Network Working Group J. Klensin
Internet-Draft October 14, 2008
Obsoletes: 3490 (if approved)
Intended status: Standards Track
Expires: April 17, 2009
Internationalized Domain Names for Applications (IDNA): Definitions and
Document Framework
draft-ietf-idnabis-defs-00.txt
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Abstract
This document is one of a collection that, together, describe the
protocol and usage context for a revision of Internationalized Domain
Names for Applications (IDNA), superseding the earlier version. It
describes the document collection and provides definitions and other
material that are common to the set.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. IDNA2008 . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1.1. Audiences . . . . . . . . . . . . . . . . . . . . . . 3
1.1.2. Normative Language . . . . . . . . . . . . . . . . . . 3
1.2. Discussion Forum . . . . . . . . . . . . . . . . . . . . . 3
1.3. Roadmap of IDNA2008 Documents . . . . . . . . . . . . . . 3
2. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. Terminology about Characters and Character Sets . . . . . 4
2.2. DNS-related Terminology . . . . . . . . . . . . . . . . . 5
2.3. Terminology Specific to IDNA . . . . . . . . . . . . . . . 6
2.3.1. Terms for IDN Label Codings . . . . . . . . . . . . . 6
2.3.2. Punycode is an Algorithm, not a Name . . . . . . . . . 9
3. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
4. Security Considerations . . . . . . . . . . . . . . . . . . . 10
5. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 11
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
6.1. Normative References . . . . . . . . . . . . . . . . . . . 11
6.2. Informative References . . . . . . . . . . . . . . . . . . 12
Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . 13
A.1. Version -00 . . . . . . . . . . . . . . . . . . . . . . . 13
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 13
Intellectual Property and Copyright Statements . . . . . . . . . . 14
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1. Introduction
1.1. IDNA2008
This document is one of a collection that, together, describe the
protocol and usage context for a revision of Internationalized Domain
Names for Applications (IDNA) that was largely completed in 2008,
known within the series and elsewhere as IDNA2008. The series
replaces an earlier version of IDNA, described in [RFC3490] and
[RFC3491]. It continues to use the Punycode algorithm [RFC3492] and
ACE (ASCII-compatible encoding) prefix from that earlier version.
The document collection is described in Section 1.3. As indicated
there, this document provides definitions and other material that are
common to the set.
1.1.1. Audiences
While many IETF specifications are directed exclusively to protocol
implementers, the character of IDNA requires that it be understood
and properly used by those whose responsibilities include making
decisions about what names are permitted in DNS zone files and about
policies related to names and naming. This document and those
concerned with the protocol definition, rules for rules for handling
strings that include characters written right-to-left, and the actual
list of characters and categories will be of primary interest to
protocol implementers. This document and the one containing
explanatory material will be of primary interest to others, although
they may have to fill in details of interest by reference to other
documents in the set.
1.1.2. Normative Language
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].
1.2. Discussion Forum
[[ RFC Editor: please remove this section. ]]
IDNA2008 is being discussed in the IETF "idnabis" Working Group and
on the mailing list idna-update@alvestrand.no
1.3. Roadmap of IDNA2008 Documents
IDNA2008 consists of the following documents:
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o This document, containing definitions and other material that are
needed for understanding other documents in the set.
o A document [IDNA2008-Rationale] that provides an overview of the
protocol and associated tables together with explanatory material
and some rationale for the decisions that led to IDNA2008. That
document also contains advice for registry operations and those
who use internationalized domain names. It is not normative.
o A document [IDNA2008-Protocol] that describes the core IDNA2008
protocol and its operations. In combination with the "Bidi"
document described immediately below, it explicitly updates and
replaces RFC 3490.
o A document [IDNA2008-Bidi] that specifies special rules ("Bidi")
for labels that contain characters that are written from right to
left.
o A specification [IDNA2008-Tables] of the categories and rules that
identify the code points allowed in a label written in native
character form (defined more specifically as a "U-label" in
Section 2.3.1.1 below), based on Unicode 5.1 [Unicode51] code
point assignments and additional rules unique to IDNA2008. That
specifications obsoletes RFC 3941 and IDN use of the tables to
which it refers.
