Network Working Group J. Klensin
Internet-Draft May 8, 2009
Obsoletes: 3490, 3491
(if approved)
Updates: 3492 (if approved)
Intended status: Standards Track
Expires: November 9, 2009
Internationalized Domain Names in Applications (IDNA): Protocol
draft-ietf-idnabis-protocol-12.txt
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Abstract
This document is the revised protocol definition for
internationalized domain names (IDNs). The rationale for changes,
the relationship to the older specification, and important
terminology are provided in other documents. This document specifies
the protocol mechanism, called Internationalizing Domain Names in
Applications (IDNA), for registering and looking up IDNs in a way
that does not require changes to the DNS itself. IDNA is only meant
for processing domain names, not free text.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.1. Discussion Forum . . . . . . . . . . . . . . . . . . . . . 5
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Requirements and Applicability . . . . . . . . . . . . . . . . 6
3.1. Requirements . . . . . . . . . . . . . . . . . . . . . . . 6
3.2. Applicability . . . . . . . . . . . . . . . . . . . . . . 6
3.2.1. DNS Resource Records . . . . . . . . . . . . . . . . . 7
3.2.2. Non-domain-name Data Types Stored in the DNS . . . . . 7
4. Registration Protocol . . . . . . . . . . . . . . . . . . . . 7
4.1. Input to IDNA Registration Process . . . . . . . . . . . . 8
4.2. Permitted Character and Label Validation . . . . . . . . . 8
4.2.1. Input Format . . . . . . . . . . . . . . . . . . . . . 8
4.2.2. Rejection of Characters that are not Permitted . . . . 8
4.2.3. Label Validation . . . . . . . . . . . . . . . . . . . 8
4.2.4. Registration Validation Summary . . . . . . . . . . . 9
4.3. Registry Restrictions . . . . . . . . . . . . . . . . . . 10
4.4. Punycode Conversion . . . . . . . . . . . . . . . . . . . 10
4.5. Insertion in the Zone . . . . . . . . . . . . . . . . . . 10
5. Domain Name Lookup Protocol . . . . . . . . . . . . . . . . . 10
5.1. Label String Input . . . . . . . . . . . . . . . . . . . . 11
5.2. Conversion to Unicode . . . . . . . . . . . . . . . . . . 11
5.3. Character Changes in Preprocessing or the User
Interface . . . . . . . . . . . . . . . . . . . . . . . . 11
5.4. A-label Input . . . . . . . . . . . . . . . . . . . . . . 12
5.5. Validation and Character List Testing . . . . . . . . . . 13
5.6. Punycode Conversion . . . . . . . . . . . . . . . . . . . 14
5.7. DNS Name Resolution . . . . . . . . . . . . . . . . . . . 14
6. Security Considerations . . . . . . . . . . . . . . . . . . . 14
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
8. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 15
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 15
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 16
10.1. Normative References . . . . . . . . . . . . . . . . . . . 16
10.2. Informative References . . . . . . . . . . . . . . . . . . 17
Appendix A. Local Mapping Alternatives . . . . . . . . . . . . . 18
A.1. Transitional Mapping Model . . . . . . . . . . . . . . . . 18
A.1.1. Fallback Lookup . . . . . . . . . . . . . . . . . . . 19
A.1.2. Two-step Lookup . . . . . . . . . . . . . . . . . . . 19
A.2. Internationalized Resource Identifier (IRI) Mapping
Model . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Appendix B. Summary of Major Changes from IDNA2003 . . . . . . . 21
Appendix C. Change Log . . . . . . . . . . . . . . . . . . . . . 21
C.1. Changes between Version -00 and -01 of
draft-ietf-idnabis-protocol . . . . . . . . . . . . . . . 21
C.2. Version -02 . . . . . . . . . . . . . . . . . . . . . . . 22
C.3. Version -03 . . . . . . . . . . . . . . . . . . . . . . . 22
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C.4. Version -04 . . . . . . . . . . . . . . . . . . . . . . . 22
C.5. Version -05 . . . . . . . . . . . . . . . . . . . . . . . 22
C.6. Version -06 . . . . . . . . . . . . . . . . . . . . . . . 23
C.7. Version -07 . . . . . . . . . . . . . . . . . . . . . . . 23
C.8. Version -08 . . . . . . . . . . . . . . . . . . . . . . . 23
C.9. Version -09 . . . . . . . . . . . . . . . . . . . . . . . 24
C.10. Version -10 . . . . . . . . . . . . . . . . . . . . . . . 24
C.11. Version -11 . . . . . . . . . . . . . . . . . . . . . . . 24
C.12. Version -12 . . . . . . . . . . . . . . . . . . . . . . . 25
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 25
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1. Introduction
This document supplies the protocol definition for internationalized
domain names. Essential definitions and terminology for
understanding this document and a road map of the collection of
documents that make up IDNA2008 appear in [IDNA2008-Defs].
