Email Address Internationalization Y. YONEYA, Ed.
(EAI) K. Fujiwara, Ed.
Internet-Draft JPRS
Intended status: Experimental November 18, 2007
Expires: May 21, 2008
Downgrading mechanism for Email Address Internationalization
draft-ietf-eai-downgrade-05.txt
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This Internet-Draft will expire on May 21, 2008.
Copyright Notice
Copyright (C) The IETF Trust (2007).
Abstract
Traditional mail systems handle only ASCII characters in SMTP
envelope and mail header fields. The Email Address
Internationalization (UTF8SMTP) extension allows UTF-8 characters in
SMTP envelope and mail header fields. To avoid bouncing
internationalized Email messages when a server in the delivery path
does not support the UTF8SMTP extension, some sort of converting
mechanism is required. This document describes a downgrading
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mechanism for Email Address Internationalization. Note that this is
a way to downgrade, not tunnel. There is no associated up-conversion
mechanism, although internationalized email clients might use
original internationalized addresses or other data when displaying or
replying to downgraded messages.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. New header fields definition . . . . . . . . . . . . . . . . . 5
3.1. Envelope information preservation headers . . . . . . . . 5
3.2. Address header field preservation headers . . . . . . . . 5
3.3. Unknown header fields preservation headers . . . . . . . . 6
4. SMTP Downgrading . . . . . . . . . . . . . . . . . . . . . . . 7
5. Email header fields downgrading . . . . . . . . . . . . . . . 8
5.1. Downgrading method for each header field . . . . . . . . . 9
6. MIME body part headers downgrading . . . . . . . . . . . . . . 11
7. Security considerations . . . . . . . . . . . . . . . . . . . 11
8. Implementation notes . . . . . . . . . . . . . . . . . . . . . 12
8.1. Trivial downgrading . . . . . . . . . . . . . . . . . . . 12
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 15
11. Change History . . . . . . . . . . . . . . . . . . . . . . . . 15
11.1. draft-yoneya-ima-downgrade: Version 00 . . . . . . . . . . 15
11.2. draft-yoneya-ima-downgrade: Version 01 . . . . . . . . . . 15
11.3. draft-ietf-eai-downgrade: Version 00 . . . . . . . . . . . 16
11.4. draft-ietf-eai-downgrade: Version 01 . . . . . . . . . . . 16
11.5. draft-ietf-eai-downgrade: Version 02 . . . . . . . . . . . 16
11.6. draft-ietf-eai-downgrade: Version 03 . . . . . . . . . . . 16
11.7. draft-ietf-eai-downgrade: Version 04 . . . . . . . . . . . 16
11.8. draft-ietf-eai-downgrade: Version 05 . . . . . . . . . . . 17
12. Normative References . . . . . . . . . . . . . . . . . . . . . 17
Appendix A. Displaying downgraded message . . . . . . . . . . . . 18
A.1. Displaying technique 1 . . . . . . . . . . . . . . . . . . 18
A.2. Displaying technique 2 . . . . . . . . . . . . . . . . . . 19
Appendix B. Examples . . . . . . . . . . . . . . . . . . . . . . 19
B.1. Downgrading example 1 . . . . . . . . . . . . . . . . . . 19
B.2. Displaying example . . . . . . . . . . . . . . . . . . . . 22
B.2.1. Displaying technique 1 example . . . . . . . . . . . . 23
B.2.2. Displaying technique 2 example . . . . . . . . . . . . 24
B.3. Downgrading example 2 . . . . . . . . . . . . . . . . . . 27
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 28
Intellectual Property and Copyright Statements . . . . . . . . . . 30
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1. Introduction
Traditional mail systems which are defined by [RFC2821] and [RFC2822]
allow ASCII characters in SMTP envelope and mail header field values.
The UTF8SMTP extension [RFC4952], [I-D.ietf-eai-utf8headers] and
[I-D.ietf-eai-smtpext] allows UTF-8 characters in SMTP envelope and
mail header field values.
If an envelope address or header field contains non-ASCII characters,
the message cannot be delivered unless every system in the delivery
path supports UTF8SMTP. To avoid bouncing such messages when a
server is encountered which does not support the UTF8SMTP extension,
this document describes a downgrading mechanism. Downgrading a
message converts envelope and header fields to an all-ASCII
representation.
[I-D.ietf-eai-utf8headers] allows UTF-8 characters to be used in mail
header fields and MIME header fields. The downgrading mechanism
specified here converts mail header fields and MIME header fields to
ASCII.
This document does not change any protocols except by defining new
header fields. It describes the conversion method from the
internationalized email envelopes/messages which are defined in
[RFC4952] [I-D.ietf-eai-utf8headers] [I-D.ietf-eai-smtpext] to the
traditional email envelopes/messages which are defined in [RFC2821]
[RFC2822].
[I-D.ietf-eai-smtpext] section 2.2 defines when downgrading occurs.
If the SMTP client has an UTF8SMTP envelope or an internationalized
message and the SMTP server doesn't support the UTF8SMTP SMTP
extension, then the SMTP client MUST NOT send a UTF8SMTP envelope or
an internationalized message to the SMTP server. The section shows 4
choices. The fourth choice is downgrading, as described here.
