[Docs] [txt|pdf|xml|html] [Tracker] [WG] [Email] [Diff1] [Diff2] [Nits]

Versions: 00 01 02 03 04 05 06 07 08 09 10 11 12 RFC 5504

Email Address Internationalization                      K. Fujiwara, Ed.
(EAI)                                                     Y. YONEYA, Ed.
Internet-Draft                                                      JPRS
Intended status: Experimental                              March 2, 2009
Expires: September 3, 2009


      Downgrading mechanism for Email Address Internationalization
                    draft-ietf-eai-downgrade-12.txt

Status of this Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-
   Drafts.

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

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt.

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

   This Internet-Draft will expire on September 3, 2009.

Copyright Notice

   Copyright (c) 2009 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 in effect on the date of
   publication of this document (http://trustee.ietf.org/license-info).
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.

Abstract

   Traditional mail systems handle only ASCII characters in SMTP
   envelope and mail header fields.  The Email Address



Fujiwara & YONEYA       Expires September 3, 2009               [Page 1]

Internet-Draft             UTF8SMTP Downgrade                 March 2009


   Internationalization (UTF8SMTP) extension allows UTF-8 characters in
   SMTP envelope and mail header fields.  To avoid rejecting
   internationalized Email messages when a server in the delivery path
   does not support the UTF8SMTP extension, some sort of converting
   mechanism is required.  This document describes a downgrading
   mechanism for Email Address Internationalization.  Note that this is
   a way to downgrade, not tunnel.  There is no associated up-conversion
   mechanism, although internationalized email clients might use
   original internationalized addresses or other data when displaying or
   replying to downgraded messages.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  5
   3.  New header fields definition . . . . . . . . . . . . . . . . .  5
     3.1.   Envelope information preservation header fields . . . . .  6
     3.2.   Address header field preservation header fields . . . . .  6
     3.3.   Unknown header fields preservation header fields  . . . .  7
   4.  SMTP Downgrading . . . . . . . . . . . . . . . . . . . . . . .  8
     4.1.   Path element downgrading  . . . . . . . . . . . . . . . .  8
     4.2.   ORCPT downgrading . . . . . . . . . . . . . . . . . . . .  9
   5.  Email header fields downgrading  . . . . . . . . . . . . . . .  9
     5.1.   Downgrading method for each ABNF element  . . . . . . . .  9
       5.1.1.  RECEIVED downgrading . . . . . . . . . . . . . . . . .  9
       5.1.2.  UNSTRUCTURED downgrading . . . . . . . . . . . . . . .  9
       5.1.3.  WORD downgrading . . . . . . . . . . . . . . . . . . . 10
       5.1.4.  COMMENT downgrading  . . . . . . . . . . . . . . . . . 10
       5.1.5.  MIME-VALUE downgrading . . . . . . . . . . . . . . . . 10
       5.1.6.  DISPLAY-NAME downgrading . . . . . . . . . . . . . . . 10
       5.1.7.  MAILBOX downgrading  . . . . . . . . . . . . . . . . . 10
       5.1.8.  ENCAPSULATION downgrading  . . . . . . . . . . . . . . 11
       5.1.9.  TYPED-ADDRESS downgrading  . . . . . . . . . . . . . . 11
     5.2.   Downgrading method for each header field  . . . . . . . . 11
       5.2.1.  Address header fields which contain <address>s . . . . 11
       5.2.2.  Address header fields with typed addresses . . . . . . 12
       5.2.3.  Downgrading Non-ASCII in comments  . . . . . . . . . . 12
       5.2.4.  Received header field  . . . . . . . . . . . . . . . . 12
       5.2.5.  MIME Content header fields . . . . . . . . . . . . . . 12
       5.2.6.  Non-ASCII in <unstructured>  . . . . . . . . . . . . . 13
       5.2.7.  Non-ASCII in <phrase>  . . . . . . . . . . . . . . . . 13
       5.2.8.  Other header fields  . . . . . . . . . . . . . . . . . 13
   6.  MIME body part header fields downgrading . . . . . . . . . . . 13
   7.  Security considerations  . . . . . . . . . . . . . . . . . . . 14
   8.  Implementation notes . . . . . . . . . . . . . . . . . . . . . 15
     8.1.   RFC 2047 encoding . . . . . . . . . . . . . . . . . . . . 15
     8.2.   Trivial downgrading . . . . . . . . . . . . . . . . . . . 16



Fujiwara & YONEYA       Expires September 3, 2009               [Page 2]

Internet-Draft             UTF8SMTP Downgrade                 March 2009


     8.3.   7bit transport consideration  . . . . . . . . . . . . . . 16
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 17
   10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 19
   11. Change History . . . . . . . . . . . . . . . . . . . . . . . . 19
     11.1.  draft-yoneya-ima-downgrade: Version 00  . . . . . . . . . 19
     11.2.  draft-yoneya-ima-downgrade: Version 01  . . . . . . . . . 19
     11.3.  draft-ietf-eai-downgrade: Version 00  . . . . . . . . . . 20
     11.4.  draft-ietf-eai-downgrade: Version 01  . . . . . . . . . . 20
     11.5.  draft-ietf-eai-downgrade: Version 02  . . . . . . . . . . 20
     11.6.  draft-ietf-eai-downgrade: Version 03  . . . . . . . . . . 20
     11.7.  draft-ietf-eai-downgrade: Version 04  . . . . . . . . . . 20
     11.8.  draft-ietf-eai-downgrade: Version 05  . . . . . . . . . . 20
     11.9.  draft-ietf-eai-downgrade: Version 06  . . . . . . . . . . 21
     11.10. draft-ietf-eai-downgrade: Version 07  . . . . . . . . . . 21
     11.11. draft-ietf-eai-downgrade: Version 08  . . . . . . . . . . 21
     11.12. draft-ietf-eai-downgrade: Version 09  . . . . . . . . . . 21
     11.13. draft-ietf-eai-downgrade: Version 10  . . . . . . . . . . 21
     11.14. draft-ietf-eai-downgrade: Version 11  . . . . . . . . . . 21
     11.15. draft-ietf-eai-downgrade: Version 12  . . . . . . . . . . 21
   12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 22
     12.1.  Normative References  . . . . . . . . . . . . . . . . . . 22
     12.2.  Informative References  . . . . . . . . . . . . . . . . . 23
   Appendix A.  Examples  . . . . . . . . . . . . . . . . . . . . . . 23
     A.1.   Downgrading example 1 . . . . . . . . . . . . . . . . . . 23
     A.2.   Downgrading example 2 . . . . . . . . . . . . . . . . . . 26
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 28

