Appendix B discusses the relationship between this specification and
the earlier version of IDNA (referred to here as "IDNA2003") and the
rationale for these changes, along with considerable explanatory
material and advice to zone administrators who support IDNs is
provided in another documents, notably [IDNA2008-Rationale].
IDNA works by allowing applications to use certain ASCII string
labels (beginning with a special prefix) to represent non-ASCII name
labels. Lower-layer protocols need not be aware of this; therefore
IDNA does not changes any infrastructure. In particular, IDNA does
not depend on any changes to DNS servers, resolvers, or protocol
elements, because the ASCII name service provided by the existing DNS
can be used for IDNA.
IDNA applies only to DNS labels. The base DNS standards [RFC1034]
[RFC1035] and their various updates specify how to combine labels
into fully-qualified domain names and parse labels out of those
names.
This document describes two separate protocols, one for IDN
registration (Section 4) and one for IDN lookup (Section 5), that
share some terminology, reference data and operations. [[anchor2:
Note in draft: See the note in the introduction to.]]Section 5
1.1. Discussion Forum
[[anchor4: RFC Editor: please remove this section.]]
This work is being discussed in the IETF IDNABIS WG and on the
mailing list idna-update@alvestrand.no
2. Terminology
Terminology used in IDNA, but also in Unicode or other character set
standards and the DNS, appears in [IDNA2008-Defs]. Terminology that
is required as part of the IDNA definition, including the definitions
of "ACE", appears in that document as well. Readers of this document
are assumed to be familiar with [IDNA2008-Defs] and with the DNS-
specific terminology in RFC 1034 [RFC1034].
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
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"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in BCP 14, RFC 2119
[RFC2119].
3. Requirements and Applicability
3.1. Requirements
IDNA makes the following requirements:
1. Whenever a domain name is put into an IDN-unaware domain name
slot (see Section 2 and [IDNA2008-Defs]), it MUST contain only
ASCII characters (i.e., must be either an A-label or an NR-LDH-
label), unless the DNS application is not subject to historical
recommendations for "hostname"-style names (see [RFC1034] and
Section 3.2.1).
2. Labels MUST be compared using equivalent forms: either both
A-Label forms or both U-Label forms. Because A-labels and
U-labels can be transformed into each other without loss of
information, these comparisons are equivalent. A pair of
A-labels MUST be compared as case-insensitive ASCII (as with all
comparisons of ASCII DNS labels). U-labels must be compared
as-is, without case-folding or other intermediate steps. Note
that it is not necessary to validate labels in order to compare
them. In many cases, validation may be important for other
reasons and SHOULD be performed.
3. Labels being registered MUST conform to the requirements of
Section 4. Labels being looked up and the lookup process MUST
conform to the requirements of Section 5.
3.2. Applicability
IDNA applies to all domain names in all domain name slots in
protocols except where it is explicitly excluded. It does not apply
to domain name slots which do not use the Letter/Digit/Hyphen (LDH)
syntax rules.
Because it uses the DNS, IDNA applies to many protocols that were
specified before it was designed. IDNs occupying domain name slots
in those older protocols MUST be in A-label form until and unless
those protocols and implementations of them are explicitly upgraded
to be aware of IDNs in Unicode. IDNs actually appearing in DNS
queries or responses MUST be A-labels.
IDNA is not defined for extended label types (see RFC 2671, Section 3
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[RFC2671]).
3.2.1. DNS Resource Records
IDNA applies only to domain names in the NAME and RDATA fields of DNS
resource records whose CLASS is IN. See RFC 1034 [RFC1034] for
precise definitions of these terms.
The application of IDNA to DNS resource records depends entirely on
the CLASS of the record, and not on the TYPE except as noted below.
This will remain true, even as new types are defined, unless a new
type defines type-specific rules. Special naming conventions for SRV
records (and "underscore names" more generally) are incompatible with
IDNA coding. The first two labels on a SRV type record (the ones
required to start in "_") MUST NOT be A-labels or U-labels, because
conversion to an A-label would lose information (since the underscore
is not a letter, digit, or hyphen and is consequently DISALLOWED in
IDNs). Of course, those labels may be part of a domain that uses IDN
labels at higher levels in the tree.
3.2.2. Non-domain-name Data Types Stored in the DNS
Although IDNA enables the representation of non-ASCII characters in
domain names, that does not imply that IDNA enables the
representation of non-ASCII characters in other data types that are
stored in domain names, specifically in the RDATA field for types
that have structured RDATA format. For example, an email address
local part is stored in a domain name in the RNAME field as part of
the RDATA of an SOA record (hostmaster@example.com would be
represented as hostmaster.example.com). IDNA does not update the
existing email standards, which allow only ASCII characters in local
parts. Even though work is in progress to define
internationalization for email addresses [RFC4952], changes to the
email address part of the SOA RDATA would require action in, or
updates to, other standards, specifically those that specify the
format of the SOA RR.
