draft-ietf-eai-popimap-downgrade-07.txt   draft-ietf-eai-popimap-downgrade-08.txt 
Email Address Internationalization K. Fujiwara Email Address Internationalization K. Fujiwara
(EAI) JPRS (EAI) JPRS
Internet-Draft July 31, 2012 Internet-Draft Oct 22, 2012
Intended status: Standards Track Intended status: Standards Track
Expires: February 1, 2013 Expires: April 25, 2013
Post-delivery Message Downgrading for Internationalized Email Messages Post-delivery Message Downgrading for Internationalized Email Messages
draft-ietf-eai-popimap-downgrade-07.txt draft-ietf-eai-popimap-downgrade-08.txt
Abstract Abstract
The Email Address Internationalization (SMTPUTF8) extension to SMTP The Email Address Internationalization (SMTPUTF8) extension to SMTP
allows UTF-8 characters in mail header fields. Upgraded POP and IMAP allows UTF-8 characters in mail header fields. Upgraded POP and IMAP
servers support internationalized Email messages. If a POP/IMAP servers support internationalized Email messages. If a POP/IMAP
client does not support Email Address Internationalization, POP/IMAP client does not support Email Address Internationalization, POP/IMAP
servers cannot deliver Internationalized Email Headers to the client servers cannot deliver Internationalized Email Headers to the client
and cannot remove the message. To avoid the situation, this document and cannot remove the message. To avoid the situation, this document
describes a conversion mechanism for internationalized Email messages describes a conversion mechanism for internationalized Email messages
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Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on February 1, 2013. This Internet-Draft will expire on April 25, 2013.
Copyright Notice Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of (http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
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include Simplified BSD License text as described in Section 4.e of include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License. described in the Simplified BSD License.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Problem statement . . . . . . . . . . . . . . . . . . . . 4 1.1. Problem statement . . . . . . . . . . . . . . . . . . . . 4
1.2. Possible solutions . . . . . . . . . . . . . . . . . . . . 4 1.2. Possible solutions . . . . . . . . . . . . . . . . . . . . 4
1.3. Approach taken in this specification . . . . . . . . . . . 4 1.3. Approach taken in this specification . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 6
3. Email Header Fields Downgrading . . . . . . . . . . . . . . . 6 3. Email Header Fields Downgrading . . . . . . . . . . . . . . . 6
3.1. Downgrading Method for Each ABNF Element . . . . . . . . . 6 3.1. Downgrading Method for Each ABNF Element . . . . . . . . . 6
3.1.1. <UNSTRUCTURED> Downgrading . . . . . . . . . . . . . . 6 3.1.1. UNSTRUCTURED Downgrading . . . . . . . . . . . . . . . 6
3.1.2. <WORD> Downgrading . . . . . . . . . . . . . . . . . . 6 3.1.2. WORD Downgrading . . . . . . . . . . . . . . . . . . . 6
3.1.3. <COMMENT> Downgrading . . . . . . . . . . . . . . . . 6 3.1.3. COMMENT Downgrading . . . . . . . . . . . . . . . . . 6
3.1.4. <MIME-VALUE> Downgrading . . . . . . . . . . . . . . . 6 3.1.4. MIME-VALUE Downgrading . . . . . . . . . . . . . . . . 7
3.1.5. <DISPLAY-NAME> Downgrading . . . . . . . . . . . . . . 7 3.1.5. DISPLAY-NAME Downgrading . . . . . . . . . . . . . . . 7
3.1.6. <Domain> Downgrading . . . . . . . . . . . . . . . . . 7 3.1.6. DOMAIN Downgrading . . . . . . . . . . . . . . . . . . 7
3.1.7. <GROUP> Downgrading . . . . . . . . . . . . . . . . . 7 3.1.7. GROUP Downgrading . . . . . . . . . . . . . . . . . . 7
3.1.8. <MAILBOX> Downgrading . . . . . . . . . . . . . . . . 7 3.1.8. MAILBOX Downgrading . . . . . . . . . . . . . . . . . 8
3.1.9. <TYPED-ADDRESS> Downgrading . . . . . . . . . . . . . 8 3.1.9. TYPED-ADDRESS Downgrading . . . . . . . . . . . . . . 8
3.2. Downgrading Method for Each Header Field . . . . . . . . . 8 3.1.10. ENCAPSULATION: A Last Resort . . . . . . . . . . . . . 8
3.2.1. Address Header Fields That Contain <address>s . . . . 8 3.2. Downgrading Method for Each Header Field . . . . . . . . . 10
3.2.2. Address Header Fields with Typed Addresses . . . . . . 9 3.2.1. Address Header Fields That Contain <address>s . . . . 10
3.2.3. Downgrading Non-ASCII in Comments . . . . . . . . . . 9 3.2.2. Downgrading Non-ASCII in Comments . . . . . . . . . . 11
3.2.4. Message-ID Header Fields . . . . . . . . . . . . . . . 10 3.2.3. Message-ID Header Fields . . . . . . . . . . . . . . . 11
3.2.5. Received Header Field . . . . . . . . . . . . . . . . 10 3.2.4. Received Header Field . . . . . . . . . . . . . . . . 11
3.2.6. MIME Content Header Fields . . . . . . . . . . . . . . 10 3.2.5. MIME Content Header Fields . . . . . . . . . . . . . . 12
3.2.7. Non-ASCII in <unstructured> . . . . . . . . . . . . . 10 3.2.6. Non-ASCII in <unstructured> . . . . . . . . . . . . . 12
