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Versions: 00 01 02 03 04 05 06 07 08 09 10 11 12 RFC 5335

Network Working Group                                       Y. Abel, Ed.
Internet-Draft                                                     TWNIC
Intended status: Experimental                              July 09, 2008
Expires: January 10, 2009


                    Internationalized Email Headers
                   draft-ietf-eai-utf8headers-12.txt

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
   have been or will be disclosed, and any of which he or she becomes
   aware will be disclosed, in accordance with Section 6 of BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   The list of current Internet-Drafts can be accessed at
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   The list of Internet-Draft Shadow Directories can be accessed at
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   This Internet-Draft will expire on January 10, 2009.

Abstract

   Full internationalization of electronic mail requires not only the
   capability to transmit non-ASCII content, to encode selected
   information in specific header fields, and to use non-ASCII
   characters in envelope addresses.  It also requires being able to
   express those addresses and information based on them in mail header
   fields.  This document specifies an experimental variant of Internet
   mail that permits the use of Unicode encoded in UTF-8, rather than
   ASCII, as the base form for Internet email header field bodies.  This
   form is permitted in transmission only if authorized by an SMTP
   extension, as specified in an associated specification.  And this
   specification updates section 6.4 of [RFC2045] to conform with the
   requirements.



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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Role of this specification . . . . . . . . . . . . . . . .  3
     1.2.  Relation to other standards  . . . . . . . . . . . . . . .  3
   2.  Background and History . . . . . . . . . . . . . . . . . . . .  3
   3.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   4.  Changes on Message Header Fields . . . . . . . . . . . . . . .  5
     4.1.  UTF8 Syntax and Normalization  . . . . . . . . . . . . . .  5
     4.2.  Changes on MIME headers  . . . . . . . . . . . . . . . . .  6
     4.3.  Syntax extensions to RFC 2822  . . . . . . . . . . . . . .  6
     4.4.  Change on addr-spec syntax . . . . . . . . . . . . . . . .  8
     4.5.  Trace field syntax . . . . . . . . . . . . . . . . . . . .  9
     4.6.  message/global . . . . . . . . . . . . . . . . . . . . . .  9
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . . 11
   6.  IANA considerations  . . . . . . . . . . . . . . . . . . . . . 12
   7.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 12
   8.  Edit history . . . . . . . . . . . . . . . . . . . . . . . . . 12
     8.1.  draft-ietf-eai-utf8header-12 . . . . . . . . . . . . . . . 12
     8.2.  draft-ietf-eai-utf8header-11 . . . . . . . . . . . . . . . 12
     8.3.  draft-ietf-eai-utf8header-10 . . . . . . . . . . . . . . . 12
     8.4.  draft-ietf-eai-utf8header-09 . . . . . . . . . . . . . . . 13
     8.5.  draft-ietf-eai-utf8header-08 . . . . . . . . . . . . . . . 13
     8.6.  draft-ietf-eai-utf8header-07 . . . . . . . . . . . . . . . 13
     8.7.  draft-ietf-eai-utf8header-06 . . . . . . . . . . . . . . . 13
     8.8.  draft-ietf-eai-utf8header-05 . . . . . . . . . . . . . . . 13
     8.9.  draft-ietf-eai-utf8header-04 . . . . . . . . . . . . . . . 13
     8.10. draft-ietf-eai-utf8header-03 . . . . . . . . . . . . . . . 13
     8.11. draft-ietf-eai-utf8header-02 . . . . . . . . . . . . . . . 14
     8.12. draft-ietf-eai-utf8header-01 . . . . . . . . . . . . . . . 14
     8.13. draft-ietf-eai-utf8header-00 . . . . . . . . . . . . . . . 14
     8.14. draft-yeh-ima-utf8header-01  . . . . . . . . . . . . . . . 14
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 14
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 14
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 15
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 15
   Intellectual Property and Copyright Statements . . . . . . . . . . 17














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1.  Introduction

1.1.  Role of this specification

   Full internationalization of electronic mail requires several
   capabilities:

   o  The capability to transmit non-ASCII content, provided for as part
      of the basic MIME specification [RFC2045], [RFC2046].
   o  The capability to use international characters in envelope
      addresses, discussed in [RFC4952] and specified in
      [I-D.ietf-eai-smtpext].
   o  The capability to express those addresses, and information related
      to them and based on them, in mail header fields, defined in this
      document.