Fujiwara & YONEYA       Expires September 3, 2009               [Page 3]

Internet-Draft             UTF8SMTP Downgrade                 March 2009


1.  Introduction

   Traditional mail systems which are defined by [RFC5321] and [RFC5322]
   allow ASCII characters in SMTP envelope and mail header field values.
   The UTF8SMTP extension [RFC4952], [RFC5335] and [RFC5336] allows
   UTF-8 characters in SMTP envelope and mail header field values.

   If an envelope address or header field contains non-ASCII characters,
   the message cannot be delivered unless every system in the delivery
   path supports UTF8SMTP.  This document describes a downgrading
   mechanism to avoid rejection of such messages when a server which
   does not support the UTF8SMTP extension is encountered.  Downgrading
   mechanism converts envelope and header fields to an all-ASCII
   representation.

   [RFC5335] allows UTF-8 characters to be used in mail header fields
   and MIME header fields.  The downgrading mechanism specified here
   converts mail header fields and MIME header fields to ASCII.

   This document does not change any protocols except by defining new
   header fields.  It describes the conversion method from the
   internationalized email envelopes/messages which are defined in
   [RFC4952] [RFC5335] [RFC5336] to the traditional email envelopes/
   messages which are defined in [RFC5321] [RFC5322].

   [RFC5336] section 2.2 defines when downgrading occurs.  If the SMTP
   client has an UTF8SMTP envelope or an internationalized message and
   the SMTP server doesn't support the UTF8SMTP SMTP extension, then the
   SMTP client MUST NOT send a UTF8SMTP envelope or an internationalized
   message to the SMTP server.  The section shows 4 choices.  The fourth
   choice is downgrading, as described here.

   Downgrading may be implemented in MUAs, MSAs, MTAs which act as the
   SMTP client, or in MDAs, POP servers, IMAP servers which store or
   offer UTF8SMTP envelopes or internationalized messages to non-
   UTF8SMTP compliant systems which include message stores.

   This document tries to define the downgrading process clearly and it
   preserves the original information as much as possible.

   Downgrading in UTF8SMTP consists of the following four parts:
   o  New header fields definition
   o  SMTP downgrading
   o  Email header fields downgrading
   o  MIME header fields downgrading


   In Section 3, many header fields starting with "Downgraded-" are



Fujiwara & YONEYA       Expires September 3, 2009               [Page 4]

Internet-Draft             UTF8SMTP Downgrade                 March 2009


   introduced.  They preserve the original envelope information and the
   original header fields.

   The SMTP downgrading is described in Section 4.  It generates ASCII
   only envelope information from an UTF8SMTP envelope.

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

   The MIME header fields are expanded in [RFC5335].  The MIME header
   fields downgrading is described in Section 6.  It generates ASCII
   only MIME header fields.

   Displaying downgraded messages which originally contain
   internationalized E-mail addresses or internationalized header fields
   is described in an another document
   ([I-D.ietf-eai-downgraded-display]).


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
   EAI overview [RFC4952] or in [RFC5321][RFC5322], MIME documents
   [RFC2045] [RFC2047] [RFC2183] [RFC2231].  The terms "ASCII address",
   "internationalized email address", "non-ASCII address", "i18mail
   address", "UTF8SMTP", "message" and "mailing list" are used with the
   definitions from [RFC4952] document.

   This document depends on [RFC5335], [RFC5336], and [RFC5337].  Key
   words used in these document are used in this document, too.

   The term "non-ASCII" is an UTF-8 string which contains at least one
   non-ASCII character.

   An "UTF8SMTP envelope" has Email originator/recipient addresses
   expanded by [RFC5336] and [RFC5337].

   An "UTF8SMTP message" is Email messages expanded by [RFC5335].


3.  New header fields definition

   New header fields starting with "Downgraded-" are defined here to
   preserve those original envelope and header field values which



Fujiwara & YONEYA       Expires September 3, 2009               [Page 5]

Internet-Draft             UTF8SMTP Downgrade                 March 2009


   contain UTF-8 characters.  During downgrading, one new "Downgraded-"
   header field is added for each original envelope or header field
   which cannot be passed as-is to a server which does not support
   UTF8SMTP.  The original envelope or header field is removed or
   rewritten.  Only those envelope and header fields which contain non-
   ASCII characters are affected.  The result of this process is a
   message which is compliant with existing email specifications
   [RFC5321] and [RFC5322].  The original internationalized information
   can be retrieved by examining the "Downgraded-" header fields which
   were added.

3.1.  Envelope information preservation header fields

   SMTP envelope downgraded information <downgraded-envelope-addr>
   consists of the original non-ASCII address and the downgraded all-
   ASCII address.

   downgraded-envelope-addr = [FWS] "<" [ A-d-l ":" ] uMailbox
                              FWS "<" Mailbox ">" ">" [CFWS]

   <uMailbox> is defined in [RFC5336]; <Mailbox> and <A-d-l> are defined
   in [RFC5321], section 4.1.2.

