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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                                              Apr 13, 2012
Updates: 5322 (if approved)
Intended status: Standards Track
Expires: October 15, 2012


 Post-delivery Message Downgrading for Internationalized Email Messages
                draft-ietf-eai-popimap-downgrade-05.txt

Abstract

   The Email Address Internationalization (SMTPUTF8) extension allows
   UTF-8 characters in mail header fields.  Upgraded POP and IMAP
   servers support internationalized Email messages.  If a POP/IMAP
   client does not support Email Address Internationalization, POP/IMAP
   servers cannot 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 in traditional message format.  In the process, message
   elements requiring internationalized treatment are recoded or removed
   and receivers are able to know that they received messages containing
   such elements even if they cannot treat the internationalized
   elements.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on October 15, 2012.

Copyright Notice

   Copyright (c) 2012 IETF Trust and the persons identified as the
   document authors.  All rights reserved.




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   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
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  Problem statement  . . . . . . . . . . . . . . . . . . . .  4
     1.2.  Possible solutions . . . . . . . . . . . . . . . . . . . .  4
     1.3.  Approach taken in this specification . . . . . . . . . . .  4
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  6
   3.  Updating RFC 5322  . . . . . . . . . . . . . . . . . . . . . .  6
   4.  New Header Fields Definition . . . . . . . . . . . . . . . . .  7
   5.  Email Header Fields Downgrading  . . . . . . . . . . . . . . .  8
     5.1.  Downgrading Method for Each ABNF Element . . . . . . . . .  8
       5.1.1.  UNSTRUCTURED Downgrading . . . . . . . . . . . . . . .  8
       5.1.2.  WORD Downgrading . . . . . . . . . . . . . . . . . . .  8
       5.1.3.  COMMENT Downgrading  . . . . . . . . . . . . . . . . .  8
       5.1.4.  MIME-VALUE Downgrading . . . . . . . . . . . . . . . .  8
       5.1.5.  DISPLAY-NAME Downgrading . . . . . . . . . . . . . . .  9
       5.1.6.  GROUP Downgrading  . . . . . . . . . . . . . . . . . .  9
       5.1.7.  MAILBOX Downgrading  . . . . . . . . . . . . . . . . .  9
       5.1.8.  ENCAPSULATION Downgrading  . . . . . . . . . . . . . . 10
       5.1.9.  TYPED-ADDRESS Downgrading  . . . . . . . . . . . . . . 10
     5.2.  Downgrading Method for Each Header Field . . . . . . . . . 10
       5.2.1.  Address Header Fields That Contain <address>s  . . . . 10
       5.2.2.  Address Header Fields with Typed Addresses . . . . . . 11
       5.2.3.  Downgrading Non-ASCII in Comments  . . . . . . . . . . 11
       5.2.4.  Message-ID Header Fields . . . . . . . . . . . . . . . 11
       5.2.5.  Received Header Field  . . . . . . . . . . . . . . . . 12
       5.2.6.  MIME Content Header Fields . . . . . . . . . . . . . . 12
       5.2.7.  Non-ASCII in <unstructured>  . . . . . . . . . . . . . 12
       5.2.8.  Non-ASCII in <phrase>  . . . . . . . . . . . . . . . . 12
       5.2.9.  Other Header Fields  . . . . . . . . . . . . . . . . . 12
   6.  MIME Body-Part Header Field Downgrading  . . . . . . . . . . . 13
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 13
   8.  Implementation Notes . . . . . . . . . . . . . . . . . . . . . 14
     8.1.  RFC 2047 Encoding  . . . . . . . . . . . . . . . . . . . . 14
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 15
   10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 16
   11. Change History . . . . . . . . . . . . . . . . . . . . . . . . 16
     11.1. Version 00 . . . . . . . . . . . . . . . . . . . . . . . . 16



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     11.2. Version 01 . . . . . . . . . . . . . . . . . . . . . . . . 16
     11.3. Version 02 . . . . . . . . . . . . . . . . . . . . . . . . 16
     11.4. Version 03 . . . . . . . . . . . . . . . . . . . . . . . . 16
     11.5. Version 04 . . . . . . . . . . . . . . . . . . . . . . . . 17
     11.6. Version 05 . . . . . . . . . . . . . . . . . . . . . . . . 17
   12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 17
     12.1. Normative References . . . . . . . . . . . . . . . . . . . 17
     12.2. Informative References . . . . . . . . . . . . . . . . . . 18
   Appendix A.  Examples  . . . . . . . . . . . . . . . . . . . . . . 18
     A.1.  Downgrading Example  . . . . . . . . . . . . . . . . . . . 18









