4. Registration Protocol
This section defines the procedure for registering an IDN. The
procedure is implementation independent; any sequence of steps that
produces exactly the same result for all labels is considered a valid
implementation.
Note that, while the registration and lookup protocols (Section 5)
are very similar in most respects, they are different and
implementers should carefully follow the appropriate steps.
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4.1. Input to IDNA Registration Process
Registration processes, especially processing by entities, such as
"registrars" who deal with registrants before the request actually
reaches the zone manager ("registry") are outside the scope of these
protocols and may differ significantly depending on local needs. By
the time a string enters the IDNA registration process as described
in this specification, it is expected to be in Unicode and MUST be in
Unicode Normalization Form C (NFC [Unicode-UAX15]). Entities
responsible for zone files ("registries") are expected to accept only
the exact string for which registration is requested, free of any
mappings or local adjustments. They SHOULD avoid any possible
ambiguity by accepting registrations only for A-labels, possibly
paired with the relevant U-labels so that they can verify the
correspondence.
4.2. Permitted Character and Label Validation
4.2.1. Input Format
The registry SHOULD permit submission of labels in A-label form and
is encouraged to accept both the A-label form and the U-label one.
If it does so, it MUST perform a conversion to a U-label, perform the
steps and tests described below, and verify that the A-label produced
by the step in Section 4.4 matches the one provided as input. In
addition, if a U-label was provided, that U-label and the one
obtained by conversion of the A-label MUST match exactly. If, for
some reason, these tests fail, the registration MUST be rejected. If
the conversion to a U-label is not performed, the registry MUST still
verify that the A-label is superficially valid, i.e., that it does
not violate any of the rules of Punycode [RFC3492] encoding such as
the prohibition on trailing hyphen-minus, appearance of non-basic
characters before the delimiter, and so on. Fake A-labels, i.e.,
invalid strings that appear to be A-labels but are not, MUST NOT be
placed in DNS zones that support IDNA.
4.2.2. Rejection of Characters that are not Permitted
The candidate Unicode string MUST NOT contain characters in the
"DISALLOWED" and "UNASSIGNED" lists specified in [IDNA2008-Tables].
4.2.3. Label Validation
The proposed label (in the form of a Unicode string, i.e., a string
that at least superficially appears to be a U-label) is then
examined, performing tests that require examination of more than one
character. Character order is considered to be the on-the-wire
order, not the display order.
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4.2.3.1. Consecutive Hyphens
The Unicode string MUST NOT contain "--" (two consecutive hyphens) in
the third and fourth character positions.
4.2.3.2. Leading Combining Marks
The Unicode string MUST NOT begin with a combining mark or combining
character (see The Unicode Standard, Section 2.11 [Unicode] for an
exact definition).
4.2.3.3. Contextual Rules
The Unicode string MUST NOT contain any characters whose validity is
context-dependent, unless the validity is positively confirmed by a
contextual rule. To check this, each code-point marked as CONTEXTJ
and CONTEXTO in [IDNA2008-Tables] MUST have a non-null rule. If such
a code-point is missing a rule, it is invalid. If the rule exists
but the result of applying the rule is negative or inconclusive, the
proposed label is invalid.
NOTE: These contextual rules are required to support characters that
could be used, under some conditions, to produce misleading labels or
to cause unacceptable ambiguity in label matching and interpretation.
For example, labels containing zero-width characters may be permitted
in context with characters whose presentation forms are significantly
changed by the zero-width characters, while other labels in which
zero-width characters appear may be rejected.
[[anchor11: Note in draft: Should this note be moved to Rationale???
It has no normative consequences here.]]
4.2.3.4. Labels Containing Characters Written Right to Left
If the proposed label contains any characters that are written from
right to left it MUST meet the "bidi" criteria [IDNA2008-BIDI].
4.2.4. Registration Validation Summary
Strings that contain at least one non-ASCII character, have been
produced by the steps above, whose contents pass all of the tests in
Section 4.2, and are 63 or fewer characters long in ACE form (see
Section 4.4), are U-labels.
To summarize, tests are made in Section 4.2 for invalid characters,
invalid combinations of characters, for labels that are invalid even
if the characters they contain are valid individually, and for labels
that do not conform to the restrictions for strings containing right
to left characters.
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4.3. Registry Restrictions
In addition to the rules and tests above, there are many reasons why
a registry could reject a label. Registries at all levels of the
DNS, not just the top level, establish policies about label
registrations. Policies are likely to be informed by the local
languages and may depend on many factors including what characters
are in the label (for example, a label may be rejected based on other
labels already registered). See [IDNA2008-Rationale] for a
discussion and recommendations about registry policies.
