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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
 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
 time-sharing system (systems processes) but not performed by the
 monitor in the current model.  A typical system process is the disc
 handler or the file system.  System processes is the disc handler or
 the file system.  System processes are probably allowed to execute in
 supervisor mode, and they actually execute I/O instructions and
 perform other privileged operations that user processes are not
 allowed to perform.  In all other ways, user and system processes are
 identical.  For reasons of efficiency, it may be useful to think of

Walden [Page 2] RFC 62 IPC for Resource Sharing 3 August 1970

 system processes as being locked in core.
 Although they will be of concern later in this study, protection
 considerations are not my concern here: instead I will assume that
 all of the processes are "good" processes which never made any
 mistakes.  If the reader needs a protection structure to keep in mind
 while he reads this note, the capability system developed in
 [1][3][7][8] should be satisfying.
 Of the operations a process can call on the monitor to perform, six
 are of particular interest for providing a capability for
 interprocess communication.
 RECEIVE. This operation allows a specified process to send a message
 to the process executing the RECEIVE. The operation has four
 parameters: the port (defined below) awaiting the message -- the
 RECEIVE port; the port a message will be accepted from -- the SEND
 port; a specification of the buffer available to receive the message;
 and a location to transfer to when the transmission is complete --
 the restart location.
 SEND.  This operation sends a message from the process executing the
 SEND to a specified process.  It has four parameters: a port to send
 the message to -- the RECEIVE port; the port the message is being
 sent from -- the SEND port; a specification of the buffer containing
 the message to be sent; and the restart location.
 RECEIVE ANY.  This operations allows any process to send a message to
 the process executing the RECEIVE ANY.  The operation has four
 parameters: the port awaiting the message -- the RECEIVE port; a
 specification of the buffer available to receive the message; a
 restart location; and a location where the port which sent the
 message may be noted.
 SEND FROM ANY.  This operation allows a process to send a message to
 a process able to receive a message from any process.  It has the
 same four parameters as SEND.  (The necessity for this operation will
 be explained much later).
 SLEEP.  This operation allows the currently running process to put
 itself to sleep pending the completion of an event.  The operation
 has one optional parameter, an event to be waited for.  An example
 event is the arrival of a hardware interrupt.  The monitor never
 unilaterally puts a process to sleep as a result of the process
 executing one of the above four operations; however, if a process is
 asleep when one of the above four operations is satisfied, the
 process is awakened.

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

 UNIQUE.  This operation obtains a unique number from the monitor.
 A port is a particular data path to a process (a RECEIVE port) or
 from a process (a SEND port), and all ports have an associated unique
 port number which is used to identify the port.  Ports are used in
 transmitting messages from one process to another in the following
 manner.  Consider two processes, A and B, that wish to communicate.
 Process A executes a RECEIVE to port N from port M.  Process B
 executes a SEND to port N from port M.  The monitor matches up the
 port numbers and transfers the message from process B to process A.
 As soon as the buffer has been fully transmitted out of process B,
 process B is restarted at the location specified in the SEND
 operation.  As soon as the message is fully received at process A,
 process A is restarted at the location specified in the RECEIVE
 operation.  Just how the processes come by the correct port numbers
 with which to communicate with other processes is not the concern of
 the monitor -- this problem is left to the processes.
 When a SEND is executed, nothing happens until a matching RECEIVE is
 executed.  Somewhere in the monitor there must be a table of port
 numbers associated with processes and restart locations.  The table
 entries are cleared after each SEND/RECEIVE match is made.  If a
 proper RECEIVE is not executed for some time, the SEND is timed out
 after a while and the SENDing process is notified.  If a RECEIVE is
 executed but the matching SEND does not happen for a long time, the
 RECEIVE is timed out and the RECEIVing process is notified.
 The mechanism of timing out "unused" table entries is of little
 fundamental importance, merely providing a convenient method of
 garbage collecting the table.  There is no problem if an entry is
 timed out prematurely, because the process can always re-execute the
 operation.  However, the timeout interval should be long enough so
 that continual re-execution of an operation will cause little
 A RECEIVE ANY never times out, but may be taken back using a
 supervisor call.  A message resultant from a SEND FROM ANY is always
 sent immediately and will be discarded if a proper receiver does not
 exist.  An error message is not returned and acknowledgment, if any,
 is up to the processes.  If the table where the SEND and RECEIVE are
 matched up ever overflows, a process originating a further SEND and
 RECEIVE is notified just as if the SEND or RECEIVE timed out.
 The restart location is an interrupt entrance associated with a
 pseudo interrupt local to the process executing the operation
 specifying the restart location.  If the process is running when then
 event causing the pseudo interrupt occurs (for example, a message
 arrives satisfying a pending RECEIVE), the effect is exactly as if

