Email Address Internationalization Y. YONEYA, Ed. (EAI) K. Fujiwara, Ed. Internet-Draft JPRS Intended status: Experimental November 18, 2007 Expires: May 21, 2008 Downgrading mechanism for Email Address Internationalization draft-ietf-eai-downgrade-05.txt Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. 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." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on May 21, 2008. Copyright Notice Copyright (C) The IETF Trust (2007). Abstract Traditional mail systems handle only ASCII characters in SMTP envelope and mail header fields. The Email Address Internationalization (UTF8SMTP) extension allows UTF-8 characters in SMTP envelope and mail header fields. To avoid bouncing internationalized Email messages when a server in the delivery path does not support the UTF8SMTP extension, some sort of converting mechanism is required. This document describes a downgrading YONEYA & Fujiwara Expires May 21, 2008 [Page 1] Internet-Draft UTF8SMTP Downgrade November 2007 mechanism for Email Address Internationalization. Note that this is a way to downgrade, not tunnel. There is no associated up-conversion mechanism, although internationalized email clients might use original internationalized addresses or other data when displaying or replying to downgraded messages. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4 3. New header fields definition . . . . . . . . . . . . . . . . . 5 3.1. Envelope information preservation headers . . . . . . . . 5 3.2. Address header field preservation headers . . . . . . . . 5 3.3. Unknown header fields preservation headers . . . . . . . . 6 4. SMTP Downgrading . . . . . . . . . . . . . . . . . . . . . . . 7 5. Email header fields downgrading . . . . . . . . . . . . . . . 8 5.1. Downgrading method for each header field . . . . . . . . . 9 6. MIME body part headers downgrading . . . . . . . . . . . . . . 11 7. Security considerations . . . . . . . . . . . . . . . . . . . 11 8. Implementation notes . . . . . . . . . . . . . . . . . . . . . 12 8.1. Trivial downgrading . . . . . . . . . . . . . . . . . . . 12 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13 10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 15 11. Change History . . . . . . . . . . . . . . . . . . . . . . . . 15 11.1. draft-yoneya-ima-downgrade: Version 00 . . . . . . . . . . 15 11.2. draft-yoneya-ima-downgrade: Version 01 . . . . . . . . . . 15 11.3. draft-ietf-eai-downgrade: Version 00 . . . . . . . . . . . 16 11.4. draft-ietf-eai-downgrade: Version 01 . . . . . . . . . . . 16 11.5. draft-ietf-eai-downgrade: Version 02 . . . . . . . . . . . 16 11.6. draft-ietf-eai-downgrade: Version 03 . . . . . . . . . . . 16 11.7. draft-ietf-eai-downgrade: Version 04 . . . . . . . . . . . 16 11.8. draft-ietf-eai-downgrade: Version 05 . . . . . . . . . . . 17 12. Normative References . . . . . . . . . . . . . . . . . . . . . 17 Appendix A. Displaying downgraded message . . . . . . . . . . . . 18 A.1. Displaying technique 1 . . . . . . . . . . . . . . . . . . 18 A.2. Displaying technique 2 . . . . . . . . . . . . . . . . . . 19 Appendix B. Examples . . . . . . . . . . . . . . . . . . . . . . 19 B.1. Downgrading example 1 . . . . . . . . . . . . . . . . . . 19 B.2. Displaying example . . . . . . . . . . . . . . . . . . . . 22 B.2.1. Displaying technique 1 example . . . . . . . . . . . . 23 B.2.2. Displaying technique 2 example . . . . . . . . . . . . 24 B.3. Downgrading example 2 . . . . . . . . . . . . . . . . . . 27 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 28 Intellectual Property and Copyright Statements . . . . . . . . . . 30 YONEYA & Fujiwara Expires May 21, 2008 [Page 2] Internet-Draft UTF8SMTP Downgrade November 2007 1. Introduction Traditional mail systems which are defined by [RFC2821] and [RFC2822] allow ASCII characters in SMTP envelope and mail header field values. The UTF8SMTP extension [RFC4952], [I-D.ietf-eai-utf8headers] and [I-D.ietf-eai-smtpext] allows UTF-8 characters in SMTP envelope and mail header field values. If an envelope address or header field contains non-ASCII characters, the message cannot be delivered unless every system in the delivery path supports UTF8SMTP. To avoid bouncing such messages when a server is encountered which does not support the UTF8SMTP extension, this document describes a downgrading mechanism. Downgrading a message converts envelope and header fields to an all-ASCII representation. [I-D.ietf-eai-utf8headers] allows UTF-8 characters to be used in mail header fields and MIME header fields. The downgrading mechanism specified here converts mail header fields and MIME header fields to ASCII. This document does not change any protocols except by defining new header fields. It describes the conversion method from the internationalized email envelopes/messages which are defined in [RFC4952] [I-D.ietf-eai-utf8headers] [I-D.ietf-eai-smtpext] to the traditional email envelopes/messages which are defined in [RFC2821] [RFC2822]. [I-D.ietf-eai-smtpext] section 2.2 defines when