L2/01-328R3

Proposed Draft Unicode Technical Report #26

COMPATIBILITY enCODING sCHEME for utf-16 – 8-BIT
(CESU-8)

Version	Unicode 3.1.1
Authors	Toby Phipps (tphipps@peoplesoft.com)
Date	2001-09-04
This Version	http://www.unicode.org/unicode/reports/tr26/tr26-1.1
Previous Version	None
Latest Version	http://www.unicode.org/unicode/reports/tr26/tr26-1.1

Summary

This document specifies a 8-bit Compatibility Encoding Scheme for UTF-16 (CESU) that is intended as an alternate encoding to UTF-8 for internal use within systems processing Unicode in order to provide a ASCII-compatible 8-bit encoding that preserves UTF-16 binary collation. It is not intended nor recommended as an encoding used for open information exchange. The Unicode Consortuim, does not encourage the use of CESU-8, but does recognize the existence of data in this encoding and supplies this Technical Report to clearly define the format and to distinguish it from UTF-8. This encoding does not replace or amend the definition of UTF-8.

Status

This document has been approved by the Unicode Technical Committee for public review as a Proposed Draft Unicode Technical Report. Publication does not imply endorsement by the Unicode Consortium. This is a draft document which may be updated, replaced, or superseded by other documents at any time. This is not a stable document; it is inappropriate to cite this document as other than a work in progress.

A list of current Unicode Technical Reports is found on http://www.unicode.org/unicode/reports/. For more information about versions of the Unicode Standard, see http://www.unicode.org/unicode/standard/versions/.

Please mail corrigenda and other comments to the author(s).

1 Introduction
2 Definitions
3 Relation to ISO/IEC 10646 and UTF-8
4. IANA Registration
5. References
Modifications

1 Introduction

CESU-8 defines an encoding scheme for Unicode identical to UTF-8 except for its representation of supplementary characters. In CESU-8, supplementary characters are represented as six-byte sequences resulting from the transformation of each UTF-16 surrogate code unit into an eight-bit form similar to the UTF-8 transformation, but without first converting the input surrogate pairs to a scalar value.

CESU-8 is useful in 8-bit processing environments where binary collation with UTF-16 is required. It is designed and recommended for use only within products requiring this UTF-16 binary collation eqivalence. It is not intended nor recommended for open interchange.

The following lists the important features of this encoding form:

The CESU-8 representation of characters on the Basic Multilingual Plane (BMP) is identical to the representation of these characters in UTF-8. Only the representation of supplementary characters differs.
Only the six-byte form of supplementary characters is legal in CESU-8; the four-byte UTF-8 style supplementary character sequence is illegal.
When supplementary characters are present, a data stream can be unequivocally determined as being encoded in UTF-8 or CESU-8 based on the representation of these supplementary characters. When encoding information is not present and encoding autodetection is attempted, if the data stream consists of well-formed UTF-8 and does not contain supplementary characters, it should always be detected as UTF-8, not CESU-8 (even though the two encodings are identical when supplementary characters are not present).
A binary collation of data encoded in CESU-8 is identical to the binary collation of the same data encoded in UTF-16.

As a very small percentage of characters in a typical data stream are expected to be supplementary characters, there is a strong possibility that CESU-8 data may be misinterpreted as UTF-8. Therefore, all use of CESU-8 outside closed implementations is strongly discouraged, such as the emittance of CESU-8 in output files, markup language or other open transmission forms.

2 Definitions

The following define the CESU-8 encoding scheme. CESU-8 is not a normative part of The Unicode Standard, and therefore the definitions below do not form part of the standard. Instead, they are encapsulated in this Unicode Technical Report as an implementation-specific transformation form for use by implementors of The Unicode Standard.

2.1

(a) CESU-8 is a Compatibility Encoding Scheme for UTF-16 (CESU) that serializes a Unicode code point as a sequence of one, two, three or six bytes.
(b) Prior to transforming data into CESU-8, supplementary characters must first be converted to their surrogate pair UTF-16 representation. For example, U+F0000 must first be converted to U+DB80 U+DC00.
(c) The resulting data stream is encoded into an eight-bit form using the bit distribution table in definition 2.2. It should be noted that this bit distribution table is identical to that of UTF-8 except that the input value is a sequence of UTF-16 code units, not a scalar value, and that a four-byte transformation is disallowed.
(d) The bit pattern 11110xxx is illegal in any CESU-8 byte, effectively prohibiting the occurrence of UTF-8 four-byte surrogates in CESU-8. Thus, a data stream may not contain both CESU-8 six-byte and UTF-8 four-byte supplementary character sequences.
(e) The shortest form rules applied to UTF-8 in The Unicode Standard Definition D36 will also apply to CESU-8.
(f) Data encoded in CESU-8 should only be exchanged when it is labeled as such in a higher-level protocol or is agreed upon in an API definition. It should not be auto-detected. Use of this encoding in the absence of encoding tags or a higher level protocol describing the encoding is invalid and strongly discouraged.

CESU-8 encoding example:
In CESU-8, <U+004D, U+0061, U+10000> is serialized as <4D 61 ED AE 80 ED B0 80>

2.2

CESU-8 Bit Distribution

UTF-16 Code Unit	1^st Byte	2^nd Byte	3^rd Byte
000000000xxxxxxx	0xxxxxxx
00000yyyyyxxxxxx	110yyyyy	10xxxxxx
zzzzyyyyyyxxxxxx	1110zzzz	10yyyyyy	10xxxxxx

3 Relation to ISO/IEC 10646 and UTF-8

ISO/IEC 10646 and The Unicode Standard define the UTF-8 encoding form, which is very similar in definition to CESU-8 other than its treatment of supplementary characters. CESU-8 is an additional encoding scheme that supplements these definitions, but does not form part of either ISO/IEC 10646 or The Unicode Standard. It is intended only for use in compatibility situations where binary collation with UTF-16 is required.

4 IANA Registration

CESU-8 will be registered with the Internet Assigned Numbers Authority. This section will be updated with the IANA registered name.

Note: CESU-8 was originally proposed and discussed with the name UTF-8S, but was renamed CESU-8 by recommendation from the Unicode Technical Committee to avoid possible confusion with UTF-8.

5 References

[Reports]	Unicode Technical Reports http://www.unicode.org/unicode/reports/ For information on the status and development process for technical reports, and for a list of technical reports.
[Versions]	Versions of the Unicode Standard http://www.unicode.org/unicode/standard/versions/ For details on the precise contents of each version of the Unicode Standard, and how to cite them.

Modifications

The following summarizes modifications from the previous version of this document.

1	Created with amendments from proposed draft as approved by UTC#88.

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