Some of the modified BNF in this document makes reference to
   syntactic entities that are defined in RFC 822 and not in this
   document.  A complete formal grammar, then, is obtained by combining
   the collected grammar appendix of this document with that of RFC 822
   plus the modifications to RFC 822 defined in RFC 1123, which
   specifically changes the syntax for `return', `date' and `mailbox'.

   The term CRLF, in this document, refers to the sequence of the two
   ASCII characters CR (13) and LF (10) which, taken together, in this
   order, denote a line break in RFC 822 mail.

   The term "character set" is used in this document to refer to a
   method used with one or more tables to convert encoded text to a
   series of octets.  This definition is intended to allow various kinds
   of text encodings, from simple single-table mappings such as ASCII to
   complex table switching methods such as those that use ISO 2022's
   techniques.  However, a MIME character set name must fully specify
   the mapping to be performed.

   The term "message", when not further qualified, means either the
   (complete or "top-level") message being transferred on a network, or
   a message encapsulated in a body of type "message".

   The term "body part", in this document, means one of the parts of the
   body of a multipart entity. A body part has a header and a body, so
   it makes sense to speak about the body of a body part.

   The term "entity", in this document, means either a message or a body
   part.  All kinds of entities share the property that they have a
   header and a body.

   The term "body", when not further qualified, means the body of an
   entity, that is the body of either a message or of a body part.




 
RFC 1521                          MIME                    September 1993


      NOTE: The previous four definitions are clearly circular.  This is
      unavoidable, since the overall structure of a MIME message is
      indeed recursive.

   In this document, all numeric and octet values are given in decimal
   notation.

   It must be noted that Content-Type values, subtypes, and parameter
   names as defined in this document are case-insensitive.  However,
   parameter values are case-sensitive unless otherwise specified for
   the specific parameter.

      FORMATTING NOTE: This document has been carefully formatted for
      ease of reading.  The PostScript version of this document, in
      particular, places notes like this one, which may be skipped by
      the reader, in a smaller, italicized, font, and indents it as
      well.  In the text version, only the indentation is preserved, so
      if you are reading the text version of this you might consider
      using the PostScript version instead. However, all such notes will
      be indented and preceded by "NOTE:" or some similar introduction,
      even in the text version.

      The primary purpose of these non-essential notes is to convey
      information about the rationale of this document, or to place this
      document in the proper historical or evolutionary context.  Such
      information may be skipped by those who are focused entirely on
      building a conformant implementation, but may be of use to those
      who wish to understand why this document is written as it is.

      For ease of recognition, all BNF definitions have been placed in a
      fixed-width font in the PostScript version of this document.

3.    The MIME-Version Header Field

   Since RFC 822 was published in 1982, there has really been only one
   format standard for Internet messages, and there has been little
   perceived need to declare the format standard in use.  This document
   is an independent document that complements RFC 822. Although the
   extensions in this document have been defined in such a way as to be
   compatible with RFC 822, there are still circumstances in which it
   might be desirable for a mail-processing agent to know whether a
   message was composed with the new standard in mind.

   Therefore, this document defines a new header field, "MIME-Version",
   which is to be used to declare the version of the Internet message
   body format standard in use.

   Messages composed in accordance with this document MUST include such




 
RFC 1521                          MIME                    September 1993


   a header field, with the following verbatim text: