been encountered. It also of the revised protocols. It also provides an overview of the new
provides an overview of the new IDNA model and explanatory material IDNA model and explanatory material for it. Additional explanatory
for it. Additional explanatory material for the specific components material for the specific components of the proposals appears with
of the proposals will appear with the associated documents. the associated documents.
1.2. Discussion Forum 1.2. Discussion Forum
[[anchor4: RFC Editor: please remove this section.]] [[ RFC Editor: please remove this section. ]]
This work is being discussed in the IETF "idnabis" Working Group and IDNA2008 is being discussed in the IETF "idnabis" Working Group and
on the mailing list idna-update@alvestrand.no on the mailing list idna-update@alvestrand.no
1.3. Objectives 1.3. Terminology
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
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
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 Terminology that is critical for understanding this document and the
rest of the documents that make up IDNA2008, appears in
[IDNA2008-Defs]. That document also contains roadmap to the IDNA2008
document collection. No attempt should be made to understand this
document without the definitions and concepts that appear there.
[[anchor8: John Klensin believes that the definitions of "IDNA2003" 1.3.1. Documents and Standards
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 This document uses the term "IDNA2003" to refer to the set of
standards that make up and support the version of IDNA published in standards that make up and support the version of IDNA published in
2003, i.e., those commonly known as the IDNA base specification 2003, i.e., those commonly known as the IDNA base specification
[RFC3490], Nameprep [RFC3491], Punycode [RFC3492], and Stringprep [RFC3490], Nameprep [RFC3491], Punycode [RFC3492], and Stringprep
[RFC3454]. In this document, those names are used to refer, [RFC3454]. In this document, those names are used to refer,
conceptually, to the individual documents, with the base IDNA conceptually, to the individual documents, with the base IDNA
specification called just "IDNA". specification called just "IDNA".
The term "IDNA2008" is used to refer to a new version of IDNA as The term "IDNA2008" is used to refer to a new version of IDNA as
described in this document and in the documents described in described in this document and in the documents described in the
Section 4. References to "these specifications" are to the entire document listing of [IDNA2008-Defs]. IDNA2008 is not dependent on
any of the IDNA2003 specifications other than the one for Punycode
encoding. References to "these specifications" are to the entire
set. set.
1.5.2. Terminology about Characters and Character Sets 1.3.2. DNS "Name" Terminology
[[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
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
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.
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:
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.
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 <IMG> 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.
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 These documents depart from historical DNS terminology and usage in
important respect. Over the years, the community has talked very one important respect. Over the years, the community has talked very
casually about "names" in the DNS, beginning with calling it "the casually about "names" in the DNS, beginning with calling it "the
domain name system". That terminology is fine in the very precise domain name system". That terminology is fine in the very precise
sense that the identifiers of the DNS do provide names for objects sense that the identifiers of the DNS do provide names for objects
and addresses. But, in the context of IDNs, the term has introduced and addresses. But, in the context of IDNs, the term has introduced
some confusion, confusion that has increased further as people have some confusion, confusion that has increased further as people have
begun to speak of DNS labels in terms of the words or phrases of begun to speak of DNS labels in terms of the words or phrases of
various natural languages. various natural languages.
Historically, many, perhaps most, of the "names" in the DNS have been Historically, many, perhaps most, of the "names" in the DNS have been
mnemonics to identify some particular concept, object, or mnemonics to identify some particular concept, object, or
skipping to change at page 13, line 6 skipping to change at page 5, line 33
because they are mnemonics, they need not obey the orthographic because they are mnemonics, they need not obey the orthographic
conventions of any language: it is not a requirement that it be conventions of any language: it is not a requirement that it be
possible for them to be "words". possible for them to be "words".
This distinction is important because the reasonable goal of an IDN 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 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 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 broad range of mnemonics in ways that are as natural as possible in a
very broad range of scripts. very broad range of scripts.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 1.3.3. New Terminology and Restrictions
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119]. These documents [IDNA2008-Defs] introduce new terminology, and
precise definitions, for the terms "U-labels", "A-labels", labels
that are "IDNA-valid", and an "LDH-label" (differing from an LDH-
conformant label or fully-qualified domain name). The also introduce
a restriction, for IDNA-conformant applications and DNS zones in
which IDNA is used, on strings used as labels that contain "--" in
the third and fourth positions, essentially requiring that such
strings be IDNA-valid. This restriction on strings containing "--"
is required for three reasons:
o to prevent confusion with pre-IDNA coding forms;
o to permit future extensions that would require changing the
prefix, no matter how unlikely those might be (see Section 7.4);
and
o to reduce the opportunities for attacks via the encoding system.
1.4. 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. The objective of IDNA2008
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 is able to
completely eliminate sources of visual or textual confusion: such
confusion is possible even under the original host naming rules where
only ASCII characters were permitted. However, through the
application of different techniques at different points (see
Section 3.3), it should be possible to keep problems to an acceptable
minimum. One consequence of this general objective is that the
desire of some user or marketing community to use a particular string
--whether the reason is to try to write sentences of particular
languages in the DNS, to express a facsimile of the symbol for a
brand, or for some other purpose-- is not a primary goal within the
context of applications in the domain name space.
1.5. Applicability and Function of IDNA
The IDNA specification solves the problem of extending the repertoire
of characters that can be used in domain names to include a large
subset of the Unicode repertoire.
IDNA does not extend 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.
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 6).
A great deal of the discussion of IDN solutions has focused on
transition issues and how IDNs will work in a world where not all of
the components have been updated. Proposals that were not chosen by
the original IDN Working Group would have depended on updating of
user applications, DNS resolvers, and DNS servers 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, IDNA requires no changes to the
DNS protocol or any DNS servers or the resolvers on user's computers.
IDNA allows the graceful introduction of IDNs not only by avoiding
upgrades to existing infrastructure (such as DNS servers and mail
transport agents), but also by allowing some rudimentary use of IDNs
in applications by using the ASCII-encoded representation of the
labels containing non-ASCII characters. While such names are user-
unfriendly to read and type, and hence not optimal for user input,
they can be used as a last resort to allow rudimentary IDN usage.
For example, they might be the best choice for display if it were
known that relevant fonts were not available on the user's computer.
In order to allow user-friendly input and output of the IDNs and
acceptance of some characters as equivalent to those to be processed
according to the protocol, the applications need to be modified to
conform to this specification.
IDNA uses the Unicode character repertoire, for continuity with the
original version of IDNA.
1.6. Comprehensibility of IDNA Mechanisms and Processing 1.6. Comprehensibility of IDNA Mechanisms and Processing
One of the major goals of this work is to improve the general One of the major goals of this work is to improve the general
understanding of how IDNA works and what characters are permitted and understanding of how IDNA works and what characters are permitted and
what happens to them. Comprehensibility and predictability to users what happens to them. Comprehensibility and predictability to users
and registrants are themselves important motivations and design goals and registrants are themselves important motivations and design goals
for this effort. The effort includes some new terminology and a for this effort. The effort includes some new terminology and a
revised and extended model, both covered in this section, and some revised and extended model, both covered in this section, and some
more specific protocol, processing, and table modifications. Details more specific protocol, processing, and table modifications. Details
of the latter appear in other documents (see Section 4). of the latter appear in other documents (see [IDNA2008-Defs]).
Several issues are inherent in the application of IDNs and, indeed, Several issues are inherent in the application of IDNs and, indeed,
almost any other system that tries to handle international characters almost any other system that tries to handle international characters
and concepts. They range from the apparently trivial --e.g., one and concepts. They range from the apparently trivial --e.g., one
cannot display a character for which one does not have a font cannot display a character for which one does not have a font
available locally-- to the more complex and subtle. Many people have available locally-- to the more complex and subtle. Many people have
observed that internationalization is just a tool to enable effective observed that internationalization is just a tool to enable effective
localization while permitting some global uniformity. Issues of localization while permitting some global uniformity. Issues of
display, of exactly how various strings and characters are entered, display, of exactly how various strings and characters are entered,
and so on are inherently issues about localization and user interface and so on are inherently issues about localization and user interface
skipping to change at page 13, line 45 skipping to change at page 8, line 26
work when characters and fonts are not available, but they can only work when characters and fonts are not available, but they can only
be general recommendations and, because display functions are rarely be general recommendations and, because display functions are rarely
controlled by the types of applications that would call upon IDNA, controlled by the types of applications that would call upon IDNA,
will rarely be very effective. will rarely be very effective.
However, shifting responsibility for character mapping and other However, shifting responsibility for character mapping and other
adjustments from the protocol (where it was located in IDNA2003) to adjustments from the protocol (where it was located in IDNA2003) to
the user interface or processing before invoking IDNA raises issues the user interface or processing before invoking IDNA raises issues
about both what that processing should do and about compatibility for about both what that processing should do and about compatibility for
references prepared in an IDNA2003 context. Those issues are references prepared in an IDNA2003 context. Those issues are
discussed in Section 8. discussed in Section 6.
Operations for converting between local character sets and normalized Operations for converting between local character sets and normalized
Unicode are part of this general set of user interface issues. The Unicode are part of this general set of user interface issues. The
conversion is obviously not required at all in a Unicode-native conversion is obviously not required at all in a Unicode-native
system that maintains all strings in Normalization Form C (NFC). It system that maintains all strings in Normalization Form C (NFC). It
may, however, involve some complexity in a system that is not may, however, involve some complexity in a system that is not
Unicode-native, especially if the elements of the local character set Unicode-native, especially if the elements of the local character set
do not map exactly and unambiguously into Unicode characters or do so do not map exactly and unambiguously into Unicode characters or do so
in a way that is not completely stable over time. Perhaps more in a way that is not completely stable over time. Perhaps more
important, if a label being converted to a local character set important, if a label being converted to a local character set
skipping to change at page 14, line 29 skipping to change at page 9, line 10
systems are substantially or completely Unicode-compatible (i.e., all systems are substantially or completely Unicode-compatible (i.e., all
of the code points in them have an exact and unique mapping to of the code points in them have an exact and unique mapping to
Unicode code points). It may be even more difficult when the Unicode code points). It may be even more difficult when the
character coding system in local use is based on conceptually character coding system in local use is based on conceptually
different assumptions than those used by Unicode about, e.g., about different assumptions than those used by Unicode about, e.g., about
font encodings used for publications in some Indic scripts. Those font encodings used for publications in some Indic scripts. Those
differences may not easily yield unambiguous conversions or differences may not easily yield unambiguous conversions or
interpretations even if each coding system is internally consistent interpretations even if each coding system is internally consistent
and adequate to represent the local language and script. and adequate to represent the local language and script.
2. The Revised IDNA Model 2. Processing in IDNA2008
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 These specifications separate Domain Name Registration and Lookup in
the protocol specification. Doing so reflects current practice in the protocol specification. Doing so reflects current practice in
which per-registry restrictions and special processing are applied at which per-registry restrictions and special processing are applied at
registration time but not during lookup. Even more important in the registration time but not during lookup. Even more important in the
longer term, it facilitates incremental addition of permitted longer term, it facilitates incremental addition of permitted
character groups to avoid freezing on one particular version of character groups to avoid freezing on one particular version of
Unicode. Unicode.
The actual registration and lookup protocols for IDNA2008 are The actual registration and lookup protocols for IDNA2008 are
specified in [IDNA2008-Protocol]. specified in [IDNA2008-Protocol].
4. IDNA2008 Document List 3. Permitted Characters: An Inclusion 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.
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 This section provides an overview of the model used to establish the
algorithm and character lists of [IDNA2008-Tables] and describes the algorithm and character lists of [IDNA2008-Tables] and describes the
names and applicability of the categories used there. Note that the names and applicability of the categories used there. Note that the
inclusion of a character in the first category group does not imply inclusion of a character in the first category group does not imply
that it can be used indiscriminately; some characters are associated that it can be used indiscriminately; some characters are associated
with contextual rules that must be applied as well. with contextual rules that must be applied as well.
The information given in this section is provided to make the rules, The information given in this section is provided to make the rules,
tables, and protocol easier to understand. It is not normative. The tables, and protocol easier to understand. It is not normative. The
normative generating rules appear in [IDNA2008-Tables] and the rules normative generating rules appear in [IDNA2008-Tables] and the rules
that actually determine what labels can be registered or looked up that actually determine what labels can be registered or looked up
are in [IDNA2008-Protocol]. are in [IDNA2008-Protocol].
5.1. A Tiered Model of Permitted Characters and Labels 3.1. 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 Moving to an inclusion model requires respecifying the list of
characters that are permitted in IDNs. In IDNA2003, the role and characters that are permitted in IDNs. In IDNA2003, the role and
utility of characters are independent of context and fixed forever utility of characters are independent of context and fixed forever
(or until the standard is replaced). Making completely context- (or until the standard is replaced). Making completely context-
independent rules globally has proven impractical because some independent rules globally has proven impractical because some
characters, especially those that are called "Join_Controls" in characters, especially those that are called "Join_Controls" in
Unicode, are needed to make reasonable use of some scripts but have Unicode, are needed to make reasonable use of some scripts but have
no visible effect(s) in others. Of necessity, IDNA2003 prohibited no visible effect(s) in others. IDNA2003 prohibited those types of
those types of characters entirely. But the restrictions were much characters entirely. But the restrictions were much too severe to
too severe to permit an adequate range of mnemonics for terminology permit an adequate range of mnemonics for terminology based on some
based on some languages. The requirement to support those characters languages. The requirement to support those characters but limit
but limit their use to very specific contexts was reinforced by the their use to very specific contexts was reinforced by the observation
observation that handling of particular characters across the that handling of particular characters across the languages that use
languages that use a script, or the use of similar or identical- a script, or the use of similar or identical-looking characters in
looking characters in different scripts, is less well understood than different scripts, is less well understood than many people believed
many people believed it was several years ago. it was several years ago.
