Internationalized Email Headers
draft-ietf-eai-rfc5335bis-06

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 a 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 [RFC2045] (Freed, N. and N. Borenstein, “Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies,” November 1996.) to conform with the requirements.

Status of This Memo

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

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at http://datatracker.ietf.org/drafts/current/.

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

This Internet-Draft will expire on June 9, 2011.

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.

Table of Contents

1.  Introduction
    1.1.  Role of This Specification
    1.2.  Relation to Other Standards
2.  Background and History
3.  Terminology
4.  Changes on Message Header Fields
    4.1.  UTF-8 Syntax and Normalization
    4.2.  Changes on MIME Headers
    4.3.  Syntax Extensions to RFC 5322
    4.4.  Change on addr-spec Syntax
    4.5.  Trace Field Syntax
    4.6.  message/global
5.  Security Considerations
6.  IANA Considerations
7.  Acknowledgements
8.  Edit history
    8.1.  draft-ietf-eai-rfc5335bis-00
    8.2.  draft-ietf-eai-rfc5335bis-01
    8.3.  draft-ietf-eai-rfc5335bis-02
    8.4.  draft-ietf-eai-rfc5335bis-03
    8.5.  draft-ietf-eai-rfc5335bis-04
    8.6.  draft-ietf-eai-rfc5335bis-05
    8.7.  draft-ietf-eai-rfc5335bis-06
9.  References
    9.1.  Normative References
    9.2.  Informative References

1.  Introduction

1.1.  Role of This Specification

Full internationalization of electronic mail requires several capabilities:

• The capability to transmit non-ASCII content, provided for as part of the basic MIME specification [RFC2045] (Freed, N. and N. Borenstein, “Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies,” November 1996.), [RFC2046] (Freed, N. and N. Borenstein, “Multipurpose Internet Mail Extensions (MIME) Part Two: Media Types,” November 1996.).
• The capability to use international characters in envelope addresses, discussed in [I‑D.ietf‑eai‑frmwrk‑4952bis] (Klensin, J. and Y. Ko, “Overview and Framework for Internationalized Email,” September 2010.) and specified in [I‑D.ietf‑eai‑rfc5336bis] (Yao, J. and W. MAO, “SMTP Extension for Internationalized Email Address,” December 2010.).
• 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 a variant of Internet mail that permits the use of Unicode encoded in UTF-8 [RFC3629] (Yergeau, F., “UTF-8, a transformation format of ISO 10646,” November 2003.), 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‑rfc5336bis] (Yao, J. and W. MAO, “SMTP Extension for Internationalized Email Address,” December 2010.) or by other transport mechanisms capable of processing it.

1.2.  Relation to Other Standards

This document updates Section 6.4 of [RFC2045] (Freed, N. and N. Borenstein, “Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies,” November 1996.). 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 Section 3.4 of [RFC5322] (Resnick, P., Ed., “Internet Message Format,” October 2008.). It Extended mailbox address syntax to permit UTF-8 character in Section 4.3 (Syntax Extensions to RFC 5322).

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 [I‑D.ietf‑eai‑rfc5336bis] (Yao, J. and W. MAO, “SMTP Extension for Internationalized Email Address,” December 2010.). message/global (see Section 4.6 (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 display 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 [RFC5322] (Resnick, P., Ed., “Internet Message Format,” October 2008.) 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 display names, comments, and in free text (such as in the "Subject:" field) to be encoded (as required by MIME format [RFC2047] (Moore, K., “MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text,” November 1996.)). This specification describes a change to the email message format that is related to the SMTP message transport change described in the associated documents [I‑D.ietf‑eai‑frmwrk‑4952bis] (Klensin, J. and Y. Ko, “Overview and Framework for Internationalized Email,” September 2010.) and [I‑D.ietf‑eai‑rfc5336bis] (Yao, J. and W. MAO, “SMTP Extension for Internationalized Email Address,” December 2010.), 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‑rfc5336bis] (Yao, J. and W. MAO, “SMTP Extension for Internationalized Email Address,” December 2010.), 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 [I‑D.ietf‑eai‑rfc5721bis] (Gellens, R., Newman, C., Yao, J., and K. Fujiwara, “POP3 Support for UTF-8,” September 2010.) and [I‑D.ietf‑eai‑5378bis] (Resnick, P., Newman, C., and S. Shen, “IMAP Support for UTF-8,” November 2010.).

The objective for this protocol is to allow UTF-8 in email header fields.

3.  Terminology

A plain ASCII string is full compatible with [RFC5321] (Klensin, J., “Simple Mail Transfer Protocol,” October 2008.) and [RFC5322] (Resnick, P., Ed., “Internet Message Format,” October 2008.). In this document, non-ASCII strings are UTF-8 strings if they are in header which contain at least one <UTF8-non-ascii>.

Unless otherwise noted, all terms used here are defined in [RFC5321] (Klensin, J., “Simple Mail Transfer Protocol,” October 2008.), [RFC5322] (Resnick, P., Ed., “Internet Message Format,” October 2008.), [I‑D.ietf‑eai‑frmwrk‑4952bis] (Klensin, J. and Y. Ko, “Overview and Framework for Internationalized Email,” September 2010.), or [I‑D.ietf‑eai‑rfc5336bis] (Yao, J. and W. MAO, “SMTP Extension for Internationalized Email Address,” December 2010.).