3.2.8. Non-ASCII in <phrase> . . . . . . . . . . . . . . . . 10 3.2.7. Non-ASCII in <phrase> . . . . . . . . . . . . . . . . 12
3.2.9. List- Header Fields and Other Header Fields . . . . . 11 3.2.8. Other Header Fields . . . . . . . . . . . . . . . . . 12
4. ENCAPSULATION: A Last Resort . . . . . . . . . . . . . . . . . 11 4. MIME Downgrading . . . . . . . . . . . . . . . . . . . . . . . 12
5. MIME Body-Part Header Field Downgrading . . . . . . . . . . . 12 4.1. MIME Body-Part Header Field Downgrading . . . . . . . . . 13
6. Security Considerations . . . . . . . . . . . . . . . . . . . 12 4.2. Delivery Status Notification downgrading . . . . . . . . . 13
7. Implementation Notes . . . . . . . . . . . . . . . . . . . . . 13 5. Security Considerations . . . . . . . . . . . . . . . . . . . 13
7.1. RFC 2047 Encoding . . . . . . . . . . . . . . . . . . . . 13 6. Implementation Notes . . . . . . . . . . . . . . . . . . . . . 14
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14 6.1. RFC 2047 Encoding . . . . . . . . . . . . . . . . . . . . 14
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 15 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 15 7.1. Obsolescence of Existing Downgraded-* Header Fields . . . 15
10.1. Normative References . . . . . . . . . . . . . . . . . . . 15 7.2. Registration of New Downgraded-* Header Fields . . . . . . 15
10.2. Informative References . . . . . . . . . . . . . . . . . . 17 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 16
Appendix A. Examples . . . . . . . . . . . . . . . . . . . . . . 17 9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 16
A.1. Downgrading Example . . . . . . . . . . . . . . . . . . . 17 9.1. Normative References . . . . . . . . . . . . . . . . . . . 16
Appendix B. Change History . . . . . . . . . . . . . . . . . . . 19 9.2. Informative References . . . . . . . . . . . . . . . . . . 18
B.1. Version 00 . . . . . . . . . . . . . . . . . . . . . . . . 19 Appendix A. Examples . . . . . . . . . . . . . . . . . . . . . . 18
B.2. Version 01 . . . . . . . . . . . . . . . . . . . . . . . . 19 A.1. Downgrading Example . . . . . . . . . . . . . . . . . . . 18
B.3. Version 02 . . . . . . . . . . . . . . . . . . . . . . . . 19 Appendix B. Change History . . . . . . . . . . . . . . . . . . . 20
B.4. Version 03 . . . . . . . . . . . . . . . . . . . . . . . . 19 B.1. Version 00 . . . . . . . . . . . . . . . . . . . . . . . . 20
B.5. Version 04 . . . . . . . . . . . . . . . . . . . . . . . . 19 B.2. Version 01 . . . . . . . . . . . . . . . . . . . . . . . . 20
B.6. Version 05 . . . . . . . . . . . . . . . . . . . . . . . . 20 B.3. Version 02 . . . . . . . . . . . . . . . . . . . . . . . . 20
B.7. Version 06 . . . . . . . . . . . . . . . . . . . . . . . . 20 B.4. Version 03 . . . . . . . . . . . . . . . . . . . . . . . . 21
B.8. Version 07 . . . . . . . . . . . . . . . . . . . . . . . . 20 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
1. Introduction 1. Introduction
1.1. Problem statement 1.1. Problem statement
Traditional (legacy) mail systems, which are defined by [RFC5322] and Traditional (legacy) mail systems, which are defined by [RFC5322] and
other specifications, allow only ASCII characters in mail header other specifications, allow only ASCII characters in mail header
field values. The SMTPUTF8 extension ([RFC6530], [RFC6531] and field values. The SMTPUTF8 extension ([RFC6530], [RFC6531] and
[RFC6532]) allow raw UTF-8 in those mail header fields. [RFC6532]) allow raw UTF-8 in those mail header fields.
If a header field contains non-ASCII strings, POP/IMAP servers cannot If a header field contains non-ASCII strings, POP/IMAP servers cannot
deliver Internationalized Email Headers to legacy clients and, deliver Internationalized Email Headers to legacy clients which does
because they have no obvious or standardized way to explain what is not send UTF8 command or UTF8 capability, and because they have no
going on to those clients, cannot even safely discard the message. obvious or standardized way to explain what is going on to those
clients, cannot even safely discard the message.
1.2. Possible solutions 1.2. Possible solutions
There are four plausible approaches to the problem, with the There are four plausible approaches to the problem, with the
preferred one depending on the particular circumstances and preferred one depending on the particular circumstances and
relationship among the delivery SMTP server, the mail store, the POP relationship among the delivery SMTP server, the mail store, the POP
or IMAP server, and the users and their MUA clients: or IMAP server, and the users and their MUA clients:
1. If the delivery MTA has sufficient knowledge about the POP and/or 1. If the delivery MTA has sufficient knowledge about the POP and/or
IMAP servers and clients being used, the message may be rejected IMAP servers and clients being used, the message may be rejected
as undeliverable. as undeliverable.
