UTF-8, a transformation format of Unicode and ISO 10646

Network Working Group                                       F. YergeauRequest for Comments: 2044                           Alis TechnologiesCategory: Informational                                   October 1996        UTF-8, a transformation format of Unicode and ISO 10646Status of this Memo   This memo provides information 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 jointly   define a 16 bit character set which encompasses most of the world's   writing systems. 16-bit characters, however, are not compatible with   many current applications and protocols, and this has led to the   development of a few so-called UCS transformation formats (UTF), each   with different characteristics.  UTF-8, the object of this memo, has   the characteristic of preserving the full US-ASCII range: US-ASCII   characters are encoded in one octet having the usual US-ASCII value,   and any octet with such a value can only be an US-ASCII character.   This provides compatibility with file systems, parsers and other   software that rely on US-ASCII values but are transparent to other   values.1.  Introduction   The Unicode Standard, version 1.1 [UNICODE], and ISO/IEC 10646-1:1993   [ISO-10646] jointly define a 16 bit character set, UCS-2, which   encompasses most of the world's writing systems.  ISO 10646 further   defines a 31-bit character set, UCS-4, with currently no assignments   outside of the region corresponding to UCS-2 (the Basic Multilingual   Plane, BMP).  The UCS-2 and UCS-4 encodings, however, are hard to use   in many current applications and protocols that assume 8 or even 7   bit characters.  Even newer systems able to deal with 16 bit   characters cannot process UCS-4 data. This situation has led to the   development of so-called UCS transformation formats (UTF), each with   different characteristics.   UTF-1 has only historical interest, having been removed from ISO   10646.  UTF-7 has the quality of encoding the full Unicode repertoire   using only octets with the high-order bit clear (7 bit US-ASCII   values, [US-ASCII]), and is thus deemed a mail-safe encoding   ([RFC1642]).  UTF-8, the object of this memo, uses all bits of an   octet, but has the quality of preserving the full US-ASCII range:Yergeau                      Informational                      [Page 1]RFC 2044                         UTF-8                      October 1996   US-ASCII characters are encoded in one octet having the normal US-   ASCII value, and any octet with such a value can only stand for an   US-ASCII character, and nothing else.   UTF-16 is a scheme for transforming a subset of the UCS-4 repertoire   into a pair of UCS-2 values from a reserved range.  UTF-16 impacts   UTF-8 in that UCS-2 values from the reserved range must be treated   specially in the UTF-8 transformation.   UTF-8 encodes UCS-2 or UCS-4 characters as a varying number of   octets, where the number of octets, and the value of each, depend on   the integer value assigned to the character in ISO 10646.  This   transformation format has the following characteristics (all values   are in hexadecimal):   -  Character values from 0000 0000 to 0000 007F (US-ASCII repertoire)      correspond to octets 00 to 7F (7 bit US-ASCII values).   -  US-ASCII values do not appear otherwise in a UTF-8 encoded charac-      ter stream.  This provides compatibility with file systems or      other software (e.g. the printf() function in C libraries) that      parse based on US-ASCII values but are transparent to other val-      ues.   -  Round-trip conversion is easy between UTF-8 and either of UCS-4,      UCS-2 or Unicode.   -  The first octet of a multi-octet sequence indicates the number of      octets in the sequence.   -  Character boundaries are easily found from anywhere in an octet      stream.   -  The lexicographic sorting order of UCS-4 strings is preserved.  Of      course this is of limited interest since the sort order is not      culturally valid in either case.   -  The octet values FE and FF never appear.   UTF-8 was originally a project of the X/Open Joint   Internationalization Group XOJIG with the objective to specify a File   System Safe UCS Transformation Format [FSS-UTF] that is compatible   with UNIX systems, supporting multilingual text in a single encoding.   The original authors were Gary Miller, Greger Leijonhufvud and John   Entenmann.  Later, Ken Thompson and Rob Pike did significant work for   the formal UTF-8.Yergeau                      Informational                      [Page 2]RFC 2044                         UTF-8                      October 1996   A description can also be found in Unicode Technical Report #4 [UNI-   CODE].  The definitive reference, including provisions for UTF-16   data within UTF-8, is Annex R of ISO/IEC 10646-1 [ISO-10646].2.  UTF-8 definition   In UTF-8, characters are encoded using sequences of 1 to 6 octets.   The only octet of a "sequence" of one has the higher-order bit set to   0, the remaining 7 bits being used to encode the character value. In   a sequence of n octets, n>1, the initial octet has the n higher-order   bits set to 1, followed by a bit set to 0.  The remaining bit(s) of   that octet contain bits from the value of the character to be   encoded.  The following octet(s) all have the higher-order bit set to   1 and the following bit set to 0, leaving 6 bits in each to contain   bits from the character to be encoded.   The table below summarizes the format of these different octet types.   The letter x indicates bits available for encoding bits of the UCS-4   character value.   UCS-4 range (hex.)           