Internet DRAFT - draft-ietf-eai-popimap-downgrade
draft-ietf-eai-popimap-downgrade
Email Address Internationalization K. Fujiwara
(EAI) JPRS
Internet-Draft Oct 22, 2012
Intended status: Standards Track
Expires: April 25, 2013
Post-delivery Message Downgrading for Internationalized Email Messages
draft-ietf-eai-popimap-downgrade-08.txt
Abstract
The Email Address Internationalization (SMTPUTF8) extension to SMTP
allows UTF-8 characters in mail header fields. Upgraded POP and IMAP
servers support internationalized Email messages. If a POP/IMAP
client does not support Email Address Internationalization, POP/IMAP
servers cannot deliver 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 in traditional message format. In the process, message
elements requiring internationalized treatment are recoded or removed
and receivers are able to know that they received messages containing
such elements even if they cannot process the internationalized
elements.
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 April 25, 2013.
Copyright Notice
Copyright (c) 2012 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
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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
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
1.1. Problem statement . . . . . . . . . . . . . . . . . . . . 4
1.2. Possible solutions . . . . . . . . . . . . . . . . . . . . 4
1.3. Approach taken in this specification . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 6
3. Email Header Fields Downgrading . . . . . . . . . . . . . . . 6
3.1. Downgrading Method for Each ABNF Element . . . . . . . . . 6
3.1.1. UNSTRUCTURED Downgrading . . . . . . . . . . . . . . . 6
3.1.2. WORD Downgrading . . . . . . . . . . . . . . . . . . . 6
3.1.3. COMMENT Downgrading . . . . . . . . . . . . . . . . . 6
3.1.4. MIME-VALUE Downgrading . . . . . . . . . . . . . . . . 7
3.1.5. DISPLAY-NAME Downgrading . . . . . . . . . . . . . . . 7
3.1.6. DOMAIN Downgrading . . . . . . . . . . . . . . . . . . 7
3.1.7. GROUP Downgrading . . . . . . . . . . . . . . . . . . 7
3.1.8. MAILBOX Downgrading . . . . . . . . . . . . . . . . . 8
3.1.9. TYPED-ADDRESS Downgrading . . . . . . . . . . . . . . 8
3.1.10. ENCAPSULATION: A Last Resort . . . . . . . . . . . . . 8
3.2. Downgrading Method for Each Header Field . . . . . . . . . 10
3.2.1. Address Header Fields That Contain <address>s . . . . 10
3.2.2. Downgrading Non-ASCII in Comments . . . . . . . . . . 11
3.2.3. Message-ID Header Fields . . . . . . . . . . . . . . . 11
3.2.4. Received Header Field . . . . . . . . . . . . . . . . 11
3.2.5. MIME Content Header Fields . . . . . . . . . . . . . . 12
3.2.6. Non-ASCII in <unstructured> . . . . . . . . . . . . . 12
3.2.7. Non-ASCII in <phrase> . . . . . . . . . . . . . . . . 12
3.2.8. Other Header Fields . . . . . . . . . . . . . . . . . 12
4. MIME Downgrading . . . . . . . . . . . . . . . . . . . . . . . 12
4.1. MIME Body-Part Header Field Downgrading . . . . . . . . . 13
4.2. Delivery Status Notification downgrading . . . . . . . . . 13
5. Security Considerations . . . . . . . . . . . . . . . . . . . 13
6. Implementation Notes . . . . . . . . . . . . . . . . . . . . . 14
6.1. RFC 2047 Encoding . . . . . . . . . . . . . . . . . . . . 14
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
7.1. Obsolescence of Existing Downgraded-* Header Fields . . . 15
7.2. Registration of New Downgraded-* Header Fields . . . . . . 15
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 16
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 16
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9.1. Normative References . . . . . . . . . . . . . . . . . . . 16
9.2. Informative References . . . . . . . . . . . . . . . . . . 18
Appendix A. Examples . . . . . . . . . . . . . . . . . . . . . . 18
A.1. Downgrading Example . . . . . . . . . . . . . . . . . . . 18
Appendix B. Change History . . . . . . . . . . . . . . . . . . . 20
B.1. Version 00 . . . . . . . . . . . . . . . . . . . . . . . . 20
B.2. Version 01 . . . . . . . . . . . . . . . . . . . . . . . . 20
B.3. Version 02 . . . . . . . . . . . . . . . . . . . . . . . . 20
B.4. Version 03 . . . . . . . . . . . . . . . . . . . . . . . . 21
B.5. Version 04 . . . . . . . . . . . . . . . . . . . . . . . . 21
B.6. Version 05 . . . . . . . . . . . . . . . . . . . . . . . . 21
B.7. Version 06 . . . . . . . . . . . . . . . . . . . . . . . . 21
B.8. Version 07 . . . . . . . . . . . . . . . . . . . . . . . . 22
B.9. Version 08 . . . . . . . . . . . . . . . . . . . . . . . . 22
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1. Introduction
1.1. Problem statement
Traditional (legacy) mail systems, which are defined by [RFC5322] and
other specifications, allow only ASCII characters in mail header
field values. The SMTPUTF8 extension ([RFC6530], [RFC6531] and
[RFC6532]) allow raw UTF-8 in those mail header fields.
