Post-delivery Message Downgrading for Internationalized Email Messages
draft-ietf-eai-popimap-downgrade-03
The information below is for an old version of the document.
| Document | Type |
This is an older version of an Internet-Draft that was ultimately published as RFC 6857.
|
|
|---|---|---|---|
| Author | Kazunori Fujiwara | ||
| Last updated | 2011-06-10 (Latest revision 2011-04-18) | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Formats | |||
| Reviews |
GENART Telechat review
by Ben Campbell
Almost ready
GENART Last Call review
by Ben Campbell
On the Right Track
|
||
| Additional resources | Mailing list discussion | ||
| Stream | WG state | WG Document | |
| Document shepherd | (None) | ||
| IESG | IESG state | Became RFC 6857 (Proposed Standard) | |
| Consensus boilerplate | Unknown | ||
| Telechat date | (None) | ||
| Responsible AD | Pete Resnick | ||
| Send notices to | eai-chairs@tools.ietf.org, draft-ietf-eai-popimap-downgrade@tools.ietf.org |
draft-ietf-eai-popimap-downgrade-03
Email Address Internationalization K. Fujiwara
(EAI) JPRS
Internet-Draft Oct 31, 2011
Updates: 5322 (if approved)
Intended status: Standards Track
Expires: May 3, 2012
Post-delivery Message Downgrading for Internationalized Email Messages
draft-ietf-eai-popimap-downgrade-03.txt
Abstract
The Email Address Internationalization (UTF8SMTP) extension allows
UTF-8 characters in mail header fields. POP and IMAP servers support
internationalized email messages. If a POP/IMAP client does not
support Email Address Internationalization, POP/IMAP servers cannot
send Internationalized Email Headers to the client and cannot remove
the message. To avoid the situation, this document describes a
conversion mechanism for internationalized Email messages to be
traditional message format.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on May 3, 2012.
Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
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carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Updating RFC 5322 . . . . . . . . . . . . . . . . . . . . . . 5
4. New Header Fields Definition . . . . . . . . . . . . . . . . . 6
4.1. Preservation Header Fields . . . . . . . . . . . . . . . . 6
5. Email Header Fields Downgrading . . . . . . . . . . . . . . . 7
5.1. Downgrading Method for Each ABNF Element . . . . . . . . . 7
5.1.1. RECEIVED Downgrading . . . . . . . . . . . . . . . . . 7
5.1.2. UNSTRUCTURED Downgrading . . . . . . . . . . . . . . . 7
5.1.3. WORD Downgrading . . . . . . . . . . . . . . . . . . . 7
5.1.4. COMMENT Downgrading . . . . . . . . . . . . . . . . . 7
5.1.5. MIME-VALUE Downgrading . . . . . . . . . . . . . . . . 7
5.1.6. DISPLAY-NAME Downgrading . . . . . . . . . . . . . . . 8
5.1.7. GROUP Downgrading . . . . . . . . . . . . . . . . . . 8
5.1.8. MAILBOX Downgrading . . . . . . . . . . . . . . . . . 8
5.1.9. ENCAPSULATION Downgrading . . . . . . . . . . . . . . 9
5.1.10. TYPED-ADDRESS Downgrading . . . . . . . . . . . . . . 9
5.2. Downgrading Method for Each Header Field . . . . . . . . . 9
5.2.1. Address Header Fields That Contain <address>s . . . . 9
5.2.2. Address Header Fields with Typed Addresses . . . . . . 10
5.2.3. Downgrading Non-ASCII in Comments . . . . . . . . . . 10
5.2.4. Message-ID Header Fields . . . . . . . . . . . . . . . 10
5.2.5. Received Header Field . . . . . . . . . . . . . . . . 11
5.2.6. MIME Content Header Fields . . . . . . . . . . . . . . 11
5.2.7. Non-ASCII in <unstructured> . . . . . . . . . . . . . 11
5.2.8. Non-ASCII in <phrase> . . . . . . . . . . . . . . . . 11
5.2.9. Other Header Fields . . . . . . . . . . . . . . . . . 11
6. MIME Body-Part Header Field Downgrading . . . . . . . . . . . 12
7. Security Considerations . . . . . . . . . . . . . . . . . . . 12
8. Implementation Notes . . . . . . . . . . . . . . . . . . . . . 13
8.1. RFC 2047 Encoding . . . . . . . . . . . . . . . . . . . . 13