   This document specifies an experimental variant of Internet mail that
   permits the use of Unicode encoded in UTF-8 [RFC3629], rather than
   ASCII, as the base form for Internet email header fields.  This form
   is permitted in transmission, if authorized by the SMTP extension
   specified in [I-D.ietf-eai-smtpext] or by other transport mechanisms
   capable of processing it.

1.2.  Relation to other standards

   This document updates section 6.4 of RFC 2045.  It removes the
   blanket ban on applying a content-transfer-encoding to all subtypes
   of message/, and instead specifies that a composite subtype MAY
   specify whether or not a content-transfer-encoding can be used for
   that subtype, with "cannot be used" as the default.

   This document also updates [RFC2822] and MIME ([RFC2045]), and the
   fact that an experimental specification updates a standards-track
   spec means that people who participate in the experiment have to
   consider those standards updated.

   Allowing of use a content-transfer-encoding on subtypes of messages
   is not limited to transmissions, which are authorized by the SMTP
   extension specified in [I-D.ietf-eai-smtpext].  Message/global
   permits use of a content-transfer-encoding.


2.  Background and History

   Mailbox names often represent the names of human users.  Many of
   these users throughout the world have names that are not normally
   expressed with just the ASCII repertoire of characters, and would
   like to use more or less their real names in their mailbox names.



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   These users are also likely to use non-ASCII text in their common
   names and subjects of email messages, both in what they send and what
   they receive.  This protocol specifies UTF-8 as the encoding to
   represent email header field bodies.

   The traditional format of email messages [RFC2822] allows only ASCII
   characters in the header fields of messages.  This prevents users
   from having email addresses that contain non-ASCII characters.  It
   further forces non-ASCII text in common names, comments, and in free
   text (such as in the Subject: field) to be encoded (as required by
   MIME format [RFC2047]).  This specification describes a change to the
   email message format that is related to the SMTP message transport
   change described in the associated document [RFC4952] and
   [I-D.ietf-eai-smtpext], and that allows non-ASCII characters in most
   email header fields.  These changes affect SMTP clients, SMTP
   servers, mail user agents (MUAs), list expanders, gateways to other
   media, and all other processes that parse or handle email messages.

   As specified in [I-D.ietf-eai-smtpext], an SMTP protocol extension
   "UTF8SMTP" is used to prevent the transmission of messages with UTF-8
   header fields to systems that cannot handle such messages.

   Use of this SMTP extension helps prevents the introduction of such
   messages into message stores that might misinterpret, improperly
   display, or mangle such messages.  It should be noted that using an
   ESMTP extension does not prevent transfering email messages with
   UTF-8 header fields to other systems that use the email format for
   messages and that may not be upgraded, such as unextended POP and
   IMAP servers.  Changes to these protocols to handle UTF-8 header
   fields are addressed in [I-D.ietf-eai-pop] and
   [I-D.ietf-eai-imap-utf8] .

   The objective for this protocol is to allow UTF-8 in email header
   fields.  Issues such as how to handle messages containing UTF-8
   header fields that have to be delivered to systems that have not been
   upgraded to support this capability are discussed in
   [I-D.ietf-eai-downgrade].


3.  Terminology

   A plain ASCII string is also a valid UTF-8 string, see [RFC3629].  In
   this document, ordinary ASCII characters are UTF-8 characters if they
   are in headers which contain <utf8-xtra-char>s.

   Unless otherwise noted, all terms used here are defined in [RFC2821],
   [RFC2822], [RFC4952], or [I-D.ietf-eai-smtpext].




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   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 [RFC2119].

   This document is discussed on the ima mailing list.  See
   https://www1.ietf.org/mailman/listinfo/ima for information about
   subscribing.  The list's archive is at
   http://www1.ietf.org/mail-archive/web/ima/index.html.


4.  Changes on Message Header Fields

   SMTP clients can send header fields in UTF-8 format, if the UTF8SMTP
   extension is advertised by the SMTP server or as permitted by other
   transport mechanisms.

   This protocol does NOT change the [RFC2822] rules for defining header
   field names.  The bodies of header fields are allowed to contain
   UTF-8 characters, but the header field names themselves must contain
   only ASCII characters.

