Network Working Group Y. YONEYA, Ed.
Internet-Draft K. Fujiwara, Ed.
Expires: April 20, 2006 JPRS
October 17, 2005
Downgrading mechanism for Internationalized eMail Address (IMA)
draft-yoneya-ima-downgrade-00.txt
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Copyright (C) The Internet Society (2005).
Abstract
Traditional mail systems handle only US-ASCII characters in SMTP
envelope and mail headers. The internationalized email address (IMA)
is implemented by allowing UTF-8 characters in the SMTP envelope and
mail headers. This document describes downgrading from
internationalized addressing with the SMTP extension and UTF-8
headers to traditional email formats and characters.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Downgrade Encoding . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Downgrading mail addresses in envelope . . . . . . . . . . 4
3.2. Downgrading mail addresses in mail header . . . . . . . . 4
3.3. Downgrading other data . . . . . . . . . . . . . . . . . . 4
4. Downgrading . . . . . . . . . . . . . . . . . . . . . . . . . 4
4.1. Timing and conditions of downgrading . . . . . . . . . . . 4
4.2. SMTP Downgrading . . . . . . . . . . . . . . . . . . . . . 5
4.3. Mail Header Downgrading . . . . . . . . . . . . . . . . . 5
4.4. Record of downgrading . . . . . . . . . . . . . . . . . . 5
5. Implementation consideration . . . . . . . . . . . . . . . . . 5
5.1. MUA . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
5.2. MDA Requirements . . . . . . . . . . . . . . . . . . . . . 6
6. Security considerations . . . . . . . . . . . . . . . . . . . 6
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 6
9. Normative References . . . . . . . . . . . . . . . . . . . . . 6
Appendix A. Examples . . . . . . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 9
Intellectual Property and Copyright Statements . . . . . . . . . . 10
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1. Introduction
Traditional Internet email systems are defined by [RFC2821] and
[RFC2822]. They allow US-ASCII characters in the SMTP envelope and
mail headers. The IMA proposal [IMA-Framework], [IMA-UTF8], [IMA-
SMTPext] allows UTF-8 characters in SMTP envelope and mail headers.
Carrying IMA from sender to recipients requires that all components
on the mail delivery route be IMA compliant. Otherwise IMA can't be
delivered. To solve the problem, this document describes a downgrade
mechanism that enables delivering IMA by converting it to
corresponding US-ASCII representation on current mail delivery
system. Not only SMTP envelope, but also UTF-8 in mail headers MUST
be converted to US-ASCII.
Converting IMA to US-ASCII SHOULD have algorithmic method and 1-to-1
mapping. Requiring users to specify both an IMA and an equivalent
US-ASCII form is inconvenient and decreases usefulness of IMA.
2. Terminology
This document assumes a reasonable understanding of the protocols and
terminology of the core email standards as documented in [RFC2821]
and [RFC2822]. It also assumes familiarity with the other IMA
documents described in [IMA-Framework].
Much of the description in this document depends on the abstractions
of "Mail Transfer Agent" ("MTA") and "Mail User Agent" ("MUA").
However, it is important to understand that those terms and the
underlying concepts postdate the design of the Internet's email
architecture and the "protocols on the wire" principle. That email
architecture, as it has evolved, and the "wire" principle have
prevented any strong and standardized distinctions about how MTAs and
MUAs interact on a given origin or destination host (or even whether
they are separate).
The final ("delivery") MTA stores Mail messages in a "message store"
or resends messages where the receiver has assigned. In this
document, this function is called Mail Delivery Agent(MDA).
In this document, an address is "all-ASCII" if every character in the
address is in the ASCII character repertoire [ASCII]; an address is
"non-ASCII" if any character is not in the ASCII character
repertoire. The term "all-ASCII" is also applied to other protocol
elements when the distinction is important, with "non-ASCII" or
"internationalized" as its opposite.
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The key words "MUST", "SHALL", "REQUIRED", "SHOULD", "RECOMMENDED",
and "MAY" in this document are to be interpreted as described in RFC
2119 [RFC2119].
3. Downgrade Encoding
3.1. Downgrading mail addresses in envelope
This section describes how to downgrade mail addresses in SMTP
envelope. The encoding described in this section is also used in the
mail header.
1. Extract mail addresses from IMA SMTP envelope according to [IMA-
SMTPext].
2. Convert mail addresses into ACE(ASCII Compatible Encoding) if it
is IMA. ACE format of IMA is below.
* Domain part (RHS of @) is encoded to Punycode of IDNA
[RFC3490].
* Local part (LHS of @) is encoded to Punycode [RFC3492] as a
whole with ACE-Prefix described in Section 7. If local part
consists exclusively of US-ASCII characters, do not convert.
3.2. Downgrading mail addresses in mail header
This section describes how to downgrade mail addresses in mail
headers.
Extract every addr-spec [RFC2822] of mailboxes from mail headers
including UTF-8. For each addr-spec, if it includes UTF-8, convert
it into ACE with the same method described in Section 3.1. Original
IMA SHOULD remain as a comment encoded by base64 of MIME with UTF-8
tag. Note that specials in addr-spec MUST be escaped. If mailbox
elements except for addr-spec include UTF-8, those MUST be encoded by
base64 with UTF-8 tag.
3.3. Downgrading other data
This section describes how to downgrade other mail headers.
1. Encode UTF-8 part of headers by base64 of MIME with UTF-8 tag.
2. encoding=UTF-8
4. Downgrading
4.1. Timing and conditions of downgrading
This section describes timing and conditions of downgrading.