Walden [Page 4] RFC 62 IPC for Resource Sharing 3 August 1970

 the hardware interrupted the process and transferred control to the
 restart location.  Enough information is saved for the process to
 continue execution at the point it was interrupted after the
 interrupt is serviced.  If the process is asleep, it is readied and
 the pseudo interrupt is saved until the process runs again and the
 interrupt is then allowed.  Any RECEIVE or RECEIVE ANY message port
 may thus be used to provide process interrupts, event channels,
 process synchronization, message transfers, etc.  The user programs
 what he wants.
 It is left as an exercise to the reader to convince himself that the
 monitor he is saddled with can be made to provide the six operations
 described above -- most monitors can since these are only additional
 supervisor calls.
 An example.  Suppose that our model time-sharing system is
 initialized to have several processes always running.  Additionally,
 these permanent processes have some universally known and permanently
 assigned ports<2>.  Suppose that two of the permanently running
 processes are the logger-process and the teletype-scanner-process.
 When the teletype-scanner-process first starts running, it puts
 itself to sleep awaiting an interrupt from the hardware teletype
 scanner.  The logger-process initially puts itself to sleep awaiting
 a message from the teletype-scanner-process via well-known permanent
 SEND and RECEIVE ports.  The teleype-scanner-process keeps a table
 indexed by teletype number, containing in each entry a pair of port
 numbers to use to send characters from that teletype to a process and
 a pair of port numbers to use to receive characters for that teletype
 from a process.  If a character arrives (waking up the teletype-
 scanner- process) and the process does not have any entry for that
 teletype, it gets a pair of unique numbers from the monitor (via
 UNIQUE) and sends a message containing this pair of numbers to the
 logger-process using the ports for which the logger-process is known
 to have a RECEIVE pending.  The scanner-process also enters the pair
 of numbers in the teletype table, and sends the character and all
 future characters from this teletype to the port with the first
 number from the port with the second number.  The scanner-process
 must also pass a second pair of unique numbers to the logger-process
 for it to use for teletype output and do a RECEIVE using these port
 numbers.  When the logger-process receives the message from the
 scanner-process, it starts up a copy of what SDS 940 TSS [6] users
 call the executive<3>, and passes the port numbers to this copy of
 the executive, so that this executive-process can also do its inputs
 and outputs to the teletype using these ports.  If the logger-process
 wants to get a job number and password from the user, it can
 temporarily use the port numbers to communicate with the user before
 it passes them on to the executive.  The scanner-process could always
 use the same port numbers for a particular teletype as long as the

Walden [Page 5] RFC 62 IPC for Resource Sharing 3 August 1970

 numbers were passed on to only one copy of the executive at a time.
 It is important to distinguish between the act of passing a port from
 one process to another and the act of passing a port number from one
 process to another.  In the previous example, where characters from a
 particular teletype are sent either to the logger-process or an
 executive-process by the teletype-scanner-process, the SEND port
 always remains in the teletype-scanner-process while the RECEIVE port
 moves from the logger-process to the executive process.  On the other
 hand, the SEND port number is passed between the logger-process and
 the executive-process to enable the RECEIVE process to do a RECEIVE
 from the correct SEND port.  It is crucial that, once a process
 transfers a port to some other process, the first process no longer
 use the port.  We could add a mechanism that enforces this.  The
 protected object system of [9] is one such mechanism.  Using this
 mechanism, a process executing a SEND would need a capability for the
 SEND port and only one capability for this SEND port would exist in
 the system at any given time.  A process executing a RECEIVE would be
 required to have a capability for the RECEIVE port, and only one
 capability for this RECEIVE port would exist at a given time.
 Without such a protection mechanism, a port implicitly moves from one
 process to another by the processes merely using the port at disjoint
 times even if the port's number is never explicitly passed.
 Of course, if the protected object system is available to us, there
 is really no need for two port numbers to be specified before a
 transmission can take place.  The fact that a process knows an
 existing RECEIVE port number could be considered prima facie evidence
 of the process' right to send to that port.  The difference between
 RECEIVE and RECEIVE ANY ports then depends solely on the number of
 copies of a particular port number that have been passed out.  A
 system based on this approach would clearly be preferable to the one
 described here if it was possible to assume that all autonomous
 time-sharing systems in a network would adopt this protection
 mechanism.  If this assumption cannot be made, it seems more
 practical to require both port numbers.
 Note that in the interprocess communication system (IPC) being
 described here, when two processes wish to communicate they set up
 the connection themselves, and they are free to do it in a mutually
 convenient manner.  For instance, they can exchange port numbers or
 one process can pick all the port numbers and instruct the other
 process which to use.  However, in a particular implementation of a
 time-sharing system, the builders of the system might choose to
 restrict the processes' execution of SENDs and RECEIVEs and might
 forbid arbitrary passing around of ports and port numbers, requiring
 instead that the monitor be called (or some other special program) to
 perform these functions.

