Email Address Internationalization
Internet Engineering Task Force (IETF)                       K. Fujiwara
(EAI)
Request for Comments: 6857                                          JPRS

Intended status:
Category: Standards Track
Expires: April 25,                                     March 2013

 Post-delivery
ISSN: 2070-1721

 Post-Delivery Message Downgrading for Internationalized Email Messages
                draft-ietf-eai-popimap-downgrade-08.txt

Abstract

   The Email Address Internationalization (SMTPUTF8) extension to SMTP
   allows UTF-8 Unicode characters encoded in UTF-8 and outside the ASCII
   repertoire in mail header fields.  Upgraded POP and IMAP servers
   support internationalized Email messages.  If a POP/IMAP POP or IMAP client does not
   support Email Address Internationalization, POP/IMAP
   servers a POP or IMAP server
   cannot deliver Internationalized Email Headers internationalized messages to the client and cannot
   remove the message.  To avoid the that situation, this document describes
   a conversion mechanism for converting internationalized Email messages
   to be in into the
   traditional message format.  In  As part of the conversion process,
   message elements requiring that require internationalized treatment are recoded
   or removed removed, and receivers are able to know recognize that they received
   messages containing such elements elements, even if they cannot process the
   internationalized elements.

Status of This Memo

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

   Internet-Drafts are working documents an Internet Standards Track document.

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

   Internet-Drafts are draft documents valid the IETF community.  It has
   received public review and has been approved for a maximum publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of six months this document, any errata,
   and how to provide feedback on it may be updated, replaced, or obsoleted by other documents obtained at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on April 25, 2013.
   http://www.rfc-editor.org/info/rfc6857.

Copyright Notice

   Copyright (c) 2012 2013 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
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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4  3
     1.1.  Problem statement Statement  . . . . . . . . . . . . . . . . . . . .  4  3
     1.2.  Possible solutions Solutions . . . . . . . . . . . . . . . . . . . .  4  3
     1.3.  Approach taken Taken in this specification This Specification . . . . . . . . . . .  4
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  6  5
   3.  Email Message Header Fields Field Downgrading . . . . . . . . . . . . . . .  6
     3.1.  Downgrading Method for Each ABNF Element . . . . . . . . .  6
       3.1.1.  UNSTRUCTURED  Unstructured Downgrading . . . . . . . . . . . . . . .  6
       3.1.2.  WORD  Word Downgrading . . . . . . . . . . . . . . . . . . .  6
       3.1.3.  COMMENT  Comment Downgrading  . . . . . . . . . . . . . . . . .  6
       3.1.4.  MIME-VALUE  MIME-Value Downgrading . . . . . . . . . . . . . . . .  7  6
       3.1.5.  DISPLAY-NAME  Display-Name Downgrading . . . . . . . . . . . . . . .  7  6
       3.1.6.  DOMAIN  Domain Downgrading . . . . . . . . . . . . . . . . . .  7  6
       3.1.7.  GROUP  Group Downgrading  . . . . . . . . . . . . . . . . . .  7
       3.1.8.  MAILBOX  Mailbox Downgrading  . . . . . . . . . . . . . . . . .  8  7
       3.1.9.  TYPED-ADDRESS  Type-Addr Downgrading  . . . . . . . . . . . . . . . .  8
       3.1.10. ENCAPSULATION: Encapsulation: A Last Resort . . . . . . . . . . . . .  8
     3.2.  Downgrading Method for Each Header Field . . . . . . . . . 10
       3.2.1.  Address Header Fields That Contain <address>s <address>
               Elements . . . . . . . . . . . . . . . . . . . . . . . 10
       3.2.2.  Downgrading  Non-ASCII Strings in Comments  . . . <comment> Elements  . . . . . . . 11
       3.2.3.  Message-ID Header Fields . . . . . . . . . . . . . . . 11
       3.2.4.  Received Header Field  . . . . . . . . . . . . . . . . 11
       3.2.5.  MIME Content Header Fields . . . . . . . . . . . . . . 12
       3.2.6.  Non-ASCII Characters in <unstructured>  . . . . . . . . . . Elements  . . . 12
       3.2.7.  Non-ASCII Characters in <phrase>  . . . . . . . . . . Elements  . . . . . . 12
       3.2.8.  Other Header Fields  . . . . . . . . . . . . . . . . . 12
   4.  MIME Downgrading . . . . . . . . . . . . . . . . . Body Parts and Delivery Status Notifications  . . . . . . 12
     4.1.  MIME Body-Part Body Part Header Field Downgrading  . . . . . . . . . 13
     4.2.  Delivery Status Notification downgrading Downgrading . . . . . . . . . 13
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . . 13
   6.  Implementation Notes Note: Encoded-Word Encoding . . . . . . . . . . . . . . . . . . . . . 14
     6.1.  RFC 2047 Encoding  . . . . . . . . . . . . . . . . . . . . 14
   7.  IANA Considerations 14
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 15
     7.1.  Obsolescence of Existing Downgraded-* Header Fields  . . . 15
     7.2.  Registration of New Downgraded-* Header Fields . . . . . . 15
   8.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 16
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 16
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 16
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 18 17
   Appendix A.  Examples  . . . . . . . . . . . . . . . . . . . . . . 18
     A.1.  Downgrading Example . . . . . . . . . . . . . . . . . . . 18
   Appendix B.  Change History  . . . . . . . . . . . . . . . . . . . 20
     B.1.  Version 00 . . . . . . . . . . . . . . . . . . . . . . . . 20
     B.2.  Version 01 . . . . . . . . . . . . . . . . . . . . . . . . 20
     B.3.  Version 02 . . . . . . . . . . . . . . . . . . . . . . . . 20
     B.4.  Version 03 . . . . . . . . . . . . . . . . . . . . . . . . 21
     B.5.  Version 04 . . . . . . . . . . . . . . . . . . . . . . . . 21
     B.6.  Version 05 . . . . . . . . . . . . . . . . . . . . . . . . 21
     B.7.  Version 06 . . . . . . . . . . . . . . . . . . . . . . . . 21
     B.8.  Version 07 . . . . . . . . . . . . . . . . . . . . . . . . 22
     B.9.  Version 08 . . . . . . . . . . . . . . . . . . . . . . . . 22