2. The message may be downgraded by the POP or IMAP server, in a way 2. The message may be downgraded by the POP or IMAP server, in a way
that preserves maximum information at the expense of some that preserves maximum information at the expense of some
complexity. complexity, and does not create security or operational problems
in the mail system.
3. Some intermediate downgrading may be applied that balances more 3. Some intermediate downgrading may be applied that balances more
information loss against lower complexity and greater ease of information loss against lower complexity and greater ease of
implementation. implementation.
4. The POP or IMAP server may fabricate a message whose intent is to 4. The POP or IMAP server may fabricate a message whose intent is to
notify the client that an internationalized message is waiting notify the client that an internationalized message is waiting
but cannot be delivered until an upgraded client is available. but cannot be delivered until an upgraded client is available.
1.3. Approach taken in this specification 1.3. Approach taken in this specification
This specification describes the second of those options. It is This specification describes the second of those options. It is
worth noticing that, at least in the general case, none of these worth noticing that, at least in the general case, none of these
options preserve sufficient information to guarantee that it is options preserve sufficient information to guarantee that it is
possible to reply to an incoming message without loss of information, possible to reply to an incoming message without loss of information,
so the choice may be considered to be among "least bad" options. 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
[I-D.ietf-eai-rfc5721bis] [I-D.ietf-eai-5738bis].
This message downgrading mechanism converts mail header fields to an This message downgrading mechanism converts mail header fields to an
all-ASCII representation. The POP/IMAP servers can use the all-ASCII representation. The POP/IMAP servers can use the
downgrading mechanism and deliver the Internationalized Email message downgrading mechanism and deliver the Internationalized Email message
as a traditional form. Receivers can know they received some as a traditional form. Receivers can know they received some
internationalized messages or some unknown/broken messages. internationalized messages or some unknown or broken messages.
[RFC6532] allows UTF-8 characters to be used in mail header fields [RFC6532] allows UTF-8 characters to be used in mail header fields
and MIME header fields. [RFC6531] allows UTF-8 characters to be used and MIME header fields. [RFC6531] allows UTF-8 characters to be used
in some trace header fields. The message downgrading mechanism in some trace header fields. The message downgrading mechanism
specified here describes the conversion method from the specified here describes the conversion method from the
internationalized messages that are defined in [RFC6530], and internationalized messages that are defined in [RFC6530], and
[RFC6532] to the traditional email messages defined in [RFC5322]. [RFC6532] to the traditional email messages defined in [RFC5322].
This document provides a precise definition of the minimum- This document provides a precise definition of the minimum-
information-loss message downgrading process. information-loss message downgrading process.
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o New header field definitions o New header field definitions
o Email header field downgrading o Email header field downgrading
o MIME header field downgrading o MIME header field downgrading
Email header field downgrading is described in Section 3. It Email header field downgrading is described in Section 3. It
generates ASCII-only header fields. generates ASCII-only header fields.
In Section 4 of this document, header fields starting with In Section 3.1.10 of this document, header fields starting with
"Downgraded-" are introduced. They preserve the information that "Downgraded-" are introduced. They preserve the information that
appeared in the original header fields. appeared in the original header fields.
The definition of MIME header fields in Internationalized Email The definition of MIME header fields in Internationalized Email
Messages is described in [RFC6532]. MIME header field downgrading is Messages is described in [RFC6532]. MIME header field downgrading is
described in Section 5. It generates ASCII-only MIME header fields. described in Section 4.1. It generates ASCII-only MIME header
fields.
Displaying downgraded messages that originally contained Displaying downgraded messages that originally contained
internationalized header fields is out of scope of this document. A internationalized header fields is out of scope of this document. A
POP/IMAP client which does not support UTF8 extensions as defined for POP/IMAP client which does not support UTF8 extensions as defined for
POP3 [UTF8 command] and IMAP ["ENABLE UTF8=ACCEPT" command] does not POP3 [UTF8 command] and IMAP ["ENABLE UTF8=ACCEPT" command] does not
know internationalized message format described in [RFC6532]. know internationalized message format described in [RFC6532].
2. Terminology 2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
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rewriting methods for each header field. rewriting methods for each header field.
3.1. Downgrading Method for Each ABNF Element 3.1. Downgrading Method for Each ABNF Element
Header field downgrading is defined below for each ABNF element. Header field downgrading is defined below for each ABNF element.
Converting the header field terminates when no non-ASCII strings Converting the header field terminates when no non-ASCII strings
remain in the header field. remain in the header field.
[RFC5322] describes ABNF elements <group>, <mailbox>, <unstructured>, [RFC5322] describes ABNF elements <group>, <mailbox>, <unstructured>,
<word>, <comment>, <display-name>. [RFC2045] describes ABNF element <word>, <comment>, <display-name>. [RFC2045] describes ABNF element
<value>. <Domain> is updated to allow non-ASCII characters in Section <value>. <domain> is updated to allow non-ASCII characters in Section
3.3 of [RFC6531] and Section 3.2 of [RFC6532]. 3.3 of [RFC6531] and Section 3.2 of [RFC6532].
3.1.1. <UNSTRUCTURED> Downgrading 3.1.1. UNSTRUCTURED Downgrading
If the header field has an <unstructured> field that contains non- If the header field has an <unstructured> field that contains non-
ASCII strings, apply [RFC2047] encoding with charset UTF-8. ASCII strings, apply [RFC2047] encoding with charset UTF-8.