If a header field contains non-ASCII strings, POP/IMAP servers cannot
deliver Internationalized Email Headers to legacy clients which does
not send UTF8 command or UTF8 capability, and because they have no
obvious or standardized way to explain what is going on to those
clients, cannot even safely discard the message.
1.2. Possible solutions
There are four plausible approaches to the problem, with the
preferred one depending on the particular circumstances and
relationship among the delivery SMTP server, the mail store, the POP
or IMAP server, and the users and their MUA clients:
1. If the delivery MTA has sufficient knowledge about the POP and/or
IMAP servers and clients being used, the message may be rejected
as undeliverable.
2. The message may be downgraded by the POP or IMAP server, in a way
that preserves maximum information at the expense of some
complexity, and does not create security or operational problems
in the mail system.
3. Some intermediate downgrading may be applied that balances more
information loss against lower complexity and greater ease of
implementation.
4. The POP or IMAP server may fabricate a message whose intent is to
notify the client that an internationalized message is waiting
but cannot be delivered until an upgraded client is available.
1.3. Approach taken in this specification
This specification describes the second of those options. It is
worth noticing that, at least in the general case, none of these
options preserve sufficient information to guarantee that it is
possible to reply to an incoming message without loss of information,
so the choice may be considered to be among "least bad" options.
While this document specifies a well designed mechanism, it is only
an interim solution while clients are being upgraded
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[I-D.ietf-eai-rfc5721bis] [I-D.ietf-eai-5738bis].
This message downgrading mechanism converts mail header fields to an
all-ASCII representation. The POP/IMAP servers can use the
downgrading mechanism and deliver the Internationalized Email message
as a traditional form. Receivers can know they received some
internationalized messages or some unknown or broken messages.
[RFC6532] allows UTF-8 characters to be used in mail header fields
and MIME header fields. [RFC6531] allows UTF-8 characters to be used
in some trace header fields. The message downgrading mechanism
specified here describes the conversion method from the
internationalized messages that are defined in [RFC6530], and
[RFC6532] to the traditional email messages defined in [RFC5322].
This document provides a precise definition of the minimum-
information-loss message downgrading process.
Downgrading consists of the following three parts:
o New header field definitions
o Email header field downgrading
o MIME header field downgrading
Email header field downgrading is described in Section 3. It
generates ASCII-only header fields.
In Section 3.1.10 of this document, header fields starting with
"Downgraded-" are introduced. They preserve the information that
appeared in the original header fields.
The definition of MIME header fields in Internationalized Email
Messages is described in [RFC6532]. MIME header field downgrading is
described in Section 4.1. 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 extensions as defined for
POP3 [UTF8 command] and IMAP ["ENABLE UTF8=ACCEPT" command] does not
know internationalized message format described in [RFC6532].
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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
Overview and Framework for Internationalized Email [RFC6530], in the
mail message specifications [RFC5322], or in the MIME documents
[RFC2045] [RFC2047] [RFC2183] [RFC2231]. The terms "U-label",
"A-label" and "IDNA" are used with the definitions from [RFC5890].
The terms "ASCII address", "non-ASCII address", "SMTPUTF8",
"message", "internationalized message" are used with the definitions
from [RFC6530]. The term "non-ASCII string" is used with the
definitions from [RFC6532].