The string produced by the above steps is checked and processed as
appropriate to local registry restrictions. Application of those
registry restrictions may result in the rejection of some labels or
the application of special restrictions to others.
4.4. Punycode Conversion
The resulting U-label is converted to an A-label. The A-label, more
precisely defined elsewhere, is the encoding of the U-label according
to the Punycode algorithm [RFC3492] with the ACE prefix "xn--" added
at the beginning of the string. The resulting string must, of
course, conform to the length limits imposed by the DNS. This
document updates RFC 3492 only to the extent of replacing the
reference to the discussion of the ACE prefix. The ACE prefix is now
specified in this document rather than as part of RFC 3490 or
Nameprep [RFC3491] but is the same in both sets of documents.
The failure conditions identified in the Punycode encoding procedure
cannot occur if the input is a U-label as determined by the steps
above.
4.5. Insertion in the Zone
The A-label is registered in the DNS by insertion into a zone.
5. Domain Name Lookup Protocol
Lookup is different from registration and different tests are applied
on the client. Although some validity checks are necessary to avoid
serious problems with the protocol, the lookup-side tests are more
permissive and rely on the assumption that names that are present in
the DNS are valid. That assumption is, however, a weak one because
the presence of wild cards in the DNS might cause a string that is
not actually registered in the DNS to be successfully looked up.
The two steps in Section 5.2 and Section 5.3 are required.
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[[anchor14: Note in Draft: Try to reorganize and renumber Section 5
(Lookup) so that it exactly parallels Section 4 (Registration). This
has not been done in drafts -10 through -12 because the task will be
much easier if the local mapping material is pulled from here (and
there is no point trying to align the section numbers twice).]]
5.1. Label String Input
The user supplies a string in the local character set, typically by
typing it or clicking on, or copying and pasting, a resource
identifier, e.g., a URI [RFC3986] or IRI [RFC3987] from which the
domain name is extracted. Alternately, some process not directly
involving the user may read the string from a file or obtain it in
some other way. Processing in this step and the next two are local
matters, to be accomplished prior to actual invocation of IDNA.
5.2. Conversion to Unicode
The string is converted from the local character set into Unicode, if
it is not already Unicode. A Unicode string may require
normalization as discussed in Section 4.1. The result MUST be a
Unicode string in NFC form.
5.3. Character Changes in Preprocessing or the User Interface
[[anchor15: Note in Draft -12. This entire section is likely to need
to be rewritten when we make final decisions about mapping.]]
The Unicode string MAY then be processed to prevent confounding of
user expectations. For instance, it might be reasonable, at this
step, to convert all upper case characters to lower case, if this
makes sense in the user's environment, but even this should be
approached with caution due to some edge cases: in the long term, it
is probably better for users to understand IDNs strictly in lower-
case, U-label, form. More generally, preprocessing may be useful to
smooth the transition from IDNA2003, especially for direct user
input, but with similar cautions. In general, IDNs appearing in
files and those transmitted across the network as part of protocols
are expected to be in either ASCII form (including A-labels) or to
contain U-labels, rather than being in forms requiring mapping or
other conversions.
Other examples of processing for localization might be applied,
especially to direct user input, at this point. They include
interpreting various characters as separating domain name components
from each other (label separators) because they either look like
periods or are used to separate sentences, mapping halfwidth or
fullwidth East Asian characters to the common form permitted in
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labels, or giving special treatment to characters whose presentation
forms are dependent only on placement in the label. Such
localization changes are also outside the scope of this
specification.
Recommendations for preprocessing for global contexts (i.e., when
local considerations do not apply or cannot be used) and for maximum
interoperability with labels that might have been specified under
liberal readings of IDNA2003 are given in [IDNA2008-Rationale]. It
is important to note that the intent of these specifications is that
labels in application protocols, files, or links are intended to be
in U-label or A-label form. Preprocessing MUST NOT map a character
that is valid in a label as specified elsewhere in this document or
in [IDNA2008-Tables] into another character. Excessively liberal use
of preprocessing, especially to strings stored in files, poses a
threat to consistent and predictable behavior for the user even if
not to actual interoperability.
Because these transformations are local, it is important that domain
names that might be passed between systems (e.g., in IRIs) be
U-labels or A-labels and not forms that might be accepted locally as
a consequence of this step. This step is not standardized as part of
IDNA, and is not further specified here.
5.4. A-label Input
If the input to this procedure appears to be an A-label (i.e., it
starts in "xn--"), the lookup application MAY attempt to convert it
to a U-label and apply the tests of Section 5.5 and the conversion of
Section 5.6 to that form. If the label is converted to Unicode
(i.e., to U-label form) using the Punycode decoding algorithm, then
the processing specified in those two sections MUST be performed, and
the label MUST be rejected if the resulting label is not identical to
the original. See the Name Server Considerations section of
[IDNA2008-Rationale] for additional discussion on this topic.
