One document matched: draft-klensin-idnabis-protocol-00.txt
Network Working Group J. Klensin
Internet-Draft November 2, 2007
Obsoletes: 3490 (if approved)
Intended status: Standards Track
Expires: May 5, 2008
Internationalizing Domain Names in Applications (IDNA): Protocol
draft-klensin-idnabis-protocol-00.txt
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Copyright Notice
Copyright (C) The IETF Trust (2007).
Abstract
This document supplies the protocol definition for a revised and
updated specification for internationalized domain names. The
rationale for these changes and relationship to the older
specification and some new terminology is provided in other
documents. This document specifies a standard method using
characters outside the ASCII repertoire in domain names. This
document defines internationalized domain names (IDNs) and a
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mechanism called Internationalizing Domain Names in Applications
(IDNA) for handling them in a standard fashion. IDNs use characters
drawn from a large subset of the Unicode repertoire, but IDNA allows
the non-ASCII characters to be represented using only the ASCII
characters already allowed in so-called host names today. This
backward-compatible representation is required in existing protocols
like DNS, so that IDNs can be introduced with no changes to the
existing infrastructure. IDNA is only meant for processing domain
names, not free text.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Requirements and Applicability . . . . . . . . . . . . . . . . 5
3.1. Requirements . . . . . . . . . . . . . . . . . . . . . . . 5
3.2. Applicability . . . . . . . . . . . . . . . . . . . . . . 5
3.2.1. DNS Resource Records . . . . . . . . . . . . . . . . . 5
3.2.2. Non-domain-name Data Types Stored in Domain Names . . 6
4. Registration Protocol . . . . . . . . . . . . . . . . . . . . 6
4.1. Proposed label . . . . . . . . . . . . . . . . . . . . . . 6
4.2. Conversion to Unicode . . . . . . . . . . . . . . . . . . 6
4.3. Permitted Character Identification . . . . . . . . . . . . 6
4.4. Additional Character String Checking and Processing . . . 7
4.5. Registry Restrictions . . . . . . . . . . . . . . . . . . 7
4.6. Punycode Conversion . . . . . . . . . . . . . . . . . . . 8
4.7. Insertion in the Zone . . . . . . . . . . . . . . . . . . 8
5. Domain Name Resolution (Lookup) Protocol . . . . . . . . . . . 8
5.1. User input . . . . . . . . . . . . . . . . . . . . . . . . 8
5.2. Conversion to Unicode . . . . . . . . . . . . . . . . . . 8
5.3. User Interface Character Changes . . . . . . . . . . . . . 9
5.4. Pre-Normalization Validation and Character List Testing . 9
5.5. Normalization . . . . . . . . . . . . . . . . . . . . . . 10
5.6. Post-Normalization Processing . . . . . . . . . . . . . . 10
5.7. Punycode Conversion . . . . . . . . . . . . . . . . . . . 10
5.8. DNS Name Resolution . . . . . . . . . . . . . . . . . . . 10
6. Entry and Display in Applications . . . . . . . . . . . . . . 10
7. Name server Considerations . . . . . . . . . . . . . . . . . . 11
7.1. Processing Non-ASCII Strings . . . . . . . . . . . . . . . 11
7.2. DNSSEC Authentication of IDN Domain Names . . . . . . . . 12
7.3. Root Server Considerations . . . . . . . . . . . . . . . . 12
8. Security Considerations . . . . . . . . . . . . . . . . . . . 12
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
10. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 13
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 14
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 14
12.1. Normative References . . . . . . . . . . . . . . . . . . . 14
12.2. Informative References . . . . . . . . . . . . . . . . . . 14
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 15
Intellectual Property and Copyright Statements . . . . . . . . . . 17
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1. Introduction
This document supplies the protocol definition for a revised and
updated specification for internationalized domain names. The
rationale for these changes and relationship to the older
specification and some new terminology is provided in other
documents, notably [IDNA200X-issues].
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 depend on changes to 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 is entirely sufficient for IDNA.
IDNA is applied only to DNS labels. Standards for combining labels
into fully-qualified domain names and parsing labels out of those
names are covered in the base DNS standards [RFC1035]. An
application may, of course, apply locally-appropriate conventions to
the presentation forms of domain names as discussed in Section 6 and
[IDNA200X-issues].