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
9.1. Statement about Downgraded- registration . . . . . . . . . 14
9.2. Existing Downgraded- registrations . . . . . . . . . . . . 14
9.3. Additional header fields . . . . . . . . . . . . . . . . . 14
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 15
11. Change History . . . . . . . . . . . . . . . . . . . . . . . . 15
11.1. Version 00 . . . . . . . . . . . . . . . . . . . . . . . . 15
11.2. Version 01 . . . . . . . . . . . . . . . . . . . . . . . . 15
11.3. Version 02 . . . . . . . . . . . . . . . . . . . . . . . . 16
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11.4. Version 03 . . . . . . . . . . . . . . . . . . . . . . . . 16
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 16
12.1. Normative References . . . . . . . . . . . . . . . . . . . 16
12.2. Informative References . . . . . . . . . . . . . . . . . . 17
Appendix A. Examples . . . . . . . . . . . . . . . . . . . . . . 17
A.1. Downgrading Example . . . . . . . . . . . . . . . . . . . 17
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1. Introduction
Traditional mail systems, which are defined by [RFC5322], allow ASCII
characters in mail header field values. The UTF8SMTP extension
([I-D.ietf-eai-frmwrk-4952bis] and [I-D.ietf-eai-rfc5335bis] allows
UTF-8 characters in mail header field values.
If a header field contains non-ASCII characters, POP/IMAP servers
cannot send Internationalized Email Headers to the client and cannot
remove the message. This message downgrading mechanism converts mail
header fields to an all-ASCII representation. The POP/IMAP servers
can use the downgrading mechanism and send the Internationalized
Email message as a traditional form.
[I-D.ietf-eai-rfc5335bis] allows UTF-8 characters to be used in mail
header fields and MIME header fields. The message downgrading
mechanism specified here describes the conversion method from the
internationalized email messages that are defined in
[I-D.ietf-eai-frmwrk-4952bis], and [I-D.ietf-eai-rfc5335bis] to the
traditional email messages defined in [RFC5322].
There is no good way to convert "From:" and "Sender:" header fields,
the draft need to update [RFC5322] to allow empty "From:" and
"Sender:" header fields and it is described in Section 3.
Message Downgrading may be implemented in POP server and IMAP server
only.
This document tries to define the message downgrading process
clearly.
Downgrading consists of the following four parts:
o Updating RFC 5322
o New header field definitions
o Email header field downgrading
o MIME header field downgrading
In Section 4 of this document, header fields starting with
"Downgraded-" are introduced. They preserve the original header
fields.
Email header field downgrading is described in Section 5. It
generates ASCII-only header fields.
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MIME header fields are expanded in [I-D.ietf-eai-rfc5335bis]. MIME
header field downgrading is described in Section 6. It generates
ASCII-only MIME header fields.
Displaying downgraded messages that originally contained
internationalized header fields is out of scope of this document. A
POP/IMAP client which does not support UTF8 extension does not know
internationalized message format described in
[I-D.ietf-eai-rfc5335bis].
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
All specialized terms used in this specification are defined in the
Email Address Internationalization (EAI) overview
[I-D.ietf-eai-frmwrk-4952bis], in the mail message specifications
[RFC5322], or in the 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
[I-D.ietf-eai-frmwrk-4952bis].
This document depends on [I-D.ietf-eai-rfc5335bis]. Key words used
in those documents are used in this document, too.
The term "non-ASCII" refers to a UTF-8 string that contains at least
one non-ASCII character.
A "UTF8SMTP message" is an email message expanded by
[I-D.ietf-eai-rfc5335bis].