Walden [Page 6] RFC 62 IPC for Resource Sharing 3 August 1970

 Flow control is provided in this IPC by the simple method of never
 starting data transmission resultant from a SEND from one process
 until a RECEIVE is executed by the receiver.  Of course, interprocess
 messages may also be sent back and forth suggesting that a process
 stop sending or that space be allocated.
 Generally, well-known permanently-assigned ports are used via RECEIVE
 ANY and SEND FROM ANY.  The permanent ports will most often be used
 for starting processes and, consequently, little data will be sent
 via them.  If a process if running (perhaps asleep), and has a
 RECEIVE ANY pending, then any process knowing the receive port number
 can talk to that process without going through loggers.  This is
 obviously essential within a local time-sharing system and seems very
 useful in a more general network if the ideal of resource sharing is
 to be reached.  For instance, in a resource sharing network, the
 programs in the subroutine libraries at all sites might have RECEIVE
 ANYs always pending over permanently assigned ports with well-known
 port numbers.  Thus, to use a particular network resource such as a
 matrix manipulation hardware, a process running anywhere in the
 network can send a message to the matrix inversion subroutine
 containing the matrix to be inverted and the port numbers to be used
 for returning the results.
 An additional example demonstrates the use of the FORTRAN compiler.
 We have already explained how a user sits down at his teletype and
 gets connected to an executive.  We go on from there.  The user is
 typing in and out of the executive which is doing SENDs and RECEIVEs.
 Eventually the user types RUN FORTRAN, and executive asks the monitor
 to start up a copy of the FORTRAN compiler and passes to FORTRAN as
 start up parameters the port numbers the executive was using to talk
 to the teletype.  (This, at least conceptually, FORTRAN is passed a
 port at which to RECEIVE characters from the teletype and a port from
 which to SEND characters to the teletype.)  FORTRAN is, of course,
 expecting these parameters and does SENDs and RECEIVEs via the
 indicated ports to discover from the user what input and output files
 the user wants to use.  FORTRAN types INPUT FILE? to the user, who
 responds F001.  FORTRAN then sends a message to the file-system-
 process, which is asleep waiting for something to do.  The message is
 sent via well-known ports and it asks the file system to open F001
 for input. The message also contains a pair of port numbers that the
 file-system process can use to send its reply.  The file-system looks
 up F001, opens it for input, make some entries in its open file
 tables, and sends back to FORTRAN a message containing the port
 numbers that FORTRAN can use to read the file.  The same procedure is
 followed for the output file.  When the compilation is complete,
 FORTRAN returns the teletype port numbers (and the ports) back to the
 executive that has been asleep waiting for a message from FORTRAN,
 and then FORTRAN halts itself.  The file-system-process goes back to

Walden [Page 7] RFC 62 IPC for Resource Sharing 3 August 1970

 sleep when it has nothing else to do<4>.
 Again, the file-system process can keep a small collection of port
 numbers which it uses over and over if it can get file system users
 to return the port numbers when they have finished with them.  Of
 course, when this collection of port numbers has eventually dribbled
 away, the file system can get some new unique numbers from the

3. A System for Interprocess Communication Between Remote Processes

 The IPC described in the previous section easily generalizes to allow
 interprocess communication between processes at geographically
 different locations as, for example, within a computer network.
 Consider first a simple configuration of processes distributed around
 the points of a star.  At each point of the star there is an
 autonomous operating system<5>.  A rather large, smart computer
 system, called the Network Controller, exists at the center of the
 star.  No processes can run in this center system, but rather it
 should be thought of as an extension of the monitor of each of the
 operating systems in the network.
 If the Network Controller is able to perform the operations SEND,
 monitors in all of the time-sharing systems in the network do not
 perform these operations themselves but rather ask the Network
 Controller to perform these operations for them, then the problem of
 interprocess communication between remote processes if solved.  No
 further changes are necessary since the Network Controller can keep
 track of which RECEIVEs have been executed and which SENDs have been
 executed and match them up just as the monitor did in the model
 time-sharing system.  A networkwide port numbering scheme is also
 possible with the Network Controller knowing where (i.e., at which
 site) a particular port is at a particular time.
 Next, consider a more complex network in which there is no common
 center point, making it necessary to distribute the functions
 performed by the Network Controller among the network nodes.  In the
 rest of this section I will show that it is possible to efficiently
 and conveniently distribute the functions performed by the star
 Network Controller among the many network sites and still enable
 general interprocess communication between remote processes.
 Some changes must be made to each of the four SEND/RECEIVE operations
 described above to adapt them for use in a distributed Network
 Controller.  To RECEIVE is added a parameter specifying a site to