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1.  Introduction

1.1.  Problem statement

   Traditional (legacy) mail systems, which are defined by [RFC5322],
   allow only ASCII characters in mail header field values.  The
   SMTPUTF8 extension ([RFC6530] and [RFC6532]) allow raw UTF-8 in those
   mail header fields.

   If a header field contains non-ASCII strings, POP/IMAP servers cannot
   send Internationalized Email Headers to legacy clients and, because
   they have no obvious or standardized way to explain what is going on
   to those clients, cannot even safely discard the message.

1.2.  Possible solutions

   Discussions leading to this specification concluded that there are
   four plausible approaches to the problem, with the preferred one
   depending on the particular circumstances and relationship among the
   delivery SMTP server, the mail store, the POP or IMAP server, and the
   users and their MUA clients:

   1.  If the delivery MTA has sufficient knowledge about the POP and/or
       IMAP servers and clients being used, the message may be rejected
       as undeliverable.

   2.  The message may be downgraded by the POP or IMAP server, in a way
       that preserves maximum information at the expense of some
       complexity.

   3.  Some intermediate downgrading may be applied that balances more
       information loss against lower complexity and greater ease of
       implementation.

   4.  The POP or IMAP server may fabricate a message whose intent is to
       notify the client that an internationalized message is waiting
       but cannot be delivered until an upgraded client is available.

1.3.  Approach taken in this specification

   This specification describes the second of those options.  It is
   worth noticing that, at least in the general case, none of these
   options preserve sufficient information to guarantee that it is
   possible to reply to an incoming message without loss of information,
   so the choice may be considered to be among "least bad" options.

   This message downgrading mechanism converts mail header fields to an
   all-ASCII representation.  The POP/IMAP servers can use the



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   downgrading mechanism and send the Internationalized Email message as
   a traditional form.  Receivers can know they received some
   internationalized messages or some unknown/broken messages.

   [RFC6532] 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
   messages that are defined in [RFC6530], and [RFC6532] to the
   traditional email messages defined in [RFC5322].

   There is no good way to convert "From:" and "Sender:" header fields,
   this document updates [RFC5322] by redefining "From:" and "Sender:"
   header fields in Section 3.

   This document provides a precise definition of the minimum-
   information-loss message downgrading process.

   Downgrading consists of the following three parts:


   o  New header field definitions

   o  Email header field downgrading

   o  MIME header field downgrading

   In Section 4 of this document, header fields starting with
   "Downgraded-" are introduced.  They preserve the information that
   appeared in the original header fields.

   Email header field downgrading is described in Section 5.  It
   generates ASCII-only header fields.

   The definition of MIME header fields in Internationalized Email
   Messages is described in [RFC6532].  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 [RFC6532].

   The purpose of post-delivery message downgrading is to enable POP/
   IMAP servers to deliver internationalized messages to traditional
   POP/IMAP clients.






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2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

   All specialized terms used in this specification are defined in the
   Overview and Framework for Internationalized Email [RFC6530], in the
   mail message specifications [RFC5322], or in the MIME documents
   [RFC2045] [RFC2047] [RFC2183] [RFC2231].  The terms "ASCII address",
   "non-ASCII address", "SMTPUTF8", "message", "internationalized
   message" are used with the definitions from [RFC6530].  The term
   "non-ASCII string" is used with the definitions from [RFC6532].

3.  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.


   from            =   "From:" address-list CRLF
   resent-from     =   "Resent-From:" address-list CRLF
   sender          =   "Sender:" address CRLF
   resent-sender   =   "Resent-Sender:" address CRLF


   This adds group syntax to "From" and "Sender" that was previously
   allowed only in destination fields such as "To" and "cc".  It is
   anticipated that when existing implementations encounter a downgraded
   field from this set, many will tolerate the appearance of a group,
   even though [RFC5322] does not permit it.  Implementations that do
   not tolerate it will fail in unpredictable ways, and they might
   refuse to process such messages.

