It should be noted that most media types are defined in terms of
   octets rather than bits, so that the mechanisms described here are
   mechanisms for encoding arbitrary octet streams, not bit streams.  If
   a bit stream is to be encoded via one of these mechanisms, it must
   first be converted to an 8bit byte stream using the network standard
   bit order ("big-endian"), in which the earlier bits in a stream
   become the higher-order bits in a 8bit byte.  A bit stream not ending
   at an 8bit boundary must be padded with zeroes. RFC 2046 provides a
   mechanism for noting the addition of such padding in the case of the
   application/octet-stream media type, which has a "padding" parameter.

   The encoding mechanisms defined here explicitly encode all data in
   US-ASCII.  Thus, for example, suppose an entity has header fields
   such as:

     Content-Type: text/plain; charset=ISO-8859-1
     Content-transfer-encoding: base64

   This must be interpreted to mean that the body is a base64 US-ASCII
   encoding of data that was originally in ISO-8859-1, and will be in
   that character set again after decoding.

   Certain Content-Transfer-Encoding values may only be used on certain
   media types.  In particular, it is EXPRESSLY FORBIDDEN to use any
   encodings other than "7bit", "8bit", or "binary" with any composite
   media type, i.e. one that recursively includes other Content-Type
   fields.  Currently the only composite media types are "multipart" and
   "message".  All encodings that are desired for bodies of type
   multipart or message must be done at the innermost level, by encoding
   the actual body that needs to be encoded.

   It should also be noted that, by definition, if a composite entity
   has a transfer-encoding value such as "7bit", but one of the enclosed
   entities has a less restrictive value such as "8bit", then either the
   outer "7bit" labelling is in error, because 8bit data are included,
   or the inner "8bit" labelling placed an unnecessarily high demand on
   the transport system because the actual included data were actually
   7bit-safe.

   NOTE ON ENCODING RESTRICTIONS:  Though the prohibition against using
   content-transfer-encodings on composite body data may seem overly
   restrictive, it is necessary to prevent nested encodings, in which
   data are passed through an encoding algorithm multiple times, and
   must be decoded multiple times in order to be properly viewed.
   Nested encodings add considerable complexity to user agents:  Aside
   from the obvious efficiency problems with such multiple encodings,
   they can obscure the basic structure of a message.  In particular,
   they can imply that several decoding operations are necessary simply




 
RFC 2045                Internet Message Bodies            November 1996


   to find out what types of bodies a message contains.  Banning nested
   encodings may complicate the job of certain mail gateways, but this
   seems less of a problem than the effect of nested encodings on user
   agents.

   Any entity with an unrecognized Content-Transfer-Encoding must be
   treated as if it has a Content-Type of "application/octet-stream",
   regardless of what the Content-Type header field actually says.

   NOTE ON THE RELATIONSHIP BETWEEN CONTENT-TYPE AND CONTENT-TRANSFER-
   ENCODING: It may seem that the Content-Transfer-Encoding could be
   inferred from the characteristics of the media that is to be encoded,
   or, at the very least, that certain Content-Transfer-Encodings could
   be mandated for use with specific media types.  There are several
   reasons why this is not the case. First, given the varying types of
   transports used for mail, some encodings may be appropriate for some
   combinations of media types and transports but not for others.  (For
   example, in an 8bit transport, no encoding would be required for text
   in certain character sets, while such encodings are clearly required
   for 7bit SMTP.)

   Second, certain media types may require different types of transfer
   encoding under different circumstances.  For example, many PostScript
   bodies might consist entirely of short lines of 7bit data and hence
   require no encoding at all.  Other PostScript bodies (especially
   those using Level 2 PostScript's binary encoding mechanism) may only
   be reasonably represented using a binary transport encoding.
   Finally, since the Content-Type field is intended to be an open-ended
   specification mechanism, strict specification of an association
   between media types and encodings effectively couples the
   specification of an application protocol with a specific lower-level
   transport.  This is not desirable since the developers of a media
   type should not have to be aware of all the transports in use and
   what their limitations are.

6.5.  Translating Encodings

   The quoted-printable and base64 encodings are designed so that
   conversion between them is possible.  The only issue that arises in
   such a conversion is the handling of hard line breaks in quoted-
   printable encoding output. When converting from quoted-printable to
   base64 a hard line break in the quoted-printable form represents a