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340 lines
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Plaintext
340 lines
12 KiB
Plaintext
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Network Working Group F. Yergeau
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Request for Comments: 2044 Alis Technologies
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Category: Informational October 1996
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UTF-8, a transformation format of Unicode and ISO 10646
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Status of this Memo
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This memo provides information for the Internet community. This memo
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does not specify an Internet standard of any kind. Distribution of
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this memo is unlimited.
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Abstract
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The Unicode Standard, version 1.1, and ISO/IEC 10646-1:1993 jointly
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define a 16 bit character set which encompasses most of the world's
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writing systems. 16-bit characters, however, are not compatible with
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many current applications and protocols, and this has led to the
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development of a few so-called UCS transformation formats (UTF), each
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with different characteristics. UTF-8, the object of this memo, has
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the characteristic of preserving the full US-ASCII range: US-ASCII
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characters are encoded in one octet having the usual US-ASCII value,
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and any octet with such a value can only be an US-ASCII character.
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This provides compatibility with file systems, parsers and other
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software that rely on US-ASCII values but are transparent to other
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values.
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1. Introduction
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The Unicode Standard, version 1.1 [UNICODE], and ISO/IEC 10646-1:1993
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[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
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defines a 31-bit character set, UCS-4, with currently no assignments
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outside of the region corresponding to UCS-2 (the Basic Multilingual
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Plane, BMP). The UCS-2 and UCS-4 encodings, however, are hard to use
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in many current applications and protocols that assume 8 or even 7
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bit characters. Even newer systems able to deal with 16 bit
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characters cannot process UCS-4 data. This situation has led to the
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development of so-called UCS transformation formats (UTF), each with
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different characteristics.
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UTF-1 has only historical interest, having been removed from ISO
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10646. UTF-7 has the quality of encoding the full Unicode repertoire
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using only octets with the high-order bit clear (7 bit US-ASCII
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values, [US-ASCII]), and is thus deemed a mail-safe encoding
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([RFC1642]). UTF-8, the object of this memo, uses all bits of an
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octet, but has the quality of preserving the full US-ASCII range:
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Yergeau Informational [Page 1]
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RFC 2044 UTF-8 October 1996
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US-ASCII characters are encoded in one octet having the normal US-
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ASCII value, and any octet with such a value can only stand for an
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US-ASCII character, and nothing else.
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UTF-16 is a scheme for transforming a subset of the UCS-4 repertoire
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into a pair of UCS-2 values from a reserved range. UTF-16 impacts
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UTF-8 in that UCS-2 values from the reserved range must be treated
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specially in the UTF-8 transformation.
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UTF-8 encodes UCS-2 or UCS-4 characters as a varying number of
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octets, where the number of octets, and the value of each, depend on
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the integer value assigned to the character in ISO 10646. This
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transformation format has the following characteristics (all values
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are in hexadecimal):
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- Character values from 0000 0000 to 0000 007F (US-ASCII repertoire)
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correspond to octets 00 to 7F (7 bit US-ASCII values).
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- US-ASCII values do not appear otherwise in a UTF-8 encoded charac-
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ter stream. This provides compatibility with file systems or
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other software (e.g. the printf() function in C libraries) that
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parse based on US-ASCII values but are transparent to other val-
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ues.
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- Round-trip conversion is easy between UTF-8 and either of UCS-4,
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UCS-2 or Unicode.
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- The first octet of a multi-octet sequence indicates the number of
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octets in the sequence.
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- Character boundaries are easily found from anywhere in an octet
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stream.
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- The lexicographic sorting order of UCS-4 strings is preserved. Of
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course this is of limited interest since the sort order is not
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culturally valid in either case.
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- The octet values FE and FF never appear.
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UTF-8 was originally a project of the X/Open Joint
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Internationalization Group XOJIG with the objective to specify a File
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System Safe UCS Transformation Format [FSS-UTF] that is compatible
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with UNIX systems, supporting multilingual text in a single encoding.
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The original authors were Gary Miller, Greger Leijonhufvud and John
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Entenmann. Later, Ken Thompson and Rob Pike did significant work for
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the formal UTF-8.
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Yergeau Informational [Page 2]
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RFC 2044 UTF-8 October 1996
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A description can also be found in Unicode Technical Report #4 [UNI-
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CODE]. The definitive reference, including provisions for UTF-16
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data within UTF-8, is Annex R of ISO/IEC 10646-1 [ISO-10646].
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2. UTF-8 definition
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In UTF-8, characters are encoded using sequences of 1 to 6 octets.
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The only octet of a "sequence" of one has the higher-order bit set to
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0, the remaining 7 bits being used to encode the character value. In
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a sequence of n octets, n>1, the initial octet has the n higher-order
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bits set to 1, followed by a bit set to 0. The remaining bit(s) of
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that octet contain bits from the value of the character to be
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encoded. The following octet(s) all have the higher-order bit set to
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1 and the following bit set to 0, leaving 6 bits in each to contain
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bits from the character to be encoded.
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The table below summarizes the format of these different octet types.
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The letter x indicates bits available for encoding bits of the UCS-4
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character value.
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UCS-4 range (hex.) UTF-8 octet sequence (binary)
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0000 0000-0000 007F 0xxxxxxx
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0000 0080-0000 07FF 110xxxxx 10xxxxxx
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0000 0800-0000 FFFF 1110xxxx 10xxxxxx 10xxxxxx
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0001 0000-001F FFFF 11110xxx 10xxxxxx 10xxxxxx 10xxxxxx
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0020 0000-03FF FFFF 111110xx 10xxxxxx 10xxxxxx 10xxxxxx 10xxxxxx
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0400 0000-7FFF FFFF 1111110x 10xxxxxx ... 10xxxxxx
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Encoding from UCS-4 to UTF-8 proceeds as follows:
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1) Determine the number of octets required from the character value
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and the first column of the table above.
