Network Working Group F. Yergeau Internet Draft Alis Technologies <draft-yergeau-utf8-rev-00.txt> 14 April 1997 Expires 14 October 1997 [Will obsolete RFC 2044] 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 ISO/IEC 10646-1 and the Unicode Standard jointly define a multi-octet character set which encompasses most of the world's writing systems. Multi-octet 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 character- istic of preserving the full US-ASCII range, providing compatibility with file systems, parsers and other software that rely on US-ASCII values but are transparent to other values. This memo updates and replaces RFC 2044, in particular addressing the question of versions of the relevant standards. Expires 14 October 1997 [Page 1]
Internet Draft UTF-8 14 April 1997 1. Introduction ISO/IEC 10646-1 [ISO-10646] and the Unicode Standard [UNICODE] 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 cur- rent applications and protocols that assume 8 or even 7 bit charac- ters. 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 charac- teristics. 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 pairs 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). A direct consequence is that a plain ASCII string is also a valid UTF-8 string. - 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. Expires 14 October 1997 [Page 2]
Internet Draft UTF-8 14 April 1997 - The first octet of a multi-octet sequence indicates the number of octets in the sequence. - The octet values FE and FF never appear. - 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 Boyer-Moore fast search algorithm can be used with UTF-8 data. - UTF-8 strings can be fairly reliably recognized as such by a sim- ple algorithm, i.e. the probability that a string of characters in any other encoding appears as valid UTF-8 is low, diminishing with increasing string length. 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 and in the Unicode Standard, version 2.0 [UNICODE]. The definitive refer- ence, 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. Expires 14 October 1997 [Page 3]
Internet Draft UTF-8 14 April 1997 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. 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], Expires 14 October 1997 [Page 4]
Internet Draft UTF-8 14 April 1997 [UNICODE] or Annex R to [ISO-10646]. 3. Versions of the standards Different versions of the Unicode standard exist: 1.0, 1.1 and 2.0 as of this writing. Each new version obsoletes and replaces the previ- ous one, but implementations, and more significantly data, are not updated instantly. Similarly, ISO 10646 is updated from time to time by published amendments, which up to now have tracked the changes in the Unicode standard, so that the two have remained in sync. In general, the changes amount to adding new characters, which does not pose particular problems with old data. Amendment 5 to ISO 10646, however, has moved and expanded the Korean Hangul block, thereby making any previous data containing Hangul characters invalid under the new version. Unicode 2.0 has the same difference from Uni- code 1.1. The official justification for allowing such an incompati- ble change was that no implementations and no data containing Hangul existed, a statement that is likely to be true but remains unprov- able. The incident has been dubbed the "Korean mess", and the rele- vant committees have pledged to never, ever again make such an incom- patible change. New versions, and in particular any incompatible changes, have conse- quences regarding MIME character encoding labels, to be discussed in section 5. 4. Examples The UCS-2 sequence "A<NOT IDENTICAL TO><ALPHA>." (0041, 2262, 0391, 002E) may be encoded as follows: 41 E2 89 A2 CE 91 2E The UCS-2 sequence representing the Hangul characters for the Korean word "hangugo" (D55C, AD6D, C5B4) may be encoded as follows: ED 95 9C EA B5 AD EC 96 B4 The UCS-2 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 9E Expires 14 October 1997 [Page 5]
Internet Draft UTF-8 14 April 1997 5. MIME registration This memo is meant to serve as the basis for registration of a MIME character set parameter (charset) [MIME]. 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/IEC 10646 encoded to a sequence of octets using the encoding scheme out- lined above. UTF-8 is suitable for use in MIME content types under the "text" top-level type. It is noteworthy that the label "UTF-8" does not contain a version identification, referring generically to ISO/IEC 10646. This is intentional, the rationale being as follows: A MIME charset label is designed to give just the information needed to interpret a sequence of bytes received on the wire into a sequence of characters, nothing more (see RFC 2045, section 2.2, in [MIME]). As long as a character set standard does not change incompatibly, version numbers serve no purpose, because one gains nothing by learn- ing from the tag that newly assigned characters may be received that one doesn't know about. The tag doesn't teach anything about the new characters, and they are going to be received anyway. Hence, as long as the standards evolve compatibly, the apparent advantage of having labels that identify the versions is only that, apparent. But there is a disadvantage to such version-dependent labels: when an older application receives data accompanied by a newer, unknown label, it may fail to recognize the label and be com- pletely unable to deal with the data, whereas a generic, known label would have triggered mostly correct processing of the data, which may well not contain any new characters. Now the "Korean mess" (ISO 10646 amendment 5) is an incompatible change, in principle contradicting the appropriateness of a version- independent MIME charset label as described above. But the compati- bility problem can only appear with data containing Korean Hangul characters encoded according to Unicode 1.1 (or equivalently ISO 10646 before amendment 5), and there is arguably no such data to worry about, this being the very reason the incompatible change was deemed acceptable. In practice, then, a version-independent label is warranted. Should the need ever arise to distinguish data containing Hangul encoded according to Unicode 1.1, then a version-dependent label, for that version only, should be registered (a suggestion would be "UNI- CODE-1-1-UTF-8"), in order to retain the advantages of a version- independent label for 2.0 and later versions. Such a version-depen- dent label could even be registered before actual need arises, pre- Expires 14 October 1997 [Page 6]
Internet Draft UTF-8 14 April 1997 emptively, but it is important to strongly recommend against creating any new Hangul-containing data without taking Amendment 5 of ISO 10646 into account. 6. 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. Dürst David Goldsmith Edwin F. Hart Kent Karlsson Markus Kuhn Michael Kung Alain LaBonté 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 R, published as Amendment 2. UTF-16 is described in Annex Q, published as Amendment 1. [MIME] N. Freed, N. Borenstein, "Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Mes- sage Bodies", RFC 2045. N. Freed, N. Borenstein, "Multipurpose Internet Mail Extensions (MIME) Part Two: Media Types", RFC 2046. K. Moore, "MIME (Multi- purpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text", RFC 2047. N. Freed, J. Klensin, J. Postel, "Multipurpose Internet Mail Extensions (MIME) Part Four: Registration Proce- dures", RFC 2048. N. Freed, N. Borenstein, "Multipur- pose Internet Mail Extensions (MIME) Part Five: Con- formance Criteria and Examples", RFC 2049. All Expires 14 October 1997 [Page 7]
Internet Draft UTF-8 14 April 1997 November 1996. [RFC1641] D. Goldsmith, M.Davis, "Using Unicode with MIME", RFC 1641, Taligent inc., July 1994. [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 -- Ver- sion 2.0", Addison-Wesley, 1996. [US-ASCII] Coded Character Set--7-bit American Standard Code for Information Interchange, ANSI X3.4-1986. Author's Address François Yergeau Alis Technologies 100, boul. Alexis-Nihon Suite 600 Montréal QC H4M 2P2 Canada Tel: +1 (514) 747-2547 Fax: +1 (514) 747-2561 EMail: fyergeau@alis.com Expires 14 October 1997 [Page 8]