   Two header fields "Downgraded-Mail-From:" and "Downgraded-Rcpt-To:"
   are defined to preserve SMTP envelope downgraded information.  The
   header field syntax is specified as follows:

   fields             =/ downgradedmailfrom / downgradedrcptto
   downgradedmailfrom =  "Downgraded-Mail-From:" unstructured CRLF
   downgradedrcptto   =  "Downgraded-Rcpt-To:"   unstructured CRLF

   The unstructured content is downgraded-envelope-addr treated as if it
   were unstructured with [RFC2047] encoding (and charset UTF-8) as
   needed.

3.2.  Address header field preservation header fields

   The address header fields preservation header fields are defined to
   preserve the original header field.  Their value field holds the
   original header field value.  The header field syntax is specified as
   follows:










Fujiwara & YONEYA       Expires September 3, 2009               [Page 6]

Internet-Draft             UTF8SMTP Downgrade                 March 2009


   fields =/ known-downgraded-headers ":" unstructured CRLF
   known-downgraded-headers = "Downgraded-" original-headers
   original-headers         = "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"


   Preserving a header field in a downgraded header field is defined as:
   1.  Generate new downgraded header field whose value is the original
       header field value.
   2.  Treat the generated header field content as if it were
       unstructured, and then apply [RFC2047] encoding with charset
       UTF-8 as necessary so the result is ASCII.

3.3.  Unknown header fields preservation header fields

   The unknown header fields preservation header fields are defined to
   encapsulate those original header fields which contain non-ASCII
   characters and are not otherwise provided for in the this
   specification.  The encapsulation header field name is the
   concatenation of "Downgraded-" and the original name.  The value
   field holds the original header field value.

   The header field syntax is specified as follows:


  fields     =/ unknown-downgraded-headers ":" unstructured CRLF
  unknown-downgraded-headers = "Downgraded-" original-header-field-name
  original-header-field-name = field-name

  field-name      =       1*ftext

  ftext           =       %d33-57 /               ; Any character except
                          %d59-126                ;  controls, SP, and
                                                  ;  ":".


   Encapsulating a header field in a "Downgraded-" header field is
   defined as:
   1.  Generate new "Downgraded-" header field whose value is the
       original header field value.





Fujiwara & YONEYA       Expires September 3, 2009               [Page 7]

Internet-Draft             UTF8SMTP Downgrade                 March 2009


   2.  Treat the generated header field content as if it were
       unstructured, and then apply [RFC2047] encoding with charset
       UTF-8 as necessary so the result is ASCII.
   3.  Remove the original header field.


4.  SMTP Downgrading

   Target of downgrading elements in SMTP envelope are below:

   o  <reverse-path> of MAIL FROM command
   o  <forward-path> of RCPT TO command
   o  ORCPT parameter of RCPT TO command

4.1.  Path element downgrading

   Downgrading the <path> of MAIL FROM and RCPT TO commands uses ALT-
   ADDRESS parameter defined in [RFC5336].  A SMTP command is
   downgradable if the <path> contains non-ASCII address and the command
   has an ALT-ADDRESS parameter which specifies an ASCII address.  Since
   only non-ASCII addresses are downgradable, specifying an ALT-ADDRESS
   value for an all-ASCII address is invalid for use with this
   specification, and no interpretation is assigned to it.  This
   restriction allows for future extension of the specification even
   though no such extensions are currently anticipated.

   Note that even if no downgrading is performed on the envelope,
   message header fields and message body MIME header fields that
   contain non-ASCII characters MUST be downgraded.  This is described
   in Section 5 and Section 6.

   When downgrading, replace each <path> which contains non-ASCII mail
   address with its specified alternative ASCII address and preserve the
   original information using "Downgraded-Mail-From" and "Downgraded-
   Rcpt-To" header fields as defined in Section 3.  Before replacing,
   decode the ALT-ADDRESS parameter value because it is encoded as xtext
   [RFC3461].

   To avoid disclosing recipient addresses, the downgrading process MUST
   NOT add "Downgraded-Rcpt-To:" header field if the SMTP downgrading
   targets multiple recipients.  See Section 7 for more detail.

   As a result of the recipient address downgrading, the domain part of
   the recipient address prior to downgrading might be different from
   the domain part of the new recipient address.  If the result of
   address resolution for the domain part of the new recipient address
   contains the server at the connection destination of the SMTP session
   for the recipient address prior to downgrading, the SMTP connection



Fujiwara & YONEYA       Expires September 3, 2009               [Page 8]

Internet-Draft             UTF8SMTP Downgrade                 March 2009


   is valid for the new recipient address.  Otherwise, the downgrading
   process MUST NOT send the downgraded message to the new recipient
   address via the connection and MUST try to send the downgraded
   message to the new recipient address.

4.2.  ORCPT downgrading

   The "RCPT TO" command can have an ORCPT parameter if the DSN
   extension [RFC3461] is supported.  If the ORCPT parameter contains an
   "utf-8" type address and the address contains raw non-ASCII
   characters, the address MUST be converted to utf-8-addr-xtext form.
   Those forms are described in [RFC5337] and clarified by successor
   documents such as [I-D.ietf-eai-dsnbis].

   Before converting to utf-8-addr-xtext form, remove xtext encoding.


5.  Email header fields downgrading

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

   [RFC5335] expands Received: header fields, [RFC5322] ABNF elements
   <mailbox>, <word>, <comment>, <unstructured>, [RFC2045] ABNF element
   <value>.

5.1.  Downgrading method for each ABNF element

   Header field downgrading is defined below for each ABNF element.
   Downgrading an unknown header field is also defined as ENCAPSULATION
   downgrading.  Converting the header field terminates when no non-
   ASCII characters remain in the header field.

5.1.1.  RECEIVED downgrading

   If the header field name is "Received:" and the FOR clause contains a
   non-ASCII addresses, remove the FOR clause from the header field.
   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 which contains non-
   ASCII characters, apply [RFC2047] encoding with charset UTF-8.