3. Updating RFC 5322
"From:" header field or "Sender:" header field may contain non-ASCII
addresses in internationalized Email messages. These non-ASCII
addresses are not allowed in [RFC5322]. The draft proposes that the
pop/imap downgrading uses <group> syntax and encodes non-ASCII
addresses into <display-name> with empty <group-list> described in
Section 5.
This specification redefines "From:", "Sender:", "Resent-From:" and
"Resent-Sender:" header fields defined in Section 3.6.2 and 3.6.6 of
[RFC5322] to allow <group> in the header fields.
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from = "From:" address-list CRLF
resent-from = "Resent-From:" address-list CRLF
sender = "Sender:" address CRLF
resent-sender = "Resent-Sender:" address CRLF
4. New Header Fields Definition
New header fields starting with "Downgraded-" are defined here to
preserve those mail header field values that contain UTF-8
characters. During downgrading, one new "Downgraded-" header field
is added for each mail header field that cannot be passed as-is to a
POP/IMAP client that does not support UTF8 extension. The original
mail header field is removed. Only those mail header fields that
contain non-ASCII characters are affected. The result of this
process is a message that is compliant with existing email
specifications [RFC5322]. The original internationalized information
can be retrieved by examining the "Downgraded-" header fields that
were added.
4.1. Preservation Header Fields
New preservation header fields are defined to preserve information
that appeared in non-ASCII text in header fields of the incoming
message. The values of the new fields holds the original header
field value in encoded form. The revised header field syntax is
specified as follows:
fields =/ known-downgraded-headers ":"
unstructured CRLF
known-downgraded-headers = "Downgraded-" original-headers
original-headers = "Message-Id" / "Resent-Message-Id" /
"In-Reply-To:" / "References:" /
"Original-Recipient" / "Final-Recipient"
To preserve a header field in a "Downgraded-" header field:
1. Generate a new "Downgraded-" header field whose value is the
original header field value.
2. Treat the generated header field content as if it were
unstructured, and then apply [RFC2047] encoding with charset
UTF-8 as necessary so that the result is ASCII.
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3. Remove the original header field.
5. Email Header Fields Downgrading
This section defines the conversion method to ASCII for each header
field that may contain non-ASCII characters.
[I-D.ietf-eai-rfc5335bis] expands "Received:" header fields;
[RFC5322] describes ABNF elements <mailbox>, <word>, <comment>,
<unstructured>; [RFC2045] describes ABNF element <value>.
5.1. Downgrading Method for Each ABNF Element
Header field downgrading is defined below for each ABNF element.
Converting the header field terminates when no non-ASCII characters
remain in the header field.
5.1.1. RECEIVED Downgrading
If the header field name is "Received:" and the FOR clause contains a
non-ASCII address, remove the FOR clause from the header field.
Other parts (not counting <comment>s) should not contain non-ASCII
values.
5.1.2. UNSTRUCTURED Downgrading
If the header field has an <unstructured> field that contains non-
ASCII characters, apply [RFC2047] encoding with charset UTF-8.
5.1.3. WORD Downgrading
If the header field has any <word> fields that contain non-ASCII
characters, apply [RFC2047] encoding with charset UTF-8.
5.1.4. COMMENT Downgrading
If the header field has any <comment> fields that contain non-ASCII
characters, apply [RFC2047] encoding with charset UTF-8.
5.1.5. 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 according to [RFC2231] with charset UTF-8 and leave the
language information empty. If the <value> element is <quoted-
string> and it contains <CFWS> outside the DQUOTE, remove the <CFWS>
before this conversion.
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5.1.6. DISPLAY-NAME Downgrading
If the header field has any <address> (<mailbox> or <group>) elements
and they have <display-name> elements that contain non-ASCII
characters, encode the <display-name> elements according to [RFC2047]
with charset UTF-8. DISPLAY-NAME downgrading is the same algorithm
as WORD downgrading.
5.1.7. GROUP Downgrading
<group> is defined in Section 3.4 of [RFC5322]. The <group> elements
may contain <mailbox>s which contain non-ASCII addresses.
