RFC1049

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

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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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  manually by the message recipient who looks at the message text
  displayed as conventional ASCII and recognizes that it is structured
  in some way that requires additional processing to be properly
  rendered.  Our proposal is designed to facilitate automatic
  processing of messages by a mail reading system.

2. Problems with Structured Messages

  Once we introduce the notion that a message body might require some
  processing other than simply painting the characters to the screen we
  raise a number of fundamental questions.  These generally arise due
  to the certainty that some receiving systems will have the facilities
  to process the received message and some will not.  The problem is
  what to do in the presence of systems with different levels of
  capability.

  First, we must recognize that the purpose of structured messages is
  to be able to send types of information, ultimately intended for
  human consumption, not expressable in plain ASCII.  Thus, there is no
  way in plain ASCII to send the italics, boldface, or greek characters
  that can be expressed in Postscript.  If some different processing is
  necessary to render these glyphs, then that is the minimum price to
  be paid in order to send them at all.

  Second, by insisting that the message body contain only ASCII, we
  insure that it will not "break" current mail reading systems which
  are not equipped to process the structure; the result on the screen
  may not be readily interpretable by the human reader, however.

  If a message sender knows that the recipient cannot process
  Postscript, he or she may prefer that the message be revised to
  eliminate the use of italics and boldface, rather than appear
  incomprehensible.  If Postscript is being used because the message
  contains passages in Greek, there may be no suitable ASCII
  equivalent, however.

  Ideally, the details of structuring the message (or not) to conform
  to the capabilities of the recipient system could be completely
  hidden from the message sender.  The distributed Internet mail system
  would somehow determine the capabilities of the recipient system, and
  convert the message automatically; or, if there was no way to send
  Greek text in ASCII, inform the sender that his message could not be
  transmitted.

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  In practice, this is a difficult task.  There are three possible
  approaches:

     1. Each mail system maintains a database of capabilities of
        remote systems it knows how to send to.  Such a database
        would be very difficult to keep up to date.

     2. The mail transport service negotiates with the receiving
        system as to its capabilities.  If the receiving system
        cannot support the specified content type, the mail is
        transformed into conventional ASCII before transmission.
        This would require changes to all existing SMTP
        implementations, and could not be implemented in the case
        where RFC-822 type messages are being forwarded via Bitnet or
        other networks which do not implement SMTP.

     3. An expanded directory service maintains information on mail
        processing capabilities of receiving hosts.  This eliminates
        the need for real-time negotiation with the final
        destination, but still requires direct interaction with the
        directory service.  Since directory querying is part of mail
        sending as opposed to mail composing/reading systems, this
        requires changes to existing mailers as well as a major
        change to the domain name directory service.

  We note in passing that the X.400 protocol implements approach number
  2, and that the Draft Recommendations for X.DS, the Directory
  Service, would support option 3.

  In the interest of facilitating early usage of structured messages,
  we choose not to recommend any of the three approaches described
  above at the present time.  In a forthcoming RFC we will propose a
  solution based on option 2, requiring modification to mailers to
  support negotiation over capabilities.  For the present, then, users
  would be obliged to keep their own private list of capabilities of
  recipients and to take care that they do not send Postscript, TROFF
  or other structured messages to recipients who cannot process them.
  The penalty for failure to do so will be the frustration of the
  recipient in trying to read a raw Postscript or TROFF file painted on
  his or her screen.  Some System Administrators may attempt to
  implement option 1 for the benefit of their users, but this does not
  impose a requirement for changes on any other mail system.

  We recognize that the long-term solution must require changes to
  mailers.  However, in order to begin now to standardize the header
  fields, and to facilitate experimentation, we issue the present RFC.

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3. The Content-type Header Field

  Whatever structuring technique is specified by the Content-type
  field, it must be known precisely to both the sender and the
  recipient of the message in order for the message to be properly
  interpreted.  In general, this means that the allowed parameter
  values for the Content-type:  field must identify a well-defined,
  standardized, document structuring technique.  We do not preclude,
  however, the use of a Content-type:  parameter value to specify a
  private structuring technique known only to the sender and the
  recipient.

  More precisely, we propose that the Content-type:  header field
  consist of up to four parameter values.  The first, or type parameter
  names the structuring technique; the second, optional, parameter is a
  version number, ver-num, which indicates a particular version or
  revision of the standardized structuring technique.  The third
  parameter is a resource reference, resource-ref, which may indicate a
  standard database of information to be used in interpreting the
  structured document.  The last parameter is a comment.

