Email Address Internationalization                        Y. YONEYA, Ed.
(EAI)                                                   K. Fujiwara, Ed.
Internet-Draft                                                      JPRS
Expires: December 28, 2006                                  Jun 26, 2006


   Downgrading mechanism for Email Address Internationalization (EAI)
                    draft-ietf-eai-downgrade-01.txt

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Copyright Notice

   Copyright (C) The Internet Society (2006).

Abstract

   Traditional mail systems handle only US-ASCII characters in SMTP
   envelope and mail headers.  The Email Address Internationalization
   (EAI) is implemented by allowing UTF-8 characters in SMTP envelope
   and mail headers.  To deliver Non-ASCII mail address through EAI
   incompliant environment, some sort of converting mechanism (i.e.
   downgrading) is required.  This document describes requirements for
   downgrading, SMTP session downgrading, header downgrading and
   implementation consideration.



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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  Downgrade Requirements . . . . . . . . . . . . . . . . . . . .  3
     3.1.  Timing and conditions of downgrading . . . . . . . . . . .  3
     3.2.  Requirements . . . . . . . . . . . . . . . . . . . . . . .  4
   4.  SMTP Downgrading . . . . . . . . . . . . . . . . . . . . . . .  4
   5.  SMTP DATA/Header downgrading . . . . . . . . . . . . . . . . .  5
     5.1.  No header downgrading  . . . . . . . . . . . . . . . . . .  6
     5.2.  Downgrading with MIME encapsulation  . . . . . . . . . . .  6
       5.2.1.  Downgrading with MIME encapsulation example  . . . . .  7
     5.3.  Header conversion  . . . . . . . . . . . . . . . . . . . .  8
       5.3.1.  Downgrading address headers  . . . . . . . . . . . . .  9
       5.3.2.  Header conversion example  . . . . . . . . . . . . . . 10
   6.  Implementation consideration . . . . . . . . . . . . . . . . . 12
     6.1.  MUA  . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
     6.2.  MDA Requirements . . . . . . . . . . . . . . . . . . . . . 12
   7.  Security considerations  . . . . . . . . . . . . . . . . . . . 12
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 12
   9.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 12
   10. Normative References . . . . . . . . . . . . . . . . . . . . . 12
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 14
   Intellectual Property and Copyright Statements . . . . . . . . . . 15



























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1.  Introduction

   Traditional mail systems which are defined by [RFC2821] and [RFC2822]
   allow US-ASCII characters in SMTP envelope and mail headers in body
   part.  The EAI proposal [EAI-Overview],[EAI-UTF8], [EAI-SMTPext]
   allows UTF-8 characters in SMTP envelope and mail headers in body
   part.

   Carrying Non-ASCII mail address from sender to recipients requires
   all components on the mail delivery route are EAI compliant.
   Otherwise Non-ASCII mail address can't be delivered.  To solve the
   problem, this document describes downgrading mechanism that enables
   delivering Non-ASCII mail address by converting it to corresponding
   US-ASCII representation on current mail delivery system.  Not only
   SMTP envelope, but also UTF-8 characters in mail headers MUST be
   converted to US-ASCII.

   Downgrading in EAI consists from following two parts:
   o  SMTP session downgrading
   o  header downgrading

   Decoding downgraded envelope/message is called 'Upgrading' in this
   document.  Each downgrading mechanism has corresponding upgrading
   mechanism.

   In this document, requirements for downgrading is described in
   section Section 3, SMTP session downgrading is described in
   Section 4, and mail header downgrading is described in Section 5.


2.  Terminology

   Terminology for this document is defined in [EAI-Overview].

   In this document, "algorithmic address" is an US-ASCII address which
   is generated by algorithmic method.


3.  Downgrade Requirements

3.1.  Timing and conditions of downgrading

   This section describes timing and conditions of downgrading.
   o  Timing: SMTP client detects that SMTP server doesn't support
      "IEmail" option at EHLO.  [EAI-SMTPext]
   o  Conditions: SMTP client detects that UTF-8 is included in the SMTP
      envelope or mail headers in the SMTP DATA.




