Internet Architecture Board (IAB) A. Sullivan
Request for Comments: 6912 Dyn, Inc.
Category: Informational D. Thaler
ISSN: 2070-1721 Microsoft
J. Klensin
O. Kolkman
NLnet Labs
April 2013
Principles for Unicode Code Point Inclusion in Labels in the DNS
Abstract
Internationalized Domain Names in Applications (IDNA) makes available
to DNS zone administrators a very wide range of Unicode code points.
Most operators of zones should probably not permit registration of
U-labels using the entire range. This is especially true of zones
that accept registrations across organizational boundaries, such as
top-level domains and, most importantly, the root. It is
unfortunately not possible to generate algorithms to determine
whether permitting a code point presents a low risk. This memo
presents a set of principles that can be used to guide the decision
of whether a Unicode code point may be wisely included in the
repertoire of permissible code points in a U-label in a zone.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This document is a product of the Internet Architecture Board (IAB)
and represents information that the IAB has deemed valuable to
provide for permanent record. It represents the consensus of the
Internet Architecture Board (IAB). Documents approved for
publication by the IAB are not a candidate for any level of Internet
Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6912.
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Copyright Notice
Copyright (c) 2013 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. Background . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. More-Restrictive Rules Going Up the DNS Tree . . . . . . 6
3. Principles Applicable to All Zones . . . . . . . . . . . . . 6
3.1. Longevity Principle . . . . . . . . . . . . . . . . . . . 6
3.2. Least Astonishment Principle . . . . . . . . . . . . . . 6
3.3. Contextual Safety Principle . . . . . . . . . . . . . . . 7
4. Principles Applicable to All Public Zones . . . . . . . . . . 7
4.1. Conservatism Principle . . . . . . . . . . . . . . . . . 7
4.2. Inclusion Principle . . . . . . . . . . . . . . . . . . . 7
4.3. Simplicity Principle . . . . . . . . . . . . . . . . . . 7
4.4. Predictability Principle . . . . . . . . . . . . . . . . 8
4.5. Stability Principle . . . . . . . . . . . . . . . . . . . 8
5. Principle Specific to the Root Zone . . . . . . . . . . . . . 8
5.1. Letter Principle . . . . . . . . . . . . . . . . . . . . 8
6. Confusion and Context . . . . . . . . . . . . . . . . . . . . 9
7. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . 9
8. Security Considerations . . . . . . . . . . . . . . . . . . . 10
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 10
10. IAB Members at the Time of Approval . . . . . . . . . . . . . 10
11. Informative References . . . . . . . . . . . . . . . . . . . 10
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1. Introduction
Operators of a DNS zone need to set policies around what Unicode code
points are allowed in labels in that zone. Typically there are a
number of important goals to consider when constructing such
policies. These include, for instance, avoiding possible visual
confusability between two labels, avoiding possible confusion between
Fully Qualified Domain Names (FQDNs) and IP address literals,
accessibility to the disabled (see "Web Content Accessibility
Guidelines (WCAG) 2.0" [WCAG20] for some discussion in a web
context), and other usability issues.
This document provides a set of principles that zone operators can
use to construct their code point policies in order to improve
usability and clarity and thereby reduce confusion.
1.1. Terminology
This document uses the following terms.
A-label: an LDH label that starts with "xn--" and meets all the
IDNA requirements, with additional restrictions as explained in
Section 2.3.2.1 of the IDNA Definitions document [RFC5890].
Character: a member of a set of elements used for the
organization, control, or representation of data. See Section 2
of the Internationalization Terminology document [RFC6365] for
more details.
Language: a way that humans communicate. The use of language
occurs in many forms, the most common of which are speech,
writing, and signing. See Section 2 of RFC 6365 for more details.
LDH label: a string consisting of ASCII letters, digits, and the
hyphen, with additional restrictions as explained in Section 2.3.1
of RFC 5890.
Public zone: in this document, a DNS zone that accepts
registration requests from organizations outside the zone
administrator's own organization. (Whether the zone performs
delegation is a separate question. What is important is the
diversity of the registration-requesting community.) Note that
under this definition, the root zone is a public zone, though one
that has a unique function in the DNS.
Rendering: the display of a string of text. See Section 5 of RFC
6365 for more details.
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Script: a set of graphic characters used for the written form of
one or more languages. See Section 2 of RFC 6365 for more
details.
U-label: a string of Unicode characters that meets all the IDNA
requirements and includes at least one non-ASCII character, with
additional restrictions as explained in Section 2.3.2.1 of RFC
5890.
Writing system: a set of rules for using one or more scripts to
write a particular language. See Section 2 of RFC 6365 for more
details.
This memo does not propose a protocol standard, and the use of words
such as "should" follow the ordinary English meaning, and not that
laid out in [RFC2119].