Independently of the characters chosen (see next subsection), the Independently of the characters chosen (see next subsection), the
theory is to divide the characters that appear in Unicode into three theory is to divide the characters that appear in Unicode into three
categories: categories:
5.1.1. PROTOCOL-VALID 3.1.1. PROTOCOL-VALID
Characters identified as "PROTOCOL-VALID" (often abbreviated Characters identified as "PROTOCOL-VALID" (often abbreviated
"PVALID") are, in general, permitted by IDNA for all uses in IDNs. "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 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 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 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 in this category that has a "right-to-left" property must be used in
context with the "Bidi" rules (see [IDNA2008-Bidi]). context with the "Bidi" rules (see [IDNA2008-Bidi]).
The term "PROTOCOL-VALID" is used to stress the fact that the 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 presence of a character in this category does not imply that a given
registry need accept registrations containing any of the characters registry need accept registrations containing any of the characters
in the category. Registries are still expected to apply judgment in the category. Registries are still expected to apply judgment
about labels they will accept and to maintain rules consistent with about labels they will accept and to maintain rules consistent with
those judgments (see [IDNA2008-Protocol] and Section 5.3). those judgments (see [IDNA2008-Protocol] and Section 3.3).
Characters that are placed in the "PROTOCOL-VALID" category are never Characters that are placed in the "PROTOCOL-VALID" category are never
removed from it unless the code points themselves are removed from removed from it unless the code points themselves are removed from
Unicode (such removal would be inconsistent with the Unicode Unicode (such removal would be inconsistent with the Unicode
stability principles (see [Unicode51], Appendix F) and hence should stability principles (see [Unicode51], Appendix F) and hence should
never occur). never occur).
[[anchor36: Placeholder: Does this topic or comment need additional 3.1.1.1. Contextual Rules
discussion or explanation?]]
5.1.1.1. Contextual Rules
Some characters may be unsuitable for general use in IDNs but Some characters may be unsuitable for general use in IDNs but
necessary for the plausible support of some scripts. The two most necessary for the plausible support of some scripts. The two most
commonly-cited examples are the zero-width joiner and non-joiner commonly-cited examples are the zero-width joiner and non-joiner
characters (ZWJ, U+200D and ZWNJ, U+200C), but provisions for characters (ZWJ, U+200D and ZWNJ, U+200C), but provisions for
unambiguous labels may require that other characters be restricted to unambiguous labels may require that other characters be restricted to
particular contexts. For example, the ASCII hyphen is not permitted particular contexts. For example, the ASCII hyphen is not permitted
to start or end a label, whether that label contains non-ASCII to start or end a label, whether that label contains non-ASCII
characters or not. characters or not.
skipping to change at page 18, line 9 skipping to change at page 11, line 15
conjunction with particular characters or scripts. In order to conjunction with particular characters or scripts. In order to
permit them to be used at all, they are specially identified as permit them to be used at all, they are specially identified as
"CONTEXTUAL RULE REQUIRED" and, when adequately understood, "CONTEXTUAL RULE REQUIRED" and, when adequately understood,
associated with a rule. In addition, the rule will define whether it 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 is to be applied on lookup as well as registration. A distinction is
made between characters that indicate or prohibit joining (known as made between characters that indicate or prohibit joining (known as
"CONTEXT-JOINER" or "CONTEXTJ") and other characters requiring "CONTEXT-JOINER" or "CONTEXTJ") and other characters requiring
contextual treatment ("CONTEXT-OTHER" or "CONTEXTO"). Only the contextual treatment ("CONTEXT-OTHER" or "CONTEXTO"). Only the
former are fully tested at lookup time. former are fully tested at lookup time.
5.1.1.2. Rules and Their Application 3.1.1.2. Rules and Their Application
The actual rules may be present or absent. If present, they may have 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 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 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 set of procedural rules that specify the context in which the
character is permitted. character is permitted.
Examples of descriptions of typical rules, stated informally and in Examples of descriptions of typical rules, stated informally and in
English, include "Must follow a character from Script XYZ", "MUST English, include "Must follow a character from Script XYZ", "Must
occur only if the entire label is in Script ABC", "MUST occur only if occur only if the entire label is in Script ABC", "Must occur only if
the previous and subsequent characters have the DFG property". the previous and subsequent characters have the DFG property".
Because it is easier to identify these characters than to know that Because it is easier to identify these characters than to know that
they are actually needed in IDNs or how to establish exactly the 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 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 version of the tables. Characters associated with null rules are not
NOT appear in putative labels for either registration or lookup. Of permitted to appear in putative labels for either registration or
course, a later version of the tables might contain a non-null rule. 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, The description of the syntax of the rules, and the rules themselves,
appears in [IDNA2008-Tables]. appears in [IDNA2008-Tables].
5.1.2. DISALLOWED 3.1.2. DISALLOWED
Some characters are sufficiently problematic for use in IDNs that Some characters are sufficiently problematic for use in IDNs that
they should be excluded for both registration and lookup (i.e., IDNA- they should be excluded for both registration and lookup (i.e., IDNA-
conforming applications performing name lookup should verify that conforming applications performing name lookup should verify that
these characters are absent; if they are present, the label strings these characters are absent; if they are present, the label strings
should be rejected rather than converted to A-labels and looked up. should be rejected rather than converted to A-labels and looked up.
Of course, this category would include code points that had been Of course, this category would include code points that had been
removed entirely from Unicode should such removals ever occur. removed entirely from Unicode should such removals ever occur.
Characters that are placed in the "DISALLOWED" category are expected Characters that are placed in the "DISALLOWED" category are expected
to never be removed from it or reclassified. If a character is to never be removed from it or reclassified. If a character is
classified as "DISALLOWED" in error and the error is sufficiently classified as "DISALLOWED" in error and the error is sufficiently
problematic, the only recourse would be either to introduce a new problematic, the only recourse would be either to introduce a new
code point into Unicode and classify it as "PROTOCOL-VALID" or for code point into Unicode and classify it as "PROTOCOL-VALID" or for
the IETF to accept the considerable costs of an incompatible change the IETF to accept the considerable costs of an incompatible change
and replace the relevant RFC with one containing appropriate and replace the relevant RFC with one containing appropriate
exceptions. exceptions.
[[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 There is provision for exception cases but, in general, characters
are placed into "DISALLOWED" if they fall into one or more of the are placed into "DISALLOWED" if they fall into one or more of the
following groups: following groups:
o The character is a compatibility equivalent for another character. o The character is a compatibility equivalent for another character.
In slightly more precise Unicode terms, application of In slightly more precise Unicode terms, application of
normalization method NFKC to the character yields some other normalization method NFKC to the character yields some other
character. character.
o The character is an upper-case form or some other form that is o The character is an upper-case form or some other form that is
mapped to another character by Unicode casefolding. mapped to another character by Unicode casefolding.
o The character is a symbol or punctuation form or, more generally, 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 something that is not a letter, digit, or a mark that is used to
form a letter or digit. form a letter or digit.
5.1.3. UNASSIGNED 3.1.3. UNASSIGNED
For convenience in processing and table-building, code points that do For convenience in processing and table-building, code points that do
not have assigned values in a given version of Unicode are treated as not have assigned values in a given version of Unicode are treated as
belonging to a special UNASSIGNED category. Such code points MUST belonging to a special UNASSIGNED category. Such code points MUST
NOT appear in labels to be registered or looked up. The category 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 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 the simple expedient of being assigned in a later version of Unicode
(at which point, they are classified into one of the other categories (at which point, they are classified into one of the other categories
as appropriate). as appropriate).
5.2. Registration Policy 3.2. 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 While these recommendations cannot and should not define registry
policies, registries SHOULD develop and apply additional restrictions policies, registries SHOULD develop and apply additional restrictions
to reduce confusion and other problems. For example, it is generally to reduce confusion and other problems. For example, it is generally
believed that labels containing characters from more than one script believed that labels containing characters from more than one script
are a bad practice although there may be some important exceptions to are a bad practice although there may be some important exceptions to
that principle. Some registries may choose to restrict registrations that principle. Some registries may choose to restrict registrations
to characters drawn from a very small number of scripts. For many to characters drawn from a very small number of scripts. For many
scripts, the use of variant techniques such as those as described in scripts, the use of variant techniques such as those as described in
[RFC3743] and [RFC4290], and illustrated for Chinese by the tables RFC 3843 [RFC3743] and RFC 4290 [RFC4290], and illustrated for
described in RFC 4713 [RFC4713] may be helpful in reducing problems Chinese by the tables described in RFC 4713 [RFC4713] may be helpful
that might be perceived by users. It is worth stressing that these in reducing problems that might be perceived by users.
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 In general, users will benefit if registries only permit characters
from scripts that are well-understood by the registry or its
advisers. If a registry decides to reduce opportunities for
confusion by constructing policies that disallow characters used in
historic writing systems or characters whose use is restricted to
specialized, highly technical contexts, some relevant information may
be found in Section 2.4 "Specific Character Adjustments", Table 4
"Candidate Characters for Exclusion from Identifiers" of
[Unicode-UAX31] and Section 3.1. "General Security Profile for
Identifiers" in [Unicode-Security].
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.
3.3. Layered Restrictions: Tables, Context, Registration, Applications
The essence of the character rules in IDNA2008 is based on the The essence of the character rules in IDNA2008 is based on the
realization that there is no magic bullet for any of the issues realization that there is no single magic bullet for any of the
associated with a multiscript DNS. Instead, the specifications issues associated with a multiscript DNS. Instead, the
define a variety of approaches that, together, constitute multiple specifications define a variety of approaches that, together,
lines of defense against ambiguity in identifiers and loss of constitute multiple lines of defense against ambiguity in identifiers
referential integrity. The actual character tables are the first and loss of referential integrity. The actual character tables are
mechanism, protocol rules about how those characters are applied or the first mechanism, protocol rules about how those characters are
restricted in context are the second, and those two in combination applied or restricted in context are the second, and those two in
constitute the limits of what can be done by a protocol alone. As combination constitute the limits of what can be done by a protocol
discussed in the previous section (Section 5.2), registries are alone. As discussed in the previous section (Section 3.2),
expected to restrict what they permit to be registered, devising and registries are expected to restrict what they permit to be
using rules that are designed to optimize the balance between registered, devising and using rules that are designed to optimize
confusion and risk on the one hand and maximum expressiveness in the balance between confusion and risk on the one hand and maximum
mnemonics on the other. expressiveness in mnemonics on the other.
In addition, there is an important role for user agents in warning In addition, there is an important role for user agents in warning
against label forms that appear unreasonable given their knowledge of against label forms that appear unreasonable given their knowledge of
local contexts and conventions. Of course, no approach based on local contexts and conventions. Of course, no approach based on
naming or identifiers alone can protect against all threats. naming or identifiers alone can protect against all threats.
6. Issues that Constrain Possible Solutions 4. Issues that Constrain Possible Solutions
6.1. Display and Network Order
[[anchor43: John Klensin asks whether the material in Bidi that 4.1. Display and Network Order
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 The correct treatment of domain names requires a clear distinction
between Network Order (the order in which the code points are sent in 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 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 displayed on a screen or paper). The order of labels in a domain
name that contains characters that are normally written right to left name that contains characters that are normally written right to left
is discussed in [IDNA2008-Bidi]. In particular, there are questions is discussed in [IDNA2008-Bidi]. In particular, there are questions
about the order in which labels are displayed if left to right and 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 right to left labels are adjacent to each other, especially if there
are also multiple consecutive appearances of one of the types. The are also multiple consecutive appearances of one of the types. The
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user agents --including web browsers, mail clients, and the like-- user agents --including web browsers, mail clients, and the like--
which may be highly localized. Even when formats are specified by which may be highly localized. Even when formats are specified by
protocols, the full composition of an Internationalized Resource protocols, the full composition of an Internationalized Resource
Identifier (IRI) [RFC3987] or Internationalized Email address Identifier (IRI) [RFC3987] or Internationalized Email address
contains elements other than the domain name. For example, IRIs contains elements other than the domain name. For example, IRIs
contain protocol identifiers and field delimiter syntax such as contain protocol identifiers and field delimiter syntax such as
"http://" or "mailto:" while email addresses contain the "@" to "http://" or "mailto:" while email addresses contain the "@" to
separate local parts from domain names. User agents are not required separate local parts from domain names. User agents are not required
to use those protocol-based forms directly but often do so. While to use those protocol-based forms directly but often do so. While
display, parsing, and processing within a label is specified by the display, parsing, and processing within a label is specified by the
IDNA protocol and the associated documents, the relationship between normative documents in the IDNA2008 collection, the relationship
fully-qualified domain names and internationalized labels is between fully-qualified domain names and internationalized labels is
unchanged from the base DNS specifications. Comments here about such unchanged from the base DNS specifications. Comments in this
full domain names are explanatory or examples of what might be done document about such full domain names are explanatory or examples of
and must not be considered normative. what might be done and must not be considered normative.
Questions remain about protocol constraints implying that the overall Questions remain about protocol constraints implying that the overall
direction of these strings will always be left to right (or right to 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 left) for an IRI or email address, or if they even should conform to
such rules. These questions also have several possible answers. such rules. These questions also have several possible answers.
Should a domain name abc.def, in which both labels are represented in 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 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, cba.fed? An IRI for clear text web access would, in network order,
begin with "http://" and the characters will appear as begin with "http://" and the characters will appear as
"http://abc.def" -- but what does this suggest about the display "http://abc.def" -- but what does this suggest about the display
order? When entering a URI to many browsers, it may be possible to 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 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 by default, assuming no tail (an approach that does not work for
other protocols). The natural display order for the typed domain protocols other than HTTP or whatever is chosen as the default). The
name on a right to left system is fed.cba. Does this change if a natural display order for the typed domain name on a right to left
protocol identifier, tail, and the corresponding delimiters are system is fed.cba. Does this change if a protocol identifier, tail,
specified? and the corresponding delimiters are specified?