3.1.2. <WORD> Downgrading 3.1.2. WORD Downgrading
If the header field has any <word> fields that contain non-ASCII If the header field has any <word> fields that contain non-ASCII
strings, apply [RFC2047] encoding with charset UTF-8. strings, apply [RFC2047] encoding with charset UTF-8.
3.1.3. <COMMENT> Downgrading 3.1.3. COMMENT Downgrading
If the header field has any <comment> fields that contain non-ASCII If the header field has any <comment> fields that contain non-ASCII
strings, apply [RFC2047] encoding with charset UTF-8. strings, apply [RFC2047] encoding with charset UTF-8.
3.1.4. <MIME-VALUE> Downgrading 3.1.4. MIME-VALUE Downgrading
If the header field has any <value> elements defined by [RFC2045] and If the header field has any <value> elements defined by [RFC2045] and
the elements contain non-ASCII strings, encode the <value> elements the elements contain non-ASCII strings, encode the <value> elements
according to [RFC2231] with charset UTF-8 and leave the language according to [RFC2231] with charset UTF-8 and leave the language
information empty. If the <value> element is <quoted-string> and it information empty. If the <value> element is <quoted-string> and it
contains <CFWS> outside the DQUOTE, remove the <CFWS> before this contains <CFWS> outside the DQUOTE, remove the <CFWS> before this
conversion. conversion.
3.1.5. <DISPLAY-NAME> Downgrading 3.1.5. DISPLAY-NAME Downgrading
If the header field has any <address> (<mailbox> or <group>) elements If the header field has any <address> (<mailbox> or <group>) elements
and they have <display-name> elements that contain non-ASCII strings, and they have <display-name> elements that contain non-ASCII strings,
encode the <display-name> elements according to [RFC2047] with encode the <display-name> elements according to [RFC2047] with
charset UTF-8. DISPLAY-NAME downgrading is the same algorithm as charset UTF-8. DISPLAY-NAME downgrading is the same algorithm as
WORD downgrading. WORD downgrading.
3.1.6. <Domain> Downgrading 3.1.6. DOMAIN Downgrading
If the header field has any <Domain> elements that contain U-labels, If the header field has any <domain> elements that contain U-labels,
rewrite the non-ASCII domain name into ASCII domain name using rewrite the non-ASCII domain name into ASCII domain name using
A-labels as specified in IDNA [RFC5891]. A-labels as specified in IDNA [RFC5891].
3.1.7. <GROUP> Downgrading 3.1.7. GROUP Downgrading
<group> is defined in Section 3.4 of [RFC5322]. The <group> elements <group> is defined in Section 3.4 of [RFC5322]. The <group> elements
may contain <mailbox>es which contain non-ASCII addresses. may contain <mailbox>es which contain non-ASCII addresses.
If the header field has any <group> elements that contain <mailbox> If a <group> element contains <mailbox> elements and one of
elements and one of <mailbox>es contains a non-ASCII <local-part>, <mailbox>es contains a non-ASCII <local-part>, rewrite the <group>
rewrite each <group> element as element as
display-name ENCODED_WORD " :;" display-name " " ENCODED_WORD " :;"
where the <ENCODED_WORD> is the original <group-list> encoded where the <ENCODED_WORD> is the original <group-list> encoded
according to [RFC2047]. according to [RFC2047].
Otherwise, the header field does not contain non-ASCII <local-part>. Otherwise, the <group> element does not contain non-ASCII <local-
If the header field contain non-ASCII <mailbox>es, they contain non- part>. If the <group> element contain non-ASCII <mailbox>es, they
ASCII domain names. Rewrite the non-ASCII domain names into ASCII contains non-ASCII domain names. Rewrite the non-ASCII domain names
domain names using A-labels as specified in IDNA [RFC5891]. into ASCII domain names using A-labels as specified in IDNA
Generated <mailbox>es contain ASCII addresses only. [RFC5891]. Generated <mailbox>es contain ASCII addresses only.
3.1.8. <MAILBOX> Downgrading 3.1.8. MAILBOX Downgrading
If the <local-part> of the <mailbox> element does not contain non- If the <local-part> of the <mailbox> element does not contain non-
ASCII characters, the <domain> element contains non-ASCII characters. ASCII characters, the <domain> element contains non-ASCII characters.
Rewrite the non-ASCII domain name into ASCII domain name using Rewrite the non-ASCII domain name into ASCII domain name using
A-labels as specified in IDNA [RFC5891]. A-labels as specified in IDNA [RFC5891].
Otherwise, the <local-part> contains non-ASCII characters. The non- Otherwise, the <local-part> contains non-ASCII characters. The non-
ASCII <local-part> has no equivalent format for ASCII addresses. ASCII <local-part> has no equivalent format for ASCII addresses. The
Rewrite each <addr-spec> element to ASCII-only group format following <addr-spec> element that contains non-ASCII strings may appear in two
the model above. The <addr-spec> element that contains non-ASCII forms as:
strings may appear in two forms as:
"<" addr-spec ">" "<" addr-spec ">"
addr-spec addr-spec
Rewrite both as: Rewrite both as:
ENCODED-WORD " :;" ENCODED-WORD " :;"
where the <ENCODED-WORD> is the original <addr-spec> encoded where the <ENCODED-WORD> is the original <addr-spec> encoded
according to [RFC2047]. according to [RFC2047].