If the header field has any <group> elements that contain <mailbox>
elements, and those <mailbox> elements in turn contain non-ASCII
addresses, rewrite each <group> element as
"Internationalized address removed" display-name ENCODED_WORD ":;"
where the <ENCODED_WORD> is the original <group-list> encoded
according to [RFC2047].
5.1.8. MAILBOX Downgrading
The <mailbox> elements have no equivalent format for non-ASCII
addresses. If the header field has any <mailbox> elements that
contain non-ASCII characters in their <addr-spec> element, rewrite
each <addr-spec> element to ASCII-only format. The <addr-spec>
element that contains non-ASCII characters may appear in two forms
as:
"<" addr-spec ">"
addr-spec
Rewrite both as:
"Internationalized address " ENCODED-WORD " removed:;"
where the <ENCODED-WORD> is the original <addr-spec> encoded
according to [RFC2047].
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5.1.9. ENCAPSULATION Downgrading
Encapsulate the header field in a "Downgraded-" header field as
described in Section 4 as a last resort.
Applying this procedure to "Received:" header field is prohibited.
ENCAPSULATION Downgrading is allowed for "Message-ID",
"In-Reply-To:", "References:", "Original-Recipient" and "Final-
Recipient" header fields.
5.1.10. TYPED-ADDRESS Downgrading
If the header field contains <utf-8-type-addr> and the <utf-8-type-
addr> contains raw non-ASCII characters, it is in utf-8-address form.
Convert it to utf-8-addr-xtext form. Those forms are described in
[I-D.ietf-eai-rfc5337bis-dsn]. COMMENT downgrading is also performed
in this case. If the address type is unrecognized and the header
field contains non-ASCII characters, then fall back to using
ENCAPSULATION downgrading on the entire header field.
5.2. 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.
5.2.1. Address Header Fields That Contain <address>s
From:
Sender:
To:
Cc:
Bcc:
Reply-To:
Resent-From:
Resent-Sender:
Resent-To:
Resent-Cc:
Resent-Bcc:
Resent-Reply-To:
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Return-Path:
Disposition-Notification-To:
If the header field contains <group> elements that contain non-ASCII
addresses, perform COMMENT downgrading, DISPLAY-NAME downgrading, and
GROUP downgrading.
If the header field contains <mailbox> elements that contain non-
ASCII addresses, perform COMMENT downgrading, DISPLAY-NAME
downgrading, and MAILBOX downgrading.
5.2.2. Address Header Fields with Typed Addresses
Original-Recipient:
Final-Recipient:
If the header field contains non-ASCII characters, perform TYPED-
ADDRESS downgrading.
5.2.3. Downgrading Non-ASCII in Comments
Date:
Resent-Date:
MIME-Version:
Content-ID:
Content-Transfer-Encoding:
Content-Language:
Accept-Language:
Auto-Submitted:
These header fields do not contain non-ASCII characters except in
comments. If the header field contains UTF-8 characters in comments,
perform COMMENT downgrading.
5.2.4. Message-ID Header Fields
Message-ID:
Resent-Message-ID:
In-Reply-To:
References:
Perform ENCAPSULATION Downgrading.
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5.2.5. Received Header Field
Received:
Perform COMMENT downgrading and RECEIVED downgrading.
5.2.6. MIME Content Header Fields
Content-Type:
Content-Disposition:
Perform MIME-VALUE downgrading and COMMENT downgrading.
5.2.7. Non-ASCII in <unstructured>
Subject:
Comments:
Content-Description:
Perform UNSTRUCTURED downgrading.
5.2.8. Non-ASCII in <phrase>
Keywords:
Perform WORD downgrading.
5.2.9. Other Header Fields
There are other header fields that contain non-ASCII characters.
They are user-defined and missing from this document, or future
defined header fields. They are treated as "Optional Fields" and
their field value are treated as unstructured described in Section
3.6.8 of [RFC5322].
Perform UNSTRUCTURED downgrading.
