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Network Working Group                                       D. GoldsmithRequest for Comments: 1641                                      M. DavisCategory: Experimental                                    Taligent, Inc.                                                               July 1994Using Unicode with MIMEStatus of this Memo   This memo defines an Experimental Protocol for the Internet   community.  This memo does not specify an Internet standard of any   kind.  Distribution of this memo is unlimited.Abstract   The Unicode Standard, version 1.1, and ISO/IEC 10646-1:1993(E)   jointly define a 16 bit character set (hereafter referred to as   Unicode) which encompasses most of the world's writing systems.   However, Internet mail (STD 11,RFC 822) currently supports only 7-   bit US ASCII as a character set. MIME (RFC 1521 andRFC 1522) extends   Internet mail to support different media types and character sets,   and thus could support Unicode in mail messages. MIME neither defines   Unicode as a permitted character set nor specifies how it would be   encoded, although it does provide for the registration of additional   character sets over time.   This document specifies the usage of Unicode within MIME.Motivation   Since Unicode is starting to see widespread commercial adoption,   users will want a way to transmit information in this character set   in mail messages and other Internet media. Since MIME was expressly   designed to allow such extensions and is on the standards track for   the Internet, it is the most appropriate means for encoding Unicode.RFC 1521 andRFC 1522 do not define Unicode as an allowed character   set, but allow registration of additional character sets.   In addition to allowing use of Unicode within MIME bodies, another   goal is to specify a way of using Unicode that allows text which   consists largely, but not entirely, of US-ASCII characters to be   represented in a way that can be read by mail clients who do not   understand Unicode. This is in keeping with the philosophy of MIME.   Such an encoding is described in another document, "UTF-7: A Mail   Safe Transformation Format of Unicode" [UTF-7].Goldsmith & Davis                                               [Page 1]

RFC 1641                Using Unicode with MIME                July 1994Overview   Several ways of using Unicode are possible. This document specifies   both guidelines for use of Unicode within MIME, and a specific usage.   The usage specified in this document is a straightforward use of   Unicode as specified in "The Unicode Standard, Version 1.1".   This encoding is intended for situations where sender and recipient   do not want to do a lot of processing, when the text does not consist   primarily of characters from the US-ASCII character set, or when   sender and receiver are known in advance to support Unicode.   Another encoding is intended for situations where the text consists   primarily of US-ASCII, with occasional characters from other parts of   Unicode. This encoding allows the US-ASCII portion to be read by all   recipients without having to support Unicode. This encoding is   specified in another document, "UTF-7: A Mail Safe Transformation   Format of Unicode" [UTF-7].   Finally, in keeping with the principles set forth inRFC 1521, text   which can be represented using the US-ASCII or ISO-8859-x character   sets should be so represented where possible, for maximum   interoperability.Definitions   The definition of character set Unicode:      The 16 bit character set Unicode is defined by "The Unicode      Standard, Version 1.1". This character set is identical with the      character repertoire and coding of the international standard      ISO/IEC 10646-1:1993(E); Coded Representation Form=UCS-2;      Subset=300; Implementation Level=3.      Note. Unicode 1.1 further specifies the use and interaction of      these character codes beyond the ISO standard. However, any valid      10646 BMP (Basic Multilingual Plane) sequence is a valid Unicode      sequence, and vice versa; Unicode supplies interpretations of      sequences on which the ISO standard is silent as to      interpretation.      This character set is encoded as sequences of octets, two per 16-      bit character, with the most significant octet first. Text with an      odd number of octets is ill-formed.      Rationale. ISO/IEC 10646-1:1993(E) specifies that when characters      in the UCS-2 form are serialized as octets, that the most      significant octet appear first.  This is also in keeping withGoldsmith & Davis                                               [Page 2]

RFC 1641                Using Unicode with MIME                July 1994      common network practice of choosing a canonical format for      transmission.General Specification of Unicode Character Sets Within MIME   The Unicode Standard is currently at version 1.1. Although new   versions should be compatible with old implementations if an   implementation is compliant with the standard, some implementations   may choose to check the version of the character set that is being   used. In order to allow some implementations to check the version   number and allow others to ignore it, all registrations of Unicode   variants and versions for MIME usage should have MIME charset names   which conform to one of the two following patterns:      UNICODE-major-minor      UNICODE-major-minor-variant   Where major and minor are strings of decimal digits (0 through 9)   specifying the major and minor version number of the Unicode standard   to which the text in question conforms. In the interests of   interoperability, the lowest version number compatible with the text   should be used. The lowest acceptable version number is UNICODE-1-1,   corresponding to "The Unicode Standard, Version 1.1". The optional   trailing string "variant" describes the particular transformation   format of Unicode that the registration describes; its content is up   to the particular registration. If there is no trailing variant   string, the charset name refers to the basic two octet form of   Unicode as described in "The Unicode Standard".   Example. A hypothetical charset which referred to the UTF-8   transformation format of Unicode/10646 (also known as UTF-2 or UTF-   FSS) might be named UNICODE-1-1-UTF-8.Encoding Character Set Unicode Within MIME   Character set Unicode uses 16 bit characters, and therefore would   normally be used with the Binary or Base64 content transfer encodings   of MIME. In header fields, it would normally be used with the B   content transfer encoding. The MIME character set identifier is   UNICODE-1-1.   Example. Here is a text portion of a MIME message containing the   Japanese word "nihongo" (hexadecimal 65E5,672C,8A9E) written in Han   characters.   Content-Type: text/plain; charset=UNICODE-1-1   Content-Transfer-Encoding: base64Goldsmith & Davis                                               [Page 3]