3.1.9. <TYPED-ADDRESS> Downgrading 3.1.9. TYPED-ADDRESS Downgrading
If the header field contains <utf-8-type-addr> and the <utf-8-type- 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. 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 Convert it to utf-8-addr-xtext form. Those forms are described in
[RFC6533]. COMMENT downgrading is also performed in this case. If [RFC6533]. COMMENT downgrading is also performed in this case. If
the address type is unrecognized and the header field contains non- the address type is unrecognized and the header field contains non-
ASCII strings, then fall back to using ENCAPSULATION on the entire ASCII strings, then fall back to using ENCAPSULATION on the entire
header field specified in Section 4. 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:
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.
3.2. Downgrading Method for Each Header Field 3.2. Downgrading Method for Each Header Field
[RFC4021] establishes a registry of header fields. This section [RFC4021] establishes a registry of header fields. This section
describes the downgrading method for each header field. describes the downgrading method for each header field.
If the whole mail header field does not contain non-ASCII strings, If the whole mail header field does not contain non-ASCII strings,
email header field downgrading is not required. Each header field's email header field downgrading is not required. Each header field's
downgrading method is described below. downgrading method is described below.
skipping to change at page 9, line 4 skipping to change at page 10, line 25
From: From:
Sender: Sender:
To: To:
Cc: Cc:
Bcc: Bcc:
Reply-To: Reply-To:
Resent-From: Resent-From:
Resent-Sender: Resent-Sender:
Resent-To: Resent-To:
Resent-Cc: Resent-Cc:
Resent-Bcc: Resent-Bcc:
Resent-Reply-To: Resent-Reply-To:
Return-Path: Return-Path:
Disposition-Notification-To: Disposition-Notification-To:
If the header field contains <group> elements that contain non-ASCII If the header field contains non-ASCII characters, first perform
addresses, perform <COMMENT> downgrading, <DISPLAY-NAME> downgrading, COMMENT downgrading and DISPLAY-NAME downgrading as described in the
and <GROUP> downgrading as described in the corresponding subsections corresponding subsections of Section 3.1. If the header field still
of Section 3.1. Optionally add those words where appropriate to the contains non-ASCII characters after that, do the following two steps:
next paragraph, but I think once will make it clear.
If the header field contains <mailbox> elements that contain non- 1. If the header field contains <group> elements that contain non-
ASCII addresses, perform <COMMENT> downgrading, <DISPLAY-NAME> ASCII addresses, perform GROUP downgrading on those elements.
downgrading, and <MAILBOX> downgrading.
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:", This procedure may generate empty <group> elements in "From:",
"Sender:" and "Reply-To:" header fields. "Sender:" and "Reply-To:" header fields.
[I-D.leiba-5322upd-from-group] updates [RFC5322] to allow (empty) [I-D.leiba-5322upd-from-group] updates [RFC5322] to allow (empty)
<group> elements in "From:", "Sender:" and "Reply-To:" header fields. <group> elements in "From:", "Sender:" and "Reply-To:" header fields.
3.2.2. Address Header Fields with Typed Addresses 3.2.2. Downgrading Non-ASCII in Comments
Original-Recipient:
Final-Recipient:
If the header field contains non-ASCII strings, perform <TYPED-
ADDRESS> downgrading.
3.2.3. Downgrading Non-ASCII in Comments
Date: Date:
Resent-Date: Resent-Date:
MIME-Version: MIME-Version:
Content-ID: Content-ID:
Content-Transfer-Encoding: Content-Transfer-Encoding:
Content-Language: Content-Language:
Accept-Language: Accept-Language:
Auto-Submitted: Auto-Submitted:
These header fields do not contain non-ASCII strings except in These header fields do not contain non-ASCII strings except in
comments. If the header field contains UTF-8 characters in comments, comments. If the header field contains UTF-8 characters in comments,
perform <COMMENT> downgrading. perform COMMENT downgrading.
3.2.4. Message-ID Header Fields 3.2.3. Message-ID Header Fields
Message-ID: Message-ID:
Resent-Message-ID: Resent-Message-ID:
In-Reply-To: In-Reply-To:
References: References:
Perform ENCAPSULATION as specified in Section 4. Perform ENCAPSULATION as specified in Section 3.1.10.
3.2.5. Received Header Field 3.2.4. Received Header Field
Received: Received:
If <Domain> elements or <Mailbox> elements contains U-labels, perform If <domain> elements or <mailbox> elements contains U-labels, perform
<Domain> downgrading specified in Section 3.1.6. Comments may DOMAIN downgrading specified in Section 3.1.6. Comments may contain
contain non-ASCII strings, perform <COMMENT> downgrading. non-ASCII strings, perform COMMENT downgrading.
After the <Domain> downgrading and the <COMMENT> downgrading, if the After the DOMAIN downgrading and the COMMENT downgrading, if the FOR
FOR clause contains a non-ASCII <local-part>, remove the "FOR" clause contains a non-ASCII <local-part>, remove the "FOR" clause.
clause. If the ID clause contains a non-ASCII values, remove the If the ID clause contains a non-ASCII values, remove the "ID" clause.
