Internet Draft                                     Patrik Faltstrom
draft-ietf-idn-idna-07.txt                                    Cisco
February 24, 2002                                      Paul Hoffman
Expires in six months                                    IMC & VPNC
                                                   Adam M. Costello
                                                        UC Berkeley

       Internationalizing Domain Names in Applications (IDNA)

Status of this Memo

This document is an Internet-Draft and is in full conformance with all
provisions of Section 10 of RFC2026.

Internet-Drafts are working documents of the Internet Engineering Task
Force (IETF), its areas, and its working groups. Note that other groups
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and may be updated, replaced, or obsoleted by other documents at any
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Until now, there has been no standard method for domain names to use
characters outside the ASCII repertoire. This document defines
internationalized domain names (IDNs) and a mechanism called IDNA for
handling them in a standard fashion. IDNs use characters drawn from a
large repertoire (Unicode), but IDNA allows the non-ASCII characters to
be represented using the same octets used in so-called host names
today. IDNA is only meant for processing domain names, not free

1. Introduction

IDNA works by allowing applications to use certain ASCII name 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
require changes to any infrastructure. In particular, IDNA does not
require any changes to DNS servers, resolvers, or protocol elements,
because the ASCII name service provided by the existing DNS is entirely

This document does not require any applications to conform to IDNA,
but applications can elect to use IDNA in order to support IDN while
maintaining interoperability with existing infrastructure. Adding IDNA
support to an existing application entails changes to the application
only, and leaves room for flexibility in the user interface.

A great deal of the discussion of IDN solutions has focused on
transition issues and how IDN will work in a world where not all of the
components have been updated. Other proposals would require that user
applications, resolvers, and DNS servers be updated in order for a user
to use an internationalized domain name. Rather than require widespread
updating of all components, IDNA requires only user applications to be
updated; no changes are needed to the DNS protocol or any DNS servers or
the resolvers on user's computers.

1.1 Interaction of protocol parts

IDNA requires that implementations process input strings with Nameprep
[NAMEPREP], which is a profile of Stringprep [STRINGPREP], and then with
Punycode [PUNYCODE]. Implementations of IDNA MUST fully implement
Nameprep and Punycode; neither Nameprep nor Punycode are optional.

2 Terminology

The key words "MUST", "SHALL", "REQUIRED", "SHOULD", "RECOMMENDED", and
"MAY" in this document are to be interpreted as described in RFC 2119

A code point is an integral value associated with a character in a coded
character set.

Unicode [UNICODE] is a coded character set containing tens of thousands
of characters. A single Unicode code point is denoted by "U+" followed
by four to six hexadecimal digits, while a range of Unicode code points
is denoted by two hexadecimal numbers separated by "..", with no

ASCII means US-ASCII, a coded character set containing 128 characters
associated with code points in the range 0..7F. Unicode is an extension
of ASCII: it includes all the ASCII characters and associates them with
the same code points.

The term "LDH code points" is defined in this document to mean the code
points associated with ASCII letters, digits, and the hyphen-minus; that
is, U+002D, 30..39, 41..5A, and 61..7A. "LDH" is an abbreviation for
"letters, digits, hyphen".

[STD13] talks about "domain names" and "host names", but many people use
the terms interchangeably. Further, because [STD13] was not terribly
clear, many people who are sure they know the exact definitions of each
of these terms disagree on the definitions.

A label is an individual part of a domain name. Labels are usually shown
separated by dots; for example, the domain name "" is
composed of three labels: "www", "example", and "com". (The zero-length
root label that is implied in domain names, as described in [STD13], is
not considered a label in this specification.) Throughout this document
the term "label" is shorthand for "text label", and "every label" means
"every text label". In IDNA, not all text strings can be labels.