A good deal of the background material that appeared in RFC 3490 has
been removed from this update. That material is either of historical
interest only or has been covered from a more recent perspective in
RFC 4690 [RFC4690] and [IDNA200X-issues].
[[anchor2: This version of this draft, and the corresponding version
of the issues/rationale document, mark the first step in separating
protocol-specific materials about IDNA from discussions of rationale
and user interface issues. The documents are intermediate works in
progress with a number of loose ends, some but not all of which are
explicitly identified. They are posted for the convenience of the
community and to solicit the community's help in identifying the
boundary between materials that should correctly be in each document.
A revised set of drafts can be anticipated significantly before IETF
70.]]
2. Terminology
General terminology applicable to IDNA, but with meanings familiar to
those who have worked with Unicode or other character set standards
and the DNS, appears in [IDNA200X-issues]. Terminology that is an
integral, normative, part of the IDNA definition appears in that
document as well. Familiarity with the terminology materials in that
document is assumed for reading this one.
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[[anchor3: May want to copy some definitions from "issues" back to
here to avoid or reduce normative dependencies. But, of course, that
would risk the two sets of definitions becoming inconsistent.
Tradeoff...]]
The key words "MUST", "SHALL", "REQUIRED", "SHOULD", "RECOMMENDED",
and "MAY" in this document are to be interpreted as described in BCP
14, RFC 2119 [RFC2119].
3. Requirements and Applicability
3.1. Requirements
IDNA conformance means adherence to the following requirements:
1. Whenever a domain name is put into an IDN-unaware domain name
slot (see Section 2 and [IDNA200X-issues]), it MUST contain only
ASCII characters (i.e., must be either an A-label, an LDH-label,
or a label associated with a DNS application that is not subject
to IDNA.
2. Comparing two labels MUST be done on the A-label form, using an
ASCII case-insensitive comparison.
3.2. Applicability
IDNA is applicable to all domain names in all domain name slots
except where it is explicitly excluded. It is not applicable to
domain name slots which do not use the LDH syntax rules.
This implies that IDNA is applicable to many protocols that predate
IDNA. Note that IDNs occupying domain name slots in those protocols
MUST be in A-label form.
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.
There are currently no other exclusions on the applicability of IDNA
to DNS resource records; it depends entirely on the CLASS, and not on
the TYPE. This will remain true, even as new types are defined,
unless there is a compelling reason for a new type to complicate
matters by imposing type-specific rules. It is worth noting that the
special naming conventions applicable to SRV records are precisely
such type-specific rules.
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3.2.2. Non-domain-name Data Types Stored in Domain Names
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. For example, an email address local part is
sometimes stored in a domain label (hostmaster@example.com would be
represented as hostmaster.example.com in the RDATA field of an SOA
record). IDNA does not update the existing email standards, which
allow only ASCII characters in local parts. Other work is under
development to define internationalization for email addresses
[RFC4952], but changes to that part of the SOA RDATA would require
action in other standards.
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.
4.1. Proposed label
The registrant submits a request for an IDN. The user typically
produces the request string by the keyboard entry of a character
sequence.
4.2. Conversion to Unicode
Some system routine, or a localized front-end to the IDNA process,
ensures that the proposed label is a Unicode string. As a local
implementation choice, the implementation may choose to map some
forbidden characters to permitted characters (for instance mapping
uppercase characters to lowercase ones), displaying the result to the
user, and allowing processing to continue.
4.3. Permitted Character Identification
The Unicode string is examined to prohibit characters that IDNA does
not permit in input. Those characters are identified in the "never"
list that is discussed in [IDNA200X-issues] and [IDNA200X-Permitted].
In addition to what is explicitly identified in those documents,
characters or sequences that are unassigned in Unicode MUST NOT be
part of labels registered in the DNS.
[[anchor9: The previous sentence should be redundant with the text in
the tables and issues documents, but I want to leave it here until we
are sure those are stable --JcK]]
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4.4. Additional Character String Checking and Processing
All characters produced as output of the preceding step are then
verified for permissibility by IDNA. Conceptually, these tests are,
in order:
1. Each code point is verified to be assigned in the version of
Unicode in use (See [IDNA200X-issues]).