downgrading occurs. If the SMTP client has an UTF8SMTP envelope or an internationalized message and the SMTP server doesn't support the UTF8SMTP SMTP extension, then the SMTP client MUST NOT send a UTF8SMTP envelope or an internationalized message to the SMTP server. The section shows 4 choices. The fourth choice is downgrading, as described here. Downgrading may be implemented in MUAs, MSAs, MTAs which act as the SMTP client, or in MDAs, POP servers, IMAP servers which store or offer UTF8SMTP envelopes or internationalized messages to non- UTF8SMTP compliant systems which include message stores. This document tries to define the downgrading process clearly and it preserves the original information as much as possible. Downgrading in UTF8SMTP consists of the following four parts: o New header fields definition o SMTP downgrading YONEYA & Fujiwara Expires May 21, 2008 [Page 3] Internet-Draft UTF8SMTP Downgrade November 2007 o Email header fields downgrading o MIME header fields downgrading In Section 3, many header fields starting with "downgraded" are introduced. They preserve the original envelope information and the original header fields. The SMTP downgrading is described in Section 4. It generates ASCII only envelope information from an UTF8SMTP envelope. The Email header fields downgrading is described in Section 5. It generates ASCII only header fields. The MIME header fields are expanded in [I-D.ietf-eai-utf8headers]. The MIME header fields downgrading is described in Section 6. It generates ASCII only MIME header fields. Decoding downgraded message is described in Appendix A. This mechanism is for use by internationalized email clients. Once a message has been downgraded, there is no up-conversion which can be applied on the SMTP delivery path. 2. Terminology The key words "MUST", "SHALL", "REQUIRED", "SHOULD", "RECOMMENDED", and "MAY" in this document are to be interpreted as described in RFC 2119 [RFC2119]. All specialized terms used in this specification are defined in the EAI overview [RFC4952] or in [RFC2821][RFC2822], 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 [RFC4952] document. This document depends on [I-D.ietf-eai-smtpext], [I-D.ietf-eai-utf8headers], and [I-D.ietf-eai-dsn]. Key words used in these document are used in this document, too. The term "non-ASCII" is an UTF-8 string which contains at least one non-ASCII character. An "UTF8SMTP envelope" has Email originator/recipient addresses expanded by [I-D.ietf-eai-smtpext] and [I-D.ietf-eai-dsn]. An "UTF8SMTP message" is Email messages expanded by YONEYA & Fujiwara Expires May 21, 2008 [Page 4] Internet-Draft UTF8SMTP Downgrade November 2007 [I-D.ietf-eai-utf8headers]. 3. New header fields definition New header fields starting with "downgraded" are defined here to preserve those original envelope and header values which contain UTF-8 characters. During downgrading, one new "downgraded" header field is added for each original envelope or header field which cannot be passed as-is to a server which does not support UTF8SMTP. The original envelope or header field is removed or altered. Only those envelope and header fields which contain non-ASCII characters are affected. The result of this process is a message which is compliant with existing email specifications [RFC2821] and [RFC2822]. The original internationalized information can be retrieved by examining the "downgraded" header fields which were added. Even though the information is not lost, the original message cannot be perfectly reconstructed. Hence, downgrading is a one-way process. However, an internationalized client might use the information in the "downgraded" header fields when processing a downgraded message, for example, such as displaying or composing a reply. 3.1. Envelope information preservation headers Two headers "Downgraded-Mail-From:" and "Downgraded-Rcpt-To:" are defined to preserve SMTP envelope downgraded information. SMTP envelope downgraded information consists of the original non-ASCII address and the downgraded all-ASCII address. The header field syntax is specified as follows: fields =/ downgradedmailfrom / downgradedrcptto downgradedmailfrom = "Downgraded-Mail-From:" [FWS] "<" uPath ">" 1*[FWS] "<" Mailbox ">" [FWS] CRLF downgradedrcptto = "Downgraded-Rcpt-To:" [FWS] "<" uPath ">" 1*[FWS] "<" Mailbox ">" [FWS] CRLF Original non-ASCII address is defined in [I-D.ietf-eai-smtpext]; it is treated as unstructured in this header and it is encoded according to [RFC2047]. is defined in [RFC2821], section 4.1.2. 3.2. Address header field preservation headers The address header fields preservation headers are defined to preserve the original header field. Their value field holds the original header field value. Any original header field value is treated as an and it MUST be encoded according to [RFC2047] with charset='UTF-8'. The header field syntax is specified YONEYA & Fujiwara Expires May 21, 2008 [Page 5] Internet-Draft UTF8SMTP Downgrade November 2007 as follows: fields =/ known-downgraded-headers ":" unstructued CRLF known-downgraded-headers = "Downgraded-" original-headers original-headers = "From" / "To" / "Cc" / "Bcc" / "Sender" / "Reply-To" / "Resent-From" / "Resent-Sender" / "Resent-To" / "Resent-Cc" / "Return-Path" Preserving a header field in a downgraded header field is defined as: 1. Generate new downgraded header field whose value is the original header field value. 