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[mascara-docs.git] / lang / C / web / utf-8 / rfc3629.txt

Network Working Group                                         F. Yergeau
Request for Comments: 3629                             Alis Technologies
STD: 63                                                    November 2003
Obsoletes: 2279
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 (2003).  All Rights Reserved.

Abstract

   ISO/IEC 10646-1 defines a large character set called the Universal
   Character Set (UCS) which encompasses most of the world's writing
   systems.  The originally proposed encodings of the UCS, however, were
   not compatible with many current applications and protocols, and this
   has led to the development of UTF-8, the object of this memo.  UTF-8
   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 obsoletes and replaces RFC 2279.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  2
   2.  Notational conventions . . . . . . . . . . . . . . . . . . . .  3
   3.  UTF-8 definition . . . . . . . . . . . . . . . . . . . . . . .  4
   4.  Syntax of UTF-8 Byte Sequences . . . . . . . . . . . . . . . .  5
   5.  Versions of the standards  . . . . . . . . . . . . . . . . . .  6
   6.  Byte order mark (BOM)  . . . . . . . . . . . . . . . . . . . .  6
   7.  Examples . . . . . . . . . . . . . . . . . . . . . . . . . . .  8
   8.  MIME registration  . . . . . . . . . . . . . . . . . . . . . .  9
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 10
   10. Security Considerations  . . . . . . . . . . . . . . . . . . . 10
   11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 11
   12. Changes from RFC 2279  . . . . . . . . . . . . . . . . . . . . 11
   13. Normative References . . . . . . . . . . . . . . . . . . . . . 12



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   14. Informative References . . . . . . . . . . . . . . . . . . . . 12
   15. URI's  . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
   16. Intellectual Property Statement  . . . . . . . . . . . . . . . 13
   17. Author's Address . . . . . . . . . . . . . . . . . . . . . . . 13
   18. Full Copyright Statement . . . . . . . . . . . . . . . . . . . 14

1. Introduction

   ISO/IEC 10646 [ISO.10646] defines a large character set called the
   Universal Character Set (UCS), which encompasses most of the world's
   writing systems.  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 implementers.  Up to the present time, changes in Unicode and
   amendments and additions 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.

   ISO/IEC 10646 and Unicode define several encoding forms of their
   common repertoire: UTF-8, UCS-2, UTF-16, UCS-4 and UTF-32.  In an
   encoding form, each character is represented as one or more encoding
   units.  All standard UCS encoding forms except UTF-8 have an encoding
   unit larger than one octet, making them hard to use in many current
   applications and protocols that assume 8 or even 7 bit characters.

   UTF-8, the object of this memo, has a one-octet encoding unit.  It
   uses all bits of an octet, but has the quality of preserving the full
   US-ASCII [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 a US-ASCII character, and nothing else.

   UTF-8 encodes UCS 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 (the character number,
   a.k.a. code position, code point or Unicode scalar value).  This
   encoding form has the following characteristics (all values are in
   hexadecimal):

   o  Character numbers from U+0000 to U+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.








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   o  US-ASCII octet 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.

   o  Round-trip conversion is easy between UTF-8 and other encoding
      forms.

   o  The first octet of a multi-octet sequence indicates the number of
      octets in the sequence.

   o  The octet values C0, C1, F5 to FF never appear.

   o  Character boundaries are easily found from anywhere in an octet
      stream.

   o  The byte-value lexicographic sorting order of UTF-8 strings is the
      same as if ordered by character numbers.  Of course this is of
      limited interest since a sort order based on character numbers is
      almost never culturally valid.

   o  The Boyer-Moore fast search algorithm can be used with UTF-8 data.

   o  UTF-8 strings can be fairly reliably recognized as such by a
      simple algorithm, i.e., the probability that a string of
      characters in any other encoding appears as valid UTF-8 is low,
      diminishing with increasing string length.

   UTF-8 was devised in September 1992 by Ken Thompson, guided by design
   criteria specified by Rob Pike, with the objective of defining a UCS
   transformation format usable in the Plan9 operating system in a non-
   disruptive manner.  Thompson's design was stewarded through
   standardization by the X/Open Joint Internationalization Group XOJIG
   (see [FSS_UTF]), bearing the names FSS-UTF (variant FSS/UTF), UTF-2
   and finally UTF-8 along the way.