   To permit UTF-8 characters in field values, the header definition in
   [RFC2822] must be extended to support new format.  The following ABNF
   is defined to substitute those definition in [RFC2822].

   The syntax rules not covered in this section remain as defined in
   [RFC2822].

4.1.  UTF8 Syntax and Normalization

   UTF-8 characters can be defined in terms of octets using the
   following ABNF, taken from [RFC3629]:

   UTF8-xtra-char  =   UTF8-2 / UTF8-3 / UTF8-4

   UTF8-2          =   %xC2-DF UTF8-tail

   UTF8-3          =   %xE0 %xA0-BF UTF8-tail /
                       %xE1-EC 2(UTF8-tail) /
                       %xED %x80-9F UTF8-tail /
                       %xEE-EF 2(UTF8-tail)

   UTF8-4          =   %xF0 %x90-BF 2( UTF8-tail ) /
                       %xF1-F3 3( UTF8-tail ) /
                       %xF4 %x80-8F 2( UTF8-tail )

   UTF8-tail       =   %x80-BF




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   These are normatively defined in [RFC3629], but kept in this document
   for reasons of convenience.

   See [RFC5198] for a discussion of normalization, the use of
   normalization form NFC is RECOMMENDED.

4.2.  Changes on MIME headers

   This specification updates section 6.4 of [RFC2045].  [RFC2045]
   prohibits applying a content-transfer-encoding to all subtypes of
   message/.  This specification relaxes the rule, allowing newly
   defined MIME types to permit content-transfer-encoding, and permits
   content-transfer-encoding for message/global (see Section 4.6).

   Background: Normally, transfer of message/global will be done in
   8-bit-clean channels, and body parts will have "identity" encodings,
   that is, no decoding is necessary.  In the case where a message
   containing a message/global is downgraded from 8-bit to 7-bit as
   described in [RFC1652]., an encoding may be applied to the message;
   if the message travels multiple times between a 7-bit environment and
   an environment implementing UTF8SMTP, multiple levels of encoding may
   occur.  This is expected to be rarely seen in practice, and the
   potential complexity of other ways of dealing with the issue are
   thought to be larger than the complexity of allowing nested encodings
   where necessary.

4.3.  Syntax extensions to RFC 2822

   The following rules intended to extend the corresponding rules in
   [RFC2822] to allow UTF8 characters.


   FWS     =  <see [RFC 2822], folding white space>

   CFWS    =  <see [RFC 2822], folding white space>

   ctext   =/  UTF8-xtra-char

   utext   =/  UTF8-xtra-char

   comment = "(" *([FWS] utf8-ccontent) [FWS] ")"

   word    = utf8-atom / utf8-quoted-string


   This means that all the [RFC2822] constructs that build upon these
   will permit UTF-8 characters, including comments and quoted strings.
   We do not change the syntax of <atext> in order to allow UTF8



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   characters in <addr-spec>, because this would also allow UTF8
   characters in <message-id>, it is not allowed due to the limitation
   described in Section 4.5.  Instead, <utf8-atext> is added to meet
   this requirement.

   utf8-text   =  %d1-9 /         ; all UTF-8 characters except
              %d11-12 /       ; US-ASCII NUL, CR and LF
              %d14-127 /
              UTF8-xtra-char

   utf8-quoted-pair   = ("\" utf8-text) / obs-qp

   utf8-qcontent      = utf8-qtext / utf8-quoted-pair

   utf8-quoted-string = [CFWS]
                           DQUOTE *([FWS] utf8-qcontent) [FWS] DQUOTE
                           [CFWS]

   utf8-ccontent = ctext / utf8-quoted-pair / comment
   utf8-qtext= qtext / UTF8-xtra-char


   utf8-atext   =  ALPHA / DIGIT /
                   "!" / "#" /     ; Any character except
                   "$" / "%" /     ; controls, SP, and specials.
                   "&" / "'" /     ; Used for atoms
                   "*" / "+" /
                   "-" / "/" /
                   "=" / "?" /
                   "^" / "_" /
                   "`" / "{" /
                   "|" / "}" /
                   "~" /
                   UTF8-xtra-char

   utf8-atom     = [CFWS] 1*utf8-atext [CFWS]

   utf8-dot-atom = [CFWS] utf8-dot-atom-text [CFWS]