2. Definitions
2.1. Terminology about Characters and Character Sets
[[anchor8: Formerly Section 1.5.2 of Rationale-03]]
A code point is an integer value associated with a character in a
coded character set.
Unicode [Unicode51] is a coded character set containing about 100,000
characters as of the current version. A single Unicode code point is
denoted in these documents by "U+" followed by four to six
hexadecimal digits, while a range of Unicode code points is denoted
by two four to six digit hexadecimal numbers separated by "..", with
no prefixes.
ASCII means US-ASCII [ASCII], a coded character set containing 128
characters associated with code points in the range 0000..007F.
Unicode may be thought of as an extension of ASCII; it includes all
the ASCII characters and associates them with equivalent code points.
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"Letters" are, informally, generalizations from the ASCII and common-
sense understanding of that term, i.e., characters that are used to
write text that are not digits, symbols, or punctuation. Formally,
they are characters with a Unicode General Category value starting in
"L" (see Section 4.5 of [Unicode51]).
2.2. DNS-related Terminology
[[anchor10: Formerly Section 1.5.3 of Rationale-03.]]
When discussing the DNS, this document generally assumes the
terminology used in the DNS specifications [RFC1034] [RFC1035]. The
term "lookup" is used to describe the combination of operations
performed by the IDNA2008 protocol and those actually performed by a
DNS resolver. The process of placing an entry into the DNS is
referred to as "registration", similar to common contemporary usage
in other contexts. Consequently, any DNS zone administration is
described as a "registry", regardless of the actual administrative
arrangements or level in the DNS tree. More detail about that
relationship is included in the "Rationale" document.
The term "LDH code point" is defined in this document to refer to the
code points associated with ASCII letters, digits, and the hyphen-
minus; that is, U+002D, 0030..0039, 0041..005A, and 0061..007A. "LDH"
is an abbreviation for "letters, digits, hyphen".
The base DNS specifications [RFC1034] [RFC1035] discuss "domain
names" and "host names", but many people use the terms
interchangeably, as do sections of these specifications. Lack of
clarity about that terminology has contributed to confusion about
intent in some cases. These documents generally use the term "domain
name". When they refer to, e.g., host name syntax restrictions, they
explicitly cites the relevant defining documents. The remaining
definitions in this subsection are essentially a review: if there is
any perceived difference between those definitions and the
definitions in the base DNS documents or those cited below, the
definitions in the other documents take precedence.
A label is an individual component of a domain name. Labels are
usually shown separated by dots; for example, the domain name
"www.example.com" is composed of three labels: "www", "example", and
"com". (The zero-length root label described in RFC 1123 [RFC1123],
which can be explicit as in "www.example.com." or implicit as in
"www.example.com", is not considered in this specification.) IDNA
extends the set of usable characters in labels that are treated as
text (as distinct from the binary string labels discussed in RFC 1035
and RFC 2181 [RFC2181] and the bitstring ones described in RFC 2673
[RFC2673]). For the rest of this document and in the related ones,
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the term "label" is shorthand for "text label", and "every label"
means "every text label".
2.3. Terminology Specific to IDNA
[[anchor11: Formerly Section 1.5.4 of Rationale-03 with some material
removed and left in that document.]]
This section defines some terminology to reduce dependence on terms
and definitions that have been problematic in the past.