Walden [Page 8] RFC 62 IPC for Resource Sharing 3 August 1970

 which the RECEIVE is to be sent.  To the SEND FROM ANY and SEND
 messages is added a site to send the SEND to although this is
 normally the local site.  Both RECEIVE and RECEIVE ANY have added the
 provision for obtaining the source site of any received message.
 Thus, when a RECEIVE is executed, the RECEIVE is sent to the site
 specified, possibly a remote site.  Concurrently a SEND is sent to
 the same site, normally the local site of the process executing the
 SEND.  At this site, called the rendezvous site, the RECEIVE is
 matched with the proper SEND and the message transmission is allowed
 to take place from the SEND site to the site from whence the RECEIVE
 A RECEIVE ANY never leaves its originating site and therein lies the
 necessity for SEND FROM ANY, since it must be possible to send a
 message to a RECEIVE ANY port and not have the message blocked
 waiting for a RECEIVE at the sending site.  It is possible to
 construct a system so the SEND/RECEIVE rendezvous takes place at the
 RECEIVE site and eliminates the SEND FROM ANY operation, but in my
 judgment the ability to block a normal SEND transmission at the
 source site more than makes up for the added complexity.
 At each site a rendezvous table is kept.  This table contains an
 entry for each unmatched SEND or RECEIVE received at that site and
 also an entry for all RECEIVE ANYs given at that site.  A matching
 SEND/RECEIVE pair is cleared from the table as soon as the match
 takes place.  As in the similar table kept in the model time-sharing,
 SEND and RECEIVE entries are timed out if unmatched for too long and
 the originator is notified.  RECEIVE ANY entries are cleared from the
 table when a fulfilling message arrives.
 The final change necessary to distribute the Network Controller
 functions is to give each site a portion of the unique numbers to
 distribute via its UNIQUE operation.  I'll discuss this topic further
 To make it clear to the reader how the distributed Network Controller
 works, an example follows.  The details of what process picks port
 numbers, etc., are only exemplary and are not a standard specified as
 part of the IPC.
 Suppose that, for two sites in the network, K and L, process A at
 site K wishes to communicate with process B at site L.  Process B has
 a RECEIVE ANY pending at port M.

Walden [Page 9] RFC 62 IPC for Resource Sharing 3 August 1970

                      SITE K                        SITE L
                      ______                        ______
                     /      \                      /      \
                    /        \                    /        \
                   /          \                  /          \
                  /            \                /            \
                 |              |              |              |
                 |   Process A  |              |   Process B  |
                 |              |              |              |
                  \            /                \            /
                   \          /      RECEIVE--> port M      /
                    \        /       ANY          \        /
                     \______/                      \______/
 Process A, fortunately, knows of the existence of port M at site L and
 sends a message using the SEND FROM ANY operation from port N to port
 M.  The message contains two port numbers and instructions for process
 B to SEND messages for process A to port P from port Q.  Site K's site
 number is appended to this message along with the message's SEND port N.
                      SITE K                        SITE L
                      ______                        ______
                     /      \                      /      \
                    /        \                    /        \
                   /          \                  /          \
                  /            \                /            \
                 |              |              |              |
                 |   Process A  |              |   Process B  |
                 |              |              |              |
                  \   port N   /                \   port M   /
                   \          /--->SEND FROM --->\          /
                    \        /        ANY         \        /
                     \______/                      \______/
                                 to port M, site L
                                 containing K,N,P, & Q
 Process A now executes a RECEIVE at port P from port Q.  Process A
 specifies the rendezvous site to be site L.

Walden [Page 10] RFC 62 IPC for Resource Sharing 3 August 1970

                      SITE K                        SITE L
                      ______                        ______
                     /      \                      /      \
                    /        \                    /        \
                   /          \        Rendezvous/          \
                  /            \            table            \
                 |              |              |              |
                 |   Process A  |           ^  |   Process B  |
                 |              |           |  |              |
                  \   port P   /            |   \            /
                   \          /             |    \          /
                    \        / <--RECEIVE __/     \        /
                     \______/     MESSAGE          \______/
                                  to site L
                                  containing P, Q, & K
 A RECEIVE message is sent from site K to site L and is entered in the
 rendezvous table at site L.  At some other time, process B executes a
 SEND to port P from port Q specifying site L as the rendezvous site.
                      SITE K                        SITE L
                      ______                       ______
                     /      \                     /      \
                    /        \                   /        \
                   /          \       Rendezvous/          \
                  /            \           table            \
                 |              |             |              |
                 |   Process A  |             |   Process B  |
                 |              |             |              |
                  \   port P   /        <--------- port Q   /
                   \          /                 \          /
                    \        /        SEND       \        /
                     \______/                     \______/
                                      to site L
                                      containing P & Q
 A rendezvous is made, the rendezvous table is cleared, and the
 transmission to port P at site K takes place.  The SEND site number
 (and conceivably the SEND port number) is appended to the messages of
 the transmission for the edification of the receiving process.