2. Background
In recent communications [IABCOMM1] [IABCOMM2], the IAB has
emphasized the importance of conservatism in allocating labels
conforming to IDNA2008 [RFC5890] [RFC5891] [RFC5892] [RFC5893]
[RFC5894] [RFC5895] in DNS zones, and especially in the root zone.
Traditional LDH labels in the root zone used only alphabetic
characters (i.e., ASCII a-z, which under the DNS also match A-Z).
Matters are more complicated with U-labels, however. The IAB
communications recommended that U-labels permit only code points with
a General_Category (gc) of Ll (Lowercase_Letter), Lo (Other_Letter),
or Lm (Modifier_Letter), but noted that for practical considerations
other code points might be permitted on a case-by-case basis.
The IAB recommendations do, however, leave some issues open that need
to be addressed. It is not clear that all code points permitted
under IDNA2008 that have a General_Category of Lo or Lm are
appropriate for a zone such as the root zone. To take but one
example, the code point U+02BC (MODIFIER LETTER APOSTROPHE) has a
General_Category of Lm. In practically every rendering (and we are
unaware of an exception), U+02BC is indistinguishable from U+2019
(RIGHT SINGLE QUOTATION MARK), which has a General_Category of Pf
(Final_Punctuation). U+02BC will also be read by large numbers of
people as being the same character as U+0027 (APOSTROPHE), which has
a General_Category of Po (Other_Punctuation), and some computer
systems may treat U+02BC as U+0027. U+02BC is PROTOCOL VALID
(PVALID) under IDNA2008 (see the IDNA Code Points document
[RFC5892]), whereas both other code points are DISALLOWED. So, to
begin with, it is plain that not every code point with a
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General_Category of Ll, Lo, or Lm is consistent with the type of
conservatism principle discussed in Section 4.1 below or the previous
IAB recommendations.
To make matters worse, some languages are dependent on code points
with General_Category Mc (Spacing_Mark) or General_Category Mn
(Nonspacing_Mark). This dependency is particularly common in Indic
languages, though not exclusive to them. (At the risk of vastly
oversimplifying, the overarching issue is mostly the interaction of
complex writing systems and the way Unicode works.) To restrict
users of those languages to only code points with General_Category of
Ll, Lo, or Lm would be extremely limiting. While DNS labels are not
words, or sentences, or phrases (as noted in the next steps for IDN
[RFC4690]), they are intended to support useful mnemonics. Mnemonics
that diverge wildly from the usual conventions are poor ones, because
in not following the usual conventions they are not easy to remember.
Also, wide divergence from usual conventions, if not well-justified
(and especially in a shared namespace like the root), invites
political controversy.
Many of the issues above turn out to be relevant to all public zones.
Moreover, the overall issue of developing a policy for code point
permission is common to all zones that accept A-labels or U-labels
for registration. As Section 4.3 of the IDNA Protocol document
[RFC5891] says, every registry at every level of the DNS is "expected
to establish policies about label registrations".
For reasons of sound management, it is not desirable to decide
whether to permit a given code point only when an application
containing that code point is pending. That approach reduces
predictability and is bound to appear subject to special pleas. It
is better instead to produce the rules governing acceptance of code
points in advance.
As is evident from the foregoing discussion about the Letter and Mark
categories, it is simply not possible to make code point decisions
algorithmically. If it were possible to develop such an algorithm,
it would already exist: the DNS is hardly unique in needing to impose
restrictions on code points while accommodating many different
linguistic communities. Nevertheless, new guidelines can be made by
starting from overarching principles. These guidelines act more as
meta-rules, leading to the establishment of other rules about the
inclusion and exclusion of particular code points in labels in a
given zone, always based on the list of code points permitted by
IDNA.
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2.1. More-Restrictive Rules Going Up the DNS Tree
A set of principles derived from the above ideas follows in Sections
3 through 5 below. Such principles fall into three categories. Some
principles apply to every DNS zone. Some additional principles apply
to all public zones, including the root zone. Finally, other
principles apply only to the root zone. This means that zones higher
in the DNS tree tend to have more restrictive rules (since additional
principles apply), and zones lower in the DNS tree tend to have less
restrictive rules, since they are used within a more narrow context.
In general, the relevant context for a principle is that of the zone,
not that of a given subset of the user community; for the root zone,
for example, the context is "the entire Internet population".
3. Principles Applicable to All Zones
3.1. Longevity Principle
Unicode properties of a code point ought to be stable across the
versions of Unicode that users of the zone are likely to have
installed. Because it is possible for the properties of a code point
to change between Unicode versions, a good way to predict such
stability is to ensure that a code point has in fact been stable for
multiple successive versions of Unicode. This principle is related
to the Stability Principle in Section 4.5.
