Format for a graphical communication protocol
RFC 965

Document Type RFC - Unknown (December 1985; No errata)
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Network Working Group                                    Lorenzo Aguilar
Request for Comments: 965                              SRI International
                                                           December 1985

            A Format for a Graphical Communication Protocol

STATUS OF THIS MEMO

   This paper describes the requirements for a graphical format on which
   to base a graphical on-line communication protocol.  The proposal is
   an Interactive Graphical Communication Format using the GKSM session
   metafile.  Distribution of this memo is unlimited.

ABSTRACT

   This paper describes the requirements for a graphical format on which
   to base a graphical on-line communication protocol. It is argued that
   on-line graphical communication is similar to graphical session
   capture, and thus we propose an Interactive Graphical Communication
   Format using the GKSM session metafile.

   We discuss the items that we believe complement the GKSM metafile as
   a format for on-line interactive exchanges. One key application area
   of such a format is multi-media on-line conferencing; therefore, we
   present a conferencing software architecture for processing the
   proposed format. We make this format specification available to those
   planning multi-media conferencing systems as a contribution toward
   the development of a graphical communication protocol that will
   permit the interoperation of these systems.

   We hope this contribution will encourage the discussion of multimedia
   data exchange and the proposal of solutions. At SRI, we stay open to
   the exploration of alternatives and we will continue our research and
   development work in this problem area.

ACKNOWLEDGEMENTS

   The author wants to thank Andy Poggio of SRI who made many insightful
   and valuable suggestions that trimmed and improved level U. His
   expertise in multi-media communication systems and his encouragement
   were a most positive input to the creation of this IGCF. Dave
   Worthington of SRI also participated in the project discussions
   involving this IGCF. Thanks are also due to Tom Powers, chairman of
   ANSI X3H33, who opened this forum to the presentation of an earlier
   version of this paper, thereby providing an opportunity for the
   invaluable feedback of the X3H33 members. Jon Postel of ISI
   recommended a number of changes that made this paper more coherent
   and accessible.

Aguilar                                                         [Page 1]



RFC 965                                                    December 1985
A Format for a Graphical Communication Protocol

   Most of the work reported in this paper was sponsored by the U.S.
   Navy, Naval Electronic Systems Command, Washington D.C., under
   Contract No. N00039-83-K-0623.

I.  INTRODUCTION

   A. Use of a Graphical Communication Protocol

      In the field of computer communications, a protocol is a procedure
      executed by two cooperating processes in order to attain a
      meaningful exchange of information. A graphical communication
      protocol is needed to exchange interactive vector graphics
      information, possibly in conjunction with other information media
      like voice, text, and video. Within this multi-media communication
      environment, computer vector graphics plays a key role because it
      takes full advantage of the processing capabilities of
      communicating computers and human users, and thus it is far more
      compact than digital images which are not generated from data
      structures containing positional information. Vector graphical
      communication trades intensive use of storage and processing, at
      the communicating ends, in return for a low volume of exchanged
      data, because workstations with graphical hardware exchange
      graphics commands in conjunction with large data structures at the
      transmitter and receivers. In this manner, the transmission of a
      single command can produce extensive changes in the data displayed
      at the sending and receiving ends.

      It is helpful to situate the aforesaid protocol at one of the
      functional levels of the ISO Open Systems Interconnection
      Reference Model [1]. Within such a model, a graphical protocol
      functionality belongs primarily in the application level, though
      some of it fits in the presentation level.  We can distinguish the
      following components of a communication protocol:

         a) a data format
         b) rules to interpret transmitted data
         c) state information tables
         d) message exchange rules

      A format for a graphical protocol should provide the layout of the
      transmitted data, and indicate how the formated data are
      associated with interpretation rules. The choice of format
      influences the state tables to be maintained for the correct
      processing of the transmitted data stream. The graphical format
      has a minor influence on the exchange rules, which should provide
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