2. Encode the generated header according to [RFC2047] with charset='UTF-8'. 3.3. Unknown header fields preservation headers The unknown header fields preservation headers are defined to encapsulate those original header field which contains non-ASCII characters and are not otherwise provided for in the this specification. The encapsulation header field name is the concatenation of "Downgraded-" and the original name. The value field holds the original header field value. Any original header field value is treated as an unstructured value and it MUST be encoded according to [RFC2047] with charset='UTF-8'. The header field syntax is specified as follows: fields =/ unknown-downgraded-headers ":" unstructued CRLF unknown-downgraded-headers = "Downgraded-" original-header-field-name original-header-field-name = field-name field-name = 1*ftext ftext = %d33-57 / ; Any character except %d59-126 ; controls, SP, and ; ":". Encapsulating a header field in a Downgraded header field is defined as: 1. Generate new Downgraded header whose value is the original header field value. YONEYA & Fujiwara Expires May 21, 2008 [Page 6] Internet-Draft UTF8SMTP Downgrade November 2007 2. Encode the generated header field according to [RFC2047] with charset='UTF-8'. 3. Remove the original header field. 4. SMTP Downgrading Target of downgrading elements in SMTP envelope are below: o MAIL FROM: o RCPT TO: o ORCPT parameter Downgrading the SMTP envelope uses ALT-ADDRESS parameter defined in [I-D.ietf-eai-smtpext]. An address is downgradable if the address is non-ASCII address and has ASCII address specified by ALT-ADDRESS parameter. Since only non-ASCII addresses are downgradable, specifying an ALT-ADDRESS value for an all-ASCII address is invalid for use with this specification, and no interpretation is assigned to it. This restriction allows for future extension of the specification even though no such extensions are currently anticipated. Note that even if no downgrading is performed on the envelope, message header fields and message body MIME header fields that contain non-ASCII characters MUST be downgraded. This is described in Section 5 and Section 6. When downgrading, replace each non-ASCII mail address in the envelope with its specified alternative ASCII address and preserve the original information using "Downgraded-Mail-From" and "Downgraded- Rcpt-To" header fields as defined in Section 3. Before replacing, decode the ALT-ADDRESS parameter value because it is encoded as xtest [RFC3461]. To avoid disclosing recipient addresses, the downgrading process MUST NOT add "Downgraded-Rcpt-To:" header if the SMTP downgrading targets multiple recipients. See Section 7 for more detail. The "RCPT TO" command may have an ORCPT parameter when the recipient address is downgraded. The ORCPT parameter is used for DSN [RFC3461]. If the ORCPT parameter contains an "utf-8" address and the address contains non-ASCII characters, the ORCPT parameter MUST be converted to utf-8-addr-xtext form or utf-8-addr-unitext form which are described in [I-D.ietf-eai-dsn]. YONEYA & Fujiwara Expires May 21, 2008 [Page 7] Internet-Draft UTF8SMTP Downgrade November 2007 5. Email header fields downgrading This section defines the conversion method to ASCII for each header field which may contain non-ASCII characters. [I-D.ietf-eai-utf8headers] expands Received: header fields, [RFC2822] ABNF elements , , , , [RFC2045] ABNF element . Header field downgrading is defined below for each ABNF element. Downgrading an unknown header field is also defined as ENCAPSULATION downgrading. Converting the header field terminates when no non- ASCII characters remain in the header field. RECEIVED downgrading: If the header field name is "Received:" and the FOR clause contains a non-ASCII addresses, remove the FOR clause from the header field. The other part except in the comments does not contain non-ASCII values. UNSTRUCTURED downgrading: If the header field has an field which contains non-ASCII characters, encode the field according to [RFC2047] with charset='UTF-8'. WORD downgrading: If the header field has any fields which contains non-ASCII characters, encode the fields according to [RFC2047] with charset='UTF-8'. COMMENT downgrading: If the header field has any fields which contains non-ASCII characters, encode the fields according to [RFC2047] with charset='UTF-8'. MIME-VALUE downgrading: If the header field has any elements defined by [RFC2045] and the elements contain non-ASCII characters, encode the elements by [RFC2231] with charset='UTF-8' and the Language information empty. If the element is and it contains outside the DQUOTE, remove the before this conversion. DISPLAY-NAME