That conversion and testing SHOULD be performed if the domain name
will later be presented to the user in native character form (this
requires that the lookup application be IDNA-aware). If those steps
are not performed, the lookup process SHOULD at least make tests to
determine that the string is actually an A-label, examining it for
the invalid formats specified in the Punycode decoding specification.
Applications that are not IDNA-aware will obviously omit that
testing; others MAY treat the string as opaque to avoid the
additional processing at the expense of providing less protection and
information to users.
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5.5. Validation and Character List Testing
As with the registration procedure described in Section 4, the
Unicode string is checked to verify that all characters that appear
in it are valid as input to IDNA lookup processing. As discussed
above and in [IDNA2008-Rationale], the lookup check is more liberal
than the registration one. Putative labels with any of the following
characteristics MUST BE rejected prior to DNS lookup:
o Labels containing code points that are unassigned in the version
of Unicode being used by the application, i.e.,in the UNASSIGNED
category of [IDNA2008-Tables].
o Labels that are not in NFC form as defined in [Unicode-UAX15].
o Labels containing prohibited code points, i.e., those that are
assigned to the "DISALLOWED" category in the permitted character
table [IDNA2008-Tables].
o Labels containing code points that are identified in
[IDNA2008-Tables] as "CONTEXTJ", i.e., requiring exceptional
contextual rule processing on lookup, but that do not conform to
that rule. Note that this implies that a rule much be defined,
not null: a character that requires a contextual rule but for
which the rule is null is treated in this step as having failed to
conform to the rule.
o Labels containing code points that are identified in
[IDNA2008-Tables] as "CONTEXTO", but for which no such rule
appears in the table of rules. Applications resolving DNS names
or carrying out equivalent operations are not required to test
contextual rules for "CONTEXTO" characters, only to verify that a
rule is defined (although they MAY make such tests to provide
better protection or give better information to the user).
o Labels whose first character is a combining mark (see
Section 4.2.3.2).
In addition, the application SHOULD apply the following test. The
test may be omitted in special circumstances, such as when the lookup
application knows that the conditions are enforced elsewhere, because
an attempt to look up and resolve such strings will almost certainly
lead to a DNS lookup failure except when wildcards are present in the
zone. However, applying the test is likely to give much better
information about the reason for a lookup failure -- information that
may be usefully passed to the user when that is feasible -- than DNS
resolution failure information alone. In any event, lookup
applications should avoid attempting to resolve labels that are
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invalid under that test.
o Verification that the string is compliant with the requirements
for right to left characters, specified in [IDNA2008-BIDI].
For all other strings, the lookup application MUST rely on the
presence or absence of labels in the DNS to determine the validity of
those labels and the validity of the characters they contain. If
they are registered, they are presumed to be valid; if they are not,
their possible validity is not relevant. While a lookup application
may reasonably issue warnings about strings it believes may be
problematic, applications that decline to process a string that
conforms to the rules above (i.e., does not look it up in the DNS)
are not in conformance with this protocol.
5.6. Punycode Conversion
The string that has now been validated for lookup is converted to ACE
form using the Punycode algorithm (with the ACE prefix added). With
the understanding that this summary is not normative (the steps above
are), the string is either
o in Unicode NFC form that contains no leading combining marks,
contains no DISALLOWED or UNASSIGNED code points, has rules
associated with any code points in CONTEXTJ or CONTEXTO, and, for
those in CONTEXTJ, to satisfies the conditions of the rules; or
o in A-label form, was supplied under circumstances in which the
U-label conversions and tests have not been performed (see
Section 5.4).
5.7. DNS Name Resolution
That resulting validated string is looked up in the DNS, using normal
DNS resolver procedures. That lookup can obviously either succeed
(returning information) or fail.
6. Security Considerations
Security Considerations for this version of IDNA, except for the
special issues associated with right to left scripts and characters,
are described in [IDNA2008-Defs]. Specific issues for labels
containing characters associated with scripts written right to left
appear in [IDNA2008-BIDI].
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7. IANA Considerations
IANA actions for this version of IDNA are specified in
[IDNA2008-Tables] and discussed informally in [IDNA2008-Rationale].
The components of IDNA described in this document do not require any
IANA actions.
8. Contributors
While the listed editor held the pen, the original versions of this
document represent the joint work and conclusions of an ad hoc design
team consisting of the editor and, in alphabetic order, Harald
Alvestrand, Tina Dam, Patrik Faltstrom, and Cary Karp. This document
draws significantly on the original version of IDNA [RFC3490] both
conceptually and for specific text. This second-generation version
would not have been possible without the work that went into that
first version and its authors, Patrik Faltstrom, Paul Hoffman, and
Adam Costello. While Faltstrom was actively involved in the creation
of this version, Hoffman and Costello were not and should not be held
responsible for any errors or omissions.
