Email Address Internationalization Y. YONEYA, Ed.
(EAI) K. Fujiwara, Ed.
Internet-Draft JPRS
Intended status: Experimental Aug 16, 2006
Expires: February 17, 2007
Downgrading mechanism for Email Address Internationalization (EAI)
draft-ietf-eai-downgrade-02.txt
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Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
Traditional mail systems handle only US-ASCII characters in SMTP
envelope and mail headers. The Email Address Internationalization
(EAI) is implemented by allowing UTF-8 characters in SMTP envelope
and mail headers. To deliver Non-ASCII mail address through EAI
incompliant environment, some sort of converting mechanism (i.e.
downgrading) is required. This document describes requirements for
downgrading, SMTP Downgrading, SMTP DATA/Header Downgrading and
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implementation consideration.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Downgrade Requirements . . . . . . . . . . . . . . . . . . . . 3
3.1. Timing and conditions of downgrading . . . . . . . . . . . 3
3.2. Requirements . . . . . . . . . . . . . . . . . . . . . . . 4
4. SMTP Downgrading . . . . . . . . . . . . . . . . . . . . . . . 4
5. SMTP DATA/Header Downgrading . . . . . . . . . . . . . . . . . 5
5.1. Header conversion . . . . . . . . . . . . . . . . . . . . 6
5.1.1. Downgrading address headers . . . . . . . . . . . . . 7
6. Implementation consideration . . . . . . . . . . . . . . . . . 8
6.1. MUA . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
7. Security considerations . . . . . . . . . . . . . . . . . . . 8
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 8
10. Change History . . . . . . . . . . . . . . . . . . . . . . . . 8
10.1. draft-yoneya-ima-downgrade: Version 00 . . . . . . . . . . 8
10.2. draft-yoneya-ima-downgrade: Version 01 . . . . . . . . . . 9
10.3. draft-ietf-eai-downgrade: Version 00 . . . . . . . . . . . 9
10.4. draft-ietf-eai-downgrade: Version 01 . . . . . . . . . . . 9
10.5. draft-ietf-eai-downgrade: Version 02 . . . . . . . . . . . 9
11. Normative References . . . . . . . . . . . . . . . . . . . . . 9
Appendix A. Examples . . . . . . . . . . . . . . . . . . . . . . 10
A.1. SMTP Downgrading Example . . . . . . . . . . . . . . . . . 10
A.2. Header conversion downgrading example . . . . . . . . . . 12
A.3. Header conversion upgrading example . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 16
Intellectual Property and Copyright Statements . . . . . . . . . . 17
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1. Introduction
Traditional mail systems which are defined by [RFC2821] and [RFC2822]
allow US-ASCII characters in SMTP envelope and mail headers in body
part. The EAI proposal [I-D.ietf-eai-framework],
[I-D.ietf-eai-utf8headers] and [I-D.ietf-eai-smtpext] allows UTF-8
characters in SMTP envelope and mail headers in body part.
Carrying Non-ASCII mail address from sender to recipients requires
all components on the mail delivery route are EAI compliant.
Otherwise Non-ASCII mail address can't be delivered. To solve the
problem, this document describes downgrading mechanism that enables
delivering Non-ASCII mail address by converting it to corresponding
US-ASCII representation on current mail delivery system. Not only
SMTP envelope, but also UTF-8 characters in mail headers MUST be
converted to US-ASCII.
Downgrading in EAI consists from following two parts:
o SMTP Downgrading
o SMTP DATA/Header Downgrading
Decoding downgraded envelope/message is called 'Upgrading' in this
document. Each downgrading mechanism has corresponding upgrading
mechanism.
In this document, requirements for downgrading is described in
section Section 3, SMTP Downgrading is described in Section 4, and
SMTP DATA/Header Downgrading is described in Section 5.
2. Terminology
Terminology for this document is defined in [I-D.ietf-eai-framework].
3. Downgrade Requirements
3.1. Timing and conditions of downgrading
Followings are timing and conditions of downgrading.
Timing: SMTP client detects that SMTP server doesn't support
"UTF8SMTP" option at EHLO. [I-D.ietf-eai-smtpext]
Conditions: SMTP client detects that UTF-8 is included in the SMTP
envelope or mail headers in the SMTP DATA.
Note: If the UTF8SMTP header exists, downgrading will be performed.
If UTF-8 characters exist in mail headers without the UTF8SMTP
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header, this is a protocol error, and handling of this situation is
outside the scope of this specification.
