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Network Working Group Dave Walden Request for Comments: 61 Bolt Beranek and Newman

                                                         July 17, 1970
                A Note on Interprocess Communication
              in a Resource Sharing Computer Network
 The attached note is a draft of a study I am still working on.  It
 may be of general interest to network participants.

Walden [Page 1] RFC 61 Interprocess Communication in a Computer Network July 1970

                      Interprocess Communication
                                 in a
                   Resource Sharing Computer Network


 "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 each
 other computer system.  That is, a program running in one computer
 system should be able to call on 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 [1].
 The actual act of resource sharing can be performed in two ways: in a
 pairwise 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 [2] and therefore view the fundamental problem of
 resource sharing to be the problem of interprocess communication.  I
 also share with Carr, Crocker, and Cerf [3] the view that
 interprocess communication over a network is a subcase of general
 interprocess communication in a multiprogrammed environment.
 These views pervade this study and have led to a two part study.
 First, a model for a time-sharing system having capabilities
 particularly suitable for enabling interprocess communication is
 constructed.  Next, it is shown that these capabilities can be easily
 used in a generalized manner which permits interprocess communication
 between processes distributed over a computer network.
 This note contains ideas based on many sources.  Particularly
 influential were -- 1) an early sketch of a Host protocol for the
 ARPA Network [1][3][4] by W. Crowther of Bolt Beranek and Newman Inc.
 (BBN) and S. Crocker of UCLA; 2) Ackerman and Plummer's paper on the
 MIT PDP-1 time sharing system [5]; and 3) discussion with R. Kahn of
 BBN about Host protocol, message control, and 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 this system.
 It must be emphasized that this note does not represent an official
 BBN position on Host protocol for the ARPA Computer Network.

Walden [Page 2] RFC 61 Interprocess Communication in a Computer Network July 1970


 This section describes a model time-sharing system which I think is
 particularly suitable for performing interprocess communication.  The
 basic structure of this model time-sharing system is not original
 The model time-sharing system has two pieces: the monitor and the
 processes.  The monitor performs several functions, including
 switching control from process to process as appropriate (e.g., when
 a process has used "enough" time or when an interrupt occurs),
 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, etc.
 The processes perform most of the functions normally thought of as
 being supervisor functions in a time-sharing system (system
 processes) as well as the normal user functions (user processes).  A
 typical system process is the disc handler or the file system.  For
 efficiency reasons it may be useful to think of system processes as
 being locked in core.
 A process can call on the monitor to perform several functions: start
 another, equal, autonomous process (i.e., load a program or find a
 copy of a program somewhere that can be shared, start it, and pass it
 some initial parameters); halt the running process; put the current
 process to sleep pending a specified event; send a message to a
 specified process; become available to receive a message from a
 specified process; become available to receive a message from any
 process; send a message to a process able to receive from any
 process; and request a unique number.  There undoubtedly should also
 be other monitor functions.  It is left as an exercise to the reader
 to convince himself that the monitor he is saddled with can be made
 to provide these functions -- most can.
 I will not concern myself with protection considerations here, but
 instead will assume all of the processes are "good" processes which
 never make any mistakes.  If the reader needs a protection structure
 to keep in mind while he reads this note, the _capability_ system
 described in [5][6][7][8] should be satisfying.
 We now look a little closer at the eight operations listed above that
 a process can ask the monitor to perform.

Walden [Page 3] RFC 61 Interprocess Communication in a Computer Network July 1970

 START.  This operation starts another process.   It has two
 parameters -- some kind of identification for the program that is to
 be loaded and a parameter list for that program.   Once the program
 is loaded, it is started at its given entry point and passed its
 parameter list in some well known manner.  The process will continue
 to exist until it halts itself.
 HALT.  This operation puts the currently running process to sleep
 pending the completion of some event.  The operation has one
 parameter, the event to be waited for.  Sample events are arrival of
 a hardware interrupt, arrival of a message from another process, etc.
 The process is restarted at the instruction after the SLEEP command.
 The monitor never unilaterally puts a process to sleep except when
 the process overflows its quantum.
 RECEIVE.  This operation allows another process to send a message to
 this process.  The operation has four parameters: the port (defined
 below) awaiting the message, the port a message will be accepted
 from, a specification of the buffer available to receive the message,
 and a location of transfer to when the transmission is complete.  [In
 other words, an interrupt location.  Any message port may be used to
 allow interrupts, event channels, etc.  The user programs what he
 SEND.  This operation sends a message to some other process.  [I
 suppose a process could also send a message to itself.]  It has four
 parameters: a port to send the message to, the port the message is
 being sent from, the message, and a location to transfer to when the
 transmission is complete.
 RECEIVE ANY.  This operation allows any process to send a message to
 this process.  The operation has four parameters: the port awaiting
 the message, the buffer available to receive the message, a location
 to transfer to when the message is received, 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 discussed below.
 UNIQUE.  This operation obtains a unique number from the monitor.
 A _port_ is a particular data path to or from a process.  All ports
 have an associated unique number which is used to identify the port.
 Ports are used in transmitting messages from one process to another
 in the following fashion.  Consider two processes, A and B, wishing
 to communicate.  Process A executes a RECEIVE at port N from port M.

Walden [Page 4] RFC 61 Interprocess Communication in a Computer Network July 1970

 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.
 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.  [Or perhaps there is only one permanently known port
 which belongs to a directory-process which keeps a table of
 permanent-process/well-known-port associations.]  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 teletype-
 scanner-process keeps a table indexed by teletype number containing
 in each entry a port to send characters from that teletype to, and a
 port at which to receive characters for that teletype.  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
 that the logger-process is known to have a RECEIVE pending for.
 [Actually, the scanner process could always use the same pair of port
 numbers for a particular teletype as long as they were passed on to
 only one copy of the executive at a time.]  The scanner-process also
 enters the pair of numbers in the teletype table, and sends the
 characters and all future characters from this teletype to the port
 with the first number from the port with the second number.  The
 scanner-process probably also passes a second pair of unique numbers
 to the logger-process for it to use for teletype output and does a
 RECEIVE using these numbers.  The logger-process when it receives the
 message from the scanner-process, starts up a copy of what SDS 940
 TSS [12] users call the executive (that program which prints file
 directories, tells who is on other teletypes, runs subsystems, etc.)
 and passes this copy of the executive, the port numbers so this
 executive-process can also do its in's and out's 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

Walden [Page 5] RFC 61 Interprocess Communication in a Computer Network July 1970

 _Port numbers_ are often passed among processes.  More rarely, a port
 is transferred to another process.  It is crucial that once a process
 transfers a _port_ to some other process that the first process no
 longer use the port.  We could add a mechanism that enforces this.
 The protected object system of [8] is one such mechanism.  [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 is 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 all of the autonomous time-sharing
 system 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 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.  Also note
 that if a process is running (perhaps asleep), and has RECEIVE ANY
 pending, then any process knowing the receive port number can talk to
 that process without going through loggers or any of that.  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.
 When a SEND is executed, nothing happens until a matching RECEIVE is
 executed.  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.
 A RECEIVE ANY never times out, but may be taken back.  A SEND FROM
 ANY message 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 or RECEIVE is notified just as if the SEND
 or RECEIVE timed out.
 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 going and consequently little data
 will be sent via them.

Walden [Page 6] RFC 61 Interprocess Communication in a Computer Network July 1970

 Still another example, this time a demonstration of 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 the
 executive asks the monitor to start up a copy of the FORTRAN compiler
 and passes to FORTRAN as start up parameters the two ports the
 executive was using to talk to the teletype.  FORTRAN is of course
 expecting these parameters and does SENDs and RECEIVEs to these ports
 to discover 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 ports that the file-system-process
 can use to send its reply.  The file-system looks up F001, opens it
 for input, makes some entries in its open file tables, and sends a
 message back to FORTRAN which contains the ports which 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 back to the executive which has been asleep waiting for
 a message from FORTRAN, and then FORTRAN halts itself.  The file-
 system-process goes back to sleep when it has nothing else to do.
 [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.]
 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 are done 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 monitor.
 Note that 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.  Of course, 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 port numbers, requiring instead
 that the monitor be called (or some other special program) to perform
 these functions.