"ID" clause.
3.2.6. MIME Content Header Fields 3.2.5. MIME Content Header Fields
Content-Type: Content-Type:
Content-Disposition: Content-Disposition:
Perform <MIME-VALUE> downgrading and <COMMENT> downgrading. Perform MIME-VALUE downgrading and COMMENT downgrading.
3.2.7. Non-ASCII in <unstructured> 3.2.6. Non-ASCII in <unstructured>
Subject: Subject:
Comments: Comments:
Content-Description: Content-Description:
Perform <UNSTRUCTURED> downgrading. Perform UNSTRUCTURED downgrading.
3.2.8. Non-ASCII in <phrase> 3.2.7. Non-ASCII in <phrase>
Keywords: Keywords:
Perform <WORD> downgrading. Perform WORD downgrading.
3.2.9. List- Header Fields and Other Header Fields 3.2.8. Other Header Fields
There are other header fields that contain non-ASCII strings. They There are other header fields that contain non-ASCII strings. They
are user-defined and missing from this document, or future defined are user-defined and missing from this document, or future defined
header fields. They are treated as "Optional Fields" and their field header fields. They are treated as "Optional Fields" and their field
values are treated as unstructured described in Section 3.6.8 of values are treated as unstructured described in Section 3.6.8 of
[RFC5322]. [RFC5322].
Perform <UNSTRUCTURED> downgrading. Perform UNSTRUCTURED downgrading.
If the software understands the header field's structure and a If the software understands the header field's structure and a
downgrading algorithm other than UNSTRUCTURED is applicable, that downgrading algorithm other than UNSTRUCTURED is applicable, that
software SHOULD use that algorithm; UNSTRUCTURED downgrading is used software SHOULD use that algorithm; UNSTRUCTURED downgrading is used
as a last resort. as a last resort.
Mailing list header fields (those that start in "List-") are part of Mailing list header fields (those that start in "List-") are part of
this category. this category.
4. ENCAPSULATION: A Last Resort 4. MIME Downgrading
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:
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. Both MIME Body-Part header fields and contents of a delivery status
notification may contain non-ASCII characters.
5. MIME Body-Part Header Field Downgrading 4.1. MIME Body-Part Header Field Downgrading
MIME body-part header fields may contain non-ASCII strings [RFC6532]. MIME body-part header fields may contain non-ASCII strings [RFC6532].
This section defines the conversion method to ASCII-only header This section defines the conversion method to ASCII-only header
fields for each MIME header field that contains non-ASCII strings. fields for each MIME header field that contains non-ASCII strings.
Parse the message body's MIME structure at all levels and check each 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 MIME header field to see whether it contains non-ASCII strings. If
the header field contains non-ASCII strings in the header field the header field contains non-ASCII strings in the header field
value, the header field is a target of the MIME body-part header value, the header field is a target of the MIME body-part header
field's downgrading. Each MIME header field's downgrading method is field's downgrading. Each MIME header field's downgrading method is
described below. COMMENT downgrading, MIME-VALUE downgrading, and described below. COMMENT downgrading, MIME-VALUE downgrading, and
UNSTRUCTURED downgrading are described in Section 3. UNSTRUCTURED downgrading are described in Section 3.
Content-ID: Content-ID:
The "Content-ID:" header field does not contain non-ASCII strings The "Content-ID:" header field does not contain non-ASCII strings
except in comments. If the header field contains UTF-8 characters except in comments. If the header field contains UTF-8 characters
in comments, perform <COMMENT> downgrading. in comments, perform COMMENT downgrading.
Content-Type: Content-Type:
Content-Disposition: Content-Disposition:
Perform <MIME-VALUE> downgrading and <COMMENT> downgrading. Perform MIME-VALUE downgrading and COMMENT downgrading.
Content-Description: Perform <UNSTRUCTURED> downgrading. Content-Description:
Perform UNSTRUCTURED downgrading.
6. Security Considerations 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 The purpose of post-delivery message downgrading is to allow POP/IMAP
servers to deliver internationalized messages to traditional POP/IMAP servers to deliver internationalized messages to traditional POP/IMAP
clients and permit the clients to display those messages. Users who clients and permit the clients to display those messages. Users who
receive such messages can know that they were internationalized. It receive such messages can know that they were internationalized. It
does not permit receivers to read the messages in their original form does not permit receivers to read the messages in their original form
and, in general, will not permit generating replies, at least without and, in general, will not permit generating replies, at least without
significant user intervention. significant user intervention.
A downgraded message's header fields contain ASCII characters only. A downgraded message's header fields contain ASCII characters only.
skipping to change at page 13, line 24 skipping to change at page 14, line 19
spoofing by malicious senders. However, the rewritten header field spoofing by malicious senders. However, the rewritten header field
values are preserved in equivalent MIME form or in newly defined values are preserved in equivalent MIME form or in newly defined
header fields for which traditional MUAs have no special processing header fields for which traditional MUAs have no special processing
procedures. procedures.