3.2. Requirements
Followings are requirements of downgrading.
1. Downgrading must be performed only once.
2. Upgrading must be performed at minimized place such as final
destination like recipient MUA.
3. Downgrading and upgrading must be automated.
4. Downgrading and upgrading should be easy and lightweight as it is
possible to do with MTA like 8BITMIME encapsulation.
5. Downgrade and upgrade method must be defined clearly.
6. Downgrading and upgrading should preserve all header information.
7. Downgrading must support SPF and DKIM.
8. Downgrading occurrence must be recorded.
4. SMTP Downgrading
Downgrading MUST be performed in each SMTP session. Target of
downgrading elements in SMTP envelope are below:
o MAIL FROM:
o RCPT TO:
Downgrading in SMTP envelope uses ALT-ADDRESS parameter proposed in
[I-D.ietf-eai-smtpext]. An address is downgradable if the address is
all-ASCII address or the address has all-ASCII address specified by
ALT-ADDRESS parameter.
If the return address in the envelope ("MAIL FROM:") is not
downgradable, downgrading fails.
One SMTP session may contain multiple recipients. Downgrading SHOULD
be performed for each SMTP recipient address. First, split a
multiple recipients session to each sessions. If the recipient
address is downgradable, the SMTP session to the recipient is
downgradable.
When MUA/MTA is transferring mail and finding its envelope contains
Non-ASCII addresses, it MUST decide to bounce or downgrade if
receiving MTA is EAI incompliant.
Even if no downgrading is performed for envelope from/to, MUA/MTA
MUST downgrade mail headers including UTF-8 or bounce. This is
described in next section.
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MTA replaces Non-ASCII mail address with specified alternative US-
ASCII address. Then appends replaced information with EAI-
Downgraded-From: and EAI-Downgraded-To: header in mail header
(outgoing SMTP DATA).
EAI-Downgraded-From: <Non-ASCII {US-ASCII}> <US-ASCII>
EAI-Downgraded-To: <Non-ASCII {US-ASCII}> <US-ASCII>
Note that when downgrading, not to disclose whole recipient address,
MUA/MTA SHOULD make SMTP connection per each recipient address.
Also note that by appending EAI-Downgraded-From/To: headers, MUA/MTA
MUST perform SMTP DATA/Header Downgrading. This is described in next
section.
Case study: SPF check
SPF checks domainname of the envelope from and SMTP connection IP
address. If ALT-ADDRESS domainname is Punycode/IDNA form of Non-
ASCII domainname, it will be compatible with current SPF. In this
case, SPF check will be performed correctly. Otherwise, more
detailed consideration is required.
NOTE: ORCPT
ESMTP ORCPT parameter is used for Delivery Status Notifications
(DSNs) [RFC3461]. ORCPT parameters contain mail addresses. After
extending ORCPT parameter to support Non-ASCII mail addresses, ORCPT
parameter downgrading should be defined here.
5. SMTP DATA/Header Downgrading
Target and non-target of downgrading elements in mail headers (SMTP
data) are below:
Originator address(es): Non-ASCII mail addresses in From:,
Reply-To:, Sender: and their Resent- headers MUST be target of
downgrading.
Destination address(es): Non-ASCII mail addresses in To:, CC:, Bcc:
and their Resent- headers MUST be target of downgrading.
IDs: IDs such as Message-ID:, Date:, In-Reply-To: and References:
MUST NOT be target of downgrading.
Trace headers: Received: headers MUST NOT be target of downgrading
because they do not contain Non-ASCII mail addresses.
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other headers: UTF-8 in other headers MUST be target of downgrading.
UTF8SMTP header: Identification of internationalized email header
requires special consideration.
5.1. Header conversion
This section defines conversion method to US-ASCII for each header
which may contain Non-ASCII characters. Each header has its own
downgrading method.
To preserve all header information, define generic encapsulation
header: "Downgraded: HeaderName: HeaderValue". The header value is
encoded by [RFC2047] with UTF-8 tag.
Downgrading:
* Check if the mail has 'UTF8SMTP' header and its value is
"UTF8". Otherwise, downgrading is not required.
* Check each header whether it contains UTF-8 characters or not.
If the header contains UTF-8 characters,
+ If the header is an address header which is described in
Section 5.1.1,
- Preserve the header in 'Downgraded' header.
- Downgrade the header defined in Section 5.1.1.
+ The other header case, encode the header by [RFC2047] with
UTF-8 tag.
* Change 'UTF8SMTP' header value to "Downgraded".
Upgrading:
* Check if the mail has 'UTF8SMTP' header and its value is
"Downgraded". Otherwise, upgrading is not required.
* Decode all 'Downgraded' headers.
+ Decode header value field string which is [RFC2047] encoded.
+ If the header is address header described in Section 5.1.1,
- Apply address header downgrading to the decoded header.
- Remove the header line which is the same with the
downgraded line.
+ Remove the 'Downgraded' header.
+ Add decoded header to mail header.
* If each mail header has [RFC2047] encoded part and which
encoding is "UTF-8", it is a downgraded header, so decode it.
* Change 'UTF8SMTP' header value to "UTF8".
Followings are pros and cons of this method.
Pros:
* EAI incompliant MUA displays the downgraded mail body except
original Non-ASCII mail addresses.
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* EAI incompliant MUA displays and handles the sender specified
alternative address (ALT-ADDRESS).
* EAI compliant MUA displays and handles original headers.
Cons:
* Implementation and processing cost is not so easy and
lightweight because MUA/MTA must parse each header and encode
it by defined method.
* Hard to preserve whole information AS IS. The address headers
are preserved but the other headers which is [RFC2047] encoded
with UTF-8 tag are not distinguished that it is downgraded or
it is encoded by sender's MUA. Also, restoration order of the
downgraded headers is not guaranteed. Therefore, to check DKIM
requires special consideration.
[[Reference to [I-D.ietf-eai-scenarios] and evaluation of each case
should be described here.]]
5.1.1. Downgrading address headers
This section targets From:, Sender:, Reply-To:, To:, CC:, Bcc:,
Resent-From:, Resent-To:, Resent-CC:, Resent-Bcc:, Resent-sender:
headers which contains Originator/Destination address(es).
The header value is composed of single or multiple mailbox/angle-addr
fields defined in [I-D.ietf-eai-utf8headers].
If the header contains UTF-8 characters, downgrading method is
follows.
1. Extract every field and downgrade mailbox/angle-addr described
below.
2. By mailbox/angle-addr downgrading, if the field became empty, the
field should be removed.
3. If all header field is removed, remove the header.
4. If From header is removed, generate new From: header from
envelope-from address.
EAI angle-addr defined in [I-D.ietf-eai-utf8headers] consists of 3
forms. Downgrading method is defined for each form.
1. <US-ASCII>
US-ASCII mail address case, preserve it.
2. <Non-ASCII>
Non-ASCII mail address without ALT-ADDRESS parameter case,
remove this angle-addr.
3. <Non-ASCII {US-ASCII}>
Non-ASCII mail address with sender-specified US-ASCII address
case, replace it as <US-ASCII>.
"mailbox" is defined as "DISPLAY NAME angle-addr" in
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[I-D.ietf-eai-utf8headers]. The "DISPLAY NAME" field should be
encoded by [RFC2047] with UTF-8 tag, if necessary. If the angle-addr
is removed, remove the field including "DISPLAY NAME".
6. Implementation consideration
6.1. MUA
EAI compliant MUA MUST implement downgrading mechanism for sending.
MUA MAY encode UTF-8 in Subject header with the same encoding of body
part while downgrading.
EAI compliant MUA MUST upgrade downgraded mail and MUST show Non-
ASCII mail addresses on display.
7. Security considerations
See the extended security considerations discussion in
[I-D.ietf-eai-framework]
8. IANA Considerations
IANA is requested to add the "EAI-Downgraded-From:",
"EAI-Downgraded-To:" and "Downgraded:" new headers to the registry
with the entries pointing to this specification for its definition.
9. Acknowledgements
John Klensin, Harald Alvestrand, Chris Newman, Charles Lindsey,
Marcos Sanz, Alexey Melnikov and JET members.
10. Change History
This section is used for tracking the update of this document. Will
be removed after finalize.
10.1. draft-yoneya-ima-downgrade: Version 00
o Initial version
o Fllowed draft-yeh-ima-utf8headers-00 and draft-yao-smtpext-00
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10.2. draft-yoneya-ima-downgrade: Version 01
o Document structure was changed
o Fllowed draft-yeh-ima-utf8headers-01 and draft-yao-smtpext-02
o Downgrading requirements were added
o SMTP DATA encapsulation method was proposed
o Downgrading examples was provided
10.3. draft-ietf-eai-downgrade: Version 00
o Fllowed draft-yeh-ima-utf8headers-01 and draft-ietf-eai-smtpext-00
o No header downgrading method was proposed
o Header encapsulation method was proposed
10.4. draft-ietf-eai-downgrade: Version 01
o Followed draft-ietf-eai-utf8headers-00
o Header conversion and encapsulation method was merged
o Header conversion method was defined in detail
10.5. draft-ietf-eai-downgrade: Version 02
o Followed draft-ietf-eai-utf8headers-01 and
draft-ietf-eai-smtpext-01
o Specification about algorithmic generated address is removed
o No header downgrading method was removed
o SMTP DATA encapsulation method was removed
11. Normative References
[I-D.ietf-eai-framework]
Klensin, J. and Y. Ko, "Overview and Framework for
Internationalized Email", draft-ietf-eai-framework-01
(work in progress), June 2006.
[I-D.ietf-eai-scenarios]
Alvestrand, H., "UTF-8 Mail: Scenarios",
draft-ietf-eai-scenarios-01 (work in progress), June 2006.
[I-D.ietf-eai-smtpext]
Yao, J. and W. Mao, "SMTP extension for internationalized
email address", draft-ietf-eai-smtpext-01 (work in
progress), July 2006.
[]
Yeh, J., "Internationalized Email Headers",
draft-ietf-eai-utf8headers-01 (work in progress),
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August 2006.
[RFC2047] Moore, K., "MIME (Multipurpose Internet Mail Extensions)
Part Three: Message Header Extensions for Non-ASCII Text",
RFC 2047, November 1996.
[RFC2821] Klensin, J., "Simple Mail Transfer Protocol", RFC 2821,
April 2001.
[RFC2822] Resnick, P., "Internet Message Format", RFC 2822,
April 2001.
[RFC3461] Moore, K., "Simple Mail Transfer Protocol (SMTP) Service
Extension for Delivery Status Notifications (DSNs)",
RFC 3461, January 2003.
Appendix A. Examples
A.1. SMTP Downgrading Example
This section shows a SMTP Downgrading example. Consider a
downgradable mail message.
o The sender address is "NON-ASCII-FROM" which is Non-ASCII address.
Its ASCII alternative is "ASCII-FROM".
o The "To" address is "NON-ASCII-TO" which is Non-ASCII address.
Its ASCII alternative is "ASCII-TO".
o The "CC" address is "NON-ASCII-CC" which is Non-ASCII address. It
has no ASCII alternative address.
o The Subject header is "UTF8-Subject" which contains Non-ASCII
characters.
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Original EAI message SMTP session
MAIL From: <NON-ASCII-FROM> ALT-ADDRESS <ASCII-FROM>
RCPT TO: <NON-ASCII-TO> ALT-ADDRESS <ASCII-TO>
RCPT TO: <NON-ASCII-CC>
-------------------------------------------------------------
UTF8SMTP: UTF8
Message-Id: MESSAGE_ID
Mime-Version: 1.0
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: 8bit
Subject: UTF8-SUBJECT
From: <NON-ASCII-FROM {ASCII-FROM}>
To: <NON-ASCII-TO {ASCII-TO}>
CC: <NON-ASCII-CC>
Date: DATE
MAIL_BODY
Figure 1: Original EAI message
In this example, there are two sessions, one is To:, the other is
CC:. The CC: session does not contain ASCII alternative address, it
is not downgradable and bounced. The To: session can be downgraded
to the next example Figure 2.
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After SMTP Downgrading
MAIL From: <ASCII-FROM>
RCPT TO: <ASCII-TO>
-------------------------------------------------------------
EAI-Downgraded-From: <NON-ASCII-FROM {ASCII-FROM}> <ASCII-FROM>
EAI-Downgraded-To: <NON-ASCII-TO {ASCII-TO}> <ASCII-TO>
UTF8SMTP: UTF8
Message-Id: MESSAGE_ID
Mime-Version: 1.0
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: 8bit
Subject: UTF8-SUBJECT
From: <NON-ASCII-FROM {ASCII-FROM}>
To: <NON-ASCII-TO {ASCII-TO}>
CC: <NON-ASCII-CC>
Date: DATE
MAIL_BODY
Figure 2: SMTP Downgraded EAI message
A.2. Header conversion downgrading example
Original EAI message
UTF8SMTP: UTF8
Message-Id: MESSAGE_ID
Mime-Version: 1.0
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: 8bit
Subject: UTF-8_SUBJECT
From: <NON-ASCII-FROM {ASCII-FROM}>
To: <NON-ASCII-TO {ASCII-TO}>
CC: <NON-ASCII-CC>
Date: DATE
MAIL_BODY
Figure 3: Original EAI message
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SMTP Downgrading adds EAI-Downgraded-From: and EAI-Downgraded-To:
headers.
EAI-Downgraded-From: <Non-ASCII {DOWNGRADED_FROM}> <DOWNGRADED_FROM>
EAI-Downgraded-To: <Non-ASCII {DOWNGRADED_TO}> <DOWNGRADED_TO>
Figure 4: Headers added by SMTP Downgrading
Result of the header conversion downgrading.
EAI-Downgraded-From:
MIME(<Non-ASCII {DOWNGRADED_FROM}) <DOWNGRADED_FROM>
EAI-Downgraded-To:
MIME(<Non-ASCII {DOWNGRADED_TO}) <DOWNGRADED_TO>
UTF8SMTP: Downgraded
Message-Id: MESSAGE_ID
Mime-Version: 1.0
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: 8bit
Subject: MIME(UTF-8_SUBJECT)
Downgraded: From: MIME(<NON-ASCII-FROM {ASCII-FROM}>)
From: <ASCII-FROM>
Downgraded: To: MIME(<NON-ASCII-TO {ASCII-TO}>)
To: <ASCII-TO>
Downgraded: CC: MIME(<NON-ASCII-CC>)
Date: DATE
MAIL_BODY
Figure 5: Header conversion downgraded message
MIME() stands for [RFC2047] encoding.
A.3. Header conversion upgrading example
This example shows upgrading process of Figure 5.
First, check 'UTF8SMTP:' header. Its value is "Downgraded", it is
EAI downgraded message.
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Decode all 'Downgraded:' headers.
Downgraded: From: MIME(<NON-ASCII-FROM {ASCII-FROM}>)
Downgraded: To: MIME(<NON-ASCII-TO {ASCII-TO}>)
Downgraded: CC: MIME(<NON-ASCII-CC>)
Figure 6: Upgrading: selecting Downgraded headers
Decode header value field string which is [RFC2047] encoded.
From: <NON-ASCII-FROM {ASCII-FROM}>
To: <NON-ASCII-TO {ASCII-TO}>
CC: <NON-ASCII-CC>
Figure 7: Upgrading: upgraded Downgraded headers
Apply address header downgrading to the decoded header.
From: <ASCII-FROM>
To: <ASCII-TO>
Figure 8: Upgrading: downgraded upgraded Downgraded headers
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Remove the header line which is the same with the downgraded line.
EAI-Downgraded-From:
MIME(<Non-ASCII {DOWNGRADED_FROM}) <DOWNGRADED_FROM>
EAI-Downgraded-To:
MIME(<Non-ASCII {DOWNGRADED_TO}) <DOWNGRADED_TO>
UTF8SMTP: Downgraded
Message-Id: MESSAGE_ID
Mime-Version: 1.0
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: 8bit
Subject: MIME(UTF-8_SUBJECT)
Downgraded: From: MIME(<NON-ASCII-FROM {ASCII-FROM}>)
Downgraded: To: MIME(<NON-ASCII-TO {ASCII-TO}>)
Downgraded: CC: MIME(<NON-ASCII-CC>)
Date: DATE
MAIL_BODY
Figure 9: Upgrading: Removing duplicated headers
Remove the 'Downgraded' header and add decoded Downgraded headers If
each mail header has [RFC2047] encoded part and which encoding is
"UTF-8", it is a downgraded header, so decode it. Change 'UTF8SMTP'
header value to "UTF8".
EAI-Downgraded-From: <Non-ASCII {DOWNGRADED_FROM}> <DOWNGRADED_FROM>
EAI-Downgraded-To: <Non-ASCII {DOWNGRADED_TO}> <DOWNGRADED_TO>
UTF8SMTP: UTF8
Message-Id: MESSAGE_ID
Mime-Version: 1.0
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: 8bit
Subject: UTF-8_SUBJECT
From: <NON-ASCII-FROM {ASCII-FROM}>
To: MIME(<NON-ASCII-TO {ASCII-TO}>
CC: MIME(<NON-ASCII-CC>
Date: DATE
MAIL_BODY
Figure 10: Header conversion upgraded message
As a result, in this example, all headers are preserved.
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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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