Internationalized Domain Names in Applications (IDNA): Registry Restrictions and Recommendations
draft-klensin-idna-rfc5891bis-06
| Document | Type | Active Internet-Draft (individual in art area) | |
|---|---|---|---|
| Authors | John Klensin , Asmus Freytag | ||
| Last updated | 2021-03-08 (latest revision 2020-07-13) | ||
| Stream | Internet Engineering Task Force (IETF) | ||
| Intended RFC status | Proposed Standard | ||
| Formats | pdf htmlized bibtex | ||
| Reviews | |||
| Stream | WG state | (None) | |
| Document shepherd | Pete Resnick | ||
| Shepherd write-up | Show (last changed 2019-08-30) | ||
| IESG | IESG state | IESG Evaluation::AD Followup | |
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| Consensus Boilerplate | Yes | ||
| Telechat date |
Has a DISCUSS. Needs 6 more YES or NO OBJECTION positions to pass. |
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| Responsible AD | Murray Kucherawy | ||
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| IANA | IANA review state | Version Changed - Review Needed | |
Network Working Group J. Klensin
Internet-Draft
Updates: 5890, 5891, 5894 (if approved) A. Freytag
Intended status: Standards Track ASMUS, Inc.
Expires: January 14, 2021 July 13, 2020
Internationalized Domain Names in Applications (IDNA): Registry
Restrictions and Recommendations
draft-klensin-idna-rfc5891bis-06
Abstract
The IDNA specifications for internationalized domain names combine
rules that determine the labels that are allowed in the DNS without
violating the protocol itself and an assignment of responsibility,
consistent with earlier specifications, for determining the labels
that are allowed in particular zones. Conformance to IDNA by
registries and other implementations requires both parts. Experience
strongly suggests that the language describing those responsibilities
was insufficiently clear to promote safe and interoperable use of the
specifications and that more details and discussion of circumstances
would have been helpful. Without making any substantive changes to
IDNA, this specification updates two of the core IDNA documents (RFCs
5890 and 5891) and the IDNA explanatory document (RFC 5894) to
provide that guidance and to correct some technical errors in the
descriptions.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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Internet-Drafts are draft documents valid for a maximum of six months
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on January 14, 2021.
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Copyright Notice
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This document is subject to BCP 78 and the IETF Trust's Legal
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Registry Restrictions in IDNA2008 . . . . . . . . . . . . . . 4
3. Progressive Subsets of Allowed Characters . . . . . . . . . . 5
4. Considerations for Domains Operated Primarily for the
Financial Benefit of the Registry Owner or Operator
Organization . . . . . . . . . . . . . . . . . . . . . . . . 7
5. Other corrections and updates . . . . . . . . . . . . . . . . 9
5.1. Updates to RFC 5890 . . . . . . . . . . . . . . . . . . . 9
5.2. Updates to RFC 5891 . . . . . . . . . . . . . . . . . . . 10
6. Related Discussions . . . . . . . . . . . . . . . . . . . . . 11
7. Security Considerations . . . . . . . . . . . . . . . . . . . 11
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 11
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 12
10.1. Normative References . . . . . . . . . . . . . . . . . . 12
10.2. Informative References . . . . . . . . . . . . . . . . . 13
Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . 15
A.1. Changes from version -00 (2017-03-11) to -01 . . . . . . 15
A.2. Changes from version -01 (2017-09-12) to -02 . . . . . . 15
A.3. Changes from version -02 (2019-07-06) to -03 . . . . . . 16
A.4. Changes from version -03 (2019-07-22) to -04 . . . . . . 16
A.5. Changes from version -04 (2019-08-02) to -05 . . . . . . 16
A.6. Changes from version -05 (2019-08-29) to -06 . . . . . . 16
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 17
1. Introduction
Parts of the specifications for Internationalized Domain Names in
Applications (IDNA) [RFC5890] [RFC5891] [RFC5894] (collectively
known, along with RFC 5892 [RFC5892], RFC 5893 [RFC5893] and updates
to them, as "IDNA2008" (or just "IDNA") impose a requirement that
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domain name system (DNS) registries restrict the characters they
allow in domain name labels (see Section 2 below), and the contents
and structure of those labels. That requirement and restriction are
consistent with the "duty to serve the community" described in the
original specification for DNS naming and authority [RFC1591]. The
restrictions are intended to limit the permitted characters and
strings to those for which the registries or their advisers have a
thorough understanding and for which they are willing to take
responsibility.