1.  Introduction

1.1.  Problem statement Statement

   Traditional (legacy) mail systems, which are defined by the Internet
   Message Format [RFC5322] and other specifications, allow only ASCII
   characters in mail header field values.  The SMTPUTF8 extension ([RFC6530],
   [RFC6530] [RFC6531] and
   [RFC6532]) allow raw [RFC6532] allows Unicode characters encoded in
   UTF-8 [RFC3629] in those these mail header fields.  "Raw non-ASCII strings"
   refers to strings of those characters in which at least one of them
   is not part of the ASCII repertoire.

   If a header field contains non-ASCII strings, POP/IMAP servers a POP or IMAP server
   cannot deliver Internationalized Email Headers internationalized messages to legacy clients which does that do
   not send UTF8 command commands or have UTF8 capability, and capability.  Also, because they
   have no obvious or standardized way to explain what is going on to those
   clients, a POP or IMAP server cannot even safely discard the message.

1.2.  Possible solutions Solutions

   There are four plausible approaches to the problem, with the problem.  The preferred one depending
   approach depends 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: Mail User Agent (MUA) clients.  The
   four approaches are as follows:

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

   2.  The  A new, surrogate, message may be downgraded created by downgrading the
       original one in the POP or IMAP server, server in a way that preserves
       maximum information at the expense of some
       complexity, complexity and that
       does not create security or operational problems in the mail
       system.  These surrogate messages are referred to as "downgraded"
       in this specification and as "surrogate messages" elsewhere.

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

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

1.3.  Approach taken Taken in this specification This Specification

   This specification describes the second of those these options.  It is
   worth noticing noting that, at least in the general case, none of these
   options preserve preserves 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 one of the available "least bad"
   options.  While this document specifies a well designed well-designed mechanism, it
   is only an interim solution while clients are being upgraded
   [I-D.ietf-eai-rfc5721bis] [I-D.ietf-eai-5738bis].
   [RFC6855] [RFC6856].

   This message downgrading mechanism converts mail header fields to an
   all-ASCII representation.  The POP/IMAP servers POP or IMAP server can use the
   downgrading mechanism and then deliver the Internationalized Email internationalized message
   as
   in a traditional form.  Receivers can form, which allows receivers to know they received some whether a
   message is internationalized messages or some unknown or broken messages. broken.