Walden [Page 11] RFC 62 IPC for Resource Sharing 3 August 1970

                      SITE K                         SITE L
                      ______                        ______
                     /      \                      /      \
                    /        \                    /        \
                   /          \                  /          \
                  /            \                /            \
                 |              |              |              |
                 |   Process A  |              |   Process B  |
                 |              |              |              |
                  \   port P   /                \   port Q   /
                   \          /<--transmission<--\          /
                    \        /                    \        /
                     \______/   to port P, site K  \______/
                                containing data and L
 Process B may simultaneously wish to execute a RECEIVE from port N at
 port M.
 Note that there is only one important control message in this system
 which moves between sites, the type of message that is called a
 Host/Host protocol message in [2].  This control message is the
 RECEIVE message.  There are two other possible intersite control
 messages: an error message to the originating site when a RECEIVE or
 SEND is timed out, and the SEND message in the rare case when the
 rendezvous site is not the SEND site.  There must also be a standard
 format for messages between ports.  For example, the following:

Walden [Page 12] RFC 62 IPC for Resource Sharing 3 August 1970

       _________________           __________________      _____________
      | rendezvous site |  <6>    | destination site |    | source site |
      |-----------------|         |------------------|    |-------------|
      |    RECEIVE port |         |   RECEIVE port   |    | RECEIVE port|
      |-----------------|         |------------------|    |-------------|
      |    SEND port    |         |   SEND port      |    | SEND port   |
      |-----------------|         |------------------|    |-------------|
      |                 |         |   source site    |    |             |
      |                 |         |------------------|    |             |
      |                 |         |                  |    |             |
      |                 |         |                  |    |             |
      |                 |         |                  |    |             |
      |                 |         |                  |    |             |
      |     data        |         |     data         |    |   data      |
      |                 |         |                  |    |             |
      |                 |         |                  |    |             |
      |                 |         |                  |    |             |
      |                 |         |                  |    |             |
      |_________________|         |__________________|    |_____________|
       transmitted                 transmitted             received
       by SEND                     by Network              by RECEIVE
       process                     Controller              process
 In the model time-sharing system it was possible to pass a port form
 process to process.  This is still possible with a distributed Network
 Remember that, for a message to be sent from one process to another, a
 SEND to port M from port N and a RECEIVE at port M from port N must
 rendezvous, normally at the SEND site.  Both processes keep track of
 where they think the rendezvous site is and supply this site as a
 parameter of appropriate operations.  The RECEIVE process thinks it is
 the SEND site also.  Since once a SEND and a RECEIVE rendezvous the
 transmission is sent to the source of the RECEIVE and the entry in the
 rendezvous table is cleared and must be set up again for each further
 transmission from N to M, it is easy for a RECEIVE port to be moved.
 If a process sends both the port numbers and the rendezvous site
 number to a new process at some other site which executes a RECEIVE
 using these same old port numbers and rendezvous site specification,
 the SENDer never knows the RECEIVEr has moved.  It is slightly harder
 for a send port to move.  However, if it does, the pair of port
 numbers that has been being used for a SEND and the original
 rendezvous site number are passed to the new site.  The process at the
 new SEND site specifies the old rendezvous site with the first SEND
 from the new site.  The RECEIVE process will also still think the
 rendezvous site is the old site, so the SEND and RECEIVE will meet at
 the old site.  When they meet, the entry in the table at that site is
 cleared, and both the SEND and RECEIVE messages are sent to the new

Walden [Page 13] RFC 62 IPC for Resource Sharing 3 August 1970

 SEND site just as if they had been destined for there in the first
 place.  The SEND and RECEIVE then meet again at the new rendezvous
 site and transmission may continue as if the port had never moved.
 Since all transmissions contain the source site number, further
 RECEIVEs will be sent to the new rendezvous site.  It is possible to
 discover that this special manipulation must take place because a SEND
 message is received at a site that did not originate the SEND
 message<7>.  Note that the SEND port and the RECEIVE port can move
 Of course, all of this could have also been done if the processes had
 sent messages back and forth announcing any potential moves and the
 new site numbers.
 A problem that may have occurred to the reader is how the SEND and
 RECEIVE buffers get matched for size.  The easiest solution would be
 to require that all buffers have a common size but this is
 unacceptable since it does not easily extend to a situation where
 processes in autonomous operating systems are attempting to
 communicate.  A second solution is for the processes to pass messages
 specifying buffer sizes.  If this solution is adopted, excessive data
 sent from the SEND process and unable to fix into the RECEIVE buffer
 is discarded and the RECEIVE process notified.  The solution has great
 appeal on account of its simplicity.  A third solution would be for
 the RECEIVE buffer size to be passed to the SEND site with RECEIVE
 message and to notify the SEND process when too much data is sent or
 even to pass the RECEIVE buffer size on to the SEND process.  This
 last method would also permit the Network Controller at the SEND site
 to make two or more SENDs out of one, if that was necessary to match a
 smaller RECEIVE buffer size.
 The maintenance of unique numbers is also a problem when the processes
 are geographically distributed.  Three solutions to this problem are
 presented here.  The first possibility is for the autonomous operating
 systems to ask the Network Controller for the unique numbers
 originally and then guarantee the integrity of any unique numbers
 currently owned by local processes and programs using whatever means
 are at the operating system's disposal.  In this case, the Network
 Controller would provide a method for a unique number to be sent from
 one site to another and would vouch for the number's identity at the
 new site.  The second method is simply to give the unique numbers to
 the processes that are using them, depending on the non-malicious
 behavior of the processes to preserve the unique numbers, or if an
 accident should happen, the two passwords (SEND and RECEIVE port
 numbers) that are required to initiate a transmission.  If the unique
 numbers are given out in a non-sequential manner and are reasonably
 long (say 32 bits), there is little danger.  In the final method, a
 user identification is included in the port numbers and the individual