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   Note: If the i-Email header exists, downgrading will be performed.
   If UTF-8 characters exist in mail headers without the i-Email header,
   this is a protocol error, and handling of this situation is outside
   the scope of this specification.

3.2.  Requirements

   1.  Downgrading must be performed only once.
   2.  Upgrading must be performed at minimized place such as final
       destination like recipient MUA.
   3.  Downgrading and upgrading must be automated.
   4.  Downgrading and upgrading should be easy and lightweight as it is
       possible to do with MTA like 8BITMIME encapsulation.
   5.  Downgrade and upgrade method must be defined clearly.
   6.  Downgrading and upgrading should preserve all header information.
   7.  Downgrading must support SPF and DKIM.
   8.  Downgrading occurrence must be recorded.


4.  SMTP Downgrading

   Downgrading MUST be performed in each SMTP session.  Target of
   downgrading elements in SMTP envelope are below:

   o  MAIL FROM:
   o  RCPT TO:

   Downgrading in SMTP envelope uses ALT-ADDR and ATOMIC option proposed
   in [EAI-SMTPext].

   Downgrading is possible only when a mail sender's MUA appends ALT-
   ADDR or ATOMIC option to all Non-ASCII envelope addresses to denote
   their alternative US-ASCII address.

   When MUA/MTA is transferring mail and finding its envelope is Non-
   ASCII, it MUST decide to bounce or downgrade if receiving MTA is EAI
   incompliant.

   Both ALT-ADDR parameter and ATOMIC parameter is specified in one
   envelope from/to, use ALT-ADDR parameter and ignore ATOMIC parameter.

   MTA generates alternative US-ASCII address when ALT-ADDR option is
   not specified and ATOMIC is "y".

   Further, even if no downgrading is performed for envelope from/to,
   MUA/MTA MUST downgrade mail headers including UTF-8 or bounce.  This
   is described in next section.




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   Algorithmic address generation method is below:
   domain-part: Punycode/IDNA [RFC3490]
   local-part: Punycode[RFC3492] without normalization.  Prefix MUST be
      assigned by IANA (which is not "xn--").

   MTA replaces Non-ASCII mail address with specified or generated
   alternative US-ASCII address.  Then appends replaced information with
   EAI-Downgraded-From and EAI-Downgraded-To header in mail header
   (outgoing SMTP DATA).
      EAI-Downgraded-From: <Non-ASCII,ATOMIC> <US-ASCII>
      EAI-Downgraded-From: <Non-ASCII,US-ASCII> <US-ASCII>
      EAI-Downgraded-To: <Non-ASCII,ATOMIC> <US-ASCII>
      EAI-Downgraded-To: <Non-ASCII,US-ASCII> <US-ASCII>

   Note that when downgrading, not to disclose whole recipient address,
   MUA/MTA SHOULD make SMTP connection per each recipient address.

   Also note that by appending EAI-Downgraded-From/To headers, MUA/MTA
   MUST perform SMTP DATA/Header downgrading.  This is described in next
   section.

   Downgraded local-part is parsed only in MDA.  MDA delivers the mail
   to final mailbox.

   Case study: SPF check

   SPF checks domainname of the envelope from and smtp connection IP
   address.  If ALT-ADDR domainname is Punycode/IDNA form of Non-ASCII
   domainname, it will be compatible with current SPF.  In this case,
   SPF check will be performed correctly.  Otherwise, more detailed
   consideration is required.


5.  SMTP DATA/Header downgrading

   In this section, three methods for SMTP DATA/Header downgrading is
   proposed.  Working group should select one.

   o  No header downgrading
   o  Encapsulating whole SMTP DATA
   o  Translating each header

   Target and non-target of downgrading elements in mail headers (SMTP
   data) are below:







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   Originator address(es): Non-ASCII mail addresses in From, Reply-To,
      Sender and their Resent- headers MUST be target of downgrading.
   Destination address(es): Non-ASCII mail addresses in To, CC, Bcc and
      their Resent- headers MUST be target of downgrading.
   IDs: IDs such as Message-ID, Date, In-Reply-To and References MUST
      NOT be target of downgrading.
   Trace headers: Received headers which contains Non-ASCII mail
      addresses MUST be target of downgrading.
   other headers: UTF-8 in other headers MUST be target of downgrading.

