Email Address Internationalization K. Fujiwara (EAI) JPRS Internet-Draft Oct 31, 2011 Updates: 5322 (if approved) Intended status: Standards Track Expires: May 3, 2012 Post-delivery Message Downgrading for Internationalized Email Messages draft-ietf-eai-popimap-downgrade-03.txt Abstract The Email Address Internationalization (UTF8SMTP) extension allows UTF-8 characters in mail header fields. POP and IMAP servers support internationalized email messages. If a POP/IMAP client does not support Email Address Internationalization, POP/IMAP servers cannot send Internationalized Email Headers to the client and cannot remove the message. To avoid the situation, this document describes a conversion mechanism for internationalized Email messages to be traditional message format. Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on May 3, 2012. Copyright Notice Copyright (c) 2011 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents Fujiwara Expires May 3, 2012 [Page 1] Internet-Draft POP/IMAP Downgrade Oct 2011 carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5 3. Updating RFC 5322 . . . . . . . . . . . . . . . . . . . . . . 5 4. New Header Fields Definition . . . . . . . . . . . . . . . . . 6 4.1. Preservation Header Fields . . . . . . . . . . . . . . . . 6 5. Email Header Fields Downgrading . . . . . . . . . . . . . . . 7 5.1. Downgrading Method for Each ABNF Element . . . . . . . . . 7 5.1.1. RECEIVED Downgrading . . . . . . . . . . . . . . . . . 7 5.1.2. UNSTRUCTURED Downgrading . . . . . . . . . . . . . . . 7 5.1.3. WORD Downgrading . . . . . . . . . . . . . . . . . . . 7 5.1.4. COMMENT Downgrading . . . . . . . . . . . . . . . . . 7 5.1.5. MIME-VALUE Downgrading . . . . . . . . . . . . . . . . 7 5.1.6. DISPLAY-NAME Downgrading . . . . . . . . . . . . . . . 8 5.1.7. GROUP Downgrading . . . . . . . . . . . . . . . . . . 8 5.1.8. MAILBOX Downgrading . . . . . . . . . . . . . . . . . 8 5.1.9. ENCAPSULATION Downgrading . . . . . . . . . . . . . . 9 5.1.10. TYPED-ADDRESS Downgrading . . . . . . . . . . . . . . 9 5.2. Downgrading Method for Each Header Field . . . . . . . . . 9 5.2.1. Address Header Fields That Contain
s . . . . 9 5.2.2. Address Header Fields with Typed Addresses . . . . . . 10 5.2.3. Downgrading Non-ASCII in Comments . . . . . . . . . . 10 5.2.4. Message-ID Header Fields . . . . . . . . . . . . . . . 10 5.2.5. Received Header Field . . . . . . . . . . . . . . . . 11 5.2.6. MIME Content Header Fields . . . . . . . . . . . . . . 11 5.2.7. Non-ASCII in . . . . . . . . . . . . . 11 5.2.8. Non-ASCII in . . . . . . . . . . . . . . . . 11 5.2.9. Other Header Fields . . . . . . . . . . . . . . . . . 11 6. MIME Body-Part Header Field Downgrading . . . . . . . . . . . 12 7. Security Considerations . . . . . . . . . . . . . . . . . . . 12 8. Implementation Notes . . . . . . . . . . . . . . . . . . . . . 13 8.1. RFC 2047 Encoding . . . . . . . . . . . . . . . . . . . . 13 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13 9.1. Statement about Downgraded- registration . . . . . . . . . 14 9.2. Existing Downgraded- registrations . . . . . . . . . . . . 14 9.3. Additional header fields . . . . . . . . . . . . . . . . . 14 10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 15 11. Change History . . . . . . . . . . . . . . . . . . . . . . . . 15 11.1. Version 00 . . . . . . . . . . . . . . . . . . . . . . . . 15 11.2. Version 01 . . . . . . . . . . . . . . . . . . . . . . . . 15 11.3. Version 02 . . . . . . . . . . . . . . . . . . . . . . . . 16 Fujiwara Expires May 3, 2012 [Page 2] Internet-Draft POP/IMAP Downgrade Oct 2011 11.4. Version 03 . . . . . . . . . . . . . . . . . . . . . . . . 16 12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 16 12.1. Normative References . . . . . . . . . . . . . . . . . . . 16 12.2. Informative References . . . . . . . . . . . . . . . . . . 17 Appendix A. Examples . . . . . . . . . . . . . . . . . . . . . . 