While logic, precedent, and reality suggest that these are questions While logic, precedent, and reality suggest that these are questions
for user interface design, not IETF protocol specifications, for user interface design, not IETF protocol specifications,
experience in the 1980s and 1990s with mixing systems in which domain 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 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 which those labels were read right to left would predict a great deal
of confusion, and heuristics that sometimes fail, if each of confusion, and heuristics that sometimes fail, if each
implementation of each application makes its own decisions on these implementation of each application makes its own decisions on these
issues. issues.
It should be obvious that any revision of IDNA, including the current Any version of IDNA, including the current one, must be written in
one, must be clear about the network (transmission on the wire) order terms of the network (transmission on the wire) order of characters
of characters in labels and for the labels in complete (fully- in labels and for the labels in complete (fully-qualified) domain
qualified) domain names. In order to prevent user confusion and, in names and must be quite precise about those relationships. While
particular, to reduce the chances for inconsistent transcription of some strong suggestions about display order would be desirable to
domain names from printed form, it is likely that some strong reduce the chances for inconsistent transcription of domain names
suggestions should be made about display order as well. from printed form, such suggestions are beyond the scope of these
specifications.
6.2. Entry and Display in Applications 4.2. Entry and Display in Applications
Applications can accept domain names using any character set or sets Applications can accept domain names using any character set or sets
desired by the application developer or specified by the operating desired by the application developer, specified by the operating
system, and can display domain names in any charset. That is, the system, or dictated by other constraints, and can display domain
names in any character set or character coding system. That is, the
IDNA protocol does not affect the interface between users and IDNA protocol does not affect the interface between users and
applications. applications.
An IDNA-aware application can accept and display internationalized An IDNA-aware application can accept and display internationalized
domain names in two formats: the internationalized character set(s) domain names in two formats: the internationalized character set(s)
supported by the application (i.e., an appropriate local supported by the application (i.e., an appropriate local
representation of a U-label), and as an A-label. Applications MAY 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 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 as an interface for special purposes, possibly for debugging, or to
cope with display limitations. In general, they SHOULD allow, but cope with display limitations. In general, they SHOULD allow, but
not encourage, user input of that label form. A-labels are opaque 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 ugly and malicious variations on them are not easily detected by
and in contexts in which they are absolutely needed. Because IDN users. Where possible, they should thus only be exposed to users and
labels can be rendered either as A-labels or U-labels, the in contexts in which they are absolutely needed. Because IDN labels
application may reasonably have an option for the user to select the can be rendered either as A-labels or U-labels, the application may
preferred method of display; if it does, rendering the U-label should reasonably have an option for the user to select the preferred method
normally be the default. 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 Domain names are often stored and transported in many places. For
example, they are part of documents such as mail messages and web example, they are part of documents such as mail messages and web
pages. They are transported in many parts of many protocols, such as 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 both the control commands of SMTP and associated the message body
the headers and the body content in HTTP. It is important to parts, and in the headers and the body content in HTTP. It is
remember that domain names appear both in domain name slots and in important to remember that domain names appear both in domain name
the content that is passed over protocols. slots and in the content that is passed over protocols.
In protocols and document formats that define how to handle In protocols and document formats that define how to handle
specification or negotiation of charsets, labels can be encoded in specification or negotiation of charsets, labels can be encoded in
any charset allowed by the protocol or document format. If a any charset allowed by the protocol or document format. If a
protocol or document format only allows one charset, the labels MUST protocol or document format only allows one charset, the labels MUST
be given in that charset. Of course, not all charsets can properly be given in that charset. Of course, not all charsets can properly
represent all labels. If a U-label cannot be displayed in its represent all labels. If a U-label cannot be displayed in its
entirety, the only choice (without loss of information) may be to entirety, the only choice (without loss of information) may be to
display the A-label. display the A-label.
skipping to change at page 23, line 17 skipping to change at page 16, line 19
transmitted using whatever character encoding and escape mechanism transmitted using whatever character encoding and escape mechanism
the protocol or document format uses at that place. This provision the protocol or document format uses at that place. This provision
is intended to prevent situations in which, e.g., UTF-8 domain names is intended to prevent situations in which, e.g., UTF-8 domain names
appear embedded in text that is otherwise in some other character appear embedded in text that is otherwise in some other character
coding. coding.
All protocols that use domain name slots already have the capacity All protocols that use domain name slots already have the capacity
for handling domain names in the ASCII charset. Thus, A-labels can for handling domain names in the ASCII charset. Thus, A-labels can
inherently be handled by those protocols. inherently be handled by those protocols.
6.3. Linguistic Expectations: Ligatures, Digraphs, and Alternate 4.3. Linguistic Expectations: Ligatures, Digraphs, and Alternate
Character Forms Character Forms
[[anchor14: There is some internal redundancy and repetition in the
material in this section. Specific suggestions about to reduce or
eliminate redundant text for -05 would be appreciated.]]
Users often have expectations about character matching or equivalence Users often have expectations about character matching or equivalence
that are based on their languages and the orthography of those that are based on their own languages and the orthography of those
languages. These expectations may not be consistent with forms or languages. These expectations may not be consistent with forms or
actions that can be naturally accommodated in a character coding actions that can be naturally accommodated in a character coding
system, especially if multiple languages are written using the same system, especially if multiple languages are written using the same
script but using different conventions. A Norwegian user might script but using different conventions. A Norwegian user might
expect a label with the ae-ligature to be treated as the same label 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 as one using the Swedish spelling with a-diaeresis even though
that mapping to English would be astonishing to users. A user in applying that mapping to English would be astonishing to users. A
German might expect a label with an o-umlaut and a label that had user in German might expect a label with an o-umlaut and a label that
"oe" substituted, but was otherwise the same, treated as equivalent had "oe" substituted, but was otherwise the same, treated as
even though that substitution would be a clear error in Swedish. A equivalent even though that substitution would be a clear error in
Chinese user might expect automatic matching of Simplified and Swedish. A Chinese user might expect automatic matching of
Traditional Chinese characters, but applying that matching for Korean Simplified and Traditional Chinese characters, but applying that
or Japanese text would create considerable confusion. For that matching for Korean or Japanese text would create considerable
matter, an English user might expect "theater" and "theatre" to confusion. For that matter, an English user might expect "theater"
match. and "theatre" to match.
Related issues arise because there are a number of languages written Related issues arise because there are a number of languages written
with alphabetic scripts in which single phonemes are written using with alphabetic scripts in which single phonemes are written using
two characters, termed a "digraph", for example, the "ph" in two characters, termed a "digraph", for example, the "ph" in
"pharmacy" and "telephone". (Note that characters paired in this "pharmacy" and "telephone". (Note that characters paired in this
manner can also appear consecutively without forming a digraph, as in manner can also appear consecutively without forming a digraph, as in
"tophat".) Certain digraphs are normally indicated typographically "tophat".) Certain digraphs are normally indicated typographically
by setting the two characters closer together than they would be if by setting the two characters closer together than they would be if
used consecutively to represent different phonemes. Some digraphs used consecutively to represent different phonemes. Some digraphs
are fully joined as ligatures (strictly designating setting totally are fully joined as ligatures (strictly designating setting totally
without intervening white space, although the term is sometimes without intervening white space, although the term is sometimes
applied to close set pairs). An example of this may be seen when the 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 word "encyclopaedia" is set with a U+00E6 LATIN SMALL LIGATURE AE
(and some would not consider that word correctly spelled unless the (and some would not consider that word correctly spelled unless the
ligature form was used or the "a" was dropped entirely). When these ligature form was used or the "a" was dropped entirely). When these
ligature and digraph forms have the same interpretation across all ligature and digraph forms have the same interpretation across all
languages that use a given script, application of Unicode languages that use a given script, application of Unicode
normalization generally resolves the differences and causes them to normalization generally resolves the differences and causes them to
match. When they have different interpretations, any requirements match. When they have different interpretations, any requirements
for matching must utilize other methods or users must be educated to for matching must utilize other methods, presumably at the registry
understand that matching will not occur. level, or users must be educated to understand that matching will not
occur.
Difficulties arise from the fact that a given ligature may be a Difficulties arise from the fact that a given ligature may be a
completely optional typographic convenience for representing a completely optional typographic convenience for representing a
digraph in one language (as in the above example with some spelling digraph in one language (as in the above example with some spelling
conventions), while in another language it is a single character that conventions), while in another language it is a single character that
may not always be correctly representable by a two-letter sequence may not always be correctly representable by a two-letter sequence
(as in the above example with different spelling conventions). This (as in the above example with different spelling conventions). This
can be illustrated by many words in the Norwegian language, where the can be illustrated by many words in the Norwegian language, where the
"ae" ligature is the 27th letter of a 29-letter extended Latin "ae" ligature is the 27th letter of a 29-letter extended Latin
alphabet. It is equivalent to the 28th letter of the Swedish alphabet. It is equivalent to the 28th letter of the Swedish
skipping to change at page 24, line 31 skipping to change at page 17, line 39
current orthographic standards. current orthographic standards.
That character (U+00E4) is also part of the German alphabet where, That character (U+00E4) is also part of the German alphabet where,
unlike in the Nordic languages, the two-character sequence "ae" is unlike in the Nordic languages, the two-character sequence "ae" is
usually treated as a fully acceptable alternate orthography for the usually treated as a fully acceptable alternate orthography for the
"umlauted a" character. The inverse is however not true, and those "umlauted a" character. The inverse is however not true, and those
two characters cannot necessarily be combined into an "umlauted a". two characters cannot necessarily be combined into an "umlauted a".
This also applies to another German character, the "umlauted o" This also applies to another German character, the "umlauted o"
(U+00F6 LATIN SMALL LETTER O WITH DIAERESIS) which, for example, (U+00F6 LATIN SMALL LETTER O WITH DIAERESIS) which, for example,
cannot be used for writing the name of the author "Goethe". It is 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", also a letter in the Swedish alphabet where, like the "a with
it cannot be correctly represented as "oe" and in the Norwegian diaeresis", it cannot be correctly represented as "oe" and in the
alphabet, where it is represented, not as "umlauted o", but as Norwegian alphabet, where it is represented, not as "o with
"slashed o", U+00F8. diaeresis", but as "slashed o", U+00F8.
Some of the ligatures that have explicit code points in Unicode were Some of the ligatures that have explicit code points in Unicode were
given special handling in IDNA2003 and now pose additional problems given special handling in IDNA2003 and now pose additional problems
as people argue that they should have been treated differently to in transition. See Section 7.2.
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.
Additional cases with alphabets written right to left are described Additional cases with alphabets written right to left are described
in Section 6.5. in Section 4.5.
Whether ligatures and digraphs are to be treated as a sequence of Whether ligatures and digraphs are to be treated as a sequence of
characters or as a single standalone one constitute a problem that characters or as a single standalone one constitute a problem that
cannot be resolved solely by operating on scripts. They are, cannot be resolved solely by operating on scripts. They are,
however, a key concern in the IDN context. Their satisfactory however, a key concern in the IDN context. Their satisfactory
resolution will require support in policies set by registries, which resolution will require support in policies set by registries, which
therefore need to be particularly mindful not just of this specific therefore need to be particularly mindful not just of this specific
issue, but of all other related matters that cannot be dealt with on issue, but of all other related matters that cannot be dealt with on
an exclusively algorithmic basis. an exclusively algorithmic and global basis.
Just as with the examples of different-looking characters that may be 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 assumed to be the same, it is in general impossible to deal with
these situations in a system such as IDNA -- or with Unicode these situations in a system such as IDNA -- or with Unicode
normalization generally -- since determining what to do requires normalization generally -- since determining what to do requires
information about the language being used, context, or both. information about the language being used, context, or both.
Consequently, these specifications make no attempt to treat these Consequently, these specifications make no attempt to treat these
combined characters in any special way. However, their existence combined characters in any special way. However, their existence
provides a prime example of a situation in which a registry that is 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, aware of the language context in which labels are to be registered,
and where that language sometimes (or always) treats the two- and where that language sometimes (or always) treats the two-
character sequences as equivalent to the combined form, should give character sequences as equivalent to the combined form, should give
serious consideration to applying a "variant" model [RFC3743] serious consideration to applying a "variant" model [RFC3743]
[RFC4290] to reduce the opportunities for user confusion and fraud [RFC4290], or to prohibiting registration of one the forms entirely,
that would result from the related strings being registered to to reduce the opportunities for user confusion and fraud that would
different parties. result from the related strings being registered to different
parties.
6.4. Case Mapping and Related Issues 4.4. Case Mapping and Related Issues
Traditionally in the DNS, ASCII letters have been stored with their In the DNS, ASCII letters are stored with their case preserved.
case preserved. Matching during the query process has been case- Matching during the query process is case-independent, but none of
independent, but none of the information that might be represented by the information that might be represented by choices of case has been
choices of case has been lost. That model has been accidentally lost. That model has been accidentally helpful because, as people
helpful because, as people have created DNS labels by catenating have created DNS labels by catenating words (or parts of words) to
words (or parts of words) to form labels, case has often been used to form labels, case has often been used to distinguish among components
distinguish among components and make the labels more memorable. and make the labels more memorable.
