Internet Draft                                     Patrik Faltstrom
draft-ietf-idn-idna-03.txt                                    Cisco
July 20, 2001                                          Paul Hoffman
Expires in six months                                    IMC & VPNC

             Internationalizing Host 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.

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The current DNS infrastructure does not provide a way to use
internationalized host names (IDN). This document describes a mechanism
that requires no changes to any DNS server or resolver that will allow
internationalized host names to be used by end users with changes only
to applications. It allows flexibility for user input and display, and
assures that host names that have non-ASCII characters are not sent to
DNS servers or resolvers.

1. Introduction

In the discussion of IDN solutions, a great deal of discussion has
focused on transition issues and how IDN will work in a world where not
all of the components have been updated. Earlier proposed solutions
require that user applications, resolvers, and DNS servers to be updated
in order for a user to use an internationalized host name. Instead of
this requirement for widespread updating of all components, the current
proposal is that only user applications be updated; no changes are
needed to the DNS protocol or any DNS servers or the resolvers on user's

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1.1 Design philosophy

Many proposals for IDN protocols have required that DNS servers be
updated to handle internationalized host names. Because of this, the
person who wanted to use an internationalized host name had to be sure
that their request went to a DNS server that was updated for IDN.
Further, that server could only send queries to other servers that had
been updated for IDN because the queries contain new protocol elements
to differentiate IDN name parts from current host parts. In addition,
these proposals require that resolvers must be updated to use the new
protocols, and in most cases the applications would need to be updated
as well.

These proposals would require that the application protocols that use
host names as protocol elements to change. This is due to the
assumptions and requirements made in those protocols about the
characters that have always been used for host names, and the encoding
of those characters. Other proposals for IDN protocols do not require
changes to DNS servers but still require changes to most application
protocols to handle the new names.

Updating all (or even a significant percentage) of the existing servers
in the world will be difficult, to say the least. Updating applications,
application gateways, and clients to handle changes to the application
protocols is also daunting. Because of this, we have designed a protocol
that requires no updating of any name servers. IDNA still requires the
updating of applications, but only for input and display of names, not
for changes to the protocols. Once a user has updated these, she or he
could immediately start using internationalized host names. The cost of
implementing IDN may thus be much lower, and the speed of implementation
could be much higher.

1.2 Terminology

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

2. Structural Overview

In IDNA, users' applications are updated to perform the processing
needed to input internationalized host names from users, display
internationalized host names that are returned from the DNS to users,
and process the inputs and outputs from the DNS.

2.1 Interfaces between DNS components in IDNA

The interfaces in IDNA can be represented pictorially as:

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

This document uses the generic term "ACE" for an ASCII-compatible
encoding. After the IDN Working Group has chosen a specific ACE, this
document will be updated to refer to just that single ACE. Until that
time, an implementor creating experimental software must choose an ACE
to use, such as RACE or LACE or DUDE.

2.1.1 Entry and display in applications

Applications can accept host names using any character set or sets
desired by the application developer, and can display host names in any
charset. That is, this protocol does not affect the interface between
users and applications.

An IDNA-aware application can accept and display internationalized host
names in two formats: the internationalized character set(s) supported
by the application, and in an ACE. Applications MAY allow ACE input and
output, 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 3. Because name
parts encoded with ACE 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 default.

Host 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 the 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.

In protocols and document formats that define how to handle
specification or negotiation of charsets, IDN host name parts can be
encoded in any charset allowed by the protocol or document format. If a
protocol or document format only allows one charset, IDN host name parts
must be given in that charset. In any place where a protocol or document
format allows transmition of the characters in IDN host name parts, IDN
host name parts SHOULD be transmitted using whatever character encoding
and escape mechanism that the protocol or document format uses at that

All protocols that have host names as protocol elements already have the
capacity for handling host names in the ASCII charset. Thus, IDN host
name parts can be specified in those protocols in the ACE charset, which
is a superset of the ASCII charset that uses the same set of octets.

2.1.2 Applications and resolvers

Applications communicate with resolver libraries through a programming
interface (API). Typically, the IETF does not standardize APIs, although
there are non-standard APIs specified for IPv6. This protocol does not
specify a specific API, but instead specifies only the input and output
formats of the host names to the resolver library.

