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Versions: 00 rfc2279                                     Standards Track
Network Working Group                                       F. Yergeau
Internet Draft                                       Alis Technologies
<draft-yergeau-utf8-rev-00.txt>                          14 April 1997
Expires 14 October 1997

[Will obsolete RFC 2044]

        UTF-8, a transformation format of Unicode and ISO 10646

Status of this Memo

   This document is an Internet-Draft.  Internet-Drafts are working doc-
   uments of the Internet Engineering Task Force (IETF), its areas, and
   its working groups. Note that other groups may also distribute work-
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   Distribution of this document is unlimited.


   ISO/IEC 10646-1 and the Unicode Standard jointly define a multi-octet
   character set which encompasses most of the world's writing systems.
   Multi-octet characters, however, are not compatible with many current
   applications and protocols, and this has led to the development of a
   few so-called UCS transformation formats (UTF), each with different
   characteristics.  UTF-8, the object of this memo, has the character-
   istic 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 updates and
   replaces RFC 2044, in particular addressing the question of versions
   of the relevant standards.

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1.  Introduction

   ISO/IEC 10646-1 [ISO-10646] and the Unicode Standard [UNICODE]
   jointly define a 16-bit character set, UCS-2, which encompasses most
   of the world's writing systems.  ISO 10646 further defines a 31-bit
   character set, UCS-4, with currently no assignments outside of the
   region corresponding to UCS-2 (the Basic Multilingual Plane, BMP).
   The UCS-2 and UCS-4 encodings, however, are hard to use in many cur-
   rent applications and protocols that assume 8 or even 7 bit charac-
   ters.  Even newer systems able to deal with 16 bit characters cannot
   process UCS-4 data. This situation has led to the development of so-
   called UCS transformation formats (UTF), each with different charac-

   UTF-1 has only historical interest, having been removed from ISO
   10646.  UTF-7 has the quality of encoding the full Unicode repertoire
   using only octets with the high-order bit clear (7 bit US-ASCII val-
   ues, [US-ASCII]), and is thus deemed a mail-safe encoding
   ([RFC1642]).  UTF-8, the object of this memo, uses all bits of an
   octet, but has the quality of preserving the full US-ASCII range: US-
   ASCII characters are encoded in one octet having the normal US-ASCII
   value, and any octet with such a value can only stand for an US-ASCII
   character, and nothing else.

   UTF-16 is a scheme for transforming a subset of the UCS-4 repertoire
   into pairs of UCS-2 values from a reserved range.  UTF-16 impacts
   UTF-8 in that UCS-2 values from the reserved range must be treated
   specially in the UTF-8 transformation.

   UTF-8 encodes UCS-2 or UCS-4 characters as a varying number of
   octets, where the number of octets, and the value of each, depend on
   the integer value assigned to the character in ISO 10646.  This
   transformation format has the following characteristics (all values
   are in hexadecimal):

   -  Character values from 0000 0000 to 0000 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

   -  US-ASCII values do not appear otherwise in a UTF-8 encoded charac-
      ter 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 val-

   -  Round-trip conversion is easy between UTF-8 and either of UCS-4,
      UCS-2 or Unicode.

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

   -  The octet values FE and FF never appear.

   -  Character boundaries are easily found from anywhere in an octet

   -  The lexicographic sorting order of UCS-4 strings is preserved.  Of
      course this is of limited interest since the sort order is not
      culturally valid in either case.

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

   -  UTF-8 strings can be fairly reliably recognized as such by a sim-
      ple 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 originally a project of the X/Open Joint Internationaliza-
   tion Group XOJIG with the objective to specify a File System Safe UCS
   Transformation Format [FSS-UTF] that is compatible with UNIX systems,
   supporting multilingual text in a single encoding.  The original
   authors were Gary Miller, Greger Leijonhufvud and John Entenmann.
   Later, Ken Thompson and Rob Pike did significant work for the formal

   A description can also be found in Unicode Technical Report #4 and in
   the Unicode Standard, version 2.0 [UNICODE].  The definitive refer-
   ence, including provisions for UTF-16 data within UTF-8, is Annex R
   of ISO/IEC 10646-1 [ISO-10646].