9. Acknowledgments
This revision to IDNA would have been impossible without the
accumulated experience since RFC 3490 was published and resulting
comments and complaints of many people in the IETF, ICANN, and other
communities, too many people to list here. Nor would it have been
possible without RFC 3490 itself and the efforts of the Working Group
that defined it. Those people whose contributions are acknowledged
in RFC 3490, [RFC4690], and [IDNA2008-Rationale] were particularly
important.
Specific textual changes were incorporated into this document after
suggestions from the other contributors, Stephane Bortzmeyer, Vint
Cerf, Lisa Dusseault, Mark Davis, Paul Hoffman, Kent Karlsson, Erik
van der Poel, Marcos Sanz, Andrew Sullivan, Ken Whistler, and other
WG participants. Special thanks are due to Paul Hoffman for
permission to extract material from his Internet-Draft to form the
basis for Appendix B.
10. References
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10.1. Normative References
[IDNA2008-BIDI]
Alvestrand, H. and C. Karp, "An updated IDNA criterion for
right-to-left scripts", July 2008, .
[IDNA2008-Defs]
Klensin, J., "Internationalized Domain Names for
Applications (IDNA): Definitions and Document Framework",
February 2009, .
[IDNA2008-Tables]
Faltstrom, P., "The Unicode Codepoints and IDNA",
July 2008, .
A version of this document is available in HTML format at
http://stupid.domain.name/idnabis/
draft-ietf-idnabis-tables-02.html
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, November 1987.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[RFC1123] Braden, R., "Requirements for Internet Hosts - Application
and Support", STD 3, RFC 1123, October 1989.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3492] Costello, A., "Punycode: A Bootstring encoding of Unicode
for Internationalized Domain Names in Applications
(IDNA)", RFC 3492, March 2003.
[Unicode-PropertyValueAliases]
The Unicode Consortium, "Unicode Character Database:
PropertyValueAliases", March 2008, .
[Unicode-RegEx]
The Unicode Consortium, "Unicode Technical Standard #18:
Unicode Regular Expressions", May 2005,
.
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[Unicode-Scripts]
The Unicode Consortium, "Unicode Standard Annex #24:
Unicode Script Property", February 2008,
.
[Unicode-UAX15]
The Unicode Consortium, "Unicode Standard Annex #15:
Unicode Normalization Forms", 2006,
.
10.2. Informative References
[ASCII] American National Standards Institute (formerly United
States of America Standards Institute), "USA Code for
Information Interchange", ANSI X3.4-1968, 1968.
ANSI X3.4-1968 has been replaced by newer versions with
slight modifications, but the 1968 version remains
definitive for the Internet.
[IDNA2008-Rationale]
Klensin, J., Ed., "Internationalizing Domain Names for
Applications (IDNA): Issues, Explanation, and Rationale",
February 2009, .
[RFC2136] Vixie, P., Thomson, S., Rekhter, Y., and J. Bound,
"Dynamic Updates in the Domain Name System (DNS UPDATE)",
RFC 2136, April 1997.
[RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS
Specification", RFC 2181, July 1997.
[RFC2535] Eastlake, D., "Domain Name System Security Extensions",
RFC 2535, March 1999.
[RFC2671] Vixie, P., "Extension Mechanisms for DNS (EDNS0)",
RFC 2671, August 1999.
[RFC3490] Faltstrom, P., Hoffman, P., and A. Costello,
"Internationalizing Domain Names in Applications (IDNA)",
RFC 3490, March 2003.
[RFC3491] Hoffman, P. and M. Blanchet, "Nameprep: A Stringprep
Profile for Internationalized Domain Names (IDN)",
RFC 3491, March 2003.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
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Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, January 2005.
[RFC3987] Duerst, M. and M. Suignard, "Internationalized Resource
Identifiers (IRIs)", RFC 3987, January 2005.
[RFC4690] Klensin, J., Faltstrom, P., Karp, C., and IAB, "Review and
Recommendations for Internationalized Domain Names
(IDNs)", RFC 4690, September 2006.
[RFC4952] Klensin, J. and Y. Ko, "Overview and Framework for
Internationalized Email", RFC 4952, July 2007.
[Unicode] The Unicode Consortium, "The Unicode Standard, Version
5.0", 2007.
Boston, MA, USA: Addison-Wesley. ISBN 0-321-48091-0
Appendix A. Local Mapping Alternatives
The subsections of this appendix are temporary and represent
different sketches of possible replacements for Section 5.3. They do
not represent an assertion of WG consensus or any assertion about the
possibility of including one of them as part of the WG's work
program. Instead, they are supplied only for purposes of comparison,
discussion, and, should it be relevant, refinement.
The first paragraph of each subsection describes how the material
would be placed relative to the existing main document text.