   The Internationalized Mail Header specification [RFC6532] allows
   UTF-8 characters (see Section 2) to be used in mail header fields and
   MIME header fields.  The Internationalized Mail Transport
   specification [RFC6531] allows UTF-8 characters to be used in some
   trace header fields.  The message downgrading mechanism specified
   here describes the conversion method from the by which internationalized messages that are defined in [RFC6530], and
   [RFC6530] [RFC6532] are converted to the traditional email messages defined in
   [RFC5322].

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

   Downgrading consists of the following three two parts:

   o  New header field definitions

   o  Email header field downgrading

   o  MIME header field downgrading

   Email header field downgrading is described in Section 3.  It
   generates ASCII-only header fields.

   In Section 3.1.10 of this document, header

   Header fields starting with
   "Downgraded-" Downgraded- are introduced. introduced in
   Section 3.1.10.  They preserve the information that appeared in the
   original header fields.

   The definition of MIME header fields in Internationalized Email
   Messages internationalized messages is
   described in [RFC6532]. RFC 6532.  A delivery status notification may contain
   non-ASCII addresses.  MIME header field downgrading is described in
   Section 4.1.  Delivery status notification downgrading is described
   in Section 4.2.  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
   POP or IMAP client which that does not support UTF8 extensions as defined
   for POP3 [UTF8 command] "UTF8 command" and IMAP ["ENABLE UTF8=ACCEPT" command] "ENABLE UTF8=ACCEPT command" does
   not
   know recognize the internationalized message format described in [RFC6532].

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

   Many of the specialized terms used in this specification are defined
   in the
   Overview other documents.  They include "Overview and Framework for
   Internationalized Email Email" [RFC6530], in the
   mail message specifications Internet Message Format
   specification [RFC5322], or in and some of the basic MIME documents
   [RFC2045] [RFC2183].  This specification makes extensive use of the
   MIME Message Header Extensions [RFC2047] [RFC2183] and extended MIME parameter
   encodings [RFC2231].  For convenience, both are described as
   "encoded-words" or "encoded-word encoding".  All of the encoded-words
   generated according to this specification use UTF-8 as their charset.

   The terms "U-label",
   "A-label" "A-label", and "IDNA" are used with as defined in the definitions from
   IDNA Definitions document [RFC5890].  The terms "ASCII address",
   "non-ASCII address", "SMTPUTF8", "message", and "internationalized
   message" are used with the definitions
   from [RFC6530]. as defined RFC 6530.  The term "non-ASCII string"
   is used with the
   definitions from definition provided in the Internationalized Email
   Headers document [RFC6532].  The term "UTF-8 character" is used
   informally in this document to denote a Unicode character, encoded in
   UTF-8, outside the ASCII repertoire.  Such characters are more
   formally described using the ABNF element <UTF8-non-ascii>, defined
   in RFC 6532.

   This document refers to the Augmented Backus-Naur Form (ABNF)
   [RFC5234] elements that appear in RFC 5322 and RFC 2045.  RFC 5322
   describes the ABNF elements <CFWS>, <comment>, <display-name>,
   <group>, <id-left>, <id-right>, <mailbox>, <quoted-string>,
   <unstructured>, and <word>.  RFC 2045 describes the ABNF element
   <value>.  Section 3.3 of the Internationalized Mail Transport
   specification [RFC6531] and Section 3.2 of the Internationalized
   Email Headers document [RFC6532] updated <domain> to allow non-ASCII
   characters.

   Some additional terms are defined locally in-line below.

3.  Email Message Header Fields Field Downgrading

   This section defines the conversion method to ASCII for converting each header field that
   may contain non-ASCII strings. strings into ASCII.  Section 3.1 describes
   rewriting the
   methods for rewriting each ABNF element.  Section 3.2 describes
   rewriting the
   methods for rewriting each header field.

3.1.  Downgrading Method for Each ABNF Element

   Header field downgrading is defined below for each ABNF element.
   Converting
   Conversion of the header field terminates when no non-ASCII strings characters other
   than those in the ASCII repertoire remain in the header field.

   [RFC5322] describes ABNF elements <group>, <mailbox>, <unstructured>,
   <word>, <comment>, <display-name>.  [RFC2045] describes ABNF element
   <value>. <domain> is updated to allow non-ASCII characters in Section
   3.3 of [RFC6531] and Section 3.2 of [RFC6532].

