< draft-ietf-idnabis-rationale-01.txt   draft-ietf-idnabis-rationale-02.txt >
Network Working Group J. Klensin Network Working Group J. Klensin
Internet-Draft July 12, 2008 Internet-Draft September 12, 2008
Intended status: Standards Track Intended status: Standards Track
Expires: January 13, 2009 Expires: March 16, 2009
Internationalized Domain Names for Applications (IDNA): Definitions, Internationalized Domain Names for Applications (IDNA): Definitions,
Background and Rationale Background and Rationale
draft-ietf-idnabis-rationale-01.txt draft-ietf-idnabis-rationale-02.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.
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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. Objectives . . . . . . . . . . . . . . . . . . . . . . . . 4
1.4. Applicability and Function of IDNA . . . . . . . . . . . . 5 1.4. Applicability and Function of IDNA . . . . . . . . . . . . 5
1.5. Terminology . . . . . . . . . . . . . . . . . . . . . . . 6 1.5. Terminology . . . . . . . . . . . . . . . . . . . . . . . 6
1.5.1. Documents and Standards . . . . . . . . . . . . . . . 6 1.5.1. Documents and Standards . . . . . . . . . . . . . . . 6
1.5.2. Terminology about Characters and Character Sets . . . 6 1.5.2. Terminology about Characters and Character Sets . . . 6
1.5.3. DNS-related Terminology . . . . . . . . . . . . . . . 7 1.5.3. DNS-related Terminology . . . . . . . . . . . . . . . 7
1.5.4. Terminology Specific to IDNA . . . . . . . . . . . . . 7 1.5.4. Terminology Specific to IDNA . . . . . . . . . . . . . 7
1.5.5. Punycode is an Algorithm, not a Name . . . . . . . . . 10 1.5.5. Punycode is an Algorithm, not a Name . . . . . . . . . 11
1.5.6. Other Terminology Issues . . . . . . . . . . . . . . . 11 1.5.6. Other Terminology Issues . . . . . . . . . . . . . . . 11
1.6. Comprehensibility of IDNA Mechanisms and Processing . . . 12 1.6. Comprehensibility of IDNA Mechanisms and Processing . . . 12
2. Summary of Major Changes from IDNA2003 . . . . . . . . . . . . 13 2. The Revised IDNA Model . . . . . . . . . . . . . . . . . . . . 13
3. The Revised IDNA Model . . . . . . . . . . . . . . . . . . . . 14 3. Processing in IDNA2008 . . . . . . . . . . . . . . . . . . . . 14
4. Processing in IDNA2008 . . . . . . . . . . . . . . . . . . . . 14 4. IDNA2008 Document List . . . . . . . . . . . . . . . . . . . . 14
5. IDNA2008 Document List . . . . . . . . . . . . . . . . . . . . 14 5. Permitted Characters: An Inclusion List . . . . . . . . . . . 15
6. Permitted Characters: An Inclusion List . . . . . . . . . . . 15 5.1. A Tiered Model of Permitted Characters and Labels . . . . 15
6.1. A Tiered Model of Permitted Characters and Labels . . . . 15 5.1.1. PROTOCOL-VALID . . . . . . . . . . . . . . . . . . . . 15
6.1.1. PROTOCOL-VALID . . . . . . . . . . . . . . . . . . . . 16 5.1.2. DISALLOWED . . . . . . . . . . . . . . . . . . . . . . 17
6.1.2. DISALLOWED . . . . . . . . . . . . . . . . . . . . . . 17 5.1.3. UNASSIGNED . . . . . . . . . . . . . . . . . . . . . . 18
6.1.3. UNASSIGNED . . . . . . . . . . . . . . . . . . . . . . 18 5.2. Registration Policy . . . . . . . . . . . . . . . . . . . 18
6.2. Registration Policy . . . . . . . . . . . . . . . . . . . 19 5.3. Layered Restrictions: Tables, Context, Registration,
6.3. Layered Restrictions: Tables, Context, Registration,
Applications . . . . . . . . . . . . . . . . . . . . . . . 19 Applications . . . . . . . . . . . . . . . . . . . . . . . 19
7. Issues that Constrain Possible Solutions . . . . . . . . . . . 19 6. Issues that Constrain Possible Solutions . . . . . . . . . . . 19
7.1. Display and Network Order . . . . . . . . . . . . . . . . 19 6.1. Display and Network Order . . . . . . . . . . . . . . . . 19
7.2. Entry and Display in Applications . . . . . . . . . . . . 21 6.2. Entry and Display in Applications . . . . . . . . . . . . 20
7.3. Linguistic Expectations: Ligatures, Digraphs, and 6.3. Linguistic Expectations: Ligatures, Digraphs, and
Alternate Character Forms . . . . . . . . . . . . . . . . 22 Alternate Character Forms . . . . . . . . . . . . . . . . 21
7.4. Case Mapping and Related Issues . . . . . . . . . . . . . 24 6.4. Case Mapping and Related Issues . . . . . . . . . . . . . 24
7.5. Right to Left Text . . . . . . . . . . . . . . . . . . . . 25 6.5. Right to Left Text . . . . . . . . . . . . . . . . . . . . 25
8. IDNs and the Robustness Principle . . . . . . . . . . . . . . 25 7. IDNs and the Robustness Principle . . . . . . . . . . . . . . 25
9. Front-end and User Interface Processing . . . . . . . . . . . 26 8. Front-end and User Interface Processing . . . . . . . . . . . 26
10. Migration and Version Synchronization . . . . . . . . . . . . 29 9. Relationship to IDNA2003 and Earlier Versions of Unicode . . . 28
10.1. Design Criteria . . . . . . . . . . . . . . . . . . . . . 29 9.1. Summary of Major Changes from IDNA2003 . . . . . . . . . . 29
10.1.1. General IDNA Validity Criteria . . . . . . . . . . . . 29 9.2. Migration and Version Synchronization . . . . . . . . . . 29
10.1.2. Labels in Registration . . . . . . . . . . . . . . . . 30 9.2.1. Design Criteria . . . . . . . . . . . . . . . . . . . 29
10.1.3. Labels in Resolution (Lookup) . . . . . . . . . . . . 31 9.2.2. More Flexibility in User Agents . . . . . . . . . . . 33
10.2. More Flexibility in User Agents . . . . . . . . . . . . . 32 9.2.3. The Question of Prefix Changes . . . . . . . . . . . . 34
10.3. The Question of Prefix Changes . . . . . . . . . . . . . . 33 9.2.4. Stringprep Changes and Compatibility . . . . . . . . . 36
10.3.1. Conditions Requiring a Prefix Change . . . . . . . . . 33 9.2.5. The Symbol Question . . . . . . . . . . . . . . . . . 37
10.3.2. Conditions Not Requiring a Prefix Change . . . . . . . 34 9.2.6. Migration Between Unicode Versions: Unassigned
10.3.3. Implications of Prefix Changes . . . . . . . . . . . . 35 Code Points . . . . . . . . . . . . . . . . . . . . . 38
10.4. Stringprep Changes and Compatibility . . . . . . . . . . . 35 9.2.7. Other Compatibility Issues . . . . . . . . . . . . . . 39
10.5. The Symbol Question . . . . . . . . . . . . . . . . . . . 36 10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 39
10.6. Migration Between Unicode Versions: Unassigned Code 11. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 40
Points . . . . . . . . . . . . . . . . . . . . . . . . . . 37 12. Internationalization Considerations . . . . . . . . . . . . . 40
10.7. Other Compatibility Issues . . . . . . . . . . . . . . . . 38 13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 41
11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 39 13.1. IDNA Character Registry . . . . . . . . . . . . . . . . . 41
12. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 39 13.2. IDNA Context Registry . . . . . . . . . . . . . . . . . . 41
13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 40 13.3. IANA Repository of IDN Practices of TLDs . . . . . . . . . 41
13.1. IDNA Character Registry . . . . . . . . . . . . . . . . . 40 14. Security Considerations . . . . . . . . . . . . . . . . . . . 42
13.2. IDNA Context Registry . . . . . . . . . . . . . . . . . . 40 15. Change Log . . . . . . . . . . . . . . . . . . . . . . . . . . 43
13.3. IANA Repository of IDN Practices of TLDs . . . . . . . . . 40 15.1. Changes between Version -00 and Version -01 of
14. Security Considerations . . . . . . . . . . . . . . . . . . . 41 draft-ietf-idnabis-rationale . . . . . . . . . . . . . . . 43
15. Change Log . . . . . . . . . . . . . . . . . . . . . . . . . . 42 15.2. Version -02 . . . . . . . . . . . . . . . . . . . . . . . 44
15.1. Version -01 of draft-klensin-idnabis-issues . . . . . . . 42 16. References . . . . . . . . . . . . . . . . . . . . . . . . . . 44
15.2. Version -02 of draft-klensin-idnabis-issues . . . . . . . 42 16.1. Normative References . . . . . . . . . . . . . . . . . . . 44
15.3. Version -03 of draft-klensin-idnabis-issues . . . . . . . 43 16.2. Informative References . . . . . . . . . . . . . . . . . . 46
15.4. Version -04 of draft-klensin-idnabis-issues . . . . . . . 43 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 47
15.5. Version -05 of draft-klensin-idnabis-issues . . . . . . . 43 Intellectual Property and Copyright Statements . . . . . . . . . . 48
15.6. Version -06 of draft-klensin-idnabis-issues . . . . . . . 43
15.7. Version -07 of draft-klensin-idnabis-issues . . . . . . . 44
15.8. Version -00 of draft-ietf-idnabis-rationale . . . . . . . 44
15.9. Version -01 of draft-ietf-idnabis-rationale . . . . . . . 45
16. References . . . . . . . . . . . . . . . . . . . . . . . . . . 46
16.1. Normative References . . . . . . . . . . . . . . . . . . . 46
16.2. Informative References . . . . . . . . . . . . . . . . . . 47
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 48
Intellectual Property and Copyright Statements . . . . . . . . . . 49
1. Introduction 1. Introduction
1.1. Context and Overview 1.1. Context and Overview
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 a subset During that time, a number of issues have arisen, including a subset
of those described in a recent IAB report [RFC4690] and the need to of those described in a recent IAB report [RFC4690] and the need to
update the system to deal with newer versions of Unicode. Those update the system to deal with newer versions of Unicode. Those
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[RFC0810]) struck a balance between the creation of useful mnemonics [RFC0810]) struck a balance between the creation of useful mnemonics
and the introduction of parsing problems or general confusion in the and the introduction of parsing problems or general confusion in the
contexts in which domain names are used. Our objective is to contexts in which domain names are used. Our objective is to
preserve that balance while expanding the character repertoire to preserve that balance while expanding the character repertoire to
include extended versions of Roman-derived scripts and scripts that include extended versions of Roman-derived scripts and scripts that
are not Roman in origin. No work of this sort will be able to are not Roman in origin. No work of this sort will be able to
completely eliminate sources of visual or textual confusion: such completely eliminate sources of visual or textual confusion: such
confusion is possible even under the original rules where only ASCII confusion is possible even under the original rules where only ASCII
characters were permitted. However, one can hope, through the characters were permitted. However, one can hope, through the
application of different techniques at different points (see application of different techniques at different points (see
Section 6.3), to keep problems to an acceptable minimum. One Section 5.3), to keep problems to an acceptable minimum. One
consequence of this general objective is that the desire of some user consequence of this general objective is that the desire of some user
or marketing community to use a particular string --whether the or marketing community to use a particular string --whether the
reason is to try to write sentences of particular languages in 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 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 other purpose-- is not a primary goal within the context of
applications in the domain name space. applications in the domain name space.