   utf8-dot-atom-text = 1*utf8-atext *("." 1*utf8-atext)

   qcontent      = utf8-qcontent

   To allow the use of UTF-8 in a Content-Description header field
   [RFC2045], the following syntax is used:

   description   = "Content-Description:" unstructured CRLF




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   The <utext> syntax is extended above to allow UTF-8 in all
   <unstructured> header fields.
   Note, however, this does not remove any constraint on the character
   set of protocol elements; for instance, all the allowed values for
   timezone in the Date: headers are still expressed in ASCII.  And
   also, none of this revised syntax changes what is allowed in a
   <msg-id>, which will still remain in pure ASCII.

4.4.  Change on addr-spec syntax

   Internationalized email addresses are represented in UTF-8.  Thus,
   all header fields containing <mailbox>es are updated to permit UTF-8
   as well as an additional, optional all-ascii alternate address.  Note
   that MSAs and MTAs may downgrade internationalized messages as
   needed.  The procedure for doing so is described in
   [I-D.ietf-eai-downgrade].

mailbox        =  name-addr / addr-spec / utf8-addr-spec

angle-addr     =/ [CFWS] "<" utf8-addr-spec [ alt-address ] ">" [CFWS] /
                  obs-angle-addr

utf8-addr-spec =  utf8-local-part "@" utf8-domain

utf8-local-part=  utf8-dot-atom / utf8-quoted-string / obs-local-part

utf8-domain    =  utf8-dot-atom / domain-literal / obs-domain

alt-address    =  FWS "<" addr-spec ">"

   Below list a few possible <mailbox> representation as example.


      "DISPLAY_NAME" <ASCII@ASCII>
         ; traditional mailbox format

      "DISPLAY_NAME" <non-ASCII@non-ASCII>
         ; UTF8SMTP but no ALT-ADDRESS parameter provided,
         ; message will bounce if UTF8SMTP extension is not supported

      <non-ASCII@non-ASCII>
             ; without DISPLAY_NAME and quoted string
             ; UTF8SMTP but no ALT-ADDRESS parameter provided,
         ; message will bounce if UTF8SMTP extension is not supported

      "DISPLAY_NAME" <non-ASCII@non-ASCII <ASCII@ASCII>>
         ; UTF8SMTP with ALT-ADDRESS parameter provided,
         ; ALT-ADDRESS can be used if downgrade is necessary



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4.5.  Trace field syntax

   "For" fields containing internationalized addresses are allowed, by
   use of the new uFor syntax.  UTF-8 information may be needed in
   Received fields.  Such information is therefore allowed to preserve
   the integrity of those fields.  The uFor syntax retains the original
   UTF-8 email address between EAI-aware MTAs.  Note that, should
   downgrading be required, the uFor parameter is dropped per the
   procedure specified in [I-D.ietf-eai-downgrade].

   The "Return-Path" header provides the email return address in the
   mail delivery.  Thus, it is augmented to carry UTF8 addresses (see
   the revised syntax of <angle-addr> in Section 4.4 of this document).
   This will not break the rule of trace field integrity, because it is
   added at the last MTA.

   The <item-value> on "Received:" syntax is augmented to allow UTF-8
   email address on "For" clause. <angle-addr> is augmented to include
   UTF-8 email address.  To allow UTF-8 email address also on syntax
   corresponding of <addr-spec> on original syntax, <utf8-addr-spec> is
   added to <item-value>.



   item-value      =/      utf8-addr-spec


4.6.  message/global

   Internationalized messages must only be transmitted as authorized by
   [I-D.ietf-eai-smtpext] or within a non-SMTP environment which
   supports these messages.  A message is a "message/global message", if
   o  it contains UTF-8 header values as specified in this document, or
   o  it contains UTF-8 values in the headers fields of body parts.

   The type message/global is similar to message/rfc822, except that it
   contains a message that can contain UTF-8 characters in the headers
   of the message or body parts.  If this type is sent to a 7-bit-only
   system, it has to be encoded in [RFC2045].  (Note that a system
   compliant with MIME that doesn't recognize message/global would treat
   it as "application/octet-stream" as described in Section 5.2.4 of
   [RFC2046].)

   Alternatively, SMTP servers and other systems which transfer a
   message/global body part MAY choose to down-convert it to a message/
   rfc822 body part using the rules described in
   [I-D.ietf-eai-downgrade].