Walden [Page 7] RFC 61 Interprocess Communication in a Computer Network July 1970

 Flow control is provided in this system by the simple method of never
 starting a SEND from one process until a RECEIVE is executed by the
 receiver.  Of course, interprocess messages may be sent back and
 forth suggesting that a process stop sending or that space be
 allocated, etc.


 The system 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 time-sharing system.  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 time-
 sharing system in the network.
 It should be obvious to the reader that if the Network Controller is
 able to perform the operations SEND, RECEIVE, SEND FROM ANY, RECEIVE
 ANY, and UNIQUE and that if all of the 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 we have solved the problem of interprocess
 communication between remote processes.  We have no further change to
 The reason everything continues to work when we postulate the
 existence of the Network Controller is that 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.  To
 RECEIVE is added a parameter specifying a site to which the RECEIVE

Walden [Page 8] RFC 61 Interprocess Communication in a Computer Network July 1970

 is to be sent.  To SEND FROM ANY and SEND 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 obtain 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 to the site from whence the
 RECEIVE came.
 A RECEIVE ANY never leaves its originating site and therein lies the
 necessity for SEND FROM ANY.  It must be possible to send a message
 to a RECEIVE ANY port and not have the message blocked waiting for
 RECEIVE at the sending site.  Of course, it would be possible to
 construct the system so the SEND/RECEIVE rendezvous takes place at
 the RECEIVE site and eliminate 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.
 Somewhere 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 or perhaps when the transmission is complete.  As
 in the similar table kept in the model time-sharing system, 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 system.
 Suppose there are 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 61 Interprocess Communication in a Computer Network July 1970

                  SITE K              SITE L
                ________            ________
               /        \          /        \
              /          \        /          \
             /            \      /            \
            |  Process A   |    |   Process B  |
            |              |    |              |
            |              |    |              |
             \            /      \            /
              \          /        \   port M /
               \________/          \____^___/
                                    RECEIVE ANY
 Process A, fortunately, knows of the existence of port M at site L
 and sends messages 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 to 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  /--->SEND FROM --->\  port M  /
              \________/       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 61 Interprocess Communication in a Computer Network July 1970

                  SITE K                         SITE L
                ________                 R    ________
               /        \                e   /        \
              /          \               n T/          \
             /            \              d a            \
            |              |             e b  Process B |
            |  Process A   |             z l            |
            |              |             v e            |
             \            /              o \            /
              \  port P  /  RECEIVE ---> u  \          /
               \________/   MESSAGE      s   \________/
                              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
                ________                 R    ________
               /        \                e   /        \
              /          \               n T/          \
             /            \              d a            \
            |              |             e b  Process B |
            |  Process A   |             z l            |
            |              |             v e            |
             \            /              o \            /
              \  port P  /               u <--- port Q /
               \________/    SEND        s   \________/
                              to site L
                              containing P & Q
 A rendezvous is made, the rendezvous table entry is cleared, and the
 transmission to port P at site K takes place.  The SEND site number
 (and conceivably the SEND port number) are appended to the messages
 of the transmission for the edification of the receiving process.