Downgrading may be implemented in MUAs, MSAs, MTAs which act as the
SMTP client, or in MDAs, POP servers, IMAP servers which store or
offer UTF8SMTP envelopes or internationalized messages to non-
UTF8SMTP compliant systems which include message stores.
This document tries to define the downgrading process clearly and it
preserves the original information as much as possible.
Downgrading in UTF8SMTP consists of the following four parts:
o New header fields definition
o SMTP downgrading
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o Email header fields downgrading
o MIME header fields downgrading
In Section 3, many header fields starting with "downgraded" are
introduced. They preserve the original envelope information and the
original header fields.
The SMTP downgrading is described in Section 4. It generates ASCII
only envelope information from an UTF8SMTP envelope.
The Email header fields downgrading is described in Section 5. It
generates ASCII only header fields.
The MIME header fields are expanded in [I-D.ietf-eai-utf8headers].
The MIME header fields downgrading is described in Section 6. It
generates ASCII only MIME header fields.
Decoding downgraded message is described in Appendix A. This
mechanism is for use by internationalized email clients. Once a
message has been downgraded, there is no up-conversion which can be
applied on the SMTP delivery path.
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].
All specialized terms used in this specification are defined in the
EAI overview [RFC4952] or in [RFC2821][RFC2822], MIME documents
[RFC2045] [RFC2047] [RFC2183] [RFC2231]. The terms "ASCII address",
"internationalized email address", "non-ASCII address", "i18mail
address", "UTF8SMTP", "message" and "mailing list" are used with the
definitions from [RFC4952] document.
This document depends on [I-D.ietf-eai-smtpext],
[I-D.ietf-eai-utf8headers], and [I-D.ietf-eai-dsn]. Key words used
in these document are used in this document, too.
The term "non-ASCII" is an UTF-8 string which contains at least one
non-ASCII character.
An "UTF8SMTP envelope" has Email originator/recipient addresses
expanded by [I-D.ietf-eai-smtpext] and [I-D.ietf-eai-dsn].
An "UTF8SMTP message" is Email messages expanded by
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[I-D.ietf-eai-utf8headers].
3. New header fields definition
New header fields starting with "downgraded" are defined here to
preserve those original envelope and header values which contain
UTF-8 characters. During downgrading, one new "downgraded" header
field is added for each original envelope or header field which
cannot be passed as-is to a server which does not support UTF8SMTP.
The original envelope or header field is removed or altered. Only
those envelope and header fields which contain non-ASCII characters
are affected. The result of this process is a message which is
compliant with existing email specifications [RFC2821] and [RFC2822].
The original internationalized information can be retrieved by
examining the "downgraded" header fields which were added. Even
though the information is not lost, the original message cannot be
perfectly reconstructed. Hence, downgrading is a one-way process.
However, an internationalized client might use the information in the
"downgraded" header fields when processing a downgraded message, for
example, such as displaying or composing a reply.
3.1. Envelope information preservation headers
Two headers "Downgraded-Mail-From:" and "Downgraded-Rcpt-To:" are
defined to preserve SMTP envelope downgraded information. SMTP
envelope downgraded information consists of the original non-ASCII
address and the downgraded all-ASCII address. The header field
syntax is specified as follows:
fields =/ downgradedmailfrom / downgradedrcptto
downgradedmailfrom = "Downgraded-Mail-From:" [FWS] "<" uPath ">" 1*[FWS]
"<" Mailbox ">" [FWS] CRLF
downgradedrcptto = "Downgraded-Rcpt-To:" [FWS] "<" uPath ">" 1*[FWS]
"<" Mailbox ">" [FWS] CRLF
Original non-ASCII address <uPath> is defined in
[I-D.ietf-eai-smtpext]; it is treated as unstructured in this header
and it is encoded according to [RFC2047]. <Mailbox> is defined in
[RFC2821], section 4.1.2.
3.2. Address header field preservation headers
The address header fields preservation headers are defined to
preserve the original header field. Their value field holds the
original header field value. Any original header field value is
treated as an <unstructured> and it MUST be encoded according to
[RFC2047] with charset='UTF-8'. The header field syntax is specified
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as follows:
fields =/ known-downgraded-headers ":" unstructued CRLF
known-downgraded-headers = "Downgraded-" original-headers
original-headers = "From" / "To" / "Cc" / "Bcc" /
"Sender" / "Reply-To" /
"Resent-From" / "Resent-Sender" /
"Resent-To" / "Resent-Cc" / "Return-Path"
Preserving a header field in a downgraded header field is defined as:
1. Generate new downgraded header field whose value is the original
header field value.
2. Encode the generated header according to [RFC2047] with
charset='UTF-8'.
3.3. Unknown header fields preservation headers
The unknown header fields preservation headers are defined to
encapsulate those original header field which contains non-ASCII
characters and are not otherwise provided for in the this
specification. The encapsulation header field name is the
concatenation of "Downgraded-" and the original name. The value
field holds the original header field value.
Any original header field value is treated as an unstructured value
and it MUST be encoded according to [RFC2047] with charset='UTF-8'.
The header field syntax is specified as follows:
fields =/ unknown-downgraded-headers ":" unstructued CRLF
unknown-downgraded-headers = "Downgraded-" original-header-field-name
original-header-field-name = field-name
field-name = 1*ftext
ftext = %d33-57 / ; Any character except
%d59-126 ; controls, SP, and
; ":".
