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 Fujiwara Expires April 25, 2013 [Page 1] Internet-Draft POP/IMAP Downgrade Oct 2012 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
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 . . . . . . . . . . . . . 12 3.2.7. Non-ASCII in . . . . . . . . . . . . . . . . 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 Fujiwara Expires April 25, 2013 [Page 2] Internet-Draft POP/IMAP Downgrade Oct 2012 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 Fujiwara Expires April 25, 2013 [Page 3] Internet-Draft POP/IMAP Downgrade Oct 2012 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 Fujiwara Expires April 25, 2013 [Page 4] Internet-Draft POP/IMAP Downgrade Oct 2012 [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]. Fujiwara Expires April 25, 2013 [Page 5] Internet-Draft POP/IMAP Downgrade Oct 2012 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 , , , , , . [RFC2045] describes ABNF element . 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 field that contains non- ASCII strings, apply [RFC2047] encoding with charset UTF-8. 3.1.2. WORD Downgrading If the header field has any fields that contain non-ASCII strings, apply [RFC2047] encoding with charset UTF-8. 3.1.3. COMMENT Downgrading If the header field has any fields that contain non-ASCII strings, apply [RFC2047] encoding with charset UTF-8. Fujiwara Expires April 25, 2013 [Page 6] Internet-Draft POP/IMAP Downgrade Oct 2012 3.1.4. MIME-VALUE Downgrading If the header field has any elements defined by [RFC2045] and the elements contain non-ASCII strings, encode the elements according to [RFC2231] with charset UTF-8 and leave the language information empty. If the element is and it contains outside the DQUOTE, remove the before this conversion. 3.1.5. DISPLAY-NAME Downgrading If the header field has any
( or ) elements and they have elements that contain non-ASCII strings, encode the 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 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 is defined in Section 3.4 of [RFC5322]. The elements may contain es which contain non-ASCII addresses. If a element contains elements and one of es contains a non-ASCII , rewrite the element as display-name " " ENCODED_WORD " :;" where the is the original encoded according to [RFC2047]. Otherwise, the element does not contain non-ASCII . If the element contain non-ASCII 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 es contain ASCII addresses only. Fujiwara Expires April 25, 2013 [Page 7] Internet-Draft POP/IMAP Downgrade Oct 2012 3.1.8. MAILBOX Downgrading If the of the element does not contain non- ASCII characters, the 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 contains non-ASCII characters. The non- ASCII has no equivalent format for ASCII addresses. The element that contains non-ASCII strings may appear in two forms as: "<" addr-spec ">" addr-spec Rewrite both as: ENCODED-WORD " :;" where the is the original encoded according to [RFC2047]. 3.1.9. TYPED-ADDRESS Downgrading If the header field contains and the 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: Fujiwara Expires April 25, 2013 [Page 8] Internet-Draft POP/IMAP Downgrade Oct 2012 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. Fujiwara Expires April 25, 2013 [Page 9] Internet-Draft POP/IMAP Downgrade Oct 2012 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
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 elements that contain non- ASCII addresses, perform GROUP downgrading on those elements. 2. If the header field contains elements that contain non- ASCII addresses, perform MAILBOX downgrading on those elements. This procedure may generate empty elements in "From:", "Sender:" and "Reply-To:" header fields. [I-D.leiba-5322upd-from-group] updates [RFC5322] to allow (empty) elements in "From:", "Sender:" and "Reply-To:" header fields. Fujiwara Expires April 25, 2013 [Page 10] Internet-Draft POP/IMAP Downgrade Oct 2012 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 elements or 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 , remove the "FOR" clause. If the ID clause contains a non-ASCII values, remove the "ID" clause. Fujiwara Expires April 25, 2013 [Page 11] Internet-Draft POP/IMAP Downgrade Oct 2012 3.2.5. MIME Content Header Fields Content-Type: Content-Disposition: Perform MIME-VALUE downgrading and COMMENT downgrading. 3.2.6. Non-ASCII in Subject: Comments: Content-Description: Perform UNSTRUCTURED downgrading. 3.2.7. Non-ASCII in 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. Fujiwara Expires April 25, 2013 [Page 12] Internet-Draft POP/IMAP Downgrade Oct 2012 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 Fujiwara Expires April 25, 2013 [Page 13] Internet-Draft POP/IMAP Downgrade Oct 2012 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 Fujiwara Expires April 25, 2013 [Page 14] Internet-Draft POP/IMAP Downgrade Oct 2012 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) Fujiwara Expires April 25, 2013 [Page 15] Internet-Draft POP/IMAP Downgrade Oct 2012 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 Fujiwara Expires April 25, 2013 [Page 16] Internet-Draft POP/IMAP Downgrade Oct 2012 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. Fujiwara Expires April 25, 2013 [Page 17] Internet-Draft POP/IMAP Downgrade Oct 2012 [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". Fujiwara Expires April 25, 2013 [Page 18] Internet-Draft POP/IMAP Downgrade Oct 2012 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: Received: from ... by ... for Received: from ... by ... for From: DISPLAY-LOCAL To: DISPLAY-REMOTE1 , DISPLAY-REMOTE2 Cc: DISPLAY-REMOTE3 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. Fujiwara Expires April 25, 2013 [Page 19] Internet-Draft POP/IMAP Downgrade Oct 2012 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 syntax in From: and Sender o Added GROUP Downgrading Fujiwara Expires April 25, 2013 [Page 20] Internet-Draft POP/IMAP Downgrade Oct 2012 B.4. Version 03 o Replaced with o Added updating RFC 5322 to allow 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" Fujiwara Expires April 25, 2013 [Page 21] Internet-Draft POP/IMAP Downgrade Oct 2012 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 Fujiwara Expires April 25, 2013 [Page 22]