RFC 1049 (rfc1049) - Page 2 of 8
Content-type header field for Internet messages
Alternative Format: Original Text Document
RFC 1049 Mail Content Type March 1988
document with embedded TROFF formatting commands. A more
sophisticated example would be a message body encoded in a Page
Description Language (PDL) such as Postscript. In both cases, simply
mapping the ASCII characters to the screen or printer in the usual
fashion will not render the document image intended by the sender; an
additional processing step is required to produce an image of the
message text on a display device or a piece of paper.
In both of these examples, the message body contains only the legal
character set, but the content has a structure which produces some
desirable result after appropriate processing by the recipient. If a
message header field could be used to indicate the structuring
technique used in the message body, then a sophisticated mail system
could use such a field to automatically invoke the appropriate
processing of the message body. For example, a header field which
indicated that the message body was encoded using Postscript could be
used to direct a mail system running under Sun Microsystem's NEWS
window manager to process the Postscript to produce the appropriate
page image on the screen.
Private header fields (beginning with "X-") are already being used by
some systems to affect such a result (e.g., the Andrew Message System
developed at Carnegie Mellon University). However, the widespread
use of such techniques will require general agreement on the name and
allowed parameter values for a header field to be used for this
purpose.
We propose that a new header field, "Content-type:" be recognized as
the standard field for indicating the structure of the message body.
The contents of the "Content-Type:" field are parameters which
specify what type of structure is used in the message body.
Note that we are not proposing that the message body contain anything
other than ASCII characters as specified in RFC-822. Whatever
structuring is contained in the message body must be represented
using only the allowed ASCII characters. Thus, this proposal should
have no impact on existing mailers, only on mail reading systems.
At the same time, this restriction eliminates the use of more general
structuring techniques such as Abstract Syntax Notation, (CCITT
Recommendation X.409) as used in the X.400 messaging standard, which
are octet-oriented.
This is not the first proposal for structuring message bodies.
RFC-767 discusses a proposed technique for structuring multi-media
mail messages. We are also aware that many users already employ mail
to send TROFF, SCRIBE, TEX, Postscript or other structured
information. Such postprocessing as is required must be invoked
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