Encapsulating a header field in a Downgraded header field is defined
as:
1. Generate new Downgraded header whose value is the original header
field value.
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2. Encode the generated header field according to [RFC2047] with
charset='UTF-8'.
3. Remove the original header field.
4. SMTP Downgrading
Target of downgrading elements in SMTP envelope are below:
o MAIL FROM:
o RCPT TO:
o ORCPT parameter
Downgrading the SMTP envelope uses ALT-ADDRESS parameter defined in
[I-D.ietf-eai-smtpext]. An address is downgradable if the address is
non-ASCII address and has ASCII address specified by ALT-ADDRESS
parameter. Since only non-ASCII addresses are downgradable,
specifying an ALT-ADDRESS value for an all-ASCII address is invalid
for use with this specification, and no interpretation is assigned to
it. This restriction allows for future extension of the
specification even though no such extensions are currently
anticipated.
Note that even if no downgrading is performed on the envelope,
message header fields and message body MIME header fields that
contain non-ASCII characters MUST be downgraded. This is described
in Section 5 and Section 6.
When downgrading, replace each non-ASCII mail address in the envelope
with its specified alternative ASCII address and preserve the
original information using "Downgraded-Mail-From" and "Downgraded-
Rcpt-To" header fields as defined in Section 3. Before replacing,
decode the ALT-ADDRESS parameter value because it is encoded as xtest
[RFC3461].
To avoid disclosing recipient addresses, the downgrading process MUST
NOT add "Downgraded-Rcpt-To:" header if the SMTP downgrading targets
multiple recipients. See Section 7 for more detail.
The "RCPT TO" command may have an ORCPT parameter when the recipient
address is downgraded. The ORCPT parameter is used for DSN
[RFC3461]. If the ORCPT parameter contains an "utf-8" address and
the address contains non-ASCII characters, the ORCPT parameter MUST
be converted to utf-8-addr-xtext form or utf-8-addr-unitext form
which are described in [I-D.ietf-eai-dsn].
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5. Email header fields downgrading
This section defines the conversion method to ASCII for each header
field which may contain non-ASCII characters.
[I-D.ietf-eai-utf8headers] expands Received: header fields, [RFC2822]
ABNF elements <mailbox>, <word>, <comment>, <unstructured>, [RFC2045]
ABNF element <value>.
Header field downgrading is defined below for each ABNF element.
Downgrading an unknown header field is also defined as ENCAPSULATION
downgrading. Converting the header field terminates when no non-
ASCII characters remain in the header field.
RECEIVED downgrading: If the header field name is "Received:" and
the FOR clause contains a non-ASCII addresses, remove the FOR
clause from the header field. The other part except in the
comments does not contain non-ASCII values.
UNSTRUCTURED downgrading: If the header field has an <unstructured>
field which contains non-ASCII characters, encode the field
according to [RFC2047] with charset='UTF-8'.
WORD downgrading: If the header field has any <word> fields which
contains non-ASCII characters, encode the fields according to
[RFC2047] with charset='UTF-8'.
COMMENT downgrading: If the header field has any <comment> fields
which contains non-ASCII characters, encode the fields according
to [RFC2047] with charset='UTF-8'.
MIME-VALUE downgrading: If the header field has any <value>
elements defined by [RFC2045] and the elements contain non-ASCII
characters, encode the <value> elements by [RFC2231] with
charset='UTF-8' and the Language information empty. If the
<value> element is <quoted-string> and it contains <CFWS> outside
the DQUOTE, remove the <CFWS> before this conversion.
DISPLAY-NAME downgrading: If the header field has any <mailbox>
elements and they have <display-name> elements which contain non-
ASCII characters, encode the <display-name> elements according to
[RFC2047] with charset='UTF-8'.
MAILBOX downgrading: The <mailbox> elements have no equivalent
format for non-ASCII addresses. If the header field has any
<mailbox> elements which contain non-ASCII characters, preserve
the header field in each Address header field preservation header
defined in Section 3.2, and rewrite each <mailbox> element to
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ASCII only format. The <mailbox> element which contains non-ASCII
characters is one of three formats.
* [ Display-name ] "<" Utf8-addr-spec 1*FCS "<" Addr-spec ">>"
Rewrite it as
[ Display-name ] "<" Addr-spec ">"
* [ Display-name ] "<" Utf8-addr-spec ">"
* Utf8-addr-spec
Rewrite both as
[ Display-name ] "Internationalized Address " Encoded-word
" Removed:;"
where the <Encoded-word> is the original <Utf8-addr-spec>
encoded according to [RFC2047].
ENCAPSULATION downgrading: if the header field contains non-ASCII
characters and for which no rule is given above, encapsulate it in
a Downgraded header field described in Section 3.3 as a last
resort.
5.1. Downgrading method for each header field
Header fields are listed in [RFC4021]. This section describes the
downgrading method for each header field.