3.1.1.  UNSTRUCTURED  Unstructured Downgrading

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

3.1.2.  WORD  Word Downgrading

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

3.1.3.  COMMENT  Comment Downgrading

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

3.1.4.  MIME-VALUE  MIME-Value Downgrading

   If the header field has any <value> elements defined by [RFC2045] and
   the elements that contain
   non-ASCII strings, remove any <CFWS> that appear outside DQUOTE
   [RFC5234] that appear in those elements, then encode the <value>
   elements
   according to as extended MIME parameter encodings [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.

3.1.5.  DISPLAY-NAME  Display-Name Downgrading

   If the header field has any <address> (<mailbox> or <group>) elements
   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 as encoded-
   words.  Display-Name downgrading is uses the same algorithm as
   WORD Word
   downgrading.

3.1.6.  DOMAIN  Domain Downgrading

   If the header field has any <domain> elements that contain U-labels,
   rewrite the non-ASCII domain name into an ASCII domain name using
   A-labels as specified in IDNA [RFC5891].

3.1.7.  GROUP  Group Downgrading

   <group> is defined in Section 3.4 of the Internet Message Format
   specification [RFC5322].  The <group> elements element may contain <mailbox>es which <mailbox>
   elements that contain non-ASCII addresses.

   If a <group> element contains <mailbox> elements and one of
   <mailbox>es those
   <mailbox> elements contains a non-ASCII <local-part>, rewrite the
   <group> element as

   display-name " " ENCODED_WORD " :;"

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

   Otherwise, the <group> element does not contain non-ASCII <local-
   part>. contains an ASCII-only <local-part>.
   If the <group> element contain contains non-ASCII <mailbox>es, <mailbox> elements, they
   contains
   contain non-ASCII domain names.  Rewrite the non-ASCII domain names
   into ASCII domain names using A-labels as specified in IDNA [RFC5891].  Generated <mailbox>es
   <mailbox> elements contain ASCII addresses only.

3.1.8.  MAILBOX  Mailbox Downgrading

   If the <local-part> of the <mailbox> element does not contain non- contains no characters
   other than those in the ASCII characters, repertoire, the <domain> element contains may
   contain non-ASCII characters.  Rewrite the non-ASCII domain name names
   into ASCII domain name names using A-labels as specified in IDNA [RFC5891].

   Otherwise, the <local-part> contains may contain non-ASCII characters.  The non-
   ASCII
   <local-part> that contains characters outside the ASCII repertoire
   has no equivalent format for ASCII addresses.  The <addr-spec>
   element that contains non-ASCII strings may appear in two forms as:

   "<" addr-spec ">"

   or

   addr-spec

   Rewrite both as:

   ENCODED-WORD " :;"

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

3.1.9.  TYPED-ADDRESS  Type-Addr Downgrading

   If the header field contains <utf-8-type-addr> and the <utf-8-type-
   addr>
   <utf-8-type-addr> contains raw non-ASCII strings, strings (<UTF8-non-ascii>),
   it is in utf-8-address form. form [RFC6533].  Convert it to
   utf-8-addr-xtext form.  Those forms are described in form [RFC6533].  COMMENT  Comment downgrading is also
   performed in this case.  If the address type is unrecognized and the
   header field contains non-
   ASCII non-ASCII strings, then fall back to using ENCAPSULATION
   Encapsulation on the entire header field as specified in
   Section 3.1.10.

3.1.10.  ENCAPSULATION:  Encapsulation: A Last Resort

   As a last resort resort, when header fields cannot be converted as discussed
   in the previous section, subsection, the fields are deleted and replaced by
   specialized new header fields.  Those fields are defined to preserve,
   in encoded form, as much information as possible from the header
   field values of the incoming message.  This mechanism is known as
   Encapsulation downgrading in this specification because it preserves
   the original information in a different form.  The syntax of these
   new header fields is:

   fields                   =/ downgraded

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

   Applying this procedure to the "Received:" header field is
   prohibited.
   ENCAPSULATION Downgrading  Encapsulation downgrading is allowed for "Message-ID", "Message-ID:",
   "In-Reply-To:", "References:", "Original-Recipient" "Original-Recipient:", and "Final-
   Recipient"
   "Final-Recipient:" header fields.

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

   1.  Generate a new header field.

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

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

   2.  Treat the initial new header field value as if it were
       unstructured, and then apply [RFC2047] the encoded-word 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.