2.  UTF-8 definition

   In UTF-8, characters are encoded using sequences of 1 to 6 octets.
   The only octet of a "sequence" of one has the higher-order bit set to
   0, the remaining 7 bits being used to encode the character value. In
   a sequence of n octets, n>1, the initial octet has the n higher-order
   bits set to 1, followed by a bit set to 0.  The remaining bit(s) of
   that octet contain bits from the value 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 UCS-4
   character value.

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   UCS-4 range (hex.)           UTF-8 octet sequence (binary)
   0000 0000-0000 007F   0xxxxxxx
   0000 0080-0000 07FF   110xxxxx 10xxxxxx
   0000 0800-0000 FFFF   1110xxxx 10xxxxxx 10xxxxxx

   0001 0000-001F FFFF   11110xxx 10xxxxxx 10xxxxxx 10xxxxxx
   0020 0000-03FF FFFF   111110xx 10xxxxxx 10xxxxxx 10xxxxxx 10xxxxxx
   0400 0000-7FFF FFFF   1111110x 10xxxxxx ... 10xxxxxx

   Encoding from UCS-4 to UTF-8 proceeds as follows:

   1) Determine the number of octets required from the character value
      and the first column of the table above.

   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 value,
      starting from the lower-order bits of the character value and
      putting them first in the last octet of the sequence, then the
      next to last, etc. until all x bits are filled in.

      The algorithm for encoding UCS-2 (or Unicode) to UTF-8 can be
      obtained from the above, in principle, by simply extending each
      UCS-2 character with two zero-valued octets.  However, UCS-2 val-
      ues between D800 and DFFF, being actually UCS-4 characters trans-
      formed through UTF-16, need special treatment: the UTF-16 trans-
      formation must be undone, yielding a UCS-4 character that is then
      transformed as above.

      Decoding from UTF-8 to UCS-4 proceeds as follows:

   1) Initialize the 4 octets of the UCS-4 character with all bits set
      to 0.

   2) Determine which bits encode the character value from the number of
      octets in the sequence and the second column of the table above
      (the bits marked x).

   3) Distribute the bits from the sequence to the UCS-4 character,
      first the lower-order bits from the last octet of the sequence and
      proceeding to the left until no x bits are left.

      If the UTF-8 sequence is no more than three octets long, decoding
      can proceed directly to UCS-2 (or equivalently Unicode).

      A more detailed algorithm and formulae can be found in [FSS_UTF],

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      [UNICODE] or Annex R to [ISO-10646].

3.  Versions of the standards

   Different versions of the Unicode standard exist: 1.0, 1.1 and 2.0 as
   of this writing.  Each new version obsoletes and replaces the previ-
   ous one, but implementations, and more significantly data, are not
   updated instantly.  Similarly, ISO 10646 is updated from time to time
   by published amendments, which up to now have tracked the changes in
   the Unicode standard, so that the two have remained in sync.

   In general, the changes amount to adding new characters, which does
   not pose particular problems with old data.  Amendment 5 to ISO
   10646, however, has 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 Uni-
   code 1.1. The official justification for allowing such an incompati-
   ble change was that no implementations and no data containing Hangul
   existed, a statement that is likely to be true but remains unprov-
   able.  The incident has been dubbed the "Korean mess", and the rele-
   vant committees have pledged to never, ever again make such an incom-
   patible change.

   New versions, and in particular any incompatible changes, have conse-
   quences regarding MIME character encoding labels, to be discussed in
   section 5.

4.  Examples

   The UCS-2 sequence "A<NOT IDENTICAL TO><ALPHA>." (0041, 2262, 0391,
   002E) may be encoded as follows:

   41 E2 89 A2 CE 91 2E

   The UCS-2 sequence representing the Hangul characters for the Korean
   word "hangugo" (D55C, AD6D, C5B4) may be encoded as follows:

   ED 95 9C EA B5 AD EC 96 B4

   The UCS-2 sequence representing the Han characters for the Japanese
   word "nihongo" (65E5, 672C, 8A9E) may be encoded as follows:

   E6 97 A5 E6 9C AC E8 AA 9E

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5.  MIME registration

   This memo is meant to serve as the basis for registration of a MIME
   character set parameter (charset) [MIME].  The proposed charset
   parameter value is "UTF-8".  This string would label media types con-
   taining text consisting of characters from the repertoire of ISO/IEC
   10646 encoded to a sequence of octets using the encoding scheme out-
   lined 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 RFC 2045, section 2.2, in [MIME]).
   As long as a character set standard does not change incompatibly,
   version numbers serve no purpose, because one gains nothing by learn-
   ing from the tag that newly assigned characters may be received that
   one doesn't know about.  The tag doesn't teach anything about the new
   characters, and they 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 com-
   pletely 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 10646 amendment 5) is an incompatible
   change, in principle contradicting the appropriateness of a version-
   independent MIME charset label as described above.  But the compati-
   bility problem can only appear with data containing Korean Hangul
   characters encoded according to Unicode 1.1 (or equivalently ISO
   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.

   In practice, then, a version-independent label is warranted.  Should
   the need ever arise to distinguish data containing Hangul encoded
   according to Unicode 1.1, then a version-dependent label, for that
   version only, should be registered (a suggestion would be "UNI-
   CODE-1-1-UTF-8"), in order to retain the advantages of a version-
   independent label for 2.0 and later versions.  Such a version-depen-
   dent label could even be registered before actual need arises, pre-

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   emptively, but it is important to strongly recommend against creating
   any new Hangul-containing data without taking Amendment 5 of ISO
   10646 into account.

6.  Security Considerations

   Security issues are not discussed in this memo.


   The following have participated in the drafting and discussion of
   this memo:

   James E. Agenbroad   Andries Brouwer
   Martin J. Dürst      David Goldsmith
   Edwin F. Hart        Kent Karlsson
   Markus Kuhn          Michael Kung
   Alain LaBonté        Murray Sargent
   Keld Simonsen        Arnold Winkler


   [FSS_UTF]      X/Open CAE Specification C501 ISBN 1-85912-082-2 28cm.
                  22p. pbk. 172g.  4/95, X/Open Company Ltd., "File Sys-
                  tem Safe UCS Transformation Format (FSS_UTF)", X/Open
                  Preleminary Specification, Document Number P316.  Also
                  published in Unicode Technical Report #4.

   [ISO-10646]    ISO/IEC 10646-1:1993. International Standard -- Infor-
                  mation technology -- Universal Multiple-Octet Coded
                  Character Set (UCS) -- Part 1: Architecture and Basic
                  Multilingual Plane.  UTF-8 is described in Annex R,
                  published as Amendment 2.  UTF-16 is described in
                  Annex Q, published as Amendment 1.

   [MIME]         N. Freed, N. Borenstein, "Multipurpose Internet Mail
                  Extensions (MIME) Part One:  Format of Internet Mes-
                  sage Bodies", RFC 2045.  N. Freed, N. Borenstein,
                  "Multipurpose Internet Mail Extensions (MIME) Part
                  Two:  Media Types", RFC 2046.  K. Moore, "MIME (Multi-
                  purpose Internet Mail Extensions) Part Three: Message
                  Header Extensions for Non-ASCII Text", RFC 2047.  N.
                  Freed, J. Klensin, J. Postel, "Multipurpose Internet
                  Mail Extensions (MIME) Part Four: Registration Proce-
                  dures", RFC 2048.  N. Freed, N. Borenstein, "Multipur-
                  pose Internet Mail Extensions (MIME) Part Five:  Con-
                  formance Criteria and Examples", RFC 2049.  All

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                  November 1996.

   [RFC1641]      D. Goldsmith, M.Davis, "Using Unicode with MIME", RFC
                  1641, Taligent inc., July 1994.

   [RFC1642]      D. Goldsmith, M. Davis, "UTF-7: A Mail-safe Transfor-
                  mation Format of Unicode", RFC 1642, Taligent inc.,
                  July 1994.

   [UNICODE]      The Unicode Consortium, "The Unicode Standard -- Ver-
                  sion 2.0", Addison-Wesley, 1996.

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

Author's Address

   François Yergeau
   Alis Technologies
   100, boul. Alexis-Nihon
   Suite 600
   Montréal  QC  H4M 2P2

   Tel: +1 (514) 747-2547
   Fax: +1 (514) 747-2561
   EMail: fyergeau@alis.com

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