The solution of keeping the characters separate but doing matching The solution of keeping the characters separate but doing matching
independent of case is not feasible with an IDNA-like model because independent of case is not feasible with IDNA or any IDNA-like model
the matching would then have to be done on the server rather than because the matching would then have to be done on the server rather
have characters mapped on the client. That situation was recognized than have characters mapped on the client. That situation was
in IDNA2003 and nothing in IDNA2008 fundamentally changes it or could recognized in IDNA2003 and nothing in IDNA2008 fundamentally changes
do so. In IDNA2003, all upper-case characters are mapped to lower- it or could do so. In IDNA2003, all characters are case-folded and
case ones and, in general, all code points that represent alternate mapped. That results in upper-case characters being mapped to lower-
forms of the same character are mapped to that character (including case ones and in some other transformations of alternate forms of
mapping Greek final form sigma to the medial form). IDNA2008 characters, especially those that do not have (or did not have)
permits, at the risk of some incompatibility, slightly more upper-case forms. For example, Greek Final Form Sigma (U+03C2) is
flexibility in this area. That additional flexibility still does not mapped to the medial form (U+03C3) and Eszett (German Sharp S,
solve the problem with final form sigma and other characters that U+00DF) is mapped to "ss". Neither of these mappings is reversible
Unicode treats as completely separate characters that match only because the upper case of U+03C3 is the Upper Case Sigma (U+03A3) and
under casemapping if at all. Many people now believe these should be "ss" is an ASCII string. IDNA2008 permits, at the risk of some
handled as separate characters so information about them can be incompatibility, slightly more flexibility in this area by avoid case
preserved in the transformations to A-labels and back. However folding and treating these characters as themselves. Approaches to
making a change to permit that behavior would create a situation in handling the incompatibility are discussed in Section 7.2. Although
which the same string, valid in both protocols, would be interpreted information is lost in IDNA2003's ToASCII operation so that, in some
differently by IDNA2003 and IDNA2008. In principle, that would sense, Final Sigma Eszett cannot be represented in an IDN at all, its
violate one of the conditions discussed in Section 9.2.3.1 and hence guarantee of mapping when those characters are used as input can be
require a prefix change. Of course, if a prefix change were made (at interpreted as violating one of the conditions discussed in
the costs discussed in Section 9.2.3.3) there would be several Section 7.4.1 and hence requiring a prefix change. The consensus was
options, including, if desired, assigning the characer to the to not make a prefix change in spite of this issue. Of course, had a
CONTEXTUAL RULE REQUIRED category and requiring that it only be used prefix change been made (at the costs discussed in Section 7.4.3)
in carefully-selected contexts. there would have been several options, including, if desired,
assignment of the character to the CONTEXTUAL RULE REQUIRED category
and requiring that it only be used in carefully-selected contexts.
6.5. Right to Left Text 4.5. Right to Left Text
In order to be sure that the directionality of right to left text is 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 unambiguous, IDNA2003 required that any label in which right to left
characters appear both starts and ends with them, may not include any characters appear both starts and ends with them, not include any
characters with strong left to right properties (which excludes other characters with strong left to right properties (which excludes other
alphabetic characters but permits European digits), and rejects any alphabetic characters but permits European digits), and rejects any
other string that contains a right to left character. This is one of 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 the few places where the IDNA algorithms (both in IDNA2003 and in
required to look at an entire label, not just at individual IDAN2008) are required to examine an entire label, not just
characters. The algorithmic model used in IDNA2003 rejects the label individual characters. The algorithmic model used in IDNA2003
when the final character in a right to left string requires a rejects the label when the final character in a right to left string
combining mark in order to be correctly represented. requires a combining mark in order to be correctly represented.
This problem manifests itself in languages written with consonantal That prohibition is not acceptable for writing systems for languages
alphabets to which diacritical vocalic systems are applied, and in written with consonantal alphabets to which diacritical vocalic
languages with orthographies derived from them where the combining systems are applied, and for languages with orthographies derived
marks may have different functionality. In both cases the combining from them where the combining marks may have different functionality.
marks can be essential components of the orthography. Examples of In both cases the combining marks can be essential components of the
this are Yiddish, written with an extended Hebrew script, and Dhivehi orthography. Examples of this are Yiddish, written with an extended
(the official language of Maldives) which is written in the Thaana Hebrew script, and Dhivehi (the official language of Maldives) which
script (which is, in turn, derived from the Arabic script). The new is written in the Thaana script (which is, in turn, derived from the
rules for right to left scripts are described in [IDNA2008-Bidi]. Arabic script). IDNA2008 removes the restriction on final combining
characters with a new set of rules for right to left scripts and
their characters. Those new rules are specified in [IDNA2008-Bidi].
7. IDNs and the Robustness Principle 5. IDNs and the Robustness Principle
The model of IDNs described in this document can be seen as a The model of IDNs described in this document can be seen as a
particular instance of the "Robustness Principle" that has been so particular instance of the "Robustness Principle" that has been so
important to other aspects of Internet protocol design. This important to other aspects of Internet protocol design. This
principle is often stated as "Be conservative about what you send and 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 liberal in what you accept" (See, e.g., Section 1.2.2 of the
applications-layer Host Requirements specification [RFC1123]). For
[RFC1123]). For IDNs to work well, not only must the protocol be IDNs to work well, not only must the protocol be carefully designed
carefully designed and implemented, but zone administrators and implemented, but zone administrators (registries) must have and
(registries) must have and require sensible policies about what is require sensible policies about what is registered -- conservative
registered -- conservative policies -- and implement and enforce policies -- and implement and enforce them.
them.
Conversely, lookup applications can (and SHOULD or maybe MUST) reject Conversely, lookup applications are expected to reject labels that
labels that clearly violate global (protocol) rules (no one has ever clearly violate global (protocol) rules (no one has ever seriously
seriously claimed that being liberal in what is accepted requires claimed that being liberal in what is accepted requires being
being stupid). However, once one gets past such global rules and stupid). However, once one gets past such global rules and deals
deals with anything sensitive to script or locale, it is necessary to 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 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 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 than guessing about whether it should have been permitted to be
registered. registered.
As mentioned elsewhere, if a string cannot be successfully found in As mentioned elsewhere, if a string cannot be successfully found in
the DNS after the lookup processing described here, it makes no the DNS after the lookup processing described here, it makes no
difference whether it simply wasn't registered or was prohibited by difference whether it simply wasn't registered or was prohibited by
some rule. some rule at the registry. Applications should, however, be
sensitive to the fact that, because of the possibility of DNS
wildcards, the ability to successfully resolve a name does not
guarantee that it was actually registered.
If lookup applications, as a user interface (UI) or other local If lookup applications, as a user interface (UI) or other local
matter, decide to warn about some strings that are valid under the matter, decide to warn about some strings that are valid under the
global rules but that they perceive as dangerous, that is their global rules but that they perceive as dangerous, that is their
prerogative and we can only hope that the market (and maybe prerogative and we can only hope that the market (and maybe
regulators) will reinforce the good choices and discourage the poor regulators) will reinforce the good choices and discourage the poor
ones. In this context, a lookup application that decides a string ones. In this context, a lookup application that decides a string
that is valid under the protocol is dangerous and refuses to look it 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 up is in violation of the protocols; one that is willing to look
something up, but warns against it, is exercising a local choice. something up, but warns against it, is exercising a local choice.
8. Front-end and User Interface Processing 6. Front-end and User Interface Processing
Domain names may be identified and processed in many contexts. They 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 may be typed in by users either by themselves or embedded in an
IRIs. They may occur in running text or be processed by one system identifier structured for a particular protocol or class of protocols
after being provided in another. Systems may wish to try to such a email addresses, URIs, or IRIs. They may occur in running
normalize URLs so as to determine (or guess) whether a reference is text or be processed by one system after being provided in another.
valid or two references point to the same object without actually Systems may wish to try to normalize URLs so as to determine (or
looking the objects up and comparing them (that is necessary, not guess) whether a reference is valid or two references point to the
just a choice, for URI types that are not intended to be resolved). same object without actually looking the objects up and comparing
Some of these goals may be more easily and reliably satisfied than them (that is necessary, not just a choice, for URI types that are
others. While there are strong arguments for any domain name that is not intended to be resolved). Some of these goals may be more easily
placed "on the wire" -- transmitted between systems -- to be in the and reliably satisfied than others. While there are strong arguments
minimum-ambiguity forms of A-labels, U-labels, or LDH-labels, it is for any domain name that is placed "on the wire" -- transmitted
inevitable that programs that process domain names will encounter between systems -- to be in the minimum-ambiguity forms of A-labels,
variant forms. U-labels, or LDH-labels, it is inevitable that programs that process
domain names will encounter variant forms.
One source of such forms will be labels created under IDNA2003 One source of such forms will be labels created under IDNA2003
because that protocol allowed labels that were transformed before because that protocol allowed labels that were transformed before
they were turned from native-character into ACE ("xn--...") format. they were turned from native-character into ACE ("xn--...") format by
One consequence of the transformations was that, when the ToUnicode mapping some characters into other. One consequence of the
and ToASCII operations of IDNA2003 were applied, transformations was that, when the ToUnicode and ToASCII operations
ToUnicode(ToASCII(original-label)) often did not produce the of IDNA2003 were applied, ToUnicode(ToASCII(original-label)) often
original-label. IDNA2008 explicitly defines A-labels and U-labels as did not produce the original label. IDNA2008 explicitly defines
different forms of the same abstract label, forms that are stable A-labels and U-labels as different forms of the same abstract label,
when conversions are performed between them, without mappings. A forms that are stable when conversions are performed between them,
different way of explaining this is that there are, today, domain without mappings. A different way of explaining this is that there
names in files on the Internet that use characters that cannot be are, today, domain names in files on the Internet that use characters
represented directly in, or recovered from, (A-label) domain names that cannot be represented directly in, or recovered from, (A-label)
but for which interpretations are provided by IDNA2003. There are domain names but for which interpretations are provided by IDNA2003.
two major categories of such characters, those that are removed by There are two major categories of such characters, those that are
NFKC normalization and those upper-case characters that are mapped to removed by NFKC normalization and those upper-case characters that
lower-case (there are also a few characters that are given special- are mapped to lower-case (there are also a few characters that are
case mapping treatment in Stringprep). given special-case mapping treatment in Stringprep including lower-
case characters that are case-folded into other lower-case characters
or strings).
Other issues in domain name identification and processing arise Other issues in domain name identification and processing arise
because IDNA2003 specified that several other characters be treated because IDNA2003 specified that several other characters be treated
as equivalent to the ASCII period (dot, full stop) character used as 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 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 in an arbitrary context (such as running text), it is difficult, even
with only ASCII characters, to know whether an actual domain name (or 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 a protocol parameter like a URI) is present and where it starts and
ends. When using Unicode, this gets even more difficult if treatment ends. When using Unicode, this gets even more difficult if treatment
of certain special characters (like the dot that separates labels in of certain special characters (like the dot that separates labels in
a domain name) depends on context (e.g., prior knowledge of whether a domain name) depends on context (e.g., prior knowledge of whether
the string represents a domain name or not). That knowledge is not the string represents a domain name or not). That knowledge is not
available if the primary heuristic for identifying the presence of available if the primary heuristic for identifying the presence of
domain names in strings depends on the presence of dots separating domain names in strings depends on the presence of dots separating
groups of characters with no intervening spaces. groups of characters with no intervening spaces.
[[anchor45: Above text is a substitute for an earlier (pre -01) [[anchor16: Above text is a substitute for an earlier (pre -01)
version and is hoped to be more clear. Comments and improvements version and is hoped to be more clear. Comments and improvements
welcome.]] welcome.]]
As discussed elsewhere in this document, the IDNA2008 model removes As discussed elsewhere in this document, the IDNA2008 model removes
all of these mappings and interpretations, including the equivalence all of these mappings and interpretations, including the equivalence
of different forms of dots, from the protocol, discouraging such of different forms of dots, from the protocol, discouraging such
mappings and leaving them, when necessary, to local processing. This mappings and leaving them, when necessary, to local processing. This
should not be taken to imply that local processing is optional or can should not be taken to imply that local processing is optional or can
be avoided entirely. Instead, unless the program context is such be avoided entirely, even if doing so might have been desirable in a
that it is known that any IDNs that appear will be either U-labels or world without IDNA2003 IDNs in files and archives. Instead, unless
A-labels, or that other forms can safely be rejected, some local the program context is such that it is known that any IDNs that
processing of apparent domain name strings will be required, both to appear will be either U-labels or A-labels, or that other forms can
maintain compatibility with IDNA2003 and to prevent user safely be rejected, some local processing of apparent domain name
astonishment. Such local processing, while not specified in this strings will be required, both to maintain compatibility with
document or the associated ones, will generally take one of two IDNA2003 and to prevent user astonishment. Such local processing,
forms: while not specified in this document or the associated ones, will
generally take one of two forms:
o Generic Preprocessing. o Generic Preprocessing.
When the context in which the program or system that processes When the context in which the program or system that processes
domain names operates is global, a reasonable balance must be domain names operates is global, a reasonable balance must be
found that is sensitive to the broad range of local needs and found that is sensitive to the broad range of local needs and
assumptions while, at the same time, not sacrificing the needs of assumptions while, at the same time, not sacrificing the needs of
one language, script, or user population to those of another. one language, script, or user population to those of another.
For this case, the best practice will usually be to apply NFKC and For this case, the best practice will usually be to apply NFKC and
case-mapping (or, perhaps better yet, Stringprep itself), plus case-mapping (or, perhaps better yet, Stringprep itself), plus
skipping to change at page 29, line 49 skipping to change at page 23, line 10
utterly refuse to handle "strange" characters at all if they utterly refuse to handle "strange" characters at all if they
appear in U-label form. None of those local decisions are a appear in U-label form. None of those local decisions are a
threat to interoperability as long as (i) only U-labels and threat to interoperability as long as (i) only U-labels and
A-labels are used in interchange with systems outside the local A-labels are used in interchange with systems outside the local
environment, (ii) no character that would be valid in a U-label as environment, (ii) no character that would be valid in a U-label as
itself is mapped to something else, (iii) any local mappings are itself is mapped to something else, (iii) any local mappings are
applied as a preprocessing step (or, for conversions from U-labels applied as a preprocessing step (or, for conversions from U-labels
or A-labels to presentation forms, postprocessing), not as part of or A-labels to presentation forms, postprocessing), not as part of
IDNA processing proper, and (iv) appropriate consideration is IDNA processing proper, and (iv) appropriate consideration is
given to labels that might have entered the environment in given to labels that might have entered the environment in
conformance to IDNA2003. [[anchor46: Placeholder: there have been conformance to IDNA2003.
suggestions that this text be removed entirely. Comments (or
improved text) welcome.]]