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Timing: SMTP client detects SMTP server doesn't support IMA by its
absence from the EHLO response.
Conditions: SMTP client detects UTF-8 is included in SMTP envelope
or mail headers.
4.2. SMTP Downgrading
Target of downgrading elements in SMTP envelope are below:
1. MAIL FROM:
2. RCPT TO:
Downgrading MUST be performed in each SMTP session.
4.3. Mail Header Downgrading
Target of downgrading elements in mail headers (SMTP data) are below:
Originator address(es): IMA in From, Reply-To and Sender and their
Resent- headers MUST be converted into ACE form.
Destination address(es): IMA in To and Cc and their Recent- headers
MUST be converted into ACE form.
IDs: IDs such as Message-ID, In-Reply-To and Reference MUST NOT be
the target of downgrading, i.e., these headers MUST NOT be
downgraded.
Other headers: UTF-8 in other headers such as Subject and Received
MUST be converted into base64 of MIME with UTF-8 tag.
Note that the MTA which performs downgrading MUST be responsible for
base64 encoding of preceding Received headers.
4.4. Record of downgrading
MTA which performs downgrading MUST add downgrading header to record
which headers are downgraded. Downgrading header is specified as
"X-IMA-Downgraded", and elements of the header is list of downgraded
header names separated by white space characters.
SMTP envelope downgrading is recorded as Envelope-To and Envelope-
From.
X-IMA-Downgraded: Envelope-From Envelope-To From To CC
5. Implementation consideration
5.1. MUA
An IMA compliant MUA MUST implement downgrade mechanism for sending.
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The MUA MAY encode UTF-8 in Subject header with the same encoding of
body part while downgrading.
IMA compliant MUA MUST decode downgraded mail headers and MUST show
IMA on display. MUA MAY use X-IMA-Downgraded header to determine
which headers were downgraded.
5.2. MDA Requirements
This section describes downgrading in MDA.
1. MDA MUST NOT convert downgraded header to UTF-8.
2. Record Return-Path header in ACE form.
3. Perform downgrading for each Storage/Back-end-Process. If and
only if MDA knows MUA is IMA compliant, then no downgrading is
performed.
4. If MDA detects downgraded IMA in SMTP envelope, then MDA MUST
decode IMA and perform the same processing as if it were IMA.
MDA MAY normalize or canonicalize local-part before processing
it.
6. Security considerations
See the extended security considerations discussion in [IMA-
Framework]
7. IANA Considerations
To distinguish downgraded IMA in ACE form, it MUST have ACE-Prefix.
The ACE-Prefix MUST be differ from IDNA ACE-Prefix to avoid possible
confusion. IANA will assign IMA ACE-Prefix when RFC is published.
IANA should register the new header [X-]IMA-Downgraded in the
registry of email header types.
8. Acknowledgements
...To be supplied ...
9. Normative 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.
[Hoffman-IMAA]
Hoffman, P. and A. Costello, "Internationalizing Mail
Addresses in Applications (IMAA)", draft-hoffman-imaa-03
(work in progress), October 2003.
[IMA-Framework]
Klensin, J. and Y. Ko, "Overview and Framework for
Internationalized Email", draft-klensin-ima-framework-00
(work in progress), September 2005.
[IMA-SMTPext]
Yao, J., Ed., "SMTP Extension for Internationalized Email
Address", draft-yao-smtpext-00 (work in progress),
September 2005.
[IMA-UTF8]
Yeh, J., "Transmission of Email Headers in UTF-8
Encoding", draft-yeh-ima-utf8headers-00 (work in
progress), October 2005.
[JET-IMA] Yao, J. and J. Yeh, "Internationalized eMail Address
(IMA)", draft-lee-jet-ima-00 (work in progress),
June 2005.
[Klensin-emailaddr]
Klensin, J., "Internationalization of Email Addresses",
draft-klensin-emailaddr-i18n-03 (work in progress),
July 2005.
[RFC1123] Braden, R., "Requirements for Internet Hosts - Application
and Support", STD 3, RFC 1123, October 1989.
[RFC1651] Klensin, J., Freed, N., Rose, M., Stefferud, E., and D.
Crocker, "SMTP Service Extensions", RFC 1651, July 1994.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2821] Klensin, J., "Simple Mail Transfer Protocol", RFC 2821,
April 2001.
[RFC2822] Resnick, P., "Internet Message Format", RFC 2822,
April 2001.
[RFC3490] Faltstrom, P., Hoffman, P., and A. Costello,
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"Internationalizing Domain Names in Applications (IDNA)",
RFC 3490, March 2003.
[RFC3492] Costello, A., "Punycode: A Bootstring encoding of Unicode
for Internationalized Domain Names in Applications
(IDNA)", RFC 3492, March 2003.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, November 2003.
Appendix A. Examples
...to be supplied...
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Authors' Addresses
Yoshiro YONEYA (editor)
JPRS
Chiyoda First Bldg. East 13F, 3-8-1 Nishi-Kanda
Chiyoda-ku, Tokyo 101-0065
Japan
Phone: +81 3 5215 8451
Email: yone@jprs.co.jp
Kazunori Fujiwara (editor)
JPRS
Chiyoda First Bldg. East 13F, 3-8-1 Nishi-Kanda
Chiyoda-ku, Tokyo 101-0065
Japan
Phone: +81 3 5215 8451
Email: fujiwara@jprs.co.jp
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