If the whole mail header field does not contain non-ASCII characters,
email header field downgrading is not required. Each header field's
downgrading method is described below.
o Address header fields which contain <mailbox>s
From:
Sender:
Reply-To:
To:
Cc:
Bcc:
Resent-From:
Resent-Sender:
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Resent-To:
Resent-Cc:
Return-Path:
If the header field contains <mailbox> elements which contains
non-ASCII addresses, preserve the header field in a downgraded
header before the conversion. Then do COMMENT downgrading,
DISPLAY-NAME downgrading and MAILBOX downgrading.
o Downgrading Non-ASCII in comments
Date:
Message-ID:
In-Reply-To:
References:
Resent-Date:
Resent-Message-ID:
MIME-Version:
Content-ID:
These header fields do not contain non-ASCII characters except in
comments. If the header contains UTF-8 characters in comments, do
COMMENT downgrading.
o Trace header fields
Received:
do COMMENT downgrading and do RECEIVED downgrading.
o MIME Content header fields
Content-Type:
Content-Disposition:
Do MIME-VALUE downgrading.
o Non-ASCII in <unstructured>
Subject:
Comments:
Content-Description:
Do UNSTRUCTURED downgrading.
o Non-ASCII in <unstructured> or <phrase>
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Keywords:
Do WORD downgrading.
o Other header fields
All other header fields which contains non-ASCII characters are
user-defined, missing from this draft or future defined header
fields. Do ENCAPSULATION downgrading as a last resort.
6. MIME body part headers downgrading
MIME body part header fields may contain non-ASCII characters
[I-D.ietf-eai-utf8headers]. This section defines the conversion
method to ASCII only header fields for each MIME header field which
contains non-ASCII characters. Parse the message body's MIME
structure for all levels and check each MIME header field whether it
contains non-ASCII characters. If the header field contains non-
ASCII characters in the header value, the header is a target of the
MIME body part headers downgrading. Each MIME header field's
downgrading method is described below. COMMENT downgrading, MIME-
VALUE downgrading, UNSTRUCTURED downgrading are described in
Section 5.
Content-ID:
The Content-ID: header does not contain non-ASCII characters
except in comments. If the header contains UTF-8 characters in
comments, do COMMENT downgrading.
Content-Type:
Content-Disposition:
Do MIME-VALUE downgrading.
Content-Description:
Do UNSTRUCTURED downgrading.
7. Security considerations
o A Downgraded message's header fields contain ASCII characters
only. But they still contain MIME encapsulated header fields
which contains non-ASCII UTF-8 characters. And more, the body
part may contain UTF-8 message. The recipients need to accept
UTF-8 mail body and UTF-8 header fields which is MIME encoded.
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o Rewriting headers increases the opportunities for undetected
spoofing.
o Recipients addresses can be undisclosed if those addresses are
listed on Bcc or group address. Those undisclosed addresses are
used only in the Envelope. Copying information from the Envelope
into headers risks inadvertent information disclosure (not just
about addresses). See Section 4 for instructions on recipient
address handling.
o The techniques described here invalidates methods that depend on
signatures over the envelope or any part of the message which
includes the top-level header or body part headers. Depending on
the specific message being downgraded, DKIM especially, but also
possibly S/MIME, PGP, and similar techniques are all likely to
break.
o Many gateways and servers on the Internet will discard headers
with which they are not familiar. To the extent to which the
downgrade procedures depend on new headers (e.g., "Downgraded") to
avoid information loss, then the risk of having those headers
dropped and its implications must be identified. In particular,
it appears to me that, if the Downgraded headers are dropped,
there is no possibility of reconstructing the original information
at any point (before, during, or after delivery).
See "Security considerations" section in [RFC4952] for more
discussion.
8. Implementation notes
Downgrading is an alternative to rejection for delivering messages
which require UTF8SMTP support to a server which does not provide
this. Implementing the full specification of this document is
desirable, but a partial implementation is also possible.
If a partial downgrading implementation confronts an unsupported
downgrading target, the implementation MUST NOT send the message to
server which does not support UTF8SMTP. Instead, it MUST reject the
message or generate a notification of non-deliverability.
8.1. Trivial downgrading
A partial downgrading, Trivial downgrading is discussed. It does not
support non-ASCII addresses in SMTP envelope and address header
fields, unknown header fields downgrading, the MIME body part headers
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downgrading. It supports
o some simple header fields downgrading: Subject
o comments and display name downgrading: From, To, CC
o trace header field downgrading: Received
Otherwise, the downgrading fails.
Trivial downgrading targets mail messages which are generated by
UTF8SMTP aware MUAs and contain non-ASCII characters in comments,
display names, unstructured parts without using non-ASCII E-mail
addresses. This E-mail message does not contain non-ASCII addresses
in SMTP Envelope and its header fields. But it is not deliverable
via UTF8SMTP un-aware MTA. Implementing full spec downgrading may be
hard, but trivial downgrading saves mail messages without using non-
ASCII addresses.
9. IANA Considerations
IANA is requested to register the following header fields in the
Permanent Message Header Field Repository, in accordance with the
procedures set out in [RFC3864].