That provision is centrally important because it recognized that
historical relationships and variations among scripts and writing
systems, the continuing evolution of those systems, differences in
the uses of characters among languages (and locations) that use the
same script, and so on make it impossible for a single list of
characters and simple rules to be able to generate an "if we use
these, we will be safe from confusion and various attacks" guideline.
Instead, the algorithm and rules of RFCs 5891 and 5892 eliminate many
of the most dangerous and otherwise problematic cases, but cannot
eliminate the need for registries and registrars to understand what
they are doing and taking responsibility for the decisions they make.
The way in which the IDNA2008 specifications expressed these
requirements may have under emphasized the intention that they
actually are requirements. Section 2.3.2.3 of the Definitions
document [RFC5890] mentions the need for the restrictions, indicates
that they are mandatory, and points the reader to section 4.3 of the
Protocol document [RFC5891], which in turn points to Section 3.2 of
the Rationale document [RFC5894], with each document providing
further detail, discussion, and clarification.
At the same time, the Internet has evolved significantly since the
management assumptions for the DNS were established with RFC 1591 and
earlier. In particular, the management and use of domain names have
gone through several transformations. Recounting of those changes is
beyond the scope of this document but one of them has had significant
practical impact on the degree to which the requirement for registry
knowledge and responsibility is observed in practice. When RFC 1591
was written, the assumption was that domains at all levels of the DNS
would be operated in the best interest of the registrants in the
domain and of the Internet as a whole. There were no notions about
domains being operated for a profit, much less with a business model
that made them more profitable the more names that could be
registered (or even, under some circumstances, reserved and not
registered). At the time RFC 1591 was written, there was also no
notion that domains would be considered more successful based on the
number of names registered and delegated from them. While rarely
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reflected in the DNS protocols, the distinction between domains
operated primarily as a revenue source of the organizations operating
the registry and ones that are operated for, e.g., use within an
enterprise or otherwise as a service have become very important
today. See Section 4 for a discussion on how those issues affect
this specification.
This specification is intended to unify and clarify these
requirements for registry decisions and responsibility and to
emphasize the importance of registry restrictions at all levels of
the DNS. It also makes a specific recommendation for character
repertoire subsetting that is intermediate between the code points
allowed by RFCs 5891 and 5892 and those allowed by individual
registries. It does not alter the basic IDNA2008 protocols and rules
themselves in any way.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
2. Registry Restrictions in IDNA2008
As mentioned above, IDNA2008 specifies that the registries for each
zone in the DNS that supports IDN labels are required to develop and
apply their own rules to restrict the allowable labels, including
limiting characters they allow to be used in labels in that zone.
The chosen list MUST be a subset of the collection of code points
specified as "PVALID", "CONTEXTJ", and "CONTEXTO" by the rules
established by the protocols themselves. Labels containing any
characters from the two CONTEXT categories or any characters that are
normally part of a script written right to left [RFC5893] require
that additional rules, specified in the protocols and known as
"contextual rules" and "bidi rules", be applied. The entire
collection of rules and restrictions required by the IDNA2008
protocols themselves are known as "protocol restrictions".
As mentioned above, registries may apply (and generally are required
to apply) additional rules to further restrict the list of permitted
code points, contextual rules (perhaps applied to normally PVALID
code points) that apply additional restrictions, and/or restrictions
on labels as distinct from code points. The most obvious of those
restrictions include provisions for restricting suggested new
registrations based on conflicts with labels already registered in
the zone, so as to avoid homograph attacks [Gabrilovich2002] and
other issues. The specifications of what constitutes such conflicts,
as well as the definition of "conflict" based on the properties of
the labels in question, is the responsibility of each registry. They
further include prohibitions on code points and labels that are not
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consistent with the intended function of the zone, the subtree in
which the zone is embedded (see Section 3), or limitations on where
allowable code points may be placed in a label.