3.2.  Downgrading Method for Each Header Field

   The Mail and MIME Header Fields document [RFC4021] establishes a
   registry of header fields.  This section describes the downgrading
   method for each header field. field listed in that registry as of the date
   of publication of this specification.

   If the whole entire mail header field does not contain non-ASCII strings, contains no characters other than
   those in the ASCII repertoire, email header field downgrading is not
   required.  Each header field's downgrading method is described below.

3.2.1.  Address Header Fields That Contain <address>s <address> Elements

   From:
   Sender:
   To:
   Cc:
   Bcc:
   Reply-To:
   Resent-From:
   Resent-Sender:
   Resent-To:
   Resent-Cc:
   Resent-Bcc:
   Resent-Reply-To:
   Return-Path:
   Disposition-Notification-To:

   If the header field contains non-ASCII characters, first perform
   COMMENT
   Comment downgrading and DISPLAY-NAME Display-Name downgrading as described in the
   corresponding subsections of Section 3.1.  If the header field still
   contains non-ASCII characters after that, do characters, complete the following two steps:

   1.  If the header field contains <group> elements that contain non-
       ASCII
       non-ASCII addresses, perform GROUP Group downgrading on those elements.

   2.  If the header field contains <mailbox> elements that contain non-
       ASCII
       non-ASCII addresses, perform MAILBOX Mailbox downgrading on those
       elements.

   This procedure may generate empty <group> elements in "From:",
   "Sender:" the "From:" and "Reply-To:"
   "Sender:" header fields.
   [I-D.leiba-5322upd-from-group]  The Group Syntax document [RFC6854] updates
   the Internet Message Format specification [RFC5322] to allow (empty)
   <group> elements in "From:", "Sender:" the "From:" and "Reply-To:" "Sender:" header fields.

3.2.2.  Downgrading  Non-ASCII Strings in Comments <comment> Elements

   Date:
   Resent-Date:
   MIME-Version:
   Content-ID:
   Content-Transfer-Encoding:
   Content-Language:
   Accept-Language:
   Auto-Submitted:

   These

   Except in comments, these header fields do not contain non-ASCII strings except characters
   other than those in
   comments. the ASCII repertoire.  If the header field
   contains UTF-8 characters in comments, perform COMMENT Comment downgrading.

3.2.3.  Message-ID Header Fields

   Message-ID:
   Resent-Message-ID:
   In-Reply-To:
   References:

   Perform ENCAPSULATION as specified

   If there are non-ASCII strings in Section 3.1.10. <id-left> or <id-right> elements,
   perform Encapsulation.  Otherwise, the header field contains UTF-8
   characters in comments and Comment downgrading should be performed.

3.2.4.  Received Header Field

   Received:

   If <domain> elements or <mailbox> elements contains contain U-labels, perform
   DOMAIN
   Domain downgrading as specified in Section 3.1.6.  Comments may
   contain non-ASCII strings, strings; if so, perform COMMENT Comment downgrading.

   After the DOMAIN Domain downgrading and the COMMENT Comment downgrading, if the FOR
   "FOR" clause contains a non-ASCII <local-part>, remove the "FOR" FOR
   clause.  If the ID "ID" clause contains a non-ASCII values, value, remove the "ID" ID
   clause.

3.2.5.  MIME Content Header Fields

   Content-Type:
   Content-Disposition:

   Perform MIME-VALUE downgrading

   If there are non-ASCII strings in <value> or <CFWS> elements, perform
   MIME-Value and COMMENT Comment downgrading.

3.2.6.  Non-ASCII Characters in <unstructured> Elements

   Subject:
   Comments:
   Content-Description:

   Perform UNSTRUCTURED

   If non-ASCII strings are present in <unstructured> elements, perform
   Unstructured downgrading.

3.2.7.  Non-ASCII Characters in <phrase> Elements

   Keywords:

   Perform WORD

   If non-ASCII strings are present in <phrase> elements, perform Word
   downgrading.

3.2.8.  Other Header Fields

   There are other

   Other header fields that contain non-ASCII strings.  They are user-defined and missing from not covered in this document, document (such as
   implementation-specific or future defined user-defined fields) might also contain
   non-ASCII strings.  Any header fields.  They are treated as "Optional Fields" and their field
   values are that does not have a conversion
   method defined above will be in this category and treated as unstructured described follows.