The techniques described here invalidate methods that depend on The techniques described here invalidate methods that depend on
digital signatures over any part of the message, which includes the digital signatures over any part of the message, which includes the
top-level header fields and body-part header fields. Depending on top-level header fields and body-part header fields. Depending on
the specific message being downgraded, at least the following the specific message being downgraded, at least the following
techniques are likely to break: DomainKeys Identified Mail (DKIM), techniques are likely to break: DomainKeys Identified Mail (DKIM),
and possibly S/MIME and Pretty Good Privacy (PGP). Receivers may and possibly S/MIME and Pretty Good Privacy (PGP). The downgrade
know they need to update their MUAs, or they don't care. 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 While information in any email header field should usually be treated
with some suspicion, current email systems commonly employ various with some suspicion, current email systems commonly employ various
mechanisms and protocols to make the information more trustworthy. mechanisms and protocols to make the information more trustworthy.
Information in the new Downgraded-* header fields is not inspected by Information in the new Downgraded-* header fields is not inspected by
traditional MUAs, and may be even less trustworthy than the traditional MUAs, and may be even less trustworthy than the
traditional header fields. Note that the Downgraded-* header fields traditional header fields. Note that the Downgraded-* header fields
could have been inserted with malicious intent (and with content could have been inserted with malicious intent (and with content
unrelated to the traditional header fields), however traditional MUAs unrelated to the traditional header fields), however traditional MUAs
do not parse Downgraded-* header fields. do not parse Downgraded-* header fields.
In addition, if an Authentication-Results header field [RFC5451] is In addition, if an Authentication-Results header field [RFC5451] is
present, traditional MUAs may treat that the digital signatures are present, traditional MUAs may treat that the digital signatures are
valid. valid.
See the "Security Considerations" section in See the "Security Considerations" section in
[I-D.leiba-5322upd-from-group] and [RFC6530] for more discussion. [I-D.leiba-5322upd-from-group] and [RFC6530] for more discussion.
7. Implementation Notes 6. Implementation Notes
7.1. RFC 2047 Encoding 6.1. RFC 2047 Encoding
While [RFC2047] has a specific algorithm to deal with whitespace in While [RFC2047] has a specific algorithm to deal with whitespace in
adjacent encoded words, there are a number of deployed adjacent encoded words, there are a number of deployed
implementations that fail to implement the algorithm correctly. As a implementations that fail to implement the algorithm correctly. As a
result, whitespace behavior is somewhat unpredictable in practice result, whitespace behavior is somewhat unpredictable in practice
when multiple encoded words are used. While RFC 5322 states that when multiple encoded words are used. While RFC 5322 states that
implementations SHOULD limit lines to not more than 78 characters, implementations SHOULD limit lines to not more than 78 characters,
implementations MAY choose to allow overly long encoded words in implementations MAY choose to allow overly long encoded words in
order to work around faulty [RFC2047] implementations. order to work around faulty [RFC2047] implementations.
Implementations that choose to do so SHOULD have an optional Implementations that choose to do so SHOULD have an optional
mechanism to limit line length to 78 characters. mechanism to limit line length to 78 characters.
8. IANA Considerations 7. IANA Considerations
[[RFC Editor: Please change "should now be" and "should be" to "have [[RFC Editor: Please change "is asked to" to "has" (and change the
been" when the IANA actions are complete.]] verb correspondingly) when the IESG approval and IANA actions are
complete.]]
[RFC5504] registered many "Downgraded-" header fields and requested [RFC5504] specified that no new header fields be registered that
that 'IANA will refuse registration of all field names that start begin with "Downgraded-". That restriction is now lifted, and this
with "Downgraded-", to avoid possible conflict with the procedure for document makes a new set of registrations, replacing the experimental
unknown header fields' preservation described in Section 3.3 of fields with standard ones.
[RFC5504].' However [RFC6530] obsoleted [RFC5504] and this document
does not use all "Downgraded-" header fields registered by [RFC5504]. 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 The following header fields should be registered in the Permanent
Message Header Field registry, in accordance with the procedures set Message Header Field registry, in accordance with the procedures set
out in [RFC3864]. out in [RFC3864].
Header field name: Downgraded-Message-Id Header field name: Downgraded-Message-Id
Applicable protocol: mail Applicable protocol: mail
Status: standard Status: standard
Author/change controller: IETF Author/change controller: IETF
Specification document(s): This document (Section 4) Specification document(s): This document (Section 3.1.10)
Header field name: Downgraded-In-Reply-To Header field name: Downgraded-In-Reply-To
Applicable protocol: mail Applicable protocol: mail
Status: standard Status: standard
Author/change controller: IETF Author/change controller: IETF
Specification document(s): This document (Section 4) Specification document(s): This document (Section 3.1.10)
Header field name: Downgraded-References Header field name: Downgraded-References
Applicable protocol: mail Applicable protocol: mail
Status: standard Status: standard
Author/change controller: IETF Author/change controller: IETF
Specification document(s): This document (Section 4) Specification document(s): This document (Section 3.1.10)
Header field name: Downgraded-Original-Recipient Header field name: Downgraded-Original-Recipient
Applicable protocol: mail Applicable protocol: mail
Status: standard Status: standard
Author/change controller: IETF Author/change controller: IETF
Specification document(s): This document (Section 4) Specification document(s): This document (Section 3.1.10)
Header field name: Downgraded-Final-Recipient Header field name: Downgraded-Final-Recipient
Applicable protocol: mail Applicable protocol: mail
Status: standard Status: standard
Author/change controller: IETF Author/change controller: IETF
Specification document(s): This document (Section 4) Specification document(s): This document (Section 3.1.10)
9. Acknowledgements 8. Acknowledgements
This document draws heavily from the experimental in-transit message This document draws heavily from the experimental in-transit message
downgrading procedure described in RFC 5504 [RFC5504]. The downgrading procedure described in RFC 5504 [RFC5504]. The
contribution of the co-author of that earlier document, Y. Yoneya, contribution of the co-author of that earlier document, Y. Yoneya,
are gratefully acknowledged. Significant comments and suggestions are gratefully acknowledged. Significant comments and suggestions
were received from John Klensin, Barry Leiba, Randall Gellens, Pete were received from John Klensin, Barry Leiba, Randall Gellens, Pete
Resnick, Martin J. Durst, and other WG participants. Resnick, Martin J. Durst, and other WG participants.