   [[ Notes in Draft: If this update is rejected, one possible solution
   is to rewrite each <addr-spec> element in "From" and "Sender" header
   fields as


   ENCODED-WORD "<" NON_EXISTING_ADDRESS ">"



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   where the <ENCODED-WORD> is the original <addr-spec> encoded
   according to [RFC2047] and NON_EXISTING_ADDRESS is an ASCII email
   address which does not exist, should, as illustrated in the example
   below, always generate an error and is specified by the administrator
   of the POP3 or IMAP server.

   For example, if the local-part of the "From:" address were the
   Russian (in Cyrillic) equivalent of Ivan, with domain-part
   "foo.example.net" and the IMAP server being used by the recipient was
   "imap.example.com", the encoded word from suggested in this note
   might appear as:


   From: =?UTF-8?Q?=d0=b8=d0=b2=d0=b0=d0=bd@foo.example.net?=
         <invalid-i18n-address@imap.example.com>


   That would lead to immediate rejection if a user attempted to reply
   uncritically to the message. ]]

4.  New Header Fields Definition

   New header fields are defined to preserve information that appeared
   in non-ASCII strings 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                   =/ downgraded

   downgraded =  "Downgraded-Message-Id:"         unstructured CRLF /
                 "Downgraded-Resent-Message-Id:"  unstructured CRLF /
                 "Downgraded-In-Reply-To:"        unstructured CRLF /
                 "Downgraded-References:"         unstructured CRLF /
                 "Downgraded-Original-Recipient:" unstructured CRLF /
                 "Downgraded-Final-Recipient:"    unstructured CRLF


   To preserve a header field in a "Downgraded-" header field:

   1.  Generate a new header field.

       *  The field name is a concatenation of "Downgraded-" and the
          original field name.

       *  The initial new field value is the original header field
          value.



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   2.  Treat the initial new header field value as if it were
       unstructured, and then apply [RFC2047] encoding with charset
       UTF-8 as necessary so that the resulting new header field value
       is completely in ASCII.

   3.  Remove the original header field.

5.  Email Header Fields Downgrading

   This section defines the conversion method to ASCII for each header
   field that may contain non-ASCII strings.

   [RFC6532] 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 strings
   remain in the header field.

5.1.1.  UNSTRUCTURED Downgrading

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

5.1.2.  WORD Downgrading

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

5.1.3.  COMMENT Downgrading

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

5.1.4.  MIME-VALUE Downgrading

   If the header field has any <value> elements defined by [RFC2045] and
   the elements contain non-ASCII strings, 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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5.1.5.  DISPLAY-NAME Downgrading

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

5.1.6.  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


   display-name ENCODED_WORD " :;"


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

5.1.7.  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 strings in their <addr-spec> element, rewrite each
   <addr-spec> element to ASCII-only format.  The <addr-spec> element
   that contains non-ASCII strings may appear in two forms as:


   "<" addr-spec ">"
   addr-spec


   Rewrite both as:


   ENCODED-WORD " :;"


   where the <ENCODED-WORD> is the original <addr-spec> encoded
   according to [RFC2047].






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5.1.8.  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.

5.1.9.  TYPED-ADDRESS Downgrading

   If the header field contains <utf-8-type-addr> and the <utf-8-type-
   addr> contains raw non-ASCII strings, it is in utf-8-address form.
   Convert it to utf-8-addr-xtext form.  Those forms are described in
   [RFC6533].  COMMENT downgrading is also performed in this case.  If
   the address type is unrecognized and the header field contains non-
   ASCII strings, then fall back to using ENCAPSULATION downgrading on
   the entire header field.

5.2.  Downgrading Method for Each Header Field

   [RFC4021] establishes a registry of header fields.  This section
   describes the downgrading method for each header field.

   If the whole mail header field does not contain non-ASCII strings,
   email header field downgrading is not required.  Each header field's
   downgrading method is described below.

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 strings, 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 strings 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:

   If the FOR clause contains a non-ASCII address, remove the FOR clause
   from the header field.  Comments may contain non-ASCII strings,
   Perform COMMENT downgrading.  Other parts should not contain non-
   ASCII strings.

