UTF-8, a transformation format of Unicode and ISO 10646
RFC 2044
Document | Type |
RFC - Informational
(October 1996; No errata)
Obsoleted by RFC 2279
Was draft-yergeau-utf8 (individual)
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Author | François Yergeau | ||
Last updated | 2013-03-02 | ||
Stream | Legacy | ||
Formats | plain text html pdf htmlized bibtex | ||
Stream | Legacy state | (None) | |
Consensus Boilerplate | Unknown | ||
RFC Editor Note | (None) | ||
IESG | IESG state | RFC 2044 (Informational) | |
Telechat date | |||
Responsible AD | (None) | ||
Send notices to | (None) |
Network Working Group F. Yergeau Request for Comments: 2044 Alis Technologies Category: Informational October 1996 UTF-8, a transformation format of Unicode and ISO 10646 Status 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-16Show full document text