2.  Notational conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

   UCS characters are designated by the U+HHHH notation, where HHHH is a
   string of from 4 to 6 hexadecimal digits representing the character
   number in ISO/IEC 10646.





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3.  UTF-8 definition

   UTF-8 is defined by the Unicode Standard [UNICODE].  Descriptions and
   formulae can also be found in Annex D of ISO/IEC 10646-1 [ISO.10646]

   In UTF-8, characters from the U+0000..U+10FFFF range (the UTF-16
   accessible range) are encoded using sequences of 1 to 4 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 number.  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 contain bits from the number of the character to be
   encoded.  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 contain
   bits from the character to be encoded.

   The table below summarizes the format of these different octet types.
   The letter x indicates bits available for encoding bits of the
   character number.

   Char. number range  |        UTF-8 octet sequence
      (hexadecimal)    |              (binary)
   --------------------+---------------------------------------------
   0000 0000-0000 007F | 0xxxxxxx
   0000 0080-0000 07FF | 110xxxxx 10xxxxxx
   0000 0800-0000 FFFF | 1110xxxx 10xxxxxx 10xxxxxx
   0001 0000-0010 FFFF | 11110xxx 10xxxxxx 10xxxxxx 10xxxxxx

   Encoding a character to UTF-8 proceeds as follows:

   1.  Determine the number of octets required from the character number
       and the first column of the table above.  It is important to note
       that the rows of the table are mutually exclusive, i.e., there is
       only one valid way to encode a given character.

   2.  Prepare the high-order bits of the octets as per the second
       column of the table.

   3.  Fill in the bits marked x from the bits of the character number,
       expressed in binary.  Start by putting the lowest-order bit of
       the character number in the lowest-order position of the last
       octet of the sequence, then put the next higher-order bit of the
       character number in the next higher-order position of that octet,
       etc.  When the x bits of the last octet are filled in, move on to
       the next to last octet, then to the preceding one, etc. until all
       x bits are filled in.





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   The definition of UTF-8 prohibits encoding character numbers between
   U+D800 and U+DFFF, which are reserved for use with the UTF-16
   encoding form (as surrogate pairs) and do not directly represent
   characters.  When encoding in UTF-8 from UTF-16 data, it is necessary
   to first decode the UTF-16 data to obtain character numbers, which
   are then encoded in UTF-8 as described above.  This contrasts with
   CESU-8 [CESU-8], which is a UTF-8-like encoding that is not meant for
   use on the Internet.  CESU-8 operates similarly to UTF-8 but encodes
   the UTF-16 code values (16-bit quantities) instead of the character
   number (code point).  This leads to different results for character
   numbers above 0xFFFF; the CESU-8 encoding of those characters is NOT
   valid UTF-8.

   Decoding a UTF-8 character proceeds as follows:

   1.  Initialize a binary number with all bits set to 0.  Up to 21 bits
       may be needed.

   2.  Determine which bits encode the character number 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 binary number, first
       the lower-order bits from the last octet of the sequence and
       proceeding to the left until no x bits are left.  The binary
       number is now equal to the character number.

   Implementations of the decoding algorithm above MUST protect against
   decoding invalid sequences.  For instance, a naive implementation may
   decode the overlong UTF-8 sequence C0 80 into the character U+0000,
   or the surrogate pair ED A1 8C ED BE B4 into U+233B4.  Decoding
   invalid sequences may have security consequences or cause other
   problems.  See Security Considerations (Section 10) below.

4.  Syntax of UTF-8 Byte Sequences

   For the convenience of implementors using ABNF, a definition of UTF-8
   in ABNF syntax is given here.

   A UTF-8 string is a sequence of octets representing a sequence of UCS
   characters.  An octet sequence is valid UTF-8 only if it matches the
   following syntax, which is derived from the rules for encoding UTF-8
   and is expressed in the ABNF of [RFC2234].