1.4. Applicability and Function of IDNA 1.4. Applicability and Function of IDNA
The IDNA standard does not require any applications to conform to it, The IDNA standard does not require any applications to conform to it,
nor does it retroactively change those applications. An application nor does it retroactively change those applications. An application
can elect to use IDNA in order to support IDN while maintaining can elect to use IDNA in order to support IDN while maintaining
interoperability with existing infrastructure. If an application interoperability with existing infrastructure. If an application
wants to use non-ASCII characters in domain names, IDNA is the only wants to use non-ASCII characters in domain names, IDNA is the only
currently-defined option. Adding IDNA support to an existing currently-defined option. Adding IDNA support to an existing
application entails changes to the application only, and leaves room application entails changes to the application only, and leaves room
for flexibility in front-end processing and more specifically in the for flexibility in front-end processing and more specifically in the
user interface (see Section 9). user interface (see Section 8).
A great deal of the discussion of IDN solutions has focused on 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 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 components have been updated. Proposals that were not chosen by
the original IDN Working Group would depend on user applications, the original IDN Working Group would depend on user applications,
resolvers, and DNS servers being updated in order for a user to apply resolvers, and DNS servers being updated in order for a user to apply
an internationalized domain name in any form or coding acceptable an internationalized domain name in any form or coding acceptable
under that method. While processing must be performed prior to or 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 after access to the DNS, no changes are needed to the DNS protocol or
any DNS servers or the resolvers on user's computers. any DNS servers or the resolvers on user's computers.
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repertoire of characters potentially makes the set of misspellings repertoire of characters potentially makes the set of misspellings
larger, especially given that in some cases the same appearance, for larger, especially given that in some cases the same appearance, for
example on a business card, might visually match several Unicode code example on a business card, might visually match several Unicode code
points or several sequences of code points. points or several sequences of code points.
IDNA allows the graceful introduction of IDNs not only by avoiding IDNA allows the graceful introduction of IDNs not only by avoiding
upgrades to existing infrastructure (such as DNS servers and mail upgrades to existing infrastructure (such as DNS servers and mail
transport agents), but also by allowing some rudimentary use of IDNs transport agents), but also by allowing some rudimentary use of IDNs
in applications by using the ASCII representation of the non-ASCII in applications by using the ASCII representation of the non-ASCII
name labels. While such names are user-unfriendly to read and type, name labels. While such names are user-unfriendly to read and type,
and hence not optimal for user input, they allow (for instance) and hence not optimal for user input, they can be used as a last
replying to email and clicking on URLs even though the domain name resort to allow rudimentary IDN usage. For example, they might be
displayed is incomprehensible to the user. In order to allow user- 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 friendly input and output of the IDNs and acceptance of some
characters as equivalent to those to be processed according to the characters as equivalent to those to be processed according to the
protocol, the applications need to be modified to conform to this protocol, the applications need to be modified to conform to this
specification. specification.
IDNA uses the Unicode character repertoire, for continuity with IDNA uses the Unicode character repertoire, for continuity with the
IDNA2003. original version of IDNA.
1.5. Terminology 1.5. Terminology
1.5.1. Documents and Standards 1.5.1. Documents and Standards
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
Section 5. References to "these specifications" are to the entire Section 4. References to "these specifications" are to the entire
set. set.
1.5.2. Terminology about Characters and Character Sets 1.5.2. Terminology about Characters and Character Sets
A code point is an integer value associated with a character in a A code point is an integer value associated with a character in a
coded character set. coded character set.
Unicode [Unicode51] is a coded character set containing almost Unicode [Unicode51] is a coded character set containing almost
100,000 characters as of the current version. A single Unicode code 100,000 characters as of the current version. A single Unicode code
point is denoted by "U+" followed by four to six hexadecimal digits, point is denoted by "U+" followed by four to six hexadecimal digits,
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"Letters" are, informally, generalizations from the ASCII and common- "Letters" are, informally, generalizations from the ASCII and common-
sense understanding of that term, i.e., characters that are used to sense understanding of that term, i.e., characters that are used to
write text that are not digits, symbols, or punctuation. Formally, write text that are not digits, symbols, or punctuation. Formally,
they are characters with a Unicode General Category value starting in they are characters with a Unicode General Category value starting in
"L" (see Section 4.5 of [Unicode51]). "L" (see Section 4.5 of [Unicode51]).
1.5.3. DNS-related Terminology 1.5.3. DNS-related Terminology
When discussing the DNS, this document generally assumes the When discussing the DNS, this document generally assumes the
terminology used in the DNS specifications [RFC1034] [RFC1035]. The terminology used in the DNS specifications [RFC1034] [RFC1035]. The
terms "lookup" and "resolution" are used interchangeably and the terms "lookup" is used to describe the combination of operations
process or application component that performs DNS resolution is performed by this protocol and those actually performed by a DNS
called a "resolver". The process of placing an entry into the DNS is resolver. The process of placing an entry into the DNS is referred
referred to as "registration" paralleling common contemporary usage to as "registration", similar to common contemporary usage in other
in other contexts. Consequently, any DNS zone administration is contexts. Consequently, any DNS zone administration is described as
described as a "registry", regardless of that actual administrative a "registry", regardless of the actual administrative arrangements or
arrangements or level in the tree. A note about that relationship is level in the DNS tree. A note about that relationship is included in
included in the text below where it seems particularly significant. the text below where it seems particularly significant.
The term "LDH code points" is defined in this document to mean the The term "LDH code points" is defined in this document to mean the
code points associated with ASCII letters, digits, and the hyphen- code points associated with ASCII letters, digits, and the hyphen-
minus; that is, U+002D, 0030..0039, 0041..005A, and 0061..007A. "LDH" minus; that is, U+002D, 0030..0039, 0041..005A, and 0061..007A. "LDH"
is an abbreviation for "letters, digits, hyphen". is an abbreviation for "letters, digits, hyphen".
The base DNS specifications [RFC1034] [RFC1035] discuss "domain The base DNS specifications [RFC1034] [RFC1035] discuss "domain
names" and "host names", but many people and sections of these names" and "host names", but many people and sections of these
specifications use the terms interchangeably. Further, because those specifications use the terms interchangeably. Lack of clarity about
documents were not terribly clear, many people who are sure they know that terminology has contributed to confusion about intent in some
the exact definitions of each of these terms disagree on the cases. This document generally uses the term "domain name". When it
definitions. This document generally uses the term "domain name". refers to, e.g., host name syntax restrictions, it explicitly cites
When it refers to, e.g., host name syntax restrictions, it explicitly the relevant defining documents. The remaining definitions in this
cites the relevant defining documents. The remaining definitions in subsection are essentially a review.
this subsection are essentially a review.
A label is an individual component of a domain name. Labels are A label is an individual component of a domain name. Labels are
usually shown separated by dots; for example, the domain name usually shown separated by dots; for example, the domain name
"www.example.com" is composed of three labels: "www", "example", and "www.example.com" is composed of three labels: "www", "example", and
"com". (The zero-length root label described in [RFC1123], which can "com". (The zero-length root label described in RFC 1123 [RFC1123],
be explicit as in "www.example.com." or implicit as in which can be explicit as in "www.example.com." or implicit as in
"www.example.com", is not considered a label in this specification.) "www.example.com", is not considered in this specification.) IDNA
IDNA extends the set of usable characters in labels that are text. extends the set of usable characters in labels that are treated as
For the rest of this document, the term "label" is shorthand for text (as distinct from the binary string labels discussed in RFC 1035
"text label", and "every label" means "every text label". 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 1.5.4. Terminology Specific to IDNA
This section defines some terminology to reduce dependence on terms This section defines some terminology to reduce dependence on terms
and definitions that have been problematic in the past. and definitions that have been problematic in the past.
1.5.4.1. Terms for IDN Label Codings 1.5.4.1. Terms for IDN Label Codings
1.5.4.1.1. IDNA-valid strings, A-label, and U-label 1.5.4.1.1. IDNA-valid strings, A-label, and U-label
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subsection. In the next, it defines a historical one to be slightly subsection. In the next, it defines a historical one to be slightly
more precise for IDNA contexts. more precise for IDNA contexts.
o A string is "IDNA-valid" if it meets all of the requirements of o A string is "IDNA-valid" if it meets all of the requirements of
these specifications for an IDNA label. IDNA-valid strings may these specifications for an IDNA label. IDNA-valid strings may
appear in either of two forms, defined immediately below. It is appear in either of two forms, defined immediately below. It is
expected that specific reference will be made to the form expected that specific reference will be made to the form
appropriate to any context in which the distinction is important. appropriate to any context in which the distinction is important.
o An "A-label" is the ASCII-Compatible Encoding (ACE, see o An "A-label" is the ASCII-Compatible Encoding (ACE, see
Section 1.5.4.4) form of an IDNA-valid string. It must be a 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 complete label: IDNA is defined for labels, not for parts of them
and not for complete domain names. This means, by definition, and not for complete domain names. This means, by definition,
that every A-label will begin with the IDNA ACE prefix, "xn--", that every A-label will begin with the IDNA ACE prefix, "xn--",
followed by a string that is a valid output of the Punycode followed by a string that is a valid output of the Punycode
algorithm and hence a maximum of 59 ASCII characters in length. algorithm and hence a maximum of 59 ASCII characters in length.
