RFC 1521                          MIME                    September 1993


   future, it is strongly recommended that new user agents explicitly
   specify a character set via the Content-Type header field.  "US-
   ASCII" does not indicate an arbitrary seven-bit character code, but
   specifies that the body uses character coding that uses the exact
   correspondence of codes to characters specified in ASCII.  National
   use variations of ISO 646 [ISO-646] are NOT ASCII and their use in
   Internet mail is explicitly discouraged. The omission of the ISO 646
   character set is deliberate in this regard.  The character set name
   of "US-ASCII" explicitly refers to ANSI X3.4-1986 [US-ASCII] only.
   The character set name "ASCII" is reserved and must not be used for
   any purpose.

      NOTE: RFC 821 explicitly specifies "ASCII", and references an
      earlier version of the American Standard.  Insofar as one of the
      purposes of specifying a Content-Type and character set is to
      permit the receiver to unambiguously determine how the sender
      intended the coded message to be interpreted, assuming anything
      other than "strict ASCII" as the default would risk unintentional
      and incompatible changes to the semantics of messages now being
      transmitted.  This also implies that messages containing
      characters coded according to national variations on ISO 646, or
      using code-switching procedures (e.g., those of ISO 2022), as well
      as 8-bit or multiple octet character encodings MUST use an
      appropriate character set specification to be consistent with this
      specification.

   The complete US-ASCII character set is listed in [US-ASCII].  Note
   that the control characters including DEL (0-31, 127) have no defined
   meaning apart from the combination CRLF (ASCII values 13 and 10)
   indicating a new line.  Two of the characters have de facto meanings
   in wide use: FF (12) often means "start subsequent text on the
   beginning of a new page"; and TAB or HT (9) often (though not always)
   means "move the cursor to the next available column after the current
   position where the column number is a multiple of 8 (counting the
   first column as column 0)." Apart from this, any use of the control
   characters or DEL in a body must be part of a private agreement
   between the sender and recipient.  Such private agreements are
   discouraged and should be replaced by the other capabilities of this
   document.

      NOTE: Beyond US-ASCII, an enormous proliferation of character sets
      is possible. It is the opinion of the IETF working group that a
      large number of character sets is NOT a good thing.  We would
      prefer to specify a single character set that can be used
      universally for representing all of the world's languages in
      electronic mail.  Unfortunately, existing practice in several
      communities seems to point to the continued use of multiple
      character sets in the near future.  For this reason, we define




 
RFC 1521                          MIME                    September 1993


      names for a small number of character sets for which a strong
      constituent base exists.

   The defined charset values are:

   US-ASCII -- as defined in [US-ASCII].

        ISO-8859-X -- where "X" is to be replaced, as necessary, for the
             parts of ISO-8859 [ISO-8859].  Note that the ISO 646
             character sets have deliberately been omitted in favor of
             their 8859 replacements, which are the designated character
             sets for Internet mail.  As of the publication of this
             document, the legitimate values for "X" are the digits 1
             through 9.

   The character sets specified above are the ones that were relatively
   uncontroversial during the drafting of MIME.  This document does not
   endorse the use of any particular character set other than US-ASCII,
   and recognizes that the future evolution of world character sets
   remains unclear.  It is expected that in the future, additional
   character sets will be registered for use in MIME.

   Note that the character set used, if anything other than US-ASCII,
   must always be explicitly specified in the Content-Type field.

   No other character set name may be used in Internet mail without the
   publication of a formal specification and its registration with IANA,
   or by private agreement, in which case the character set name must
   begin with "X-".

   Implementors are discouraged from defining new character sets for
   mail use unless absolutely necessary.

   The "charset" parameter has been defined primarily for the purpose of
   textual data, and is described in this section for that reason.
   However, it is conceivable that non-textual data might also wish to
   specify a charset value for some purpose, in which case the same
   syntax and values should be used.