Internet Engineering Task Force (IETF)                       K. Fujiwara
Request for Comments: 5825                                          JPRS
Category: Experimental                                          B. Leiba
ISSN: 2070-1721                                      Huawei Technologies
                                                             April 2010


Displaying Downgraded Messages for Email Address Internationalization

Abstract

  This document describes a method for displaying downgraded messages
  that originally contained internationalized email addresses or
  internationalized header fields.

Status of This Memo

  This document is not an Internet Standards Track specification; it is
  published for examination, experimental implementation, and
  evaluation.

  This document defines an Experimental Protocol for the Internet
  community.  This document is a product of the Internet Engineering
  Task Force (IETF).  It represents the consensus of the IETF
  community.  It has received public review and has been approved for
  publication by the Internet Engineering Steering Group (IESG).  Not
  all documents approved by the IESG are a candidate for any level of
  Internet Standard; see Section 2 of RFC 5741.

  Information about the current status of this document, any errata,
  and how to provide feedback on it may be obtained at
  http://www.rfc-editor.org/info/rfc5825.

Copyright Notice

  Copyright (c) 2010 IETF Trust and the persons identified as the
  document authors.  All rights reserved.

  This document is subject to BCP 78 and the IETF Trust's Legal
  Provisions Relating to IETF Documents
  (http://trustee.ietf.org/license-info) in effect on the date of
  publication of this document.  Please review these documents
  carefully, as they describe your rights and restrictions with respect
  to this document.  Code Components extracted from this document must
  include Simplified BSD License text as described in Section 4.e of
  the Trust Legal Provisions and are provided without warranty as
  described in the Simplified BSD License.




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Table of Contents

  1. Introduction ....................................................2
  2. Terminology .....................................................2
  3. Converting Downgraded Message Headers for Display ...............3
     3.1. Considerations .............................................3
     3.2. The Process ................................................3
          3.2.1. No Reconstruction of the Envelope
                 Information Preservation ............................4
          3.2.2. Reconstructing the Address Header Fields'
                 Preservation Header .................................4
          3.2.3. The Unknown Header Fields' Preservation
                 Header Fields .......................................5
  4. Security Considerations .........................................6
  5. Acknowledgements ................................................6
  6. References ......................................................6
     6.1. Normative References .......................................6
     6.2. Informative References .....................................7
  Appendix A.  Examples ..............................................8
    A.1.  Displaying Example ........................................11

1.  Introduction

  The Email Address Internationalization (UTF8SMTP) extension document
  set [RFC4952] [RFC5336] [RFC5335] [RFC5337] expands Email address
  structure, syntax, and email header format.  To avoid rejection of
  internationalized email messages, the downgrading mechanism [RFC5504]
  converts an internationalized message to a traditional email message
  when a server in the delivery path does not support the UTF8SMTP
  extension.  The downgraded message is a traditional email message,
  except the message has "Downgraded-" header fields.

  A perfect reverse-function of the downgrading does not exist because
  the encoding defined in [RFC2047] is not exactly reversible and
  "Received" header field downgrading may remove FOR clause
  information.  The restoration of the downgrading should be done once
  at the final destination of the downgraded message such as Mail User
  Agents (MUAs) or IMAP servers.  This document describes the
  restoration methods for displaying downgraded messages in MUAs.

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].






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  Specialized terms used in this specification are defined in the EAI
  overview [RFC4952] or in [RFC5321], [RFC5322], or the MIME documents
  [RFC2045], [RFC2047], [RFC2183], and [RFC2231].

  This document depends on [RFC5335] and [RFC5504].  Key words used in
  those documents are used in this document, too.

  The term "MIME decode" is used for both "encoded-word" decoding
  defined by [RFC2047] and MIME parameter value decoding defined by
  [RFC2231].

3.  Converting Downgraded Message Headers for Display

3.1.  Considerations

  The order of some header fields (such as "Resent-*" fields) is
  significant.  The process of regenerating the original fields from
  the downgraded ones MUST NOT reorder the fields.

  In order to regenerate a field from a specific downgraded header
  field, it's necessary to find the corresponding replacement in the
  current message.  If the corresponding field cannot be found, the
  downgraded header field in question cannot be regenerated and used.