These per-registry (or per-zone) rules are commonly known as
"registry restrictions" to distinguish them from the protocol
restrictions described above. By necessity, protocol restrictions
are somewhat generic, having to cater both to the union of the needs
for all zones as well as to the desires of the most permissive zones.
In consequence, additional registry restrictions are essential to
provide for the necessary security in the face of the tremendous
variations and differences in writing systems and their ongoing
evolution and development, as well as the human ability to recognize
and distinguish characters in different scripts around the world and
under different circumstances.
3. Progressive Subsets of Allowed Characters
The algorithm and rules of RFCs 5891 and 5892 determine the set of
code points that are possible for inclusion in domain name labels;
registries MUST NOT permit code points in labels unless they are part
of that set. Labels that contain code points that are normally
written from right to left MUST also conform to the requirements of
RFC 5893. Each registry that intends to allow IDN registrations MUST
then determine the strict subset of that set of code points that will
be allowed by that registry. It SHOULD also consider additional
rules, including contextual and whole label restrictions that provide
further protection for registrants and users. For example, the
widely-used principle that bars labels containing characters from
more than one script is not an IDNA2008 requirement. It has been
adopted by many registries but there may be circumstances in which is
it not required or appropriate.
In formulating their own rules, registries should normally consult
carefully-developed consensus recommendations about global maximum
repertoires to be used such as the ICANN Maximal Starting Repertoire
4 (MSR-4) for the Development of Label Generation Rules for the Root
Zone [ICANN-MSR4] (or its successor documents). Additional
recommendations of similar quality about particular scripts or
languages exist, including, but not limited to, the RFCs for Cyrillic
[RFC5992], Arabic Language [RFC5564], or script-based repertoires
from the approved ICANN Root Zone Label Generation Rules (LGR-3)
[ICANN-LGR3] (or its successor documents). Many of these
recommendations also cover rules about relationships among code
points that may be particularly important for complex scripts. They
also interact with recommendations about how labels that appear to be
the same should be handled.
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It is the responsibility of the registry to determine which, if any,
of those recommendations are applicable and to further subset or
extend them as needed. For example, several of the recommendations
are designed for the root zone and therefore exclude digits and
U+002D HYPHEN-MINUS; this restriction is not generally appropriate
for other zones. On the other hand, some zones may be designed to
not cater for all users of a given script, but perhaps only for the
needs of selected languages, in which case a more selective
repertoire may be appropriate.
In making these determinations, a registry SHOULD follow the IAB
guidance in RFC 6912 [RFC6912]. Those guidelines include a number of
principles for use in making decisions about allowable code points.
In addition, that document notes that the closer a particular zone is
to the root, the more restrictive the space of permitted labels
should be. RFC 5894 provides some suggestions for any registry that
may decide to reduce opportunities for confusion or attacks by
constructing policies that disallow characters used in historic
writing systems (whether these be archaic scripts or extensions of
modern scripts for historic or obsolete orthographies) or characters
whose use is restricted to specialized, or highly technical contexts.
These suggestions were among the principles guiding the design of
ICANN's Maximal Starting Repertoires (MSR) [LGR-Procedure].
A registry decision to allow only those code points in the full
repertoire of the MSR (plus digits and hyphen) would already avoid a
number of issues inherent in a more permissive policy such as "use
anything permitted by IDNA2008", while still supporting the native
languages and scripts for the vast majority of users today. However,
it is unlikely, by itself, to fully satisfy the mandate set out above
for three reasons.
1. The MSR, like the set of code points permissible under IDNA2008
itself, was conceived merely as a boundary condition on
permissible letter code points (it excludes digits and the
hyphen). It was always intended to be used as a starting point
for setting registry policy, with the expectation that some of
the code points in the MSR would not be included in the final
registry policy, whether for lack of actual usage, or for being
inherently problematic.