Fujiwara & YONEYA       Expires September 3, 2009               [Page 9]

Internet-Draft             UTF8SMTP Downgrade                 March 2009


5.1.3.  WORD downgrading

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

5.1.4.  COMMENT downgrading

   If the header field has any <comment> fields which contains 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 by [RFC2231] with charset UTF-8 and the Language information
   empty.  If the <value> element is <quoted-string> and it contains
   <CFWS> outside the DQUOTE, remove the <CFWS> before this conversion.

5.1.6.  DISPLAY-NAME downgrading

   If the header field has any <address> (<mailbox> and <group>)
   elements and they have <display-name> elements which 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.  MAILBOX downgrading

   The <mailbox> elements have no equivalent format for non-ASCII
   addresses.  If the header field has any <mailbox> elements which
   contain non-ASCII characters, preserve the header field in each
   Address header field preservation header field defined in
   Section 3.2, and rewrite each <mailbox> element to ASCII only format.
   The <mailbox> element which contains non-ASCII characters is one of
   three formats.

   o  [ Display-name ] "<" Utf8-addr-spec 1*FCS "<" Addr-spec ">>"

      Rewrite it as

      [ Display-name ] "<" Addr-spec ">"

   o  [ Display-name ] "<" Utf8-addr-spec ">"
   o  Utf8-addr-spec

      Rewrite both as
      [ Display-name ] "Internationalized Address " Encoded-word
      " Removed:;"



Fujiwara & YONEYA       Expires September 3, 2009              [Page 10]

Internet-Draft             UTF8SMTP Downgrade                 March 2009


      where the <Encoded-word> is the original <Utf8-addr-spec> encoded
      according to [RFC2047].

5.1.8.  ENCAPSULATION downgrading

   if the header field contains non-ASCII characters and for which no
   rule is given above, encapsulate it in a Downgraded header field
   described in Section 3.3 as a last resort.

   Applying this procedure to "Received" header field is prohibited.

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 characters, it is utf-8-address form and
   convert it to utf-8-addr-xtext form as described in Section 4.2.
   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 which contain <address>s

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







Fujiwara & YONEYA       Expires September 3, 2009              [Page 11]

Internet-Draft             UTF8SMTP Downgrade                 March 2009


   Return-Path:
   Disposition-Notification-To:

   If the header field contains <mailbox> elements which contains non-
   ASCII addresses, preserve the header field in a downgraded header
   field before the conversion.  Then 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:
   Message-ID:
   Resent-Message-ID:
   In-Reply-To:
   References:
   Resent-Date:
   Resent-Message-ID:
   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.  Received header field

   Received:

   perform COMMENT downgrading and RECEIVED downgrading.

5.2.5.  MIME Content header fields








Fujiwara & YONEYA       Expires September 3, 2009              [Page 12]

Internet-Draft             UTF8SMTP Downgrade                 March 2009


   Content-Type:
   Content-Disposition:

   Perform MIME-VALUE downgrading and COMMENT downgrading.

5.2.6.  Non-ASCII in <unstructured>

   Subject:
   Comments:
   Content-Description:

   Perform UNSTRUCTURED downgrading.

5.2.7.  Non-ASCII in <phrase>

   Keywords:

   Perform WORD downgrading.

5.2.8.  Other header fields

   All other header fields which contains non-ASCII characters are user-
   defined, missing from this draft or future defined header fields.
   Perform ENCAPSULATION downgrading.

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

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


6.  MIME body part header fields downgrading

   MIME body part header fields may contain non-ASCII characters
   [RFC5335].  This section defines the conversion method to ASCII only
   header fields for each MIME header field which contains non-ASCII
   characters.  Parse the message body's MIME structure for all levels
   and check each MIME header field whether it contains non-ASCII
   characters.  If the header field contains non-ASCII characters in the
   header field value, the header field is a target of the MIME body
   part header fields downgrading.  Each MIME header field's downgrading
   method is described below.  COMMENT downgrading, MIME-VALUE
   downgrading, UNSTRUCTURED downgrading are described in Section 5.





Fujiwara & YONEYA       Expires September 3, 2009              [Page 13]

Internet-Draft             UTF8SMTP Downgrade                 March 2009


   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

   A Downgraded message's header fields contain ASCII characters only.
   But they still contain MIME encapsulated header fields which contains
   non-ASCII UTF-8 characters.  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 which are MIME
   encoded.  Thus it inherits the security considerations of MIME
   encoded header fields [RFC2047] and [RFC3629].

   Rewriting header fields increases the opportunities for undetected
   spoofing by the malicious senders.  However rewritten header fields
   are preserved into Downgraded-* header fields and parsing
   Downgraded-* header fields enables detecting spoofing caused by
   downgrading.

   Addresses that do not appear in the message header fields may appear
   in the RCPT commands to an SMTP server for a number of reasons.
   Copying information from the Envelope into header fields risks
   inadvertent information disclosure (see [RFC5321] and Section 4).
   Mitigating inadvertent information disclosure is discussed in same
   place.

   The techniques described here invalidates methods that depend on
   digital signatures over the envelope or any part of the message which
   includes the top-level header fields or body part header fields.
   Depending on the specific message being downgraded, DKIM especially,
   but also possibly S/MIME, PGP, and similar techniques are all likely
   to break.  The two obvious mitigations are to stick to 7-bit
   transport when using these techniques (as most/all of them presently
   require), or make sure you have UTF8SMTP end-to-end when needed.