If the software understands the header field's structure and a
downgrading algorithm other than UNSTRUCTURED is applicable, that
software SHOULD use that algorithm; UNSTRUCTURED downgrading is used
as a last resort.
Mailing list header fields (those that start in "List-") are part of
this category.
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6. MIME Body-Part Header Field Downgrading
MIME body-part header fields may contain non-ASCII characters
[I-D.ietf-eai-rfc5335bis]. This section defines the conversion
method to ASCII-only header fields for each MIME header field that
contains non-ASCII characters. Parse the message body's MIME
structure at all levels and check each MIME header field to see
whether it contains non-ASCII characters. If the header field
contains non-ASCII characters in the header field value, the header
field is a target of the MIME body-part header field's downgrading.
Each MIME header field's downgrading method is described below.
COMMENT downgrading, MIME-VALUE downgrading, and UNSTRUCTURED
downgrading are described in Section 5.
Content-ID:
The "Content-ID:" header field does not contain non-ASCII
characters except in comments. If the header field contains UTF-8
characters in comments, perform COMMENT downgrading.
Content-Type:
Content-Disposition:
Perform MIME-VALUE downgrading and COMMENT downgrading.
Content-Description: Perform UNSTRUCTURED downgrading.
7. Security Considerations
Existing clients do not know new From: and Sender: header fields
syntax updated by Section 3 and may get wrong when they confront
<group> syntax in From: and Sender: fields.
A downgraded message's header fields contain ASCII characters only.
But they still contain MIME-encapsulated header fields that contain
non-ASCII UTF-8 characters. Furthermore, the body part may contain
UTF-8 characters. Implementations parsing Internet messages need to
accept UTF-8 body parts and UTF-8 header fields that are MIME-
encoded. Thus, this document inherits the security considerations of
MIME-encoded header fields ([RFC2047] and [RFC3629]).
Rewriting header fields increases the opportunities for undetected
spoofing by malicious senders. However, rewritten header fields are
preserved into Downgraded-* header fields, and parsing Downgraded-*
header fields enables the detection of spoofing caused by
downgrading.
The techniques described here invalidate methods that depend on
digital signatures over any part of the message, which includes the
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top-level header fields and body-part header fields. Depending on
the specific message being downgraded, the following techniques are
likely to break: DomainKeys Identified Mail (DKIM), and possibly
S/MIME and Pretty Good Privacy (PGP). The two obvious mitigations
are to stick to 7-bit transport when using these techniques (as most/
all of them presently require) or to make sure to have UTF8SMTP end-
to-end when needed.
While information in any email header field should usually be treated
with some suspicion, current email systems commonly employ various
mechanisms and protocols to make the information more trustworthy.
Currently, information in the new Downgraded-* header fields is
usually not inspected by these mechanisms, and may be even less
trustworthy than the traditional header fields. Note that the
Downgraded-* header fields could have been inserted with malicious
intent (and with content unrelated to the traditional header fields).
See the "Security Considerations" section in
[I-D.ietf-eai-frmwrk-4952bis] for more discussion.
8. Implementation Notes
8.1. RFC 2047 Encoding
While [RFC2047] has a specific algorithm to deal with whitespace in
adjacent encoded words, there are a number of deployed
implementations that fail to implement the algorithm correctly. As a
result, whitespace behavior is somewhat unpredictable in practice
when multiple encoded words are used. While RFC 5322 states that
implementations SHOULD limit lines to not more than 78 characters,
implementations MAY choose to allow overly long encoded words in
order to work around faulty [RFC2047] implementations.
Implementations that choose to do so SHOULD have an optional
mechanism to limit line length to 78 characters.
9. IANA Considerations
[[RFC Editor: Please change "should now be" and "should be" to "have
been" when the IANA actions are complete.]]
[[ Notes in draft: this section is not finished, to be reviewed with
IANA. ]]
Following instructions in the now-obsolete [RFC5504], IANA has made a
series of entries in the the Permanent Message Header Field registry.