The more diverse the community using the zone, the greater the
importance of following this principle. The policy for a leaf zone
in the DNS might only require stability across two Unicode versions,
whereas a more public zone might require stability across four or
more releases before the code point's properties are considered long-
lived and stable.
3.2. Least Astonishment Principle
Every zone administrator should be sensitive to the likely use of a
code point to be permitted, particularly taking into account the
population likely to use the zone. Zone administrators should
especially consider whether a candidate code point could present
difficulty if the code point is encountered outside the usual
linguistic circumstances. By the same token, the failure to support
a code point that is normal in some linguistic circumstances could be
very surprising for users likely to encounter the names in that
circumstance.
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3.3. Contextual Safety Principle
Every zone administrator should be sensitive to ways in which a code
point that is permitted could be used in support of malicious
activity. This is not a completely new problem: the digit 1 and the
lowercase letter l are, for instance, easily confused in many
contexts. The very large repertoire of code points in Unicode (even
just the subset permitted for IDNs) makes the problem somewhat worse,
just because of the scale.
4. Principles Applicable to All Public Zones
4.1. Conservatism Principle
Public zones are, by definition, zones that are shared by different
groups of people. Therefore, any decision to permit a code point in
a public zone (including the root) should be as conservative as
practicable. Doubts should always be resolved in favor of rejecting
a code point for inclusion rather than in favor of including it, in
order to minimize risk.
4.2. Inclusion Principle
Just as IDNA2008 starts from the principle that the Unicode range is
excluded, and then adds code points according to derived properties
of the code points, so a public zone should only permit inclusion of
a code point if it is known to be "safe" in terms of usability and
confusability within the context of that zone. The default treatment
of a code point should be that it is excluded.
4.3. Simplicity Principle
The rules for determining whether a code point is to be included
should be simple enough that they are readily understood by someone
with a moderate background in the DNS and Unicode issues. This
principle does not mean that a completely naive person needs to be
able to understand the rationale for including a code point, but it
does mean that if the reason for inclusion of a very peculiar code
point, even a safe one, is too difficult to understand, the code
point would not be permitted.
The meaning of "simple" or "readily understood" is context-dependent.
For instance, the root zone has to serve everyone in the world; for
practical purposes, this means that the reasons for including a code
point need to be comprehensible even to people who cannot use the
script where the code point is found. In a zone that permits a
constrained subset of Unicode characters (for instance, only those
needed to write a single alphabetic language) and that supports a
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clearly delineated linguistic community (for instance, the speakers
of a single language with well-understood written conventions), more
complicated rules might be acceptable. Compare this principle with
the Least Astonishment Principle in Section 3.2.
4.4. Predictability Principle
The rules for determining whether a code point is to be included
should be predictable enough that those with the requisite
understanding of DNS, IDNA, and Unicode will usually reach the same
conclusion. This is not a requirement for algorithmic treatment of
code points; as previously noted, that is not possible. Rather, it
is to say that the consistent application of professional judgment is
likely to yield the same results; combined with the principle in
Section 4.1, when results are not predictable, the anomalous code
point would not be permitted.
Just as in Section 4.3, this principle tends to cause more
restriction the more diverse the community using the zone; it is most
restrictive for the root zone. This is because what is predictable
within a given language community is possibly very surprising across
languages.
4.5. Stability Principle
Once a code point is permitted, it is at least very hard to stop
permitting that code point. In public zones (including the root),
the list of code points to be permitted should change very slowly, if
at all, and usually only in the direction of permitting an addition
as time and experience indicate that inclusion of such a code point
is both safe and consistent with these principles.
5. Principle Specific to the Root Zone
5.1. Letter Principle
"Requirements for Internet Hosts - Application and Support" [RFC1123]
notes that top-level labels "will be alphabetic". In the absence of
widespread agreement about the force of that note, prudence suggests
that U-labels in the root zone should exclude code points that are
not normally used to write words, or that are in some cases normally
used for purposes other than writing words. This is not the same as
using Unicode's General_Category to include only letters. It is a
restriction that expands the possible class of included code points
beyond the Unicode letters, but only expands so far as to include the
things that are normally used to create words. Under this principle,
code points with (for example) General_Category Mn (Nonspacing_Mark)
might be included -- but only those that are used to write words and
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not (for instance) musical symbols. In addition, such marks should
only be used within a label in ways that they would be used when
making a word: combinations that would be nonsense when used in a
word should also be rejected when tried in DNS labels. This
principle should be applied as narrowly as possible; as the next
steps for IDN document [RFC4690] says, "While DNS labels may
conveniently be used to express words in many circumstances, the goal
is not to express words (or sentences or phrases), but to permit the
creation of unambiguous labels with good mnemonic value".