Header field name: Downgraded-Mail-From
Applicable protocol: mail
Status: experimental
Author/change controller: IETF
Specification document(s): This document (Section 3)
Header field name: Downgraded-Rcpt-To
Applicable protocol: mail
Status: experimental
Author/change controller: IETF
Specification document(s): This document (Section 3)
Header field name: Downgraded-From
Applicable protocol: mail
Status: experimental
Author/change controller: IETF
Specification document(s): This document (Section 3)
Header field name: Downgraded-Sender
Applicable protocol: mail
Status: experimental
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Author/change controller: IETF
Specification document(s): This document (Section 3)
Header field name: Downgraded-To
Applicable protocol: mail
Status: experimental
Author/change controller: IETF
Specification document(s): This document (Section 3)
Header field name: Downgraded-Cc
Applicable protocol: mail
Status: experimental
Author/change controller: IETF
Specification document(s): This document (Section 3)
Header field name: Downgraded-Reply-To
Applicable protocol: mail
Status: experimental
Author/change controller: IETF
Specification document(s): This document (Section 3)
Header field name: Downgraded-Bcc
Applicable protocol: mail
Status: experimental
Author/change controller: IETF
Specification document(s): This document (Section 3)
Header field name: Downgraded-Resent-From
Applicable protocol: mail
Status: experimental
Author/change controller: IETF
Specification document(s): This document (Section 3)
Header field name: Downgraded-Resent-To
Applicable protocol: mail
Status: experimental
Author/change controller: IETF
Specification document(s): This document (Section 3)
Header field name: Downgraded-Resent-Cc
Applicable protocol: mail
Status: experimental
Author/change controller: IETF
Specification document(s): This document (Section 3)
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Header field name: Downgraded-Resent-Sender
Applicable protocol: mail
Status: experimental
Author/change controller: IETF
Specification document(s): This document (Section 3)
Header field name: Downgraded-Return-Path
Applicable protocol: mail
Status: experimental
Author/change controller: IETF
Specification document(s): This document (Section 3)
And more, IANA is requested to reserve all the field names that start
by "Downgraded-" for unknown header fields downgrading described in
Section 3.3, in accordance with the procedures set out in [RFC3864].
Header field name: Names which starts by "Downgraded-"
Applicable protocol: mail
Status: experimental
Author/change controller: IETF
Specification document(s): This document (Section 3)
10. Acknowledgements
Significant comments and suggestions were received from John Klensin,
Harald Alvestrand, Chris Newman, Randall Gellens, Charles Lindsey,
Marcos Sanz, Alexey Melnikov, Frank Ellermann, Edward Lewis, S.
Moonesamy and JET members.
11. Change History
This section is used for tracking the update of this document. Will
be removed after finalize.
11.1. draft-yoneya-ima-downgrade: Version 00
o Initial version
o Followed draft-yeh-ima-utf8headers-00 and draft-yao-smtpext-00
11.2. draft-yoneya-ima-downgrade: Version 01
o Document structure was changed
o Followed draft-yeh-ima-utf8headers-01 and draft-yao-smtpext-02
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o Downgrading requirements were added
o SMTP DATA encapsulation method was proposed
o Downgrading examples was provided
11.3. draft-ietf-eai-downgrade: Version 00
o Followed draft-yeh-ima-utf8headers-01 and
draft-ietf-eai-smtpext-00
o No header downgrading method was proposed
o Header encapsulation method was proposed
11.4. 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
11.5. 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
11.6. draft-ietf-eai-downgrade: Version 03
o Followed draft-ietf-eai-utf8headers-03 and
draft-ietf-eai-smtpext-03
o Downgraded: and Envelope-Downgraded: headers definition was added
o Mail header fields downgrading method was refined
o Examples in Appendix A were refined
11.7. draft-ietf-eai-downgrade: Version 04
o Followed draft-ietf-eai-utf8headers-06, draft-ietf-eai-smtpext-07
and draft-ietf-eai-dsn-02
o Downgrading requirements and conditions were moved to
Introduction.
o Descriptions about upgrading were removed.
o SPF and DKIM discussion were removed.
o Added many header fields downgrading.
o Allow address literal rewriting without alternate ASCII address in
header fields.
o Added MIME body part headers downgrading.
o Added ORCPT downgrading.
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11.8. draft-ietf-eai-downgrade: Version 05
o fixed examples
* ALT-ADDRESS parameter mistake
* RFC2047(x) notation was changed to encoded-word format
o Added implementation consideration section and trivial downgrading
o Downgraded: and Envelope-Downgraded: headers are separated for
each original headers.
o Removed list-* header fields downgrading
o Changed the way of writing the header field downgrading section
12. Normative References
[I-D.ietf-eai-dsn]
Newman, C. and A. Melnikov, "International Delivery and
Disposition Notifications", draft-ietf-eai-dsn-05 (work in
progress), November 2007.
[I-D.ietf-eai-smtpext]
Yao, J. and W. Mao, "SMTP extension for internationalized
email address", draft-ietf-eai-smtpext-09 (work in
progress), November 2007.
[I-D.ietf-eai-utf8headers]
Yeh, J., "Internationalized Email Headers",
draft-ietf-eai-utf8headers-07 (work in progress),
September 2007.
[RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message
Bodies", RFC 2045, November 1996.
[RFC2047] Moore, K., "MIME (Multipurpose Internet Mail Extensions)
Part Three: Message Header Extensions for Non-ASCII Text",
RFC 2047, November 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2183] Troost, R., Dorner, S., and K. Moore, "Communicating
Presentation Information in Internet Messages: The
Content-Disposition Header Field", RFC 2183, August 1997.
[RFC2231] Freed, N. and K. Moore, "MIME Parameter Value and Encoded
Word Extensions:
Character Sets, Languages, and Continuations", RFC 2231,
November 1997.
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[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.