2. Each code point is checked for its presence as "Always permitted"
in the table of included characters for registration or, if
appropriate for the specific registry, as "Maybe permitted" (see
[IDNA200X-Permitted]).
3. Each code point is checked for its presence as "Context Required"
in the table of included characters for registration. If that
indication appears, the table of contextual rules is checked for
a rule for that character. If no rule is found, the proposed
label is rejected. If one is found, it is applied (typically as
a test on the entire label or adjacent characters). If the
application of the rule does not conclude that the character is
valid in context, the proposed label must be rejected. (See the
IANA Considerations: IDNA Context Registry section of
[IDNA200X-issues]).
4. Additional special tests for right-to-left strings are applied
(See [IDNA200X-BIDI].
Strings that have been produced by the steps above, and whose
contents pass the above tests, are U-labels.
To summarize, tests are made here for invalid combinations of
characters, and for labels that are invalid even if the individual
characters they contain are all valid. For example, labels
containing invisible ("zero-width") characters may be permitted in
context with characters whose presentation forms are significantly
changed by the presence or absence of the zero-width characters,
while other labels in which zero-width characters appear may be
rejected. Additional transformations that do not occur as the result
of the steps above may be specified at this point by IDNA200x. As
the list of characters permitted to be registered expands, new rules,
similar to those suggested for zero-width characters, may accompany
them.
4.5. Registry Restrictions
Registries at all levels of the DNS, not just the top level, are
expected to establish policies about the labels that may be
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registered, and for the processes associated with that action. As
discussed in [IDNA200X-issues]), such restrictions have always
existed in the DNS.
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.
4.6. Punycode Conversion
The resulting U-label is converted to an A-label (i.e., the encoding
of that label according to the Punycode algorithm [RFC3492] with the
prefix included, i.e., the "xn--..." form).
4.7. Insertion in the Zone
The A-label is then registered in the DNS by insertion into a zone.
5. Domain Name Resolution (Lookup) Protocol
Resolution is conceptually different from registration and different
tests are applied on the client. The resolution-side tests are more
permissive and rely heavily on the assumption that names that are
present in the DNS are valid. Among other things, this distinction
facilitates expansion of the permitted character lists to include new
scripts and accommodate new version of Unicode.
5.1. User input
The user supplies a string in the local character set, typically by
typing it or clicking on, or cutting and pasting, a resource
identifier, e.g., a URI [RFC3986] or IRI [RFC3987]. Processing in
this step and the next two are local matters, to be accomplished
prior to actual invocation of IDNAbis, but at least this one and the
next one must be accomplished in some way.
5.2. Conversion to Unicode
The local character set, character coding conventions, and, as
necessary, display and presentation conventions, are converted to
Unicode (without surrogates), paralleling the process above
(Section 4.2).
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5.3. User Interface Character Changes
The Unicode string MAY then be processed, in a way specific to the
local environment, to make the result of the IDNA processing match
user expectations. For instance, at this step, it would be
reasonable to case-fold all upper case characters to lower case, if
this makes sense in the user's environment.
Other examples of processing for localization that might be applied,
if appropriate, at this point (but even further outside the scope of
this specification) include interpreting the KANA MIDDLE DOT to
separate domain name components from each other, standardizing
different "width" forms of the same character, or giving special
treatment to characters whose presentation forms are dependent only
on placement in the label.
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, and not
specified further here.
5.4. Pre-Normalization Validation and Character List Testing
In parallel to the registration procedure, the Unicode string is
checked to verify that all characters that appear in it are valid for
IDNA resolution input. As discussed in [IDNA200X-issues], the
resolution check is more liberal than that of the registration one:
characters that fall into the "Maybe" (see [IDNA200X-issues])
categories in the inclusion tables do not lead to label rejection on
resolution. Putative labels containing code points with any of the
following characteristics MUST BE rejected prior to DNS lookup:
o Code points that are unassigned in the version of Unicode being
used by the application.
o Prohibited code points, i.e., those that are assigned to the
"Never" category in the permitted character table.
o Code points that are shown in the permitted character table as
requiring a contextual rule, but for which no such rule appears in
the table of rules.