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] (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.).

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 [RFC5322] (Resnick, P., Ed., “Internet Message Format,” October 2008.) 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 [RFC5322] (Resnick, P., Ed., “Internet Message Format,” October 2008.) is extended to support the new format. The following ABNF is defined to substitute those definitions in [RFC5322] (Resnick, P., Ed., “Internet Message Format,” October 2008.).

The syntax rules not covered in this section remain as defined in [RFC5322] (Resnick, P., Ed., “Internet Message Format,” October 2008.).

4.1.  UTF-8 Syntax and Normalization

UTF-8 characters can be defined in terms of octets using the following ABNF [RFC5234] (Crocker, D. and P. Overell, “Augmented BNF for Syntax Specifications: ABNF,” January 2008.), taken from [RFC3629] (Yergeau, F., “UTF-8, a transformation format of ISO 10646,” November 2003.):

UTF8-non-ascii  =   UTF8-2 / UTF8-3 / UTF8-4

UTF8-2          =   <See Section 4 of RFC3629>

UTF8-3          =   <See Section 4 of RFC3629>

UTF8-4          =   <See Section 4 of RFC3629>

See [RFC5198] (Klensin, J. and M. Padlipsky, “Unicode Format for Network Interchange,” March 2008.) for a discussion of normalization; the use of normalization form [NFC] (Davis, M. and K. Whistler, “Unicode Standard Annex #15: Unicode Normalization Forms,” September 2010.) is RECOMMENDED. Actually, if one is going to do internationalization properly, one of the most often-cited goals is to permit people to spell their names correctly. Since many mailbox local parts reflect personal names, that principle applies as well. And NFKC is not recommended because it may lose information that is needed to correctly spell some names in unusual circumstances.

4.2.  Changes on MIME Headers

This specification updates Section 6.4 of [RFC2045] (Freed, N. and N. Borenstein, “Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies,” November 1996.). [RFC2045] (Freed, N. and N. Borenstein, “Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies,” November 1996.) prohibits applying a content-transfer-encoding to any 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 (message/global)).

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] (Klensin, J., Freed, N., Rose, M., Stefferud, E., and D. Crocker, “SMTP Service Extension for 8bit-MIMEtransport,” July 1994.), 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 5322

The following rules are intended to extend the corresponding rules in [RFC5322] (Resnick, P., Ed., “Internet Message Format,” October 2008.) in order to allow UTF-8 characters.

FWS     =  <see Section 3.2.2 of RFC 5322>

CFWS    =  <see Section 3.2.2 of RFC 5322>

ctext   =/  UTF8-non-ascii
            ; Extending ctext in RFC 5322, Section 3.2.2
comment =   "(" *([FWS] uCcontent) [FWS] ")"

word    =   uAtom / uQuoted-String

This means that all the [RFC5322] (Resnick, P., Ed., “Internet Message Format,” October 2008.) 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 UTF-8 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 (Trace Field Syntax). Instead, <uAtext> is added to meet this requirement.

uText          = %d1-9 /    ; all UTF-8 characters except
                 %d11-12 /  ; US-ASCII NUL, CR, and LF
                 %d14-127 /
                 UTF8-non-ascii

uQuoted-Pair   = ("\" (VCHAR / WSP / UTF8-non-ascii )) / obs-qp

VCHAR          = <See appendix B.1 of RFC 5234>

WSP            = <See appendix B.1 of RFC 5234>

obs-qp         = <See Section 4.1 of RFC 5322>

uQcontent      = uQtext / uQuoted-Pair

DQUOTE         = <See appendix B.1 of RFC 5234>

uCcontent      = ctext / uQuoted-Pair / comment

uQtext         = qtext / UTF8-non-ascii

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

uAtom          = [CFWS] 1*uAtext [CFWS]

uDot-Atom      = [CFWS] uDot-Atom-text [CFWS]

uDot-Atom-text = 1*uAtext *("." 1*uAtext)

qcontent       =/ uQcontent

To allow the use of UTF-8 in a Content-Description header field [RFC2045] (Freed, N. and N. Borenstein, “Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies,” November 1996.), the following syntax is used:

description    = "Content-Description" ":" *uText
                ; Replace description in RFC 2045, Section 8

The <uText> syntax is extended above to allow UTF-8 in all <description> 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:" header fields 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 from [RFC5321] (Klensin, J., “Simple Mail Transfer Protocol,” October 2008.) Section 4.1.2 to permit UTF-8 addresses.

mailbox	       = name-addr / addr-spec / uAddr-Spec
                 ; Replace mailbox in RFC 5322, Section 3.4

angle-addr     =/ [CFWS] "<" uAddr-Spec ">" [CFWS]
                 ; Extending angle-addr in RFC 5322, Section 3.4

uAddr-Spec     = uLocal-Part "@" uDomain

uLocal-Part    = uDot-Atom / uQuoted-String / obs-local-part
                 ; Replace Local-Part in RFC 5322, Section 3.4.1

uQuoted-String = [CFWS] DQUOTE *([FWS] uQcontent) [FWS] DQUOTE [CFWS]

obs-local-part = <See Section 4.4 of RFC 5322>
uDomain        = uDot-Atom / domain-literal / obs-domain

domain-literal = <See Section 3.4.1 of RFC 5322>

Below are a few examples of possible <mailbox> representations.