Those registrations should now be changed as follows:
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9.1. Statement about Downgraded- registration
The statement about refusing any "Downgraded-" registrations should
be updated to refer to this document and to provide for registering
such fields as specified in Section 9.3.
[[ Note in draft: The restriction may become useless if unknown
header fields may be treated as unstructured. ]]
9.2. Existing Downgraded- registrations
Individual existing registrations for
Downgraded-Bcc
Downgraded-Cc
Downgraded-Disposition-Notification-To
Downgraded-From
Downgraded-Mail-From
Downgraded-Rcpt-To
Downgraded-Reply-To
Downgraded-Resent-Bcc
Downgraded-Resent-Cc
Downgraded-Resent-From
Downgraded-Resent-Reply-To
Downgraded-Resent-Sender
Downgraded-Resent-To
Downgraded-Return-Path
Downgraded-Sender
Downgraded-To
should be updated to replace "experimental" with "obsoleted" and to
reference this document.
9.3. Additional header fields
The following header fields should be registered in the Permanent
Message Header Field registry, in accordance with the procedures set
out in [RFC3864].
Header field name: Downgraded-Message-Id
Applicable protocol: mail
Status: standard
Author/change controller: IETF
Specification document(s): This document (Section 4)
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Header field name: Downgraded-In-Reply-To
Applicable protocol: mail
Status: standard
Author/change controller: IETF
Specification document(s): This document (Section 4)
Header field name: Downgraded-References
Applicable protocol: mail
Status: standard
Author/change controller: IETF
Specification document(s): This document (Section 4)
Header field name: Downgraded-Original-Recipient
Applicable protocol: mail
Status: standard
Author/change controller: IETF
Specification document(s): This document (Section 4)
Header field name: Downgraded-Final-Recipient
Applicable protocol: mail
Status: standard
Author/change controller: IETF
Specification document(s): This document (Section 4)
10. Acknowledgements
This document draws heavily from the experimental in-transit message
downgrading procedure described in RFC 5504 [RFC5504]. The
contribution of the co-author of that earlier document, Y. Yoneya,
are gratefully acknowledged.
11. Change History
This section is used for tracking the update of this document. Will
be removed after finalize.
11.1. Version 00
o Initial version
o Imported header field downgrading from RFC 5504
11.2. Version 01
o same as Version 00
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11.3. Version 02
o Added updating RFC 5322 to allow <group> syntax in From: and
Sender
o Added GROUP Downgrading
11.4. Version 03
o Replaced <utf8-addr-spec> with <addr-spec>
o Added updating RFC 5322 to allow <group> syntax in From: and
Sender
o Added one sentence in Security considerations
o Updated IANA considerations
12. References
12.1. Normative References
[I-D.ietf-eai-frmwrk-4952bis] Klensin, J. and Y. Ko, "Overview and
Framework for Internationalized
Email",
draft-ietf-eai-frmwrk-4952bis-12 (work
in progress), October 2011.
[I-D.ietf-eai-rfc5335bis] Yang, A., Steele, S., and N. Freed,
"Internationalized Email Headers",
draft-ietf-eai-rfc5335bis-13 (work in
progress), October 2011.
[I-D.ietf-eai-rfc5337bis-dsn] Hansen, T., Newman, C., and A.
Melnikov, "Internationalized Delivery
Status and Disposition Notifications",
draft-ietf-eai-rfc5337bis-dsn-05 (work
in progress), October 2011.
[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-
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ASCII Text", RFC 2047, November 1996.
[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.
[RFC5322] Resnick, P., Ed., "Internet Message
Format", RFC 5322, October 2008.
[RFC3629] Yergeau, F., "UTF-8, a transformation
format of ISO 10646", STD 63,
RFC 3629, November 2003.
[RFC4021] Klyne, G. and J. Palme, "Registration
of Mail and MIME Header Fields",
RFC 4021, March 2005.
[RFC2119] Bradner, S., "Key words for use in
RFCs to Indicate Requirement Levels",
BCP 14, RFC 2119, March 1997.