3. Email Header Fields Downgrading
This section defines the conversion method to ASCII for each header
field that may contain non-ASCII strings. Section 3.1 describes
rewriting methods for each ABNF element. Section 3.2 describes
rewriting methods for each header field.
3.1. 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 strings
remain in the header field.
[RFC5322] describes ABNF elements <group>, <mailbox>, <unstructured>,
<word>, <comment>, <display-name>. [RFC2045] describes ABNF element
<value>. <domain> is updated to allow non-ASCII characters in Section
3.3 of [RFC6531] and Section 3.2 of [RFC6532].
3.1.1. UNSTRUCTURED Downgrading
If the header field has an <unstructured> field that contains non-
ASCII strings, apply [RFC2047] encoding with charset UTF-8.
3.1.2. WORD Downgrading
If the header field has any <word> fields that contain non-ASCII
strings, apply [RFC2047] encoding with charset UTF-8.
3.1.3. COMMENT Downgrading
If the header field has any <comment> fields that contain non-ASCII
strings, apply [RFC2047] encoding with charset UTF-8.
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3.1.4. MIME-VALUE Downgrading
If the header field has any <value> elements defined by [RFC2045] and
the elements contain non-ASCII strings, 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.
3.1.5. DISPLAY-NAME Downgrading
If the header field has any <address> (<mailbox> or <group>) elements
and they have <display-name> elements that contain non-ASCII strings,
encode the <display-name> elements according to [RFC2047] with
charset UTF-8. DISPLAY-NAME downgrading is the same algorithm as
WORD downgrading.
3.1.6. DOMAIN Downgrading
If the header field has any <domain> elements that contain U-labels,
rewrite the non-ASCII domain name into ASCII domain name using
A-labels as specified in IDNA [RFC5891].
3.1.7. GROUP Downgrading
<group> is defined in Section 3.4 of [RFC5322]. The <group> elements
may contain <mailbox>es which contain non-ASCII addresses.
If a <group> element contains <mailbox> elements and one of
<mailbox>es contains a non-ASCII <local-part>, rewrite the <group>
element as
display-name " " ENCODED_WORD " :;"
where the <ENCODED_WORD> is the original <group-list> encoded
according to [RFC2047].
Otherwise, the <group> element does not contain non-ASCII <local-
part>. If the <group> element contain non-ASCII <mailbox>es, they
contains non-ASCII domain names. Rewrite the non-ASCII domain names
into ASCII domain names using A-labels as specified in IDNA
[RFC5891]. Generated <mailbox>es contain ASCII addresses only.
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3.1.8. MAILBOX Downgrading
If the <local-part> of the <mailbox> element does not contain non-
ASCII characters, the <domain> element contains non-ASCII characters.
Rewrite the non-ASCII domain name into ASCII domain name using
A-labels as specified in IDNA [RFC5891].
Otherwise, the <local-part> contains non-ASCII characters. The non-
ASCII <local-part> has no equivalent format for ASCII addresses. The
<addr-spec> element that contains non-ASCII strings may appear in two
forms as:
"<" addr-spec ">"
addr-spec
Rewrite both as:
ENCODED-WORD " :;"
where the <ENCODED-WORD> is the original <addr-spec> encoded
according to [RFC2047].
3.1.9. TYPED-ADDRESS Downgrading
If the header field contains <utf-8-type-addr> and the <utf-8-type-
addr> contains raw non-ASCII strings, it is in utf-8-address form.
Convert it to utf-8-addr-xtext form. Those forms are described in
[RFC6533]. COMMENT downgrading is also performed in this case. If
the address type is unrecognized and the header field contains non-
ASCII strings, then fall back to using ENCAPSULATION on the entire
header field specified in Section 3.1.10.
3.1.10. ENCAPSULATION: A Last Resort
As a last resort when header fields cannot be converted as discussed
in the previous section, the fields are deleted and replaced by
specialized new header fields. Those fields are defined to preserve,
in encoded form, as much information as possible from the header
field values of the incoming message. The syntax of these new header
fields is:
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fields =/ downgraded
downgraded = "Downgraded-Message-Id:" unstructured CRLF /
"Downgraded-Resent-Message-Id:" unstructured CRLF /
"Downgraded-In-Reply-To:" unstructured CRLF /
"Downgraded-References:" unstructured CRLF /
"Downgraded-Original-Recipient:" unstructured CRLF /
"Downgraded-Final-Recipient:" unstructured CRLF
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.