17 A.1. Downgrading Example . . . . . . . . . . . . . . . . . . . 17 Fujiwara Expires May 3, 2012 [Page 3] Internet-Draft POP/IMAP Downgrade Oct 2011 1. Introduction Traditional mail systems, which are defined by [RFC5322], allow ASCII characters in mail header field values. The UTF8SMTP extension ([I-D.ietf-eai-frmwrk-4952bis] and [I-D.ietf-eai-rfc5335bis] allows UTF-8 characters in mail header field values. If a header field contains non-ASCII characters, POP/IMAP servers cannot send Internationalized Email Headers to the client and cannot remove the message. This message downgrading mechanism converts mail header fields to an all-ASCII representation. The POP/IMAP servers can use the downgrading mechanism and send the Internationalized Email message as a traditional form. [I-D.ietf-eai-rfc5335bis] allows UTF-8 characters to be used in mail header fields and MIME header fields. The message downgrading mechanism specified here describes the conversion method from the internationalized email messages that are defined in [I-D.ietf-eai-frmwrk-4952bis], and [I-D.ietf-eai-rfc5335bis] to the traditional email messages defined in [RFC5322]. There is no good way to convert "From:" and "Sender:" header fields, the draft need to update [RFC5322] to allow empty "From:" and "Sender:" header fields and it is described in Section 3. Message Downgrading may be implemented in POP server and IMAP server only. This document tries to define the message downgrading process clearly. Downgrading consists of the following four parts: o Updating RFC 5322 o New header field definitions o Email header field downgrading o MIME header field downgrading In Section 4 of this document, header fields starting with "Downgraded-" are introduced. They preserve the original header fields. Email header field downgrading is described in Section 5. It generates ASCII-only header fields. Fujiwara Expires May 3, 2012 [Page 4] Internet-Draft POP/IMAP Downgrade Oct 2011 MIME header fields are expanded in [I-D.ietf-eai-rfc5335bis]. MIME header field downgrading is described in Section 6. It generates ASCII-only MIME header fields. Displaying downgraded messages that originally contained internationalized header fields is out of scope of this document. A POP/IMAP client which does not support UTF8 extension does not know internationalized message format described in [I-D.ietf-eai-rfc5335bis]. 2. Terminology The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119]. All specialized terms used in this specification are defined in the Email Address Internationalization (EAI) overview [I-D.ietf-eai-frmwrk-4952bis], in the mail message specifications [RFC5322], or in the MIME documents [RFC2045] [RFC2047] [RFC2183] [RFC2231]. The terms "ASCII address", "internationalized email address", "non-ASCII address", "i18mail address", "UTF8SMTP", "message", and "mailing list" are used with the definitions from [I-D.ietf-eai-frmwrk-4952bis]. This document depends on [I-D.ietf-eai-rfc5335bis]. Key words used in those documents are used in this document, too. The term "non-ASCII" refers to a UTF-8 string that contains at least one non-ASCII character. A "UTF8SMTP message" is an email message expanded by [I-D.ietf-eai-rfc5335bis]. 3. Updating RFC 5322 "From:" header field or "Sender:" header field may contain non-ASCII addresses in internationalized Email messages. These non-ASCII addresses are not allowed in [RFC5322]. The draft proposes that the pop/imap downgrading uses syntax and encodes non-ASCII addresses into with empty described in Section 5. This specification redefines "From:", "Sender:", "Resent-From:" and "Resent-Sender:" header fields defined in Section 3.6.2 and 3.6.6 of [RFC5322] to allow in the header fields. Fujiwara Expires May 3, 2012 [Page 5] Internet-Draft POP/IMAP Downgrade Oct 2011 from = "From:" address-list CRLF resent-from = "Resent-From:" address-list CRLF sender = "Sender:" address