In either case, it is vital that user interface designs and, where In either case, it is vital that user interface designs and, where
the interfaces are not sufficient, users, be aware that the only the interfaces are not sufficient, users, be aware that the only
forms of domain names that this protocol anticipates will resolve forms of domain names that this protocol anticipates will resolve
globally or compare equal when crude methods (i.e., those not 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 conforming to the strict definition of label equivalence given in
native-script labels are in U-label form. Forms that assume mapping [IDNA2008-Defs]) are used are those in which all native-script labels
will occur, especially forms that were not valid under IDNA2003, may are in U-label form. Forms that assume mapping will occur,
or may not function in predictable ways across all implementations. 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.
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 7. Migration from IDNA2003 and Unicode Version Synchronization
9.2.1. Design Criteria 7.1. Design Criteria
As mentioned above and in RFC 4690, two key goals of this work are to As mentioned above and in RFC 4690, two key goals of the IDNA2008
enable applications to be agnostic about whether they are being run design are to enable applications to be agnostic about whether they
in environments supporting any Unicode version from 3.2 onward and to are being run in environments supporting any Unicode version from 3.2
permit incrementally adding permitted scripts and other character onward and to permit incrementally adding new characters, character
collections without disruption or, subsequent to this version, groups, scripts, and other character collections as they are
"heavy" processes such as formation of an IETF WG. The mechanisms incorporated into Unicode, without disruption and, in the long term,
that support this are outlined above, but this section reviews them without "heavy" processes such as those involving IETF consensus.
in a context that may be more helpful to those who need to understand (An IETF consensus process is required by the IDNA2008 specifications
the approach and make plans for it. and is expected to be required and used until significant experience
accumulates with IDNA operations and new versions of Unicode.) The
mechanisms that support this are outlined above and elsewhere in the
IDNA2008 document set, 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 7.1.1. General IDNA Validity Criteria
The general criteria for a putative label, and the collection of The general criteria for a putative label, and the collection of
characters that make it up, to be considered IDNA-valid are: characters that make it up, to be considered IDNA-valid are (the
actual rules are rigorously defined in the "Protocol" and "Tables"
documents):
o The characters are "letters", marks needed to form letters, o The characters are "letters", marks needed to form letters,
numerals, or other code points used to write words in some numerals, or other code points used to write words in some
language. Symbols, drawing characters, and various notational language. Symbols, drawing characters, and various notational
characters are permanently excluded -- some because they are characters are permanently excluded -- some because they are
actively dangerous in URI, IRI, or similar contexts and others actively dangerous in URI, IRI, or similar contexts and others
because there is no evidence that they are important enough to because there is no evidence that they are important enough to
Internet operations or internationalization to justify inclusion Internet operations or internationalization to justify expansion
and the complexities that would come with it (additional of domain names beyond the general principle of "letters, digits,
discussion and rationale for the symbol decision appears in and hyphen" and the complexities that would come with it
Section 9.2.5). (additional discussion and rationale for the symbol decision
appears in Section 7.6).
o Other than in very exceptional cases, e.g., where they are needed o Other than in very exceptional cases, e.g., where they are needed
to write substantially any word of a given language, punctuation to write substantially any word of a given language, punctuation
characters are excluded as well. The fact that a word exists is 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 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 are not expected to be usable for multiple-word phrases (although
they are certainly not prohibited if the conventions and they are certainly not prohibited if the conventions and
orthography of a particular language cause that to be possible). orthography of a particular language cause that to be possible).
Even for English, very common constructions -- contractions like Even for English, very common constructions -- contractions like
"don't" or "it's", names that are written with apostrophes such as "don't" or "it's", names that are written with apostrophes such as
"O'Reilly" or characters for which apostrophes are common "O'Reilly", or characters for which apostrophes are common
substitutes, and words whose usually-preferred spellings retain substitutes cannot be represented in DNS labels. Words in English
diacritical marks from earlier forms -- cannot be represented in whose usually-preferred spellings include diacritical marks cannot
DNS labels. be represented under the original hostname rules, but most can be
represented if treated as IDNs.
o Characters that are unassigned (have no character assignment at o Characters that are unassigned (have no character assignment at
all) in the version of Unicode being used by the registry or all) in the version of Unicode being used by the registry or
application are not permitted, even on lookup. There are at least application are not permitted, even on lookup. There are at least
two reasons for this. Tests involving the context of characters two reasons for this.
(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 * Tests involving the context of characters (e.g., some
or by a current version of NFKC is prohibited as input to IDNA characters being permitted only adjacent to ones of specific
(for either registration or lookup). Implementers of user types but otherwise invisible or very problematic for other
interfaces to applications are free to make those conversions when reasons) and integrity tests on complete labels are needed.
they consider them suitable for their operating system Unassigned code points cannot be permitted because one cannot
environments, context, or users. 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.
* 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 a current
version of NFKC is prohibited as input to IDNA (for either
registration or lookup). With a few exceptions, this principle
excludes any character mapped to another by Nameprep [RFC3491].
Tables used to identify the characters that are IDNA-valid are Tables used to identify the characters that are IDNA-valid are
expected to be driven by the principles above (described in more expected to be driven by the principles above, principles that are
precise form in [IDNA2008-Tables]). The principles are not just an specified exactly in [IDNA2008-Tables]). The rules given there are
interpretation of the tables. normative, rather than being just an interpretation of the tables.
9.2.1.2. Labels in Registration 7.1.2. Labels in Registration
Anyone entering a label into a DNS zone must properly validate that 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 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 order for applications to work as intended. This principle is not
new: for example, zone administrators are expected to verify that new. For example, since the DNS was first deployed, zone
names meet "hostname" [RFC0952] or special service location formats administrators have been expected to verify that names meet
[RFC2782] where necessary for the expected applications. For zones "hostname" [RFC0952] where necessary for the expected applications.
that will contain IDNs, support for Unicode version-independence Later addition of special service location formats [RFC2782] imposed
requires restrictions on all strings placed in the zone. In new requirements on zone administrators for the use of labels that
particular, for such zones: conform to the requirements of those formats. 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 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 starts in "xn--", MUST be IDNA-valid, i.e., they MUST be valid
valid A-labels, as discussed in Section 2 above. A-labels, as discussed in Section 2 above.
o The Unicode tables (i.e., tables of code points, character o The Unicode tables (i.e., tables of code points, character
classes, and properties) and IDNA tables (i.e., tables of classes, and properties) and IDNA tables (i.e., tables of
contextual rules such as those described above), MUST be contextual rules such as those that appear in the Tables
consistent on the systems performing or validating labels to be document), MUST be consistent on the systems performing or
registered. Note that this does not require that tables reflect validating labels to be registered. Note that this does not
the latest version of Unicode, only that all tables used on a require that tables reflect the latest version of Unicode, only
given system are consistent with each other. 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 Under this model, a registry (or entity communicating with a registry
to accomplish name registrations) will need to update its tables -- to accomplish name registrations) will need to update its tables --
both the Unicode-associated tables and the tables of permitted IDN both the Unicode-associated tables and the tables of permitted IDN
characters -- to enable a new script or other set of new characters. characters -- to enable a new script or other set of new characters.
It will not be affected by newer versions of Unicode, or newly- It will not be affected by newer versions of Unicode, or newly-
authorized characters, until and unless it wishes to make those authorized characters, until and unless it wishes to make those
registrations. The registration side is also responsible --under the registrations. The zone administrator is also responsible -- under
protocol and to registrants and users-- for much more careful the protocol and to registrants and users -- for both checking as
checking than is expected of applications systems that look names up, required by the protocol and verification that whatever policies it
both checking as required by the protocol and checking required by develops are complied with, whether those policies are for minimizing
whatever policies it develops for minimizing risks due to confusable risks due to confusable characters and sequences, for preserving
characters and sequences and preserving language or script integrity. language or script integrity, or for other purposes. Those checking
and verification procedures are more extensive than those that are is
expected of applications systems that look names up.
Systems looking up or resolving DNS labels, especially IDN DNS Systems looking up or resolving DNS labels, especially IDN DNS
labels, MUST be able to assume that applicable registration rules labels, MUST be able to assume that applicable registration rules
were followed for names entered into the DNS. were followed for names entered into the DNS.
9.2.1.3. Labels in Lookup 7.1.3. 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 Anyone looking up a label in a DNS zone is required to
o MUST maintain a consistent set of tables, as discussed above. As o Maintain a consistent set of tables, as discussed above. As with
with registration, the tables need not reflect the latest version registration, the tables need not reflect the latest version of
of Unicode but they MUST be consistent. Unicode but they must be consistent.
o MUST validate the characters in labels to be looked up only to the o Validate the characters in labels to be looked up only to the
extent of determining that the U-label does not contain either extent of determining that the U-label does not contain either
code points prohibited by IDNA (categorized as "DISALLOWED") or code points prohibited by IDNA (categorized as "DISALLOWED") or
code points that are unassigned in its version of Unicode. code points that are unassigned in its version of Unicode.
o MUST validate the label itself for conformance with a small number o Validate the label itself for conformance with a small number of
of whole-label rules, notably verifying that there are no leading whole-label rules, notably verifying that there are no leading
combining marks, that the "bidi" conditions are met if right to combining marks, that the "bidi" conditions are met if right to
left characters appear, that any required contextual rules are left characters appear, that any required contextual rules are
available and that, if such rules are associated with Joiner available and that, if such rules are associated with Joiner
Controls, they are tested. Controls, they are tested.
o MUST NOT validate other contextual rules about characters, o Avoid validating other contextual rules about characters,
including mixed-script label prohibitions, although such rules MAY including mixed-script label prohibitions, although such rules may
be used to influence presentation decisions in the user interface. be used to influence presentation decisions in the user interface.
By avoiding applying its own interpretation of which labels are valid By avoiding applying its own interpretation of which labels are valid
as a means of rejecting lookup attempts, the lookup application as a means of rejecting lookup attempts, the lookup application
becomes less sensitive to version incompatibilities with the becomes less sensitive to version incompatibilities with the
particular zone registry associated with the domain name. particular zone registry associated with the domain name.
An application or client that looks processes names according to this An application or client that processes names according to this
protocol and then resolves them in the DNS will be able to locate any 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 name that is validly registered, as long as its version of the
Unicode-associated tables is sufficiently up-to-date to interpret all Unicode-associated tables is sufficiently up-to-date to interpret all
of the characters in the label. It SHOULD distinguish, in its of the characters in the label. Messages to users should distinguish
messages to users, between "label contains an unallocated code point" between "label contains an unallocated code point" and other types of
and other types of lookup failures. A failure on the basis of an old lookup failures. A failure on the basis of an old version of Unicode
version of Unicode may lead the user to a desire to upgrade to a may lead the user to a desire to upgrade to a newer version, but will
newer version, but will have no other ill effects (this is consistent have no other ill effects (this is consistent with behavior in the
with behavior in the transition to the DNS when some hosts could not transition to the DNS when some hosts could not yet handle some forms
yet handle some forms of names or record types). of names or record types).
9.2.2. More Flexibility in User Agents 7.2. Changes in Character Interpretations
[[anchor19: Note in Draft: This subsection is completely new in
version -04 of this document. It could almost certainly use
improvement. It also contains some material that is redundant with
material in other sections. I have not tried to remove that material
and will not do so until the WG concludes that this section is
relatively stable, but would appreciate help in identifying what
should be removed or how this might be enhanced to contain more of
that other material. --JcK]]
In those scripts that make case distinctions, there are a few
characters for which an obvious and unique upper case character has
not historically been available to match a lower case one or vice
versa. For those characters, the mappings used in constructing the
Stringprep tables for IDNA2003, performed using the Unicode CaseFold
operation (See Section 5.8 of the Unicode Standard [Unicode51]),
generate different characters or sets of characters. Those
operations are not reversible and lose even more information than
traditional upper case or lower case transformations, but are more
useful than those transformations for comparison purposes. Two
notable characters of this type are the German character Eszett
(Sharp S, U+00DF) and the Greek Final Form Sigma (U+03C2). The
former is case-folded to the ASCII string "ss", the latter to a
medial (Lower Case) Sigma (U+03C3).
The decision to eliminate mappings, including case folding, from the
IDNA2008 protocol in order to make A-labels and U-labels idempotent
made these characters problematic. If they were to be disallowed,
important words and mnemonics could not be written in
orthographically reasonable ways. If they were to be permitted as
characters distinct from the forms produced by case folding, there
would be no information loss and registries would have maximum
flexibility, but labels using those characters that were looked up
according to IDNA2003 rules would be transformed into A-labels using
their case-mapped variations while lookup according to IDNA2008 rules
would be based on different A-labels that represented the actual
characters.
With the understanding that there would be incompatibility either way
but a judgment that the incompatibility was not significant enough to
just a prefix change, the WG concluded that Eszett and Final Form
Sigma should be treated as distinct and Protocol-Valid characters.
The decision faces registries, especially registries maintaining
zones for third parties, with a variation on what has become a
familiar problem: how to introduce a new service in a way that does
not create confusion or significantly weaken or invalidate existing
identifiers.
While it is beyond the scope of these documents to specify a
preference for any of them, or to suggest that there are not other
possibilities, there have traditionally been several approaches to
problems of this type:
o Do not permit use of the newly-available character at the registry
level. This might cause lookup failures if a domain name were
written with the expectation of the IDNA2003 mapping behavior, but
would eliminate any possibility of false matches.
o Hold a "sunrise" arrangement in which holders of the previously-
mapped labels (labels containing "ss" in the Eszett case or ones
containing Lower Case Sigma in the Final Sigma case) are given
priority (and perhaps other benefits) for registering the
corresponding string containing the newly-available characters.
o Adopt some sort of "variant" approach in which registrants either
obtained labels with both character forms or one of them was
blocked from registration by anyone but the registrant of the
other form.
In principle, lookup applications could also compensate for the
difference in interpretation by looking up the string according to
the IDNA208 interpretation and then, if that failed, doing the lookup
with the mapping, simulating the IDNA2003 interpretation. The risk
of false positives is such that this is generally to be discouraged
unless the application is able to engage in a "did you really mean"
dialogue with the end user.