The prefix and string together must conform to all requirements The prefix and string together must conform to all requirements
for a label that can be stored in the DNS including conformance to for a label that can be stored in the DNS including conformance to
the LDH ("host name") rule described in RFC 1034, RFC 1123 and the rules for the preferred form described in RFC 1034, RFC 1035,
elsewhere. and RFC 1123.
o A "U-label" is an IDNA-valid string of Unicode characters, o A "U-label" is an IDNA-valid string of Unicode characters,
including at least one non-ASCII character, expressed in a including at least one non-ASCII character, expressed in a
standard Unicode Encoding Form, normally UTF-8 in an Internet standard Unicode Encoding Form -- normally UTF-8 in an Internet
transmission context, and subject to the constraint below. transmission context -- and subject to the constraint below.
Conversions between valid U-labels and valid A-labels is performed Conversions between U-labels and A-labels are performed according
according to the specification in [RFC3492], adding or removing to the "Punycode" specification [RFC3492], adding or removing the
the ACE prefix (see Section 1.5.4.4) as needed. ACE prefix (see Section 1.5.4.5) as needed.
To be valid, U-labels and A-labels must obey an important symmetry To be valid, U-labels and A-labels must obey an important symmetry
constraint. While that constraint may be tested in any of several constraint. While that constraint may be tested in any of several
ways, an A-label must be capable of being produced by conversion from 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 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 conversion from an A-label. Among other things, this implies that
both U-labels and A-labels must represent strings in normalized form. both U-labels and A-labels must be strings in Unicode NFC
These strings MUST contain only characters specified elsewhere in [Unicode-UAX15] normalized form. These strings MUST contain only
this document and its companion documents, and only in the contexts characters specified elsewhere in this document and its companion
indicated as appropriate. documents, and only in the contexts indicated as appropriate.
Any rules or conventions that apply to DNS labels in general, such as 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 rules about lengths of strings, apply to whichever of the U-label or
A-label would be more restrictive. For the U-label, constraints A-label would be more restrictive. For the U-label, constraints
imposed by existing protocols and their presentation forms make the imposed by existing protocols and their presentation forms make the
length restriction apply to the length in octets of the UTF-8 form of 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 those labels (which will always be greater than or equal to the
length in code points). The exception to this, of course, is that length in code points). The exception to this, of course, is that
the restriction to ASCII characters does not apply to the U-label. the restriction to ASCII characters does not apply to the U-label.
skipping to change at page 9, line 29 skipping to change at page 9, line 33
o cannot be processed as U-labels or A-labels as described in these o cannot be processed as U-labels or A-labels as described in these
specifications, specifications,
are invalid in IDNA-conformant applications as labels in domain names are invalid in IDNA-conformant applications as labels in domain names
that identify Internet hosts or similar resources. This restriction that identify Internet hosts or similar resources. This restriction
on strings containing "--" is required for three reasons: on strings containing "--" is required for three reasons:
o to prevent confusion with pre-IDNA coding forms; o to prevent confusion with pre-IDNA coding forms;
o to permit future extensions that would require changing the o to permit future extensions that would require changing the
prefix, no matter how unlikely those might be (see Section 10.3); prefix, no matter how unlikely those might be (see Section 9.2.3);
and and
o to reduce the opportunities for attacks via the encoding system. o to reduce the opportunities for attacks via the encoding system.
1.5.4.2. LDH-label and Internationalized Label 1.5.4.2. LDH-label and Internationalized Label
In the hope of further clarifying discussions about IDNs, these In the hope of further clarifying discussions about IDNs, these
specifications use the term "LDH-label" strictly to refer to an all- specifications use the term "LDH-label" strictly to refer to an all-
ASCII label that obeys the "hostname" (LDH) conventions and that is ASCII label that obeys the preferred syntax (often known as
not an IDN. In other words, only "U-label" and "A-label" refer to "hostname" (from RFC 952 [RFC0952]) or "LDH") conventions and that is
IDNs; LDH-labels are not IDNs. "Internationalized label" is used not an IDN. It should be stressed that an A-label obeys the
when a term is needed to refer to any of the three categories. There "hostname" rules and is sometimes described as "LDH-conformant" or in
are some standardized DNS label formats, such as those for service similar language but that it is not an LDH-label as used in this
location (SRV) records [RFC2782] that do not fall into any of the document.
three categories and hence are not internationalized labels.
1.5.4.3. Equivalence 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. 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 If the A-labels are equal in a case-independent comparison, then the
labels are considered equivalent, no matter how they are represented. labels are considered equivalent, no matter how they are represented.
Traditional LDH labels already have a notion of equivalence: within Traditional LDH labels already have a notion of equivalence: within
that list of characters, upper case and lower case are considered that list of characters, upper case and lower case are considered
equivalent. The IDNA notion of equivalence is an extension of that equivalent. The IDNA notion of equivalence is an extension of that
older notion. Equivalent labels in IDNA are treated as alternate older notion. Equivalent labels in IDNA are treated as alternate
forms of the same label, just as "foo" and "Foo" are treated as forms of the same label, just as "foo" and "Foo" are treated as
alternate forms of the same label. alternate forms of the same label.
1.5.4.4. ACE Prefix 1.5.4.5. ACE Prefix
The "ACE prefix" is defined in this document to be a string of ASCII 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. characters "xn--" that appears at the beginning of every A-label.
"ACE" stands for "ASCII-Compatible Encoding". "ACE" stands for "ASCII-Compatible Encoding".
1.5.4.5. Domain Name Slot 1.5.4.6. Domain Name Slot
A "domain name slot" is defined in this document to be a protocol 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) element or a function argument or a return value (and so on)
explicitly designated for carrying a domain name. Examples of domain explicitly designated for carrying a domain name. Examples of domain
name slots include: the QNAME field of a DNS query; the name argument name slots include: the QNAME field of a DNS query; the name argument
of the gethostbyname() or getaddrinfo() standard C library functions; of the gethostbyname() or getaddrinfo() standard C library functions;
the part of an email address following the at-sign (@) in the 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 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 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 attribute of an HTML <IMG> tag. General text that just happens to
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negotiation in an interactive session). negotiation in an interactive session).
An "IDN-unaware domain name slot" is defined in this document to be 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. any domain name slot that is not an IDN-aware domain name slot.
Obviously, this includes any domain name slot whose specification Obviously, this includes any domain name slot whose specification
predates IDNA. predates IDNA.
1.5.5. Punycode is an Algorithm, not a Name 1.5.5. Punycode is an Algorithm, not a Name
There has been some confusion about whether a "Punycode string" does There has been some confusion about whether a "Punycode string" does
or does not include the prefix and about whether it is required that or does not include the ACE prefix and about whether it is required
such strings could have been the output of ToASCII (see RFC 3490, that such strings could have been the output of the ToASCII operation
Section 4 [RFC3490]). This specification discourages the use of the (see RFC 3490, Section 4 [RFC3490]). This specification discourages
term "Punycode" to describe anything but the encoding method and the use of the term "Punycode" to describe anything but the encoding
algorithm of [RFC3492]. The terms defined above are preferred as method and algorithm of [RFC3492]. The terms defined above are
much more clear than terms such as "Punycode string". preferred as much more clear than terms such as "Punycode string".
1.5.6. Other Terminology Issues 1.5.6. Other Terminology Issues
The document departs from historical DNS terminology and usage in one The document departs from historical DNS terminology and usage in one
important respect. Over the years, the community has talked very 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
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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.
An "internationalized domain name" (IDN) is a domain name that may The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
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 resolution 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 6.2.
"The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119]. document are to be interpreted as described in RFC 2119 [RFC2119].
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 5). of the latter appear in other documents (see Section 4).
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 12, line 41 skipping to change at page 13, line 10
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 9. discussed in Section 8.
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
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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. Summary of Major Changes from IDNA2003 2. The Revised IDNA Model
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.
9. Make bidirectional domain names in a paragraph display in a non-
surprising fashion.[[anchor17: Is this statement necessary or is
it redundant with the previous one?]]
10. Remove the dot separator from the mandatory part of the
protocol.
11. Make some currently-valid labels that are not actually IDNA
labels invalid.
3. The Revised IDNA Model
IDNA is a client-side protocol, i.e., almost all of the processing is 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 performed by the client. The strings that appear in, and are
resolved by, the DNS conform to the traditional rules for the naming resolved by, the DNS conform to the traditional rules for the naming
of hosts, and consist of ASCII letters, digits, and hyphens. This of hosts, and consist of ASCII letters, digits, and hyphens. This
approach permits IDNA to be deployed without modifications to the DNS approach permits IDNA to be deployed without modifications to the DNS
itself. That, in turn, avoids both having to upgrade the entire itself. That, in turn, avoids both having to upgrade the entire
Internet to support IDNs and needing to incur the unknown risks to Internet to support IDNs and needing to incur the unknown risks to
deployed systems of DNS structural or design changes especially if deployed systems of DNS structural or design changes especially if
those changes need to be deployed all at the same time. those changes need to be deployed all at the same time.
4. Processing in IDNA2008 [[anchor17: This paragraph is somewhat redundant with material
above.It will be dropped in -03 if there are not strong arguments for
keeping it here.]]
These specifications separate Domain Name Registration and Resolution 3. Processing in IDNA2008
in the protocol specification. Doing so reflects current practice in
These specifications separate Domain Name Registration and Lookup in
the protocol specification. Doing so reflects current practice in
which per-registry restrictions and special processing are applied at which per-registry restrictions and special processing are applied at
registration time but not on resolution. 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].
5. IDNA2008 Document List 4. IDNA2008 Document List
[[anchor19: This section will need to be extensively revised or [[anchor19: This section will need to be extensively revised or
removed before publication.]] removed before publication.]]
