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Internet Engineering Task Force (IETF)                           A. YangRequest for Comments: 6532                                         TWNICObsoletes:5335                                                S. SteeleUpdates:2045                                                  MicrosoftCategory: Standards Track                                       N. FreedISSN: 2070-1721                                                   Oracle                                                           February 2012Internationalized Email HeadersAbstract   Internet mail was originally limited to 7-bit ASCII.  MIME added   support for the use of 8-bit character sets in body parts, and also   defined an encoded-word construct so other character sets could be   used in certain header field values.  However, full   internationalization of electronic mail requires additional   enhancements to allow the use of Unicode, including characters   outside the ASCII repertoire, in mail addresses as well as direct use   of Unicode in header fields like "From:", "To:", and "Subject:",   without requiring the use of complex encoded-word constructs.  This   document specifies an enhancement to the Internet Message Format and   to MIME that allows use of Unicode in mail addresses and most header   field content.   This specification updatesSection 6.4 of RFC 2045 to eliminate the   restriction prohibiting the use of non-identity content-transfer-   encodings on subtypes of "message/".Status of This Memo   This is an Internet Standards Track document.   This document is a product of the Internet Engineering Task Force   (IETF).  It represents the consensus of the IETF community.  It has   received public review and has been approved for publication by the   Internet Engineering Steering Group (IESG).  Further information on   Internet Standards is available inSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://www.rfc-editor.org/info/rfc6532.Yang, et al.                 Standards Track                    [Page 1]

RFC 6532             Internationalized Email Headers       February 2012Copyright Notice   Copyright (c) 2012 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 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 Contents1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .32.  Terminology Used in This Specification . . . . . . . . . . . .33.  Changes to Message Header Fields . . . . . . . . . . . . . . .43.1.  UTF-8 Syntax and Normalization . . . . . . . . . . . . . .43.2.  Syntax Extensions toRFC 5322  . . . . . . . . . . . . . .53.3.  Use of 8-bit UTF-8 in Message-IDs  . . . . . . . . . . . .53.4.  Effects on Line Length Limits  . . . . . . . . . . . . . .53.5.  Changes to MIME Message Type Encoding Restrictions . . . .63.6.  Use of MIME Encoded-Words  . . . . . . . . . . . . . . . .63.7.  The message/global Media Type  . . . . . . . . . . . . . .74.  Security Considerations  . . . . . . . . . . . . . . . . . . .85.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .96.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .97.  References . . . . . . . . . . . . . . . . . . . . . . . . . .107.1.  Normative References . . . . . . . . . . . . . . . . . . .107.2.  Informative References . . . . . . . . . . . . . . . . . .10Yang, et al.                 Standards Track                    [Page 2]

RFC 6532             Internationalized Email Headers       February 20121.  Introduction   Internet mail distinguishes a message from its transport and further   divides a message between a header and a body [RFC5322].  Internet   mail header field values contain a variety of strings that are   intended to be user-visible.  The range of supported characters for   these strings was originally limited to [ASCII] in 7-bit form.  MIME   [RFC2045] [RFC2046] [RFC2047] provides the ability to use additional   character sets, but this support is limited to body part data and to   special encoded-word constructs that were only allowed in a limited   number of places in header field values.   Globalization of the Internet requires support of the much larger set   of characters provided by Unicode [RFC5198] in both mail addresses   and most header field values.  Additionally, complex encoding schemes   like encoded-words introduce inefficiencies as well as significant   opportunities for processing errors.  And finally, native support for   the UTF-8 charset is now available on most systems.  Hence, it is   strongly desirable for Internet mail to support UTF-8 [RFC3629]   directly.   This document specifies an enhancement to the Internet Message Format   [RFC5322] and to MIME that permits the direct use of UTF-8, rather   than only ASCII, in header field values, including mail addresses.  A   new media type, message/global, is defined for messages that use this   extended format.  This specification also lifts the MIME restriction   on having non-identity content-transfer-encodings on any subtype of   the message top-level type so that message/global parts can be safely   transmitted across existing mail infrastructure.   This specification is based on a model of native, end-to-end support   for UTF-8, which depends on having an "8-bit-clean" environment   assured by the transport system.  Support for carriage across legacy,   7-bit infrastructure and for processing by 7-bit receivers requires   additional mechanisms that are not provided by these specifications.   This specification is a revision of and replacement for [RFC5335].Section 6 of [RFC6530] describes the change in approach between this   specification and the previous version.2.  Terminology Used in This Specification   A plain ASCII string is fully compatible with [RFC5321] and   [RFC5322].  In this document, non-ASCII strings are UTF-8 strings if   they are in header field values that contain at least one   <UTF8-non-ascii> (seeSection 3.1).Yang, et al.                 Standards Track                    [Page 3]