7.3. More Flexibility in User Agents
These specifications do not perform mappings between one character or These specifications do not perform mappings between one character or
code point and others for any reason. Instead, they prohibits the code point and others for any reason. Instead, they prohibit the
characters that would be mapped to others by normalization, case characters that would be mapped to others by normalization, case
folding, or other rules. As examples, while mathematical characters folding (with exceptions for lower case characters that have no upper
based on Latin ones are accepted as input to IDNA2003, they are case form, which are retained), or other rules. As examples, while
prohibited in IDNA2008. Similarly, double-width characters and other mathematical characters based on Latin ones are accepted as input to
variations are prohibited as IDNA input. 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 Since the rules in [IDNA2008-Tables] have the effect that only
are stable under NFKC are valid, if it is convenient for an strings that are not transformed by NFKC are valid, if an application
application to perform NFKC normalization before lookup, that chooses to perform NFKC normalization before lookup, that operation
operation is safe since this will never make the application unable is safe since this will never make the application unable to look up
to look up any valid string. any valid string. However, as discussed above, the application
cannot guarantee that any other application will perform that
mapping, so it should be used only with caution and for informed
users.
In many cases these prohibitions should have no effect on what the In many cases these prohibitions should have no effect on what the
user can type as input to the lookup process. It is perfectly user can type as input to the lookup process. It is perfectly
reasonable for systems that support user interfaces to perform some reasonable for systems that support user interfaces to perform some
character mapping that is appropriate to the local environment. This character mapping that is appropriate to the local environment. This
would normally be done prior to actual invocation of IDNA. At least would normally be done prior to actual invocation of IDNA. At least
conceptually, the mapping would be part of the Unicode conversions conceptually, the mapping would be part of the Unicode conversions
discussed above and in [IDNA2008-Protocol]. However, those changes discussed above and in [IDNA2008-Protocol]. However, those changes
will be local ones only -- local to environments in which users will will be local ones only -- local to environments in which users will
clearly understand that the character forms are equivalent. For use clearly understand that the character forms are equivalent. For use
in interchange among systems, it appears to be much more important 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 that U-labels and A-labels can be mapped back and forth without loss
of information. of information.
One specific, and very important, instance of this strategy arises One specific, and very important, instance of this strategy arises
with case-folding. In the ASCII-only DNS, names are looked up and 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. 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 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, any mixture of them) and that form is preserved, returned in queries,
and so on. IDNA2003 simulated that behavior by performing case- and so on. IDNA2003 simulated that behavior for non-ASCII strings by
mapping at registration time (resulting in only lower-case IDNs in performing case-folding at registration time (resulting in only
the DNS) and when names were looked up. lower-case IDNs in the DNS) and when names were looked up.
As suggested earlier in this section, it appears to be desirable to As suggested earlier in this section, it appears to be desirable to
do as little character mapping as possible consistent with having do as little character mapping as possible consistent with having
Unicode work correctly (e.g., NFC mapping to resolve different Unicode work correctly (e.g., NFC mapping to resolve different
codings for the same character is still necessary although the codings for the same character is still necessary although the
specifications require that it be performed prior to invoking the specifications require that it be performed prior to invoking the
protocol) and to make the mapping between A-labels and U-labels protocol) and to make the mapping between A-labels and U-labels
idempotent. Case-mapping is not an exception to this principle. If idempotent. Case-mapping is not an exception to this principle. If
only lower case characters can be registered in the DNS (i.e., be only lower case characters can be registered in the DNS (i.e., be
present in a U-label), then IDNA2008 should prohibit upper-case present in a U-label), then IDNA2008 should prohibit upper-case
characters as input. Some other considerations reinforce this characters as input. Some other considerations reinforce this
conclusion. For example, an essential element of the ASCII case- conclusion. For example, an essential element of the ASCII case-
mapping functions is that uppercase(character) must be equal to mapping functions is that uppercase(character) must be equal to
uppercase(lowercase(character)). That requirement may not be uppercase(lowercase(character)). That requirement may not be
satisfied with IDNs. The relationship between upper case and lower satisfied with IDNs. For example, there are some characters in
scripts that use case distinction that do not have counterparts in
one case or the other. The relationship between upper case and lower
case may even be language-dependent, with different languages (or case may even be language-dependent, with different languages (or
even the same language in different areas) expecting different even the same language in different areas) expecting different
mappings. Of course, the expectations of users who are accustomed to mappings. Of course, the expectations of users who are accustomed to
a case-insensitive DNS environment will probably be well-served if a case-insensitive DNS environment will probably be well-served if
user agents perform case mapping prior to IDNA processing, but the user agents perform case folding prior to IDNA processing, but the
IDNA procedures themselves should neither require such mapping nor IDNA procedures themselves should neither require such mapping nor
expect them when they are not natural to the localized environment. expect them when they are not natural to the localized environment.
9.2.3. The Question of Prefix Changes 7.4. The Question of Prefix Changes
The conditions that would require a change in the IDNA "prefix" The conditions that would require a change in the IDNA ACE prefix
("xn--" for the version of IDNA specified in [RFC3490]) have been a ("xn--" for the version of IDNA specified in [RFC3490]) have been a
great concern to the community. A prefix change would clearly be great concern to the community. A prefix change would clearly be
necessary if the algorithms were modified in a manner that would necessary if the algorithms were modified in a manner that would
create serious ambiguities during subsequent transition in create serious ambiguities during subsequent transition in
registrations. This section summarizes our conclusions about the registrations. This section summarizes our conclusions about the
conditions under which changes in prefix would be necessary and the conditions under which changes in prefix would be necessary and the
implications of such a change. implications of such a change.
9.2.3.1. Conditions Requiring a Prefix Change 7.4.1. Conditions Requiring a Prefix Change
An IDN prefix change is needed if a given string would be looked up An IDN prefix change is needed if a given string would be looked up
or otherwise interpreted differently depending on the version of the or otherwise interpreted differently depending on the version of the
protocol or tables being used. Consequently, work to update IDNs protocol or tables being used. Consequently, work to update IDNs
would require a prefix change if, and only if, one of the following would require a prefix change if, and only if, one of the following
four conditions were met: four conditions were met:
1. The conversion of an A-label to Unicode (i.e., a U-label) yields 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 one string under IDNA2003 (RFC3490) and a different string under
IDNA2008. IDNA2008.
2. An input string that is valid under IDNA2003 and also valid under 2. An input string that is valid under IDNA2003 and also valid under
IDNA2008 yields two different A-labels with the different IDNA2008 yields two different A-labels with the different
versions of IDNA. This condition is believed to be essentially versions of IDNA. This condition is believed to be essentially
equivalent to the one above. equivalent to the one above except for a very small number of
edge cases which may not, pragmatically, justify a prefix change
(See Section 7.2).
Note, however, that if the input string is valid under one Note, however, that if the input string is valid under one
version and not valid under the other, this condition does not version and not valid under the other, this condition does not
apply. See the first item in Section 9.2.3.2, below. apply. See the first item in Section 7.4.2, below.
3. A fundamental change is made to the semantics of the string that 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 is inserted in the DNS, e.g., if a decision were made to try to
include language or specific script information in that string, include language or specific script information in that string,
rather than having it be just a string of characters. rather than having it be just a string of characters.
4. A sufficiently large number of characters is added to Unicode so 4. A sufficiently large number of characters is added to Unicode so
that the Punycode mechanism for block offsets no longer has that the Punycode mechanism for block offsets no longer has
enough capacity to reference the higher-numbered planes and enough capacity to reference the higher-numbered planes and
blocks. This condition is unlikely even in the long term and blocks. This condition is unlikely even in the long term and
certain not to arise in the next few years. certain not to arise in the next few years.
9.2.3.2. Conditions Not Requiring a Prefix Change 7.4.2. Conditions Not Requiring a Prefix Change
In particular, as a result of the principles described above, none of In particular, as a result of the principles described above, none of
the following changes require a new prefix: the following changes require a new prefix:
1. Prohibition of some characters as input to IDNA. This may make 1. Prohibition of some characters as input to IDNA. This may make
names that are now registered inaccessible, but does not require names that are now registered inaccessible, but does not require
a prefix change. a prefix change.
2. Adjustments in Stringprep tables or IDNA actions, including 2. Adjustments in IDNA tables or actions, including normalization
normalization definitions, that affect characters that were definitions, that affect characters that were already invalid
already invalid under IDNA2003. 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 3. Changes in the style of the IDNA definition that does not alter
Stringprep or Nameprep, the third condition above and part of the the actions performed by IDNA.
second are moot.
9.2.3.3. Implications of Prefix Changes 7.4.3. Implications of Prefix Changes
While it might be possible to make a prefix change, the costs of such 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 a change are considerable. Even if they wanted to do so, all
registries could not convert all IDNA2003 ("xn--") registrations to a registries could not convert all IDNA2003 ("xn--") registrations to a
new form at the same time and synchronize that change with new form at the same time and synchronize that change with
applications supporting lookup. Unless all existing registrations applications supporting lookup. Unless all existing registrations
were simply to be declared invalid, and perhaps even then, systems were simply to be declared invalid (and perhaps even then) systems
that needed to support both labels with old prefixes and labels with that needed to support both labels with old prefixes and labels with
new ones would first process a putative label under the IDNA2008 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 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 the label under IDNA2003 rules and look it up again. That
process could significantly slow down all processing that involved process could significantly slow down all processing that involved
IDNs in the DNS especially since, in principle, a fully-qualified IDNs in the DNS especially since, in principle, a fully-qualified
name could contain a mixture of labels that were registered with the 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 old and new prefixes, a situation that would make the use of DNS
caching very difficult. In addition, looking up the same input caching very difficult. In addition, looking up the same input
string as two separate A-labels would create some potential for string as two separate A-labels would create some potential for
confusion and attacks, since they could, in principle, map to confusion and attacks, since they could, in principle, map to
different targets and then resolve to different DNS label nodes. different targets and then resolve to different DNS label nodes.
Consequently, a prefix change is to be avoided if at all possible, Consequently, a prefix change is to be avoided if at all possible,
even if it means accepting some IDNA2003 decisions about character even if it means accepting some IDNA2003 decisions about character
distinctions as irreversible. distinctions as irreversible and/or giving special treatment to edge
cases.
9.2.4. Stringprep Changes and Compatibility 7.5. Stringprep Changes and Compatibility
Concerns have been expressed about problems for non-DNS uses of The Nameprep [RFC3491] specification, a key part of IDNA2003, is a
Stringprep being caused by changes to the specification intended to profile of Stringprep [RFC3454]. While Nameprep is a Stringprep
improve the handling of IDNs, most notably as this might affect profile specific to IDNA, Stringprep is used by a number of other
identification and authentication protocols. Section 9.2.3, above, protocols. Concerns have been expressed about problems for non-DNS
essentially also applies in this context. The proposed new inclusion uses of Stringprep being caused by changes to the specification
tables [IDNA2008-Tables], the reduction in the number of characters intended to improve the handling of IDNs, most notably as this might
permitted as input for registration or lookup (Section 5), and even affect identification and authentication protocols. The proposed new
the proposed changes in handling of right to left strings inclusion tables [IDNA2008-Tables], the reduction in the number of
characters permitted as input for registration or lookup (Section 3),
and even the proposed changes in handling of right to left strings
[IDNA2008-Bidi] either give interpretations to strings prohibited [IDNA2008-Bidi] either give interpretations to strings prohibited
under IDNA2003 or prohibit strings that IDNA2003 permitted. Strings under IDNA2003 or prohibit strings that IDNA2003 permitted. The
that are valid under both IDNA2003 and IDNA2008, and the IDNA2008 protocol does not use either Nameprep or Stringprep at all,
corresponding versions of Stringprep, are not changed in so there are no side-effect changes to other protocols.
interpretation. This protocol does not use either Nameprep or
Stringprep as specified in IDNA2003.
It is particularly important to keep IDNA processing separate from It is particularly important to keep IDNA processing separate from
processing for various security protocols because some of the processing for various security protocols because some of the
constraints that are necessary for smooth and comprehensible use of constraints that are necessary for smooth and comprehensible use of
IDNs may be unwanted or undesirable in other contexts. For example, IDNs may be unwanted or undesirable in other contexts. For example,
the criteria for good passwords or passphrases are very different the criteria for good passwords or passphrases are very different
from those for desirable IDNs. Similarly, internationalized SCSI from those for desirable IDNs: passwords should be hard to guess,
identifiers and other protocol components are likely to have while domain names should normally be easily memorable. Similarly,
different requirements than IDNs. 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 7.6. The Symbol Question
One of the major differences between this specification and the One of the major differences between this specification and the
original version of IDNA is that the original version permitted non- original version of IDNA is that the original version permitted non-
letter symbols of various sorts, including punctuation and line- letter symbols of various sorts, including punctuation and line-
drawing symbols, in the protocol. They were always discouraged in drawing symbols, in the protocol. They were always discouraged in
practice. In particular, both the "IESG Statement" about IDNA and practice. In particular, both the "IESG Statement" about IDNA and
all versions of the ICANN Guidelines specify that only language all versions of the ICANN Guidelines specify that only language
characters be used in labels. This specification disallows symbols characters be used in labels. This specification disallows symbols
entirely. There are several reasons for this, which include: entirely. There are several reasons for this, which include:
o As discussed elsewhere, the original IDNA specification assumed o As discussed elsewhere, the original IDNA specification assumed
that as many Unicode characters as possible should be permitted, that as many Unicode characters as possible should be permitted,
directly or via mapping to other characters, in IDNs. This directly or via mapping to other characters, in IDNs. This
specification operates on an inclusion model, extrapolating from specification operates on an inclusion model, extrapolating from
the LDH rules --which have served the Internet very well-- to a the LDH rules -- which have served the Internet very well -- to a
Unicode base rather than an ASCII base. Unicode base rather than an ASCII base.
o Most Unicode names for letters are, in most cases, fairly o Most Unicode names for letters are, in most cases, fairly
intuitive, unambiguous and recognizable to users of the relevant intuitive, unambiguous and recognizable to users of the relevant
script. Symbol names are more problematic because there may be no script. Symbol names are more problematic because there may be no
general agreement on whether a particular glyph matches a symbol; general agreement on whether a particular glyph matches a symbol;
there are no uniform conventions for naming; variations such as there are no uniform conventions for naming; variations such as
outline, solid, and shaded forms may or may not exist; and so on. 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 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 in a logo that might be read as "I love...". While the user might
read such a logo as "I love..." or "I heart...", considerable read such a logo as "I love..." or "I heart...", considerable
knowledge of the coding distinctions made in Unicode is needed to knowledge of the coding distinctions made in Unicode is needed to
know that there more than one "heart" character (e.g., U+2665, 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 U+2661, and U+2765) and how to describe it. These issues are of
particular importance if strings are expected to be understood or particular importance if strings are expected to be understood or
transcribed by the listener after being read out loud. transcribed by the listener after being read out loud.