For all other strings, the resolver 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.
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5.5. Normalization
The validated Unicode string is normalized (using NFC); no case-
mapping is performed. If a code point unassigned in the current
version of Unicode is actually assigned in a later version, the
resolver and the application containing it and calling it should be
upgraded when possible; the protocol cannot automatically provide
that upgrade. See [IDNA200X-issues] for more discussion on this
issue.
5.6. Post-Normalization Processing
Any necessary processing or filtering is applied to the normalized
output string produced by the above. In the cases that can be
anticipated, this step will be null. It is included in the model in
case, e.g., full-label checks are needed on lookup.
5.7. Punycode Conversion
The validated string, a U-label, is converted to an A-label.
5.8. DNS Name Resolution
The A-label is looked up in the DNS, using normal DNS procedures.
6. Entry and Display in Applications
[[anchor12: It may be appropriate to move this entire section to
"issues". Opinions welcome.]]
Applications can accept domain names using any character set or sets
desired by the application developer, and can display domain names in
any charset. That is, the IDNA protocol does not affect the
interface between users and applications.
An IDNA-aware application can accept and display internationalized
domain names in two formats: the internationalized character set(s)
supported by the application (i.e., an appropriate local
representation of a U-label), and as an A-label. Applications MAY
allow input from, and display to, the user of A-labels, but are not
encouraged to do so except as an interface for special purposes,
possibly for debugging, or to cope with display limitations.
A-labels are opaque and ugly, and, where possible, should thus only
be exposed to users who absolutely need them. Because IDN labels can
be rendered either as the A-labels or U-labels, the application may
reasonably have an option for the user to select the preferred method
of display; if it does, rendering the U-label should normally not be
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the default.
Domain names are often stored and transported in many places. For
example, they are part of documents such as mail messages and web
pages. They are transported in many parts of many protocols, such as
both the control commands and the RFC 2822 body parts of SMTP, and
the headers and the body content in HTTP. It is important to
remember that domain names appear both in domain name slots and in
the content that is passed over protocols.
In protocols and document formats that define how to handle
specification or negotiation of charsets, labels can be encoded in
any charset allowed by the protocol or document format. If a
protocol or document format only allows one charset, the labels MUST
be given in that charset. Of course, not all charsets can properly
represent all labels. If a U-label cannot be displayed in its
entirety, the only choice (without loss of information) may be to
display the A-label.
In any place where a protocol or document format allows transmission
of the characters in internationalized labels, internationalized
labels SHOULD be transmitted using whatever character encoding and
escape mechanism that the protocol or document format uses at that
place.
All protocols that use domain name slots already have the capacity
for handling domain names in the ASCII charset. Thus, A-labels can
inherently be handled by those protocols.
7. Name server Considerations
7.1. Processing Non-ASCII Strings
Existing DNS servers do not know the IDNA rules for handling non-
ASCII forms of IDNs, and therefore need to be shielded from them.
All existing channels through which names can enter a DNS server
database (for example, master files (as described in RFC 1034) and
DNS update messages [RFC2136]) are IDN-unaware because they predate
IDNA. Other sections of this document provide the needed shielding
by ensuring that internationalized domain names entering DNS server
databases through such channels have already been converted to their
equivalent ASCII A-label forms.
It is imperative that there be only one ASCII encoding for a
particular domain name. Because of the design of the U-label and
A-label definitions and transformations, there are no ACE labels that
decode to ASCII labels (LDH or otherwise), and therefore name servers
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cannot contain multiple ASCII encodings of the same domain name.
[RFC2181] explicitly allows domain labels to contain octets beyond
the ASCII range (0..7F), and this document does not change that.
Note, however, that there is no defined interpretation of octets
80..FF as characters. If labels containing these octets are returned
to applications, unpredictable behavior could result. The A-label
(ASCII) form, which cannot contain those characters, is the only
standard representation for internationalized labels in the current
DNS protocol.