  In the Extended Backus Naur Form of RFC-822, we have:

  Content-Type:= type [";" ver-num [";" 1#resource-ref]] [comment]

3.1. Type Values

  Initially, the type parameter would be limited to the following set
  of values:

  type:=           "POSTSCRIPT"/"SCRIBE"/"SGML"/"TEX"/"TROFF"/
                   "DVI"/"X-"atom

  These values are not case sensitive.  POSTSCRIPT, Postscript, and
  POStscriPT are all equivalent.

  POSTSCRIPT      Indicates the enclosed document consists of
                  information encoded using the Postscript Page
                  Definition Language developed by Adobe Systems,
                  Inc. [1]

  SCRIBE          Indicates the document contains embedded formatting
                  information according to the syntax used by the
                  Scribe document formatting language distributed by
                  the Unilogic Corporation. [6]

  SGML            Indicates the document contains structuring
                  information to according the rules specified for

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                  the Standard Generalized Markup Language, IS 8879,
                  as published by the International Organization for
                  Standardization. [3] Documents structured according
                  to the ISO DIS 8613--Office Docment Architecture and
                  Interchange Format--may also be encoded using SGML
                  syntax.

  TEX             Indicates the document contains embedded formatting
                  information according to the syntax of the TEX
                  document production language. [4]

  TROFF           Indicates the document contains embedded formatting
                  information according to the syntax specified for the
                  TROFF formatting package developed by AT&T Bell
                  Laboratories. [5]

  DVI             Indicates the document contains information according
                  to the device independent file format produced by
                  TROFF or TEX.

  "X-"atom        Any type value beginning with the characters "X-" is
                  a private value.

3.2. Version Number

  Since standard structuring techniques in fact evolve over time, we
  leave room for specifying a version number for the content type.
  Valid values will depend upon the type parameter.

  ver-num:=      local-part

    In particular, we have the following valid values:

    For type=POSTSCRIPT

  ver-num:= "1.0"/"2.0"/"null"

    For type=SCRIBE

  ver-num:= "3"/"4"/"5"/"null"

    For type=SGML

  ver-num:="IS.8879.1986"/"null"

3.3. Resource Reference

  resource-ref:=  local-part

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  As Apple has demonstrated with their implementation of the
  Laserwriter, a very general document structuring technique can be
  made more efficient by defining a set of macros or other similar
  resources to be used in interpreting any transmitted stream.  The
  Macintosh transmits a LaserPrep file to the Laserwriter containing
  font and macro definitions which can be called upon by subsequent
  documents.  The result is that documents as sent to the Laserwriter
  are considerably more compact than if they had to include the
  LaserPrep file each time.  The Resource Reference parameter allows
  specification of a well known resource, such as a LaserPrep file,
  which should be used by the receiving system when processing the
  message.

  Resource references could also include macro packages for use with
  TEX or references to preprocessors such as eqn and tbl for use with
  troff.  Allowed values will vary according to the type parameter.

    In particular, we propose the following values:

    For type = POSTSCRIPT

  resource-ref:=  "laserprep2.9"/"laserprep3.0"/"laserprep3.1"/
                  "laserprep4.0"/local-part

    For type = TROFF

  resource-ref:=  "eqn"/"tbl"/"me"/local-part

3.4. Comment

  The comment field can be any additional comment text the user
  desires.  Comments are enclosed in parentheses as specified in
  RFC-822.

4. Conclusion

  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 strcutured 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 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.

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RFC 1049 Mail Content Type March 1988

  In the near term, the major use of a Content-Type:  header field is
  likely to be for designating the message body as containing a Page
  Definition Language representation such as Postscript.

  Additional type values shall be registered with Internet Assigned
  Numbers Coordinator at USC-ISI.  Please contact:

                  Joyce K. Reynolds
                  USC Information Sciences Institute
                  4676 Admiralty Way
                  Marina del Rey, CA  90292-6695

                  213-822-1511    [email protected]

                               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.

  3.  ISO TC97/SC18.  Standard Generalized Markup Language.
      Tech. Rept. DIS 8879, ISO, 1986.

  4.  Knuth, Donald E.  The TEXbook.  Addison-Wesley, Reading, Mass.,
      1984.

  5.  Ossanna, Joseph F. NROFF/TROFF User's Manual.  Bell
      Laboratories, Murray Hill, New Jersey, 1976.  Computing Science
      Technical Report No.54.

  6.  Unilogic.  SCRIBE Document Production Software.  Unilogic, 1985.
      Fourth Edition.

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