   Rewriting Received header is prohibited in [RFC2821] Section 4.4
   Trace field.  But downgrading may be considered as the 'Mail
   Gatewaying' which is described in [RFC2821] Section 3.8.  If it is
   true, these downgrading methods are acceptable.

5.1.  No header downgrading

   Most MTAs support 8bit characters in mail headers.  Currently, mail
   systems in some countries or languages use raw 8bit header value in
   their local encoding.  This method does not care about using UTF-8
   headers in existing mail systems.

   Pros:
      *  Easy to implement.
   Cons:
      *  This method may break existing mail infrastructure.

5.2.  Downgrading with MIME encapsulation

   This downgrading method requires new MIME 'Content-Type:' which
   express EAI.  This document assumes 'Content-Type: Message/EAI'
   existence.

   Downgrading:
      *  If mail header contains UTF-8 data, downgrade whole message to
         be MIME encoded.  Whole message becomes new MIME part (Message/
         EAI).
      *  Message-ID, Subject, Date headers are copied from original
         header.
      *  From header is generated with downgraded Envelope-from.
      *  To header is generated with single downgraded Envelope-to.
      *  If Subject header contains UTF-8, it is replaced to a certain
         message or encoded by MIME [RFC2047].
      *  Message-ID, Date headers are preserved.
      As a result, new body contains one new MIME part (Message/EAI).






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   Upgrading:
      *  If mail message contains only one MIME part and its Content-
         Type is 'Message/EAI', it may be a downgraded message.  To
         check if it is a downgraded message, compare mail body's
         message-id and MIME part's message-id.  If message-ids are the
         same, it is a downgraded message.  Then, treat MIME part as
         entire mail message.
      *  When checking trace field, checker SHOULD check Received header
         both in wrapping headers and headers in encapsulated part.

   Case study: DKIM

   DKIM checker performs upgrading the downgraded message first.

   Pros:
      *  MTA does not need to decode each header carefully.
      *  Whole headers can be submitted AS IS.
   Cons:
      *  Non-ASCII from/to can not distinguish from downgraded mail
         headers.
      *  EAI incompliant MUA can not treat any downgraded mail.

   [[Reference to [EAI-Scenarios] and evaluation of each case should be
   described here.]]

5.2.1.  Downgrading with MIME encapsulation example

























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   Downgrading example

   Message-Id: MESSAGE_ID
   Mime-Version: 1.0
   Content-Type: Multipart/Mixed;
      boundary="--Next_Part(unique_string)--"
   Content-Transfer-Encoding: 8bit
   Subject: DOWNGRADED_SUBJECT
   From: <US-ASCII_FROM>
   To: <US-ASCII_TO>
   Date: DATE

   ----Next_Part(unique_string)--
   Content-Type: Message/EAI
   Content-Transfer-Encoding: 8bit
   Content-Disposition: inline

   EAI-Downgraded-From: <Non-ASCII,ATOMIC> <US-ASCII_FROM>
   EAI-Downgraded-To: <Non-ASCII,ATOMIC> <US-ASCII_TO>
   Received: ...
   Received: ...
   Message-Id: MESSAGE_ID
   Mime-Version: 1.0
   Subject: UTF-8_SUBJECT
   From: <Non-ASCII,ATOMIC>
   To: <Non-ASCII,ATOMIC>
   Date: DATE

   MAIL_BODY

   ----Next_Part(unique_string)----


5.3.  Header conversion

   Define conversion method to US-ASCII for each header which may
   contain Non-ASCII characters.  Each header has its own downgrading
   method.

   To preserve all header information, define generic encapsulation
   header: "Downgraded: HeaderName: HeaderValue".  The header value is
   encoded by [RFC2047] with UTF-8 tag.

   Downgrading:
      *  For all headers, check if the header contains UTF-8 characters.
      *  Encapsulate 'i-Email' header in Downgraded header.





