    Network Working Group                                       Bob Bressler
    Request for Comments: 333                           MIT/Dynamic Modeling
    NIC # 9926                                                    Dan Murphy
    Category: C9 (experimentation)                                 BBN/TENEX
    Obsoletes: 62                                                Dave Walden
    Updates: none                                                    BBN/IMP
                                                                 15 May 1972

            A PROPOSED EXPERIMENT WITH A MESSAGE SWITCHING PROTOCOL

    CONTENTS

       Introduction ..................................................  1
       Some Background ...............................................  2
       References ....................................................  3
       MSP Specification .............................................  4
       Issue .........................................................  8
       Message Header ................................................ 10
       Examples ...................................................... 15
       TELNET ........................................................ 16
       The Information Operator ...................................... 16
       Unique Port Numbers ........................................... 20
       Flow Chart .................................................... 23
       MSP Variations ................................................ 25
       Appendix ...................................................... 26

    INTRODUCTION

       A message switching protocol (MSP) is a system whose function is to
       switch messages among its ports.

       For example, there is an implementation of an MSP in each Interface
       Message Processor.  We believe that the effective utilization of
       communications networks by computer operating systems will require a
       better understanding of MSPs.  In particular, we feel that Network
       Control Programs (NCPs), as they have been implemented on the ARPA
       Computer Network (ARPANET), do not adequately emphasize the
       communications aspects of networking -- i.e., they reflect a certain
       reluctance on the part of systems people to move away from what we
       term "the stream orientation".  We propose, as an aside the network
       development using the current NCPs, to rethink the design of NCP-
       level software beginning with a consideration of MSPs.

       The thrust of this note is to sketch how one would organize the
       lowest level host-host protocol in the ARPANET around MSPs and how
       this organization would affect the implementation of host software.

    Bressler, et al.            Experimentation                     [Page 1]

    RFC 333          MESSAGE SWITCHING PROTOCOL EXPERIMENT          May 1972

    SOME BACKGROUND

       Over the past several weeks there has been considerable informal
       discussion about the possibility of implementing, on an experimental
       basis, in several of the ARPA Network Host Computers, NCPs which
       follow a protocol based on the concept of message switching rather
       than the concept of line switching (see the parenthetical sentence in
       the first paragraph of page 6 of NIC document 8246, Host/Host
       Protocol for the ARPA Network).  Party to this discussion have been
       Bob Bressler (MIT/Dynamic Modeling) Steve Crocker (ARPA), Will
       Crowther (BBN/IMP), Tom Knight (MIT/AI), Alex McKenzie (BBN/IMP), Bob
       Metcalfe (MIT/Dynamic Modeling), Dan Murphy (BBN/TENEX), Jon Postel
       (UCLA/NMC), and Dave Walden (BBN/IMP).

       Several interesting points and conclusions have been made during this
       discussion:

          1. Bressler has implemented a message switched interprocess
             communication system for the Dynamic Modeling PDP-10 and has
             extended it so it could be used for interprocess communication
             between processes in the Dynamic Modeling PDP-10 and the AI
             PDP-10.  He reports that it is something like an order of
             magnitude smaller than his NCP.

          2. Murphy has noted that a Host/Host protocol based on message
             switching could be implemented experimentally and run in
             parallel with the real Host/Host protocol using some of the
             links set aside for experimentation.  Further, Murphy has noted
             that if this experimental message switching protocol were
             implemented in TENEX, a number of (TENEX) sites could easily
             participate in the experiment.

          3. It is the consensus of the discussants that Bressler should
             take a crack at specifying a message switching protocol* and
             that if this specification looked relatively easy to implement,
             a serious attempt should be made by Murphy and Bressler to find
             the resources to implement the experimental protocol on the two
             BBN TENEX and the MIT Dynamic Modeling and AI machines.

          4. MSP was chosen as the acronym for Message Switching Protocol,
             and links 192-195 were reserved for use in an MSP experiment.

       -------------
       *This note fulfills any obligation Bressler may have incurred to
       produce an MSP specification.

    Bressler, et al.            Experimentation                     [Page 2]

    RFC 333          MESSAGE SWITCHING PROTOCOL EXPERIMENT          May 1972

       We solicit comments and suggestions from the Network Working Group
       with regard to this experiment.  However, although we will very much
       appreciate comments and suggestions, because this is a limited
       experiment and not an attempt to specify a protocol to supersede the
       present Host/Host protocol for the ARPA Network, we may arbitrarily
       reject suggestions.

    REFERENCES

       Familiarly with the following references will be helpful to the
       reading of the rest of this note.

          1) NIC document 8246, HOST/HOST PROTOCOL FOR THE ARPA NETWORK

          2) NIC document 9348 on the Telnet Protocol

          3) NIC document 7101, OFFICIAL INITIAL CONNECTION PROTOCOL,
             DOCUMENT # 2

          4) a system of interprocess communication in a resource sharing
             computer network, CACM, April, 1972.