2.3.1. Terms for IDN Label Codings
2.3.1.1. IDNA-valid strings, A-label, and U-label
To improve clarity, this subsection of the document introduces three
new terms. In the next subsection, it defines a historical term to
be slightly more precise for IDNA contexts.
o A string is "IDNA-valid" if it meets all of the requirements of
these specifications for an IDNA label. IDNA-valid strings may
appear in either of the two forms, defined immediately below, or
may, trivially, be ASCII strings that conform to the traditional
"hostname" (or "LDH") rule and that do not contain "--" as the
third and fourth character. These documents make specific
reference to the form appropriate to any context in which the
distinction is important.
o An "A-label" is the ASCII-Compatible Encoding (ACE, see
Section 2.3.1.5) form of an IDNA-valid string. It must be a
complete label: IDNA is defined for labels, not for parts of them
and not for complete domain names. This means, by definition,
that every A-label will begin with the IDNA ACE prefix, "xn--"
(see Section 2.3.1.5), followed by a string that is a valid output
of the Punycode algorithm and hence a maximum of 59 ASCII
characters in length. The prefix and string together must conform
to all requirements for a label that can be stored in the DNS
including conformance to the rules for the preferred form
described in RFC 1034, RFC 1035, and RFC 1123.
o A "U-label" is an IDNA-valid string of Unicode characters,
including at least one non-ASCII character, expressed in a
standard Unicode Encoding Form -- in an Internet transmission
context this will normally be UTF-8 -- and subject to the
constraint below. Conversions between U-labels and A-labels are
performed according to the "Punycode" specification [RFC3492],
adding or removing the ACE prefix as needed.
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To be valid, U-labels and A-labels must obey an important symmetry
constraint. While that constraint may be tested in any of several
ways, an A-label must be capable of being produced by conversion from
a U-label and a U-label must be capable of being produced by
conversion from an A-label. Among other things, this implies that
both U-labels and A-labels must be strings in Unicode NFC
[Unicode-UAX15] normalized form. These strings MUST contain only
characters specified elsewhere in this document series, and only in
the contexts indicated as appropriate.
Any rules or conventions that apply to DNS labels in general, such as
rules about lengths of strings, apply to whichever of the U-label or
A-label would be more restrictive. For the U-label, constraints
imposed by existing protocols and their presentation forms make the
length restriction apply to the length in octets of the UTF-8 form of
those labels (which will always be greater than or equal to the
length in code points). The exception to this, of course, is that
the restriction to ASCII characters does not apply to the U-label.
A different way to look at these terms, which may be more clear to
some readers, is that U-labels, A-labels, and LDH-labels (see the
next subsection) are disjoint categories that, together, make up the
forms of legitimate strings for use in domain names that describe
hosts. Of the three, only A-labels and LDH-labels can actually
appear in DNS zone files or queries; U-labels can appear, along with
the other two, in presentation and user interface forms and in
selected protocols other than those of the DNS itself. Strings that
do not conform to the rules for one of these three categories and, in
particular, strings that contain "--" in the third and fourth
character position but are:
o not A-labels or
o cannot be processed as U-labels or A-labels as described in these
specifications,
are invalid in IDNA-conformant applications as labels in domain names
that identify Internet hosts or similar resources.
2.3.1.2. LDH-label and Internationalized Label
In the hope of further clarifying discussions about IDNs, these
specifications use the term "LDH-label" strictly to refer to an all-
ASCII label that obeys the preferred syntax (often known as
"hostname" (from RFC 952 [RFC0952]) or "LDH") conventions and that is
not an IDN. It should be stressed that an A-label obeys the
"hostname" rules and is sometimes described as "LDH-conformant", or
in similar language, but it is not an LDH-label as that term is
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defined in these specifications.