[[anchor54: The above paragraph remains controversial as to [[anchor20: The above paragraph remains controversial as to
whether it is valid. The WG will need to make a decision if this whether it is valid. The WG will need to make a decision if this
section is not dropped entirely.]] section is not dropped entirely.]]
o As a simplified example of this, assume one wanted to use a o As a simplified example of this, assume one wanted to use a
"heart" or "star" symbol in a label. This is problematic because "heart" or "star" symbol in a label. This is problematic because
the those names are ambiguous in the Unicode system of naming (the those names are ambiguous in the Unicode system of naming (the
actual Unicode names require far more qualification). A user or actual Unicode names require far more qualification). A user or
would-be registrant has no way to know --absent careful study of would-be registrant has no way to know -- absent careful study of
the code tables-- whether it is ambiguous (e.g., where there are the code tables -- whether it is ambiguous (e.g., where there are
multiple "heart" characters) or not. Conversely, the user seeing multiple "heart" characters) or not. Conversely, the user seeing
the hypothetical label doesn't know whether to read it --try to the hypothetical label doesn't know whether to read it -- try to
transmit it to a colleague by voice-- as "heart", as "love", as transmit it to a colleague by voice -- as "heart", as "love", as
"black heart", or as any of the other examples below. "black heart", or as any of the other examples below.
o The actual situation is even worse than this. There is no o The actual situation is even worse than this. There is no
possible way for a normal, casual, user to tell the difference 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 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 and U+2729 or the without somehow knowing to look for a
distinction. We have a white heart (U+2661) and few black hearts distinction. We have a white heart (U+2661) and few black hearts.
and describing a label containing a heart symbol is hopelessly Consequently, describing a label as containing a heart hopelessly
ambiguous. In cities where "Square" is a popular part of a ambiguous: we can only know that it contains one of several
location name, one might well want to use a square symbol in a characters that look like hearts or have "heart" in their names.
label as well and there are far more squares of various flavors in In cities where "Square" is a popular part of a location name, one
Unicode than there are hearts or stars. 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 o The consequence of these ambiguities of description and
dependencies on distinctions that were, or were not, made in dependencies on distinctions that were, or were not, made in
Unicode codings, is that symbols are a very poor basis for Unicode codings is that symbols are a very poor basis for reliable
reliable communication. Consistent with this conclusion, the communication. Consistent with this conclusion, the Unicode
Unicode standard recommends that strings used in identifiers not standard recommends that strings used in identifiers not contain
contain symbols or punctuation [Unicode-UAX31]. Of course, these symbols or punctuation [Unicode-UAX31]. Of course, these
difficulties with symbols do not arise with actual pictographic difficulties with symbols do not arise with actual pictographic
languages and scripts which would be treated like any other languages and scripts which would be treated like any other
language characters; the two should not be confused. language characters; the two should not be confused.
[[anchor55: Note in Draft: Should the above section be significantly 7.7. Migration Between Unicode Versions: Unassigned Code Points
trimmed or eliminated?]]
9.2.6. Migration Between Unicode Versions: Unassigned Code Points
In IDNA2003, labels containing unassigned code points are looked up In IDNA2003, labels containing unassigned code points are looked up
on the theory that, if they appear in labels and can be mapped and on the theory that, if they appear in labels and can be mapped and
then resolved, the relevant standards must have changed and the then resolved, the relevant standards must have changed and the
registry has properly allocated only assigned values. registry has properly allocated only assigned values.
In this specification, strings containing unassigned code points MUST In IDNA2008, strings containing unassigned code points MUST NOT be
NOT be either looked up or registered. There are several reasons for either looked up or registered. There are several reasons for this,
this, with the most important ones being: with the most important ones being:
o It cannot be known with sufficient reliability in advance that a o It cannot be known with sufficient reliability in advance that a
code point that was not previously assigned will not be assigned code point that was not previously assigned will not be assigned
to a compatibility character. In IDNA2003, since there is no to a compatibility character. In IDNA2003, since there is no
direct dependency on NFKC (Stringprep's tables are based on NFKC, direct dependency on NFKC (Stringprep's tables are based on NFKC,
but IDNA2003 depends only on Stringprep), allocation of a but IDNA2003 depends only on Stringprep), allocation of a
compatibility character might produce some odd situations, but it compatibility character might produce some odd situations, but it
would not be a problem. In IDNA2008, where compatibility would not be a problem. In IDNA2008, where compatibility
characters are generally assigned to DISALLOWED, permitting characters are generally assigned to DISALLOWED, permitting
strings containing unassigned characters to be looked up would strings containing unassigned characters to be looked up would
skipping to change at page 40, line 48 skipping to change at page 34, line 42
obscure characters or archaic scripts. Unfortunately, that does not obscure characters or archaic scripts. Unfortunately, that does not
appear to be a safe assumption for at least two reasons. First, much 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 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 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 world may still be very important to those who use it. Cultural and
linguistic preservation principles make it inappropriate to declare linguistic preservation principles make it inappropriate to declare
the script of no importance in IDNs. Second, we already have the script of no importance in IDNs. Second, we already have
counterexamples in, e.g., the relationships associated with new Han counterexamples in, e.g., the relationships associated with new Han
characters being added (whether in the BMP or in Unicode Plane 2). characters being added (whether in the BMP or in Unicode Plane 2).
9.2.7. Other Compatibility Issues 7.8. Other Compatibility Issues
The existing (2003) IDNA model includes several odd artifacts of the The existing (2003) IDNA model includes several odd artifacts of the
context in which it was developed. Many, if not all, of these are context in which it was developed. Many, if not all, of these are
potential avenues for exploits, especially if the registration potential avenues for exploits, especially if the registration
process permits "source" names (names that have not been processed process permits "source" names (names that have not been processed
through IDNA and nameprep) to be registered. As one example, since through IDNA and Nameprep) to be registered. As one example, since
the character Eszett, used in German, is mapped by IDNA2003 into the the character Eszett, used in German, is mapped by IDNA2003 into the
sequence "ss" rather than being retained as itself or prohibited, a sequence "ss" rather than being retained as itself or prohibited, a
string containing that character but that is otherwise in ASCII is 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 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 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 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 a user appears to be an IDN. The proposed IDNA2008 eliminates this
artifact. A character is either permitted as itself or it is artifact. A character is either permitted as itself or it is
prohibited; special cases that make sense only in a particular prohibited; special cases that make sense only in a particular
linguistic or cultural context can be dealt with as localization linguistic or cultural context can be dealt with as localization
matters where appropriate. matters where appropriate.
10. Acknowledgments 8. Acknowledgments
The editor and contributors would like to express their thanks to The editor and contributors would like to express their thanks to
those who contributed significant early (pre-WG) review comments, those who contributed significant early (pre-WG) review comments,
sometimes accompanied by text, especially Mark Davis, Paul Hoffman, sometimes accompanied by text, especially Mark Davis, Paul Hoffman,
Simon Josefsson, and Sam Weiler. In addition, some specific ideas Simon Josefsson, and Sam Weiler. In addition, some specific ideas
were incorporated from suggestions, text, or comments about sections were incorporated from suggestions, text, or comments about sections
that were unclear supplied by Frank Ellerman, Michael Everson, Asmus that were unclear supplied by Frank Ellerman, Michael Everson, Asmus
Freytag, Erik van der Poel, Michel Suignard, and Ken Whistler, Freytag, Erik van der Poel, Michel Suignard, and Ken Whistler,
although, as usual, they bear little or no responsibility for the although, as usual, they bear little or no responsibility for the
conclusions the editor and contributors reached after receiving their conclusions the editor and contributors reached after receiving their
skipping to change at page 41, line 46 skipping to change at page 35, line 41
Technical Consortium. The discussions at and subsequent to that Technical Consortium. The discussions at and subsequent to that
meeting were very helpful in focusing the issues and in refining the meeting were very helpful in focusing the issues and in refining the
specifications. The active participants at that meeting were (in specifications. The active participants at that meeting were (in
alphabetic order as usual) Harald Alvestrand, Vint Cerf, Tina Dam, alphabetic order as usual) Harald Alvestrand, Vint Cerf, Tina Dam,
Mark Davis, Lisa Dusseault, Patrik Faltstrom (by telephone), Cary Mark Davis, Lisa Dusseault, Patrik Faltstrom (by telephone), Cary
Karp, John Klensin, Warren Kumari, Lisa Moore, Erik van der Poel, Karp, John Klensin, Warren Kumari, Lisa Moore, Erik van der Poel,
Michel Suignard, and Ken Whistler. We express our thanks to Google Michel Suignard, and Ken Whistler. We express our thanks to Google
for support of that meeting and to the participants for their for support of that meeting and to the participants for their
contributions. 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 Useful comments and text on the WG versions of the draft were
received from many participants in the IETF "IDNABIS" WG and a number received from many participants in the IETF "IDNABIS" WG and a number
of document changes resulted from mailing list discussions made by of document changes resulted from mailing list discussions made by
that group. Marcos Sanz provided specific analysis and suggestions that group. Marcos Sanz provided specific analysis and suggestions
that were exceptionally helpful in refining the text. that were exceptionally helpful in refining the text, as did Mark
Davis, Martin Duerst, Ken Whistler, and Andrew Sullivan.
11. Contributors 9. Contributors
While the listed editor held the pen, this core of this document and While the listed editor held the pen, this core of this document and
the initial WG version represents the joint work and conclusions of the initial WG version represents the joint work and conclusions of
an ad hoc design team consisting of the editor and, in alphabetic an ad hoc design team consisting of the editor and, in alphabetic
order, Harald Alvestrand, Tina Dam, Patrik Faltstrom, and Cary Karp. order, Harald Alvestrand, Tina Dam, Patrik Faltstrom, and Cary Karp.
In addition, there were many specific contributions and helpful In addition, there were many specific contributions and helpful
comments from those listed in the Acknowledgments section and others comments from those listed in the Acknowledgments section and others
who have contributed to the development and use of the IDNA who have contributed to the development and use of the IDNA
protocols. protocols.
12. Internationalization Considerations 10. Internationalization Considerations
DNS labels and fully-qualified domain names provide mnemonics that DNS labels and fully-qualified domain names provide mnemonics that
assist in identifying and referring to resources on the Internet. assist in identifying and referring to resources on the Internet.
IDNs expand the range of those mnemonics to include those based on IDNs expand the range of those mnemonics to include those based on
languages and character sets other than Western European and Roman- languages and character sets other than Western European and Roman-
derived ones. But domain "names" are not, in general, words in any derived ones. But domain "names" are not, in general, words in any
language. The recommendations of the IETF policy on character sets language. The recommendations of the IETF policy on character sets
and languages, BCP 18 [RFC2277] are applicable to situations in which and languages, BCP 18 [RFC2277] are applicable to situations in which
language identification is used to provide language-specific language identification is used to provide language-specific
contexts. The DNS is, by contrast, global and international and contexts. The DNS is, by contrast, global and international and
ultimately has nothing to do with languages. Adding languages (or ultimately has nothing to do with languages. Adding languages (or
similar context) to IDNs generally, or to DNS matching in particular, similar context) to IDNs generally, or to DNS matching in particular,
would imply context dependent matching in DNS, which would be a very would imply context dependent matching in DNS, which would be a very
significant change to the DNS protocol itself. It would also imply significant change to the DNS protocol itself. It would also imply
that users would need to identify the language associated with a that users would need to identify the language associated with a
particular label in order to look that label up, a decision that particular label in order to look that label up, a decision that
would be impossible in many or most cases. would be impossible in many or most cases.
13. IANA Considerations 11. IANA Considerations
This section gives an overview of registries required for IDNA. The This section gives an overview of registries required for IDNA. The
actual definitions of the first two appear in [IDNA2008-Tables]. actual definitions of the first two appear in [IDNA2008-Tables].
13.1. IDNA Character Registry 11.1. IDNA Character Registry
The distinction among the three major categories "UNASSIGNED", The distinction among the three major categories "UNASSIGNED",
"DISALLOWED", and "PROTOCOL-VALID" is made by special categories and "DISALLOWED", and "PROTOCOL-VALID" is made by special categories and
rules that are integral elements of [IDNA2008-Tables]. Convenience rules that are integral elements of [IDNA2008-Tables]. Convenience
in programming and validation requires a registry of characters and in programming and validation requires a registry of characters and
scripts and their categories, updated for each new version of Unicode scripts and their categories, updated for each new version of Unicode
and the characters it contains. The details of this registry are and the characters it contains. The details of this registry are
specified in [IDNA2008-Tables]. specified in [IDNA2008-Tables].
13.2. IDNA Context Registry 11.2. IDNA Context Registry
For characters that are defined in the IDNA Character Registry list For characters that are defined in the IDNA Character Registry list
as PROTOCOL-VALID but requiring a contextual rule (i.e., the types of 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 rule described in Section 3.1.1.1), IANA will create and maintain a
list of approved contextual rules. The details for those rules list of approved contextual rules. The details for those rules
appear in [IDNA2008-Tables]. appear in [IDNA2008-Tables].