2. It was recognized that many scripts require contextual rules for
many more code points than are covered by CONTEXTO or CONTEXTJ
rules defined in IDNA2008. This is particularly true for
combining marks, typically used to encode diacritics, tone marks,
vowel signs and the like. While, theoretically, any combining
mark may occur in any context in Unicode, in practice rendering
and other software that users rely on in viewing or entering
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labels will not support arbitrary combining sequences, or indeed
arbitrary combinations of code points, in the case of complex
scripts.
Contextual rules are needed in order to limit allowable code
point sequences to those that can be expected to be rendered
reliably. Identifying those requires knowledge about the way
code points are used in a script, whence the mandate for
registries to only support code points they understand. In this,
some of the other recommendations, such as the Informational RFCs
for specific scripts (e.g., Cyrillic [RFC5992]) or languages
(e.g., Arabic [RFC5564] or Chinese [RFC4713]), or the Root Zone
LGRs developed by ICANN, may provide useful guidance.
3. Third, because of the widely accepted practice of limiting any
given label to a single script, a universal repertoire, such as
the MSR, would have to be divided on a per-script basis into
subrepertoires to make it useful, with some of those repertoires
overlapping, for example, in the case of East Asian shared usage
of the Han ideographs.
Registries choosing to make exceptions -- allow code points that
recommendations such as the MSR do not allow -- should make such
decisions only with great care and only if they have considerable
understanding of, and great confidence in, their appropriateness.
The obvious exception from the MSR would be to allow digits and the
hyphen. Neither were allowed by the MSR, but only because they are
not allowed in the Root Zone.
Nothing in this document permits a registry to allow code points or
labels that are disallowed or otherwise prohibited by IDNA2008.
4. Considerations for Domains Operated Primarily for the Financial
Benefit of the Registry Owner or Operator Organization
As discussed in the Introduction (Section 1), the distributed
administrative structure of the DNS today can be described by
dividing zones into two categories depending on how they are
administered and for whom. These categories are not precise -- some
zones may not fall neatly into one category or the other -- but are
useful in understanding the practical applicability of this
specification. They are:
Zones operating primarily or exclusively within a country,
organization, or enterprise and responsible to the Internet users
in that country or the management of the organization or
enterprise. DNS operations, including registrations and
delegations, will typically occur in support of the purpose of
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that country, organization or enterprise rather than being its
primary purpose.
Zones operating primarily as all or part of a business of selling
names for the financial benefit of entities responsible for the
registry. For these domains, most delegations of subdomains are
to entities with little or no affiliation with the registry
operator other than contractual agreements about operation of
those subdomains. These zones are often known as "public domains"
or with similar terms, but those terms often have other semantics
and may not cover all cases. In particular, a country code domain
operated primarily in the interest of registrants and Internet
users and in service to the broader Internet community is often
considered a "public domain" but would fall into the first
category, not the second.
Rules requiring strict registry responsibility, including either
thorough understanding of scripts and related issues in domain name
labels being considered for registration or local naming rules that
have the same effect, typically come naturally to registries for
zones of the first type. Registration of labels that would prove
problematic for any reason hurts the relevant organization or
enterprise or its customers or users within the relevant country and
more broadly. More generally, there are strong incentives to be
extremely conservative about labels that might be registered and few,
if any, incentives favoring adventures into labels that might be
considered clever, much less ones that are hard to type, render, or,
where it is relevant to users, remember correctly.
By contrast, in a zone in which the profits are derived exclusively,
or almost exclusively, from selling or reserving (including
"blocking") names, there may be perceived incentives to register
whatever names would-be registrants "want" or fears that any
restrictions will cut into the available namespace. In such
situations, restrictions are unlikely to be applied unless they meet
at least one of two criteria: (i) they are easy to apply and can be
applied algorithmically or otherwise automatically and/or (ii) there
is clear evidence that the particular label would cause harm.