   Many gateways and servers on the Internet will discard header fields
   with which they are not familiar.  To the extent to which the



Fujiwara & YONEYA       Expires September 3, 2009              [Page 14]

Internet-Draft             UTF8SMTP Downgrade                 March 2009


   downgrade procedures depend on new header fields (e.g.,
   "Downgraded-") to avoid information loss, the risk of having those
   header fields dropped and its implications must be identified.  In
   particular, if the Downgraded header fields are dropped, there is no
   possibility of reconstructing the original information at any point
   (before, during, or after delivery).  Such gateways violate [RFC2979]
   and can be upgraded to correct the problem.

   Even though the information is not lost, the original message cannot
   be perfectly reconstructed because some downgrading methods remove
   information (see Section 5.1.1 and Section 5.1.5).  Hence,
   downgrading is a one-way process.

   While information in any email header field should usually 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).

   If an internationalized MUA would simply try to "upgrade" the message
   for display purposes (that is, display the information in the
   Downgraded-* header fields instead of the traditional header fields),
   the effectiveness of the deployed mechanisms and protocols is likely
   to be reduced, and the user may be exposed to additional risks.  More
   guidance on how to display downgraded messages will be given in
   [I-D.ietf-eai-downgraded-display].

   Concerns about the trustworthiness of the Downgraded-* header fields
   are not limited to displaying and replying in MUAs, and should be
   carefully considered before using them for other purposes as well.

   See "Security considerations" section in [RFC4952] 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



Fujiwara & YONEYA       Expires September 3, 2009              [Page 15]

Internet-Draft             UTF8SMTP Downgrade                 March 2009


   implementations SHOULD limit lines to not more than 78 characters,
   implementations MAY choose to allow overlong 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.

8.2.  Trivial downgrading

   Downgrading is an alternative to avoid the rejection of messages
   which require UTF8SMTP support by a server which does not provide
   this.  Implementing the full specification of this document is
   desirable, but a partial implementation is also possible.

   If a partial downgrading implementation confronts an unsupported
   downgrading target, the implementation MUST NOT send the message to a
   server which does not support UTF8SMTP.  Instead, it MUST reject the
   message or generate a notification of non-deliverability.

   A partial downgrading, Trivial downgrading is discussed.  It does not
   support non-ASCII addresses in SMTP envelope and address header
   fields, unknown header fields downgrading, the MIME body part header
   fields downgrading.  It supports
   o  some simple header fields downgrading: Subject
   o  comments and display name downgrading: From, To, Cc
   o  trace header field downgrading: Received

   Otherwise, the downgrading fails.

   Trivial downgrading targets mail messages which are generated by
   UTF8SMTP aware MUAs and contain non-ASCII characters in comments,
   display names, unstructured parts without using non-ASCII E-mail
   addresses.  This mail message does not contain non-ASCII E-mail
   addresses in the SMTP Envelope and its header fields.  But it is not
   deliverable via a UTF8SMTP un-aware SMTP server.  Implementing full
   specification downgrading may be hard, but trivial downgrading saves
   mail messages without using non-ASCII addresses.

8.3.  7bit transport consideration

   The SMTP client may encounter a SMTP server which does not support
   the 8BITMIME SMTP extension [RFC1652].  The server does not support
   "8bit" or "binary" data.  Implementers need to consider converting
   "8bit" data to "base64" or "quoted-printable" encoded form and adjust
   the "Content-Transfer-Encoding" header field accordingly.  If the
   body contains multiple MIME parts, this conversion MUST be performed
   for each MIME part.





Fujiwara & YONEYA       Expires September 3, 2009              [Page 16]

Internet-Draft             UTF8SMTP Downgrade                 March 2009


9.  IANA Considerations

   IANA is requested to register the following header fields in the
   Permanent Message Header Field Repository, in accordance with the
   procedures set out in [RFC3864].

   Header field name:  Downgraded-Mail-From
   Applicable protocol:  mail
   Status:  experimental
   Author/change controller:  IETF
   Specification document(s):  This document (Section 3)

   Header field name:  Downgraded-Rcpt-To
   Applicable protocol:  mail
   Status:  experimental
   Author/change controller:  IETF
   Specification document(s):  This document (Section 3)

   Header field name:  Downgraded-From
   Applicable protocol:  mail
   Status:  experimental
   Author/change controller:  IETF
   Specification document(s):  This document (Section 3)

   Header field name:  Downgraded-Sender
   Applicable protocol:  mail
   Status:  experimental
   Author/change controller:  IETF
   Specification document(s):  This document (Section 3)

   Header field name:  Downgraded-To
   Applicable protocol:  mail
   Status:  experimental
   Author/change controller:  IETF
   Specification document(s):  This document (Section 3)

   Header field name:  Downgraded-Cc
   Applicable protocol:  mail
   Status:  experimental
   Author/change controller:  IETF
   Specification document(s):  This document (Section 3)

   Header field name:  Downgraded-Bcc
   Applicable protocol:  mail







Fujiwara & YONEYA       Expires September 3, 2009              [Page 17]

Internet-Draft             UTF8SMTP Downgrade                 March 2009


   Status:  experimental
   Author/change controller:  IETF
   Specification document(s):  This document (Section 3)

   Header field name:  Downgraded-Reply-To
   Applicable protocol:  mail
   Status:  experimental
   Author/change controller:  IETF
   Specification document(s):  This document (Section 3)

   Header field name:  Downgraded-Resent-From
   Applicable protocol:  mail
   Status:  experimental
   Author/change controller:  IETF
   Specification document(s):  This document (Section 3)

   Header field name:  Downgraded-Resent-Sender
   Applicable protocol:  mail
   Status:  experimental
   Author/change controller:  IETF
   Specification document(s):  This document (Section 3)

   Header field name:  Downgraded-Resent-To
   Applicable protocol:  mail
   Status:  experimental
   Author/change controller:  IETF
   Specification document(s):  This document (Section 3)

   Header field name:  Downgraded-Resent-Cc
   Applicable protocol:  mail
   Status:  experimental
   Author/change controller:  IETF
   Specification document(s):  This document (Section 3)

