RFC 2049                    MIME Conformance               November 1996


    (2)   Conversion to canonical form.

          The entire body, including "out-of-band" information
          such as record lengths and possibly file attribute
          information, is converted to a universal canonical
          form.  The specific media type of the body as well as
          its associated attributes dictate the nature of the
          canonical form that is used.  Conversion to the proper
          canonical form may involve character set conversion,
          transformation of audio data, compression, or various
          other operations specific to the various media types.
          If character set conversion is involved, however, care
          must be taken to understand the semantics of the media
          type, which may have strong implications for any
          character set conversion, e.g. with regard to
          syntactically meaningful characters in a text subtype
          other than "plain".

          For example, in the case of text/plain data, the text
          must be converted to a supported character set and
          lines must be delimited with CRLF delimiters in
          accordance with RFC 822.  Note that the restriction on
          line lengths implied by RFC 822 is eliminated if the
          next step employs either quoted-printable or base64
          encoding.

    (3)   Apply transfer encoding.

          A Content-Transfer-Encoding appropriate for this body
          is applied.  Note that there is no fixed relationship
          between the media type and the transfer encoding.  In
          particular, it may be appropriate to base the choice of
          base64 or quoted-printable on character frequency
          counts which are specific to a given instance of a
          body.

    (4)   Insertion into entity.

          The encoded body is inserted into a MIME entity with
          appropriate headers. The entity is then inserted into
          the body of a higher-level entity (message or
          multipart) as needed.

   Conversion from entity form to local form is accomplished by
   reversing these steps. Note that reversal of these steps may produce
   differing results since there is no guarantee that the original and
   final local forms are the same.





 
RFC 2049                    MIME Conformance               November 1996


   It is vital to note that these steps are only a model; they are
   specifically NOT a blueprint for how an actual system would be built.
   In particular, the model fails to account for two common designs:

    (1)   In many cases the conversion to a canonical form prior
          to encoding will be subsumed into the encoder itself,
          which understands local formats directly.  For example,
          the local newline convention for text bodies might be
          carried through to the encoder itself along with
          knowledge of what that format is.

    (2)   The output of the encoders may have to pass through one
          or more additional steps prior to being transmitted as
          a message.  As such, the output of the encoder may not
          be conformant with the formats specified by RFC 822.
          In particular, once again it may be appropriate for the
          converter's output to be expressed using local newline
          conventions rather than using the standard RFC 822 CRLF
          delimiters.

   Other implementation variations are conceivable as well.  The vital
   aspect of this discussion is that, in spite of any optimizations,
   collapsings of required steps, or insertion of additional processing,
   the resulting messages must be consistent with those produced by the
   model described here.  For example, a message with the following
   header fields:

     Content-type: text/foo; charset=bar
     Content-Transfer-Encoding: base64

   must be first represented in the text/foo form, then (if necessary)
   represented in the "bar" character set, and finally transformed via
   the base64 algorithm into a mail-safe form.