RFC 2044 (rfc2044) - Page 2 of 6

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

RFC 2044                         UTF-8                      October 1996


   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 a pair 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).

   -  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-
      ues.

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

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

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

   -  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 octet values FE and FF never appear.

   UTF-8 was originally a project of the X/Open Joint
   Internationalization 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 UTF-8.





Yergeau                      Informational