An "internationalized domain name" (IDN) is a domain name for which the
ToASCII operation (see section 4) can be applied to each label without
failing. This document does not attempt to define an "internationalized
host name". It is expected that protocols and name-handling bodies will
want to limit the characters allowed in IDNs further than what is
specified in this document, such as to prohibit additional characters
that they feel are unneeded or harmful in registered domain names.

An "internationalized label" is a label composed of characters from the
Unicode character set; note, however, that not every string of Unicode
characters can be an internationalized label. To allow internationalized
labels to be handled by existing applications, IDNA uses an "ACE label"
(ACE stands for ASCII Compatible Encoding), which can be represented
using only ASCII characters but is equivalent to a label containing
non-ASCII characters. More rigorously, an ACE label is defined to be any
label that the ToUnicode operation would alter (see section 4.2). For
every internationalized label that cannot be directly represented in
ASCII, there is an equivalent ACE label. The conversion of labels to and
from the ACE form is specified in section 4.

The "ACE prefix" is defined in this document to be a string of ASCII
characters that appears at the beginning of every ACE label. It is
specified in section 5.

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() library function; the part of an email address following
the at-sign (@) in the From: field of an email message header; and the host
portion of the URI in the src attribute of an HTML <IMG> tag.
General text that just happens to contain a domain name is not 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 "internationalized 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).

A "generic domain name slot" is defined in this document to be any
domain name slot that is not an internationalized domain name slot.
Obviously, this includes any domain name slot whose specification
predates IDNA.

3. Requirements

IDNA conformance means adherence of the following three requirements:

1) Whenever a domain name is put into a generic domain name slot (see
section 2), every label MUST contain only ASCII characters. Given an
internationalized domain name (IDN), an equivalent domain name
satisfying this requirement can be obtained by applying the ToASCII
operation (see section 4) to each label.

2) ACE labels obtained from domain name slots SHOULD be hidden from
users except when the use of the non-ASCII form would cause problems or
when the ACE form is explicitly requested.  Given an internationalized
domain name, an equivalent domain name containing no ACE labels can be
obtained by applying the ToUnicode operation (see section 4) to each
label.  When requirements 1 and 2 both apply, requirement 1 takes

3) Whenever two labels are compared, they MUST be considered to
match if and only if their ASCII forms (obtained by applying ToASCII)
match using a case-insensitive ASCII comparison.

4. Conversion operations

This section specifies the ToASCII and ToUnicode operations. Each one
operates on a sequence of Unicode code points (but remember that all
ASCII code points are also Unicode code points). When domain names are
represented using character sets other than Unicode and ASCII, they will
need to first be transcoded to Unicode before these operations can be
applied, and might need to be transcoded back afterwards.


The ToASCII operation takes a sequence of Unicode code points and
transforms it into a sequence of code points in the ASCII range (0..7F).
The original sequence and the resulting sequence are equivalent labels.
(If the original is an internationalized label that cannot be directly
represented in ASCII, the result will be the equivalent ACE label.)

ToASCII fails if any step of it fails. If any step fails, the original
sequence MUST NOT be used as a label in an IDN.

The inputs to ToASCII are a sequence of code points; a flag indicating
whether to prohibit unassigned code points (see [STRINGPREP]); and a
flag indicating whether to apply the host name syntax rules. The output
of ToASCII is either a sequence of ASCII code points or a failure

ToASCII never alters a sequence of code points that are all in the ASCII
range to begin with (although it could fail).

ToASCII consists of the following steps:

    1. If all code points in the sequence are in the ASCII range (0..7F)
       then skip to step 3.

    2. Perform the steps specified in [NAMEPREP] and fail if there is
       an error.

    3. If the label is part of a host name (or is subject to the host
       name syntax rules) then perform these checks:

         (a) Verify the absence of non-LDH ASCII code points; that is,
             the absence of 0..2C, 2E..2F, 3A..40, 5B..60, and 7B..7F.

         (b) Verify the absence of leading and trailing hyphen-minus;
             that is, the absence of U+002D at the beginning and end of
             the sequence.

