Systems for Interprocess Communication in a Resource Sharing Computer Network
RFC 62

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Network Working Group                                       D. C. Walden
Request for Comments: 62                                        BBN Inc.
Supercedes NWG/RFC #61                                     3 August 1970

                A System for Interprocess Communication
                                 in a
                   Resource Sharing Computer Network

1.  Introduction

   If you are working to develop methods of communications within a
   computer network, you can engage in one of two activities.  You can
   work with others, actually constructing a computer network, being
   influenced, perhaps influencing your colleagues.  Or you can
   construct an intellectual position of how things should be done in an
   ideal network, one better than the one you are helping to construct,
   and then present this position for the designers of future networks
   to study.  The author has spent the past two years engaged in the
   first activity.  This paper results from recent engagement in the
   second activity.

   "A resource sharing computer network is defined to be a set of
   autonomous, independent computer systems, interconnected so as to
   permit each computer system to utilize all of the resources of the
   other computer systems much as it would normally call a subroutine."
   This definition of a network and the desirability of such a network
   is expounded upon by Roberts and Wessler in [9].

   The actual act of resource sharing can be performed in two ways:  in
   an ad hoc manner between all pairs of computer systems in the
   network; or according to a systematic network-wide standard.  This
   paper develops one possible network-wide system for resource sharing.

   I believe it is natural to think of resources as being associated
   with processes<1> and available only through communication with these
   processes.  Therefore, I view the fundamental problem of resource
   sharing to be the problem of interprocess communication.  I also
   share with Carr, Crocker, and Cerf [2] the view that interprocess
   communication over a network is a subcase of general interprocess
   communication in a multi-programmed environment.

   These views have led me to perform a two-part study.  First, a set of
   operations enabling interprocess communication within a single time-
   sharing system is constructed.  This set of operations eschews many
   of the interprocess communications techniques currently in use within
   time-sharing systems -- such as communication through shared memory
   -- and relies instead on techniques that can be easily generalized to

Walden                                                         [Page 1]
RFC 62                  IPC for Resource Sharing          3 August 1970

   permit communication between remote processes.  The second part of
   the study presents such a generalization.  The application of this
   generalized system to the ARPA Computer Network [9] is also
   discussed.

   The ideas enlarged upon in this paper came from many sources.
   Particularly influential were -- 1) an early sketch of a Host
   protocol for the ARPA Network by S. Crocker of UCLA and W. Crowther
   of Bolt Beranek and Newman Inc. (BBN); 2) Ackerman and Plummer's
   paper on the MIT PDP-1 time-sharing system [1]; and 3) discussions
   with W. Crowther and R. Kahn of BBN about Host protocol, flow
   control, and message routing for the ARPA Network.  Hopefully, there
   are also some original ideas in this note.  I alone am responsible
   for the collection of all of these ideas into the system described
   herein, and I am therefore responsible for any inconsistencies or
   bugs in the system.

   It must be emphasized that this paper does not represent an official
   BBN position on Host protocol for the ARPA Computer Network.

2.  A System for Interprocess Communication within a Time-Sharing System

   This section describes a set of operations enabling interprocess
   communication within a time-sharing system.  Following the notation
   of [10], I call this interprocess communication facility an IPC.  As
   an aid to the presentation of this IPC, a model for a time-sharing
   system is described; this model is then used to illustrate the use of
   the interprocess communication operations.

   The model time-sharing has two pieces: the monitor and the processes.
   The monitor performs such functions as switching control from one
   process to another process when a process has used "enough" time,
   fielding hardware interrupts, managing core and the swapping medium,
   controlling the passing of control from one process to another (i.e.,
   protection mechanisms), creating processes,caring for sleeping
   processes, and providing to the processes a set of machine extending
   operations (often called Supervisor or Monitor Calls).  The processes
   perform the normal user functions (user processes) as well as the
   functions usually thought of as being supervisor functions in a