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      *  If the header contains UTF-8 characters,
         +  If the header is an address header which is described in
            Section 5.3.1,
            -  Preserve the header in 'Downgraded' header.
            -  Downgrade the header defined in Section 5.3.1.
         +  The other header case, encode the header by [RFC2047] with
            UTF-8 tag.
   Upgrading:
      *  If the mail has 'Downgraded' headers, the mail is a downgraded
         EAI mail message.
      *  Decode all 'Downgraded' header.
         +  Decode header value field string which is [RFC2047] encoded.
         +  If the header is address headers described in Section 5.3.1,
            -  Apply address header downgrading to the decoded header.
            -  Remove the header line which is same to the downgraded
               line.
         +  Remove the 'Downgraded' header.
         +  Add decoded header to mail header.  "HeaderName:
            HeaderValue".
      *  If each mail header has [RFC2047] encoded part and which
         encoding is "UTF-8", it is a downgraded header, so decode it.

   Pros:
      *  EAI incompliant MUA displays the downgraded mail body except
         original Non-ASCII mail addresses.
      *  EAI incompliant MUA displays and handles the sender specified
         or algorithmic address.
      *  EAI compliant MUA displays and handles original headers.
   Cons:
      *  Implementation and processing cost is higher than 'Header
         Encapsulation' defined in Section 5.2 because MUA/MTA must
         parse each header and encode it by defined method.
      *  Hard to preserve whole information AS IS.  The address headers
         are preserved but the other headers which is [RFC2047] encoded
         with UTF-8 tag are not distinguished that it is downgraded or
         it is encoded by sender's MUA.  Therefore, to check DKIM
         requires special consideration.

   [[Reference to [EAI-Scenarios] and evaluation of each case should be
   described here.]]

5.3.1.  Downgrading address headers

   This section targets From, Sender, Reply-To, To, CC, BCC, Resent-
   From, Resent-To, Resent-CC, Resent-Bcc, Resent-sender headers which
   contains Originator/Destination address(es).

   The header value is composed of single or multiple mailbox/angle-addr



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   fields defined in [EAI-UTF8].

   If the header contains UTF-8 characters, downgrading method is
   follows.
   1.  Extract every field and downgrade mailbox/angle-addr described
       below.
   2.  By mailbox/angle-addr downgrading, if the field became empty, the
       field should be removed.
   3.  If all header field is removed, remove the header.
   4.  If From header is removed, generate new From header from
       envelope-from address.

   EAI angle-addr defined in [EAI-UTF8] consists of 4 forms.
   Downgrading method is defined for each form.
   1.  <Non-ASCII>
          Non-ASCII mail address without ALT-ADDR and ATOMIC parameter
          case, remove this angle-addr.
   2.  <Non-ASCII,US-ASCII>
          Non-ASCII mail address with sender-specified US-ASCII address
          case, replace it as <US-ASCII>.
   3.  <Non-ASCII,ATOMIC>
          Non-ASCII mail address with ATOMIC parameter case, generate
          the algorithmic address from Non-ASCII mail address and
          replace it as <ALG-ASCII>.
   4.  <US-ASCII>
          US-ASCII mail address case, preserve it.

   "mailbox" is defined as "DISPLAY NAME angle-addr" in [EAI-UTF8].  The
   "DISPLAY NAME" field should be encoded by [RFC2047] with UTF-8 tag,
   if necessary.  If the angle-addr is removed, remove the field
   including "DISPLAY NAME".

5.3.2.  Header conversion example


















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   Original EAI message

   i-Email: 1.0
   Message-Id: MESSAGE_ID
   Mime-Version: 1.0
   Content-Type: text/plain; charset="UTF-8"
   Content-Transfer-Encoding: 8bit
   Subject: UTF-8_SUBJECT
   From: <NON-ASCII-FROM,ASCII-FROM>
   To: <NON-ASCII-TO,ASCII-TO>
   CC: <NON-ASCII-CC,ASCII-CC>
   Date: DATE

   MAIL_BODY


   SMTP downgrading adds EAI-Downgraded-From, EAI-Downgraded-To headers.