CRLF resent-sender = "Resent-Sender:" address CRLF 4. New Header Fields Definition New header fields starting with "Downgraded-" are defined here to preserve those mail header field values that contain UTF-8 characters. During downgrading, one new "Downgraded-" header field is added for each mail header field that cannot be passed as-is to a POP/IMAP client that does not support UTF8 extension. The original mail header field is removed. Only those mail header fields that contain non-ASCII characters are affected. The result of this process is a message that is compliant with existing email specifications [RFC5322]. The original internationalized information can be retrieved by examining the "Downgraded-" header fields that were added. 4.1. Preservation Header Fields New preservation header fields are defined to preserve information that appeared in non-ASCII text in header fields of the incoming message. The values of the new fields holds the original header field value in encoded form. The revised header field syntax is specified as follows: fields =/ known-downgraded-headers ":" unstructured CRLF known-downgraded-headers = "Downgraded-" original-headers original-headers = "Message-Id" / "Resent-Message-Id" / "In-Reply-To:" / "References:" / "Original-Recipient" / "Final-Recipient" To preserve a header field in a "Downgraded-" header field: 1. Generate a new "Downgraded-" header field whose value is the original header field value. 2. Treat the generated header field content as if it were unstructured, and then apply [RFC2047] encoding with charset UTF-8 as necessary so that the result is ASCII. Fujiwara Expires May 3, 2012 [Page 6] Internet-Draft POP/IMAP Downgrade Oct 2011 3. Remove the original header field. 5. Email Header Fields Downgrading This section defines the conversion method to ASCII for each header field that may contain non-ASCII characters. [I-D.ietf-eai-rfc5335bis] expands "Received:" header fields; [RFC5322] describes ABNF elements , , , ; [RFC2045] describes ABNF element . 5.1. Downgrading Method for Each ABNF Element Header field downgrading is defined below for each ABNF element. Converting the header field terminates when no non-ASCII characters remain in the header field. 5.1.1. RECEIVED Downgrading If the header field name is "Received:" and the FOR clause contains a non-ASCII address, remove the FOR clause from the header field. Other parts (not counting s) should not contain non-ASCII values. 5.1.2. UNSTRUCTURED Downgrading If the header field has an field that contains non- ASCII characters, apply [RFC2047] encoding with charset UTF-8. 5.1.3. WORD Downgrading If the header field has any fields that contain non-ASCII characters, apply [RFC2047] encoding with charset UTF-8. 5.1.4. COMMENT Downgrading If the header field has any fields that contain non-ASCII characters, apply [RFC2047] encoding with charset UTF-8. 5.1.5. MIME-VALUE Downgrading If the header field has any elements defined by [RFC2045] and the elements contain non-ASCII characters, 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. Fujiwara Expires May 3, 2012 [Page 7] Internet-Draft POP/IMAP Downgrade Oct 2011 5.1.6. DISPLAY-NAME Downgrading If the header field has any
( or ) elements and they have elements that contain non-ASCII characters, encode the elements according to [RFC2047] with charset UTF-8. DISPLAY-NAME downgrading is the same algorithm as WORD downgrading. 5.1.7. GROUP Downgrading is defined in Section 3.4 of [RFC5322]. The elements may contain s which contain non-ASCII addresses. If the header field has any elements that contain elements, and those elements in turn contain non-ASCII addresses, rewrite each element as "Internationalized address removed" display-name ENCODED_WORD ":;" where the is the original encoded according to [RFC2047]. 