13.3. IANA Repository of IDN Practices of TLDs 11.3. IANA Repository of IDN Practices of TLDs
This registry, historically described as the "IANA Language Character This registry, historically described as the "IANA Language Character
Set Registry" or "IANA Script Registry" (both somewhat misleading Set Registry" or "IANA Script Registry" (both somewhat misleading
terms) is maintained by IANA at the request of ICANN. It is used to terms) is maintained by IANA at the request of ICANN. It is used to
provide a central documentation repository of the IDN policies used provide a central documentation repository of the IDN policies used
by top level domain (TLD) registries who volunteer to contribute to by top level domain (TLD) registries who volunteer to contribute to
it and is used in conjunction with ICANN Guidelines for IDN use. it and is used in conjunction with ICANN Guidelines for IDN use.
It is not an IETF-managed registry and, while the protocol changes It is not an IETF-managed registry and, while the protocol changes
specified here may call for some revisions to the tables, these specified here may call for some revisions to the tables, these
specifications have no direct effect on that registry and no IANA specifications have no direct effect on that registry and no IANA
action is required as a result. action is required as a result.
14. Security Considerations 12. 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, 12.1. General Security Issues with IDNA
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.
This problem is not solved by security protocols like TLS that do not This document in the IDNA2008 series is purely explanatory and
take local character sets into account. informational and consequently introduces no new security issues. It
would, of course, be a poor idea for someone to try to implement from
it; such an attempt would almost certainly lead to interoperability
problems and might lead to security ones. A discussion of security
issues with IDNA2008, and IDNA generally, appears in [IDNA2008-Defs].
To help prevent confusion between characters that are visually 12.2. Security Differences from IDNA2003
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 The registration and lookup models described in this set of documents
[IDNA2008-Protocol] change the mechanisms available for lookup change the mechanisms available for lookup applications to determine
applications to determine the validity of labels they encounter. In the validity of labels they encounter. In some respects, the ability
some respects, the ability to test is strengthened. For example, to test is strengthened. For example, putative labels that contain
putative labels that contain unassigned code points will now be unassigned code points will now be rejected, while IDNA2003 permitted
rejected, while IDNA2003 permitted them (something that is now them (something that is now recognized as a considerable source of
recognized as a considerable source of risk). On the other hand, the risk). On the other hand, the protocol specification no longer
protocol specification no longer assumes that the application that assumes that the application that looks up a name will be able to
looks up a name will be able to determine, and apply, information determine, and apply, information about the protocol version used in
about the protocol version used in registration. In theory, that may registration. In theory, that may increase risk since the
increase risk since the application will be able to do less pre- application will be able to do less pre-lookup validation. In
lookup validation. In practice, the protection afforded by that test practice, the protection afforded by that test has been largely
has been largely illusory for reasons explained in RFC 4690 and illusory for reasons explained in RFC 4690 and above.
above.
Any change to Stringprep or, more broadly, the IETF's model of the Any change to Stringprep or, more broadly, the IETF's model of the
use of internationalized character strings in different protocols, use of internationalized character strings in different protocols,
creates some risk of inadvertent changes to those protocols, creates some risk of inadvertent changes to those protocols,
invalidating deployed applications or databases, and so on. Our invalidating deployed applications or databases, and so on. The same
current hypothesis is that the same considerations that would require considerations that would require changing the IDN prefix (see the
changing the IDN prefix (see Section 9.2.3.2) are the ones that discussion of prefix changes in Section 7.4) are the ones that would,
would, e.g., invalidate certificates or hashes that depend on e.g., invalidate certificates or hashes that depend on Stringprep,
Stringprep, but those cases require careful consideration and but those cases require careful consideration and evaluation. More
evaluation. More important, it is not necessary to change important, it is not necessary to change Stringprep at all in order
Stringprep2003 at all in order to make the IDNA changes contemplated to create a definition or implementation of IDNA as specified in this
here. It is far preferable to create a separate document, or set of documents. Because these documents do not depend on
separate profile components, for IDN work, leaving the question of Stringprep at all, the question of upgrading other protocols that do
upgrading to other protocols to experts on them and eliminating any depend on Stringprep can be left to experts on those protocols: there
possible synchronization dependency between IDNA changes and possible is no dependency between IDNA changes and possible upgrades to
upgrades to security protocols or conventions. 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.
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 13. References
16.1. Normative References 13.1. Normative References
[ASCII] American National Standards Institute (formerly United [ASCII] American National Standards Institute (formerly United
States of America Standards Institute), "USA Code for States of America Standards Institute), "USA Code for
Information Interchange", ANSI X3.4-1968, 1968. Information Interchange", ANSI X3.4-1968, 1968.
ANSI X3.4-1968 has been replaced by newer versions with ANSI X3.4-1968 has been replaced by newer versions with
slight modifications, but the 1968 version remains slight modifications, but the 1968 version remains
definitive for the Internet. definitive for the Internet.
[IDNA2008-Bidi] [IDNA2008-Bidi]
Alvestrand, H. and C. Karp, "An updated IDNA criterion for Alvestrand, H. and C. Karp, "An updated IDNA criterion for
right to left scripts", July 2008, <https:// right to left scripts", July 2008, <https://
datatracker.ietf.org/drafts/draft-ietf-idnabis-bidi/>. datatracker.ietf.org/drafts/draft-ietf-idnabis-bidi/>.
[IDNA2008-Defs]
Klensin, J., "Internationalized Domain Names for
Applications (IDNA): Definitions and Document Framework",
November 2008, <https://datatracker.ietf.org/drafts/
draft-ietf-idnabis-defs/>.
[IDNA2008-Protocol] [IDNA2008-Protocol]
Klensin, J., "Internationalized Domain Names in Klensin, J., "Internationalized Domain Names in
Applications (IDNA): Protocol", September 2008, <https:// Applications (IDNA): Protocol", November 2008, <https://
datatracker.ietf.org/drafts/draft-ietf-idnabis-protocol/>. datatracker.ietf.org/drafts/draft-ietf-idnabis-protocol/>.
[IDNA2008-Tables] [IDNA2008-Tables]
Faltstrom, P., "The Unicode Code Points and IDNA", Faltstrom, P., "The Unicode Code Points and IDNA",
July 2008, <https://datatracker.ietf.org/drafts/ July 2008, <https://datatracker.ietf.org/drafts/
draft-ietf-idnabis-tables/>. draft-ietf-idnabis-tables/>.
A version of this document is available in HTML format at A version of this document is available in HTML format at
http://stupid.domain.name/idnabis/ http://stupid.domain.name/idnabis/
draft-ietf-idnabis-tables-02.html 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, [RFC3490] Faltstrom, P., Hoffman, P., and A. Costello,
"Internationalizing Domain Names in Applications (IDNA)", "Internationalizing Domain Names in Applications (IDNA)",
RFC 3490, March 2003. 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 [RFC3492] Costello, A., "Punycode: A Bootstring encoding of Unicode
for Internationalized Domain Names in Applications for Internationalized Domain Names in Applications
(IDNA)", RFC 3492, March 2003. (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, <https://
datatracker.ietf.org/drafts/draft-ietf-idnabis-tables/>.
[Unicode-UAX15] [Unicode-UAX15]
The Unicode Consortium, "Unicode Standard Annex #15: The Unicode Consortium, "Unicode Standard Annex #15:
Unicode Normalization Forms", March 2008, Unicode Normalization Forms", March 2008,
<http://www.unicode.org/reports/tr15/>. <http://www.unicode.org/reports/tr15/>.
[Unicode51] [Unicode51]
The Unicode Consortium, "The Unicode Standard, Version The Unicode Consortium, "The Unicode Standard, Version
5.1.0", 2008. 5.1.0", 2008.
defined by: The Unicode Standard, Version 5.0, Boston, MA, defined by: The Unicode Standard, Version 5.0, Boston, MA,
Addison-Wesley, 2007, ISBN 0-321-48091-0, as amended by Addison-Wesley, 2007, ISBN 0-321-48091-0, as amended by
Unicode 5.1.0 Unicode 5.1.0
(http://www.unicode.org/versions/Unicode5.1.0/). (http://www.unicode.org/versions/Unicode5.1.0/).
16.2. Informative References 13.2. Informative References
[BIG5] Institute for Information Industry of Taiwan, "Computer [BIG5] Institute for Information Industry of Taiwan, "Computer
Chinese Glyph and Character Code Mapping Table, Technical Chinese Glyph and Character Code Mapping Table, Technical
Report C-26", 1984. Report C-26", 1984.
There are several forms and variations and a closely- There are several forms and variations and a closely-
related standard, CNS 11643. See the discussion in related standard, CNS 11643. See the discussion in
Chapter 3 of Lunde, K., CJKV Information Processing, Chapter 3 of Lunde, K., CJKV Information Processing,
O'Reilly & Associates, 1999 O'Reilly & Associates, 1999
skipping to change at page 49, line 5 skipping to change at page 40, line 30
[RFC2277] Alvestrand, H., "IETF Policy on Character Sets and [RFC2277] Alvestrand, H., "IETF Policy on Character Sets and
Languages", BCP 18, RFC 2277, January 1998. Languages", BCP 18, RFC 2277, January 1998.
[RFC2673] Crawford, M., "Binary Labels in the Domain Name System", [RFC2673] Crawford, M., "Binary Labels in the Domain Name System",
RFC 2673, August 1999. RFC 2673, August 1999.
[RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for [RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
specifying the location of services (DNS SRV)", RFC 2782, specifying the location of services (DNS SRV)", RFC 2782,
February 2000. February 2000.
[RFC3454] Hoffman, P. and M. Blanchet, "Preparation of
Internationalized Strings ("stringprep")", RFC 3454,
December 2002.
[RFC3491] Hoffman, P. and M. Blanchet, "Nameprep: A Stringprep
Profile for Internationalized Domain Names (IDN)",
RFC 3491, March 2003.
[RFC3743] Konishi, K., Huang, K., Qian, H., and Y. Ko, "Joint [RFC3743] Konishi, K., Huang, K., Qian, H., and Y. Ko, "Joint
Engineering Team (JET) Guidelines for Internationalized Engineering Team (JET) Guidelines for Internationalized
Domain Names (IDN) Registration and Administration for Domain Names (IDN) Registration and Administration for
Chinese, Japanese, and Korean", RFC 3743, April 2004. Chinese, Japanese, and Korean", RFC 3743, April 2004.
[RFC3987] Duerst, M. and M. Suignard, "Internationalized Resource [RFC3987] Duerst, M. and M. Suignard, "Internationalized Resource
Identifiers (IRIs)", RFC 3987, January 2005. Identifiers (IRIs)", RFC 3987, January 2005.
[RFC4290] Klensin, J., "Suggested Practices for Registration of [RFC4290] Klensin, J., "Suggested Practices for Registration of
Internationalized Domain Names (IDN)", RFC 4290, Internationalized Domain Names (IDN)", RFC 4290,
December 2005. December 2005.
[RFC4690] Klensin, J., Faltstrom, P., Karp, C., and IAB, "Review and [RFC4690] Klensin, J., Faltstrom, P., Karp, C., and IAB, "Review and
Recommendations for Internationalized Domain Names Recommendations for Internationalized Domain Names
(IDNs)", RFC 4690, September 2006. (IDNs)", RFC 4690, September 2006.
[RFC4713] Lee, X., Mao, W., Chen, E., Hsu, N., and J. Klensin, [RFC4713] Lee, X., Mao, W., Chen, E., Hsu, N., and J. Klensin,
"Registration and Administration Recommendations for "Registration and Administration Recommendations for
Chinese Domain Names", RFC 4713, October 2006. Chinese Domain Names", RFC 4713, October 2006.
[Unicode-Security]
The Unicode Consortium, "Unicode Technical Standard #39:
Unicode Security Mechanisms", August 2008,
<http://www.unicode.org/reports/tr39/>.
[Unicode-UAX31] [Unicode-UAX31]
The Unicode Consortium, "Unicode Standard Annex #31: The Unicode Consortium, "Unicode Standard Annex #31:
Unicode Identifier and Pattern Syntax", March 2008, Unicode Identifier and Pattern Syntax", March 2008,
<http://www.unicode.org/reports/tr31/>. <http://www.unicode.org/reports/tr31/>.
[Unicode-UTR36] [Unicode-UTR36]
The Unicode Consortium, "Unicode Technical Report #36: The Unicode Consortium, "Unicode Technical Report #36:
Unicode Security Considerations", July 2008, Unicode Security Considerations", July 2008,
<http://www.unicode.org/reports/tr36/>. <http://www.unicode.org/reports/tr36/>.
Appendix A. Change Log
[[ RFC Editor: Please remove this appendix. ]]
A.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 7.1.2)
and related text about IDNA-validity (in the "Defs" document as of
-04 of this one) 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 7.1
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.
A.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.
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.
A.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.
A.4. Version -04
o Definitional and other normative material moved to new document
(draft-ietf-idnabis-defs). Version -03 annotations removed.
o Material on differences between IDNA2003 and IDNA2003 moved to an
appendix in Protocol.
o Material left over from the origins of this document as a
preliminary proposal has been removed or rewritten.
o Changes made to reflect consensus call results, including removing
several placeholder notes for discussion.
o Added more material, including discussion of historic scripts, to
Section 3.2 on registration policies.
o Added a new section (Section 7.2) to contain specific discussion
of handling of characters that are interpreted differently in
input to IDNA2003 and 2008.
o Some material, including this section/appendix, rearranged.
Author's Address Author's Address
John C Klensin John C Klensin
1770 Massachusetts Ave, Ste 322 1770 Massachusetts Ave, Ste 322
Cambridge, MA 02140 Cambridge, MA 02140
USA USA
Phone: +1 617 245 1457 Phone: +1 617 245 1457
Email: john+ietf@jck.com Email: john+ietf@jck.com
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