Internet Draft                                     Patrik Faltstrom
draft-ietf-idn-idna-01.txt                                    Cisco
                                                       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

To date, the 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.

Updating all (or even a significant percentage) of the DNS servers in
the world will be difficult, to say the least. Because of this, we have
designed a protocol that requires no updating of any name servers. IDNA
still requires the updating of applications, but once a user has updated
these, she or he could immediately start using internationalized host
names. The cost of implementing IDN would thus be much lower, and the
speed of implementation will 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 |                         | End system
      |         +-------------+
      |                ^
      |                |   API call and return: nameprepped ACE
      |                v
      |          +----------+
      |          | Resolver |
      |          +----------+                           |
                       |   DNS query and response: nameprepped ACE
                | DNS 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 Users and 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
advanced users, 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. Since ACE can be rendered
either as the encoded ASCII glyphs or the proper decoded character
glyphs, the rendering engine for an application SHOULD have an option
for the user to select the preferred display; if it does, rendering the
ACE SHOULD NOT be the default.

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 parts 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.

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.

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 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

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.

[RFC2279] Francois Yergeau, "UTF-8, a transformation format of ISO
10646", January 1998, RFC 2279.

[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.

B. Changes from the -00 draft

Throughout: Changed "RACE" to "ACE" and removed RACE-specific wording.

1: Added pointer to the mailing list.

1.3: Removed the [IDNCOMP] comparison section.

2.1: Added the last paragraph discussing ACEs.

2.1.2: Changed the discussion of IPv6 APIs to say they are not
standards. Added reference to [STD3].

2.1.3: Added reference to [STD3]. Removed the sentence about making
things smaller.

3: Added reference to [STD3].

6: Added [STD3], and updated the reference info for [STD13].
Removed [IDNCOMP].

C. Authors' Addresses

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

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

Patrik F„ltstr÷m <>                         Cisco Systems
Consulting Engineer                                  Office of the CSO
Phone: (Stockholm) +46-8-4494212            (San Jose) +1-408-525-8509
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