   EAI-Downgraded-From: <Non-ASCII,DOWNGRADED_FROM> <DOWNGRADED_FROM>
   EAI-Downgraded-To: <Non-ASCII,DOWNGRADED_TO> <DOWNGRADED_TO>


   Result of the header conversion downgrading.


   EAI-Downgraded-From:
       MIME(<Non-ASCII,DOWNGRADED_FROM>) <DOWNGRADED_FROM>
   EAI-Downgraded-To:
       MIME(<Non-ASCII,DOWNGRADED_TO>) <DOWNGRADED_TO>
   Downgraded: i-Email: 1.0
   Message-Id: MESSAGE_ID
   Mime-Version: 1.0
   Content-Type: text/plain; charset="UTF-8"
   Content-Transfer-Encoding: 8bit
   Subject: MIME(UTF-8_SUBJECT)
   Downgraded: From: MIME(<NON-ASCII-FROM,ASCII-FROM>)
   From: <ASCII-FROM>
   Downgraded: To: MIME(<NON-ASCII-TO,ASCII-TO>)
   To: <ASCII-TO>
   Downgraded: CC: MIME(<NON-ASCII-CC,ASCII-CC>)
   CC: <ASCII-CC>
   Date: DATE

   MAIL_BODY


   MIME() stands for [RFC2047] encoding.



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6.  Implementation consideration

6.1.  MUA

   EAI compliant MUA MUST implement downgrading mechanism for sending.

   MUA MAY encode UTF-8 in Subject header with the same encoding of body
   part while downgrading.

   EAI compliant MUA MUST upgrade downgraded mail and MUST show Non-
   ASCII mail addresses on display.

6.2.  MDA Requirements

   This section describes downgrading in MDA.
   1.  MDA MUST NOT upgrade.
   2.  Perform downgrading for each Storage/Back-end-Process.  If and
       only if MDA knows recipient's MUA is EAI compliant, then no
       downgrading is performed.
   3.  If MDA detects that SMTP recipient address is an algorithmic
       address, then MDA MUST decode it and perform the same processing
       as if it were Non-ASCII mail address.  MDA MAY normalize or
       canonicalize local-part before processing it.


7.  Security considerations

   See the extended security considerations discussion in [EAI-Overview]


8.  IANA Considerations

   To distinguish downgraded Non-ASCII mail addresses in ACE form, it
   MUST have ACE-Prefix.  The ACE-Prefix MUST differ from IDNA ACE-
   Prefix to avoid possible confusion.  IANA will assign Non-ASCII mail
   address ACE-Prefix when RFC is published.


9.  Acknowledgements

   John Klensin, Harald Alvestrand, Chris Newman, Charles Lindsey,
   Marcos Sanz, Alexey Melnikov, and JET members.

10.  Normative References

   [EAI-Overview]
              Klensin, J. and Y. Ko, "Overview and Framework for
              Internationalized Email", draft-ietf-eai-framework-01



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              (work in progress).

   [EAI-SMTPext]
              Yao, J., Ed., "SMTP extension for internationalized email
              address", draft-ietf-eai-smtpext-00 (work in progress),
              Febrary 2006.

   [EAI-Scenarios]
              Alvestrand, H., "Internationalized Email Addresses:
              Scenarios", draft-ietf-eai-scenarios-00 (work in
              progress), May 2006.

   [EAI-UTF8]
              Yeh, J., "Internationalized Email Headers",
              draft-yeh-ima-utf8headers-01 (work in progress),
              February 2006.

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

   [RFC2821]  Klensin, J., "Simple Mail Transfer Protocol", RFC 2821,
              April 2001.

   [RFC2822]  Resnick, P., "Internet Message Format", RFC 2822,
              April 2001.

   [RFC3490]  Faltstrom, P., Hoffman, P., and A. Costello,
              "Internationalizing Domain Names in Applications (IDNA)",
              RFC 3490, March 2003.

   [RFC3492]  Costello, A., "Punycode: A Bootstring encoding of Unicode
              for Internationalized Domain Names in Applications
              (IDNA)", RFC 3492, March 2003.

















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Authors' Addresses

   Yoshiro YONEYA (editor)
   JPRS
   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


   Kazunori Fujiwara (editor)
   JPRS
   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































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