[RFC3864] Klyne, G., Nottingham, M., and J. Mogul, "Registration
Procedures for Message Header Fields", BCP 90, RFC 3864,
September 2004.
[RFC4021] Klyne, G. and J. Palme, "Registration of Mail and MIME
Header Fields", RFC 4021, March 2005.
[RFC4952] Klensin, J. and Y. Ko, "Overview and Framework for
Internationalized Email", RFC 4952, July 2007.
Appendix A. Displaying downgraded message
A perfect reverse-function of the downgrading does not exist because
the encoding defined in [RFC2047] is not exactly reversible. The
restoration of the downgrading should be done once at the final
destination of the downgraded message such as MUAs or IMAP servers.
This document describes the restoration methods as displaying
technique.
Displaying downgraded message is mostly retrieved by MIME decoding
according to [RFC2047] and [RFC2231]. Result of MIME decoding,
downgraded header fields are still in Downgraded-*: header fields,
but the header field values are MIME decoded. These decoded
Downgraded-*: header fields contain the original header field values
and the recipient can read them. But the recipient's MUA cannot use
the original header fields automatically.
Additionally, MUAs can process Downgraded-*: header. It is described
in Appendix A.1 and Appendix A.2.
A.1. Displaying technique 1
MUA can remove 'Downgraded-' from decoded 'Downgraded-*:' header
fields' name. With this technique, The address header fields may be
displayed twice, one is ASCII-only downgraded header field and the
other is from decoded Downgraded-*: header.
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A.2. Displaying technique 2
MUA can decode and regenerate headers which contains the original
non-ASCII addresses. It is described below:
o For each Downgraded-*: header field, generate new header field
which field name is the original header name and the field value
is the decoded header field value.
* If the header is an address header described in Section 5,
+ Generate ASCII only header fields defined in Section 5 from
the decoded header fields without re-generating
Downgraded-*: header fields.
+ Remove the header field which is the same with the generated
ASCII only header from the header fields. This comparison
requires considerations for foldings and [RFC2047] encoded
parts.
Appendix B. Examples
B.1. Downgrading example 1
This section shows an SMTP Downgrading example. Consider a mail
message.
o The sender address is "NON-ASCII-local@example.com" which is non-
ASCII address. Its ASCII alternative is "ASCII-local@example.com"
and its display-name is "DISPLAY-local".
o The "To" address is "NON-ASCII-remote1@example.net" which is non-
ASCII address. Its ASCII alternative is
"ASCII-remote1@example.net" and its display-name is "DISPLAY-
remote1".
o The "CC" address is non-ASCII address
"NON-ASCII-remote2@example.org" without alternative ASCII address.
Its display-name is "DISPLAY-remote2".
o Three display-names contains non-ASCII characters.
o The Subject header is "NON-ASCII-SUBJECT" which contains non-ASCII
characters.
o assume To: recipient's MTA (example.net) does not support
UTF8SMTP.
o assume Cc: recipient's MTA (example.org) supports UTF8SMTP.
The example SMTP envelope/message is showin in Figure 4. In this
example, the To: recipient's session is fucused.
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MAIL From: <NON-ASCII-local@example.com>
ALT-ADDRESS=ASCII-local@example.com
RCPT TO: <NON-ASCII-remote1@example.net>
ALT-ADDRESS=ASCII-remote1@example.net
RCPT TO: <NON-ASCII-remote2@example.org>
-------------------------------------------------------------
Message-Id: MESSAGE_ID
Mime-Version: 1.0
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: 8bit
Subject: NON-ASCII-SUBJECT
From: DISPLAY-local <NON-ASCII-local@example.com
<ASCII-local@example.com>>
To: DISPLAY-remote1 <NON-ASCII-remote1@example.net
<ASCII-remote1@example.net>>
CC: DISPLAY-remote2 <NON-ASCII-remote2@example.org>
Date: DATE
MAIL_BODY
Figure 4: Original envelope/message (example 1)
In this example, there are two SMTP recipients, one is To:, the other
is CC:. The SMTP downgrading treats To: session downgrading.
Figure 5 shows SMTP downgraded example.
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MAIL From: <ASCII-local@example.com>
RCPT TO: <ASCII-remote1@example.net>
-------------------------------------------------------------
Downgraded-Mail-From: =?UTF-8?Q?<NON-ASCII-local@example.com>?=
<ASCII-local@example.com>
Downgraded-Rcpt-To: =?UTF-8?Q?<NON-ASCII-remote1@example.net>?=
<ASCII-remote1@example.net>
Message-Id: MESSAGE_ID
Mime-Version: 1.0
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: 8bit
Subject: NON-ASCII-SUBJECT
From: DISPLAY-local <NON-ASCII-local@example.com
<ASCII-local@example.com>>
To: DISPLAY-remote1 <NON-ASCII-remote1@example.net
<ASCII-remote1@example.net>>
CC: DISPLAY-remote2 <NON-ASCII-remote2@example.org>
Date: DATE
MAIL_BODY
Figure 5: SMTP Downgraded envelope/message (example 1)
After SMTP downgrading, header fields downgrading is performed.
Final downgraded message is shown in Figure 6. Return-Path header
will be added by the final destination MTA.