downgrading: If the header field has any elements and they have elements which contain non- ASCII characters, encode the elements according to [RFC2047] with charset='UTF-8'. MAILBOX downgrading: The elements have no equivalent format for non-ASCII addresses. If the header field has any elements which contain non-ASCII characters, preserve the header field in each Address header field preservation header defined in Section 3.2, and rewrite each element to YONEYA & Fujiwara Expires May 21, 2008 [Page 8] Internet-Draft UTF8SMTP Downgrade November 2007 ASCII only format. The element which contains non-ASCII characters is one of three formats. * [ Display-name ] "<" Utf8-addr-spec 1*FCS "<" Addr-spec ">>" Rewrite it as [ Display-name ] "<" Addr-spec ">" * [ Display-name ] "<" Utf8-addr-spec ">" * Utf8-addr-spec Rewrite both as [ Display-name ] "Internationalized Address " Encoded-word " Removed:;" where the is the original encoded according to [RFC2047]. ENCAPSULATION downgrading: if the header field contains non-ASCII characters and for which no rule is given above, encapsulate it in a Downgraded header field described in Section 3.3 as a last resort. 5.1. 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. o Address header fields which contain s From: Sender: Reply-To: To: Cc: Bcc: Resent-From: Resent-Sender: YONEYA & Fujiwara Expires May 21, 2008 [Page 9] Internet-Draft UTF8SMTP Downgrade November 2007 Resent-To: Resent-Cc: Return-Path: If the header field contains elements which contains non-ASCII addresses, preserve the header field in a downgraded header before the conversion. Then do COMMENT downgrading, DISPLAY-NAME downgrading and MAILBOX downgrading. o Downgrading Non-ASCII in comments Date: Message-ID: In-Reply-To: References: Resent-Date: Resent-Message-ID: MIME-Version: Content-ID: These header fields do not contain non-ASCII characters except in comments. If the header contains UTF-8 characters in comments, do COMMENT downgrading. o Trace header fields Received: do COMMENT downgrading and do RECEIVED downgrading. o MIME Content header fields Content-Type: Content-Disposition: Do MIME-VALUE downgrading. o Non-ASCII in Subject: Comments: Content-Description: Do UNSTRUCTURED downgrading. o Non-ASCII in or YONEYA & Fujiwara Expires May 21, 2008 [Page 10] Internet-Draft UTF8SMTP Downgrade November 2007 Keywords: Do WORD downgrading. o Other header fields All other header fields which contains non-ASCII characters are user-defined, missing from this draft or future defined header fields. Do ENCAPSULATION downgrading as a last resort. 6. MIME body part headers downgrading MIME body part header fields may contain non-ASCII characters [I-D.ietf-eai-utf8headers]. This section defines the conversion method to ASCII only header fields for each MIME header field which contains non-ASCII characters. Parse the message body's MIME structure for all levels and check each MIME header field whether it contains non-ASCII characters. If the header field contains non- ASCII characters in the header value, the header is a target of the MIME body part headers downgrading. Each MIME header field's downgrading method is described below. COMMENT downgrading, MIME- VALUE downgrading, UNSTRUCTURED downgrading are described in Section 5. Content-ID: The Content-ID: header does not contain non-ASCII characters except in comments. If the header contains UTF-8 characters in comments, do COMMENT downgrading. Content-Type: Content-Disposition: Do MIME-VALUE downgrading. Content-Description: Do UNSTRUCTURED downgrading. 7. Security considerations o A Downgraded message's header fields contain ASCII characters only. But they still contain MIME encapsulated header fields which contains non-ASCII UTF-8 characters. And more, the body part may contain UTF-8 message. The recipients need to accept UTF-8 mail body and UTF-8 header fields which is MIME encoded. YONEYA & Fujiwara Expires May 21, 2008 [Page 11] Internet-Draft UTF8SMTP Downgrade November 2007 o Rewriting headers increases the opportunities for undetected spoofing. o Recipients addresses can be undisclosed if those addresses are listed on Bcc or group address. Those undisclosed addresses are used only in the Envelope. Copying information from the Envelope into headers risks inadvertent information disclosure (not just about addresses). See Section 4 for instructions on recipient address handling. o The techniques described here invalidates methods that depend on signatures over the envelope or any part of the message which includes the top-level header or body part headers. Depending on the specific message being downgraded, DKIM especially, but also possibly S/MIME, PGP, and similar techniques are all likely to break. o Many gateways and servers on the Internet will discard headers with which they are not familiar. To the extent to which the downgrade procedures depend on new headers (e.g., "Downgraded") to avoid information loss, then the risk of having those headers dropped and its implications must be identified. In particular, it appears to me that, if the Downgraded headers are dropped, there is no possibility of reconstructing the original information at any point (before, during, or after delivery). See "Security considerations" section in [RFC4952] for more discussion. 