Walden [Page 11] RFC 61 Interprocess Communication in a Computer Network July 1970

              SITE K                                SITE L
            ________                              ________
           /        \                            /        \
          /          \                          /          \
         /            \                        /            \
        |   Process A  |                      |  Process B   |
        |              |                      |              |
        |              |                      |              |
         \  port P    /                        \  port Q    /
          \          / <---- transmission <---- \          /
           \________/    to port T, 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 [3].  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.
 Of course there must also be a standard format for messages between
 ports.  For example, the following:

Walden [Page 12] RFC 61 Interprocess Communication in a Computer Network July 1970

  +-----------------+  +-----------------+  +-----------------+
  | rendezvous site |  | destination site|  |  source site    |
  +-----------------+  +-----------------+  +-----------------+
  | RECEIVE port    |  | RECEIVE port    |  |  RECEIVE port   |
  +-----------------+  +-----------------+  +-----------------+
  | SEND port       |  | SEND port       |  |   SEND port     |
  +-----------------+  +-----------------+  +-----------------+
  |                 |  | source port     |  |                 |
  |                 |  +-----------------+  |                 |
  |                 |  |                 |  |                 |
  |                 |  |                 |  |                 |
  |                 |  |                 |  |                 |
  |                 |  |                 |  |                 |
  |                 |  |                 |  |                 |
  |     data        |  |      data       |  |     data        |
  |                 |  |                 |  |                 |
  |                 |  |                 |  |                 |
  |                 |  |                 |  |                 |
  |                 |  |                 |  |                 |
  |                 |  |                 |  |                 |
  |                 |  |                 |  |                 |
  +-----------------+  +-----------------+  +-----------------+
     transmitted           transmitted          received
     by SEND               by Network           by RECEIVE
     process               Controller           process
 Note: for a SEND FROM ANY message, the rendezvous site is the
 destination site.
 In the model time-sharing system it was possible to pass a port from
 process to process.  This is still possible with a distributed
 Network Controller.  [The reader unconvinced of the utility of port
 passing is directed to read the section on reconnection in [11].]
 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 and the SEND process normally 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

Walden [Page 13] RFC 61 Interprocess Communication in a Computer Network July 1970

 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, the rendezvous site number for the
 SEND message is changed to the site which originated the SEND message
 and both the SEND and RECEIVE messages are sent to the new 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 which did not originate the SEND message.
 Everything is so easily changed because there are no permanent
 connections to break and move as in the once proposed reconnection
 scheme for the ARPA network [10][11] 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.
 Of course, all of this could have been done by the processes sending
 messages back and forth announcing any potential moves and the new
 site numbers.

Walden [Page 14] RFC 61 Interprocess Communication in a Computer Network July 1970


 [1]  L. Roberts and B. Wessler, Computer Network Development to
      achieve Resource Sharing, Proceedings 1970 SJCC.
 [2]  V. Vyssotsky, F. F.  Corbato, and R. Graham, Structure of the
      MULTICS Supervisor, Proceedings 1965 FJCC.
 [3]  C. Carr, S. Crocker, and V. Cerf, Host/Host Communication
      Protocol in the ARPA Network, Proceedings 1970 SJCC.
 [4]  F. Heart, et al, The Interface Message Processor for the ARPA
      Computer Network, Proceedings 1970 SJCC.
 [5]  W. Ackerman and W. Plummer, An Implementation of Multi-
      processing Computer System, Proceedings Gatlinburg Symposium on
      Operating System Principles.
 [6]  J. Dennis and E. Van Horn, Programming Semantics for
      Multiprogramming Computation, Proceedings of the San Dimes
      Conference on Programming Language and Pragmatics.
 [7]  B. Lampson, Dynamic Protection Structures, Proceedings FJCC
 [8]  B. Lampson, An Overview of the CAL Time-Sharing System, Computer
      Center, University of Calif., Berkeley.
 [9]  P. Hansen, The Nucleus of a Multiprogramming System, CACM, April
 [10] S. Crocker, ARPA Network Working Group Note #36.
 [11] J. Postel and S. Crocker, ARPA Network Working Group Note #48.
 [12] B. Lampson, 940 Lectures.