   Header field name:  Downgraded-Resent-Bcc
   Applicable protocol:  mail
   Status:  experimental
   Author/change controller:  IETF
   Specification document(s):  This document (Section 3)

   Header field name:  Downgraded-Resent-Reply-To
   Applicable protocol:  mail
   Status:  experimental
   Author/change controller:  IETF







Fujiwara & YONEYA       Expires September 3, 2009              [Page 18]

Internet-Draft             UTF8SMTP Downgrade                 March 2009


   Specification document(s):  This document (Section 3)

   Header field name:  Downgraded-Return-Path
   Applicable protocol:  mail
   Status:  experimental
   Author/change controller:  IETF
   Specification document(s):  This document (Section 3)

   Header field name:  Downgraded-Disposition-Notification-To
   Applicable protocol:  mail
   Status:  experimental
   Author/change controller:  IETF
   Specification document(s):  This document (Section 3)


   Furthermore, IANA is requested to refuse registration of all the
   field names that start with "Downgraded-" for unknown header fields
   downgrading described in Section 3.3 to avoid conflicts with existing
   IETF activity (Email Address Internationalization).


10.  Acknowledgements

   Significant comments and suggestions were received from John Klensin,
   Harald Alvestrand, Chris Newman, Randall Gellens, Charles Lindsey,
   Marcos Sanz, Alexey Melnikov, Frank Ellermann, Edward Lewis, S.
   Moonesamy and JET members.


11.  Change History

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

11.1.  draft-yoneya-ima-downgrade: Version 00

   o  Initial version
   o  Followed draft-yeh-ima-utf8headers-00 and draft-yao-smtpext-00

11.2.  draft-yoneya-ima-downgrade: Version 01

   o  Document structure was changed
   o  Followed draft-yeh-ima-utf8headers-01 and draft-yao-smtpext-02
   o  Downgrading requirements were added
   o  SMTP DATA encapsulation method was proposed
   o  Downgrading examples was provided





Fujiwara & YONEYA       Expires September 3, 2009              [Page 19]

Internet-Draft             UTF8SMTP Downgrade                 March 2009


11.3.  draft-ietf-eai-downgrade: Version 00

   o  Followed draft-yeh-ima-utf8headers-01 and
      draft-ietf-eai-smtpext-00
   o  No header field downgrading method was proposed
   o  Header encapsulation method was proposed

11.4.  draft-ietf-eai-downgrade: Version 01

   o  Followed draft-ietf-eai-utf8headers-00
   o  Header conversion and encapsulation method was merged
   o  Header conversion method was defined in detail

11.5.  draft-ietf-eai-downgrade: Version 02

   o  Followed draft-ietf-eai-utf8headers-01 and
      draft-ietf-eai-smtpext-01
   o  Specification about algorithmic generated address is removed
   o  No header field downgrading method was removed
   o  SMTP DATA encapsulation method was removed

11.6.  draft-ietf-eai-downgrade: Version 03

   o  Followed draft-ietf-eai-utf8headers-03 and
      draft-ietf-eai-smtpext-03
   o  Downgraded: and Envelope-Downgraded: headers definition was added
   o  Mail header fields downgrading method was refined
   o  Examples in Appendix A were refined

11.7.  draft-ietf-eai-downgrade: Version 04

   o  Followed draft-ietf-eai-utf8headers-06, draft-ietf-eai-smtpext-07
      and draft-ietf-eai-dsn-02
   o  Downgrading requirements and conditions were moved to
      Introduction.
   o  Descriptions about upgrading were removed.
   o  SPF and DKIM discussion were removed.
   o  Added many header fields downgrading.
   o  Allow address literal rewriting without alternate ASCII address in
      header fields.
   o  Added MIME body part headers downgrading.
   o  Added ORCPT downgrading.

11.8.  draft-ietf-eai-downgrade: Version 05

   o  fixed examples





Fujiwara & YONEYA       Expires September 3, 2009              [Page 20]

Internet-Draft             UTF8SMTP Downgrade                 March 2009


      *  ALT-ADDRESS parameter mistake
      *  RFC2047(x) notation was changed to encoded-word format
   o  Added implementation consideration section and trivial downgrading
   o  Downgraded: and Envelope-Downgraded: headers are separated for
      each original headers.
   o  Removed list-* header fields downgrading
   o  Changed the way of writing the header field downgrading section

11.9.  draft-ietf-eai-downgrade: Version 06

   o  Moved decoding downgraded messages as a separate document
   o  Added a text to UNSTRUCTURED downgrading
   o  Added "replacing SMTP connection" if necessary to SMTP
      downgrading.
   o  fixed examples

11.10.  draft-ietf-eai-downgrade: Version 07

   o  Fixed some typos
   o  Added a text about 7bit transport

11.11.  draft-ietf-eai-downgrade: Version 08

   o  Comments from the working group last call (wording)

11.12.  draft-ietf-eai-downgrade: Version 09

   o  References

11.13.  draft-ietf-eai-downgrade: Version 10

   o  Comments from AD Review

11.14.  draft-ietf-eai-downgrade: Version 11

   o  IETF Last call: Comments from Gen-ART and IANA
   o  Added new downgraded header field definitions for Resent-Reply-To,
      Recent-Bcc and Disposition-Notification-To
   o  Separated "Email header fields downgrading" section into
      subsections
   o  Updated ORCPT and TYPED-ADDRESS downgrading

11.15.  draft-ietf-eai-downgrade: Version 12

   o  Comments from IESG
   o  rewrite all 'header' to 'header field'.





Fujiwara & YONEYA       Expires September 3, 2009              [Page 21]

Internet-Draft             UTF8SMTP Downgrade                 March 2009


12.  References

12.1.  Normative References

   [RFC1652]  Klensin, J., Freed, N., Rose, M., Stefferud, E., and D.
              Crocker, "SMTP Service Extension for 8bit-MIMEtransport",
              RFC 1652, July 1994.