5.1.8. MAILBOX Downgrading The elements have no equivalent format for non-ASCII addresses. If the header field has any elements that contain non-ASCII characters in their element, rewrite each element to ASCII-only format. The element that contains non-ASCII characters may appear in two forms as: "<" addr-spec ">" addr-spec Rewrite both as: "Internationalized address " ENCODED-WORD " removed:;" where the is the original encoded according to [RFC2047]. Fujiwara Expires May 3, 2012 [Page 8] Internet-Draft POP/IMAP Downgrade Oct 2011 5.1.9. ENCAPSULATION Downgrading Encapsulate the header field in a "Downgraded-" header field as described in Section 4 as a last resort. Applying this procedure to "Received:" header field is prohibited. ENCAPSULATION Downgrading is allowed for "Message-ID", "In-Reply-To:", "References:", "Original-Recipient" and "Final- Recipient" header fields. 5.1.10. TYPED-ADDRESS Downgrading If the header field contains and the contains raw non-ASCII characters, it is in utf-8-address form. Convert it to utf-8-addr-xtext form. Those forms are described in [I-D.ietf-eai-rfc5337bis-dsn]. COMMENT downgrading is also performed in this case. If the address type is unrecognized and the header field contains non-ASCII characters, then fall back to using ENCAPSULATION downgrading on the entire header field. 5.2. Downgrading Method for Each Header Field Header fields are listed in [RFC4021]. This section describes the downgrading method for each header field. If the whole mail header field does not contain non-ASCII characters, email header field downgrading is not required. Each header field's downgrading method is described below. 5.2.1. Address Header Fields That Contain
s From: Sender: To: Cc: Bcc: Reply-To: Resent-From: Resent-Sender: Resent-To: Resent-Cc: Resent-Bcc: Resent-Reply-To: Fujiwara Expires May 3, 2012 [Page 9] Internet-Draft POP/IMAP Downgrade Oct 2011 Return-Path: Disposition-Notification-To: If the header field contains elements that contain non-ASCII addresses, perform COMMENT downgrading, DISPLAY-NAME downgrading, and GROUP downgrading. If the header field contains elements that contain non- ASCII addresses, perform COMMENT downgrading, DISPLAY-NAME downgrading, and MAILBOX downgrading. 5.2.2. Address Header Fields with Typed Addresses Original-Recipient: Final-Recipient: If the header field contains non-ASCII characters, perform TYPED- ADDRESS downgrading. 5.2.3. Downgrading Non-ASCII in Comments Date: Resent-Date: MIME-Version: Content-ID: Content-Transfer-Encoding: Content-Language: Accept-Language: Auto-Submitted: These header fields do not contain non-ASCII characters except in comments. If the header field contains UTF-8 characters in comments, perform COMMENT downgrading. 5.2.4. Message-ID Header Fields Message-ID: Resent-Message-ID: In-Reply-To: References: Perform ENCAPSULATION Downgrading. Fujiwara Expires May 3, 2012 [Page 10] Internet-Draft POP/IMAP Downgrade Oct 2011 5.2.5. Received Header Field Received: Perform COMMENT downgrading and RECEIVED downgrading. 5.2.6. MIME Content Header Fields Content-Type: Content-Disposition: Perform MIME-VALUE downgrading and COMMENT downgrading. 5.2.7. Non-ASCII in Subject: Comments: Content-Description: Perform UNSTRUCTURED downgrading. 5.2.8. Non-ASCII in Keywords: Perform WORD downgrading. 5.2.9. Other Header Fields There are other header fields that contain non-ASCII characters. They are user-defined and missing from this document, or future defined header fields. They are treated as "Optional Fields" and their field value are treated as unstructured described in Section 3.6.8 of [RFC5322]. Perform UNSTRUCTURED downgrading. If the software understands the header field's structure and a downgrading algorithm other than UNSTRUCTURED is applicable, that software SHOULD use that algorithm; UNSTRUCTURED downgrading is used as a last resort. Mailing list header fields (those that start in "List-") are part of this category. Fujiwara Expires May 3, 2012 [Page 11] Internet-Draft