As suggested above, the two categories above are not precise. In
particular, there may be domains that, despite being set up to
operate to produce revenue about actual costs, are sufficiently
conservative about their operations to more closely resemble the
first group in practice than the second one.
The requirement of IDNA that is discussed at length elsewhere in this
specification stands: IDNA (and IDNs generally) would work better and
Internet users would be better protected and more secure if
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registries and registrars (of any type) confined their registrations
to scripts and code point sequences that they understood thoroughly.
While the IETF rarely gives advice to those who choose to violate
IETF Standards, some advice to zones in the second category above may
be in order. That advice is that significant conservatism in what is
allowed to be registered, even for reservation purposes, and even
more conservatism about what labels are actually entered into zones
and delegated, is the best option for the Internet and its users. If
practical considerations do not allow that much conservatism, then it
is desirable to consult and utilize the many lists and tables that
have been, and continue to be, developed to advise on what might be
sensible for particular scripts and languages. These include ICANN's
twin efforts of creating per-script Root Zone Label Generation Rules
[RZ-LGR-3] and Second Level Reference Label Generation Rules
[SL-REF-LGR] (the latter of which may be per language). They also
include other lists of code points or code point relationships that
may be particularly problematic and that should be treated with extra
caution or prohibited entirely such as the proposed "troublesome
character" list [Freytag-troublesome]. See also Section 6 below.
5. Other corrections and updates
After the initial IDNA2008 documents were published (and RFC 5892 was
updated for Unicode 6.0 by RFC 6452 [RFC6452]) several errors or
instances of confusing text were noted. For the convenience of the
community, the relevant corrections for RFCs 5890 and 5891 are noted
below and update the corresponding documents. There are no errata
for RFC 5893 or 5894 as of the date this document was published.
Because further updates to RFC 5892 would require addressing other
pending issues, the outstanding erratum for that document is not
considered here. For consistency with the original documents,
references to Unicode 5.0 are preserved in this document.
5.1. Updates to RFC 5890
The outstanding errata against RFC 5890 (Errata ID 4695, 4696, 4823,
and 4824 [RFC-Editor-5890Errata]) are all associated with the same
issue, the number of Unicode characters that can be associated with a
maximum-length (63 octet) A-label. In retrospect and contrary to
some of the suggestions in the errata, that value should not be
expressed in octets because RFC 5890 and the other IDNA 2008
documents are otherwise careful to not specify Unicode encoding forms
but, instead, work exclusively with Unicode code points.
Consequently the relevant material in RFC 5890 should be corrected as
follows:
Section 2.3.2.1
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Old: expansion of the A-label form to a U-label may produce
strings that are much longer than the normal 63 octet DNS limit
(potentially up to 252 characters).
New: expansion of the A-label form to a U-label may produce
strings that are much longer than the normal 63 octet DNS limit
(See Section 4.2).
Comment: If the length limit is going to be a source of confusion
or careful calculations, it should appear in only one place.
Section 4.2
Old: Because A-labels (the form actually used in the DNS) are
potentially much more compressed than UTF-8 (and UTF-8 is, in
general, more compressed that UTF-16 or UTF-32), U-labels that
obey all of the relevant symmetry (and other) constraints of
these documents may be quite a bit longer, potentially up to
252 characters (Unicode code points).
New: A-labels (the form actually used in the DNS) and the
Punycode algorithm used as part of the process to produce them
[RFC3492] are strings that are potentially much more compressed
than any standard Unicode Encoding Form. A 63 octet A-label
cannot represent more than 58 Unicode code points (four octet
overhead and the requirement that at least one character lie
outside the ASCII range) but implementations allocating buffer
space for the conversion should allow significantly more space
(i.e., extra octets) depending on the encoding form they are
using.
5.2. Updates to RFC 5891
Errata ID 3969: Improve reference for combining marks. There is only
one erratum for RFC 5891, Errata ID 3969 [RFC5891Erratum].
Combining marks are explained in the cited section, but not, as
the text indicates, exactly defined.