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Return-Path: <ASCII-local@example.com>
Downgraded-Mail-From: =?UTF-8?Q?<NON-ASCII-local@example.com>?=
<ASCII-local@example.com>
Downgraded-Rcpt-To: =?UTF-8?Q?<NON-ASCII-remote1@example.net>?=
<ASCII-remote1@example.net>
Message-Id: MESSAGE_ID
Mime-Version: 1.0
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: 8bit
Subject: =?UTF-8?Q?NON-ASCII-SUBJECT?=
Downgraded-From: =?UTF-8?Q?DISPLAY-local?=
=?UTF-8?Q?<NON-ASCII-local@example.com?= <ASCII-local@example.com>>
From: =?UTF-8?Q?DISPLAY-local?= <ASCII-local@example.com>
To: =?UTF-8?Q?DISPLAY-remote1?= <ASCII-remote1@example.net>
Downgraded-To: =?UTF-8?Q?DISPLAY-remote1?=
=?UTF-8?Q?<NON-ASCII-remote1@example.net?= <ASCII-remote1@example.net>>
CC: =?UTF-8?Q?DISPLAY-remote2?= Internationalized address
=?UTF-8?Q?NON-ASCII-remote2@example.org?= removed:;
Downgraded-CC: =?UTF-8?Q?DISPLAY-remote2?=
=?UTF-8?Q?<NON-ASCII-remote2@example.org>?=
Date: DATE
MAIL_BODY
Figure 6: Downgraded message (example 1)
B.2. Displaying example
Figure 7 shows MIME decoded message of Figure 6.
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Return-Path: <ASCII-local@example.com>
Downgraded-Mail-From: <NON-ASCII-local@example.com>
<ASCII-local@example.com>
Downgraded-Rcpt-To: <NON-ASCII-remote1@example.net>
<ASCII-remote1@example.net>
Message-Id: MESSAGE_ID
Mime-Version: 1.0
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: 8bit
Subject: NON-ASCII-SUBJECT
Downgraded-From: DISPLAY-local<NON-ASCII-local@example.com
<ASCII-local@example.com>>
From: DISPLAY-local <ASCII-local@example.com>
Downgraded-To: DISPLAY-remote1<NON-ASCII-remote1@example.net
<ASCII-remote1@example.net>>
To: DISPLAY-remote1 <ASCII-remote1@example.net>
Downgraded-CC: DISPLAY-remote2<NON-ASCII-remote2@example.org>
CC: DISPLAY-remote2 Internationalized address
NON-ASCII-remote2@example.org removed:;
Date: DATE
MAIL_BODY
Figure 7: MIME decoded message
B.2.1. Displaying technique 1 example
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After removing 'Downgraded-' from decoded 'Downgraded-*:' header
fields
Return-Path: <ASCII-local@example.com>
Downgraded-Mail-From: <NON-ASCII-local@example.com>
<ASCII-local@example.com>
Downgraded-Rcpt-To: <NON-ASCII-remote1@example.net>
<ASCII-remote1@example.net>
Message-Id: MESSAGE_ID
Mime-Version: 1.0
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: 8bit
Subject: NON-ASCII-SUBJECT
From: DISPLAY-local<NON-ASCII-local@example.com
<ASCII-local@example.com>>
From: DISPLAY-local <ASCII-local@example.com>
To: DISPLAY-remote1<NON-ASCII-remote1@example.net
<ASCII-remote1@example.net>>
To: DISPLAY-remote1 <ASCII-remote1@example.net>
CC: DISPLAY-remote2<NON-ASCII-remote2@example.org>
CC: DISPLAY-remote2 Internationalized address
NON-ASCII-remote2@example.org removed:;
Date: DATE
MAIL_BODY
Figure 8: Displaying technique 1
B.2.2. Displaying technique 2 example
This example shows displaying process of Appendix A.2 for Figure 6.
First, check downgraded address header fields existence.
Downgraded-From: =?UTF-8?Q?DISPLAY-local?=
=?UTF-8?Q?<NON-ASCII-local@example.com?= <ASCII-local@example.com>>
Downgraded-To: =?UTF-8?Q?DISPLAY-remote1?=
=?UTF-8?Q?<NON-ASCII-remote1@example.net?= <ASCII-remote1@example.net>>
Downgraded-CC: =?UTF-8?Q?DISPLAY-remote2?=
=?UTF-8?Q?<NON-ASCII-remote2@example.org>?=
Figure 9: Displaying technique 2: selecting downgraded address header
fields
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Then, decode MIME encoded parts.
Downgraded-From: DISPLAY-local<NON-ASCII-local@example.com
<ASCII-local@example.com>>
Downgraded-To: DISPLAY-remote1<NON-ASCII-remote1@example.net
<ASCII-remote1@example.net>>
Downgraded-CC: DISPLAY-remote2<NON-ASCII-remote2@example.org>
Figure 10: Displaying technique 2: decode downgraded address header
fields
Apply header fields downgrading to the decoded header without re-
generating Downgraded-*: header.
From: =?UTF-8?Q?DISPLAY-local?= <ASCII-local@example.com>
To: =?UTF-8?Q?DISPLAY-remote1?= <ASCII-remote1@example.net>
CC: =?UTF-8?Q?DISPLAY-remote2?= Internationalized address
=?UTF-8?Q?NON-ASCII-remote2@example.org?= removed:;
Figure 11: Displaying technique 2: downgraded decoded 'Downgraded-*:'
Remove the header fields which is the same with Figure 11 from
Figure 6.