Walden [Page 15] RFC 61 Interprocess Communication in a Computer Network July 1970


 Only one resource sharing computer network currently exists, the
 aforementioned ARPA network.  In this Appendix, I hope to show that
 the system that was described in this note can be applied to the ARPA
 network.  A significant body of work exists on interprocess
 communication within the ARPA network.  This work comes in several
 almost distinct pieces: the Host/IMP protocol, IMP/IMP protocol, and
 the Host/Host protocol.  I assume familiarity with this work in the
 subsequent discussion.  [See references [1][3][4][10][11];
 Specifications for the Inter-connection of a Host to an IMP, BBN
 Report No. 1822; and ARPA Network Working Group Notes #37, 38, 39,
 42, 44, 46, 47, 48, 49, 50, 54, 55, 56, 57, 56, 59.]
 In the ARPA network, the IMP's 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.  Application of the interprocess
 communication system I have described leads me to different
 allocation of responsibility.  The IMP still continues to correctly
 move bits from one site to another, 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 perhaps they use
 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
 It is perhaps easiest to step through the five operations again.
 SEND.  The Host gives the IMP a SEND port number, a RECEIVE port
 number, the rendezvous site, and a buffer specification=20 (e.g.,
 start and end, beginning and length).  The SEND is sent to the
 rendezvous site, normally the local site.  When the matching RECEIVE
 arrives, the Host is notified of the RECEIVE port of the just arrived
 receive message.  This port number is sufficient to identify the
 SENDing process although a given time-sharing system may have to keep
 internal tables mapping this port number into useful internal process
 identifiers.  Simultaneously, the IMP will begin to ask the Host for
 specific chunks of the data buffer.  These chunks will be sent off to
 the destination as the IMP's RFNM control allows.  If a RFNM is not
 received for too long, implying a message has been lost in the
 network, the Host is asked for the same chunk of data again [which
 also allows messages to be completely thrown away by the IMP network

Walden [Page 16] RFC 61 Interprocess Communication in a Computer Network July 1970

 if that should ever be useful], but the Host has the option to abort
 the transmission at this time.  While a transmission is taking place,
 the Host may ask the IMP 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 Host.  If a SEND times out, an error is returned
 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.  When chunks of 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 the data in its input buffer.  When the last of
 the SEND buffer is passed into the Host, it is marked accordingly and
 the Host can then detect this.  A second RECEIVE over the same port
 pair is allowed.  A third results in an error message to the Host.
 The mechanism described in this and the previous paragraphs allows a
 pair of processes to always 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 providing complete flow control.  (It is
 conceivable that the RECEIVE message could allocate a piece of
 network bandwidth while making its network traverse to the rendezvous
 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.
 RFNM's are tied to the transmission of a particular chunk of buffer
 just as acknowledgments are now tied to packets and they perform the
 same function.  If the Hosts allow the IMPs to reassemble buffers in
 the Hosts by the IMP telling the Host where it should put a buffer
 chunk as described above, chunks of a single buffer can be
 transmitted in parallel and several RFNMs can be outstanding
 simultaneously.  Packet reassembly is still done in the IMPs.
 A final operation must be provided by the IMP -- the UNIQUE
 operation.  There are many ways to maintain unique numbers and three
 are presented here.  The first possibility is for the Hosts to ask
 the IMPs for the unique numbers originally and then guarantee the

Walden [Page 17] RFC 61 Interprocess Communication in a Computer Network July 1970

 integrity of any unique numbers currently owned by local processes
 and programs using whatever means the Host has at its disposal.  In
 this case the IMPs would provide a method for a unique number to be
 sent from one host to another and would vouch for the number's
 identity at the new site.
 The second method is to simply 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 ports)
 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 time-sharing 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 [3].
 Random Contents.  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.  The IMPs can
 stop a specific host transmission (by not asking for the next chunk
 for a while) if that should seem necessary to alleviate congestion
 problems in the communications subnet.  And 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 it becomes
 imminent that a message for that process will be arriving.
       [ This RFC was put into machine readable form for entry ]
       [ into the online RFC archives by Katsunori Tanaka 4/99 ]

Walden [Page 18]

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