Old: The Unicode string MUST NOT begin with a combining mark or
combining character (see The Unicode Standard, Section 2.11
[UnicodeA] for an exact definition).
New: The Unicode string MUST NOT begin with a combining mark or
combining character (see The Unicode Standard, Section 2.11
[UnicodeA] for an explanation and Section 3.6, definition D52
[UnicodeB]) for an exact definition).
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Comment: When RFC 5891 is actually updated, the references in the
text should be updated to the current version of Unicode and
the section numbers checked.
6. Related Discussions
This document is one of a series of measures that have been suggested
to address IDNA issues raised in other documents and discussions.
Those other discussions and associated documents include suggested
mechanisms for dealing with combining sequences and single-code point
characters with the same appearance, ones that normalization neither
combines nor decomposes as IDNA2008 assumed. That topic was
discussed further in [IDNA-Unicode] and in the IAB response to that
issue [IAB-2015]. Those and other documents also discuss issues with
IDNA and character graphemes for which abstractions exist in Unicode
in precomposed form but that can be generated from combining
sequences. Another approach is a suggested registry of code points
known to be problematic [Freytag-troublesome]. In combination, the
various discussions of combining sequences and non-decomposing
characters may lay the foundation for an actual update to the IDNA
code points document [RFC5892]. Such an update would presumably also
address the existing errata against that document.
At a much higher-level, discussions are ongoing to consider issues,
demands, and proposals for new uses of the DNS.
7. Security Considerations
As discussed in IAB recommendations about internationalized domain
names [RFC4690], [RFC6912], and elsewhere, poor choices of strings
for DNS labels can lead to opportunities for attacks, user confusion,
and other issues less directly related to security. This document
clarifies the importance of registries carefully establishing design
policies for the labels they will allow and that having such policies
and taking responsibility for them is a requirement, not an option.
If that clarification is useful in practice, the result should be an
improvement in security.
8. Acknowledgments
Many thanks to Patrik Faltstrom who provided an important review on
the initial version, to Jaap Akkerhuis, Don Eastlake, Barry Leiba,
and Alessandro Vesely who did reviews that improved the text and to
Pete Resnick who acted as document shepherd and did an additional
careful review.
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9. IANA Considerations
[[CREF1: RFC Editor: Please remove this section before publication.]]
This memo includes no requests to or actions for IANA. In
particular, it does not contain any provisions that would alter any
IDNA-related registries or tables.
10. References
10.1. Normative References
[ICANN-LGR3]
ICANN, "Root Zone Label Generation Rules (LGR-1)", July
2019,
<https://www.icann.org/news/announcement-2-2019-04-25-en>.
[ICANN-MSR4]
ICANN, "Maximal Starting Repertoire Version 4 (MSR-4) for
the Development of Label Generation Rules for the Root
Zone", January 2019,
<https://www.icann.org/news/announcement-2019-02-07-en>.
[RFC1591] Postel, J., "Domain Name System Structure and Delegation",
RFC 1591, DOI 10.17487/RFC1591, March 1994,
<https://www.rfc-editor.org/info/rfc1591>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC5890] Klensin, J., "Internationalized Domain Names for
Applications (IDNA): Definitions and Document Framework",
RFC 5890, DOI 10.17487/RFC5890, August 2010,
<https://www.rfc-editor.org/info/rfc5890>.
[RFC5891] Klensin, J., "Internationalized Domain Names in
Applications (IDNA): Protocol", RFC 5891,
DOI 10.17487/RFC5891, August 2010,
<https://www.rfc-editor.org/info/rfc5891>.
[RFC5891Erratum]
"RFC 5891, "Internationalized Domain Names in Applications
(IDNA): Protocol"", Errata ID 3969, April 2014,
<http://www.rfc-editor.org/errata_search.php?rfc=5891>.
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[RFC5893] Alvestrand, H., Ed. and C. Karp, "Right-to-Left Scripts
for Internationalized Domain Names for Applications
(IDNA)", RFC 5893, DOI 10.17487/RFC5893, August 2010,
<https://www.rfc-editor.org/info/rfc5893>.