Subsequent paragraphs are the actual suggestions, although in
incomplete sketch form.
A.1. Transitional Mapping Model
If this subsection were adopted, Section 5.3 would be deleted and
this one would be inserted after, or integrated with, Section 5.7.
This specification does not support the extensive mappings from one
character to another, including Unicode Case Folding and
Compatibility Character mapping, of IDNA2003. It also changes the
interpretations of a small number of characters relative to IDNA2003.
Most applications, especially those with which IDNs have been used
for some time, will need to maintain reasonable compatibility with
files created under IDNA2003 and user interfaces designed for it.
This section specifies additional steps to be taken to provide
maximum IDNA2003 compatibility.
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If an application requires IDNA2003 backward compatibility, it MUST
execute the steps in one of the two subsections that immediately
follow.
A.1.1. Fallback Lookup
If the string validates and the resolution attempt in Section 5.7
successfully returns a result, the lookup process terminates with
that result. If validation succeeds but resolution fails, the
validated string is proceeded through the ToASCII operation specified
in IDNA2003 [RFC3490]. Assuming it produces a valid result, the
resulting string is compared to the previous validated one. If they
are not identical, a resolution attempt is made with the ToASCII
output and the result of that attempt is returned as the result of
the lookup operation.
Should IDNA2008 validation fail, the string is processed through
ToASCII and, assuming the result is valid, the resulting string is
resolved and the result of that attempt returned as the result of the
lookup operation.
If ToASCII (IDNA2003) conversion is attempted and fails, the lookup
operation behaves as if no name was found in the DNS.
Note that this procedure involves, at most, one DNS lookup
(resolution attempt). If IDNA2008 string validation, conversion, and
resolution succeed, no attempt is made to use IDNA2003 mechanisms.
The procedure does, however, require that lookup applications fully
support both IDNA2008 and IDNA2003 lookup operations so that the
fallback can occur.
A.1.2. Two-step Lookup
Prior to the resolution attempt in Section 5.7, ACE strings are
computed using both IDNA2003 (ToASCII) and IDNA2008 methods (as
specified here). Assuming both validate, those strings are compared.
If they are identical, or only one was valid, then a single DNS
resolution is performed and its result is the result of the lookup
operation. If both are valid but they are not identical, one
resolution attempt is made with each of the two ACE strings.
If neither string is valid as an IDN, then the lookup operation
fails.
When two resolutions are attempted, if one of the two is successful
and the other is not, the successful value is used as the result of
the lookup. If both are successful, the user or calling application
must be presented with a choice in some way.
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This procedure will require two DNS lookups (resolution attempts) in
all cases except those in which the label string fails IDNA2008
validation, neither IDNA2003 or IDNA2008 can validate the string and
translate it to ACE form, or the strings obtained from the two
conversions are identical. As with the prior option, IDNA
implementations will need to support both the IDNA2003 algorithm and
tables and the IDNA2008 one. The question of how multiple results
from different interpretations of the same input string should be
handled by applications is a difficult one, with potential false
positive and security attack vector implications as well as the
possibility of general confusion.
In particular, if both interpretations of the name return values, the
lookup application has no practical way to tell whether the relevant
registry has applied "variant" or "bundling" techniques to ensure
that both domain names are under the same control or not. From that
perspective, the approach in the previous subsection assumes that has
been done (if the IDNA2003-interpretation label is present at all)
while this one assumes that such bundling is unlikely to have
occurred.
[[anchor25: Note in Draft: If this appendix is used, RFC3490 must be
moved from Informative to Normative.]]
A.2. Internationalized Resource Identifier (IRI) Mapping Model
This subsection is intended to be descriptive of an approach that
lies outside IDNA, rather than a normative component of it. If it
were adopted, Section 5.3 would be deleted and the material below
would be referenced, either as a non-normative Appendix in Protocol
or, more reasonably, as a section of Rationale.
IDNA2003 supported extensive mappings from one character to another,
including Unicode Case Folding and Compatibility Character mapping.
Those mappings are no longer supported on registration and are
inconsistent with the "exact match" lookups that people expect from
the DNS. Some mapping should still be supported, both for
compatibility with applications that assume IDNA2003 and to avoid
confounding user expectations. The specific mappings involved are
not part of IDNA, but are expected to be specified as part of a
revision to the IRI specification [RFC3987] and the conversion from
IRI form to URI form. That change leaves mapping unspecified and
prohibited for actual domain names, however, in practice, most domain
names, especially in the web applications that appear to have been
most important for IDNs between the publication of IDNA2003 and the
release of this specification, are not interpreted as themselves but
as abbreviated form of URIs or IRIs and hence subject to the
transformation rules of the latter.
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Appendix B. 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 less
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.
Appendix C. Change Log
[[anchor28: RFC Editor: Please remove this appendix.]]