Walden [Page 14] RFC 62 IPC for Resource Sharing 3 August 1970

 operating systems guarantee the integrity of these identification
 bits.  Thus a process, while not able to be sure that the correct port
 is transmitting to him, can be sure that some port of the correct user
 is transmitting.  This is the so-called virtual net concept suggested
 by W. Crowther [2].<8>
 A third difficult problem arises when remote processes wish to
 communicate, the problem of maintaining high bandwidth connections
 between the remote processes.  The solution to this problem lies in
 allowing the processes considerable information about the state of an
 on-going transmission.  First, we examine a SEND process in detail.
 When a process executes a SEND, the local portion of the Network
 Controller passes the SEND on to the rendezvous site, normally the
 local site.  When a RECEIVE arrives matching a pending SEND, the
 Network Controller notifies the SEND process by causing an interrupt
 to the specified restart location.  Simultaneously the Network
 Controller starts shipping the SEND buffer to the RECEIVE site.  When
 transmission is complete, a flag is set which the SEND process can
 test.  While a transmission is taking place, the process may ask the
 Network Controller to perform other operations, including other SENDs.
 A second SEND over a pair of ports already in the act of transmission
 is noted and the SEND becomes active as soon as the first transmission
 is complete.  A third identical SEND results in an error message to
 the SENDing process.  Next, we examine a RECEIVE process in detail.
 When a process executes a RECEIVE, the RECEIVE is sent to the
 rendezvous site.  When data resultant from this RECEIVE starts to
 arrive at the RECEIVE site, the RECEIVE process is notified via an
 interrupt to the specified restart location.  When the transmission is
 complete, a flag is set which the RECEIVE process can test.  A second
 RECEIVE over the same port pair is allowed.  A third results in an
 error message to the RECEIVE process.  Thus, there is sufficient
 machinery to allow a pair of processes always to have both a
 transmission in progress and the next one pending.  Therefore, no
 efficiency is lost.  On the other hand, each transmission must be
 preceded by a RECEIVE into a specified buffer, thus continuing to
 provide complete flow control.

4. A Potential Application

 Only one  resource sharing computer network currently exists, the
 ARPA Computer Network.  In this section, I discuss application of the
 system described in this paper to the ARPA Network [2][5][9].
 The ARPA Network currently incorporates ten sites spread across the
 United States.  Each site consists of one to three (potentially four)
 independent computer systems called Hosts and one communications
 computer system called an IMP.  All of the Hosts at a site are