7.2. DNSSEC Authentication of IDN Domain Names
DNS Security [RFC2535] is a method for supplying cryptographic
verification information along with DNS messages. Public Key
Cryptography is used in conjunction with digital signatures to
provide a means for a requester of domain information to authenticate
the source of the data. This ensures that it can be traced back to a
trusted source, either directly, or via a chain of trust linking the
source of the information to the top of the DNS hierarchy.
IDNA specifies that all internationalized domain names served by DNS
servers that cannot be represented directly in ASCII must use the
A-label form. Conversion to A-labels must be performed prior to a
zone being signed by the private key for that zone. Because of this
ordering, it is important to recognize that DNSSEC authenticates a
domain name containing A-labels or conventional LDH-labels, not
U-labels. In the presence of DNSSEC, no form of a zone file or query
response that contains a U-label may be signed or validated against.
One consequence of this for sites deploying IDNA in the presence of
DNSSEC is that any special purpose proxies or forwarders used to
transform user input into IDNs must be earlier in the resolution flow
than DNSSEC authenticating nameservers for DNSSEC to work.
7.3. Root Server Considerations
IDNs are likely to be somewhat longer than current domain names, so
the bandwidth needed by the root servers is likely to go up by a
small amount. Also, queries and responses for IDNs will probably be
somewhat longer than typical queries today, so more queries and
responses may be forced to go to TCP instead of UDP.
8. Security Considerations
The general security principles and issues for IDNA appear in
[IDNA200X-issues]. The comments below are specific to this protocol,
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but should be read in the context of that material and the
specifications, identified there, on which this one depends.
This memo describes an algorithm which encodes characters that are
not valid according to the base DNS specifications (STD13 [RFC1034]
[RFC1035] and Host Requirements [RFC1123]) into octet values that are
valid. No security issues such as string length increases or new
allowed values are introduced by the encoding process or the use of
these encoded values, apart from those introduced by the ACE encoding
itself.
Domain names (or portions of them) are sometimes compared against a
set of privileged or anti-privileged domains. In such situations it
is especially important that the comparisons be done properly, as
specified in requirement 2 of Section 3.1. For labels already in
ASCII form, the proper comparison reduces to the same case-
insensitive ASCII comparison that has always been used for ASCII
labels.
The introduction of IDNA means that any existing labels that start
with the ACE prefix would be construed as U-labels, at least until
they failed one of the relevant tests, whether or not that was the
intent of the zone administrator or registrant. There is no evidence
that this has caused any practical problems since RFC 3490 was
adopted, but the risk still exists in principle.
9. IANA Considerations
IANA actions for this version of IDNA are specified in
[IDNA200X-issues].
10. Contributors
While the listed editor held the pen, this document represents 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.
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11. Acknowledgements
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 [IDNA200X-issues] were particularly
important.
12. References
12.1. Normative References
[IDNA200X-BIDI]
Alvestrand, H. and C. Karp, "An IDNA problem in right-to-
left scripts", October 2006, <http://www.ietf.org/
internet-drafts/draft-alvestrand-idna-bidi-00.txt>.
[IDNA200X-issues]
Klensin, J., Ed., "Internationalizing Domain Names for
Applications (IDNA): Issues and Rationale", November 2007.
[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.
12.2. 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
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slight modifications, but the 1968 version remains
definitive for the Internet.
[IDNA200X-Permitted]
Faltstrom, P., "The Unicode Codepoints and IDN",
February 2007, <http://stupid.domain.name/idnabis/
draft-faltstrom-idnabis-tables-02.txt>.
A version of this document, is available in HTML format at
http://stupid.domain.name/idnabis/
draft-faltstrom-idnabis-tables-02.txt
[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.
[RFC3490] Faltstrom, P., Hoffman, P., and A. Costello,
"Internationalizing Domain Names in Applications (IDNA)",
RFC 3490, March 2003.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
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
Klensin Expires May 5, 2008 [Page 15]
Internet-Draft IDNA200X Protocol November 2007
Author's Address
John C Klensin
1770 Massachusetts Ave, Ste 322
Cambridge, MA 02140
USA
Phone: +1 617 245 1457
Fax:
Email: john+ietf@jck.com
URI:
Klensin Expires May 5, 2008 [Page 16]
Internet-Draft IDNA200X Protocol November 2007
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Klensin Expires May 5, 2008 [Page 17]
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