To preserve a header field in a "Downgraded-" header field:
1. Generate a new header field.
* The field name is a concatenation of "Downgraded-" and the
original field name.
* The initial new field value is the original header field
value.
2. Treat the initial new header field value as if it were
unstructured, and then apply [RFC2047] encoding with charset
UTF-8 as necessary so that the resulting new header field value
is completely in ASCII.
3. Remove the original header field.
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3.2. Downgrading Method for Each Header Field
[RFC4021] establishes a registry of header fields. This section
describes the downgrading method for each header field.
If the whole mail header field does not contain non-ASCII strings,
email header field downgrading is not required. Each header field's
downgrading method is described below.
3.2.1. 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:
Return-Path:
Disposition-Notification-To:
If the header field contains non-ASCII characters, first perform
COMMENT downgrading and DISPLAY-NAME downgrading as described in the
corresponding subsections of Section 3.1. If the header field still
contains non-ASCII characters after that, do the following two steps:
1. If the header field contains <group> elements that contain non-
ASCII addresses, perform GROUP downgrading on those elements.
2. If the header field contains <mailbox> elements that contain non-
ASCII addresses, perform MAILBOX downgrading on those elements.
This procedure may generate empty <group> elements in "From:",
"Sender:" and "Reply-To:" header fields.
[I-D.leiba-5322upd-from-group] updates [RFC5322] to allow (empty)
<group> elements in "From:", "Sender:" and "Reply-To:" header fields.
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3.2.2. 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 strings except in
comments. If the header field contains UTF-8 characters in comments,
perform COMMENT downgrading.
3.2.3. Message-ID Header Fields
Message-ID:
Resent-Message-ID:
In-Reply-To:
References:
Perform ENCAPSULATION as specified in Section 3.1.10.
3.2.4. Received Header Field
Received:
If <domain> elements or <mailbox> elements contains U-labels, perform
DOMAIN downgrading specified in Section 3.1.6. Comments may contain
non-ASCII strings, perform COMMENT downgrading.
After the DOMAIN downgrading and the COMMENT downgrading, if the FOR
clause contains a non-ASCII <local-part>, remove the "FOR" clause.
If the ID clause contains a non-ASCII values, remove the "ID" clause.
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3.2.5. MIME Content Header Fields
Content-Type:
Content-Disposition:
Perform MIME-VALUE downgrading and COMMENT downgrading.
3.2.6. Non-ASCII in <unstructured>
Subject:
Comments:
Content-Description:
Perform UNSTRUCTURED downgrading.
3.2.7. Non-ASCII in <phrase>
Keywords:
Perform WORD downgrading.
3.2.8. Other Header Fields
There are other header fields that contain non-ASCII strings. They
are user-defined and missing from this document, or future defined
header fields. They are treated as "Optional Fields" and their field
values 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.
4. MIME Downgrading
Both MIME Body-Part header fields and contents of a delivery status
notification may contain non-ASCII characters.
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4.1. MIME Body-Part Header Field Downgrading
MIME body-part header fields may contain non-ASCII strings [RFC6532].
This section defines the conversion method to ASCII-only header
fields for each MIME header field that contains non-ASCII strings.
Parse the message body's MIME structure at all levels and check each
MIME header field to see whether it contains non-ASCII strings. If
the header field contains non-ASCII strings 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 3.
Content-ID:
The "Content-ID:" header field does not contain non-ASCII strings
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.
4.2. Delivery Status Notification downgrading
If the message contains a delivery status notification defined at
Section 6 of [RFC3461], perform the following tests and conversions.
If there are "Original-Recipient:" and "Final-Recipient:" header
fields, and the header fields contain non-ASCII strings, perform
TYPED-ADDRESS downgrading.
5. Security Considerations
The purpose of post-delivery message downgrading is to allow POP/IMAP
servers to deliver internationalized messages to traditional POP/IMAP
clients and permit the clients to display those messages. Users who
receive such messages can know that they were internationalized. It
does not permit receivers to read the messages in their original form
and, in general, will not permit generating replies, at least without
significant user intervention.
A downgraded message's header fields contain ASCII characters only.