8. Implementation notes Downgrading is an alternative to rejection for delivering messages which require UTF8SMTP support to a server which does not provide this. Implementing the full specification of this document is desirable, but a partial implementation is also possible. If a partial downgrading implementation confronts an unsupported downgrading target, the implementation MUST NOT send the message to server which does not support UTF8SMTP. Instead, it MUST reject the message or generate a notification of non-deliverability. 8.1. Trivial downgrading A partial downgrading, Trivial downgrading is discussed. It does not support non-ASCII addresses in SMTP envelope and address header fields, unknown header fields downgrading, the MIME body part headers YONEYA & Fujiwara Expires May 21, 2008 [Page 12] Internet-Draft UTF8SMTP Downgrade November 2007 downgrading. It supports o some simple header fields downgrading: Subject o comments and display name downgrading: From, To, CC o trace header field downgrading: Received Otherwise, the downgrading fails. Trivial downgrading targets mail messages which are generated by UTF8SMTP aware MUAs and contain non-ASCII characters in comments, display names, unstructured parts without using non-ASCII E-mail addresses. This E-mail message does not contain non-ASCII addresses in SMTP Envelope and its header fields. But it is not deliverable via UTF8SMTP un-aware MTA. Implementing full spec downgrading may be hard, but trivial downgrading saves mail messages without using non- ASCII addresses. 9. IANA Considerations IANA is requested to register the following header fields in the Permanent Message Header Field Repository, in accordance with the procedures set out in [RFC3864]. Header field name: Downgraded-Mail-From Applicable protocol: mail Status: experimental Author/change controller: IETF Specification document(s): This document (Section 3) Header field name: Downgraded-Rcpt-To Applicable protocol: mail Status: experimental Author/change controller: IETF Specification document(s): This document (Section 3) Header field name: Downgraded-From Applicable protocol: mail Status: experimental Author/change controller: IETF Specification document(s): This document (Section 3) Header field name: Downgraded-Sender Applicable protocol: mail Status: experimental YONEYA & Fujiwara Expires May 21, 2008 [Page 13] Internet-Draft UTF8SMTP Downgrade November 2007 Author/change controller: IETF Specification document(s): This document (Section 3) Header field name: Downgraded-To Applicable protocol: mail Status: experimental Author/change controller: IETF Specification document(s): This document (Section 3) Header field name: Downgraded-Cc Applicable protocol: mail Status: experimental Author/change controller: IETF Specification document(s): This document (Section 3) Header field name: Downgraded-Reply-To Applicable protocol: mail Status: experimental Author/change controller: IETF Specification document(s): This document (Section 3) Header field name: Downgraded-Bcc Applicable protocol: mail Status: experimental Author/change controller: IETF Specification document(s): This document (Section 3) Header field name: Downgraded-Resent-From Applicable protocol: mail Status: experimental Author/change controller: IETF Specification document(s): This document (Section 3) Header field name: Downgraded-Resent-To Applicable protocol: mail Status: experimental Author/change controller: IETF Specification document(s): This document (Section 3) Header field name: Downgraded-Resent-Cc Applicable protocol: mail Status: experimental Author/change controller: IETF Specification document(s): This document (Section 3) YONEYA & Fujiwara Expires May 21, 2008 [Page 14] Internet-Draft UTF8SMTP Downgrade November 2007 Header field name: Downgraded-Resent-Sender Applicable protocol: mail Status: experimental Author/change controller: IETF Specification document(s): This document (Section 3) Header field name: Downgraded-Return-Path Applicable protocol: mail Status: experimental Author/change controller: IETF Specification document(s): This document (Section 3) And more, IANA is requested to reserve all the field names that start by "Downgraded-" for unknown header fields downgrading described in Section 3.3, in accordance with the procedures set out in [RFC3864]. Header field name: Names which starts by "Downgraded-" Applicable protocol: mail Status: experimental Author/change controller: IETF Specification document(s): This document (Section 3) 10. Acknowledgements Significant comments and suggestions were received from John Klensin, Harald Alvestrand, Chris Newman, Randall Gellens, Charles Lindsey, Marcos Sanz, Alexey Melnikov, Frank Ellermann, Edward Lewis, S. Moonesamy and JET members. 