       Reference 4 is a revision of RFC 62.  We strongly suggest the reader
       be familiar with reference 4 before he attempts to read the present
       RFC; a reprint of reference 4 is attached as an appendix.

    Bressler, et al.            Experimentation                     [Page 3]

    RFC 333          MESSAGE SWITCHING PROTOCOL EXPERIMENT          May 1972

    MSP SPECIFICATION

       Our MSP is essentially a generalization of the interprocess
       communication system outlined in Section 3 of the fourth reference.
       (Henceforth, if we are required to mention the interprocess
       communication system presented in Section 3 of reference 4, we shall
       call it "the IPC".)  For two processes to communicate using the MSP,
       the process desiring to send must in some sense execute a SEND and
       the process desiring to receive must in some sense execute a RECEIVE.
       The SEND and RECEIVE, in effect, rendezvous somewhere and
       transmission is allowed to take place.  With the RECEIVE are
       specified (among other things) a FROM-TO-PORT-ID, a TO-PORT-ID, and a
       RENDEZVOUS HOST.  With SEND are specified a from-port-id, a to-port-
       id, a rendezvous Host, and (possibly) some data to be transmitted.
       Using SEND and RECEIVE, sending a message from a SENDER PROCESS to a
       RECEIVER PROCESS takes place as follows.  The sender process executes
       a SEND which causes an OUT-MESSAGE plus the specified data to be
       transmitted to the Host specified as the rendezvous Host in the SEND.
       Concurrently (although not necessarily simultaneously)the receiver
       process executes a RECEIVE which causes an IN-MESSAGE to be sent to
       the Host specified as the rendezvous Host in the RECEIVE.  At the
       rendezvous Host, OUT-messages and IN-messages are entered in a table
       called the RENDEZVOUS TABLE.  When an OUT-message and an IN-message
       are detected with matching to-port-id, from-port-id, and rendezvous
       Host, three things are done:  1)  the OUT-message plus the data is
       forwarded to the Host which was the source of the IN-message, 2)  the
       IN-message is forwarded to the Host which was the source of the OUT-
       message, and 3)  the IN-message and OUT-message plus the data are
       deleted from the rendezvous table in the rendezvous Host.

       The process is greatly simplified if the rendezvous Host is also
       either the send Host or receive Host.  Specific algorithms
       enumerating these sequences appear later in this note.

       To clarify the basic concepts, let us look at a case involving three
       Hosts, to which we shall give the names SND, RCV, and RNDZ.  At Host
       SND, process S is doing a send, and at Host RCV, process R is doing a
       receive.  Both specify rendezvous at Host RNDZ.

    Bressler, et al.            Experimentation                     [Page 4]

    RFC 333          MESSAGE SWITCHING PROTOCOL EXPERIMENT          May 1972

    +--------------------+     +----------+     +--------------------+
    |HOST SND            |     |          |     |HOST RCV            |
    |                    |     |          |     |                    |
    |                    |     |          |     |                    |
    |       (PROCESS)    |     +----------+     |                    |
    |       (   S   )    |         HOST         |                    |
    |              \     |         RNDZ         |          (PROCESS) |
    |              [DATA]|                      |          (  R    ) |
    +--------------------+                      +--------------------+

    Process S now executes a SEND with

         from-port-id = S, to-port-id = R, and rendezvous-Host = RNDZ.

    Host SND then creates a table entry in its rendezvous table.

    +-----------------------------------+
    |HOST SND            MSP   _ _ _    |
    |           ------------->|_ _ _|   |
    |         /        ^      |_ _ _| <-|-------RENDEZVOUS
    |        /         |      |_ _ _|   |         TABLE
    |(PROCESS)         |                |
    |(   S   )         +-- SEND (from=S to=R; rend=RNDZ)
    |        \                          |
    |         [DATA]                    |
    +-----------------------------------+

    Host SND now sends an "OUT" message with S's data to Host RNDZ.

      HOST SND                               HOST RNDZ
    +------------+                    +---------------------------+
    |         MSP|  "OUT" + DATA      |MSP  _____  RENDEZVOUS     |
    |            |--------------------|--> |_ _ _| TABLE          |
    |            |  from=S; to=R      | \  |_ _ _|                |
    |            |                    |  \ |_ _ _|                |
    +------------+                    |   \             __        |
                                      |    \---------->|  | DATA  |
                                      |                |__|BUFFER |
                                      |                           |
                                      +---------------------------+

       Concurrently process R at Host RCV executes a RECEIVE with from-
       port-id = S, to-port-id = R, and rendezvous-Host = RNDZ.  As above,
       Host RCV creates a table entry in its rendezvous table and sends an
       "IN" message to Host RNDZ (see following figure).

    Bressler, et al.            Experimentation                     [Page 5]

    RFC 333          MESSAGE SWITCHING PROTOCOL EXPERIMENT          May 1972

       (Don't panic now about buffering in an intermediate Host.  The time
       to panic is afer you've read and understood the rest of our
       arguments.)