The following documents are being produced as part of the IDNA2008 The following documents are being produced as part of the IDNA2008
effort. effort.
o A revised version of this document, containing an overview, o A revised version of this document, containing an overview,
rationale, and conformance conditions. rationale, and conformance conditions.
o A separate document, drawn from material in early versions of this o A separate document, drawn from material in early versions of this
one, that explicitly updates and replaces RFC 3490 but which has one, that explicitly updates and replaces RFC 3490 but which has
most rationale material from that document moved to this one most rationale material from that document moved to this one
[IDNA2008-Protocol]. [IDNA2008-Protocol].
o A document describing the "Bidi problem" with Stringprep and o A document describing the "Bidi problem" with Stringprep and
proposing a solution [IDNA2008-Bidi]. proposing a solution [IDNA2008-Bidi].
o A specification of the categories and rules that identify the code o A specification of the categories and rules that identify the code
points allowed in a U-label, based on Unicode 5.0 code points allowed in a U-label, based on Unicode 5.0 code
assignments. See Section 6 and [IDNA2008-Tables]. assignments. See Section 5 and [IDNA2008-Tables].
o One or more documents containing guidance and suggestions for o One or more documents containing guidance and suggestions for
registries (in this context, those responsible for establishing registries (in this context, those responsible for establishing
policies for any zone file in the DNS, not only those at the top 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 or second level). The documents in this category may not be IETF
products and may be prepared and completed asynchronously with products and may be prepared and completed asynchronously with
those described above. those described above.
6. Permitted Characters: An Inclusion List 5. Permitted Characters: An Inclusion List
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].
6.1. A Tiered Model of Permitted Characters and Labels 5.1. A Tiered Model of Permitted Characters and Labels
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. Of necessity, IDNA2003 prohibited
those types of characters entirely. But the restrictions were much those types of characters entirely. But the restrictions were much
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but limit their use to very specific contexts was reinforced by the but limit their use to very specific contexts was reinforced by the
observation that handling of particular characters across the observation that handling of particular characters across the
languages that use a script, or the use of similar or identical- languages that use a script, or the use of similar or identical-
looking characters in different scripts, is less well understood than looking characters in different scripts, is less well understood than
many people believed it was several years ago. many people believed 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:
6.1.1. PROTOCOL-VALID 5.1.1. 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 group 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 6.3). those judgments (see [IDNA2008-Protocol] and Section 5.3).
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).
[[anchor21: Placeholder: Does this topic or comment need additional [[anchor21: Placeholder: Does this topic or comment need additional
discussion or explanation?]] discussion or explanation?]]
6.1.1.1. Contextual Rules 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 (ZWNJ, U+200C, and ZWJ, U+200D), 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.
These characters must not appear in IDNs without additional These characters must not appear in IDNs without additional
restrictions, typically because they have no visible consequences in restrictions, typically because they have no visible consequences in
most scripts but affect format or presentation in a few others or most scripts but affect format or presentation in a few others or
because they are combining characters that are safe for use only in because they are combining characters that are safe for use only in
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.
6.1.1.2. Rules and Their Application 5.1.1.2. 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 label), "False" (character may not be used in any label), or may be a
an extended regular expression that specifies the context in which set of procedural rules that specify the context in which the
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 MUST
NOT appear in putative labels for either registration or lookup. Of NOT appear in putative labels for either registration or lookup. Of
course, a later version of the tables might contain a non-null rule. course, a later version of the tables might contain a non-null rule.
[[anchor23: Definition of regular expression language to be supplied The description of the syntax of the rules, and the rules themselves,
or replaced with a description of the definitional technique. It may appears in [IDNA2008-Tables].
be useful to more more of this material to Tables as part of moving
the rules from Protocol to Tables.]]
6.1.2. DISALLOWED 5.1.2. 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., they should be excluded for both registration and lookup (i.e., IDNA-
conforming applications performing name resolution 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.
[[anchor24: Note in Draft: the permanence of DISALLOWED was still [[anchor23: Note in Draft: the permanence of DISALLOWED was still
under discussion in the WG when this draft was posted. The text under discussion in the WG when this draft was posted. The text
above reflects the editor's opinion about the emerging consensus but above reflects the editor's opinion about the emerging consensus but
is subject to change as the discussion continues.]] 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.
6.1.3. UNASSIGNED 5.1.3. 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).
6.2. Registration Policy 5.2. Registration Policy
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 [RFC3743] and [RFC4290], and illustrated for Chinese by the tables
described in RFC 4713 [RFC4713] may be helpful in reducing problems described in RFC 4713 [RFC4713] may be helpful in reducing problems
that might be perceived by users. It is worth stressing that these that might be perceived by users. It is worth stressing that these
principles of policy development and application apply at all levels principles of policy development and application apply at all levels
of the DNS, not only, e.g., TLD registrations. 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..
6.3. Layered Restrictions: Tables, Context, Registration, Applications 5.3. 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 magic bullet for any of the issues
associated with a multiscript DNS. Instead, the specifications associated with a multiscript DNS. Instead, the specifications
define a variety of approaches that, together, constitute multiple define a variety of approaches that, together, constitute multiple
lines of defense against ambiguity in identifiers and loss of lines of defense against ambiguity in identifiers and loss of
referential integrity. The actual character tables are the first referential integrity. The actual character tables are the first
mechanism, protocol rules about how those characters are applied or mechanism, protocol rules about how those characters are applied or
restricted in context are the second, and those two in combination restricted in context are the second, and those two in combination
constitute the limits of what can be done by a protocol alone. As constitute the limits of what can be done by a protocol alone. As
discussed in the previous section (Section 6.2), registries are discussed in the previous section (Section 5.2), registries are
expected to restrict what they permit to be registered, devising and expected to restrict what they permit to be registered, devising and
using rules that are designed to optimize the balance between using rules that are designed to optimize the balance between
confusion and risk on the one hand and maximum expressiveness in confusion and risk on the one hand and maximum expressiveness in
mnemonics on the other. 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.
[[anchor25: Note in Draft: the last sentence above basically
duplicates a comment in Security Considerations. Is it worth having
in both places??]]
7. Issues that Constrain Possible Solutions 6. Issues that Constrain Possible Solutions
7.1. Display and Network Order 6.1. Display and Network Order
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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issues. issues.
It should be obvious that any revision of IDNA, including the current It should be obvious that any revision of IDNA, including the current
one, must be clear about the network (transmission on the wire) order one, must be clear about the network (transmission on the wire) order
of characters in labels and for the labels in complete (fully- of characters in labels and for the labels in complete (fully-
qualified) domain names. In order to prevent user confusion and, in qualified) domain names. In order to prevent user confusion and, in
particular, to reduce the chances for inconsistent transcription of particular, to reduce the chances for inconsistent transcription of
domain names from printed form, it is likely that some strong domain names from printed form, it is likely that some strong
suggestions should be made about display order as well. suggestions should be made about display order as well.
7.2. Entry and Display in Applications 6.2. 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 or specified by the operating
system, and can display domain names in any charset. That is, the system, and can display domain names in any charset. 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 and user input of A-labels, but are encouraged to allow the display of A-labels, but are encouraged to not do so except
not do so except as an interface for special purposes, possibly for as an interface for special purposes, possibly for debugging, or to
debugging, or to cope with display limitations. A-labels are opaque cope with display limitations. In general, they SHOULD allow, but
not encourage, user input of that label form. A-labels are opaque
and ugly, and, where possible, should thus only be exposed to users and ugly, and, where possible, should thus only be exposed to users
and in contexts in which they are absolutely needed. Because IDN and in contexts in which they are absolutely needed. Because IDN
labels can be rendered either as the A-labels or U-labels, the labels can be rendered either as A-labels or U-labels, the
application may reasonably have an option for the user to select the application may reasonably have an option for the user to select the
preferred method of display; if it does, rendering the U-label should preferred method of display; if it does, rendering the U-label should
normally be the default. 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 and the RFC 2822 body parts of SMTP, and
the headers and the body content in HTTP. It is important to the headers and the body content in HTTP. It is important to
remember that domain names appear both in domain name slots and in remember that domain names appear both in domain name slots and in
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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.
7.3. Linguistic Expectations: Ligatures, Digraphs, and Alternate 6.3. Linguistic Expectations: Ligatures, Digraphs, and Alternate
Character Forms Character Forms
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 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-umlaut even though applying
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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, in parallel to the also a letter in the Swedish alphabet where, like the "umlauted a",
"umlauted a", it cannot be correctly represented as "oe" and in the it cannot be correctly represented as "oe" and in the Norwegian
Norwegian alphabet, where it is represented, not as "umlauted o", but alphabet, where it is represented, not as "umlauted o", but as
as "slashed o", U+00F8. "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 as people argue that they should have been treated differently to
preserve important information. For example, the German character preserve important information. For example, the German character
Eszett (Sharp S, U+00DF) is retained as itself by NFKC but case- Eszett (Sharp S, U+00DF) is retained as itself by NFKC but case-
folded by Stringprep to "ss", but the closely-related, but less folded by Stringprep to "ss", but the closely-related, but less
frequently seen, character "Long S T" (U+FB05) is a compatibility frequently seen, character "Long S T" (U+FB05) is a compatibility
character that is mapped out by NFKC. Unless exceptions are made, character that is mapped out by NFKC. Unless exceptions are made,
both will be treated as DISALLOWED by IDNA2008. But there is both will be treated as DISALLOWED by IDNA2008. But there is
significant interest in an exception, especially for Eszett. significant interest in an exception, especially for Eszett.
Depending on what the exception was, making it would either raise Depending on what the exception was, making it would either raise
some backward compatibility problems with IDNA2003 or create an some backward compatibility problems with IDNA2003 or create an
unusual special case that would highlight differences in preferred unusual special case that would highlight differences in preferred
orthography between German as written in Germany and German as orthography between German as written in Germany and German as
written in some other countries, notably Switzerland. Additional written in some other countries, notably Switzerland. Additional
discussion of issues with Eszett appear in Section 10.7. 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 7.5. in Section 6.5.
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 basis.
skipping to change at page 24, line 33 skipping to change at page 24, line 16
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] to reduce the opportunities for user confusion and fraud
that would result from the related strings being registered to that would result from the related strings being registered to
different parties. different parties.