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   Type name:  message

   Subtype name:  global

   Required parameters:  none

   Optional parameters:  none

   Encoding considerations:  Any content-transfer-encoding is permitted.
      The 8-bit or binary content-transfer-encodings are recommended
      where permitted.

   Security considerations:  See Section 5

   Interoperability considerations:  The media type provides
      functionality similar to the message/rfc822 content type for email
      messages with international email headers.  When there is a need
      to embed or return such content in another message, there is
      generally an option to use this media type and leave the content
      unchanged or downconvert the content to message/rfc822.  Both of
      these choices will interoperate with the installed base, but with
      different properties.  Systems unaware of international headers
      will typically treat a message/global body part as an unknown
      attachment, while they will understand the structure of a message/
      rfc822.  However, systems which understand message/global will
      provide functionality superior to the result of a down-conversion
      to message/rfc822.  The most interoperable choice depends on the
      deployed software.

   Published specification:  RFC XXXX

   Applications that use this media type:  SMTP servers and email
      clients that support multipart/report generation or parsing.
      Email clients which forward messages with international headers as
      attachments.

   Additional information:

   Magic number(s):  none

   File extension(s):  The extension ".u8msg" is suggested.

   Macintosh file type code(s):  A uniform type identifier (UTI) of
      "public.utf8-email-message" is suggested.  This conforms to
      "public.message" and "public.composite-content" but does not
      necessarily conform to "public.utf8-plain-text".





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   Person & email address to contact for further information:  See the
      Author's address section of this document.

   Intended usage:  COMMON

   Restrictions on usage:  This is a structured media type which embeds
      other MIME media types.  The 8-bit or binary content-transfer-
      encoding MUST be used unless this media type is sent over a 7-bit
      only transport.

   Author:  See Author's Address section of this document.

   Change controller:  IETF Standards Process


5.  Security Considerations

   If a user has a non-ASCII mailbox address and an ASCII mailbox
   address, a digital certificate that identifies that user may have
   both addresses in the identity.  Having multiple email addresses as
   identities in a single certificate is already supported in PKIX and
   OpenPGP.

   Because UTF-8 often requires several octets to encode a single
   character, internationalized local parts may cause mail addresses to
   become longer.  As specified in [RFC2822], each line of characters
   MUST be no more 998 octets, excluding the CRLF.

   Because internationalized local parts may cause email addresses to be
   longer, processes which parse, store, or handle email addresses or
   local parts must take extra care not to overflow buffers, truncate
   addresses, exceed storage allotments, or, when comparing, fail to use
   the entire length.

   In this specification, a user could provide an ASCII alternative
   address for a non-ASCII address.  However, it is possible these two
   address go to different mailboxes, or even different persons.  This
   configuration may be based on a user's personal choice, or based on
   administration policy.  We recognize that if ASCII and non-ASCII
   email is delivered to two different destinations, based on MTA
   capability, this may violate the principle of least astonishment, but
   this is not a "protocol problem".

   The security impact of UTF-8 headers on email signature systems such
   as DKIM, S/MIME and OpenPGP is discussed in RFC 4952 section 9.  A
   subsequent document [I-D.ietf-eai-downgrade] will cover the impact of
   downgrading on these systems.



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6.  IANA considerations

   IANA is asked to register the message/global MIME type using the
   registration form contained in Section 4.4.


7.  Acknowledgements

   This document incorporates many ideas first described in Internet
   Draft form by Paul Hoffman, although many details have changed from
   that earlier work.

   The author especially thank Jeff Yeh for their efforts and
   contributions on editing previous versions.

   Most of the content of this document is provided by John C Klensin.
   Also some significant comments and suggestions were received from
   Charles H. Lindsey, Kari Hurtta, Pete Resnick, Alexey Melnikov, Chris
   Newman, Yangwoo KO, Yoshiro YONEYA, and other members of the JET team
   and were incorporated into the document.  The editor is much great
   thanks to their contribution sincerely.