11. Change History This section is used for tracking the update of this document. Will be removed after finalize. 11.1. draft-yoneya-ima-downgrade: Version 00 o Initial version o Followed draft-yeh-ima-utf8headers-00 and draft-yao-smtpext-00 11.2. draft-yoneya-ima-downgrade: Version 01 o Document structure was changed o Followed draft-yeh-ima-utf8headers-01 and draft-yao-smtpext-02 YONEYA & Fujiwara Expires May 21, 2008 [Page 15] Internet-Draft UTF8SMTP Downgrade November 2007 o Downgrading requirements were added o SMTP DATA encapsulation method was proposed o Downgrading examples was provided 11.3. draft-ietf-eai-downgrade: Version 00 o Followed draft-yeh-ima-utf8headers-01 and draft-ietf-eai-smtpext-00 o No header downgrading method was proposed o Header encapsulation method was proposed 11.4. draft-ietf-eai-downgrade: Version 01 o Followed draft-ietf-eai-utf8headers-00 o Header conversion and encapsulation method was merged o Header conversion method was defined in detail 11.5. draft-ietf-eai-downgrade: Version 02 o Followed draft-ietf-eai-utf8headers-01 and draft-ietf-eai-smtpext-01 o Specification about algorithmic generated address is removed o No header downgrading method was removed o SMTP DATA encapsulation method was removed 11.6. draft-ietf-eai-downgrade: Version 03 o Followed draft-ietf-eai-utf8headers-03 and draft-ietf-eai-smtpext-03 o Downgraded: and Envelope-Downgraded: headers definition was added o Mail header fields downgrading method was refined o Examples in Appendix A were refined 11.7. draft-ietf-eai-downgrade: Version 04 o Followed draft-ietf-eai-utf8headers-06, draft-ietf-eai-smtpext-07 and draft-ietf-eai-dsn-02 o Downgrading requirements and conditions were moved to Introduction. o Descriptions about upgrading were removed. o SPF and DKIM discussion were removed. o Added many header fields downgrading. o Allow address literal rewriting without alternate ASCII address in header fields. o Added MIME body part headers downgrading. o Added ORCPT downgrading. YONEYA & Fujiwara Expires May 21, 2008 [Page 16] Internet-Draft UTF8SMTP Downgrade November 2007 11.8. draft-ietf-eai-downgrade: Version 05 o fixed examples * ALT-ADDRESS parameter mistake * RFC2047(x) notation was changed to encoded-word format o Added implementation consideration section and trivial downgrading o Downgraded: and Envelope-Downgraded: headers are separated for each original headers. o Removed list-* header fields downgrading o Changed the way of writing the header field downgrading section 12. Normative References [I-D.ietf-eai-dsn] Newman, C. and A. Melnikov, "International Delivery and Disposition Notifications", draft-ietf-eai-dsn-05 (work in progress), November 2007. [I-D.ietf-eai-smtpext] Yao, J. and W. Mao, "SMTP extension for internationalized email address", draft-ietf-eai-smtpext-09 (work in progress), November 2007. [I-D.ietf-eai-utf8headers] Yeh, J., "Internationalized Email Headers", draft-ietf-eai-utf8headers-07 (work in progress), September 2007. [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 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. YONEYA & Fujiwara Expires May 21, 2008 [Page 17] Internet-Draft UTF8SMTP Downgrade November 2007 [RFC2821] Klensin, J., "Simple Mail Transfer Protocol", RFC 2821, April 2001. [RFC2822] Resnick, P., "Internet Message Format", RFC 2822, April 2001. [RFC3461] Moore, K., "Simple Mail Transfer Protocol (SMTP) Service Extension for Delivery Status Notifications (DSNs)", RFC 3461, January 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. [RFC4952] Klensin, J. and Y. Ko, "Overview and Framework for Internationalized Email", RFC 4952, July 2007. Appendix A. Displaying downgraded message A perfect reverse-function of the downgrading does not exist because the encoding defined in [RFC2047] is not exactly reversible. The restoration of the downgrading should be done once at the final destination of the downgraded message such as MUAs or IMAP servers. This document describes the restoration methods as displaying technique. Displaying downgraded message is mostly retrieved by MIME decoding according to [RFC2047] and [RFC2231]. Result of MIME decoding, downgraded header fields are still in Downgraded-*: header fields, but the header field values are MIME decoded. These decoded Downgraded-*: header fields contain the original header field values and the recipient can read them. But the recipient's MUA cannot use the original header fields automatically. Additionally, MUAs can process Downgraded-*: header. It is described in Appendix A.1 and Appendix A.2. A.1. Displaying technique 1 MUA can remove 'Downgraded-' from decoded 'Downgraded-*:' header fields' name. With this technique, The address header fields may be displayed twice, one is ASCII-only downgraded header field and the other is from decoded Downgraded-*: header. YONEYA & Fujiwara Expires May 21, 2008 [Page 18] Internet-Draft UTF8SMTP Downgrade November 2007 A.2. Displaying technique 2 MUA can decode and regenerate headers which contains the original non-ASCII addresses. It is described below: o For each Downgraded-*: header field, generate new header field which field name is the original header name and the field value is the decoded header field value. * If the header is an address header described in Section 5, + Generate ASCII only header fields defined in Section 5 from the decoded header fields without re-generating Downgraded-*: header fields. + Remove the header field which is the same with the generated ASCII only header from the header fields. This comparison requires considerations for foldings and [RFC2047] encoded parts. Appendix B. Examples B.1. Downgrading example 1 This section shows an SMTP Downgrading example. Consider a mail message. o The sender address is "NON-ASCII-local@example.com" which is non- ASCII address. Its ASCII alternative is "ASCII-local@example.com" and its display-name is "DISPLAY-local". o The "To" address is "NON-ASCII-remote1@example.net" which is non- ASCII address. Its ASCII alternative is "ASCII-remote1@example.net" and its display-name is "DISPLAY- remote1". o The "CC" address is non-ASCII address "NON-ASCII-remote2@example.org" without alternative ASCII address. Its display-name is "DISPLAY-remote2". o Three display-names contains non-ASCII characters. o The Subject header is "NON-ASCII-SUBJECT" which contains non-ASCII characters. o assume To: recipient's MTA (example.net) does not support UTF8SMTP. o assume Cc: recipient's MTA (example.org) supports UTF8SMTP. The example SMTP envelope/message is showin in Figure 4. In this example, the To: recipient's session is fucused. YONEYA & Fujiwara Expires May 21, 2008 [Page 19] Internet-Draft UTF8SMTP Downgrade November 2007 MAIL From: ALT-ADDRESS=ASCII-local@example.com RCPT TO: ALT-ADDRESS=ASCII-remote1@example.net RCPT TO: ------------------------------------------------------------- Message-Id: MESSAGE_ID Mime-Version: 1.0 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 8bit Subject: NON-ASCII-SUBJECT From: DISPLAY-local > To: DISPLAY-remote1 > CC: DISPLAY-remote2 Date: DATE MAIL_BODY Figure 4: Original envelope/message (example 1) In this example, there are two SMTP recipients, one is To:, the other is CC:. The SMTP downgrading treats To: session downgrading. Figure 5 shows SMTP downgraded example. YONEYA & Fujiwara Expires May 21, 2008 [Page 20] Internet-Draft UTF8SMTP Downgrade November 2007 MAIL From: RCPT TO: ------------------------------------------------------------- Downgraded-Mail-From: =?UTF-8?Q??= Downgraded-Rcpt-To: =?UTF-8?Q??= Message-Id: MESSAGE_ID Mime-Version: 1.0 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 8bit Subject: NON-ASCII-SUBJECT From: DISPLAY-local > To: DISPLAY-remote1 > CC: DISPLAY-remote2 Date: DATE MAIL_BODY Figure 5: SMTP Downgraded envelope/message (example 1) After SMTP downgrading, header fields downgrading is performed. Final downgraded message is shown in Figure 6. Return-Path header will be added by the final destination MTA. YONEYA & Fujiwara Expires May 21, 2008 [Page 21] Internet-Draft UTF8SMTP Downgrade November 2007 Return-Path: Downgraded-Mail-From: =?UTF-8?Q??= Downgraded-Rcpt-To: =?UTF-8?Q??= Message-Id: MESSAGE_ID Mime-Version: 1.0 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 8bit Subject: =?UTF-8?Q?NON-ASCII-SUBJECT?= Downgraded-From: =?UTF-8?Q?DISPLAY-local?= =?UTF-8?Q?> From: =?UTF-8?Q?DISPLAY-local?= To: =?UTF-8?Q?DISPLAY-remote1?= Downgraded-To: =?UTF-8?Q?DISPLAY-remote1?= =?UTF-8?Q?> CC: =?UTF-8?Q?DISPLAY-remote2?= Internationalized address =?UTF-8?Q?NON-ASCII-remote2@example.org?= removed:; Downgraded-CC: =?UTF-8?Q?DISPLAY-remote2?= =?UTF-8?Q??= Date: DATE MAIL_BODY Figure 6: Downgraded message (example 1) B.2. Displaying example Figure 7 shows MIME decoded message of Figure 6. YONEYA & Fujiwara Expires May 21, 2008 [Page 22] Internet-Draft UTF8SMTP Downgrade November 2007 Return-Path: Downgraded-Mail-From: Downgraded-Rcpt-To: Message-Id: MESSAGE_ID Mime-Version: 1.0 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 8bit Subject: NON-ASCII-SUBJECT Downgraded-From: DISPLAY-local> From: DISPLAY-local Downgraded-To: DISPLAY-remote1> To: DISPLAY-remote1 Downgraded-CC: DISPLAY-remote2 CC: DISPLAY-remote2 Internationalized address NON-ASCII-remote2@example.org removed:; Date: DATE MAIL_BODY Figure 7: MIME decoded message B.2.1. Displaying technique 1 example YONEYA & Fujiwara Expires May 21, 2008 [Page 23] Internet-Draft UTF8SMTP Downgrade November 2007 After removing 'Downgraded-' from decoded 'Downgraded-*:' header fields Return-Path: Downgraded-Mail-From: Downgraded-Rcpt-To: Message-Id: MESSAGE_ID Mime-Version: 1.0 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 8bit Subject: NON-ASCII-SUBJECT From: DISPLAY-local> From: DISPLAY-local To: DISPLAY-remote1> To: DISPLAY-remote1 CC: DISPLAY-remote2 CC: DISPLAY-remote2 Internationalized address NON-ASCII-remote2@example.org removed:; Date: DATE MAIL_BODY Figure 8: Displaying technique 1 B.2.2. Displaying technique 2 example This example shows displaying process of Appendix A.2 for Figure 6. First, check downgraded address header fields existence. Downgraded-From: =?UTF-8?Q?DISPLAY-local?