RFC 965 (rfc965) - Page 2 of 51

Format for a graphical communication protocol

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
      for the efficient use of transmission capacity to transport the
      data under such a format. Besides the graphical format, there are


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