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

   "DISPLAY_NAME" <non-ASCII@non-ASCII>
   ; message will be rejected if UTF8SMTP extension is not supported

   <non-ASCII@non-ASCII>
   ; without DISPLAY_NAME and quoted string
   ; message will be rejected if UTF8SMTP extension is not supported

4.5.  Trace Field Syntax

The 'uFor' clause in "Received:" fields has been allowed the use of internationalized addresses in "For" fields. It described in [I‑D.ietf‑eai‑rfc5336bis] (Yao, J. and W. MAO, “SMTP Extension for Internationalized Email Address,” December 2010.), Section 3.6.3.

The "Return-path" designates the address to which messages indicating non-delivery or other mail system failures are to be sent. Thus, the header is augmented to carry UTF-8 addresses (see the revised syntax of <angle-addr> in Section 4.4 (Change on addr-spec Syntax) of this document). This will not break the rule of trace field integrity, because the header field is added at the last MTA and described in [RFC5321] (Klensin, J., “Simple Mail Transfer Protocol,” October 2008.).

The <received-token> on "Received:" field ( described in Section 3.6.7 of [RFC5322] (Resnick, P., Ed., “Internet Message Format,” October 2008.)) 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>, <uAddr-Spec> is added to <received-token>.

received-token =/ uAddr-Spec

4.6.  message/global

Internationalized messages MUST only be transmitted as authorized by [I‑D.ietf‑eai‑rfc5336bis] (Yao, J. and W. MAO, “SMTP Extension for Internationalized Email Address,” December 2010.) or within a non-SMTP environment which supports these messages. A message is a "message/global message", if

• it contains UTF-8 header values as specified in this document, or
• it contains UTF-8 values in the headers fields of body parts.

The type message/global is similar to message/rfc822, except that it specifies that a message 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] (Freed, N. and N. Borenstein, “Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies,” November 1996.). (Note that a system compliant with MIME that doesn't recognize message/global SHOULD treat it as "application/octet-stream" as described in Section 5.2.4 of [RFC2046] (Freed, N. and N. Borenstein, “Multipurpose Internet Mail Extensions (MIME) Part Two: Media Types,” November 1996.).)

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 (Security Considerations).

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 internationalized 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 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".

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 SHOULD 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) [RFC5280] (Cooper, D., Santesson, S., Farrell, S., Boeyen, S., Housley, R., and W. Polk, “Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile,” May 2008.) and OpenPGP [RFC3156] (Elkins, M., Del Torto, D., Levien, R., and T. Roessler, “MIME Security with OpenPGP,” August 2001.).

Because UTF-8 often requires several octets to encode a single character, internationalized local parts and header value may cause mail addresses to become longer. As specified in [RFC5322] (Resnick, P., Ed., “Internet Message Format,” October 2008.), each line of characters MUST be no more 998 octets, excluding the CRLF. On the other hand, MDA (Mail Delivery Agent) 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 [I‑D.ietf‑eai‑frmwrk‑4952bis] (Klensin, J. and Y. Ko, “Overview and Framework for Internationalized Email,” September 2010.), Section 14.

6.  IANA Considerations

IANA is requested to update the registration of the message/global MIME type using the registration form contained in Section 4.6 (message/global).

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.  Edit history

[[RFC Editor: please remove this section before publishing.]]

8.1.  draft-ietf-eai-rfc5335bis-00

1. Applied Errata suggested by Alfred Hoenes.
2. Adjust [RFC2821] and [RFC2822] to [RFC5321] (Klensin, J., “Simple Mail Transfer Protocol,” October 2008.) and [RFC5322] (Resnick, P., Ed., “Internet Message Format,” October 2008.).
3. Abrogate <alt-address> in ABNF of <angle-addr>.
4. Revoke [RFC5504] from this document.
5. Upgrade some references from I-Ds to RFC.

8.2.  draft-ietf-eai-rfc5335bis-01

1. Author name revised.

8.3.  draft-ietf-eai-rfc5335bis-02

1. ABNF revised.

8.4.  draft-ietf-eai-rfc5335bis-03

1. Fix typos
2. ABNF revised
3. Improve sentence

8.5.  draft-ietf-eai-rfc5335bis-04

1. improve sentences and ABNF revised based on AD and Co-chairs

8.6.  draft-ietf-eai-rfc5335bis-05

1. ABNF revised in Section 4.4 (Change on addr-spec Syntax) based on AD comments

8.7.  draft-ietf-eai-rfc5335bis-06

1. ABNF revised
2. improve Section 6 (IANA Considerations)

9.  References

9.1. Normative References

9.2. Informative References

Authors' Addresses