UTF-8 octet sequence (binary)   0000 0000-0000 007F   0xxxxxxx   0000 0080-0000 07FF   110xxxxx 10xxxxxx   0000 0800-0000 FFFF   1110xxxx 10xxxxxx 10xxxxxx   0001 0000-001F FFFF   11110xxx 10xxxxxx 10xxxxxx 10xxxxxx   0020 0000-03FF FFFF   111110xx 10xxxxxx 10xxxxxx 10xxxxxx 10xxxxxx   0400 0000-7FFF FFFF   1111110x 10xxxxxx ... 10xxxxxx   Encoding from UCS-4 to UTF-8 proceeds as follows:   1) Determine the number of octets required from the character value      and the first column of the table above.   2) Prepare the high-order bits of the octets as per the second column      of the table.   3) Fill in the bits marked x from the bits of the character value,      starting from the lower-order bits of the character value and      putting them first in the last octet of the sequence, then the      next to last, etc. until all x bits are filled in.Yergeau                      Informational                      [Page 3]RFC 2044                         UTF-8                      October 1996      The algorithm for encoding UCS-2 (or Unicode) to UTF-8 can be      obtained from the above, in principle, by simply extending each      UCS-2 character with two zero-valued octets.  However, UCS-2 val-      ues between D800 and DFFF, being actually UCS-4 characters trans-      formed through UTF-16, need special treatment: the UTF-16 trans-      formation must be undone, yielding a UCS-4 character that is then      transformed as above.      Decoding from UTF-8 to UCS-4 proceeds as follows:   1) Initialize the 4 octets of the UCS-4 character with all bits set      to 0.   2) Determine which bits encode the character value from the number of      octets in the sequence and the second column of the table above      (the bits marked x).   3) Distribute the bits from the sequence to the UCS-4 character,      first the lower-order bits from the last octet of the sequence and      proceeding to the left until no x bits are left.      If the UTF-8 sequence is no more than three octets long, decoding      can proceed directly to UCS-2 (or equivalently Unicode).      A more detailed algorithm and formulae can be found in [FSS_UTF],      [UNICODE] or Annex R to [ISO-10646].3.  Examples   The Unicode sequence "A<NOT IDENTICAL TO><ALPHA>." (0041, 2262, 0391,   002E) may be encoded as follows:      41 E2 89 A2 CE 91 2E   The Unicode sequence "Hi Mom <WHITE SMILING FACE>!" (0048, 0069,   0020, 004D, 006F, 006D, 0020, 263A, 0021) may be encoded as follows:      48 69 20 4D 6F 6D 20 E2 98 BA 21   The Unicode sequence representing the Han characters for the Japanese   word "nihongo" (65E5, 672C, 8A9E) may be encoded as follows:      E6 97 A5 E6 9C AC E8 AA 9EYergeau                      Informational                      [Page 4]RFC 2044                         UTF-8                      October 1996MIME registrations   This memo is meant to serve as the basis for registration of a MIME   character encoding (charset) as per [RFC1521].  The proposed charset   parameter value is "UTF-8".  This string would label media types   containing text consisting of characters from the repertoire of ISO   10646-1 encoded to a sequence of octets using the encoding scheme   outlined above.Security Considerations   Security issues are not discussed in this memo.Acknowledgments   The following have participated in the drafting and discussion of   this memo:      James E. Agenbroad   Andries Brouwer      Martin J. D|rst      David Goldsmith      Edwin F. Hart        Kent Karlsson      Markus Kuhn          Michael Kung      Alain LaBonte        Murray Sargent      Keld Simonsen        Arnold WinklerBibliography   [FSS_UTF]      X/Open CAE Specification C501 ISBN 1-85912-082-2 28cm.                  22p. pbk. 172g.  4/95, X/Open Company Ltd., "File Sys-                  tem Safe UCS Transformation Format (FSS_UTF)", X/Open                  Preleminary Specification, Document Number P316.  Also                  published in Unicode Technical Report #4.   [ISO-10646]    ISO/IEC 10646-1:1993. International Standard -- Infor-                  mation technology -- Universal Multiple-Octet Coded                  Character Set (UCS) -- Part 1: Architecture and Basic                  Multilingual Plane.  UTF-8 is described in Annex R,                  adopted but not yet published.  UTF-16 is described in                  Annex Q, adopted but not yet published.   [RFC1521]      Borenstein, N., and N. Freed, "MIME (Multipurpose                  Internet Mail Extensions) Part One: Mechanisms for                  Specifying and Describing the Format of Internet Mes-                  sage Bodies", RFC 1521, Bellcore, Innosoft, September                  1993.   [RFC1641]      Goldsmith, D., and M. Davis, "Using Unicode with                  MIME", RFC 1641, Taligent inc., July 1994.Yergeau                      Informational                      [Page 5]RFC 2044                         UTF-8                      October 1996   [RFC1642]      Goldsmith, D., and M. Davis, "UTF-7: A Mail-safe                  Transformation Format of Unicode", RFC 1642,                  Taligent, Inc., July 1994.   [UNICODE]      The Unicode Consortium, "The Unicode Standard --                  Worldwide Character Encoding -- Version 1.0", Addison-                  Wesley, Volume 1, 1991, Volume 2, 1992.  UTF-8 is                  described in Unicode Technical Report #4.   [US-ASCII]     Coded Character Set--7-bit American Standard Code for                  Information Interchange, ANSI X3.4-1986.Author's Address      Francois Yergeau      Alis Technologies      100, boul. Alexis-Nihon      Suite 600      Montreal  QC  H4M 2P2      Canada      Tel: +1 (514) 747-2547      Fax: +1 (514) 747-2561      EMail: fyergeau@alis.com