Content-type header field for Internet messages
RFC 1049

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Network Working Group                                           M. Sirbu
Request for Comments:  1049                                          CMU
                                                              March 1988

           A CONTENT-TYPE HEADER FIELD FOR INTERNET MESSAGES

STATUS OF THIS MEMO

   This RFC suggests proposed additions to the Internet Mail Protocol,
   RFC-822, for the Internet community, and requests discussion and
   suggestions for improvements.  Distribution of this memo is
   unlimited.

ABSTRACT

   A standardized Content-type field allows mail reading systems to
   automatically identify the type of a structured message body and to
   process it for display accordingly.  The structured message body must
   still conform to the RFC-822 requirements concerning allowable
   characters.  A mail reading system need not take any specific action
   upon receiving a message with a valid Content-Type header field.  The
   ability to recognize this field and invoke the appropriate display
   process accordingly will, however, improve the readability of
   messages, and allow the exchange of messages containing mathematical
   symbols, or foreign language characters.

                             Table of Contents

   1. Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 1
   2. Problems with Structured Messages . . . . . . . . . . . . . . . 3
   3. The Content-type Header Field . . . . . . . . . . . . . . . . . 5
        3.1. Type Values  . . . . . . . . . . . . . . . . . . . . . . 5
        3.2. Version Number . . . . . . . . . . . . . . . . . . . . . 6
        3.3. Resource Reference . . . . . . . . . . . . . . . . . . . 6
        3.4. Comment. . . . . . . . . . . . . . . . . . . . . . . . . 7
   4. Conclusion  . . . . . . . . . . . . . . . . . . . . . . . . . . 7

1. Introduction

   As defined in RFC-822, [2], an electronic mail message consists of a
   number of defined header fields, some containing structured
   information (e.g., date, addresses), and a message body consisting of
   an unstructured string of ASCII characters.

   The success of the Internet mail system has led to a desire to use
   the mail system for sending around information with a greater degree
   of structure, while remaining within the constraints imposed by the
   limited character set.  A prime example is the use of mail to send a

Sirbu                                                           [Page 1]
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.