RFC 6532             Internationalized Email Headers       February 2012   Unless otherwise noted, all terms used here are defined in [RFC5321],   [RFC5322], [RFC6530], or [RFC6531].   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].   The term "8-bit" means octets are present in the data with values   above 0x7F.3.  Changes to Message Header Fields   To permit non-ASCII Unicode characters in field values, the header   definition in [RFC5322] is extended to support the new format.  The   following sections specify the necessary changes toRFC 5322's ABNF.   The syntax rules not mentioned below remain defined as in [RFC5322].   Note that this protocol does not change rules inRFC 5322 for   defining header field names.  The bodies of header fields are allowed   to contain Unicode characters, but the header field names themselves   must consist of ASCII characters only.   Also note that messages in this format require the use of the   SMTPUTF8 extension [RFC6531] to be transferred via SMTP.3.1.  UTF-8 Syntax and Normalization   UTF-8 characters can be defined in terms of octets using the   following ABNF [RFC5234], taken from [RFC3629]:   UTF8-non-ascii  =   UTF8-2 / UTF8-3 / UTF8-4   UTF8-2          =   <Defined inSection 4 of RFC3629>   UTF8-3          =   <Defined inSection 4 of RFC3629>   UTF8-4          =   <Defined inSection 4 of RFC3629>   See [RFC5198] for a discussion of Unicode normalization;   normalization form NFC [UNF] SHOULD be used.  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 to mailboxes as well.  The NFKC normalization form [UNF]   SHOULD NOT be used because it may lose information that is needed to   correctly spell some names in some unusual circumstances.Yang, et al.                 Standards Track                    [Page 4]

RFC 6532             Internationalized Email Headers       February 20123.2.  Syntax Extensions toRFC 5322   The following rules extend the ABNF syntax defined in [RFC5322] and   [RFC5234] in order to allow UTF-8 content.   VCHAR   =/  UTF8-non-ascii   ctext   =/  UTF8-non-ascii   atext   =/  UTF8-non-ascii   qtext   =/  UTF8-non-ascii   text    =/  UTF8-non-ascii                 ; note that this upgrades the body to UTF-8   dtext   =/  UTF8-non-ascii   The preceding changes mean that the following constructs now allow   UTF-8:   1.  Unstructured text, used in header fields like "Subject:" or       "Content-description:".   2.  Any construct that uses atoms, including but not limited to the       local parts of addresses and Message-IDs.  This includes       addresses in the "for" clauses of "Received:" header fields.   3.  Quoted strings.   4.  Domains.   Note that header field names are not on this list; these are still   restricted to ASCII.3.3.  Use of 8-bit UTF-8 in Message-IDs   Implementers of Message-ID generation algorithms MAY prefer to   restrain their output to ASCII since that has some advantages, such   as when constructing "In-reply-to:" and "References:" header fields   in mailing-list threads where some senders use internationalized   addresses and others do not.3.4.  Effects on Line Length LimitsSection 2.1.1 of [RFC5322] limits lines to 998 characters and   recommends that the lines be restricted to only 78 characters.  This   specification changes the former limit to 998 octets.  (Note that, inYang, et al.                 Standards Track                    [Page 5]

RFC 6532             Internationalized Email Headers       February 2012   ASCII, octets and characters are effectively the same, but this is   not true in UTF-8.)  The 78-character limit remains defined in terms   of characters, not octets, since it is intended to address display   width issues, not line-length issues.3.5.  Changes to MIME Message Type Encoding Restrictions   This specification updatesSection 6.4 of [RFC2045].  [RFC2045]   prohibits applying a content-transfer-encoding to any subtypes of   "message/".  This specification relaxes that rule -- it allows newly   defined MIME types to permit content-transfer-encoding, and it allows   content-transfer-encoding for message/global (seeSection 3.7).   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.   But in the case where a message containing a message/global is   downgraded from 8-bit to 7-bit as described in [RFC6152], an encoding   might have to be applied to the message.  If the message travels   multiple times between a 7-bit environment and an environment   implementing these extensions, 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 is thought to be   larger than the complexity of allowing nested encodings where   necessary.3.6.  Use of MIME Encoded-Words   The MIME encoded-words facility [RFC2047] provides the ability to   place non-ASCII text, but only in a subset of the places allowed by   this extension.  Additionally, encoded-words are substantially more   complex since they allow the use of arbitrary charsets.  Accordingly,   encoded-words SHOULD NOT be used when generating header fields for   messages employing this extension.  Agents MAY, when incorporating   material from another message, convert encoded-word use to direct use   of UTF-8.   Note that care must be taken when decoding encoded-words because the   results after replacing an encoded-word with its decoded equivalent   in UTF-8 may be syntactically invalid.  Processors that elect to   decode encoded-words MUST NOT generate syntactically invalid fields.Yang, et al.                 Standards Track                    [Page 6]

RFC 6532             Internationalized Email Headers       February 20123.7.  The message/global Media Type   Internationalized messages in this format MUST only be transmitted as   authorized by [RFC6531] or within a non-SMTP environment that   supports these messages.  A message is a "message/global message" if:   o  it contains 8-bit UTF-8 header values as specified in this      document, or   o  it contains 8-bit UTF-8 values in the header fields of body parts.   The content of a message/global part is otherwise identical to that   of a message/rfc822 part.   If an object of this type is sent to a 7-bit-only system, it MUST   have an appropriate content-transfer-encoding applied.  (Note that a   system compliant with MIME that doesn't recognize message/global is   supposed to treat it as "application/octet-stream" as described inSection 5.2.4 of [RFC2046].)   The registration is as follows:   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:  SeeSection 4.   Interoperability considerations:  This media type provides      functionality similar to the message/rfc822 content type for email      messages with internationalized 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.  Each 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/globalYang, et al.                 Standards Track                    [Page 7]