   UTF8-octets = *( UTF8-char )
   UTF8-char   = UTF8-1 / UTF8-2 / UTF8-3 / UTF8-4
   UTF8-1      = %x00-7F
   UTF8-2      = %xC2-DF UTF8-tail



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   UTF8-3      = %xE0 %xA0-BF UTF8-tail / %xE1-EC 2( UTF8-tail ) /
                 %xED %x80-9F UTF8-tail / %xEE-EF 2( UTF8-tail )
   UTF8-4      = %xF0 %x90-BF 2( UTF8-tail ) / %xF1-F3 3( UTF8-tail ) /
                 %xF4 %x80-8F 2( UTF8-tail )
   UTF8-tail   = %x80-BF

   NOTE -- The authoritative definition of UTF-8 is in [UNICODE].  This
   grammar is believed to describe the same thing Unicode describes, but
   does not claim to be authoritative.  Implementors are urged to rely
   on the authoritative source, rather than on this ABNF.

5.  Versions of the standards

   ISO/IEC 10646 is updated from time to time by publication of
   amendments and additional parts; similarly, new versions of the
   Unicode standard are published over time.  Each new version obsoletes
   and replaces the previous one, but implementations, and more
   significantly data, are not updated instantly.

   In general, the changes amount to adding new characters, which does
   not pose particular problems with old data.  In 1996, Amendment 5 to
   the 1993 edition of ISO/IEC 10646 and Unicode 2.0 moved and expanded
   the Korean Hangul block, thereby making any previous data containing
   Hangul characters invalid under the new version.  Unicode 2.0 has the
   same difference from Unicode 1.1.  The justification for allowing
   such an incompatible change was that there were no major
   implementations and no significant amounts of data containing Hangul.
   The incident has been dubbed the "Korean mess", and the relevant
   committees have pledged to never, ever again make such an
   incompatible change (see Unicode Consortium Policies [1]).

   New versions, and in particular any incompatible changes, have
   consequences regarding MIME charset labels, to be discussed in MIME
   registration (Section 8).

6.  Byte order mark (BOM)

   The UCS character U+FEFF "ZERO WIDTH NO-BREAK SPACE" is also known
   informally as "BYTE ORDER MARK" (abbreviated "BOM").  This character
   can be used as a genuine "ZERO WIDTH NO-BREAK SPACE" within text, but
   the BOM name hints at a second possible usage of the character:  to
   prepend a U+FEFF character to a stream of UCS characters as a
   "signature".  A receiver of such a serialized stream may then use the
   initial character as a hint that the stream consists of UCS
   characters and also to recognize which UCS encoding is involved and,
   with encodings having a multi-octet encoding unit, as a way to





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   recognize the serialization order of the octets.  UTF-8 having a
   single-octet encoding unit, this last function is useless and the BOM
   will always appear as the octet sequence EF BB BF.

   It is important to understand that the character U+FEFF appearing at
   any position other than the beginning of a stream MUST be interpreted
   with the semantics for the zero-width non-breaking space, and MUST
   NOT be interpreted as a signature.  When interpreted as a signature,
   the Unicode standard suggests than an initial U+FEFF character may be
   stripped before processing the text.  Such stripping is necessary in
   some cases (e.g., when concatenating two strings, because otherwise
   the resulting string may contain an unintended "ZERO WIDTH NO-BREAK
   SPACE" at the connection point), but might affect an external process
   at a different layer (such as a digital signature or a count of the
   characters) that is relying on the presence of all characters in the
   stream.  It is therefore RECOMMENDED to avoid stripping an initial
   U+FEFF interpreted as a signature without a good reason, to ignore it
   instead of stripping it when appropriate (such as for display) and to
   strip it only when really necessary.

   U+FEFF in the first position of a stream MAY be interpreted as a
   zero-width non-breaking space, and is not always a signature.  In an
   attempt at diminishing this uncertainty, Unicode 3.2 adds a new
   character, U+2060 "WORD JOINER", with exactly the same semantics and
   usage as U+FEFF except for the signature function, and strongly
   recommends its exclusive use for expressing word-joining semantics.
   Eventually, following this recommendation will make it all but
   certain that any initial U+FEFF is a signature, not an intended "ZERO
   WIDTH NO-BREAK SPACE".