POP/IMAP Downgrade Oct 2011 6. MIME Body-Part Header Field Downgrading MIME body-part header fields may contain non-ASCII characters [I-D.ietf-eai-rfc5335bis]. This section defines the conversion method to ASCII-only header fields for each MIME header field that contains non-ASCII characters. Parse the message body's MIME structure at all levels and check each MIME header field to see whether it contains non-ASCII characters. If the header field contains non-ASCII characters in the header field value, the header field is a target of the MIME body-part header field's downgrading. Each MIME header field's downgrading method is described below. COMMENT downgrading, MIME-VALUE downgrading, and UNSTRUCTURED downgrading are described in Section 5. Content-ID: The "Content-ID:" header field does not contain non-ASCII characters except in comments. If the header field contains UTF-8 characters in comments, perform COMMENT downgrading. Content-Type: Content-Disposition: Perform MIME-VALUE downgrading and COMMENT downgrading. Content-Description: Perform UNSTRUCTURED downgrading. 7. Security Considerations Existing clients do not know new From: and Sender: header fields syntax updated by Section 3 and may get wrong when they confront syntax in From: and Sender: fields. A downgraded message's header fields contain ASCII characters only. But they still contain MIME-encapsulated header fields that contain non-ASCII UTF-8 characters. Furthermore, the body part may contain UTF-8 characters. Implementations parsing Internet messages need to accept UTF-8 body parts and UTF-8 header fields that are MIME- encoded. Thus, this document inherits the security considerations of MIME-encoded header fields ([RFC2047] and [RFC3629]). Rewriting header fields increases the opportunities for undetected spoofing by malicious senders. However, rewritten header fields are preserved into Downgraded-* header fields, and parsing Downgraded-* header fields enables the detection of spoofing caused by downgrading. The techniques described here invalidate methods that depend on digital signatures over any part of the message, which includes the Fujiwara Expires May 3, 2012 [Page 12] Internet-Draft POP/IMAP Downgrade Oct 2011 top-level header fields and body-part header fields. Depending on the specific message being downgraded, the following techniques are likely to break: DomainKeys Identified Mail (DKIM), and possibly S/MIME and Pretty Good Privacy (PGP). The two obvious mitigations are to stick to 7-bit transport when using these techniques (as most/ all of them presently require) or to make sure to have UTF8SMTP end- to-end when needed. While information in any email header field should usually be treated with some suspicion, current email systems commonly employ various mechanisms and protocols to make the information more trustworthy. Currently, information in the new Downgraded-* header fields is usually not inspected by these mechanisms, and may be even less trustworthy than the traditional header fields. Note that the Downgraded-* header fields could have been inserted with malicious intent (and with content unrelated to the traditional header fields). See the "Security Considerations" section in [I-D.ietf-eai-frmwrk-4952bis] for more discussion. 8. Implementation Notes 8.1. RFC 2047 Encoding While [RFC2047] has a specific algorithm to deal with whitespace in adjacent encoded words, there are a number of deployed implementations that fail to implement the algorithm correctly. As a result, whitespace behavior is somewhat unpredictable in practice when multiple encoded words are used. While RFC 5322 states that implementations SHOULD limit lines to not more than 78 characters, implementations MAY choose to allow overly long encoded words in order to work around faulty [RFC2047] implementations. Implementations that choose to do so SHOULD have an optional mechanism to limit line length to 78 characters. 