    4. If all code points in the sequence are in the ASCII range (0..7F),
       then skip to step 8.

    5. Verify that the sequence does NOT begin with the ACE prefix.

    6. Encode the sequence using the encoding algorithm in [PUNYCODE].

    7. Prepend the ACE prefix.

    8. Verify that the number of code points is in the range 1 to 63

4.2 ToUnicode

The ToUnicode operation takes a sequence of Unicode code points and
returns a sequence of Unicode code points. If the input sequence is a
label in ACE form, then the result is an equivalent internationalized
label that is not in ACE form, otherwise the original sequence is
returned unaltered.

ToUnicode never fails. If any step fails, then the original input
sequence is returned immediately in that step.

The inputs to ToUnicode are a sequence of code points; a flag indicating
whether to prohibit unassigned code points (see [STRINGPREP]); and a
flag indicating whether to apply the host name syntax rules. The output
of ToUnicode is always a sequence of Unicode code points.

    1. If all code points in the sequence are in the ASCII range (0..7F)
       then skip to step 3.

    2. Perform the steps specified in [NAMEPREP] and fail if there is an
       error. (If step 3 of ToASCII is also performed here, it will not
       affect the overall behavior of ToUnicode, but it is not

    3. Verify that the sequence begins with the ACE prefix, and save a
       copy of the sequence.

    4. Remove the ACE prefix.

    5. Decode the sequence using decoding algorithm in [PUNYCODE]. Save
       a copy of the result of this step.

    6. Apply ToASCII.

    7. Verify that the sequence matches the saved copy from step 3, using
       a case-insensitive ASCII comparison.

    8. Return the saved copy from step 5.

5. ACE prefix

[[ Note to the IESG and Internet Draft readers: The two uses of the
string "IESG--" below are to be changed at time of publication to a
prefix which fulfills the requirements in the first paragraph. ]]

The ACE prefix, used in the conversion operations (section 4), is two
alphanumeric ASCII characters followed by two hyphen-minuses. It cannot
be any of the prefixes already used in earlier documents, which includes
the following: "bl--", "bq--", "dq--", "lq--", "mq--", "ra--", "wq--"
and "zq--". The ToASCII and ToUnicode operations MUST recognize the ACE
prefix in a case-insensitive manner.

The ACE prefix for IDNA is "IESG--".

This means that an ACE label might be "IESG--de-jg4avhby1noc0d", where
"de-jg4avhby1noc0d" is the part of the ACE label that is generated by
the encoding steps in [PUNYCODE].

6. Implications for typical applications using DNS

In IDNA, applications perform the processing needed to input
internationalized domain names from users, display internationalized
domain names to users, and process the inputs and outputs from DNS and
other protocols that carry domain names.

The components and interfaces between them can be represented
pictorially as:

                     | User |
                        | Input and display: local interface methods
                        | (pen, keyboard, glowing phosphorus, ...)
    |                   v                               |
    |          +-----------------------------+          |
    |          |        Application          |          |
    |          |  (conversion between local  |          |
    |          |  character set and Unicode  |          |
    |          |       is done here)         |          |
    |          +-----------------------------+          |
    |                    ^       ^                      | End system
    |                    |       |                      |
    |  Call to resolver: |       | Application-specific |
    |               ACE  |       | protocol:            |
    |                    v       | predefined by the    |
    |           +----------+     | protocol or defaults |
    |           | Resolver |     | to ACE               |
    |           +----------+     |                      |
    |                 ^          |                      |
        DNS protocol: |          |
                  ACE |          |
                      v          v
           +-------------+    +---------------------+
           | DNS servers |    | Application servers |
           +-------------+    +---------------------+

6.1 Entry and display in applications

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, and as an ACE label. ACE labels that are
displayed or input MUST always include the ACE prefix. Applications MAY
allow input and display of ACE labels, but are not encouraged to do so
except as an interface for special purposes, possibly for debugging. ACE
encoding is opaque and ugly, and should thus only be exposed to users
who absolutely need it. The optional use, especially during a transition
period, of ACE encodings in the user interface is described in section
6.4. Because name labels encoded as ACE name labels can be rendered
either as the encoded ASCII characters or the proper decoded characters,
the application MAY have an option for the user to select the preferred
method of display; if it does, rendering the ACE SHOULD NOT be the

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.