   In the meantime, the uncertainty unfortunately remains and may affect
   Internet protocols.  Protocol specifications MAY restrict usage of
   U+FEFF as a signature in order to reduce or eliminate the potential
   ill effects of this uncertainty.  In the interest of striking a
   balance between the advantages (reduction of uncertainty) and
   drawbacks (loss of the signature function) of such restrictions, it
   is useful to distinguish a few cases:

   o  A protocol SHOULD forbid use of U+FEFF as a signature for those
      textual protocol elements that the protocol mandates to be always
      UTF-8, the signature function being totally useless in those
      cases.

   o  A protocol SHOULD also forbid use of U+FEFF as a signature for
      those textual protocol elements for which the protocol provides
      character encoding identification mechanisms, when it is expected
      that implementations of the protocol will be in a position to
      always use the mechanisms properly.  This will be the case when



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      the protocol elements are maintained tightly under the control of
      the implementation from the time of their creation to the time of
      their (properly labeled) transmission.

   o  A protocol SHOULD NOT forbid use of U+FEFF as a signature for
      those textual protocol elements for which the protocol does not
      provide character encoding identification mechanisms, when a ban
      would be unenforceable, or when it is expected that
      implementations of the protocol will not be in a position to
      always use the mechanisms properly.  The latter two cases are
      likely to occur with larger protocol elements such as MIME
      entities, especially when implementations of the protocol will
      obtain such entities from file systems, from protocols that do not
      have encoding identification mechanisms for payloads (such as FTP)
      or from other protocols that do not guarantee proper
      identification of character encoding (such as HTTP).

   When a protocol forbids use of U+FEFF as a signature for a certain
   protocol element, then any initial U+FEFF in that protocol element
   MUST be interpreted as a "ZERO WIDTH NO-BREAK SPACE".  When a
   protocol does NOT forbid use of U+FEFF as a signature for a certain
   protocol element, then implementations SHOULD be prepared to handle a
   signature in that element and react appropriately: using the
   signature to identify the character encoding as necessary and
   stripping or ignoring the signature as appropriate.

7.  Examples

   The character sequence U+0041 U+2262 U+0391 U+002E "A<NOT IDENTICAL
   TO><ALPHA>." is encoded in UTF-8 as follows:

       --+--------+-----+--
       41 E2 89 A2 CE 91 2E
       --+--------+-----+--

   The character sequence U+D55C U+AD6D U+C5B4 (Korean "hangugeo",
   meaning "the Korean language") is encoded in UTF-8 as follows:

       --------+--------+--------
       ED 95 9C EA B5 AD EC 96 B4
       --------+--------+--------

   The character sequence U+65E5 U+672C U+8A9E (Japanese "nihongo",
   meaning "the Japanese language") is encoded in UTF-8 as follows:

       --------+--------+--------
       E6 97 A5 E6 9C AC E8 AA 9E
       --------+--------+--------



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   The character U+233B4 (a Chinese character meaning 'stump of tree'),
   prepended with a UTF-8 BOM, is encoded in UTF-8 as follows:

       --------+-----------
       EF BB BF F0 A3 8E B4
       --------+-----------

8.  MIME registration

   This memo serves as the basis for registration of the MIME charset
   parameter for UTF-8, according to [RFC2978].  The charset parameter
   value is "UTF-8".  This string labels media types containing text
   consisting of characters from the repertoire of ISO/IEC 10646
   including all amendments at least up to amendment 5 of the 1993
   edition (Korean block), encoded to a sequence of octets using the
   encoding scheme outlined above.  UTF-8 is suitable for use in MIME
   content types under the "text" top-level type.

   It is noteworthy that the label "UTF-8" does not contain a version
   identification, referring generically to ISO/IEC 10646.  This is
   intentional, the rationale being as follows:

   A MIME charset label is designed to give just the information needed
   to interpret a sequence of bytes received on the wire into a sequence
   of characters, nothing more (see [RFC2045], section 2.2).  As long as
   a character set standard does not change incompatibly, version
   numbers serve no purpose, because one gains nothing by learning from
   the tag that newly assigned characters may be received that one
   doesn't know about.  The tag itself doesn't teach anything about the
   new characters, which are going to be received anyway.

   Hence, as long as the standards evolve compatibly, the apparent
   advantage of having labels that identify the versions is only that,
   apparent.  But there is a disadvantage to such version-dependent
   labels: when an older application receives data accompanied by a
   newer, unknown label, it may fail to recognize the label and be
   completely unable to deal with the data, whereas a generic, known
   label would have triggered mostly correct processing of the data,
   which may well not contain any new characters.

   Now the "Korean mess" (ISO/IEC 10646 amendment 5) is an incompatible
   change, in principle contradicting the appropriateness of a version
   independent MIME charset label as described above.  But the
   compatibility problem can only appear with data containing Korean
   Hangul characters encoded according to Unicode 1.1 (or equivalently
   ISO/IEC 10646 before amendment 5), and there is arguably no such data
   to worry about, this being the very reason the incompatible change
   was deemed acceptable.



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   In practice, then, a version-independent label is warranted, provided
   the label is understood to refer to all versions after Amendment 5,
   and provided no incompatible change actually occurs.  Should
   incompatible changes occur in a later version of ISO/IEC 10646, the
   MIME charset label defined here will stay aligned with the previous
   version until and unless the IETF specifically decides otherwise.

9.  IANA Considerations

   The entry for UTF-8 in the IANA charset registry has been updated to
   point to this memo.

10.  Security Considerations

   Implementers of UTF-8 need to consider the security aspects of how
   they handle illegal UTF-8 sequences.  It is conceivable that in some
   circumstances an attacker would be able to exploit an incautious
   UTF-8 parser by sending it an octet sequence that is not permitted by
   the UTF-8 syntax.

   A particularly subtle form of this attack can be carried out against
   a parser which performs security-critical validity checks against the
   UTF-8 encoded form of its input, but interprets certain illegal octet
   sequences as characters.  For example, a parser might prohibit the
   NUL character when encoded as the single-octet sequence 00, but
   erroneously allow the illegal two-octet sequence C0 80 and interpret
   it as a NUL character.  Another example might be a parser which
   prohibits the octet sequence 2F 2E 2E 2F ("/../"), yet permits the
   illegal octet sequence 2F C0 AE 2E 2F.  This last exploit has
   actually been used in a widespread virus attacking Web servers in
   2001; thus, the security threat is very real.

   Another security issue occurs when encoding to UTF-8: the ISO/IEC
   10646 description of UTF-8 allows encoding character numbers up to
   U+7FFFFFFF, yielding sequences of up to 6 bytes.  There is therefore
   a risk of buffer overflow if the range of character numbers is not
   explicitly limited to U+10FFFF or if buffer sizing doesn't take into
   account the possibility of 5- and 6-byte sequences.

   Security may also be impacted by a characteristic of several
   character encodings, including UTF-8: the "same thing" (as far as a
   user can tell) can be represented by several distinct character
   sequences.  For instance, an e with acute accent can be represented
   by the precomposed U+00E9 E ACUTE character or by the canonically
   equivalent sequence U+0065 U+0301 (E + COMBINING ACUTE).  Even though
   UTF-8 provides a single byte sequence for each character sequence,
   the existence of multiple character sequences for "the same thing"
   may have security consequences whenever string matching, indexing,



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   searching, sorting, regular expression matching and selection are
   involved.  An example would be string matching of an identifier
   appearing in a credential and in access control list entries.  This
   issue is amenable to solutions based on Unicode Normalization Forms,
   see [UAX15].

11.  Acknowledgements

   The following have participated in the drafting and discussion of
   this memo: James E. Agenbroad, Harald Alvestrand, Andries Brouwer,
   Mark Davis, Martin J. Duerst, Patrick Faltstrom, Ned Freed, David
   Goldsmith, Tony Hansen, Edwin F. Hart, Paul Hoffman, David Hopwood,
   Simon Josefsson, Kent Karlsson, Dan Kohn, Markus Kuhn, Michael Kung,
   Alain LaBonte, Ira McDonald, Alexey Melnikov, MURATA Makoto, John
   Gardiner Myers, Chris Newman, Dan Oscarsson, Roozbeh Pournader,
   Murray Sargent, Markus Scherer, Keld Simonsen, Arnold Winkler,
   Kenneth Whistler and Misha Wolf.

12.  Changes from RFC 2279

   o  Restricted the range of characters to 0000-10FFFF (the UTF-16
      accessible range).

   o  Made Unicode the source of the normative definition of UTF-8,
      keeping ISO/IEC 10646 as the reference for characters.

   o  Straightened out terminology.  UTF-8 now described in terms of an
      encoding form of the character number.  UCS-2 and UCS-4 almost
      disappeared.

   o  Turned the note warning against decoding of invalid sequences into
      a normative MUST NOT.

   o  Added a new section about the UTF-8 BOM, with advice for
      protocols.

   o  Removed suggested UNICODE-1-1-UTF-8 MIME charset registration.

   o  Added an ABNF syntax for valid UTF-8 octet sequences

   o  Expanded Security Considerations section, in particular impact of
      Unicode normalization









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13.  Normative References

   [RFC2119]   Bradner, S., "Key words for use in RFCs to Indicate
               Requirement Levels", BCP 14, RFC 2119, March 1997.