7.4. Case Mapping and Related Issues 6.4. Case Mapping and Related Issues
Traditionally in the DNS, ASCII letters have been stored with their Traditionally in the DNS, ASCII letters have been stored with their
case preserved. Matching during the query process has been case- case preserved. Matching during the query process has been case-
independent, but none of the information that might be represented by independent, but none of the information that might be represented by
choices of case has been lost. That model has been accidentally choices of case has been lost. That model has been accidentally
helpful because, as people have created DNS labels by catenating helpful because, as people have created DNS labels by catenating
words (or parts of words) to form labels, case has often been used to words (or parts of words) to form labels, case has often been used to
distinguish among components and make the labels more memorable. distinguish among components 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
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permits, at the risk of some incompatibility, slightly more permits, at the risk of some incompatibility, slightly more
flexibility in this area. That additional flexibility still does not flexibility in this area. That additional flexibility still does not
solve the problem with final form sigma and other characters that solve the problem with final form sigma and other characters that
Unicode treats as completely separate characters that match only Unicode treats as completely separate characters that match only
under casemapping if at all. Many people now believe these should be under casemapping if at all. Many people now believe these should be
handled as separate characters so information about them can be handled as separate characters so information about them can be
preserved in the transformations to A-labels and back. However preserved in the transformations to A-labels and back. However
making a change to permit that behavior would create a situation in making a change to permit that behavior would create a situation in
which the same string, valid in both protocols, would be interpreted which the same string, valid in both protocols, would be interpreted
differently by IDNA2003 and IDNA2008. In principle, that would differently by IDNA2003 and IDNA2008. In principle, that would
violate one of the conditions discussed in Section 10.3.1 and hence violate one of the conditions discussed in Section 9.2.3.1 and hence
require a prefix change. Of course, if a prefix change were made (at require a prefix change. Of course, if a prefix change were made (at
the costs discussed in Section 10.3.3) there would be several the costs discussed in Section 9.2.3.3) there would be several
options, including, if desired, assigning the characer to the options, including, if desired, assigning the characer to the
CONTEXTUAL RULE REQUIRED category and requiring that it only be used CONTEXTUAL RULE REQUIRED category and requiring that it only be used
in carefully-selected contexts. in carefully-selected contexts.
7.5. Right to Left Text 6.5. 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, may 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 old and new) are
required to look at an entire label, not just at individual required to look at an entire label, not just at individual
characters. The algorithmic model used in IDNA2003 rejects the label characters. The algorithmic model used in IDNA2003 rejects the label
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This problem manifests itself in languages written with consonantal This problem manifests itself in languages written with consonantal
alphabets to which diacritical vocalic systems are applied, and in alphabets to which diacritical vocalic systems are applied, and in
languages with orthographies derived from them where the combining languages with orthographies derived from them where the combining
marks may have different functionality. In both cases the combining marks may have different functionality. In both cases the combining
marks can be essential components of the orthography. Examples of marks can be essential components of the orthography. Examples of
this are Yiddish, written with an extended Hebrew script, and Dhivehi this are Yiddish, written with an extended Hebrew script, and Dhivehi
(the official language of Maldives) which is written in the Thaana (the official language of Maldives) which is written in the Thaana
script (which is, in turn, derived from the Arabic script). The new script (which is, in turn, derived from the Arabic script). The new
rules for right to left scripts are described in [IDNA2008-Bidi]. rules for right to left scripts are described in [IDNA2008-Bidi].
8. IDNs and the Robustness Principle 7. 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., RFC 1123, Section 1.2.2
[RFC1123]). For IDNs to work well, not only must the protocol be [RFC1123]). For IDNs to work well, not only must the protocol be
carefully designed and implemented, but zone administrators carefully designed and implemented, but zone administrators
(registries) must have and require sensible policies about what is (registries) must have and require sensible policies about what is
registered -- conservative policies -- and implement and enforce registered -- conservative policies -- and implement and enforce
them. them.
Conversely, resolvers can (and SHOULD or maybe MUST) reject labels Conversely, lookup applications can (and SHOULD or maybe MUST) reject
that clearly violate global (protocol) rules (no one has ever labels that clearly violate global (protocol) rules (no one has ever
seriously claimed that being liberal in what is accepted requires seriously claimed that being liberal in what is accepted requires
being stupid). However, once one gets past such global rules and being stupid). However, once one gets past such global rules and
deals with anything sensitive to script or locale, it is necessary to deals with anything sensitive to script or locale, it is necessary to
assume that garbage has not been placed into the DNS, i.e., one must 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 doesn't resolve, it makes no As mentioned elsewhere, if a string cannot be successfully found in
the DNS after the lookup processing described here, it makes no
difference whether it simply wasn't registered or was prohibited by difference whether it simply wasn't registered or was prohibited by
some rule. some rule.
If resolvers, as a user interface (UI) or other local matter, decide If lookup applications, as a user interface (UI) or other local
to warn about some strings that are valid under the global rules but matter, decide to warn about some strings that are valid under the
that they perceive as dangerous, that is their prerogative and we can global rules but that they perceive as dangerous, that is their
only hope that the market (and maybe regulators) will reinforce the prerogative and we can only hope that the market (and maybe
good choices and discourage the poor ones. In this context, a regulators) will reinforce the good choices and discourage the poor
resolver that decides a string that is valid under the protocol is ones. In this context, a lookup application that decides a string
dangerous and refuses to look it up is in violation of the protocols; that is valid under the protocol is dangerous and refuses to look it
one that is willing to look something up, but warns against it, is up is in violation of the protocols; one that is willing to look
exercising a local choice. something up, but warns against it, is exercising a local choice.
9. Front-end and User Interface Processing 8. 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 as part of URIs or
IRIs. They may occur in running text or be processed by one system IRIs. They may occur in running text or be processed by one system
after being provided in another. Systems may wish to try to after being provided in another. Systems may wish to try to
normalize URLs so as to determine (or guess) whether a reference is normalize URLs so as to determine (or guess) whether a reference is
valid or two references point to the same object without actually valid or two references point to the same object without actually
looking the objects up and comparing them. Some of these goals may looking the objects up and comparing them (that is necessary, not
be more easily and reliably satisfied than others. While there are just a choice, for URI types that are not intended to be resolved).
strong arguments for any domain name that is placed "on the wire" -- Some of these goals may be more easily and reliably satisfied than
transmitted between systems -- to be in the minimum-ambiguity forms others. While there are strong arguments for any domain name that is
of A-labels, U-labels, or LDH-labels, it is inevitable that programs placed "on the wire" -- transmitted between systems -- to be in the
that process domain names will encounter variant forms. One source minimum-ambiguity forms of A-labels, U-labels, or LDH-labels, it is
of such forms will be labels created under IDNA2003. Because of the inevitable that programs that process domain names will encounter
way that protocol was specified, there are a significant number of variant forms.
domain names in files on the Internet that use characters that cannot
be represented directly in domain names but for which interpretations One source of such forms will be labels created under IDNA2003
are provided. There are two major categories of such characters, because that protocol allowed labels that were transformed before
those that are removed by NFKC normalization and those upper-case they were turned from native-character into ACE ("xn--...") format.
characters that are mapped to lower-case (there are also a few One consequence of the transformations was that, when the ToUnicode
characters that are given special-case mapping treatment in and ToASCII operations of IDNA2003 were applied,
Stringprep). [[anchor29: The text above is a too obscure, but was ToUnicode(ToASCII(original-label)) often did not produce the
intended to address the mapping differences between IDNA2003 and the original-label. IDNA2008 explicitly defines A-labels and U-labels as
current proposal. Patrik suggests the following, which will need different forms of the same abstract label, forms that are stable
some tuning before it can be inserted: One source of such forms will when conversions are performed between them, without mappings. A
be labels created under IDNA2003 as some allowed labels where different way of explaining this is that there are, today, domain
transformed before they where turned into its ascii (xn--) form so names in files on the Internet that use characters that cannot be
that ToUnicode(ToASCII(label)) != label. This is why IDNA2008 represented directly in, or recovered from, (A-label) domain names
explicitly define A-label and U-label being a form of the label that but for which interpretations are provided by IDNA2003. There are
is stable when converting between A-label and U-label, without two major categories of such characters, those that are removed by
mappings. A different way of explaining this is that there could be NFKC normalization and those upper-case characters that are mapped to
already today domain names in files on the Internet that use
characters that cannot be represented directly in domain names but
for which interpretations are provided. There are two major
categories of such characters, those that are removed by NFKC
normalization and those upper-case characters that are mapped to
lower-case (there are also a few characters that are given special- lower-case (there are also a few characters that are given special-
case mapping treatment in Stringprep)."]] case mapping treatment in Stringprep).
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 domain name appears in an arbitrary context a label separator. If a string that might be a domain name appears
(such as running text), it is difficult, even with only ASCII in an arbitrary context (such as running text), it is difficult, even
characters, to know whether a domain name (or a protocol parameter with only ASCII characters, to know whether an actual domain name (or
like a URI) is present and where it starts and ends. When using a protocol parameter like a URI) is present and where it starts and
Unicode this gets even more difficult if treatment of certain special ends. When using Unicode, this gets even more difficult if treatment
characters (like the dot that separates labels in a domain name) of certain special characters (like the dot that separates labels in
depends on context. That problem occurs if the dot is part of a a domain name) depends on context (e.g., prior knowledge of whether
domain name or not, which would mean that, contrary to common the string represents a domain name or not). That knowledge is not
practice today, the primary heuristic for identifying a domain name available if the primary heuristic for identifying the presence of
depends on dots separating strings with no intervening spaces. domain names in strings depends on the presence of dots separating
[[anchor30: Above text is a substitute for an earlier (pre -01) groups of characters with no intervening spaces.