  In any case where reconstruction of a particular downgraded header
  field fails, both header fields (the "downgraded-YYY" header field
  and the "YYY" header field) SHOULD be left in the message as they
  are.  The MUA MAY choose to communicate the situation to the user
  (see the "Security Considerations" section).

3.2.  The Process

  A MUA MAY decode and regenerate the original header fields of the
  message (Mail Transport Agents (MTAs) and Mail Delivery Agents (MDAs)
  SHOULD NOT attempt to do this; it SHOULD be left to the MUA).  This
  procedure can be used to approximately reverse the downgrade process,
  but it will not always construct the original header fields exactly.

  Three types of downgraded header fields are described in Section 3 of
  [RFC5504]:

  1.  "Envelope Information Preservation Header Fields", described in
      RFC5504 Section 3.1 and in Section 3.2.1, below.

  2.  "Address Header Fields' Preservation Header Fields", described in
      RFC5504 Section 3.2 and in Section 3.2.2, below.





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  3.  "Unknown Header Fields' Preservation Header Fields", described in
      RFC5504 Section 3.3 and in Section 3.2.3, below.

  After processing downgraded header fields, decode all header fields,
  as described in [RFC2047] and [RFC2231].

3.2.1.  No Reconstruction of the Envelope Information Preservation
       Header Fields

  Envelope information preservation header fields are new fields that
  might have been added by the downgrade process.  Because they do not
  represent fields that appeared in the original message, this process
  is not applicable to them.

3.2.2.  Reconstructing the Address Header Fields' Preservation Header
       Fields

  Reconstructing address header fields' preservation header fields is
  OPTIONAL, and a decision MAY be made on each field, individually.  In
  particular, it might be less important to process the "Resent-*"
  header fields, so an implementation MAY choose to skip those.

  To construct a displayable copy of a header field from one of these
  downgraded header fields, follow this procedure:

  1.  In an edit buffer, create a new header field:

      (a)  For the field name, remove the "Downgraded-" prefix from the
           downgraded field name.  For example, "Downgraded-From"
           becomes "From", and "Downgraded-Resent-To" becomes
           "Resent-To".

      (b)  For the field value, decode the MIME-encoded value of the
           downgraded field according to [RFC2047].

  2.  Apply "Email Header Fields Downgrading", defined in Section 5 of
      [RFC5504], to the field in the edit buffer.  The process
      generates two header fields, one is ASCII header field and the
      other is the Address Header Fields' Preservation Header Field.
      Put the generated ASCII header field into comparison buffer 1.

  3.  Canonicalize the header field in the comparison buffer 1:

      1.  Unfold all header field continuation lines as described in
          [RFC5322].






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      2.  Ensure that there is one space character before and one after
          the <mailbox-list> separator ",".  If a space character is
          missing, insert one.

      3.  Ensure that there is one space character before and one after
          each <comment>.  If a space character is missing, insert one.

      4.  Decode each <encoded-word> whose charset is "UTF-8".

      5.  Convert all sequences of one or more WSP characters to a
          single space character.  WSP characters here include those
          before and after a line-folding boundary.

      6.  Delete all WSP characters at the end of each unfolded header
          field value.

      7.  Delete any WSP characters remaining before and after the
          colon separating the header field name from the header field
          value, retaining the colon separator.

  4.  Locate the first instance of the corresponding field in the
      message's headers.

  5.  Canonicalize the located field as in step 3, and put the result
      into comparison buffer 2.

  6.  Compare the header field in comparison buffer 1 with the header
      field in comparison buffer 2.  If they match, go to step 8.

  7.  Locate the next instance of the corresponding field in the
      message's headers.  If one is found, go to step 5.  If none is
      found, stop: you cannot use this downgraded field because you
      can't find its replacement in the message.

  8.  Replace the located header field with the one in the edit buffer.
      You MUST NOT reorder the header fields when you do this; it's
      important to replace the field in the same place.  Remove the
      target downgraded header field in the message header.