Walden [Page 15] RFC 62 IPC for Resource Sharing 3 August 1970

 directly connected to the IMP.  The IMPs themselves are connected
 together by 50-kilobit phone lines (much higher rate lines are a
 potential), although each IMP is connected to only one to five other
 IMPs.  The IMPs provide a communications subnet through which the
 Hosts communicate.  Data is sent through the communications subnet in
 messages of arbitrary size (currently about 8000 bits) called network
 messages.  When a network message is received by the IMP at the
 destination site, that IMP sends an acknowledgment, called a RFNM, to
 the source site.
 A system for interprocess communication for the ARPA Network (let us
 call this IPC for ARPA) is currently being designed by the Network
 Working Group, under the chairmanship of S. Crocker of UCLA.  Their
 design is somewhat constrained by the communications subnet [5]<9>.
 I would like to compare point-by-point IPC for ARPA with the one
 developed in this paper; however, such a comparison would first
 require description here, almost from scratch, of the current state
 of IPC for ARPA since very little up-to-date information about IPC
 for ARPA appears in the open literature [2].  Also, IPC for ARPA is
 quite complex and the working documents describing it now run to many
 hundred pages, making any description lengthy and inappropriate for
 this paper.<10> Therefore, I shall make only a few scattered
 comparisons of the two systems, the first of which are implicit in
 this paragraph.
 The interprocess communication system being developed for the ARPA
 Network comes in several almost distinct pieces: The Host/IMP
 protocol, IMP/IMP protocol, and the Host/Host protocol.  The IMPs
 have sole responsibility for correctly transmitting bits from one
 site to another.  The Hosts have sole responsibility for making
 interprocess connections.  Both the Host and IMP are concerned and
 take a little responsibility for flow control and message sequencing.
 Applications of the interprocess communication system described in
 this paper leads me to make a different allocation of responsibility.
 The IMP still continues to move bits from on site to another
 correctly but the Network Controller also resides in the IMP, and
 flow control is completely in the hands of the processes running in
 the Hosts, although using the mechanisms provided by the IMPs.
 UNIQUE operations in slightly altered forms for the Hosts and also
 maintain the rendezvous tables, including moving of SEND ports when
 necessary.  Putting these operations in the IMP requires the
 Host/Host protocol program to be written only once, rather than many
 times as is currently being done in the ARPA Network.  It is perhaps
 useful to step through the five operations again.
 SEND.  The Host gives the IMP a SEND port number, a RECEIVE port

Walden [Page 16] RFC 62 IPC for Resource Sharing 3 August 1970

 number, the rendezvous site, and a buffer specification (e.g., start
 and end, or beginning and length).  The SEND is sent to the
 rendezvous site IMP, normally the local IMP.  When a matching RECEIVE
 arrives at the local IMP, the Host is notified of the RECEIVE port of
 the just arrived message.  This port number is sufficient to identify
 the SENDing process, although a given operating system may have to
 keep internal tables mapping this port number into a useful internal
 process identifier.  Simultaneously, the IMP begins to ask the Host
 for specific pieces of the SEND buffer, sending these pieces as
 network messages to the destination site.  If a RFNM is not received
 for too long, implying a network message has been lost in the
 network, the Host is asked for the same data again and it is
 retransmitted.<11> Except for the last piece of a buffer, the IMP
 requests pieces from the Host which are common multiplies of the word
 size of the source Host, IMP, and destination Host.  This avoids
 mid-transmission word alignment problems.
 RECEIVE.  The Host gives the IMP a SEND port, a RECEIVE port, a
 rendezvous site, and a buffer description.  The RECEIVE message is
 sent to the rendezvous site.  As the network messages making up a
 transmission arrive for the RECEIVE port, they are passed to the Host
 along with RECEIVE port number (and perhaps the SEND port number),
 and an indication to the Host where to put this data in its input
 buffer.  When the last network message of the SEND buffer is passed
 into the Host, it is marked accordingly and the Host can then detect
 this.  (It is conceivable that the RECEIVE message could also
 allocate a piece of network bandwidth while making its network
 traverse to the rendezvous site.)
 RECEIVE ANY.  The Host gives the IMP a RECEIVE port and a buffer
 descriptor.  This works the same as RECEIVE but assumes the local
 site to be the rendezvous site.
 SEND FROM ANY.  The Host gives the IMP RECEIVE and SEND ports, the
 destination site, and a buffer descriptor.  The IMP requests and
 transmits the buffer as fast as possible.  A SEND FROM ANY for a
 non-existent port is discarded at the destination site.
 In the ARPA Network, the Hosts are required by the IMPs to physically
 break their transmissions into network messages, and successive
 messages of a single transmission must be delayed until the RFNM is
 received for the previous message.  In the system described here,
 since RFNMs are tied to the transmission of a particular piece of
 buffer and since the Hosts allow the IMPs to reassemble buffers in
 the Hosts by the IMP telling the Host where to put each buffer piece
 then pieces of a single buffer can be transmitted in parallel network
 messages and several RFNMs can be outstanding simultaneously.  This
 enables The Hosts to deal with transmissions of more natural sizes

Walden [Page 17] RFC 62 IPC for Resource Sharing 3 August 1970

 and higher bandwidth for a single transmission.
 For additional efficiency, the IMP might know the approximate time it
 takes for a RECEIVE to get to a particular other site and warn the
 Host to wake up a process shortly before the arrival of a message for
 that process is imminent.
 5. Conclusion
 Since the system described in this paper has not been implemented, I
 have no clearly demonstrable conclusions nor any performance reports.
 Instead, I conclude with four openly subjective claims.
 1) The interprocess communication system described in Section 2 is
 simpler and more general than most existing systems of equivalent
 power and is more powerful than most intra time-sharing system
 communication systems currently available.
 2) Time-sharing systems structured like the model in Section 2 should
 be studied by designers of time-sharing systems who may see a
 computer network in their future, as structure seems to enable
 joining a computer network with a minimum of difficulty.
 3) As computer networks become more common, remote interprocess
 communication systems like the one described in Section 3 should be
 studied.  The system currently being developed for ARPA is a step in
 the wrong direction, being addressed, in my opinion, more to
 communication between monitors than to communication between
 processes and consequently subverting convenient resource sharing.
 4) The application of the system as described in Section 4 is much
 simpler to implement and more powerful than the system currently
 being constructed for the ARPA Network, and I suggest that
 implementation of my method be seriously considered for adoption by
 the ARPA Network.