[[anchor27: 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, leaving such mappings of different forms of dots, from the protocol, discouraging such
to local processing. This should not be taken to imply that local mappings and leaving them, when necessary, to local processing. This
processing is optional or can be avoided entirely. Instead, unless should not be taken to imply that local processing is optional or can
the program context is such that it is known that any IDNs that be avoided entirely. Instead, unless the program context is such
appear will be either U-labels or A-labels, some local processing of that it is known that any IDNs that appear will be either U-labels or
apparent domain name strings will be required, both to maintain A-labels, or that other forms can safely be rejected, some local
compatibility with IDNA2003 and to prevent user astonishment. Such processing of apparent domain name strings will be required, both to
local processing, while not specified in this document or the maintain compatibility with IDNA2003 and to prevent user
associated ones, will generally take one of two forms: astonishment. Such local processing, 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 28, line 35 skipping to change at page 28, line 18
software will be highly localized for a particular environment and software will be highly localized for a particular environment and
carefully adapted to the expectations of users in that carefully adapted to the expectations of users in that
environment. The many discussions about using the Internet to environment. The many discussions about using the Internet to
preserve and support local cultures suggest that these cases may preserve and support local cultures suggest that these cases may
be more common in the future than they have been so far. be more common in the future than they have been so far.
In these cases, we should avoid trying to tell implementers what In these cases, we should avoid trying to tell implementers what
they should do, if only because they are quite likely (and for they should do, if only because they are quite likely (and for
good reason) to ignore us. We would assume that they would map good reason) to ignore us. We would assume that they would map
characters that the intuitions of their users would suggest be characters that the intuitions of their users would suggest be
mapped. One can imagine switches about whether some sorts of mapped and would hope that they would do that mapping as early as
mappings occur, warnings before applying them or, in a slightly possible, storing A-label or U-label forms in files and
more extreme version of the approach taken in Internet Explorer transporting only those forms between systems. One can imagine
version 7 (IE7), utterly refuse to handle "strange" characters at switches about whether some sorts of mappings occur, warnings
all if they appear in U-label form. None of those local decisions before applying them or, in a slightly more extreme version of the
are a threat to interoperability as long as (i) only U-labels and approach taken in Internet Explorer version 7 (IE7), systems that
utterly refuse to handle "strange" characters at all if they
appear in U-label form. None of those local decisions are a
threat to interoperability as long as (i) only U-labels and
A-labels are used in interchange with systems outside the local 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. [[anchor31: Placeholder: there have been conformance to IDNA2003. [[anchor28: Placeholder: there have been
suggestions that this text be removed entirely. Comments (or suggestions that this text be removed entirely. Comments (or
improved text) welcome.]] improved text) welcome.]]
10. Migration and Version Synchronization In either case, it is vital that user interface designs and, where
the interfaces are not sufficient, users, be aware that the only
forms of domain names that this protocol anticipates will resolve
globally or compare equal when crude methods (i.e., those not
conforming to Section 1.5.4.4) are used are those in which all
native-script labels are in U-label form. Forms that assume mapping
will occur, especially forms that were not valid under IDNA2003, may
or may not function in predictable ways across all implementations.
10.1. Design Criteria 9. Relationship to IDNA2003 and Earlier Versions of Unicode
9.1. Summary of Major Changes from IDNA2003
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
9.2.1. 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 this work are to
enable applications to be agnostic about whether they are being run enable applications to be agnostic about whether they are being run
in environments supporting any Unicode version from 3.2 onward and to in environments supporting any Unicode version from 3.2 onward and to
permit incrementally adding permitted scripts and other character permit incrementally adding permitted scripts and other character
collections without disruption or, subsequent to this version, collections without disruption or, subsequent to this version,
"heavy" processes such as formation of an IETF WG. The mechanisms "heavy" processes such as formation of an IETF WG. The mechanisms
that support this are outlined above, but this section reviews them that support this are outlined above, but this section reviews them
in a context that may be more helpful to those who need to understand in a context that may be more helpful to those who need to understand
the approach and make plans for it. the approach and make plans for it.
10.1.1. General IDNA Validity Criteria 9.2.1.1. 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:
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 inclusion
and the complexities that would come with it (additional and the complexities that would come with it (additional
discussion and rationale for the symbol decision appears in discussion and rationale for the symbol decision appears in
Section 10.5). Section 9.2.5).
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, and words whose usually-preferred spellings retain
diacritical marks from earlier forms -- cannot be represented in diacritical marks from earlier forms -- cannot be represented in
DNS labels. DNS labels.
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 resolution (lookup). There application are not permitted, even on lookup. There are at least
are at least two reasons for this. Tests involving the context of two reasons for this. Tests involving the context of characters
characters (e.g., some characters being permitted only adjacent to (e.g., some characters being permitted only adjacent to ones of
ones of specific types but otherwise invisible or very problematic specific types but otherwise invisible or very problematic for
for other reasons) and integrity tests on complete labels are other reasons) and integrity tests on complete labels are needed.
needed. Unassigned code points cannot be permitted because one Unassigned code points cannot be permitted because one cannot
cannot determine whether particular code points will require determine whether particular code points will require contextual
contextual rules (and what those rules should be) before rules (and what those rules should be) before characters are
characters are assigned to them and the properties of those assigned to them and the properties of those characters fully
characters fully understood. Second, Unicode specifies that an understood. Second, Unicode specifies that an unassigned code
unassigned code point normalizes and case folds to itself. If the point normalizes and case folds to itself. If the code point is
code point is later assigned to a character, and particularly if later assigned to a character, and particularly if the newly-
the newly-assigned code point has a combining class that assigned code point has a combining class that determines its
determines its placement relative to other combining characters, placement relative to other combining characters, it could
it could normalize to some other code point or sequence, creating normalize to some other code point or sequence, creating confusion
confusion and/or violating other rules listed here. and/or violating other rules listed here.
o Any character that is mapped to another character by Nameprep2003 o Any character that is mapped to another character by Nameprep2003
or by a current version of NFKC is prohibited as input to IDNA or by a current version of NFKC is prohibited as input to IDNA
(for either registration or resolution). Implementers of user (for either registration or lookup). Implementers of user
interfaces to applications are free to make those conversions when interfaces to applications are free to make those conversions when
they consider them suitable for their operating system they consider them suitable for their operating system
environments, context, or users. environments, context, or users.
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 (described in more
precise form in [IDNA2008-Tables]). The principles are not just an precise form in [IDNA2008-Tables]). The principles are not just an
interpretation of the tables. interpretation of the tables.
10.1.2. Labels in Registration 9.2.1.2. 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, zone administrators are expected to verify that
names meet "hostname" [RFC0952] or special service location formats names meet "hostname" [RFC0952] or special service location formats
[RFC2782] where necessary for the expected applications. For zones [RFC2782] where necessary for the expected applications. For zones
that will contain IDNs, support for Unicode version-independence that will contain IDNs, support for Unicode version-independence
requires restrictions on all strings placed in the zone. In requires restrictions on all strings placed in the zone. In
particular, for such zones: 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., that they MUST be
valid A-labels, as discussed in Section 3 above. valid 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 described above), MUST be
consistent on the systems performing or validating labels to be consistent on the systems performing or validating labels to be
registered. Note that this does not require that tables reflect registered. Note that this does not require that tables reflect
the latest version of Unicode, only that all tables used on a the latest version of Unicode, only that all tables used on a
given system are consistent with each other. given system are consistent with each other.
[[anchor33: Note in draft: the above text was changed significantly [[anchor31: Note in draft: the above text was changed significantly
between -00 and -01 to clearly restrict its scope to zones supporting between -00 and -01 to clearly restrict its scope to zones supporting
IDNA and to eliminate comments about labels containing "--" in the IDNA and to eliminate comments about labels containing "--" in the
third and forth positions but with different prefixes. There appears third and forth positions but with different prefixes. There appears
to be consensus that more extensive rules belong in a "best to be consensus that more extensive rules belong in a "best
practices" document about appropriate DNS labels, but that document practices" document about appropriate DNS labels, but that document
is not in-scope for the IDNABIS WG.]] 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 registration side is also responsible --under the
protocol and to registrants and users-- for much more careful protocol and to registrants and users-- for much more careful
checking than is expected of applications systems that look names up, checking than is expected of applications systems that look names up,
both checking as required by the protocol and checking required by both checking as required by the protocol and checking required by
whatever policies it develops for minimizing risks due to confusable whatever policies it develops for minimizing risks due to confusable
characters and sequences and preserving language or script integrity. characters and sequences and preserving language or script integrity.
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.
10.1.3. Labels in Resolution (Lookup) 9.2.1.3. Labels in Lookup
Anyone looking up a label in a DNS zone Anyone looking up a label in a DNS zone
o MUST maintain a consistent set of tables, as discussed above. As o MUST maintain a consistent set of tables, as discussed above. As
with registration, the tables need not reflect the latest version with registration, the tables need not reflect the latest version
of Unicode but they MUST be consistent. of Unicode but they MUST be consistent.
o MUST validate the characters in labels to be looked up only to the o MUST validate the characters in labels to be looked up only to the
extent of determining that the U-label does not contain either 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
skipping to change at page 32, line 10 skipping to change at page 32, line 46
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 MUST NOT validate 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 resolver 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 names up in the DNS will be able An application or client that looks processes names according to this
to resolve any name that is validly registered, as long as its protocol and then resolves them in the DNS will be able to locate any
version of the Unicode-associated tables is sufficiently up-to-date name that is validly registered, as long as its version of the
to interpret all of the characters in the label. It SHOULD Unicode-associated tables is sufficiently up-to-date to interpret all
distinguish, in its messages to users, between "label contains an of the characters in the label. It SHOULD distinguish, in its
unallocated code point" and other types of lookup failures. A messages to users, between "label contains an unallocated code point"
failure on the basis of an old version of Unicode may lead the user and other types of lookup failures. A failure on the basis of an old
to a desire to upgrade to a newer version, but will have no other ill version of Unicode may lead the user to a desire to upgrade to a
effects (this is consistent with behavior in the transition to the newer version, but will have no other ill effects (this is consistent
DNS when some hosts could not yet handle some forms of names or with behavior in the transition to the DNS when some hosts could not
record types). yet handle some forms of names or record types).
10.2. More Flexibility in User Agents 9.2.2. 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 prohibits 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, or other rules. As examples, while mathematical characters
based on Latin ones are accepted as input to IDNA2003, they are based on Latin ones are accepted as input to IDNA2003, they are
prohibited in IDNA2008. Similarly, double-width characters and other prohibited in IDNA2008. Similarly, double-width characters and other
variations are prohibited as IDNA input. variations are prohibited as IDNA input.