9. IANA Considerations [[RFC Editor: Please change "should now be" and "should be" to "have been" when the IANA actions are complete.]] [[ Notes in draft: this section is not finished, to be reviewed with IANA. ]] Following instructions in the now-obsolete [RFC5504], IANA has made a series of entries in the the Permanent Message Header Field registry. Those registrations should now be changed as follows: Fujiwara Expires May 3, 2012 [Page 13] Internet-Draft POP/IMAP Downgrade Oct 2011 9.1. Statement about Downgraded- registration The statement about refusing any "Downgraded-" registrations should be updated to refer to this document and to provide for registering such fields as specified in Section 9.3. [[ Note in draft: The restriction may become useless if unknown header fields may be treated as unstructured. ]] 9.2. Existing Downgraded- registrations Individual existing registrations for Downgraded-Bcc Downgraded-Cc Downgraded-Disposition-Notification-To Downgraded-From Downgraded-Mail-From Downgraded-Rcpt-To Downgraded-Reply-To Downgraded-Resent-Bcc Downgraded-Resent-Cc Downgraded-Resent-From Downgraded-Resent-Reply-To Downgraded-Resent-Sender Downgraded-Resent-To Downgraded-Return-Path Downgraded-Sender Downgraded-To should be updated to replace "experimental" with "obsoleted" and to reference this document. 9.3. Additional header fields The following header fields should be registered in the Permanent Message Header Field registry, in accordance with the procedures set out in [RFC3864]. Header field name: Downgraded-Message-Id Applicable protocol: mail Status: standard Author/change controller: IETF Specification document(s): This document (Section 4) Fujiwara Expires May 3, 2012 [Page 14] Internet-Draft POP/IMAP Downgrade Oct 2011 Header field name: Downgraded-In-Reply-To Applicable protocol: mail Status: standard Author/change controller: IETF Specification document(s): This document (Section 4) Header field name: Downgraded-References Applicable protocol: mail Status: standard Author/change controller: IETF Specification document(s): This document (Section 4) Header field name: Downgraded-Original-Recipient Applicable protocol: mail Status: standard Author/change controller: IETF Specification document(s): This document (Section 4) Header field name: Downgraded-Final-Recipient Applicable protocol: mail Status: standard Author/change controller: IETF Specification document(s): This document (Section 4) 10. Acknowledgements This document draws heavily from the experimental in-transit message downgrading procedure described in RFC 5504 [RFC5504]. The contribution of the co-author of that earlier document, Y. Yoneya, are gratefully acknowledged. 11. Change History This section is used for tracking the update of this document. Will be removed after finalize. 11.1. Version 00 o Initial version o Imported header field downgrading from RFC 5504 11.2. Version 01 o same as Version 00 Fujiwara Expires May 3, 2012 [Page 15] Internet-Draft POP/IMAP Downgrade Oct 2011 11.3. Version 02 o Added updating RFC 5322 to allow syntax in From: and Sender o Added GROUP Downgrading 11.4. 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 12. References 12.1. Normative References [I-D.ietf-eai-frmwrk-4952bis] Klensin, J. and Y. Ko, "Overview and Framework for Internationalized Email", draft-ietf-eai-frmwrk-4952bis-12 (work in progress), October 2011. [I-D.ietf-eai-rfc5335bis] Yang, A., Steele, S., and N. Freed, "Internationalized Email Headers", draft-ietf-eai-rfc5335bis-13 (work in progress), October 2011. [I-D.ietf-eai-rfc5337bis-dsn] Hansen, T., Newman, C., and A. Melnikov, "Internationalized Delivery Status and Disposition Notifications", draft-ietf-eai-rfc5337bis-dsn-05 (work in progress), October 2011. [RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies", RFC 2045, November 1996. [RFC2047] Moore, K., "MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non- Fujiwara