[RFC5894] Klensin, J., "Internationalized Domain Names for
Applications (IDNA): Background, Explanation, and
Rationale", RFC 5894, DOI 10.17487/RFC5894, August 2010,
<https://www.rfc-editor.org/info/rfc5894>.
[RFC6912] Sullivan, A., Thaler, D., Klensin, J., and O. Kolkman,
"Principles for Unicode Code Point Inclusion in Labels in
the DNS", RFC 6912, DOI 10.17487/RFC6912, April 2013,
<https://www.rfc-editor.org/info/rfc6912>.
10.2. Informative References
[Freytag-troublesome]
Freytag, A., Klensin, J., and A. Sullivan, "Those
Troublesome Characters: A Registry of Unicode Code Points
Needing Special Consideration When Used in Network
Identifiers", June 2017, <draft-freytag-troublesome-
characters-01>.
[Gabrilovich2002]
Gabrilovich, E. and A. Gontmakher, "The Homograph Attack",
Communications of the ACM 45(2):128, February 2002.
[IAB-2015]
Internet Architecture Board (IAB), "IAB Statement on
Identifiers and Unicode 7.0.0", February 2015,
<https://www.iab.org/documents/correspondence-reports-
documents/2015-2/iab-statement-on-identifiers-and-unicode-
7-0-0/>.
[IDNA-Unicode]
Klensin, J. and P. Faltstrom, "IDNA Update for Unicode
7.0.0", September 2017, <draft-klensin-idna-5892upd-
unicode70-05>.
[LGR-Procedure]
Internet Corporation for Assigned Names and Numbers
(ICANN), "Procedure to Develop and Maintain the Label
Generation Rules for the Root Zone in Respect of IDNA
Labels", March 2013,
<https://www.icann.org/en/system/files/files/draft-lgr-
procedure-20mar13-en.pdf>.
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[RFC-Editor-5890Errata]
RFC Editor, "RFC Errata: RFC 5890, "Internationalized
Domain Names for Applications (IDNA): Definitions and
Document Framework", August 2010", Note to RFC
Editor: Please figure out how you would like this
referenced and make it so., Captured 2017-09-10, 2016,
<https://www.rfc-editor.org/errata_search.php?rfc=5890>.
[RFC3492] Costello, A., "Punycode: A Bootstring encoding of Unicode
for Internationalized Domain Names in Applications
(IDNA)", RFC 3492, DOI 10.17487/RFC3492, March 2003,
<https://www.rfc-editor.org/info/rfc3492>.
[RFC4690] Klensin, J., Faltstrom, P., Karp, C., and IAB, "Review and
Recommendations for Internationalized Domain Names
(IDNs)", RFC 4690, DOI 10.17487/RFC4690, September 2006,
<https://www.rfc-editor.org/info/rfc4690>.
[RFC4713] Lee, X., Mao, W., Chen, E., Hsu, N., and J. Klensin,
"Registration and Administration Recommendations for
Chinese Domain Names", RFC 4713, DOI 10.17487/RFC4713,
October 2006, <https://www.rfc-editor.org/info/rfc4713>.
[RFC5564] El-Sherbiny, A., Farah, M., Oueichek, I., and A. Al-Zoman,
"Linguistic Guidelines for the Use of the Arabic Language
in Internet Domains", RFC 5564, DOI 10.17487/RFC5564,
February 2010, <https://www.rfc-editor.org/info/rfc5564>.
[RFC5892] Faltstrom, P., Ed., "The Unicode Code Points and
Internationalized Domain Names for Applications (IDNA)",
RFC 5892, DOI 10.17487/RFC5892, August 2010,
<https://www.rfc-editor.org/info/rfc5892>.
[RFC5992] Sharikov, S., Miloshevic, D., and J. Klensin,
"Internationalized Domain Names Registration and
Administration Guidelines for European Languages Using
Cyrillic", RFC 5992, DOI 10.17487/RFC5992, October 2010,
<https://www.rfc-editor.org/info/rfc5992>.
