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Versions: 00 01 02 03 04 05 06 07 08 rfc6857             Standards Track
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



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   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 <address>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 <unstructured>  . . . . . . . . . . . . . 11
       5.2.8.  Non-ASCII in <phrase>  . . . . . . . . . . . . . . . . 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



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     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













































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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.



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   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 <group> syntax and encodes non-ASCII
   addresses into <display-name> with empty <group-list> 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 <group> in the header fields.





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   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.




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   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 <mailbox>, <word>, <comment>,
   <unstructured>; [RFC2045] describes ABNF element <value>.

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 <comment>s) should not contain non-ASCII
   values.

5.1.2.  UNSTRUCTURED Downgrading

   If the header field has an <unstructured> field that contains non-
   ASCII characters, apply [RFC2047] encoding with charset UTF-8.

5.1.3.  WORD Downgrading

   If the header field has any <word> fields that contain non-ASCII
   characters, apply [RFC2047] encoding with charset UTF-8.

5.1.4.  COMMENT Downgrading

   If the header field has any <comment> 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 <value> elements defined by [RFC2045] and
   the elements contain non-ASCII characters, encode the <value>
   elements according to [RFC2231] with charset UTF-8 and leave the
   language information empty.  If the <value> element is <quoted-
   string> and it contains <CFWS> outside the DQUOTE, remove the <CFWS>
   before this conversion.




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5.1.6.  DISPLAY-NAME Downgrading

   If the header field has any <address> (<mailbox> or <group>) elements
   and they have <display-name> elements that contain non-ASCII
   characters, encode the <display-name> elements according to [RFC2047]
   with charset UTF-8.  DISPLAY-NAME downgrading is the same algorithm
   as WORD downgrading.

5.1.7.  GROUP Downgrading

   <group> is defined in Section 3.4 of [RFC5322].  The <group> elements
   may contain <mailbox>s which contain non-ASCII addresses.

   If the header field has any <group> elements that contain <mailbox>
   elements, and those <mailbox> elements in turn contain non-ASCII
   addresses, rewrite each <group> element as


   "Internationalized address removed" display-name ENCODED_WORD ":;"


   where the <ENCODED_WORD> is the original <group-list> encoded
   according to [RFC2047].

5.1.8.  MAILBOX Downgrading

   The <mailbox> elements have no equivalent format for non-ASCII
   addresses.  If the header field has any <mailbox> elements that
   contain non-ASCII characters in their <addr-spec> element, rewrite
   each <addr-spec> element to ASCII-only format.  The <addr-spec>
   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 <ENCODED-WORD> is the original <addr-spec> encoded
   according to [RFC2047].





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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 <utf-8-type-addr> and the <utf-8-type-
   addr> 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 <address>s


   From:
   Sender:
   To:
   Cc:
   Bcc:
   Reply-To:
   Resent-From:
   Resent-Sender:
   Resent-To:
   Resent-Cc:
   Resent-Bcc:
   Resent-Reply-To:







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   Return-Path:
   Disposition-Notification-To:

   If the header field contains <group> elements that contain non-ASCII
   addresses, perform COMMENT downgrading, DISPLAY-NAME downgrading, and
   GROUP downgrading.

   If the header field contains <mailbox> 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.






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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 <unstructured>


   Subject:
   Comments:
   Content-Description:

   Perform UNSTRUCTURED downgrading.

5.2.8.  Non-ASCII in <phrase>

   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.





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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
   <group> 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



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   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:





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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)







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   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






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11.3.  Version 02

   o  Added updating RFC 5322 to allow <group> syntax in From: and
      Sender

   o  Added GROUP Downgrading

11.4.  Version 03

   o  Replaced <utf8-addr-spec> with <addr-spec>

   o  Added updating RFC 5322 to allow <group> syntax in From: and
      Sender

   o  Added one sentence in Security considerations

   o  Updated IANA considerations

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-



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                                  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:




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   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: <NON-ASCII-local@example.com>
   Received: from ... by ... for <NON-ASCII-remote1@example.net>
   Received: from ... by ... for <NON-ASCII-remote1@example.net>
   From: DISPLAY-local <NON-ASCII-local@example.com>
   To: DISPLAY-remote1 <NON-ASCII-remote1@example.net>,
       DISPLAY-remote2 <NON-ASCII-remote2@example.com>
   Cc: DISPLAY-remote3 <NON-ASCII-remote3@example.org>
   Subject: NON-ASCII-SUBJECT
   Date: 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



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   "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











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