3.2.3.  The Unknown Header Fields' Preservation Header Fields

  The unknown header fields' preservation header fields SHOULD be left
  as they are unless the MUA has special knowledge of a particular
  field.  An MUA with such knowledge MAY use the procedure similar to
  the procedure in Section 3.2.2, above, for those fields about which
  it knows.  (Note that the whitespace canonicalization rule might not
  be applicable to some header fields.)




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4.  Security Considerations

  While information in any email header should usually be treated with
  some suspicion, current email systems commonly employ various
  mechanisms and protocols to make the information more trustworthy.
  For example, an organization's boundary MTA can modify "From" lines
  so that messages arriving from outside the organization are easily
  distinguishable from internal emails.  As a result of that rewriting,
  the "From" header field might not match the "Downgraded-From" header
  field.

  A MUA MAY emphasize bogus or broken address header fields'
  preservation header fields found in step 7 of Section 3.2.2.

  Hiding the information from the actual header fields when using the
  "Downgraded-" header fields does not cause loss of information if
  generating MIME-decoded header fields in step 1 of Section 3.2.2 and
  the comparison done in step 7 are successful.  To ensure that no
  information is lost, a MUA SHOULD have a function that uses the
  actual message that was received (with/without MIME decoding) to
  render the message.

  We have focused, here, on issues with displaying downgraded messages.
  For more discussion of downgraded and internationalized messages in
  general, see the "Security Considerations" section in [RFC5504] and
  [RFC4952].

5.  Acknowledgements

  This document was separated from [RFC5504].  Both documents were
  developed in the EAI WG.  Significant comments and suggestions were
  received from John Klensin, Harald Alvestrand, Chris Newman, Randall
  Gellens, Charles Lindsey, Marcos Sanz, Alexey Melnikov, Pasi Eronen,
  Frank Ellermann, Edward Lewis, S. Moonesamy, and JET members.

6.  References

6.1.  Normative References

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

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





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  [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.

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

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

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

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

6.2.  Informative References

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
















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Appendix A.  Examples

  This section shows an example of displaying a downgraded message.
  First, an example of the original UTF8SMTP message and its downgraded
  message are shown.  The example comes from "Example 1" of [RFC5504]
  and three header fields, "Unknown-Field", "Resent-From", and
  "Resent-To", are added.  The example UTF8SMTP message is shown in
  Figure 1.

  Message-Id: MESSAGE_ID
  Mime-Version: 1.0
  Content-Type: text/plain; charset="UTF-8"
  Content-Transfer-Encoding: 8bit
  Subject: NON-ASCII-SUBJECT
  Unknown-Field: NON-ASCII-Unknown
  From: DISPLAY-local <[email protected]
   <[email protected]>>
  To: DISPLAY-remote1 <[email protected]
   <[email protected]>>
  Cc: DISPLAY-remote2 <[email protected]>
  Resent-From: DISPLAY-remote1 <[email protected]
   <[email protected]>>
  Resent-To: DISPLAY-reto <[email protected]
   <[email protected]>>
  Date: DATE

  MAIL_BODY

                       Figure 1: Original message






















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  A delivered downgraded message is shown in Figure 2.  A Return-Path
  header will be added by the final destination MTA.  Some "Received"
  header fields may be added.

Return-Path: <[email protected]>
Received: ...
Downgraded-Mail-From: =?UTF-8?Q?<[email protected]_?=
=?UTF-8?Q?<[email protected]>>?=
Downgraded-Rcpt-To: =?UTF-8?Q?<[email protected]_?=
=?UTF-8?Q?<[email protected]>>?=
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-Unknown-Field: =?UTF-8?Q?NON-ASCII-Unknown?=
From: =?UTF-8?Q?DISPLAY-local?= <[email protected]>
Downgraded-From: =?UTF-8?Q?DISPLAY-local_<[email protected]_?=
=?UTF-8?Q?<[email protected]>>?=
To: =?UTF-8?Q?DISPLAY-remote1?= <[email protected]>
Downgraded-To: =?UTF-8?Q?DISPLAY-remote1_?=
=?UTF-8?Q?<[email protected]_<[email protected]>>?=
Cc: =?UTF-8?Q?DISPLAY-remote2?= Internationalized address
[email protected]?= removed:;
Downgraded-Cc: =?UTF-8?Q?DISPLAY-remote2_?=
=?UTF-8?Q?<[email protected]>?=
Resent-From: =?UTF-8?Q?DISPLAY-remote1?= <[email protected]>
Downgraded-Resent-From: =?UTF-8?Q?DISPLAY-remote1_?=
=?UTF-8?Q?<[email protected]_<[email protected]>>?=
Resent-To: =?UTF-8?Q?DISPLAY-reto?= <[email protected]>
Downgraded-Resent-To: =?UTF-8?Q?DISPLAY-reto_?=
=?UTF-8?Q?<[email protected]_<[email protected]>>?=
Date: DATE