   If there are non-ASCII strings present in Section 3.6.8 of
   [RFC5322].

   Perform UNSTRUCTURED the header fields, perform
   Unstructured downgrading.

   If the software understands the header field's structure and a
   downgrading algorithm other than UNSTRUCTURED Unstructured is applicable, that
   software SHOULD use that algorithm; UNSTRUCTURED Unstructured downgrading is used
   as a last resort.
   when there is no other option.

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

4.  MIME Downgrading Body Parts and Delivery Status Notifications

   Both the MIME Body-Part body part header fields [RFC2045] [RFC6532] and the
   contents of a delivery status notification [RFC6533] may contain
   non-ASCII characters.

4.1.  MIME Body-Part Body Part Header Field Downgrading

   RFC 6532 specifies an extension that permits MIME body-part header fields may fields,
   including body part header fields, to contain non-ASCII strings [RFC6532]. strings.
   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 body part header
   field's downgrading.  Each  The downgrading methods used for the MIME body
   part header field's downgrading method is
   described below.  COMMENT downgrading, MIME-VALUE downgrading, fields Content-ID, Content-Type, Content-Disposition, and
   UNSTRUCTURED downgrading
   Content-Description are described in Section 3.

   Content-ID:
      The "Content-ID:" header field does not contain non-ASCII strings
      except in comments.  If the same as those used for 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. fields
   of the same name described in Section 3.2

4.2.  Delivery Status Notification downgrading Downgrading

   If the message contains a delivery status notification defined at (see Section 6
   of [RFC3461], the SMTP DSN Extension [RFC3461]), perform the following tests and
   conversions.

   If there are "Original-Recipient:" and "Final-Recipient:" header
   fields, and the header fields contain non-ASCII strings, perform
   TYPED-ADDRESS
   Type-Addr downgrading.

5.  Security Considerations

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

   A downgraded message's

   After downgrading as specified in this document, the header fields of
   a message will contain ASCII characters only.
   But they still contain MIME-encapsulated header fields that contain
   non-ASCII strings.  Furthermore, only, some of them in
   encoded-word form.  Nothing in this document or other SMTPUTF8
   specifications [RFC6530] [RFC6531] alters the body part may contain UTF-8
   characters.  Implementations parsing Internet messages need to accept
   UTF-8 body parts and UTF-8 header fields basic properties of
   MIME that are MIME-encoded. allow characters outside the ASCII repertoire in encodings
   as specified for them.  Thus, this document inherits the security
   considerations of MIME-
   encoded associated with MIME-encoded header fields ([RFC2047] as
   specified in RFC 2047 [RFC2047] and [RFC3629]). with UTF-8 itself as specified in
   RFC 3629 [RFC3629].

   Rewriting header fields increases the opportunities for undetected
   spoofing by malicious senders.  However, the rewritten header field
   values are preserved in equivalent MIME form or in newly defined
   header fields for which traditional MUAs have no special processing
   procedures.

   The techniques described here may invalidate methods that depend on
   digital signatures over any part of the message, which includes the
   top-level header fields and body-part body part header fields.  Depending on
   the specific message being downgraded, at least the following
   techniques are likely to break: DomainKeys Identified Mail (DKIM), (DKIM) and
   possibly S/MIME and Pretty Good Privacy (PGP).  The downgrade
   mechanism SHOULD NOT remove signatures even if the signatures will
   fail validation after downgrading.  As much of the information as
   possible from the original message SHOULD be preserved.  In addition,
   MUAs may be able to use the presence of an Authentication-Results
   header field [RFC5451] to assess whether the digital signatures were
   valid before the header fields were downgraded.

   While information in any email header field should usually be treated
   with some suspicion, current email systems commonly employ various
   mechanisms and protocols to make the information more trustworthy.
   Information in the new Downgraded-* header fields is not inspected by
   traditional MUAs, 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 fields); however, traditional
   MUAs do not parse evaluate 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 Security Considerations sections in
   [I-D.leiba-5322upd-from-group] the Group Syntax document
   [RFC6854] and the Internationalized Email Framework [RFC6530] for
   more discussion.