  1. Almost any of the common definitions of a process would suit the
     needs of this paper.
  2. Or perhaps there is only one permanently known port, which
     belongs to a directory-process that keeps a table of
     permanent-process/well-know-port associations.
  3. That program which prints file directories, tells who is on other

Walden [Page 18] RFC 62 IPC for Resource Sharing 3 August 1970

     teletypes, runs subsystems, etc.
  4. The reader should have noticed by now that I do not like to think
     of a new process (consisting of a new conceptual copy of a
     program) being started up each time another user wishes to use
     the program.  Rather, I like to think of the program as a single
     process which knows it is being used simultaneously by many other
     processes and consciously multiplexes among the users or delays
     service to users until it can get around to them.
  5. I use operating system rather than time-sharing system in this
     section to point up the fact that the autonomous systems at the
     network nodes may be either full blown time-sharing systems in
     their own right, and individual process in a larger
     geographically distributed time-sharing system, or merely
     autonomous sites wishing to communicate.
  6. For a SEND FROM ANY message, the rendezvous site is the
     destination site.
  7. For readers familiar with the once-proposed re-connection scheme
     for the ARPA Network, the above system is simple, comparatively,
     because there are no permanent connections to break and move;
     that is, connections only exist fleetingly in the system
     described here and can therefore be remade between any pair of
     processes which at any time happen to know each other's port
     numbers and have some clue where they each are.
  8. Crowther says this is not the virtual net concept.
  9. As one of the builders of the ARPA communications subnet, I am
     partially responsible for these constraints.
 10. The reader having access to the ARPA working documents may want
     to read Specifications for the Interconnection of a Host to
     an IMP, BBN Report No. 1822; and ARPA Network Working Group
     Notes #36, 37, 38, 39, 42, 44, 46, 47, 48, 49, 50, 54, 55, 56,
     57, 58, 59, 60.
 11. This also allows messages to be completely thrown away by the IMP
     subnet it that should ever be useful.


  1.  Ackerman, W., and Plummer, W.  An implementation of a
          multi-processing computer system.  Proc. ACM Symp. on
          Operating System Principles, Gatlinsburg, Tenn.,

Walden [Page 19] RFC 62 IPC for Resource Sharing 3 August 1970

          Oct. 1-4, 1967.
  2.  Carr, C. Crocker, S., and Cerf, V.  Host/Host communication
          protocol in the ARPA network.  Proc. AFIPS 1970 Spring
          Joint Comput. Conf., Vol. 36, AFIPS Press, Montvale, N.J.,
          pp. 589-597.
  3.  Dennis, J., and VanHorn, E.  Programming semantics for
          multiprogrammed computations.  Comm. ACM 9, 3 (March,
          1966), 143-155.
  4.  Hansen, P.B.  The nucleus of a multiprogramming system.  Comm.
          ACM 13, 4 (April, 1970), 238-241, 250.
  5.  Heart, F., Kahn, R., Ornstein, S., Crowther, W., and Walden, D.
          The interface message processor for the ARPA computer
          network.  Proc. AFIPS 1970 Spring Joint Comput. Conf., Vol.
          36, AFIPS Press, Montvale, N.J., pp. 551-567.
  6.  Lampson, B.  SDS 940 Lectures, circulated informally.
  7.  _______.  An overview of the CAL time-sharing system.  Computer
          Center, University of California, Berkeley, Calif.
  8.  _______.  Dynamic protection structures.  Proc.  AFIPS 1969 Fall
          Joint Comput. Conf., Vol. 35, AFIPS Press, Montvale, N.J.,
          pp. 27-38.
  9.  Roberts, L., and Wessler, B.  Computer network development to
          achive resource sharing.  Proc.  AFIPS 1970 Spring Joint
          Comput. Conf., Vol. 36, AFIPS Press, Monvale, N.J., pp.
 10.  Spier, M., and Organick, E.  The MULTICS interprocess
          communication facility.  Proc. ACM Second Symp. on Operating
          Systems Principles, Princeton University, Oct. 20-22, 1969.

Author's Address

 D. C. Walden
 Bolt Bernakek and Newman, Inc.
 Cambridge, Massachusetts
      [ This RFC was put into machine readable form for entry ]
      [ into the online RFC archives by Adam Costello 3/97 ]

Walden [Page 20]

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