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

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

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

   [RFC2183]  Troost, R., Dorner, S., and K. Moore, "Communicating
              Presentation Information in Internet Messages: The
              Content-Disposition Header Field", RFC 2183, August 1997.

   [RFC2231]  Freed, N. and K. Moore, "MIME Parameter Value and Encoded
              Word Extensions:
              Character Sets, Languages, and Continuations", RFC 2231,
              November 1997.

   [RFC2979]  Freed, N., "Behavior of and Requirements for Internet
              Firewalls", RFC 2979, October 2000.

   [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.

   [RFC4952]  Klensin, J. and Y. Ko, "Overview and Framework for
              Internationalized Email", RFC 4952, July 2007.




Fujiwara & YONEYA       Expires September 3, 2009              [Page 22]

Internet-Draft             UTF8SMTP Downgrade                 March 2009


   [RFC5321]  Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
              October 2008.

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

   [RFC5335]  Abel, Y., "Internationalized Email Headers", RFC 5335,
              September 2008.

   [RFC5336]  Yao, J. and W. Mao, "SMTP Extension for Internationalized
              Email Addresses", RFC 5336, September 2008.

   [RFC5337]  Newman, C. and A. Melnikov, "Internationalized Delivery
              Status and Disposition Notifications", RFC 5337,
              September 2008.

12.2.  Informative References

   [I-D.ietf-eai-downgraded-display]
              Fujiwara, K., "Displaying Downgraded Messages for Email
              Address Internationalization",
              draft-ietf-eai-downgraded-display-00 (work in progress),
              October 2008.

   [I-D.ietf-eai-dsnbis]
              Newman, C. and A. Melnikov, "Internationalized Delivery
              Status and Disposition Notifications",
              draft-ietf-eai-dsnbis-00 (work in progress),
              December 2008.


Appendix A.  Examples

A.1.  Downgrading example 1

   This section shows an SMTP Downgrading example.  Consider a mail
   message where:
   o  The sender address is "NON-ASCII-local@example.com" which is a
      non-ASCII address.  Its ASCII alternative is
      "ASCII-local@example.com" and its display-name is "DISPLAY-local".
   o  The "To:" address is "NON-ASCII-remote1@example.net" which is a
      non-ASCII address.  Its ASCII alternative is
      "ASCII-remote1@example.net" and its display-name is "DISPLAY-
      remote1".
   o  The "Cc:" address is a non-ASCII address
      "NON-ASCII-remote2@example.org" without alternative ASCII address.
      Its display-name is "DISPLAY-remote2".




Fujiwara & YONEYA       Expires September 3, 2009              [Page 23]

Internet-Draft             UTF8SMTP Downgrade                 March 2009


   o  Three display-names contain non-ASCII characters.
   o  The Subject header field is "NON-ASCII-SUBJECT" which contains
      non-ASCII characters.
   o  Assuming the "To:" recipient's MTA (example.net) does not support
      UTF8SMTP.
   o  assuming the "Cc:" recipient's MTA (example.org) supports
      UTF8SMTP.
   The example SMTP envelope/message is shown in Figure 1.  In this
   example, the "To:" recipient's session is the focus.



   MAIL FROM: <NON-ASCII-local@example.com>
               ALT-ADDRESS=ASCII-local@example.com
   RCPT TO: <NON-ASCII-remote1@example.net>
             ALT-ADDRESS=ASCII-remote1@example.net
   RCPT TO: <NON-ASCII-remote2@example.org>
   -------------------------------------------------------------
   Message-Id: MESSAGE_ID
   Mime-Version: 1.0
   Content-Type: text/plain; charset="UTF-8"
   Content-Transfer-Encoding: 8bit
   Subject: NON-ASCII-SUBJECT
   From: DISPLAY-local <NON-ASCII-local@example.com
    <ASCII-local@example.com>>
   To: DISPLAY-remote1 <NON-ASCII-remote1@example.net
    <ASCII-remote1@example.net>>
   Cc: DISPLAY-remote2 <NON-ASCII-remote2@example.org>
   Date: DATE

   MAIL_BODY


              Figure 1: Original envelope/message (example 1)

   In this example, there are two SMTP recipients, one is "To:", the
   other is "Cc:".  The SMTP downgrading treats To: session downgrading.
   Figure 2 shows SMTP downgraded example.













Fujiwara & YONEYA       Expires September 3, 2009              [Page 24]

Internet-Draft             UTF8SMTP Downgrade                 March 2009


   MAIL FROM: <ASCII-local@example.com>
   RCPT TO: <ASCII-remote1@example.net>
   -------------------------------------------------------------
   Downgraded-Mail-From: =?UTF-8?Q?<NON-ASCII-local@example.com_?=
    =?UTF-8?Q?<ASCII-local@example.com>>?=
   Downgraded-Rcpt-To: =?UTF-8?Q?<NON-ASCII-remote1@example.net_?=
    =?UTF-8?Q?<ASCII-remote1@example.net>>?=
   Message-Id: MESSAGE_ID
   Mime-Version: 1.0
   Content-Type: text/plain; charset="UTF-8"
   Content-Transfer-Encoding: 8bit
   Subject: NON-ASCII-SUBJECT
   From: DISPLAY-local <NON-ASCII-local@example.com
    <ASCII-local@example.com>>
   To: DISPLAY-remote1 <NON-ASCII-remote1@example.net
    <ASCII-remote1@example.net>>
   Cc: DISPLAY-remote2 <NON-ASCII-remote2@example.org>
   Date: DATE

   MAIL_BODY


          Figure 2: SMTP Downgraded envelope/message (example 1)

   After SMTP downgrading, header fields downgrading is performed.
   Final downgraded message is shown in Figure 3.  Return-Path header
   field will be added by the final destination MTA.
