RFC 6532             Internationalized Email Headers       February 2012      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 6532   Applications that use this media type:  SMTP servers and email      clients that support multipart/report generation or parsing.      Email clients that forward messages with internationalized 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      Authors' Addresses section of this document.   Intended usage:  COMMON   Restrictions on usage:  This is a structured media type that embeds      other MIME media types.  An 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 Authors' Addresses section of this document.   Change controller:  IETF Standards Process4.  Security Considerations   Because UTF-8 often requires several octets to encode a single   character, internationalization may cause header field values (in   general) and mail addresses (in particular) to become longer.  As   specified in [RFC5322], each line of characters MUST be no more than   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.Yang, et al.                 Standards Track                    [Page 8]

RFC 6532             Internationalized Email Headers       February 2012   There are lots of ways to use UTF-8 to represent something equivalent   or similar to a particular displayed character or group of   characters; see the security considerations in [RFC3629] for details   on the problems this can cause.  The normalization process described   inSection 3.1 is recommended to minimize these issues.   The security impact of UTF-8 headers on email signature systems such   as Domain Keys Identified Mail (DKIM), S/MIME, and OpenPGP is   discussed inSection 14 of [RFC6530].   If a user has a non-ASCII mailbox address and an ASCII mailbox   address, a digital certificate that identifies that user might have   both addresses in the identity.  Having multiple email addresses as   identities in a single certificate is already supported in PKIX   (Public Key Infrastructure using X.509) [RFC5280] and OpenPGP   [RFC3156], but there may be user-interface issues associated with the   introduction of UTF-8 into addresses in this context.5.  IANA Considerations   IANA has updated the registration of the message/global MIME type   using the registration form contained inSection 3.7.6.  Acknowledgements   This document incorporates many ideas first described in a draft   document by Paul Hoffman, although many details have changed from   that earlier work.   The authors especially thank Jeff Yeh for his efforts and   contributions on editing previous versions.   Most of the content of this document was provided by John C Klensin   and Dave Crocker.  Significant comments and suggestions were received   from Martin Duerst, Julien Elie, Arnt Gulbrandsen, Kristin Hubner,   Kari Hurtta, Yangwoo Ko, Charles H. Lindsey, Alexey Melnikov, Chris   Newman, Pete Resnick, Yoshiro Yoneya, and additional members of the   Joint Engineering Team (JET) and were incorporated into the document.   The authors wish to sincerely thank them all for their contributions.Yang, et al.                 Standards Track                    [Page 9]

RFC 6532             Internationalized Email Headers       February 20127.  References7.1.  Normative References   [ASCII]    "Coded Character Set -- 7-bit American Standard Code for              Information Interchange", ANSI X3.4, 1986.   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate              Requirement Levels",BCP 14,RFC 2119, March 1997.   [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO              10646", STD 63,RFC 3629, November 2003.   [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.   [RFC5321]  Klensin, J., "Simple Mail Transfer Protocol",RFC 5321,              October 2008.   [RFC5322]  Resnick, P., Ed., "Internet Message Format",RFC 5322,              October 2008.   [RFC6530]  Klensin, J. and Y. Ko, "Overview and Framework for              Internationalized Email",RFC 6530, February 2012.   [RFC6531]  Yao, J. and W. Mao, "SMTP Extension for Internationalized              Email",RFC 6531, February 2012.   [UNF]      Davis, M. and K. Whistler, "Unicode Standard Annex #15:              Unicode Normalization Forms", September 2010,              <http://www.unicode.org/reports/tr15/>.7.2.  Informative References   [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.Yang, et al.                 Standards Track                   [Page 10]

RFC 6532             Internationalized Email Headers       February 2012   [RFC3156]  Elkins, M., Del Torto, D., Levien, R., and T. Roessler,              "MIME Security with OpenPGP",RFC 3156, August 2001.   [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",RFC 5280, May 2008.   [RFC5335]  Yang, A., "Internationalized Email Headers",RFC 5335,              September 2008.   [RFC6152]  Klensin, J., Freed, N., Rose, M., and D. Crocker, "SMTP              Service Extension for 8-bit MIME Transport", STD 71,RFC 6152, March 2011.Authors' Addresses   Abel Yang   TWNIC   4F-2, No. 9, Sec 2, Roosevelt Rd.   Taipei  100   Taiwan   Phone: +886 2 23411313 ext 505   EMail: abelyang@twnic.net.tw   Shawn Steele   Microsoft   EMail: Shawn.Steele@microsoft.com   Ned Freed   Oracle   800 Royal Oaks   Monrovia, CA  91016-6347   USA   EMail: ned+ietf@mrochek.comYang, et al.                 Standards Track                   [Page 11]