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2) Prepare the high-order bits of the octets as per the second column
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of the table.
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3) Fill in the bits marked x from the bits of the character value,
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starting from the lower-order bits of the character value and
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putting them first in the last octet of the sequence, then the
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next to last, etc. until all x bits are filled in.
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Yergeau Informational [Page 3]
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RFC 2044 UTF-8 October 1996
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The algorithm for encoding UCS-2 (or Unicode) to UTF-8 can be
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obtained from the above, in principle, by simply extending each
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UCS-2 character with two zero-valued octets. However, UCS-2 val-
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ues between D800 and DFFF, being actually UCS-4 characters trans-
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formed through UTF-16, need special treatment: the UTF-16 trans-
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formation must be undone, yielding a UCS-4 character that is then
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transformed as above.
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Decoding from UTF-8 to UCS-4 proceeds as follows:
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1) Initialize the 4 octets of the UCS-4 character with all bits set
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to 0.
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2) Determine which bits encode the character value from the number of
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octets in the sequence and the second column of the table above
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(the bits marked x).
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3) Distribute the bits from the sequence to the UCS-4 character,
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first the lower-order bits from the last octet of the sequence and
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proceeding to the left until no x bits are left.
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If the UTF-8 sequence is no more than three octets long, decoding
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can proceed directly to UCS-2 (or equivalently Unicode).
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A more detailed algorithm and formulae can be found in [FSS_UTF],
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[UNICODE] or Annex R to [ISO-10646].
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3. Examples
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The Unicode sequence "A<NOT IDENTICAL TO><ALPHA>." (0041, 2262, 0391,
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002E) may be encoded as follows:
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41 E2 89 A2 CE 91 2E
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The Unicode sequence "Hi Mom <WHITE SMILING FACE>!" (0048, 0069,
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0020, 004D, 006F, 006D, 0020, 263A, 0021) may be encoded as follows:
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48 69 20 4D 6F 6D 20 E2 98 BA 21
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The Unicode sequence representing the Han characters for the Japanese
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word "nihongo" (65E5, 672C, 8A9E) may be encoded as follows:
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E6 97 A5 E6 9C AC E8 AA 9E
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Yergeau Informational [Page 4]
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RFC 2044 UTF-8 October 1996
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MIME registrations
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This memo is meant to serve as the basis for registration of a MIME
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character encoding (charset) as per [RFC1521]. The proposed charset
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parameter value is "UTF-8". This string would label media types
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containing text consisting of characters from the repertoire of ISO
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10646-1 encoded to a sequence of octets using the encoding scheme
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outlined above.
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Security Considerations
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Security issues are not discussed in this memo.
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Acknowledgments
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The following have participated in the drafting and discussion of
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this memo:
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James E. Agenbroad Andries Brouwer
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Martin J. D|rst David Goldsmith
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Edwin F. Hart Kent Karlsson
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Markus Kuhn Michael Kung
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Alain LaBonte Murray Sargent
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Keld Simonsen Arnold Winkler
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Bibliography
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[FSS_UTF] X/Open CAE Specification C501 ISBN 1-85912-082-2 28cm.
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22p. pbk. 172g. 4/95, X/Open Company Ltd., "File Sys-
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tem Safe UCS Transformation Format (FSS_UTF)", X/Open
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Preleminary Specification, Document Number P316. Also
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published in Unicode Technical Report #4.
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[ISO-10646] ISO/IEC 10646-1:1993. International Standard -- Infor-
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mation technology -- Universal Multiple-Octet Coded
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Character Set (UCS) -- Part 1: Architecture and Basic
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Multilingual Plane. UTF-8 is described in Annex R,
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adopted but not yet published. UTF-16 is described in
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Annex Q, adopted but not yet published.
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[RFC1521] Borenstein, N., and N. Freed, "MIME (Multipurpose
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Internet Mail Extensions) Part One: Mechanisms for
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Specifying and Describing the Format of Internet Mes-
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sage Bodies", RFC 1521, Bellcore, Innosoft, September
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1993.
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[RFC1641] Goldsmith, D., and M. Davis, "Using Unicode with
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MIME", RFC 1641, Taligent inc., July 1994.
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Yergeau Informational [Page 5]
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RFC 2044 UTF-8 October 1996
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[RFC1642] Goldsmith, D., and M. Davis, "UTF-7: A Mail-safe
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Transformation Format of Unicode", RFC 1642,
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Taligent, Inc., July 1994.
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[UNICODE] The Unicode Consortium, "The Unicode Standard --
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Worldwide Character Encoding -- Version 1.0", Addison-
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Wesley, Volume 1, 1991, Volume 2, 1992. UTF-8 is
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described in Unicode Technical Report #4.
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[US-ASCII] Coded Character Set--7-bit American Standard Code for
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Information Interchange, ANSI X3.4-1986.
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Author's Address
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Francois Yergeau
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Alis Technologies
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100, boul. Alexis-Nihon
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Suite 600
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Montreal QC H4M 2P2
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Canada
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Tel: +1 (514) 747-2547
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Fax: +1 (514) 747-2561
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EMail: fyergeau@alis.com
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Yergeau Informational [Page 6]
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