But they still contain MIME-encapsulated header fields that contain
non-ASCII strings. Furthermore, the body part may contain UTF-8
characters. Implementations parsing Internet messages need to accept
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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, the rewritten header field
values are preserved in equivalent MIME form or in newly defined
header fields for which traditional MUAs have no special processing
procedures.
The techniques described here invalidate methods that depend on
digital signatures over any part of the message, which includes the
top-level header fields and body-part header fields. Depending on
the specific message being downgraded, at least the following
techniques are likely to break: DomainKeys Identified Mail (DKIM),
and possibly S/MIME and Pretty Good Privacy (PGP). The downgrade
mechanism SHOULD NOT remove signatures even if the signatures will
fail validation after downgrading. As much of the information as
possible from the original message SHOULD be preserved.
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.
Information in the new Downgraded-* header fields is not inspected by
traditional MUAs, 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), however traditional MUAs
do not parse Downgraded-* header fields.
In addition, if an Authentication-Results header field [RFC5451] is
present, traditional MUAs may treat that the digital signatures are
valid.
See the "Security Considerations" section in
[I-D.leiba-5322upd-from-group] and [RFC6530] for more discussion.
6. Implementation Notes
6.1. 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
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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.
7. IANA Considerations
[[RFC Editor: Please change "is asked to" to "has" (and change the
verb correspondingly) when the IESG approval and IANA actions are
complete.]]
[RFC5504] specified that no new header fields be registered that
begin with "Downgraded-". That restriction is now lifted, and this
document makes a new set of registrations, replacing the experimental
fields with standard ones.
7.1. Obsolescence of Existing Downgraded-* Header Fields
The "Downgraded-*" header fields that were registered as experimental
fields in [RFC5504] are no longer in use. IANA is asked to change
the status from "experimental" to "obsoleted" for every name in the
Permanent Message Header Field registry that begins with
"Downgraded-".
7.2. Registration of New Downgraded-* Header Fields
[[RFC Editor: Please change "should be" to "have been" when the IANA
actions are complete.]]
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 3.1.10)
Header field name: Downgraded-In-Reply-To
Applicable protocol: mail
Status: standard
Author/change controller: IETF
Specification document(s): This document (Section 3.1.10)
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Header field name: Downgraded-References
Applicable protocol: mail
Status: standard
Author/change controller: IETF
Specification document(s): This document (Section 3.1.10)
Header field name: Downgraded-Original-Recipient
Applicable protocol: mail
Status: standard
Author/change controller: IETF
Specification document(s): This document (Section 3.1.10)
Header field name: Downgraded-Final-Recipient
Applicable protocol: mail
Status: standard
Author/change controller: IETF
Specification document(s): This document (Section 3.1.10)
8. 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. Significant comments and suggestions
were received from John Klensin, Barry Leiba, Randall Gellens, Pete
Resnick, Martin J. Durst, and other WG participants.
9. References
9.1. Normative References
[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
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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.
[RFC3461] Moore, K., "Simple Mail Transfer
Protocol (SMTP) Service Extension for
Delivery Status Notifications
(DSNs)", RFC 3461, January 2003.
[RFC3629] Yergeau, F., "UTF-8, a transformation
format of ISO 10646", STD 63,
RFC 3629, November 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.
[RFC5322] Resnick, P., Ed., "Internet Message
Format", RFC 5322, October 2008.
[RFC5890] Klensin, J., "Internationalized
Domain Names for Applications (IDNA):
Definitions and Document Framework",
RFC 5890, August 2010.
[RFC5891] Klensin, J., "Internationalized
Domain Names in Applications (IDNA):
Protocol", RFC 5891, August 2010.
[RFC6530] Klensin, J. and Y. Ko, "Overview and
Framework for Internationalized
Email", RFC 6530, February 2012.
[RFC6531] Yao, J. and W. Mao, "SMTP Extension
for Internationalized Email",
RFC 6531, February 2012.
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[RFC6532] Yang, A., Steele, S., and N. Freed,
"Internationalized Email Headers",
RFC 6532, February 2012.
[RFC6533] Hansen, T., Newman, C., and A.
Melnikov, "Internationalized Delivery
Status and Disposition
Notifications", RFC 6533,
February 2012.