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, ACE labels
(internationalized labels that have been processed with the ToASCII
operation) can inherently be handled by those protocols.

6.2 Applications and resolver libraries

Applications normally use functions in the operating system when they
resolve DNS queries. Those functions in the operating system are often
called "the resolver library", and the applications communicate with the
resolver libraries through a programming interface (API).

Because these resolver libraries today expect only domain names in
ASCII, applications MUST prepare labels that are passed to the resolver
library using the ToASCII operation. Labels received from the resolver
library contain only ASCII characters; internationalized labels that
cannot be represented directly in ASCII use the ACE form. ACE labels
always include the ACE prefix.

IDNA-aware applications MUST be able to work with both
non-internationalized labels (those that conform to [STD13]
and [STD3]) and internationalized labels.

It is expected that new versions of the resolver libraries in the future
will be able to accept domain names in other formats than ASCII, and
application developers might one day pass not only domain names in
Unicode, but also in local script to a new API for the resolver
libraries in the operating system.

6.3 DNS servers

An operating system might have a set of libraries for performing the
ToASCII operation. The input to such a library might be in one or more
charsets that are used in applications (UTF-8 and UTF-16 are likely
candidates for almost any operating system, and script-specific charsets
are likely for localized operating systems).

For internationalized labels that cannot be represented directly in
ASCII, DNS servers MUST use the ACE form produced by the ToASCII
operation. All IDNs served by DNS servers MUST contain only ASCII

If a signalling system which makes negotiation possible between old and
new DNS clients and servers is standardized in the future, the encoding
of the query in the DNS protocol itself can be changed from ACE to
something else, such as UTF-8. The question whether or not this should
be used is, however, a separate problem and is not discussed in this

6.4 Avoiding exposing users to the raw ACE encoding

All applications that might show the user a domain name obtained from a
domain name slot, such as from gethostbyaddr or part of a mail header,
SHOULD be updated as soon as possible in order to prevent users from
seeing the ACE.

If an application decodes an ACE name using ToUnicode but cannot show
all of the characters in the decoded name, such as if the name contains
characters that the output system cannot display, the application SHOULD
show the name in ACE format (which always includes the ACE prefix)
instead of displaying the name with the replacement character (U+FFFD).
This is to make it easier for the user to transfer the name correctly to
other programs. Programs that by default show the ACE form when they
cannot show all the characters in a name label SHOULD also have a
mechanism to show the name that is produced by the ToUnicode operation
with as many characters as possible and replacement characters in the
positions where characters cannot be displayed.

The ToUnicode operation does not alter labels that are not valid ACE
labels, even if they begin with the ACE prefix. After ToUnicode has been
applied, if a label still begins with the ACE prefix, then it is not a
valid ACE label, and is not equivalent to any of the intermediate
Unicode strings constructed by ToUnicode.

6.5 Bidirectional text in domain names

The display of domain names that contain bidirectional text is not covered
in this document. It may be covered in a future version of this
document, or may be covered in a different document.

For developers interested in displaying domain names that have
bidirectional text, the Unicode standard has an extensive discussion of
how to deal with reorder glyphs for display when dealing with
bidirectional text such as Arabic or Hebrew. See [UAX9] for more
information. In particular, all Unicode text is stored in logical order.