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 strings.  They
   are user-defined and missing from this document, or future defined
   header fields.  They are treated as "Optional Fields" and their field
   values are treated as unstructured described in Section 3.6.8 of
   [RFC5322].

   Perform UNSTRUCTURED downgrading.

   If the software understands the header field's structure and a
   downgrading algorithm other than UNSTRUCTURED is applicable, that
   software SHOULD use that algorithm; UNSTRUCTURED downgrading is used
   as a last resort.




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   Mailing list header fields (those that start in "List-") are part of
   this category.

6.  MIME Body-Part Header Field Downgrading

   MIME body-part header fields may contain non-ASCII strings [RFC6532].
   This section defines the conversion method to ASCII-only header
   fields for each MIME header field that contains non-ASCII strings.
   Parse the message body's MIME structure at all levels and check each
   MIME header field to see whether it contains non-ASCII strings.  If
   the header field contains non-ASCII strings in the header field
   value, the header field is a target of the MIME body-part header
   field's downgrading.  Each MIME header field's downgrading method is
   described below.  COMMENT downgrading, MIME-VALUE downgrading, and
   UNSTRUCTURED downgrading are described in Section 5.

   Content-ID:
      The "Content-ID:" header field does not contain non-ASCII strings
      except in comments.  If the header field contains UTF-8 characters
      in comments, perform COMMENT downgrading.

   Content-Type:

   Content-Disposition:
      Perform MIME-VALUE downgrading and COMMENT downgrading.

   Content-Description:  Perform UNSTRUCTURED downgrading.

7.  Security Considerations

   The purpose of post-delivery message downgrading is to allow POP/IMAP
   servers to deliver internationalized messages to traditional POP/IMAP
   clients and permit the clients to display those messages.  Users who
   receive such messages can know that they were internationalized.  It
   does not permit receivers to read the messages in their original form
   and, in general, will not permit generating replies, at least without
   significant user intervention.

   This specification is designed so that MUAs that receive converted
   messages may be traditional and SMTPUTF8-unaware.  The specification
   assumes that such MUAs have no special provisions for either
   "Downgraded-" header fields or the new syntax of From and Sender
   header fields described in Section 3.

   A downgraded message's header fields contain ASCII characters only.
   But they still contain MIME-encapsulated header fields that contain
   non-ASCII strings.  Furthermore, the body part may contain UTF-8
   characters.  Implementations parsing Internet messages need to accept



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   UTF-8 body parts and UTF-8 header fields that are MIME-encoded.
   Thus, this document inherits the security considerations of MIME-
   encoded header fields ([RFC2047] and [RFC3629]).

   Rewriting header fields increases the opportunities for undetected
   spoofing by malicious senders.  However, the rewritten header field
   values are preserved in equivalent MIME form or in newly defined
   header fields which traditional MUAs do not care.

   The techniques described here invalidate methods that depend on
   digital signatures over any part of the message, which includes the
   top-level header fields and body-part header fields.  Depending on
   the specific message being downgraded, at least the following
   techniques are likely to break: DomainKeys Identified Mail (DKIM),
   and possibly S/MIME and Pretty Good Privacy (PGP).  Receivers may
   know they need to update their MUAs, or they don't care.

   While information in any email header field should usually be treated
   with some suspicion, current email systems commonly employ various
   mechanisms and protocols to make the information more trustworthy.
   Information in the new Downgraded-* header fields is not inspected by
   MUAs, and may be even less trustworthy than the traditional header
   fields.  Note that the Downgraded-* header fields could have been
   inserted with malicious intent (and with content unrelated to the
   traditional header fields), however traditional MUAs do not parse
   Downgraded-* header fields.

   In addition, if an Authentication-Results header field [RFC5451] is
   present, traditional MUAs may treat that the digital signatures are
   valid.

   See the "Security Considerations" section in [RFC6530] 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.



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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. ]]

   [RFC5504] registered many "Downgraded-" header fields and requested
   that 'IANA will refuse registration of all field names that start
   with "Downgraded-", to avoid possible conflict with the procedure for
   unknown header fields' preservation described in Section 3.3 of
   [RFC5504].'  However [RFC6530] obsoleted [RFC5504] and this document
   does not use all "Downgraded-" header fields registered by [RFC5504].