10. References 9. References
10.1. Normative References 9.1. Normative References
[RFC2045] Freed, N. and N. Borenstein, [RFC2045] Freed, N. and N. Borenstein,
"Multipurpose Internet Mail "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Extensions (MIME) Part One: Format of
Internet Message Bodies", RFC 2045, Internet Message Bodies", RFC 2045,
November 1996. November 1996.
[RFC2047] Moore, K., "MIME (Multipurpose [RFC2047] Moore, K., "MIME (Multipurpose
Internet Mail Extensions) Part Three: Internet Mail Extensions) Part Three:
Message Header Extensions for Non- Message Header Extensions for Non-
skipping to change at page 16, line 5 skipping to change at page 17, line 14
Information in Internet Messages: The Information in Internet Messages: The
Content-Disposition Header Field", Content-Disposition Header Field",
RFC 2183, August 1997. RFC 2183, August 1997.
[RFC2231] Freed, N. and K. Moore, "MIME [RFC2231] Freed, N. and K. Moore, "MIME
Parameter Value and Encoded Word Parameter Value and Encoded Word
Extensions: Character Sets, Languages Extensions: Character Sets, Languages
, and Continuations", RFC 2231, , and Continuations", RFC 2231,
November 1997. 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 [RFC3629] Yergeau, F., "UTF-8, a transformation
format of ISO 10646", STD 63, format of ISO 10646", STD 63,
RFC 3629, November 2003. RFC 3629, November 2003.
[RFC3864] Klyne, G., Nottingham, M., and J. [RFC3864] Klyne, G., Nottingham, M., and J.
Mogul, "Registration Procedures for Mogul, "Registration Procedures for
Message Header Fields", BCP 90, Message Header Fields", BCP 90,
RFC 3864, September 2004. RFC 3864, September 2004.
[RFC4021] Klyne, G. and J. Palme, "Registration [RFC4021] Klyne, G. and J. Palme, "Registration
skipping to change at page 16, line 50 skipping to change at page 18, line 17
RFC 6532, February 2012. RFC 6532, February 2012.
[RFC6533] Hansen, T., Newman, C., and A. [RFC6533] Hansen, T., Newman, C., and A.
Melnikov, "Internationalized Delivery Melnikov, "Internationalized Delivery
Status and Disposition Status and Disposition
Notifications", RFC 6533, Notifications", RFC 6533,
February 2012. February 2012.
[I-D.leiba-5322upd-from-group] Leiba, B., "Update to Internet [I-D.leiba-5322upd-from-group] Leiba, B., "Update to Internet
Message Format to Allow Group Syntax Message Format to Allow Group Syntax
in the 'From:' Header Field", in the "From:" and "Sender:" Header
draft-leiba-5322upd-from-group-03 Fields",
(work in progress), July 2012. draft-leiba-5322upd-from-group-06
(work in progress), October 2012.
10.2. Informative References [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 [RFC5451] Kucherawy, M., "Message Header Field
for Indicating Message Authentication for Indicating Message Authentication
Status", RFC 5451, April 2009. Status", RFC 5451, April 2009.
[RFC5504] Fujiwara, K. and Y. Yoneya, [RFC5504] Fujiwara, K. and Y. Yoneya,
"Downgrading Mechanism for Email "Downgrading Mechanism for Email
Address Internationalization", Address Internationalization",
RFC 5504, March 2009. RFC 5504, March 2009.
skipping to change at page 21, line 5 skipping to change at page 22, line 17
o Updated by WGLC comments o Updated by WGLC comments
o Fixed Received downgrading and added to refer "RFC 6531", "RFC o Fixed Received downgrading and added to refer "RFC 6531", "RFC
5890", "RFC 5891" 5890", "RFC 5891"
o Added Domain downgrading for Received, Group and Mailbox o Added Domain downgrading for Received, Group and Mailbox
o Swapped section 3 and 4 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 Author's Address
Kazunori Fujiwara Kazunori Fujiwara
Japan Registry Services Co., Ltd. Japan Registry Services Co., Ltd.
Chiyoda First Bldg. East 13F, 3-8-1 Nishi-Kanda Chiyoda First Bldg. East 13F, 3-8-1 Nishi-Kanda
Chiyoda-ku, Tokyo 101-0065 Chiyoda-ku, Tokyo 101-0065
Japan Japan
Phone: +81 3 5215 8451 Phone: +81 3 5215 8451
EMail: fujiwara@wide.ad.jp, fujiwara@jprs.co.jp EMail: fujiwara@jprs.co.jp
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