8.  Edit history

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

8.1.  draft-ietf-eai-utf8header-12

   1.  Sentences modified
   2.  Update [RFC2045] into the Abstract
   3.  Update security mechanisms descriptions in Section 5

8.2.  draft-ietf-eai-utf8header-11

   1.  Sentences modified

8.3.  draft-ietf-eai-utf8header-10

   1.  Revise some paragraphs
   2.  Correct typos of ABNF
   3.  Note <qtext> and <text> of 2822bis
   4.  Fixed some idnits warnning







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8.4.  draft-ietf-eai-utf8header-09

   1.  Delete Section 5 (Addtional Issues)
   2.  Correct two typos of ABNF
   3.  Refine normalization issue to refer to [RFC5198]
   4.  Note <qtext> and <text> of 2822bis
   5.  Revise Section 6

8.5.  draft-ietf-eai-utf8header-08

   1.  Sentences modified

8.6.  draft-ietf-eai-utf8header-07

   1.  Modify subtype message/utf8smtp to message/global
   2.  Acknowledgements revise

8.7.  draft-ietf-eai-utf8header-06

   1.  ABNF revise.
   2.  Sentences modified
   3.  Add paragraph in Section 5
   4.  Add paragraph in Section 1.2
   5.  Modify Section 4.6

8.8.  draft-ietf-eai-utf8header-05

   1.  ABNF revise.
   2.  Remove original the section 4 (Pre-requirement)
   3.  Add UTF8SMTP message (Section 4.6)

8.9.  draft-ietf-eai-utf8header-04

   1.  ABNF revise.
   2.  Modify uFor description in Section 4.5

8.10.  draft-ietf-eai-utf8header-03

   1.  Editrial changes on terms and english.
   2.  ABNF revise.
   3.  addr-spec change, put ALT-ADDRESS inside "<" and ">" quote with
       "<" and ">".
   4.  Remove the "Header-Type" header.
   5.  Add uFor description in Section 4.5
   6.  Remove the content in IANA considerations since "Header-Type" is
       removed.





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8.11.  draft-ietf-eai-utf8header-02

   1.  Editrial changes on terms and english.
   2.  Change the header name "UTF8SMTP" to "Header-Type", and ABNF
       revise.
   3.  addr-spec change, put ALT-ADDRESS inside "<" and ">" quote with
       "[" and "]".
   4.  IANA considerations section rewrite.

8.12.  draft-ietf-eai-utf8header-01

   1.  ABNF revise.
   2.  Terminology sync with overview document.
   3.  addr-spec change, put ALT-ADDRESS inside "<" and ">" quote with
       "{" and "}".
   4.  add IANA considerations to register the new 2822 header
       "UTF8SMTP".
   5.  add Security considerations about relation of UTF8SMTP address to
       ALT-ADDRESS.

8.13.  draft-ietf-eai-utf8header-00

   1.  ABNF added.
   2.  Editrial changes.
   3.  Sent it as WG document.

8.14.  draft-yeh-ima-utf8header-01

   1.  Section re-arranged.
   2.  Remove content are not below to this document.


9.  References

9.1.  Normative References

   [I-D.ietf-eai-smtpext]
              Yao, J. and W. MAO, "SMTP extension for internationalized
              email address", draft-ietf-eai-smtpext-12 (work in
              progress), April 2008.

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

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




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

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

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

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

   [RFC5198]  Klensin, J. and M. Padlipsky, "Unicode Format for Network
              Interchange", RFC 5198, March 2008.

9.2.  Informative References

   [I-D.ietf-eai-downgrade]
              Fujiwara, K. and Y. Yoneya, "Downgrading mechanism for
              Email Address Internationalization",
              draft-ietf-eai-downgrade-07 (work in progress),
              March 2008.

   [I-D.ietf-eai-imap-utf8]
              Resnick, P. and C. Newman, "IMAP Support for UTF-8",
              draft-ietf-eai-imap-utf8-03 (work in progress),
              April 2008.

   [I-D.ietf-eai-pop]
              Newman, C. and R. Gellens, "POP3 Support for UTF-8",
              draft-ietf-eai-pop-03 (work in progress), February 2008.

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

   [RFC2046]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
              Extensions (MIME) Part Two: Media Types", RFC 2046,
              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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Author's Address

   Abel Yang (editor)
   TWNIC
   4F-2, No. 9, Sec 2, Roosvelt Rd.
   Taipei,   100
   Taiwan

   Phone: +886 2 23411313 ext 505
   Email: abelyang@twnic.net.tw









































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