[RFC3864] Klyne, G., Nottingham, M., and J.
Mogul, "Registration Procedures for
Message Header Fields", BCP 90,
RFC 3864, September 2004.
12.2. Informative References
[RFC5504] Fujiwara, K. and Y. Yoneya,
"Downgrading Mechanism for Email
Address Internationalization",
RFC 5504, March 2009.
Appendix A. Examples
A.1. Downgrading Example
This appendix shows an message downgrading example. Consider a
received mail message where:
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o The sender address is a non-ASCII address,
"NON-ASCII-local@example.com". Its display-name is "DISPLAY-
local".
o The "To:" header field contains two non-ASCII addresses,
"NON-ASCII-remote1@example.net" and
"NON-ASCII-remote2@example.com" Its display-names are "DISPLAY-
remote1" and "DISPLAY-remote2".
o The "Cc:" header field contains a non-ASCII address,
"NON-ASCII-remote3@example.org". Its display-name is "DISPLAY-
remote3".
o Four display names contain non-ASCII characters.
o The Subject header field is "NON-ASCII-SUBJECT", which contains
non-ASCII characters.
o The "Message-Id:" header field contains "NON-ASCII-MESSAGE_ID",
which contains non-ASCII characters.
o There is an unknown header field "X-Unknown-Header" which contains
non-ASCII characters.
Return-Path: <NON-ASCII-local@example.com>
Received: from ... by ... for <NON-ASCII-remote1@example.net>
Received: from ... by ... for <NON-ASCII-remote1@example.net>
From: DISPLAY-local <NON-ASCII-local@example.com>
To: DISPLAY-remote1 <NON-ASCII-remote1@example.net>,
DISPLAY-remote2 <NON-ASCII-remote2@example.com>
Cc: DISPLAY-remote3 <NON-ASCII-remote3@example.org>
Subject: NON-ASCII-SUBJECT
Date: DATE
Message-Id: NON-ASCII-MESSAGE_ID
Mime-Version: 1.0
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: 8bit
X-Unknown-Header: NON-ASCII-CHARACTERS
MAIL_BODY
Figure 1: Received message in a mail drop
The downgraded message is shown in Figure 2. "Return-Path:",
"From:", "To:" and "Cc:" header fields are rewritten. "Subject:" and
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"X-Unknown-Header:" header fields are encoded using [RFC2047].
"Message-Id:" header field is encapsulated as
"Downgraded-Message-Id:" header field.
Return-Path: Internationalized address
=?UTF-8?Q?NON-ASCII-local@example.com?= removed:;
Received: from ... by ...
Received: from ... by ...
From: =?UTF-8?Q?DISPLAY-local?= Internationalized address
=?UTF-8?Q?NON-ASCII-local@example.com?= removed:;
To: =?UTF-8?Q?DISPLAY-remote1?= Internationalized address
=?UTF-8?Q?NON-ASCII-remote1@example.net?= removed:;,
=?UTF-8?Q?DISPLAY-remote2?= Internationalized address
=?UTF-8?Q?NON-ASCII-remote2@example.com?= removed:;,
Cc: =?UTF-8?Q?DISPLAY-remote3?= Internationalized address
=?UTF-8?Q?NON-ASCII-remote3@example.org?= removed:;
Subject: =?UTF-8?Q?NON-ASCII-SUBJECT?=
Date: DATE
Downgraded-Message-Id: =?UTF-8?Q?MESSAGE_ID?=
Mime-Version: 1.0
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: 8bit
X-Unknown-Header: =?UTF-8?Q?NON-ASCII-CHARACTERS?=
MAIL_BODY
Figure 2: Downgraded message
Author's Address
Kazunori Fujiwara
Japan Registry Services Co., Ltd.
Chiyoda First Bldg. East 13F, 3-8-1 Nishi-Kanda
Chiyoda-ku, Tokyo 101-0065
Japan
Phone: +81 3 5215 8451
EMail: fujiwara@wide.ad.jp, fujiwara@jprs.co.jp
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