6.  Implementation Notes

6.1.  RFC 2047 Note: Encoded-Word Encoding

   While [RFC2047] has a the specification of encoded-words includes specific algorithm to deal rules for
   dealing with whitespace in adjacent encoded words, words [RFC2047], there
   are a number of deployed implementations that fail to implement the
   algorithm correctly.  As a result, whitespace behavior is somewhat unpredictable
   unpredictable, in practice practice, when multiple encoded words are used.

   While RFC 5322 states that implementations SHOULD limit lines to not more than 78 characters,
   characters or less, implementations MAY choose to allow overly long
   encoded words in
   order to work around faulty [RFC2047] implementations. implementations of encoded-words.
   Implementations that choose to do so SHOULD have an optional
   mechanism to limit line length to 78 characters.

7.  IANA Considerations

   [[RFC Editor: Please change "is asked to" to "has" (and change the
   verb correspondingly) when the IESG approval and IANA actions are
   complete.]]

   The experimental specification from which this document was partially
   derived [RFC5504] specified specifies that no new header fields be registered that
   begin beginning with "Downgraded-".
   Downgraded- are to be registered.  That restriction is now lifted,
   and this document makes a new set of registrations, replacing the
   experimental fields with standard ones.

7.1.  Obsolescence of Existing Downgraded-* Header Fields

   The "Downgraded-*" Downgraded-* header fields that were registered as experimental
   fields in [RFC5504] RFC 5504 are no longer in use.  IANA is asked to change has changed the status
   from "experimental" to "obsoleted" for every name in the
   Permanent "Permanent
   Message Header Field Names" registry that begins began with
   "Downgraded-". Downgraded-.

7.2.  Registration of New Downgraded-* Header Fields

   [[RFC Editor: Please change "should be" to "have been" when the IANA
   actions are complete.]]

   The following header fields should be have been registered in the Permanent "Permanent
   Message Header Field Names" registry, in accordance with the
   procedures set out in the Header Field Registration document
   [RFC3864].

   Header field name:  Downgraded-Message-Id
   Applicable protocol:  mail
   Status:  standard
   Author/change controller:  IETF
   Specification document(s):  This document (Section 3.1.10)

   Header field name:  Downgraded-In-Reply-To
   Applicable protocol:  mail
   Status:  standard
   Author/change controller:  IETF
   Specification document(s):  This document (Section 3.1.10)

   Header field name:  Downgraded-References
   Applicable protocol:  mail
   Status:  standard
   Author/change controller:  IETF
   Specification document(s):  This document (Section 3.1.10)

   Header field name:  Downgraded-Original-Recipient
   Applicable protocol:  mail
   Status:  standard
   Author/change controller:  IETF
   Specification document(s):  This document (Section 3.1.10)

   Header field name:  Downgraded-Final-Recipient
   Applicable protocol:  mail
   Status:  standard
   Author/change controller:  IETF
   Specification document(s):  This document (Section 3.1.10)

8.  Acknowledgements

   This document draws heavily from the experimental in-transit message
   downgrading procedure described in RFC 5504 [RFC5504]. 5504.  The
   contribution contributions of the co-author
   coauthor of that earlier document, Y. Yoneya, are gratefully
   acknowledged.  Significant comments and suggestions were received
   from John Klensin, Barry Leiba, Randall Gellens, Pete Resnick, Martin
   J. Durst, and other WG participants.

9.  References

9.1.  Normative References

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

   [RFC2047]  Moore, K., "MIME (Multipurpose Internet Mail Extensions)
              Part Three: Message Header Extensions for Non-
                                   ASCII 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
                                   , Languages, and Continuations", RFC 2231,
              November 1997.

   [RFC3461]  Moore, K., "Simple Mail Transfer Protocol (SMTP) Service
              Extension for Delivery Status Notifications (DSNs)",
              RFC 3461, January 2003.

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

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

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

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

   [RFC5890]  Klensin, J., "Internationalized Domain Names for
              Applications (IDNA): Definitions and Document Framework",
              RFC 5890, August 2010.

   [RFC5891]  Klensin, J., "Internationalized Domain Names in
              Applications (IDNA): Protocol", RFC 5891, August 2010.

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

   [RFC6531]  Yao, J. and W. Mao, "SMTP Extension for Internationalized
              Email", RFC 6531, February 2012.

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

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

   [I-D.leiba-5322upd-from-group]

   [RFC6854]  Leiba, B., "Update to Internet Message Format to Allow
              Group Syntax in the "From:" and "Sender:" Header
                                   Fields",
                                   draft-leiba-5322upd-from-group-06
                                   (work in progress), October 2012.

