draft-ietf-idn-mua-00.txt

bind-3.2.

Internet Draft                                             Maynard Kang
draft-ietf-idn-mua-00.txt                                   i-EMAIL.net
February 5, 2001                                
Expires on August 5, 2001                               

          Internationalizing Domain Names in Mail User Agents
 
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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Force (IETF), its areas, and its working groups. Note that other
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Abstract

This document describes a way where domain names used in Internet e-mail 
can be internationalized by making changes only to end-user Mail User 
Agents and, by doing so, avoid damaging other applications which handle
Internet e-mail, such as Message Transfer Agents and Delivery Agents.

1. Introduction

One of the proposed solutions for internationalized domain names (IDN)
involves only updating the user applications with no changes required
to the DNS protocol, servers and resolvers [IDNA] compared to other
solutions which require changes to be made to protocol, servers,
resolvers and applications.

The underlying principle of [IDNA] may be similarly applied to the
Internet e-mail system today - by effecting changes to only the Mail
User Agent (MUA) component of the e-mail system. Thus, existing
Message Transfer Agents, Delivery Agents and other applications which 
handle e-mail do not have to be changed at all.

1.1 Definitions and Conventions

Usage of terms related to the character encoding model are in
reference to Unicode Technical Report 17 [UTR17].

The terms "international character", "non-ASCII character" and 
"multilingual character", which are used interchangeably, are taken 
to mean any abstract character which is not included in the range 
specified by [US-ASCII].

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 [RFC2119].

1.3. Design Philosophy

As the Internet e-mail system is a diverse, distributed and 
heterogeneous system with many vendors deploying a vast number of 
applications, it is of utmost importance that interoperability amongst 
these various components is maintained. Thus, the ideal solution would 
be one which does not compromise or damage the operation of any of these 
existing components once internationalized domain names are encountered.

Also, solutions which call for changes to be made to many or even all
components of the Internet e-mail system would require far too much
time and effort to deploy, given that Internet e-mail has such a huge
installed base.

This solution adheres to both of the above principles, in that
interoperability is preserved and that the cost and speed of 
implementation is low. All that the user has to do to use IDNs in e-mail 
is update his or her MUA.

1.4. IDN Summary

This solution specifies an IDN architecture of arch-3 (just send ACE)
and a transition strategy of trans-1 (always do current plus new
architecture) as described in [IDNCOMP]. The choice of ACE format is not 
defined in this document, but MUST be the same as that specified in 
[IDNA] in order to maintain uniqueness and consistency.

1.5. E-mail Internationalization Summary

As many Internet e-mail standards such as the SMTP protocol [RFC821]
and the e-mail message format [RFC822] only specify usage of the 7-bit
ASCII character set [US-ASCII], international characters which use octet-
based character encoding schemes (CES) cannot be used in e-mail 
transmission, headers and bodies.

Although this issue has been addressed in [RFC2045] for message bodies
and [RFC2047] for message headers through the use of a Transfer Encoding
Syntax (TES) such as Quoted-Printable or Base64, there is no similar 
solution which extends the functionality of [RFC821] to include usage of
international characters, except for [RFC1652] which allows transmission 
of 8-bit data passed by the DATA command in an SMTP session.

[RFC1652] however, does not fully address the problem of using IDNs in
an SMTP session - the IDN may be used in areas within the SMTP session 
other than the DATA command, such as the MAIL FROM and RCPT TO commands, 
where an IDN may be part of the e-mail address(es) specified there.

Hence, this would be a major stumbling block to deploying "just-send-
8bit" IDNs for use in Internet e-mail, as these IDNs would not be able
to be used in SMTP e-mail transmissions due to [RFC821] restrictions.

2. Architectural Overview

The end-user MUA may encounter IDNs in the scenarios below:

(i)   When specifying the transmission server (i.e. SMTP server)
(ii)  When specifying the retrieval server (i.e. POP3/IMAP4/any other
      retrieval mechanism)
(iii) When specifying e-mail addresses during composition of a message
(iv)  When reading messages with e-mail addresses in it

As with [IDNA], the MUA is updated in a similar fashion to process IDNs 
which are input by users and process IDNs which are displayed to users, 
in all of the scenarios above.

For (i) and (ii), the IDN MUST be handled in the same manner as 
specified in [IDNA]. The method of handling an IDN For (iii) and (iv) is
described below in 2.1.

2.1 Interfaces between E-mail components when composing/reading a mail

The interfaces between e-mail components can be pictorially represented 
as shown below.

The example assumes the setup of a POP3/IMAP4 retrieval client and 
server, but the exact nature of end-to-end e-mail transmission may vary
accordingly (e.g. elm or pine would read directly from the mail store). 
However, these variations do not impact an accurate description of this 
solution to a large extent as no changes are required at these levels.

        +------+                                       +------+
        | User |                                       | User |
        +------+                                       +---^--|
          | User Input:          User Display: Characters/ |
          | Keyboard/Pen/etc        Glyphs on CRT or other |
    +-----v---------------+    Representation (e.g. sound) |
    | Input Method Editor |                   +------------|-----+
    +---------------------+                   | Rendering Engine |
        | Input: Any localized/               +---------^--------+
        | internationalized      Output: Any localized/ |
        | charset                     internationalized |
   +----v-----------------+                     charset |
   | +------------------+ |                  +----------|-------------+
   | | Mail Composition | |                  | +--------------+       |
   | | Interface        | | Sender's         | | Mail Reading |       |
   | +------------------+ | MUA              | | Interface    |       |
   |    |                 |                  | +--------^-----+       |
   |    | Nameprepped ACE |       Receiver's |          | Nameprepped |
   |    v                 |              MUA |          | ACE         |
   | +-------------+      |                  | +-------------------+  |
   | | SMTP Client |      |                  | | POP3/IMAP4 Client |  |
   | +-------------+      |                  | +-------------------+  |
   +----|-----------------+                  +----------^-------------+
        | Nameprepped                                   | Nameprepped
        v ACE         Nameprepped       Nameprepped     | ACE
     +-------------+  ACE   +------------+  ACE   +-------------------+
     | SMTP Server | -----> | Mail Store | -----> | POP3/IMAP4 Server |
     +-------------+        +------------+        +-------------------+

2.1.1 Interface between User and Input Method Editor

For ASCII characters, input is straightforward: the user types on the 
keyboard and whichever character that is pressed is sent to the 
application.

However, for international characters, the end-user has to use a script-
specific Input Method Editor (IME), which may or may not be built-into
the OS, to interpret what the user communicates to the system and
thereafter send the respective international characters to the 
application.