= =?UTF-8?Q?> Downgraded-To: =?UTF-8?Q?DISPLAY-remote1?= =?UTF-8?Q?> Downgraded-CC: =?UTF-8?Q?DISPLAY-remote2?= =?UTF-8?Q??= Figure 9: Displaying technique 2: selecting downgraded address header fields YONEYA & Fujiwara Expires May 21, 2008 [Page 24] Internet-Draft UTF8SMTP Downgrade November 2007 Then, decode MIME encoded parts. Downgraded-From: DISPLAY-local> Downgraded-To: DISPLAY-remote1> Downgraded-CC: DISPLAY-remote2 Figure 10: Displaying technique 2: decode downgraded address header fields Apply header fields downgrading to the decoded header without re- generating Downgraded-*: header. From: =?UTF-8?Q?DISPLAY-local?= To: =?UTF-8?Q?DISPLAY-remote1?= CC: =?UTF-8?Q?DISPLAY-remote2?= Internationalized address =?UTF-8?Q?NON-ASCII-remote2@example.org?= removed:; Figure 11: Displaying technique 2: downgraded decoded 'Downgraded-*:' Remove the header fields which is the same with Figure 11 from Figure 6. YONEYA & Fujiwara Expires May 21, 2008 [Page 25] Internet-Draft UTF8SMTP Downgrade November 2007 Return-Path: Downgraded-Mail-From: =?UTF-8?Q??= Downgraded-Rcpt-To: =?UTF-8?Q??= Message-Id: MESSAGE_ID Mime-Version: 1.0 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 8bit Subject: =?UTF-8?Q?NON-ASCII-SUBJECT?= Downgraded-From: =?UTF-8?Q?DISPLAY-local?= =?UTF-8?Q?> Downgraded-To: =?UTF-8?Q?DISPLAY-remote1?= =?UTF-8?Q?> Downgraded-CC: =?UTF-8?Q?DISPLAY-remote2?= =?UTF-8?Q??= Date: DATE MAIL_BODY Figure 12: Displaying technique 2: Removing duplicated headers Decode each 'Downgraded' header and replace it with its decoded header. If each mail header has [RFC2047] encoded part and which encoding is "UTF-8", it is a downgraded header, so decode it. Return-Path: Downgraded-Mail-From: Downgraded-Rcpt-To: Message-Id: MESSAGE_ID Mime-Version: 1.0 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 8bit Subject: NON-ASCII-SUBJECT From: DISPLAY-local> To: DISPLAY-remote1> CC: DISPLAY-remote2 Date: DATE MAIL_BODY YONEYA & Fujiwara Expires May 21, 2008 [Page 26] Internet-Draft UTF8SMTP Downgrade November 2007 Figure 13: Display technique 2: decoded message As a result, in this simple example, all original header fields are displayed in the original form. B.3. Downgrading example 2 In many cases, the sender wants to use non-ASCII address, the recipient does not support UTF8SMTP and does not have non-ASCII address. o The sender address is "NON-ASCII-local@example.com" which is non- ASCII address. Its ASCII alternative is "ASCII-local@example.com". It has a display-name "DISPLAY-local" which contains non-ASCII characters. o The "To" address is "ASCII-remote1@example.net" which is ASCII only. It has a display-name "DISPLAY-remote1" which contains non- ASCII characters. o The Subject header is "NON-ASCII-SUBJECT" which contains non-ASCII characters. The second example envelope/message is shown in Figure 14. MAIL From: ALT-ADDRESS=ASCII-local@example.com RCPT TO: ------------------------------------------------------------- Message-Id: MESSAGE_ID Mime-Version: 1.0 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 8bit Subject: NON-ASCII-SUBJECT From: DISPLAY-local > To: DISPLAY-remote1 Date: DATE MAIL_BODY Figure 14: Original message (example 2) In this example, SMTP session is downgradable. Figure 15 shows SMTP downgraded envelope/message. YONEYA & Fujiwara Expires May 21, 2008 [Page 27] Internet-Draft UTF8SMTP Downgrade November 2007 MAIL From: RCPT TO: ------------------------------------------------------------- Downgraded-Mail-From: =?UTF-8?Q??= Message-Id: MESSAGE_ID Mime-Version: 1.0 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 8bit Subject: NON-ASCII-SUBJECT From: DISPLAY-local > To: DISPLAY-remote1 Date: DATE MAIL_BODY Figure 15: SMTP Downgraded envelope/message (example 2) After SMTP downgrading, header fields downgrading is performed. The downgraded example is shown in Figure 16. Return-Path: Downgraded-Mail-From: =?UTF-8?Q??= Message-Id: MESSAGE_ID Mime-Version: 1.0 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 8bit Subject: =?UTF-8?Q?NON-ASCII-SUBJECT?= Downgraded-From: =?UTF-8?Q?DISPLAY-local?= =?UTF-8?Q?> From: =?UTF-8?Q?DISPLAY-local?= To: =?UTF-8?Q?DISPLAY-remote1?= Date: DATE MAIL_BODY Figure 16: Downgraded message (example 2) YONEYA & Fujiwara Expires May 21, 2008 [Page 28] Internet-Draft UTF8SMTP Downgrade November 2007 Authors' Addresses Yoshiro YONEYA (editor) JPRS Chiyoda First Bldg. East 13F, 3-8-1 Nishi-Kanda Chiyoda-ku, Tokyo 101-0065 Japan Phone: +81 3 5215 8451 Email: yone@jprs.co.jp Kazunori Fujiwara (editor) JPRS 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 YONEYA & Fujiwara Expires May 21, 2008 [Page 29] Internet-Draft UTF8SMTP Downgrade November 2007 Full Copyright Statement Copyright (C) The IETF Trust (2007). 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The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf-ipr@ietf.org. Acknowledgment Funding for the RFC Editor function is provided by the IETF Administrative Support Activity (IASA). YONEYA & Fujiwara Expires May 21, 2008 [Page 30]