Fujiwara & YONEYA       Expires September 3, 2009              [Page 25]

Internet-Draft             UTF8SMTP Downgrade                 March 2009


Return-Path: <ASCII-local@example.com>
Downgraded-Mail-From: =?UTF-8?Q?<NON-ASCII-local@example.com_?=
 =?UTF-8?Q?<ASCII-local@example.com>>?=
Downgraded-Rcpt-To: =?UTF-8?Q?<NON-ASCII-remote1@example.net_?=
 =?UTF-8?Q?<ASCII-remote1@example.net>>?=
Message-Id: MESSAGE_ID
Mime-Version: 1.0
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: 8bit
Subject: =?UTF-8?Q?NON-ASCII-SUBJECT?=
From: =?UTF-8?Q?DISPLAY-local?= <ASCII-local@example.com>
Downgraded-From: =?UTF-8?Q?DISPLAY-local_<NON-ASCII-local@example.com_?=
 =?UTF-8?Q?<ASCII-local@example.com>>?=
To: =?UTF-8?Q?DISPLAY-remote1?= <ASCII-remote1@example.net>
Downgraded-To: =?UTF-8?Q?DISPLAY-remote1_?=
 =?UTF-8?Q?<NON-ASCII-remote1@example.net_<ASCII-remote1@example.net>>?=
Cc: =?UTF-8?Q?DISPLAY-remote2?= Internationalized address
 =?UTF-8?Q?NON-ASCII-remote2@example.org?= removed:;
Downgraded-Cc: =?UTF-8?Q?DISPLAY-remote2_?=
 =?UTF-8?Q?<NON-ASCII-remote2@example.org>?=
Date: DATE

MAIL_BODY


                 Figure 3: Downgraded message (example 1)

A.2.  Downgrading example 2

   In many cases, the sender wants to use non-ASCII address and the
   recipient is a traditional mail user.  The SMTP server handing mail
   for the recipient and/or the recipient's MUA does not support
   UTF8SMTP extension.  Consider a mail message where:
   o  The sender address is "NON-ASCII-local@example.com" which is a
      non-ASCII address.  Its ASCII alternative is
      "ASCII-local@example.com".  It has a display-name "DISPLAY-local"
      which contains non-ASCII characters.
   o  The "To:" address is "ASCII-remote1@example.net" which is ASCII
      only.  It has a display-name "DISPLAY-remote1" which contains non-
      ASCII characters.
   o  The "Subject:" header field is "NON-ASCII-SUBJECT" which contains
      non-ASCII characters.
   The second example envelope/message is shown in Figure 4.








Fujiwara & YONEYA       Expires September 3, 2009              [Page 26]

Internet-Draft             UTF8SMTP Downgrade                 March 2009


   MAIL From: <NON-ASCII-local@example.com>
               ALT-ADDRESS=ASCII-local@example.com
   RCPT TO: <ASCII-remote1@example.net>
   -------------------------------------------------------------
   Message-Id: MESSAGE_ID
   Mime-Version: 1.0
   Content-Type: text/plain; charset="UTF-8"
   Content-Transfer-Encoding: 8bit
   Subject: NON-ASCII-SUBJECT
   From: DISPLAY-local <NON-ASCII-local@example.com
    <ASCII-local@example.com>>
   To: DISPLAY-remote1 <ASCII-remote1@example.net>
   Date: DATE

   MAIL_BODY


                  Figure 4: Original message (example 2)

   In this example, SMTP session is downgradable.  Figure 5 shows SMTP
   downgraded envelope/message.



   MAIL From: <ASCII-local@example.com>
   RCPT TO: <ASCII-remote1@example.net>
   -------------------------------------------------------------
   Downgraded-Mail-From: =?UTF-8?Q?<NON-ASCII-local@example.com_?=
    ?=UTF8?Q?<ASCII-local@example.com>>?=
   Message-Id: MESSAGE_ID
   Mime-Version: 1.0
   Content-Type: text/plain; charset="UTF-8"
   Content-Transfer-Encoding: 8bit
   Subject: NON-ASCII-SUBJECT
   From: DISPLAY-local <NON-ASCII-local@example.com
    <ASCII-local@example.com>>
   To: DISPLAY-remote1 <ASCII-remote1@example.net>
   Date: DATE

   MAIL_BODY


          Figure 5: SMTP Downgraded envelope/message (example 2)

   After SMTP downgrading, header fields downgrading is performed.  The
   downgraded example is shown in Figure 6.





Fujiwara & YONEYA       Expires September 3, 2009              [Page 27]

Internet-Draft             UTF8SMTP Downgrade                 March 2009


Return-Path: <ASCII-local@example.com>
Downgraded-Mail-From: =?UTF-8?Q?<NON-ASCII-local@example.com_?=
 =?UTF8?Q?<ASCII-local@example.com>>?=
Message-Id: MESSAGE_ID
Mime-Version: 1.0
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: 8bit
Subject: =?UTF-8?Q?NON-ASCII-SUBJECT?=
Downgraded-From: =?UTF-8?Q?DISPLAY-local_<NON-ASCII-local@example.com_?=
 =?UTF-8?Q?<ASCII-local@example.com>>?=
From: =?UTF-8?Q?DISPLAY-local?= <ASCII-local@example.com>
To: =?UTF-8?Q?DISPLAY-remote1?= <ASCII-remote1@example.net>
Date: DATE

MAIL_BODY


                 Figure 6: Downgraded message (example 2)


Authors' Addresses

   Kazunori Fujiwara (editor)
   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


   Yoshiro YONEYA (editor)
   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: yone@jprs.co.jp











Fujiwara & YONEYA       Expires September 3, 2009              [Page 28]


Html markup produced by rfcmarkup 1.108, available from http://tools.ietf.org/tools/rfcmarkup/