One document matched: draft-yergeau-utf8-00.txt
Network Working Group F. Yergeau
Internet Draft Alis Technologies
<draft-yergeau-utf8-00.txt> 27 March 1996
Expires 1 October 1996
UTF-8, a transformation format of Unicode and ISO 10646
Status of this Memo
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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
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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.
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- 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.
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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<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, 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
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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.
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[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
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