6. Confusion and Context
While many discussions of confusion have focused on characters, e.g.,
whether two characters are confusable with each other (and under what
circumstances), a focus on characters alone could lead to the
prohibition of very large numbers of labels, including many that
present little risk. Instead, the focus should be on whether one
label is confusable with another. For example, if a label contains
several characters that are distinct to a particular script, and all
of its characters are from that script, it is inherently not
confusable with a label from any other script no matter what other
characters might appear in it. Another label that lacks those
distinguishing characters might be a problem. The notion extends
from labels to domain names, in the sense that distinguishing
characters used in a higher-level label may set expectations with
respect to the characters in the lower-level labels. This
expectation might be regarded as a benefit, but it is also a problem,
since there is no technical way to require consistent policies in
delegated namespaces.
7. Conclusion
The principles outlined in this document can be applied when
considering any range of Unicode code points for possible inclusion
in a DNS zone. It is worth observing that doing anything (especially
in light of Section 4.5) implicitly disadvantages communities with a
writing system not yet well understood and not represented in the
technical and policy communities involved in the discussion. That
disadvantage is to be guarded against as much as practical, but is
effectively impossible to prevent (while still taking action) in
light of imperfect human knowledge.
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8. Security Considerations
The principles outlined in this memo are intended to improve
usability and clarity and thereby reduce confusion among different
labels. While these principles may contribute to reduction of risk,
they are not sufficient to provide a comprehensive
internationalization policy for zone management.
Additional discussion of security considerations can be found in the
Unicode Security Considerations [UTR36].
9. Acknowledgements
The authors thank the participants in the IAB Internationalization
program for the discussion of the ideas in this memo, particularly
Marc Blanchet. In addition, Stephane Bortzmeyer, Paul Hoffman,
Daniel Kalchev, Panagiotis Papaspiliopoulos, and Vaggelis Segredakis
made specific comments.
10. IAB Members at the Time of Approval
Bernard Aboba
Jari Arkko
Marc Blanchet
Ross Callon
Alissa Cooper
Spencer Dawkins
Joel Halpern
Russ Housley
David Kessens
Danny McPherson
Jon Peterson
Dave Thaler
Hannes Tschofenig
11. Informative References
[IABCOMM1] Internet Architecture Board, "IAB Statement: 'The
interpretation of rules in the ICANN gTLD Applicant
Guidebook'", February 2012, <http://www.iab.org/
documents/correspondence-reports-documents/201/>.
[IABCOMM2] Internet Architecture Board, "Response to ICANN questions
concerning 'The interpretation of rules in the ICANN gTLD
Applicant Guidebook'", March 2012, <http://www.iab.org/
documents/correspondence-reports-documents/201/>.
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[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.
[RFC4690] Klensin, J., Faltstrom, P., Karp, C., and IAB, "Review and
Recommendations for Internationalized Domain Names
(IDNs)", RFC 4690, September 2006.
[RFC5890] Klensin, J., "Internationalized Domain Names for
Applications (IDNA): Definitions and Document Framework",
RFC 5890, August 2010.
[RFC5891] Klensin, J., "Internationalized Domain Names in
Applications (IDNA): Protocol", RFC 5891, August 2010.
[RFC5892] Faltstrom, P., "The Unicode Code Points and
Internationalized Domain Names for Applications (IDNA)",
RFC 5892, August 2010.
[RFC5893] Alvestrand, H. and C. Karp, "Right-to-Left Scripts for
Internationalized Domain Names for Applications (IDNA)",
RFC 5893, August 2010.
[RFC5894] Klensin, J., "Internationalized Domain Names for
Applications (IDNA): Background, Explanation, and
Rationale", RFC 5894, August 2010.
[RFC5895] Resnick, P. and P. Hoffman, "Mapping Characters for
Internationalized Domain Names in Applications (IDNA)
2008", RFC 5895, September 2010.
[RFC6365] Hoffman, P. and J. Klensin, "Terminology Used in
Internationalization in the IETF", BCP 166, RFC 6365,
September 2011.
[UTR36] Davis, M. and M. Suignard, "Unicode Security
Considerations", Unicode Technical Report #36, July 2012.
[WCAG20] W3C, "Web Content Accessibility Guidelines (WCAG) 2.0",
W3C Recommendation, December 2008,
<http://www.w3.org/TR/2008/REC-WCAG20-20081211/>.
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Authors' Addresses
Andrew Sullivan
Dyn, Inc.
150 Dow St
Manchester, NH 03101
USA
EMail: asullivan@dyn.com
Dave Thaler
Microsoft
One Microsoft Way
Redmond, WA 98052
USA
EMail: dthaler@microsoft.com
John C Klensin
1770 Massachusetts Ave, Ste 322
Cambridge, MA 02140
USA
Phone: +1 617 491 5735
EMail: john-ietf@jck.com
Olaf Kolkman
NLnet Labs
Science Park 400
Amsterdam 1098 XH
The Netherlands
EMail: olaf@NLnetLabs.nl
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