Network Working Group F. Yergeau Internet Draft Alis Technologies 27 March 1996 Expires 1 October 1996 UTF-8, a transformation format of Unicode and ISO 10646 Status of this Memo This document is an Internet-Draft. Internet-Drafts are working doc- uments of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute work- ing documents as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months. Internet-Drafts may be updated, replaced, or obsoleted by other documents at any time. It is not appropriate to use Internet- Drafts as reference material or to cite them other than as a "working draft" or "work in progress". To learn the current status of any Internet-Draft, please check the 1id-abstracts.txt listing contained in the Internet-Drafts Shadow Directories on ds.internic.net (US East Coast), nic.nordu.net (Europe), ftp.isi.edu (US West Coast), or munnari.oz.au (Pacific Rim). Distribution of this document 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 Expires 1 October 1996 [Page 1] Internet Draft UTF-8 27 March 1996 encompasses most of the world's writing systems. ISO 10646 further defines a 32 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 charac- ters cannot process UCS-4 data. This situation has led to the devel- opment of so-called UCS transformation formats (UTF), each with dif- ferent 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 val- ues, [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: 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 007E (US-ASCII repertoire) correspond to octets 00 to 7E (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 that parse based on US-ASCII values but are trans- parent to other values. - 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. Expires 1 October 1996 [Page 2] Internet Draft UTF-8 27 March 1996 - The lexicographic sorting order of UCS-4 strings is preserved. - The octet values FE and FF never appear. UTF-8 was originally a project of the X/Open Joint Internationaliza- tion 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. 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, will be Annex P 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 is(are) used to encode bits from the initial character. 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 encode bits from the initial character. The table below shows the format of these different octet types. The letter x indicates bits available for encoding bits of the initial character. Octet Binary Free bits Max. character value 1st of 1 0xxxxxxx 7 0000 007F 1st of 2 110xxxxx 5 0000 07FF 1st of 3 1110xxxx 4 0000 FFFF 1st of 4 11110xxx 3 001F FFFF 1st of 5 111110xx 2 03FF FFFF 1st of 6 1111110x 1 7FFF FFFF 2nd to 6th 10xxxxxx 6 Encoding from UCS-4 to UTF-8 proceeds as follows: 1) determine the number of octets required from the character value and the fourth column of the table above. 2) Prepare the high-order bits of the octets as per the second column of the table. Expires 1 October 1996 [Page 3] Internet Draft UTF-8 27 March 1996 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. 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 trough UTF-16, need special treatment: the UTF-16 transfor- mation 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 the forthcoming Annex P to [ISO-10646]. 3. Examples The Unicode sequence "A." (0041, 2262, 0391, 002E) may be encoded as follows: 41 E2 89 A2 CE 91 2E The Unicode sequence "Hi Mom !" (0048, 0069, 0020, 004D, 006F, 004D, 0020, 263A, 0021) may be encoded as follows: 48 69 20 4D 6F 4D 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 E4 AA 9E Expires 1 October 1996 [Page 4] Internet Draft UTF-8 27 March 1996 MIME 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 con- taining 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: Andries Brouwer Martin J. Drst David Goldsmith Edwin F. Hart Markus Kuhn Michael Kung Alain LaBont Murray Sargent Keld Simonsen Arnold Winkler Bibliography [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 P, adopted but not yet published. UTF-16 is described in Annex O, adopted but not yet published. [RFC1521] N. Borenstein and N. Freed, "MIME (Multipurpose Inter- net Mail Extensions) Part One: Mechanisms for Specify- ing and Describing the Format of Internet Message Bod- ies", RFC 1521, Bellcore, Innosoft, September 1993. [RFC1641] D. Goldsmith, M.Davis, "Using Unicode with MIME", RFC 1641, Taligent inc., July 1994. Expires 1 October 1996 [Page 5] Internet Draft UTF-8 27 March 1996 [RFC1642] D. Goldsmith, M. Davis, "UTF-7: A Mail-safe Transfor- mation 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 Franois Yergeau Alis Technologies 100, boul. Alexis-Nihon Suite 600 Montral QC H4M 2P2 Canada Tel: +1 (514) 747-2547 Fax: +1 (514) 747-2561 EMail: fyergeau@alis.com Expires 1 October 1996 [Page 6]