MAIL_BODY

                      Figure 2: Downgraded message














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  Figure 3 shows the MIME-decoded message of Figure 2.  The recipient
  can read the original "From", "To", "Cc", "Resent-From", "Resent-To"
  and "Unknown-Field" header fields as "Downgraded-From",
  "Downgraded-To", "Downgraded-Cc", "Downgraded-Resent-From",
  "Downgraded-Resent-To", and "Downgraded-Unknown-Field" header fields.

  Return-Path: <[email protected]>
  Received: ...
  Downgraded-Mail-From: <[email protected]
   <[email protected]>>
  Downgraded-Rcpt-To: <[email protected]
   <[email protected]>>
  Message-Id: MESSAGE_ID
  Mime-Version: 1.0
  Content-Type: text/plain; charset="UTF-8"
  Content-Transfer-Encoding: 8bit
  Subject: NON-ASCII-SUBJECT
  Downgraded-Unknown-Field: NON-ASCII-Unknown
  From: DISPLAY-local <[email protected]>
  Downgraded-From: DISPLAY-local <[email protected]
   <[email protected]>>
  To: DISPLAY-remote1 <[email protected]>
  Downgraded-To: DISPLAY-remote1 <[email protected]
   <[email protected]>>
  Cc: DISPLAY-remote2 Internationalized address
   [email protected] removed:;
  Downgraded-Cc: DISPLAY-remote2 <[email protected]>
  Resent-From: DISPLAY-remote1 <[email protected]>
  Downgraded-Resent-From: DISPLAY-remote1
   <[email protected] <[email protected]>>
  Resent-To: DISPLAY-reto <[email protected]>
  Downgraded-Resent-To: DISPLAY-reto
   <[email protected] <[email protected]>>
  Date: DATE

  MAIL_BODY

                     Figure 3: MIME-decoded message













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A.1.  Displaying Example

  This example shows how to display the message in Figure 2, above,
  using the process defined in Section 3.  For simplicity, we will show
  the reconstruction of all the applicable fields at once.

  Selecting all Downgraded-* fields gives this:

Downgraded-Mail-From: =?UTF-8?Q?<[email protected]_?=
=?UTF-8?Q?<[email protected]>>?=
Downgraded-Rcpt-To: =?UTF-8?Q?<[email protected]_?=
=?UTF-8?Q?<[email protected]>>?=
Downgraded-Unknown-Field: =?UTF-8?Q?NON-ASCII-Unknown?=
Downgraded-From: =?UTF-8?Q?DISPLAY-local_<[email protected]_?=
=?UTF-8?Q?<[email protected]>>?=
Downgraded-To: =?UTF-8?Q?DISPLAY-remote1_?=
=?UTF-8?Q?<[email protected]_<[email protected]>>?=
Downgraded-Cc: =?UTF-8?Q?DISPLAY-remote2_?=
=?UTF-8?Q?<[email protected]>?=
Downgraded-Resent-From: =?UTF-8?Q?DISPLAY-remote1_?=
=?UTF-8?Q?<[email protected]_<[email protected]>>?=
Downgraded-Resent-To: =?UTF-8?Q?DISPLAY-reto_?=
=?UTF-8?Q?<[email protected]_<[email protected]>>?=

                   Figure 4: Downgraded header fields

  Two of the fields, "Downgraded-Mail-From" and "Downgraded-Rcpt-To",
  are envelope information preservation header fields, and will not be
  reconstructed.  One field, "Downgraded-Unknown-Field", is an unknown
  header fields' preservation header field and will also not be
  reconstructed.  That leaves the address header fields' preservation
  header fields to be reconstructed.