RFC 1641                Using Unicode with MIME                July 1994   ZeVnLIqe   Example. Here is a text portion of a MIME message containing the   Unicode sequence "A<NOT IDENTICAL TO><ALPHA>." (hexadecimal   0041,2262,0391,002E)   Content-Type: text/plain; charset=UNICODE-1-1   Content-Transfer-Encoding: base64   AEEiYgORAC4=Acknowledgements   Many thanks to the following people for their contributions,   comments, and suggestions. If we have omitted anyone it was through   oversight and not intentionally.         Glenn Adams         Harald T. Alvestrand         Nathaniel Borenstein         Lee Collins         Jim Conklin         Dave Crocker         Steve Dorner         Dana S. Emery         Ned Freed         John H. Jenkins         John C. Klensin         Valdis Kletnieks         Keith Moore         Masataka Ohta         Einar StefferudSecurity Considerations   Security issues are not discussed in this memo.References[UNICODE 1.1] "The Unicode Standard, Version 1.1": Version 1.0, Volume 1              (ISBN 0-201-56788-1), Version 1.0, Volume 2 (ISBN 0-201-              60845-6), and "Unicode Technical Report #4, The Unicode              Standard, Version 1.1" (available from The Unicode              Consortium, and soon to be published by Addison-Wesley).[ISO 10646]   ISO/IEC 10646-1:1993(E) Information Technology--Universal              Multiple-octet Coded Character Set (UCS).Goldsmith & Davis                                               [Page 4]

RFC 1641                Using Unicode with MIME                July 1994[UTF-7]       Goldsmith, D., and M. Davis, "UTF-7: A Mail Safe              Transformation Format of Unicode",RFC 1642, Taligent,              Inc., July 1994.[US-ASCII]    Coded Character Set--7-bit American Standard Code for              Information Interchange, ANSI X3.4-1986.[ISO-8859]    Information Processing -- 8-bit Single-Byte Coded Graphic              Character Sets -- Part 1: Latin Alphabet No. 1, ISO 8859-              1:1987.  Part 2: Latin alphabet No.  2, ISO 8859-2, 1987.              Part 3: Latin alphabet No. 3, ISO 8859-3, 1988.  Part 4:              Latin alphabet No.  4, ISO 8859-4, 1988.  Part 5:              Latin/Cyrillic alphabet, ISO 8859-5, 1988.  Part 6:              Latin/Arabic alphabet, ISO 8859-6, 1987.  Part 7:              Latin/Greek alphabet, ISO 8859-7, 1987.  Part 8:              Latin/Hebrew alphabet, ISO 8859-8, 1988.  Part 9: Latin              alphabet No. 5, ISO 8859-9, 1990.[RFC822]      Crocker, D., "Standard for the Format of ARPA Internet              Text Messages", STD 11,RFC 822, UDEL, August 1982.[RFC-1521]    Borenstein N., and N. Freed, "MIME (Multipurpose Internet              Mail Extensions) Part One:  Mechanisms for Specifying and              Describing the Format of Internet Message Bodies",RFC1521, Bellcore, Innosoft, September 1993.[RFC-1522]    Moore, K., "Representation of Non-Ascii Text in Internet              Message Headers"RFC 1522, University of Tennessee,              September 1993.[UTF-8]       X/Open Company Ltd., "File System Safe UCS Transformation              Format (FSS_UTF)", X/Open Preliminary Specification,              Document Number: P316. This information also appears in              Unicode Technical Report #4, and in a forthcoming annex to              ISO/IEC 10646.Goldsmith & Davis                                               [Page 5]

RFC 1641                Using Unicode with MIME                July 1994Authors' Addresses   David Goldsmith   Taligent, Inc.   10201 N. DeAnza Blvd.   Cupertino, CA 95014-2233   Phone: 408-777-5225   Fax: 408-777-5081   EMail: david_goldsmith@taligent.com   Mark Davis   Taligent, Inc.   10201 N. DeAnza Blvd.   Cupertino, CA 95014-2233   Phone: 408-777-5116   Fax: 408-777-5081   EMail: mark_davis@taligent.comGoldsmith & Davis                                               [Page 6]