C.1. Changes between Version -00 and -01 of draft-ietf-idnabis-protocol
o Corrected discussion of SRV records.
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o Several small corrections for clarity.
o Inserted more "open issue" placeholders.
C.2. Version -02
o Rewrote the "conversion to Unicode" text in Section 5.2 as
requested on-list.
o Added a comment (and reference) about EDNS0 to the "DNS Server
Conventions" section, which was also retitled.
o Made several editorial corrections and improvements in response to
various comments.
o Added several new discussion placeholder anchors and updated some
older ones.
C.3. Version -03
o Trimmed change log, removing information about pre-WG drafts.
o Incorporated a number of changes suggested by Marcos Sanz in his
note of 2008.07.17 and added several more placeholder anchors.
o Several minor editorial corrections and improvements.
o "Editor" designation temporarily removed because the automatic
posting machinery does not accept it.
C.4. Version -04
o Removed Contextual Rule appendices for transfer to Tables.
o Several changes, including removal of discussion anchors, based on
discussions at IETF 72 (Dublin)
o Rewrote the preprocessing material (Section 5.3) somewhat.
C.5. Version -05
o Updated part of the A-label input explanation (Section 5.4) per
note from Erik van der Poel.
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C.6. Version -06
o Corrected a few typographical errors.
o Incorporated the material (formerly in Rationale) on the
relationship between IDNA2003 and IDNA2008 as an appendix and
pointed to the new definitions document.
o Text modified in several places to recognize the dangers of
interaction between DNS wildcards and IDNs.
o Text added to be explicit about the handling of edge and failure
cases in Punycode encoding and decoding.
o Revised for consistency with the new Definitions document and to
make the text read more smoothly.
C.7. Version -07
o Multiple small textual and editorial changes and clarifications.
o Requirement for normalization clarified to apply to all cases and
conditions for preprocessing further clarified.
o Substantive change to Section 4.2.1, turning a SHOULD to a MUST
(see note from Mark Davis, 19 November, 2008 18:14 -0800).
C.8. Version -08
o Added some references and altered text to improve clarity.
o Changed the description of CONTEXTJ/CONTEXTO to conform to that in
Tables. In other words, these are now treated as distinction
categories (again), rather than as specially-flagged subsets of
PROTOCOL VALID.
o The discussion of label comparisons has been rewritten to make it
more precise and to clarify that one does not need to verify that
a string is a [valid] A-label or U-label in order to test it for
equality with another string. The WG should verify that the
current text is what is desired.
o Other changes to reflect post-IETF discussions or editorial
improvements.
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C.9. Version -09
o Removed Security Considerations material to Defs document.
o Removed the Name Server Considerations material to Rationale.
That material is not normative and not needed to implement the
protocol itself.
o Adjusted terminology to match new version of Defs.
o Removed all discussion of local mapping and option for it from
registration protocol. Such mapping is now completely prohibited
on Registration.
o Removed some old placeholders and inquiries because no comments
have been received.
o Small editorial corrections.
C.10. Version -10
o Rewrote the registration input material slightly to further
clarify the "no mapping on registration" principle.
o Added placeholder notes about several tasks, notably reorganizing
Section 4 and Section 5 so that subsection numbers are parallel.
o Cleaned up an incorrect use of the terms "A-label" and "U-label"
in the lookup phase that was spotted by Mark Davis. Inserted a
note there about alternate ways to deal with the resulting
terminology problem.
o Added a temporarily appendix (above) to document alternate
strategies for possible replacements for Section 5.3.
C.11. Version -11
o Removed dangling reference to "C-label" (editing error in prior
draft).
o Recast the last steps of the Lookup description to eliminate
"apparent" (previously "putative") terminology.
o Rewrote major portions of the temporary appendix that describes
transitional mappings to improve clarity and add context.
o Did some fine-tuning of terminology, notably in Section 3.2.1.
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C.12. Version -12
o Extensive editorial improvements, mostly due to suggestions from
Lisa Dusseault.
o Conformance statements have been made consistent, especially in
Section 4.2.1 and subsequent text, which said "SHOULD" in one
place and then said "MAY" as the result of incomplete removal of
registration-time mapping. Also clarified the definition of
"registration processes" in Section 4.1 -- the previous text had
confused several people.
o A few new "question to the WG notes have been added about
appropriateness or placement of text. If there are no comments on
the mailing list, the editor will apply his own judgment.
o Several of the usual small typos and other editorial errors have
been corrected.
o Section 5 has still not been reorganized to match Section 4 in
structure and subsection numbering -- will be done as soon as the
mapping decisions and references are final.
Author's Address
John C Klensin
1770 Massachusetts Ave, Ste 322
Cambridge, MA 02140
USA
Phone: +1 617 245 1457
Email: john+ietf@jck.com
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