2.3.1.3. Internationalized Domain Name
An "internationalized domain name" (IDN) is a domain name that may
contain any mixture of LDH-labels, A-labels, or U-labels. This
implies that every conventional domain name is an IDN (which implies
that it is possible for a domain name to be an IDN without it
containing any non-ASCII characters). Just as has been the case with
ASCII names, some DNS zone administrators may impose restrictions,
beyond those imposed by DNS or IDNA, on the characters or strings
that may be registered as labels in their zones. Because of the
diversity of characters that can be used in a U-label and the
confusion they might cause, such restrictions are mandatory for IDN
registries and zones even though the particular restrictions are not
part of these specifications. Because these restrictions, commonly
known as "registry restrictions", only affect what can be registered
and not lookup processing, they have no effect on the syntax or
semantics of DNS protocol messages; a query for a name that matches
no records will yield the same response regardless of the reason why
it is not in the zone. Clients issuing queries or interpreting
responses cannot be assumed to have any knowledge of zone-specific
restrictions or conventions. See the section on registration policy
in [IDNA2008-Rationale] for additional discussion.
"Internationalized label" is used when a term is needed to refer to a
single label of an IDN, i.e., one that might be any of an LDH-label,
A-label, or U-label. There are some standardized DNS label formats,
such as those for service location (SRV) records [RFC2782], that do
not fall into any of the three categories and hence are not
internationalized labels.
2.3.1.4. Equivalence
In IDNA, equivalence of labels is defined in terms of the A-labels.
If the A-labels are equal in a case-independent comparison, then the
labels are considered equivalent, no matter how they are represented.
Traditional LDH labels already have a notion of equivalence: within
that list of characters, upper case and lower case are considered
equivalent. The IDNA notion of equivalence is an extension of that
older notion. Equivalent labels in IDNA are treated as alternate
forms of the same label, just as "foo" and "Foo" are treated as
alternate forms of the same label.
2.3.1.5. ACE Prefix
The "ACE prefix" is defined in this document to be a string of ASCII
characters "xn--" that appears at the beginning of every A-label.
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"ACE" stands for "ASCII-Compatible Encoding".
2.3.1.6. Domain Name Slot
A "domain name slot" is defined in this document to be a protocol
element or a function argument or a return value (and so on)
explicitly designated for carrying a domain name. Examples of domain
name slots include: the QNAME field of a DNS query; the name argument
of the gethostbyname() or getaddrinfo() standard C library functions;
the part of an email address following the at-sign (@) in the
parameter to the SMTP MAIL or RCPT commands or the "From:" field of
an email message header; and the host portion of the URI in the src
attribute of an HTML ![]()
tag. A string that has the syntax of a
domain name but that appears in general text is not in a domain name
slot. For example, a domain name appearing in the plain text body of
an email message is not occupying a domain name slot.
An "IDN-aware domain name slot" is defined in this document to be a
domain name slot explicitly designated for carrying an
internationalized domain name as defined in this document. The
designation may be static (for example, in the specification of the
protocol or interface) or dynamic (for example, as a result of
negotiation in an interactive session).
An "IDN-unaware domain name slot" is defined in this document to be
any domain name slot that is not an IDN-aware domain name slot.
Obviously, this includes any domain name slot whose specification
predates IDNA.
2.3.2. Punycode is an Algorithm, not a Name
[[anchor17: Formerly Section 1.5.5 of Rationale-03.]]
There has been some confusion about whether a "Punycode string" does
or does not include the ACE prefix and about whether it is required
that such strings could have been the output of the ToASCII operation
(see RFC 3490, Section 4 [RFC3490]). This specification discourages
the use of the term "Punycode" to describe anything but the encoding
method and algorithm of [RFC3492]. The terms defined above are
preferred as much more clear than terms such as "Punycode string".
3. IANA Considerations
Actions for IANA are specified in other documents in this series
[IDNA2008-Protocol] [IDNA2008-Tables]. An overview of the
relationships among the various IANA registries appears in
[IDNA2008-Rationale]. This document does not specify any actions for
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IANA.
4. Security Considerations
Security on the Internet partly relies on the DNS. Thus, any change
to the characteristics of the DNS can change the security of much of
the Internet.
