UTF-8, a transformation format of ISO 10646
RFC 2279
Document | Type |
RFC - Draft Standard
(January 1998; No errata)
Obsoleted by RFC 3629
Obsoletes RFC 2044
Was draft-yergeau-utf8-rev (individual)
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Author | François Yergeau | ||
Last updated | 2013-03-02 | ||
Stream | Legacy stream | ||
Formats | plain text html pdf htmlized (tools) htmlized bibtex | ||
Stream | Legacy state | (None) | |
Consensus Boilerplate | Unknown | ||
RFC Editor Note | (None) | ||
IESG | IESG state | RFC 2279 (Draft Standard) | |
Telechat date | |||
Responsible AD | (None) | ||
Send notices to | (None) |
Network Working Group F. Yergeau Request for Comments: 2279 Alis Technologies Obsoletes: 2044 January 1998 Category: Standards Track UTF-8, a transformation format of ISO 10646 Status of this Memo This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited. Copyright Notice Copyright (C) The Internet Society (1998). All Rights Reserved. Abstract ISO/IEC 10646-1 defines a multi-octet character set called the Universal Character Set (UCS) 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 characteristic 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. 1. Introduction ISO/IEC 10646-1 [ISO-10646] defines a multi-octet character set called the Universal Character Set (UCS), which encompasses most of the world's writing systems. Two multi-octet encodings are defined, a four-octet per character encoding called UCS-4 and a two-octet per character encoding called UCS-2, able to address only the first 64K characters of the UCS (the Basic Multilingual Plane, BMP), outside of which there are currently no assignments. It is noteworthy that the same set of characters is defined by the Unicode standard [UNICODE], which further defines additional character properties and other application details of great interest to implementors, but does not have the UCS-4 encoding. Up to the Yergeau Standards Track [Page 1] RFC 2279 UTF-8 January 1998 present time, changes in Unicode and amendments to ISO/IEC 10646 have tracked each other, so that the character repertoires and code point assignments have remained in sync. The relevant standardization committees have committed to maintain this very useful synchronism. 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/IEC 10646. UTF-7 has the quality of encoding the full BMP 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 ([RFC2152]). 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/IEC 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 character 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 values. - Round-trip conversion is easy between UTF-8 and either of UCS-4, UCS-2. Yergeau Standards Track [Page 2] RFC 2279 UTF-8 January 1998Show full document text