   [I-D.ietf-eai-rfc5721bis]       Gellens, R., Newman, C., Yao, J., and
                                   K. Fujiwara, "POP3 Support for
                                   UTF-8", draft-ietf-eai-rfc5721bis-08
                                   (work in progress), October 2012.

   [I-D.ietf-eai-5738bis] Header Fields",
              RFC 6854, January 2013.

   [RFC6855]  Resnick, P., Newman, C., and S. Shen, "IMAP Support for
              UTF-8",
                                   draft-ietf-eai-5738bis-09 (work in
                                   progress), August 2012. RFC 6855, March 2013.

   [RFC6856]  Randy, R., Newman, C., Yao, J., and K. Fujiwara, "Post
              Office Protocol Version 3 (POP3) Support for UTF-8",
              RFC 6856, January 2013.

9.2.  Informative References

   [RFC5234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, January 2008.

   [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 a message downgrading example.  Consider a
   received mail message where:

   o  The sender address is a non-ASCII address,
      "NON-ASCII-LOCAL@example.com".  Its display-name is "DISPLAY-
      LOCAL".
      "DISPLAY-LOCAL".

   o  The "To:" header field contains two non-ASCII addresses,
      "NON-ASCII-REMOTE1@example.net" and
      "NON-ASCII-REMOTE2@example.com" Its
      "NON-ASCII-REMOTE2@example.com".  Their display-names are "DISPLAY-
      REMOTE1"
      "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".
      "DISPLAY-REMOTE3".

   o  Four display names display-names contain non-ASCII characters.

   o  The Subject "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" "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: Mon, 30 Jul 2012 01:23:45 -0000
   Message-Id: NON-ASCII-MESSAGE_ID
   Mime-Version: 1.0
   Content-Type: text/plain; charset="UTF-8"
   Content-Transfer-Encoding: 8bit
   X-Unknown-Header: NON-ASCII-CHARACTERS

   MAIL_BODY

                 Figure 1: Received message Message in a mail drop Maildrop

   The downgraded message is shown in Figure 2.  "Return-Path:",
   "From:", "To:" "To:", and "Cc:" header fields are rewritten.  "Subject:"
   and "X-Unknown-Header:" header fields are encoded using [RFC2047]. as encoded-words.
   The "Message-Id:" header field is encapsulated as a
   "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: Mon, 30 Jul 2012 01:23:45 -0000
   Downgraded-Message-Id: =?UTF-8?Q?MESSAGE_ID?=
   Mime-Version: 1.0
   Content-Type: text/plain; charset="UTF-8"
   Content-Transfer-Encoding: 8bit
   X-Unknown-Header: =?UTF-8?Q?NON-ASCII-CHARACTERS?=

   MAIL_BODY
                       Figure 2: Downgraded message

Appendix B.  Change History

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

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

B.1.  Version 00

   o  Initial version

   o  Imported header field downgrading from RFC 5504

B.2.  Version 01

   o  same as Version 00

B.3.  Version 02

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

   o  Added GROUP Downgrading

B.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

B.5.  Version 04

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

   o  Updated "Updating RFC 5322"

   o  Compacted new header field definition

   o  Compacted security considerations

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

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

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

B.6.  Version 05

   o  Some text corrections

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

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

   o  Removed 5.1.1.  RECEIVED Downgrading

B.7.  Version 06

   o  Removed "Updating RFC 5322"
   o  Added reference to draft-leiba-5322upd-from-group

B.8.  Version 07

   o  Updated by WGLC comments

   o  Fixed Received downgrading and added to refer "RFC 6531", "RFC
      5890", "RFC 5891"

   o  Added Domain downgrading for Received, Group and Mailbox

   o  Swapped section 3 and 4

B.9.  Version 08

   o  Updated by IETF Last call and IESG comments

   o  Removed "Address Header Fields with Typed Addresses" and added
      "Delivery Status Notification downgrading" in MIME downgrading

   o  Added a space between display-name and ENCODED_WORD.

   o  Moved "ENCAPSULATION: A Last Resort" from section 4 to section
      3.1.10.

   o  Updated address header fields downgrading

   o  Updated introduction, security considerations and iana
      considerations 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@jprs.co.jp