Domain names are used by users to identify and connect to Internet
servers. The security of the Internet is compromised if a user
entering a single internationalized name is connected to different
servers based on different interpretations of the internationalized
domain name.
When systems use local character sets other than ASCII and Unicode,
these specifications leave the problem of transcoding between the
local character set and Unicode up to the application or local
system. If different applications (or different versions of one
application) implement different transcoding rules, they could
interpret the same name differently and contact different servers.
This problem is not solved by security protocols, such as TLS, that
do not take local character sets into account.
To help prevent confusion between characters that are visually
similar, it is suggested that implementations provide visual
indications where a domain name contains multiple scripts. Such
mechanisms can also be used to show when a name contains a mixture of
simplified and traditional Chinese characters, or to distinguish zero
and one from O and l. DNS zone administrators may impose
restrictions (subject to the limitations identified elsewhere in
these documents) that try to minimize characters that have similar
appearance or similar interpretations. It is worth noting that there
are no comprehensive technical solutions to the problems of
confusable characters. One can reduce the extent of the problems in
various ways, but probably never eliminate it. Some specific
suggestions about identification and handling of confusable
characters appear in a Unicode Consortium publication
[Unicode-UTR36].
No mechanism involving names or identifiers alone can protect against
a wide variety of security threats and attacks that are largely
independent of the naming or identification system. These attacks
include spoofed pages, DNS query trapping and diversion, and so on.
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5. Acknowledgments
The initial version of this document was created largely by
extracting text from the "rationale" document [IDNA2008-Rationale].
See the section of this name, and the one entitled "Contributors", in
it.
6. References
6.1. Normative 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.
[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.
[Unicode-UAX15]
The Unicode Consortium, "Unicode Standard Annex #15:
Unicode Normalization Forms", March 2008,
.
[Unicode51]
The Unicode Consortium, "The Unicode Standard, Version
5.1.0", 2008.
defined by: The Unicode Standard, Version 5.0, Boston, MA,
Addison-Wesley, 2007, ISBN 0-321-48091-0, as amended by
Unicode 5.1.0
(http://www.unicode.org/versions/Unicode5.1.0/).
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6.2. Informative References
[IDNA2008-Bidi]
Alvestrand, H. and C. Karp, "An updated IDNA criterion for
right to left scripts", July 2008, .
[IDNA2008-Protocol]
Klensin, J., "Internationalized Domain Names in
Applications (IDNA): Protocol", September 2008, .
[IDNA2008-Rationale]
Klensin, J., "Internationalized Domain Names for
Applications (IDNA): Background, Explanation, and
Rationale", October 2008, .
[IDNA2008-Tables]
Faltstrom, P., "The Unicode Code Points 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
[RFC0952] Harrenstien, K., Stahl, M., and E. Feinler, "DoD Internet
host table specification", RFC 952, October 1985.
[RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS
Specification", RFC 2181, July 1997.
[RFC2673] Crawford, M., "Binary Labels in the Domain Name System",
RFC 2673, August 1999.
[RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
specifying the location of services (DNS SRV)", RFC 2782,
February 2000.
[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.
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[RFC3492] Costello, A., "Punycode: A Bootstring encoding of Unicode
for Internationalized Domain Names in Applications
(IDNA)", RFC 3492, March 2003.
[Unicode-UTR36]
The Unicode Consortium, "Unicode Technical Report #36:
Unicode Security Considerations", July 2008,
.
Appendix A. Change Log
[[RFC Editor: Please remove this appendix]]
A.1. Version -00
This document was created by pulling selected material out of
draft-ietf-idnabis-rationale-03 ("Rationale") after a WG consensus
call indicated that the rearrangement was appropriate. Mark Davis
made the major contribution of getting the process started by
identifying particular sections to be moved, even though this draft
does not completely reflect his list.
For Version -00 only, each section is identified with the associated
former section of Rationale-03. Those sections were edited after
incorporation into this document, so "Formerly" should be interpreted
very loosely.
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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