   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)

   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







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   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.  Significant comments and suggestions
   were received from John Klensin, Barry Leiba, Randall Gellens, Pete
   Resnick, Martin J. Durst, and other WG participants.

11.  Change History

   [[RFC Editor: Please remove this section prior to publication.]]

   This section is used for tracking the update of this document.  Will
   be removed after finalize.

11.1.  Version 00

   o  Initial version

   o  Imported header field downgrading from RFC 5504

11.2.  Version 01

   o  same as Version 00

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





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   o  Updated IANA considerations

11.5.  Version 04

   o  Removed "Internationalized Address removed" from GROUP and MAILBOX
      downgrading

   o  Updated "Updating RFC 5322"

   o  Compacted new header field definition

   o  Compacted security considerations

   o  Updated IANA considerations to remove obsoleting header fields
      that are registered by RFC 5504

   o  Added a discussion of alternate downgrading models for the POP and
      IMAP cases.

   o  Incorporated a large number of editorial changes to improve
      clarity.

11.6.  Version 05

   o  Some text corrections

   o  Terminology change: only to use non-ASCII address, non-ASCII
      message, non-ASCII string and imported them from RFC 6530 and RFC
      6532

   o  Replace "non-ASCII character" with "non-ASCII string"

   o  Removed 5.1.1.  RECEIVED Downgrading: It's

12.  References

12.1.  Normative References

   [RFC2045]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
              Extensions (MIME) Part One: Format of Internet Message
              Bodies", RFC 2045, November 1996.

   [RFC2047]  Moore, K., "MIME (Multipurpose Internet Mail Extensions)
              Part Three: Message Header Extensions for Non-ASCII Text",
              RFC 2047, November 1996.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.



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   [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.

   [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, RFC 3629, November 2003.

   [RFC3864]  Klyne, G., Nottingham, M., and J. Mogul, "Registration
              Procedures for Message Header Fields", BCP 90, RFC 3864,
              September 2004.

   [RFC4021]  Klyne, G. and J. Palme, "Registration of Mail and MIME
              Header Fields", RFC 4021, March 2005.

   [RFC5322]  Resnick, P., Ed., "Internet Message Format", RFC 5322,
              October 2008.

   [RFC6530]  Klensin, J. and Y. Ko, "Overview and Framework for
              Internationalized Email", RFC 6530, February 2012.

   [RFC6532]  Yang, A., Steele, S., and N. Freed, "Internationalized
              Email Headers", RFC 6532, February 2012.

   [RFC6533]  Hansen, T., Newman, C., and A. Melnikov,
              "Internationalized Delivery Status and Disposition
              Notifications", RFC 6533, February 2012.

12.2.  Informative References

   [RFC5451]  Kucherawy, M., "Message Header Field for Indicating
              Message Authentication Status", RFC 5451, April 2009.

   [RFC5504]  Fujiwara, K. and Y. Yoneya, "Downgrading Mechanism for
              Email Address Internationalization", RFC 5504, March 2009.

Appendix A.  Examples

A.1.  Downgrading Example

   This appendix shows an message downgrading example.  Consider a
   received mail message where:





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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 strings.

   o  The "Message-Id:" header field contains "NON-ASCII-MESSAGE_ID",
      which contains non-ASCII strings.

   o  There is an unknown header field "X-Unknown-Header" which contains
      non-ASCII strings.



   Return-Path: <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: =?UTF-8?Q?NON-ASCII-local@example.com?= :;
   Received: from ... by ...
   Received: from ... by ...
   From: =?UTF-8?Q?DISPLAY-local?=
         =?UTF-8?Q?NON-ASCII-local@example.com?= :;
   To:   =?UTF-8?Q?DISPLAY-remote1?=
         =?UTF-8?Q?NON-ASCII-remote1@example.net?= :;,
         =?UTF-8?Q?DISPLAY-remote2?=
         =?UTF-8?Q?NON-ASCII-remote2@example.com?= :;,
   Cc:   =?UTF-8?Q?DISPLAY-remote3?=
         =?UTF-8?Q?NON-ASCII-remote3@example.org?= :;
   Subject: =?UTF-8?Q?NON-ASCII-SUBJECT?=
   Date: 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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