[I-D.leiba-5322upd-from-group] Leiba, B., "Update to Internet
Message Format to Allow Group Syntax
in the "From:" and "Sender:" Header
Fields",
draft-leiba-5322upd-from-group-06
(work in progress), October 2012.
[I-D.ietf-eai-rfc5721bis] Gellens, R., Newman, C., Yao, J., and
K. Fujiwara, "POP3 Support for
UTF-8", draft-ietf-eai-rfc5721bis-08
(work in progress), October 2012.
[I-D.ietf-eai-5738bis] Resnick, P., Newman, C., and S. Shen,
"IMAP Support for UTF-8",
draft-ietf-eai-5738bis-09 (work in
progress), August 2012.
9.2. Informative References
[RFC5451] Kucherawy, M., "Message Header Field
for Indicating Message Authentication
Status", RFC 5451, April 2009.
[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:
o The sender address is a non-ASCII address,
"NON-ASCII-LOCAL@example.com". Its display-name is "DISPLAY-
LOCAL".
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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 strings.
o The "Message-Id:" header field contains "NON-ASCII-MESSAGE_ID",
which contains non-ASCII strings.
o There is an unknown header field "X-Unknown-Header" which contains
non-ASCII strings.
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: Mon, 30 Jul 2012 01:23:45 -0000
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
"X-Unknown-Header:" header fields are encoded using [RFC2047].
"Message-Id:" header field is encapsulated as
"Downgraded-Message-Id:" header field.
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Return-Path: =?UTF-8?Q?NON-ASCII-LOCAL@example.com?= :;
Received: from ... by ...
Received: from ... by ...
From: =?UTF-8?Q?DISPLAY-LOCAL?=
=?UTF-8?Q?NON-ASCII-LOCAL@example.com?= :;
To: =?UTF-8?Q?DISPLAY-REMOTE1?=
=?UTF-8?Q?NON-ASCII-REMOTE1@example.net?= :;,
=?UTF-8?Q?DISPLAY-REMOTE2?=
=?UTF-8?Q?NON-ASCII-REMOTE2@example.com?= :;,
Cc: =?UTF-8?Q?DISPLAY-REMOTE3?=
=?UTF-8?Q?NON-ASCII-REMOTE3@example.org?= :;
Subject: =?UTF-8?Q?NON-ASCII-SUBJECT?=
Date: Mon, 30 Jul 2012 01:23:45 -0000
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
Appendix B. Change History
[[RFC Editor: Please remove this section prior to publication.]]
This section is used for tracking the update of this document. Will
be removed after finalize.
B.1. Version 00
o Initial version
o Imported header field downgrading from RFC 5504
B.2. Version 01
o same as Version 00
B.3. Version 02
o Added updating RFC 5322 to allow <group> syntax in From: and
Sender
o Added GROUP Downgrading
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B.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
B.5. Version 04
o Removed "Internationalized Address removed" from GROUP and MAILBOX
downgrading
o Updated "Updating RFC 5322"
o Compacted new header field definition
o Compacted security considerations
o Updated IANA considerations to remove obsoleting header fields
that are registered by RFC 5504
o Added a discussion of alternate downgrading models for the POP and
IMAP cases.
o Incorporated a large number of editorial changes to improve
clarity.
B.6. Version 05
o Some text corrections
o Terminology change: only to use non-ASCII address, non-ASCII
message, non-ASCII string and imported them from RFC 6530 and RFC
6532
o Replace "non-ASCII character" with "non-ASCII string"
o Removed 5.1.1. RECEIVED Downgrading
B.7. Version 06
o Removed "Updating RFC 5322"
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o Added reference to draft-leiba-5322upd-from-group
B.8. Version 07
o Updated by WGLC comments
o Fixed Received downgrading and added to refer "RFC 6531", "RFC
5890", "RFC 5891"
o Added Domain downgrading for Received, Group and Mailbox
o Swapped section 3 and 4
B.9. Version 08
o Updated by IETF Last call and IESG comments
o Removed "Address Header Fields with Typed Addresses" and added
"Delivery Status Notification downgrading" in MIME downgrading
o Added a space between display-name and ENCODED_WORD.
o Moved "ENCAPSULATION: A Last Resort" from section 4 to section
3.1.10.
o Updated address header fields downgrading
o Updated introduction, security considerations and iana
considerations
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@jprs.co.jp
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