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Return-Path: <ASCII-local@example.com>
Downgraded-Mail-From: =?UTF-8?Q?<NON-ASCII-local@example.com>?=
<ASCII-local@example.com>
Downgraded-Rcpt-To: =?UTF-8?Q?<NON-ASCII-remote1@example.net>?=
<ASCII-remote1@example.net>
Message-Id: MESSAGE_ID
Mime-Version: 1.0
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: 8bit
Subject: =?UTF-8?Q?NON-ASCII-SUBJECT?=
Downgraded-From: =?UTF-8?Q?DISPLAY-local?=
=?UTF-8?Q?<NON-ASCII-local@example.com?= <ASCII-local@example.com>>
Downgraded-To: =?UTF-8?Q?DISPLAY-remote1?=
=?UTF-8?Q?<NON-ASCII-remote1@example.net?= <ASCII-remote1@example.net>>
Downgraded-CC: =?UTF-8?Q?DISPLAY-remote2?=
=?UTF-8?Q?<NON-ASCII-remote2@example.org>?=
Date: DATE
MAIL_BODY
Figure 12: Displaying technique 2: Removing duplicated headers
Decode each 'Downgraded' header and replace it with its decoded
header. If each mail header has [RFC2047] encoded part and which
encoding is "UTF-8", it is a downgraded header, so decode it.
Return-Path: <ASCII-local@example.com>
Downgraded-Mail-From: <NON-ASCII-local@example.com>
<ASCII-local@example.com>
Downgraded-Rcpt-To: <NON-ASCII-remote1@example.net>
<ASCII-remote1@example.net>
Message-Id: MESSAGE_ID
Mime-Version: 1.0
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: 8bit
Subject: NON-ASCII-SUBJECT
From: DISPLAY-local<NON-ASCII-local@example.com
<ASCII-local@example.com>>
To: DISPLAY-remote1<NON-ASCII-remote1@example.net
<ASCII-remote1@example.net>>
CC: DISPLAY-remote2<NON-ASCII-remote2@example.org>
Date: DATE
MAIL_BODY
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Figure 13: Display technique 2: decoded message
As a result, in this simple example, all original header fields are
displayed in the original form.
B.3. Downgrading example 2
In many cases, the sender wants to use non-ASCII address, the
recipient does not support UTF8SMTP and does not have non-ASCII
address.
o The sender address is "NON-ASCII-local@example.com" which is non-
ASCII address. Its ASCII alternative is
"ASCII-local@example.com". It has a display-name "DISPLAY-local"
which contains non-ASCII characters.
o The "To" address is "ASCII-remote1@example.net" which is ASCII
only. It has a display-name "DISPLAY-remote1" which contains non-
ASCII characters.
o The Subject header is "NON-ASCII-SUBJECT" which contains non-ASCII
characters.
The second example envelope/message is shown in Figure 14.
MAIL From: <NON-ASCII-local@example.com>
ALT-ADDRESS=ASCII-local@example.com
RCPT TO: <ASCII-remote1@example.net>
-------------------------------------------------------------
Message-Id: MESSAGE_ID
Mime-Version: 1.0
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: 8bit
Subject: NON-ASCII-SUBJECT
From: DISPLAY-local <NON-ASCII-local@example.com
<ASCII-local@example.com>>
To: DISPLAY-remote1 <ASCII-remote1@example.net>
Date: DATE
MAIL_BODY
Figure 14: Original message (example 2)
In this example, SMTP session is downgradable. Figure 15 shows SMTP
downgraded envelope/message.
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MAIL From: <ASCII-local@example.com>
RCPT TO: <ASCII-remote1@example.net>
-------------------------------------------------------------
Downgraded-Mail-From: =?UTF-8?Q?<NON-ASCII-local@example.com>?=
<ASCII-local@example.com>
Message-Id: MESSAGE_ID
Mime-Version: 1.0
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: 8bit
Subject: NON-ASCII-SUBJECT
From: DISPLAY-local <NON-ASCII-local@example.com
<ASCII-local@example.com>>
To: DISPLAY-remote1 <ASCII-remote1@example.net>
Date: DATE
MAIL_BODY
Figure 15: SMTP Downgraded envelope/message (example 2)
After SMTP downgrading, header fields downgrading is performed. The
downgraded example is shown in Figure 16.
Return-Path: <ASCII-local@example.com>
Downgraded-Mail-From: =?UTF-8?Q?<NON-ASCII-local@example.com>?=
<ASCII-local@example.com>
Message-Id: MESSAGE_ID
Mime-Version: 1.0
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: 8bit
Subject: =?UTF-8?Q?NON-ASCII-SUBJECT?=
Downgraded-From: =?UTF-8?Q?DISPLAY-local?=
=?UTF-8?Q?<NON-ASCII-local@example.com?= <ASCII-local@example.com>>
From: =?UTF-8?Q?DISPLAY-local?= <ASCII-local@example.com>
To: =?UTF-8?Q?DISPLAY-remote1?= <ASCII-remote1@example.net>
Date: DATE
MAIL_BODY
Figure 16: Downgraded message (example 2)
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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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Full Copyright Statement
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contained in BCP 78, and except as set forth therein, the authors
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