Downgraded-From: =?UTF-8?Q?DISPLAY-local_<[email protected]_?=
=?UTF-8?Q?<[email protected]>>?=
Downgraded-To: =?UTF-8?Q?DISPLAY-remote1_?=
=?UTF-8?Q?<[email protected]_<[email protected]>>?=
Downgraded-Cc: =?UTF-8?Q?DISPLAY-remote2_?=
=?UTF-8?Q?<[email protected]>?=
Downgraded-Resent-From: =?UTF-8?Q?DISPLAY-remote1_?=
=?UTF-8?Q?<[email protected]_<[email protected]>>?=
Downgraded-Resent-To: =?UTF-8?Q?DISPLAY-reto_?=
=?UTF-8?Q?<[email protected]_<[email protected]>>?=

             Figure 5: Header fields for the reconstruction






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  Now, perform step 1 to the downgraded header fields shown in Figure 5
  and create an edit buffer.

  From: DISPLAY-local <[email protected]
   <[email protected]>>
  To: DISPLAY-remote1 <[email protected]
   <[email protected]>>
  Cc: DISPLAY-remote2 <[email protected]>
  Resent-From: DISPLAY-remote1
   <[email protected] <[email protected]>>
  Resent-To: DISPLAY-reto
   <[email protected] <[email protected]>>

                 Figure 6: edit buffer: Output of step 1

  Apply "Email Header Fields Downgrading" to the "edit buffer".  It
  generates downgraded ASCII header fields and the address header
  fields' preservation header fields.  The latter fields are the same
  as the downgraded header fields.  Put the former fields into
  "comparison buffer 1".

  From:DISPLAY-local <[email protected]>
  To:DISPLAY-remote1 <[email protected]>
  Cc:DISPLAY-remote2 Internationalized address
   [email protected] removed:;
  Resent-From:DISPLAY-remote1 <[email protected]>
  Resent-To:DISPLAY-reto <[email protected]>

             Figure 7: comparison buffer 1: Output of step 3

  Perform steps 4 to 6, comparison, for each header field.  Five header
  fields, "From", "To", "Cc", "Resent-From" and "Resent-To" fields will
  match, and we will proceed to step 8.  (Step 7, iteration, does not
  apply in this example.

















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  Perform step 8, replacing all applicable fields, without changing the
  order.  Then, do MIME decoding on everything, for display.

  Return-Path: <[email protected]>
  Received: ...
  Downgraded-Mail-From: <[email protected]
   <[email protected]>>
  Downgraded-Rcpt-To: <[email protected]>
   <[email protected]>
  Message-Id: MESSAGE_ID
  Mime-Version: 1.0
  Content-Type: text/plain; charset="UTF-8"
  Content-Transfer-Encoding: 8bit
  Subject: NON-ASCII-SUBJECT
  Downgraded-Unknown-Field: NON-ASCII-Unknown
  From: DISPLAY-local <[email protected]
   <[email protected]>>
  To: DISPLAY-remote1 <[email protected]
   <[email protected]>>
  Cc: DISPLAY-remote2 <[email protected]>
  Resent-From: DISPLAY-remote1 <[email protected]
   <[email protected]>>
  Resent-To: DISPLAY-reto <[email protected]
   <[email protected]>>
  Date: DATE

                       Figure 8: The final result

  As a result, in this simple example, some original header fields are
  now displayed in their original form.  Differences between Figure 1
  and Figure 8 are Return-Path, Downgraded-Mail-From,
  Downgraded-Rcpt-To, and Downgraded-Unknown-Field.



















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RFC 5825             Displaying Downgraded Messages           April 2010


Authors' Addresses

  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: [email protected]


  Barry Leiba
  Huawei Technologies

  Phone: +1 646 827 0648
  EMail: [email protected]
  URI:   http://internetmessagingtechnology.org/

































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