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RFC 5335 - Internationalized Email Headers


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Network Working Group                                       Y. Abel, Ed.
Request for Comments: 5335                                         TWNIC
Updates: 2045, 2822                                       September 2008
Category: Experimental

                    Internationalized Email Headers

Status of This Memo

   This memo defines an Experimental Protocol for the Internet
   community.  It does not specify an Internet standard of any kind.
   Discussion and suggestions for improvement are requested.
   Distribution of this memo is unlimited.

Abstract

   Full internationalization of electronic mail requires not only the
   capabilities 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 the 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.
   This form is permitted in transmission only if authorized by an SMTP
   extension, as specified in an associated specification.  This
   specification Updates section 6.4 of RFC 2045 to conform with the
   requirements.

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.  UTF-8 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.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 12
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 13

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 [RFC5336].

   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 [RFC5336] 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
   specification means that people who participate in the experiment
   have to consider those standards updated.

   Allowing use of a content-transfer-encoding on subtypes of messages
   is not limited to transmissions that are authorized by the SMTP
   extension specified in [RFC5336].  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.

   These users are also likely to use non-ASCII text in their common
   names and subjects of email messages, both received and sent.  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 [RFC5336],
   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 [RFC5336], 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 prevent 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 transferring 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 [EAI-POP] and [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 [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 [RFC5336].

   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].

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 is 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 the new format.  The following
   ABNF is defined to substitute those definitions in [RFC2822].

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

4.1.  UTF-8 Syntax and Normalization

   UTF-8 characters can be defined in terms of octets using the
   following ABNF [RFC5234], 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

   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 -- it allows newly
   defined MIME types to permit content-transfer-encoding, and it allows
   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 are intended to extend the corresponding rules in
   [RFC2822] in order to allow UTF-8 characters.

   FWS     =  <see [RFC2822], folding white space>

   CFWS    =  <see [RFC2822], 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
   characters in <addr-spec>.  This would also allow UTF-8 characters in
   <message-id>, which 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

   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 Message Submission Servers ("MSAs") and Message Transfer Agents
   (MTAs) may downgrade internationalized messages as needed.  The
   procedure for doing so is described in [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 are a few examples of possible <mailbox> representations.

      "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

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 email address internationalization (EAI)-
   aware MTAs.  Note that, should downgrading be required, the uFor
   parameter is dropped per the procedure specified in [DOWNGRADE].

   The "Return-Path" header provides the email return address in the
   mail delivery.  Thus, the header is augmented to carry UTF-8
   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 the header is added at the last MTA and described
   in [RFC2821].

   The <item-value> on "Received:" syntax is augmented to allow UTF-8
   email address in the "For" field. <angle-addr> is augmented to
   include UTF-8 email address.  In order to allow UTF-8 email addresses
   in an <addr-spec>, <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
   [RFC5336] 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 MIME [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 [DOWNGRADE].

   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 down-convert 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 that 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 5335

   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".

   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 the 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
   (Public Key Infrastructure for X.509 Certificates) 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 that parse, store, or handle email addresses or
   local parts must take extra care not to overflow buffers, truncate
   addresses, or exceed storage allotments.  Also, they must take care,
   when comparing, to use the entire lengths of the addresses.

   In this specification, a user could provide an ASCII alternative
   address for a non-ASCII address.  However, it is possible these two
   addresses go to different mailboxes, or even different people.  This
   configuration may be based on a user's personal choice or 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 Domain Keys Identified Mail (DKIM), S/MIME, and OpenPGP is
   discussed in RFC 4952, Section 9.  A subsequent document [DOWNGRADE]
   will cover the impact of downgrading on these systems.

6.  IANA Considerations

   IANA has registered 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 thanks Jeff Yeh for his 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
   (Joint Engineering Team) and were incorporated into the document.
   The editor sincerely thanks them for their contributions.

8.  References

8.1.  Normative References

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

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

   [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.

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

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

8.2.  Informative References

   [DOWNGRADE]  Fujiwara, K. and Y. Yoneya, "Downgrading mechanism for
                Email Address Internationalization", Work in Progress,
                July 2008.

   [EAI-POP]    Newman, C. and R. Gellens, "POP3 Support for UTF-8",
                Work in Progress, July 2008.

   [IMAP-UTF8]  Resnick, P. and C. Newman, "IMAP Support for UTF-8",
                Work in Progress, April 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.

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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