Since the rules in [IDNA2008-Tables] provide that only strings that Since the rules in [IDNA2008-Tables] provide that only strings that
are stable under NFKC are valid, if it is convenient for an are stable under NFKC are valid, if it is convenient for an
application to perform NFKC normalization before lookup, that application to perform NFKC normalization before lookup, that
operation is safe since this will never make the application unable operation is safe since this will never make the application unable
to look up any valid string. to look up any valid string.
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 at resolution time. It is perfectly reasonable for user can type as input to the lookup process. It is perfectly
systems that support user interfaces to perform some character reasonable for systems that support user interfaces to perform some
mapping that is appropriate to the local environment. This would character mapping that is appropriate to the local environment. This
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
skipping to change at page 33, line 37 skipping to change at page 34, line 27
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. 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 mapping 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.
10.3. The Question of Prefix Changes 9.2.3. 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 "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.
10.3.1. Conditions Requiring a Prefix Change 9.2.3.1. Conditions Requiring a Prefix Change
An IDN prefix change is needed if a given string would resolve or An IDN prefix change is needed if a given string would be looked up
otherwise be 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.
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 10.3.2, below. apply. See the first item in Section 9.2.3.2, below.
3. A fundamental change is made to the semantics of the string that 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.
10.3.2. Conditions Not Requiring a Prefix Change 9.2.3.2. 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 Stringprep tables or IDNA actions, including
normalization definitions, that affect characters that were normalization definitions, that affect characters that were
already invalid under IDNA2003. already invalid under IDNA2003.
3. Changes in the style of definitions of Stringprep or Nameprep 3. Changes in the style of definitions of Stringprep or Nameprep
that do not alter the actions performed by them. that do not alter the actions performed by them.
Of course, because these specifications do not involve changes to Of course, because these specifications do not involve changes to
Stringprep or Nameprep, the third condition above and part of the Stringprep or Nameprep, the third condition above and part of the
second are moot. second are moot.
10.3.3. Implications of Prefix Changes 9.2.3.3. 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, resolve to confusion and attacks, since they could, in principle, map to
different targets. 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.
10.4. Stringprep Changes and Compatibility 9.2.4. Stringprep Changes and Compatibility
Concerns have been expressed about problems for non-DNS uses of Concerns have been expressed about problems for non-DNS uses of
Stringprep being caused by changes to the specification intended to Stringprep being caused by changes to the specification intended to
improve the handling of IDNs, most notably as this might affect improve the handling of IDNs, most notably as this might affect
identification and authentication protocols. Section 10.3, above, identification and authentication protocols. Section 9.2.3, above,
essentially also applies in this context. The proposed new inclusion essentially also applies in this context. The proposed new inclusion
tables [IDNA2008-Tables], the reduction in the number of characters tables [IDNA2008-Tables], the reduction in the number of characters
permitted as input for registration or resolution (Section 6), and permitted as input for registration or lookup (Section 5), and even
even the proposed changes in handling of right to left strings 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. Strings
that are valid under both IDNA2003 and IDNA2008, and the that are valid under both IDNA2003 and IDNA2008, and the
corresponding versions of Stringprep, are not changed in corresponding versions of Stringprep, are not changed in
interpretation. This protocol does not use either Nameprep or interpretation. This protocol does not use either Nameprep or
Stringprep as specified in IDNA2003. 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
skipping to change at page 36, line 15 skipping to change at page 37, line 5
different requirements than IDNs. different requirements than IDNs.
Perhaps even more important in practice, since most other known uses Perhaps even more important in practice, since most other known uses
of Stringprep encode or process characters that are already in of Stringprep encode or process characters that are already in
normalized form and expect the use of only those characters that can normalized form and expect the use of only those characters that can
be used in writing words of languages, the changes proposed here and be used in writing words of languages, the changes proposed here and
in [IDNA2008-Tables] are unlikely to have any effect at all, in [IDNA2008-Tables] are unlikely to have any effect at all,
especially not on registries and registrations that follow rules especially not on registries and registrations that follow rules
already in existence when this work started. already in existence when this work started.
10.5. The Symbol Question 9.2.5. 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:
skipping to change at page 36, line 47 skipping to change at page 37, line 37
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.
[[anchor35: The above paragraph remains controversial as to [[anchor33: 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 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
skipping to change at page 37, line 30 skipping to change at page 38, line 19
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 and describing a label containing a heart symbol is hopelessly
ambiguous. In cities where "Square" is a popular part of a ambiguous. In cities where "Square" is a popular part of a
location name, one might well want to use a square symbol in a location name, one might well want to use a square symbol in a
label as well and there are far more squares of various flavors in label as well and there are far more squares of various flavors in
Unicode than there are hearts or stars. 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 communication. Of course, these difficulties with reliable communication. Consistent with this conclusion, the
symbols do not arise with actual pictographic languages and Unicode standard recommends that strings used in identifiers not
scripts which would be treated like any other language characters; contain symbols or punctuation [Unicode-UAX31]. Of course, these
the two should not be confused. difficulties with symbols do not arise with actual pictographic
languages and scripts which would be treated like any other
language characters; the two should not be confused.
[[anchor36: Note in Draft: Should the above section be significantly [[anchor34: Note in Draft: Should the above section be significantly
trimmed or eliminated?]] trimmed or eliminated?]]
10.6. Migration Between Unicode Versions: Unassigned Code Points 9.2.6. Migration Between Unicode Versions: Unassigned Code Points
In IDNA2003, labels containing unassigned code points are resolved on In IDNA2003, labels containing unassigned code points are looked up
the theory that, if they appear in labels and can be resolved, the on the theory that, if they appear in labels and can be mapped and
relevant standards must have changed and the registry has properly then resolved, the relevant standards must have changed and the
allocated only assigned values. registry has properly allocated only assigned values.
In this specification, strings containing unassigned code points MUST In this specification, strings containing unassigned code points MUST
NOT be either looked up or registered. There are several reasons for NOT be either looked up or registered. There are several reasons for
this, with the most important ones being: this, 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
skipping to change at page 38, line 32 skipping to change at page 39, line 25
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).
10.7. Other Compatibility Issues 9.2.7. 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.
11. Acknowledgments 10. 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 39, line 34 skipping to change at page 40, line 25
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 Special thanks are due to Paul Hoffman for permission to extract
material from his Internet-Draft to form the basis for Section 2. 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. that group. Marcos Sanz provided specific analysis and suggestions
that were exceptionally helpful in refining the text.
12. Contributors 11. 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
DNS labels and fully-qualified domain names provide mnemonics that
assist in identifying and referring to resources on the Internet.
IDNs expand the range of those mnemonics to include those based on
languages and character sets other than Western European and Roman-
derived ones. But domain "names" are not, in general, words in any
language. The recommendations of the IETF policy on character sets
and languages, BCP 18 [RFC2277] are applicable to situations in which
language identification is used to provide language-specific
contexts. The DNS is, by contrast, global and international and
ultimately has nothing to do with languages. Adding languages (or
similar context) to IDNs generally, or to DNS matching in particular,
would imply context dependent matching in DNS, which would be a very
significant change to the DNS protocol itself. It would also imply
that users would need to identify the language associated with a
particular label in order to look that label up, a decision that
would be impossible in many or most cases.
13. IANA Considerations 13. 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 definition of the first one appears in [IDNA2008-Tables]. actual definitions of the first two appear in [IDNA2008-Tables].
13.1. IDNA Character Registry 13.1. 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 13.2. 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 6.1.1.1), IANA will create and maintain a rule described in Section 5.1.1.1), IANA will create and maintain a
list of approved contextual rules. Additions or changes to these list of approved contextual rules. The details for those rules
rules require IETF Review, as described in [RFC5226]. appear in [IDNA2008-Tables].
[[anchor41: Note in Draft: This section was changed between -00 and
-01 based on list discussion. Consensus needs to be verified for
that decision.]]
A table from which that registry can be initialized, and some further
discussion, appears in [RulesInit].
[[anchor42: This subsection should probably be moved to Tables along
with the Contextual rules themselves (from Protocol) when the move is
made.]]
13.3. IANA Repository of IDN Practices of TLDs 13.3. 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.
skipping to change at page 41, line 42 skipping to change at page 42, line 44
restrictions (subject to the limitations identified elsewhere in this restrictions (subject to the limitations identified elsewhere in this
document) that try to minimize characters that have similar document) that try to minimize characters that have similar
appearance or similar interpretations. It is worth noting that there appearance or similar interpretations. It is worth noting that there
are no comprehensive technical solutions to the problems of are no comprehensive technical solutions to the problems of
confusable characters. One can reduce the extent of the problems in confusable characters. One can reduce the extent of the problems in
various ways, but probably never eliminate it. Some specific various ways, but probably never eliminate it. Some specific
suggestions about identification and handling of confusable suggestions about identification and handling of confusable
characters appear in a Unicode Consortium publication characters appear in a Unicode Consortium publication
[Unicode-UTR36]. [Unicode-UTR36].
The registration and resolution models described above and in The registration and lookup models described above and in
[IDNA2008-Protocol] change the mechanisms available for applications [IDNA2008-Protocol] change the mechanisms available for lookup
and resolvers to determine the validity of labels they encounter. In applications to determine the validity of labels they encounter. In
some respects, the ability to test is strengthened. For example, some respects, the ability to test is strengthened. For example,
putative labels that contain unassigned code points will now be putative labels that contain unassigned code points will now be
rejected, while IDNA2003 permitted them (something that is now rejected, while IDNA2003 permitted them (something that is now
recognized as a considerable source of risk). On the other hand, the recognized as a considerable source of risk). On the other hand, the
protocol specification no longer assumes that the application that protocol specification no longer assumes that the application that
looks up a name will be able to determine, and apply, information looks up a name will be able to determine, and apply, information
about the protocol version used in registration. In theory, that may about the protocol version used in registration. In theory, that may
increase risk since the application will be able to do less pre- increase risk since the application will be able to do less pre-
lookup validation. In practice, the protection afforded by that test lookup validation. In practice, the protection afforded by that test
has been largely illusory for reasons explained in RFC 4690 and has been largely 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. Our
current hypothesis is that the same considerations that would require current hypothesis is that the same considerations that would require
changing the IDN prefix (see Section 10.3.2) are the ones that would, changing the IDN prefix (see Section 9.2.3.2) are the ones that
e.g., invalidate certificates or hashes that depend on Stringprep, would, e.g., invalidate certificates or hashes that depend on
but those cases require careful consideration and evaluation. More Stringprep, but those cases require careful consideration and
important, it is not necessary to change Stringprep2003 at all in evaluation. More important, it is not necessary to change
order to make the IDNA changes contemplated here. It is far Stringprep2003 at all in order to make the IDNA changes contemplated
preferable to create a separate document, or separate profile here. It is far preferable to create a separate document, or
components, for IDN work, leaving the question of upgrading to other separate profile components, for IDN work, leaving the question of
protocols to experts on them and eliminating any possible upgrading to other protocols to experts on them and eliminating any
synchronization dependency between IDNA changes and possible upgrades possible synchronization dependency between IDNA changes and possible
to security protocols or conventions. upgrades to security protocols or conventions.