6.6  DNSSEC authentication of IDN domain names

DNS Security [DNSSEC] 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 ACE
form produced by the ToASCII operation. This operation 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
the ASCII domain name, not the Unicode form or the mapping between the
Unicode form and the ASCII form. In other words, the output of ToASCII
is the canonical name. In the presence of DNSSEC, this is the name that
MUST be signed in the zone and MUST be validated against. It also SHOULD
be used for other name comparisons, such as when a browser wants to
indicate that a URL has been previously visited.

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.

6.7 Limitations of IDNA

The IDNA protocol does not solve all linguistic issues with users
inputting names in different scripts. Many important language-based and
script-based mappings are not covered in IDNA and must be handled
outside the protocol. For example, names that are entered in a mix of
traditional and simplified Chinese characters will not be mapped to a
single canonical name. Another example is Scandinavian names that are
entered with U+00F6 (LATIN SMALL LETTER O WITH DIAERESIS) will not be

7. Name Server Considerations

Internationalized domain name data in zone files (as specified by section
5 of RFC 1035) MUST be processed with ToASCII before it is entered in
the zone files.

It is imperative that there be only one ASCII encoding for a particular
domain name. ACE is an encoding for domain name labels that use non-ASCII
characters. Thus, a primary master name server MUST NOT contain an
ACE-encoded label that decodes to an ASCII label. The ToASCII operation
assures that no such names are ever output from the operation.

Name servers MUST NOT serve records with domain names that contain
non-ASCII characters; such names MUST be converted to ACE form by the
ToASCII operation in order to be served. If names that are not processed
by ToASCII are passed to an application, it will result in unpredictable
behavior. Note that [STRINGPREP] describes how to handle versioning of
unallocated codepoints.

8. Root Server Considerations

IDNs are likely to be somewhat longer than current host names, so the
bandwidth needed by the root servers should 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.

9. 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.

This memo describes an algorithm which encodes characters that are not
valid according to STD3 and STD13 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 are used by users to connect to Internet servers. The
security of the Internet would be compromised if a user entering a
single internationalized name could be connected to different servers
based on different interpretations of the internationalized domain name.

Because this document normatively refers to [NAMEPREP], it includes the
security considerations from that document as well.

A. References

[PUNYCODE] Adam Costello, "Punycode", draft-ietf-idn-punycode.

[DNSSEC] Don Eastlake, "Domain Name System Security Extensions", RFC
2535, March 1999.

[NAMEPREP] Paul Hoffman and Marc Blanchet, "Preparation of
Internationalized Domain Names", draft-ietf-idn-nameprep.

[RFC2119] Scott Bradner, "Key words for use in RFCs to Indicate
Requirement Levels", March 1997, RFC 2119.

[STD3] Bob Braden, "Requirements for Internet Hosts -- Communication
Layers" (RFC 1122) and "Requirements for Internet Hosts -- Application
and Support" (RFC 1123), STD 3, October 1989.

[STD13] Paul Mockapetris, "Domain names - concepts and facilities" (RFC
1034) and "Domain names - implementation and specification" (RFC 1035),
STD 13, November 1987.

[STRINGPREP] Paul Hoffman and Marc Blanchet, "Preparation of
Internationalized Strings ("stringprep")", draft-hoffman-stringprep,
work in progress
[UAX9] Unicode Standard Annex #9, The Bidirectional Algorithm,

[UNICODE] The Unicode Standard, Version 3.1.0: The Unicode Consortium.
The Unicode Standard, Version 3.0. Reading, MA, Addison-Wesley
Developers Press, 2000. ISBN 0-201-61633-5, as amended by: Unicode
Standard Annex #27: Unicode 3.1,

B. Authors' Addresses

Patrik Faltstrom
Cisco Systems
Arstaangsvagen 31 J
S-117 43 Stockholm  Sweden

Paul Hoffman
Internet Mail Consortium and VPN Consortium
127 Segre Place
Santa Cruz, CA  95060  USA

Adam M. Costello
University of California, Berkeley
idna-spec.amc @