Before converting the name parts into ACE, the application MUST prepare
each name part as specified in [NAMEPREP]. The application MUST use ACE
for the name parts that are sent to the resolver, and will always get
name parts encoded in ACE from the resolver.

IDNA-aware applications MUST be able to work with both
non-internationalized host name parts (those that conform to [STD13] and
[STD3]) and internationalized host name parts. An IDNA-aware application
that is resolving a non-internationalized host name part MUST NOT do
any preparation or conversion to ACE on any non-internationalized name

2.1.3 Resolvers and DNS servers

An operating system might have a set of libraries for converting host
names to nameprepped ACE. 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). The output would be either
the unchanged name part (if the input already conforms to [STD13] and
[STD3]), or the nameprepped, ACE-encoded name part.

DNS servers MUST use the ACE format for internationalized host name

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

2.1.4 Avoiding exposing users to the raw ACE encoding

All applications that might show the user a host name that was received
from a gethostbyaddr or other such lookup SHOULD update as soon as
possible in order to prevent users from seeing the ACE. However, this is
not considered a big problem because so few applications show this type
of resolution to users.

If an application decodes an ACE name 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 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 part SHOULD
also have a mechanism to show the name with as many characters as
possible and replacement characters in the positions where characters
cannot be displayed. In many cases, the application doesn't know exactly
what the underlying rendering engine can or cannot display.

In addition to the condition above, if an application decodes an ACE
name but finds that the decoded name was not properly prepared according
to [NAMEPREP] (for example, if it has illegal characters in it), the
application SHOULD show the name in ACE format and SHOULD NOT display
the name in its decoded form. This is to avoid security issues described

2.1.5 Automatic detection of ACE

An application which receives a host name SHOULD verify whether or not
the host name is in ACE. This is possible by verifying the prefix in
each of the labels, and seeing whether or not the label is in ACE. This
MUST be done regardless of whether or not the communication channel used
(such as keyboard input, cut and paste, application protocol,
application payload, and so on) is encoding with ACE.

The reason for this requirement is that many applications are not
ACE-aware. Applications that are not ACE-aware will send host names in
ACE but mark the charset as being US-ASCII or some other charset which
has the characters that are valid in [STD13] as a subset.

2.1.6 Bidirectional text

In IDNA, text storage and display follows the rules in the Unicode standard
[Unicode3.1]. In particular, all Unicode text is stored in logical order;
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.

3. Name Server Considerations

It is imperative that there be only one encoding for a particular host
name. ACE is an encoding for host name parts that use characters outside
those allowed for host names [STD13]. Thus, a primary master name server
MUST NOT contain an ACE-encoded name that decodes to a host name that is
allowed in [STD13] and [STD3].

Name servers MUST NOT have any records with host names that contain
internationalized name parts unless those name parts have be prepared
according to [NAMEPREP]. If names that are not legal in [NAMEPREP] are
passed to an application, it will result in an error being passed to the
application with no error being reported to the name server. Further, no
application will ever ask for a name that is not legal in [NAMEPREP]
because requests always go through [NAMEPREP] before getting to the DNS.
Note that [NAMEPREP] describes how to handle versioning of unallocated

The host name data in zone files (as specified by section 5 of RFC 1035)
MUST be both nameprepped and ACE encoded.

4. Root Server Considerations

Because there are no changes to the DNS protocols, adopting this
protocol has no effect on the DNS root servers.

5. Security Considerations

Much of the security of the Internet relies on the DNS. Thus, any change
to the characteristics of the DNS can change the security of much of the

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.

When detecting an ACE-encoded host name, and decoding the ACE, care must
be taken that the resulting value(s) are valid characters which can be
handled by the application. This is described in more detail in section

Host 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 host name.

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

6. References

[NAMEPREP] Paul Hoffman & Marc Blanchet, "Preparation of
Internationalized Host 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.

[UAX9] Unicode Standard Annex #9, The Bidirectional Algorithm.

[Unicode3.1] 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. Changes from the -02 draft

Editorial changes throughout

2.1.1: Major changes to the second paragraph. Added major text to fourth

2.1.4: Added to the end of the second paragraph. Added the third

2.1.6: Complete change.

6: Added [Unicode3.1] and [UAX9].

C. 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