No mechanism involving names or identifiers alone can protect a wide No mechanism involving names or identifiers alone can protect a wide
variety of security threats and attacks that are largely independent variety of security threats and attacks that are largely independent
of them including spoofed pages, DNS query trapping and diversion, of them including spoofed pages, DNS query trapping and diversion,
and so on. and so on.
15. Change Log 15. Change Log
[[anchor45: RFC Editor: Please remove this section.]] [[anchor42: RFC Editor: Please remove this section.]]
For version 00 of draft-ietf-idnabis-rationale, this list contains a
complete trace going back through the earlier, design team, drafts.
Material earlier than that described in Section 15.9 will be removed
in WG draft -02.
15.1. Version -01 of draft-klensin-idnabis-issues
Version -01 of this document is a considerable rewrite from -00.
Many sections have been clarified or extended and several new
sections have been added to reflect discussions in a number of
contexts since -00 was issued.
15.2. Version -02 of draft-klensin-idnabis-issues
o Corrected several editorial errors including an accidentally-
introduced misstatement about NFKC.
o Extensively revised the document to synchronize its terminology
with version 03 of [IDNA2008-Tables] and to provide a better
conceptual framework for its categories and how they are used.
Added new material to clarify terminology and relationships with
other efforts. More subtle changes in this version lay the
groundwork for separating the document into a conceptual overview
and a protocol specification for version 03.
15.3. Version -03 of draft-klensin-idnabis-issues
o Removed protocol materials to a separate document and incorporated
rationale and explanation materials from the original
specification in RFC 3960 into this document. Cleaned up earlier
text to reflect a more mature specification and restructured
several sections and added additional rationale material.
o Strengthened and clarified the A-label / U-label/ LDH-label
definition.
o Retitled the document to reflect its evolving role.
15.4. Version -04 of draft-klensin-idnabis-issues
o Moved more text from "protocol" and further reorganized material.
o Provided new material on "Contextual Rule Required.
o Improved consistency of terminology, both internally and with the
"tables" document.
o Improved the IANA Considerations section and discussed the
existing IDNA-related registry.
o More small changes to increase consistency.
15.5. Version -05 of draft-klensin-idnabis-issues
Changed "YES" category back to "ALWAYS" to re-synch with the tables
document and provide clearer terminology.
15.6. Version -06 of draft-klensin-idnabis-issues
o Clarified the prohibitions on strings that look like A-labels but
are not and on unassigned code points.
o Clarified length restrictions on IDN labels.
o Revised the terminology definitions to remove the impression of
circularity and removed invocations of ToASCII and ToUnicode,
which do not exist in IDNA2008.
o Added a new section on front-end processing.
o Added a new section to discuss case-mapping.
o Extended the discussion of prefix changes to identify the
implications of making one.
o Several more editorial improvements, corrected references, and
similar adjustments.
15.7. Version -07 of draft-klensin-idnabis-issues
o Added material that specifically defines the format of contextual
rules.
o Added and altered text after discussions at the 30 January meeting
(see Section 11) and the follow-up to those discussions. Among
the key decisions at that meeting were to eliminate the
distinction among the valid categories (formerly "ALWAYS", "MAYBE
YES", and "MAYBE NO"), to adjust the terminology accordingly, and
to change "CONTEXTUAL RULE REQUIRED" from a separate category in
this document and the protocol one to a modifier of what is now
called "PROTOCOL-VALID". The consequent changes resulted in
removal of several sections of explanation from this document.
o Resynchronized terminology with "protocol" and "tables" documents.
o More editorial and typographic corrections.
15.8. Version -00 of draft-ietf-idnabis-rationale
o Rewrote the abstract and introduction, and retuned the title, to
be more consistent with WG work and activities. Changed the file
name to reflect WG naming.
o Removed most of the material that explained, or compared this
approach to, IDNA2003. Some of this material may appear in the
non-WG "IDNA-alternatives" draft if it is ever completed.
o Changed IDNA200X in terminology and references to IDNA2008.
o Added a contextual rule for hyphen to the appendix, adjusted the
rule syntax slightly, and supplied draft regular expression rules.
o Responded to comments produced during the WG charter discussions
and from several individuals. In general, comments requesting a
reorganization of the collection of documents have not been
responded to pending a WG decision on that topic.
o Moved the contextual rule appendix out of here and into
"Protocol". It may not belong there either, but definitely does
not belong here, and was holding up getting this document out.
o Many small editorial improvements, including reorganization of
some material.
Editorial note: While several sections have been removed from this
version, the WG should discuss whether further cuts are desirable,
e.g., whether Section 7.3, Section 7.4, or Section 10.3 provide
enough value to be worth retaining? Can Section 10.4 be trimmed
without loss of useful information and, if so, how? Section 10.7
appears critical of IDNA2003 in undesirable ways: should it be
dropped or do people have suggestions about how to improve it?
Strong opinions have been expressed that Section 10.5 should be
trimmed significantly or removed entirely. The WG will need to
discuss that too. Are there other materials that should be trimmed
out?
15.9. Version -01 of draft-ietf-idnabis-rationale 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 o Clarified the U-label definition to note that U-labels must
contain at least one non-ASCII character. Also clarified the contain at least one non-ASCII character. Also clarified the
relationship among label types. relationship among label types.
o Rewrote the discussion of Labels in Registration (Section 10.1.2) 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 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 remove more general restrictions. Added a temporary note in line
to explain the situation. to explain the situation.
o Changed the "IDNA uses Unicode" statement to focus on o Changed the "IDNA uses Unicode" statement to focus on
compatibility with IDNA2003 and avoid more general or compatibility with IDNA2003 and avoid more general or
controversial assertions. controversial assertions.
o Added a discussion of examples to Section 10.1 o Added a discussion of examples to Section 9.2.1
o Made a number of other small editorial changes and corrections o Made a number of other small editorial changes and corrections
suggested by Mark Davis. suggested by Mark Davis.
o Added several more discussion anchors and notes and expanded or o Added several more discussion anchors and notes and expanded or
updated some existing ones. 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.
16. References 16. References
16.1. Normative References 16.1. 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, <http://www.ietf.org/ right to left scripts", July 2008, <https://
internet-drafts/draft-ietf-idnabs-bidi-01.txt>. datatracker.ietf.org/drafts/draft-ietf-idnabs-bidi/>.
[IDNA2008-Protocol] [IDNA2008-Protocol]
Klensin, J., "Internationalized Domain Names in Klensin, J., "Internationalized Domain Names in
Applications (IDNA): Protocol", July 2008, <http:// Applications (IDNA): Protocol", July 2008, <https://
www.ietf.org/internet-drafts/ datatracker.ietf.org/drafts/draft-ietf-idnabis-protocol/>.
draft-ietf-idnabis-protocol-02.txt>.
[IDNA2008-Tables] [IDNA2008-Tables]
Faltstrom, P., "The Unicode Code Points and IDNA", Faltstrom, P., "The Unicode Code Points and IDNA",
May 2008, <http://www.ietf.org/internet-drafts/ July 2008, <https://datatracker.ietf.org/drafts/
draft-ietf-idnabis-tables-01.txt>. 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-01.html draft-ietf-idnabis-tables-02.html
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997. Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3454] Hoffman, P. and M. Blanchet, "Preparation of [RFC3454] Hoffman, P. and M. Blanchet, "Preparation of
Internationalized Strings ("stringprep")", RFC 3454, Internationalized Strings ("stringprep")", RFC 3454,
December 2002. 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)",
skipping to change at page 47, line 17 skipping to change at page 45, line 43
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226, IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008. May 2008.
[RulesInit] [RulesInit]
Klensin, J., "Internationalizing Domain Names in Klensin, J., "Internationalizing Domain Names in
Applications (IDNA): Protocol, Appendix A Contextual Rules Applications (IDNA): Protocol, Appendix A Contextual Rules
Table", July 2008, <http://www.ietf.org/internet-drafts/ Table", July 2008, <http://www.ietf.org/internet-drafts/
draft-ietf-idnabis-protocol-02.txt>. draft-ietf-idnabis-protocol-02.txt>.
[Unicode-UAX15]
The Unicode Consortium, "Unicode Standard Annex #15:
Unicode Normalization Forms", March 2008,
<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 16.2. Informative References
skipping to change at page 48, line 9 skipping to change at page 46, line 39
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities", [RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, November 1987. STD 13, RFC 1034, November 1987.
[RFC1035] Mockapetris, P., "Domain names - implementation and [RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987. specification", STD 13, RFC 1035, November 1987.
[RFC1123] Braden, R., "Requirements for Internet Hosts - Application [RFC1123] Braden, R., "Requirements for Internet Hosts - Application
and Support", STD 3, RFC 1123, October 1989. and Support", STD 3, RFC 1123, October 1989.
[RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS
Specification", RFC 2181, July 1997.
[RFC2277] Alvestrand, H., "IETF Policy on Character Sets and
Languages", BCP 18, RFC 2277, January 1998.
[RFC2673] Crawford, M., "Binary Labels in the Domain Name System",
RFC 2673, August 1999.
[RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for [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.
[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
skipping to change at page 48, line 33 skipping to change at page 47, line 25
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-UAX31]
The Unicode Consortium, "Unicode Standard Annex #31:
Unicode Identifier and Pattern Syntax", March 2008,
<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", August 2006, Unicode Security Considerations", July 2008,
<http://www.unicode.org/reports/tr36/>. <http://www.unicode.org/reports/tr36/>.
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
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