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

           A CONTENT-TYPE HEADER FIELD FOR INTERNET MESSAGES

STATUS OF THIS MEMO(略)

ABSTRACT(略)

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
   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.

(以下，略)

                                REFERENCES

   1.  Adobe Systems, Inc.  Postscript Language Reference Manual.
       Addison-Wesley, Reading, Mass., 1985.

   2.  Crocker, David H.  RFC-822:  Standard for the Format of ARPA
       Internet Text Messages.  Network Information Center,
       August 13, 1982.