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rfc:std:std63

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

Yergeau Standards Track [Page 1] RFC 3629 UTF-8 November 2003

 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.

Yergeau Standards Track [Page 2] RFC 3629 UTF-8 November 2003

 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.

Yergeau Standards Track [Page 3] RFC 3629 UTF-8 November 2003

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.

Yergeau Standards Track [Page 4] RFC 3629 UTF-8 November 2003

 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

Yergeau Standards Track [Page 5] RFC 3629 UTF-8 November 2003

 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

Yergeau Standards Track [Page 6] RFC 3629 UTF-8 November 2003

 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

Yergeau Standards Track [Page 7] RFC 3629 UTF-8 November 2003

    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:
  1. -+——–+—–+–

41 E2 89 A2 CE 91 2E

  1. -+——–+—–+–
 The character sequence U+D55C U+AD6D U+C5B4 (Korean "hangugeo",
 meaning "the Korean language") is encoded in UTF-8 as follows:
  1. ——-+——–+——–

ED 95 9C EA B5 AD EC 96 B4

  1. ——-+——–+——–
 The character sequence U+65E5 U+672C U+8A9E (Japanese "nihongo",
 meaning "the Japanese language") is encoded in UTF-8 as follows:
  1. ——-+——–+——–

E6 97 A5 E6 9C AC E8 AA 9E

  1. ——-+——–+——–

Yergeau Standards Track [Page 8] RFC 3629 UTF-8 November 2003

 The character U+233B4 (a Chinese character meaning 'stump of tree'),
 prepended with a UTF-8 BOM, is encoded in UTF-8 as follows:
  1. ——-+———–

EF BB BF F0 A3 8E B4

  1. ——-+———–

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.

Yergeau Standards Track [Page 9] RFC 3629 UTF-8 November 2003

 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,

Yergeau Standards Track [Page 10] RFC 3629 UTF-8 November 2003

 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

Yergeau Standards Track [Page 11] RFC 3629 UTF-8 November 2003

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.

Yergeau Standards Track [Page 12] RFC 3629 UTF-8 November 2003

 [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
 obtain a general license or permission for the use of such
 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

Yergeau Standards Track [Page 13] RFC 3629 UTF-8 November 2003

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
 or assist in its implementation may be prepared, copied, published
 and distributed, in whole or in part, without restriction of any
 kind, provided that the above copyright notice and this paragraph are
 included on all such copies and derivative works.  However, this
 document itself may not be modified in any way, such as by removing
 the copyright notice or references to the Internet Society or other
 Internet organizations, except as needed for the purpose of
 developing Internet standards in which case the procedures for
 copyrights defined in the Internet Standards process must be
 followed, or as required to translate it into languages other than
 English.
 The limited permissions granted above are perpetual and will not be
 revoked by the Internet Society or its successors or assignees.
 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.

Yergeau Standards Track [Page 14]

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