[RFC6452] Faltstrom, P., Ed. and P. Hoffman, Ed., "The Unicode Code
Points and Internationalized Domain Names for Applications
(IDNA) - Unicode 6.0", RFC 6452, DOI 10.17487/RFC6452,
November 2011, <https://www.rfc-editor.org/info/rfc6452>.
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[RZ-LGR-3]
Internet Corporation for Assigned Names and Numbers, "Root
Zone Label Generation Rules - LGR-3: Overview and Summary,
Version 3", July 2019,
<https://www.icann.org/sites/default/files/lgr/lgr-3-
overview-10jul19-en.pdf>.
[SL-REF-LGR]
Internet Corporation for Assigned Names and Numbers
(ICANN), "Second Level Label Generation Rules", 2019,
<https://www.icann.org/resources/pages/second-level-lgr-
2015-06-21-en>.
[UnicodeA]
The Unicode Consortium, "The Unicode Standard, Version
12.1", May 2019.
Section 2.11
[UnicodeB]
The Unicode Consortium, "The Unicode Standard, Version
12.1", May 2019.
Section 3.6, definition D52
Appendix A. Change Log
RFC Editor: Please remove this appendix before publication.
A.1. Changes from version -00 (2017-03-11) to -01
o Added Acknowledgments and adjusted references.
o Filled in Section 5 with updates to respond to errata.
o Added Section 6 to discuss relationships to other documents.
o Modified the Abstract to note specifically updated documents.
o Several small editorial changes and corrections.
A.2. Changes from version -01 (2017-09-12) to -02
After a pause of nearly 34 months due to inability to get this draft
processed, including nearly a year waiting for a new directorate to
actually do anything of substance about fundamental IDNA issues, the
-02 version was posted in the hope of getting a new start. Specific
changes include:
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o Added a new section, Section 4, and some introductory material to
address the very practical issue that domains run on a for-profit
basis are unlikely to follow the very strict "understand what you
are registering" requirement if they support IDNs at all and
expect to profit from them.
o Added a pointer to draft-klensin-idna-unicode-review to the
discussion of other work.
o Editorial corrections and changes.
A.3. Changes from version -02 (2019-07-06) to -03
o Minor editorial changes in response to shepherd review.
o Additional references.
A.4. Changes from version -03 (2019-07-22) to -04
o Editorial changes after AD review and some additional changes to
improve clarity.
A.5. Changes from version -04 (2019-08-02) to -05
o Small editorial corrections, many to correct glitches found during
IETF Last Call.
o Updated acknowledgments, particularly to reflect reviews in Last
Call.
A.6. Changes from version -05 (2019-08-29) to -06
Other than some small editorial adjustments, these changes made
after, and reflect, IESG post-last-call review and comments. To the
extent it was possible to do so without making this document
inconsistent with the other IDNA documents, established IETF,
Unicode, and ICANN community i18n terminology, or well-established
IDNA or i18n practices, the first author believes that the document
responds to all previously-outstanding IESG substantive comments.
o Fixed a remaining citation issue with a Unicode document. This
version has not been updated to reflect Unicode 13, but the
document should be adjusted so that all references are
contemporary at the time of publication.
o Added reference to homograph attacks, and slightly adjusted
discussion of them, per discussion with IESG post-last-call.
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o Removed pointer to RFC 5890 from discussion of mixed-script labels
in Section 3.
o Rewrote parts of Section 4 to eliminate the term "for-profit" and
clarify the issues.
o Removed pointer to draft-klensin-idna-unicode-review because RFC
8753 has been published and is therefore no longer pending /
parallel work.
o Rewrote Section 6 to make the relationships among various
documents and efforts somewhat more clear.
o References to RFCs 5893 and 6912 moved from Informative to
Normative.
Authors' Addresses
John C Klensin
1770 Massachusetts Ave, Ste 322
Cambridge, MA 02140
USA
Phone: +1 617 245 1457
Email: john-ietf@jck.com
Asmus Freytag
ASMUS, Inc.
Email: asmus@unicode.org
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