   [ISO.10646] International Organization for Standardization,
               "Information Technology - Universal Multiple-octet coded
               Character Set (UCS)", ISO/IEC Standard 10646,  comprised
               of ISO/IEC 10646-1:2000, "Information technology --
               Universal Multiple-Octet Coded Character Set (UCS) --
               Part 1: Architecture and Basic Multilingual Plane",
               ISO/IEC 10646-2:2001, "Information technology --
               Universal Multiple-Octet Coded Character Set (UCS) --
               Part 2:  Supplementary Planes" and ISO/IEC 10646-
               1:2000/Amd 1:2002, "Mathematical symbols and other
               characters".

   [UNICODE]   The Unicode Consortium, "The Unicode Standard -- Version
               4.0",  defined by The Unicode Standard, Version 4.0
               (Boston, MA, Addison-Wesley, 2003.  ISBN 0-321-18578-1),
               April 2003, <http://www.unicode.org/unicode/standard/
               versions/enumeratedversions.html#Unicode_4_0_0>.

14.  Informative References

   [CESU-8]    Phipps, T., "Unicode Technical Report #26: Compatibility
               Encoding Scheme for UTF-16: 8-Bit (CESU-8)", UTR 26,
               April 2002,
               <http://www.unicode.org/unicode/reports/tr26/>.

   [FSS_UTF]   X/Open Company Ltd., "X/Open Preliminary Specification --
               File System Safe UCS Transformation Format (FSS-UTF)",
               May 1993, <http://wwwold.dkuug.dk/jtc1/sc22/wg20/docs/
               N193-FSS-UTF.pdf>.

   [RFC2045]   Freed, N. and N. Borenstein, "Multipurpose Internet Mail
               Extensions (MIME) Part One: Format of Internet Message
               Bodies", RFC 2045, November 1996.

   [RFC2234]   Crocker, D. and P. Overell, "Augmented BNF for Syntax
               Specifications: ABNF", RFC 2234, November 1997.

   [RFC2978]   Freed, N. and J. Postel, "IANA Charset Registration
               Procedures", BCP 19, RFC 2978, October 2000.







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   [UAX15]     Davis, M. and M. Duerst, "Unicode Standard Annex #15:
               Unicode Normalization Forms",  An integral part of The
               Unicode Standard, Version 4.0.0, April 2003, <http://
               www.unicode.org/unicode/reports/tr15>.

   [US-ASCII]  American National Standards Institute, "Coded Character
               Set - 7-bit American Standard Code for Information
               Interchange", ANSI X3.4, 1986.

15.  URIs

   [1]  <http://www.unicode.org/unicode/standard/policies.html>

16.  Intellectual Property Statement

   The IETF takes no position regarding the validity or scope of any
   intellectual property or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights
   might or might not be available; neither does it represent that it
   has made any effort to identify any such rights.  Information on the
   IETF's procedures with respect to rights in standards-track and
   standards-related documentation can be found in BCP-11.  Copies of
   claims of rights made available for publication and any assurances of
   licenses to be made available, or the result of an attempt made to
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   proprietary rights by implementors or users of this specification can
   be obtained from the IETF Secretariat.

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights which may cover technology that may be required to practice
   this standard.  Please address the information to the IETF Executive
   Director.

17.  Author's Address

   Francois Yergeau
   Alis Technologies
   100, boul. Alexis-Nihon, bureau 600
   Montreal, QC  H4M 2P2
   Canada

   Phone: +1 514 747 2547
   Fax:   +1 514 747 2561
   EMail: fyergeau@alis.com





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18.  Full Copyright Statement

   Copyright (C) The Internet Society (2003).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
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   Internet organizations, except as needed for the purpose of
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   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than
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   The limited permissions granted above are perpetual and will not be
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   This document and the information contained herein is provided on an
   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.



















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