Expires May 3, 2012 [Page 16] Internet-Draft POP/IMAP Downgrade Oct 2011 ASCII Text", RFC 2047, November 1996. [RFC2183] Troost, R., Dorner, S., and K. Moore, "Communicating Presentation Information in Internet Messages: The Content-Disposition Header Field", RFC 2183, August 1997. [RFC2231] Freed, N. and K. Moore, "MIME Parameter Value and Encoded Word Extensions: Character Sets, Languages, and Continuations", RFC 2231, November 1997. [RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322, October 2008. [RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO 10646", STD 63, RFC 3629, November 2003. [RFC4021] Klyne, G. and J. Palme, "Registration of Mail and MIME Header Fields", RFC 4021, March 2005. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC3864] Klyne, G., Nottingham, M., and J. Mogul, "Registration Procedures for Message Header Fields", BCP 90, RFC 3864, September 2004. 12.2. Informative References [RFC5504] Fujiwara, K. and Y. Yoneya, "Downgrading Mechanism for Email Address Internationalization", RFC 5504, March 2009. Appendix A. Examples A.1. Downgrading Example This appendix shows an message downgrading example. Consider a received mail message where: Fujiwara Expires May 3, 2012 [Page 17] Internet-Draft POP/IMAP Downgrade Oct 2011 o The sender address is a non-ASCII address, "NON-ASCII-local@example.com". Its display-name is "DISPLAY- local". o The "To:" header field contains two non-ASCII addresses, "NON-ASCII-remote1@example.net" and "NON-ASCII-remote2@example.com" Its display-names are "DISPLAY- remote1" and "DISPLAY-remote2". o The "Cc:" header field contains a non-ASCII address, "NON-ASCII-remote3@example.org". Its display-name is "DISPLAY- remote3". o Four display names contain non-ASCII characters. o The Subject header field is "NON-ASCII-SUBJECT", which contains non-ASCII characters. o The "Message-Id:" header field contains "NON-ASCII-MESSAGE_ID", which contains non-ASCII characters. o There is an unknown header field "X-Unknown-Header" which contains non-ASCII characters. Return-Path: Received: from ... by ... for Received: from ... by ... for From: DISPLAY-local To: DISPLAY-remote1 , DISPLAY-remote2 Cc: DISPLAY-remote3 Subject: NON-ASCII-SUBJECT Date: DATE Message-Id: NON-ASCII-MESSAGE_ID Mime-Version: 1.0 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 8bit X-Unknown-Header: NON-ASCII-CHARACTERS MAIL_BODY Figure 1: Received message in a mail drop The downgraded message is shown in Figure 2. "Return-Path:", "From:", "To:" and "Cc:" header fields are rewritten. "Subject:" and Fujiwara Expires May 3, 2012 [Page 18] Internet-Draft POP/IMAP Downgrade Oct 2011 "X-Unknown-Header:" header fields are encoded using [RFC2047]. "Message-Id:" header field is encapsulated as "Downgraded-Message-Id:" header field. Return-Path: Internationalized address =?UTF-8?Q?NON-ASCII-local@example.com?= removed:; Received: from ... by ... Received: from ... by ... From: =?UTF-8?Q?DISPLAY-local?= Internationalized address =?UTF-8?Q?NON-ASCII-local@example.com?= removed:; To: =?UTF-8?Q?DISPLAY-remote1?= Internationalized address =?UTF-8?Q?NON-ASCII-remote1@example.net?= removed:;, =?UTF-8?Q?DISPLAY-remote2?= Internationalized address =?UTF-8?Q?NON-ASCII-remote2@example.com?= removed:;, Cc: =?UTF-8?Q?DISPLAY-remote3?= Internationalized address =?UTF-8?Q?NON-ASCII-remote3@example.org?= removed:; Subject: =?UTF-8?Q?NON-ASCII-SUBJECT?= Date: DATE Downgraded-Message-Id: =?UTF-8?Q?MESSAGE_ID?= Mime-Version: 1.0 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 8bit X-Unknown-Header: =?UTF-8?Q?NON-ASCII-CHARACTERS?= MAIL_BODY Figure 2: Downgraded message Author's Address Kazunori Fujiwara Japan Registry Services Co., Ltd. Chiyoda First Bldg. East 13F, 3-8-1 Nishi-Kanda Chiyoda-ku, Tokyo 101-0065 Japan Phone: +81 3 5215 8451 EMail: fujiwara@wide.ad.jp, fujiwara@jprs.co.jp Fujiwara Expires May 3, 2012 [Page 19]