         HOST RNDZ                          HOST RCV
    +------------------------+       +-----------------------+
    |                 MSP    |       |  MSP                  |
    |       TABLE    _____   |       |   _____  TABLE        |
    |             +-|_ _ _|  |  "IN" |  |_ _ _|              |
    |             | |_ _ _|<-|----------|_ _ _|<-\           |RECEIVE
    |             | |_ _ _|  |       |  |_ _ _|   \       <--|(from=S
    |             |          |       |             \         |  to=R
    |            _V_         |       |              \        | rend=RNDZ)
    |    BUFFER |   |        |       |             (PROCESS) |
    |           |___|        |       |             (   R   ) |
    +------------------------+       +-----------------------+

       Host RNDZ now notices that the "OUT" from Host SND and the "IN" from
       R at RCV match one another and thus Host RNDZ takes three actions:

          1. Sends an "IN to Host SND (from-port-id = S, to-port-id = R,
             rendezvous-Host = RNDZ).

          2. Sends an "OUT" and the buffered data to Host RCV (from-port-id
             = S, to-port-id = R, rendezvous-Host =RNDZ)

          3. Clears the entry from its table.

       HOST SND                                           HOST RCV
       +------------------+        +------------+         +-------------+
       |                  |        |   TABLE    |         |             |
       |   TABLE  ___     |  "IN"  |    ___     |  "OUT"  |   ___  TABLE|
       |         |___|    |        |   |___|    |  + DATA |  |_ _|      |
       |         |___|<---|--------|---|___|----|---------|->|_ _|      |
       |         |___|    |        |   |___|    |         |  |_ _|      |
       | ( S )            |        +------------+         |        ( R )|
       |                  |          HOST RNDZ            |             |
       +------------------+                               +-------------+

       Host RCV gets the "OUT" and DATA and finds the matching entry in its
       table.  It gives the DATA to process R and clears the entry from its
       table.

       Host SND gets an "IN" which matches an entry in his table and clears
       that entry.  This message serves as a combined acknowledgement and go
       ahead which can be passed along to process S.

       The transmission is now complete.

    Bressler, et al.            Experimentation                     [Page 6]

    RFC 333          MESSAGE SWITCHING PROTOCOL EXPERIMENT          May 1972

       By both, one, or neither of the sender and receiver processes
       specifying a remote rendezvous Host, four important different kinds
       of transmissions can be made to take place.  These are illustrated in
       the following four figures.  In the figures crossed or parallel
       dotted lines are used to indicate rendezvous.  The site of the
       "crossed rendezvous" is the important difference between types of
       transmission illustrated in figures.  Circles indicate processes.
       Rectangles are rendezvous tables.

       The figures also show "(IN)" and "(OUT)" messages being passed into
       the processes.  The parentheses are used to indicate that the "IN"
       and "OUT" are only CONCEPTUALLY passed into the processes.  What
       actually happens is implementation dependent.  The process might be
       awakened and be given no further information if it blocked when
       issuing the SEND or RECEIVE.  The process might be interrupted and
       passed some information such as the to-port-id from the IN or the
       from-port-id of the OUT.  The process might actually be passed the
       complete IN or OUT message.

          ------         _________           ------
         (      )       |         |         (      )
         (      ) SEND  |         | RECEIVE (      )
         (      )------>|--+  +---|<--------(      )
         (      )       |   \/    |         (      )
         (      ) (IN)  |   /\    |  (OUT)  (      )
         (      )<------|--+   +--|-------->(      )
         (______)       |_________| +DATA   (______)

         |<------------- Host K ------------------>|

                   A Rendezvous at the Sender's Host

          ----         _______               ______          ----
         (    )       |       |             |      |        (    )
         (    ) SEND  |       |      IN     |      | RECEIVE(    )
         (    )------>|-+  +--|<------------|------|<-------(    )
         (    )       |  \/   |             |      |        (    )
         (    ) (IN)  |  /\   |  OUT+DATA   |      | (OUT)  (    )
         (    )<------|-+  +--|------------>|------|------->(    )
         (____)       |_______|             |______| +DATA  (____)

         |<---- Host K ------>|<-- Network-->|<----- Host L ----->|

                   A Rendezvous at the Sender's Host

    Bressler, et al.            Experimentation                     [Page 7]

    RFC 333          MESSAGE SWITCHING PROTOCOL EXPERIMENT          May 1972

          ----         ______                _______          ----
         (    )       |      |              |       |        (    )
         (    ) SEND  |      |   OUT+DATA   |       | RECEIVE(    )
         (    )------>|------|------------->|-+  +--|<-------(    )
         (    )       |      |              |  \/   |        (    )
         (    ) (IN)  |      |      IN      |  /\   | (OUT)  (    )
         (    )<------|------|<-------------|-+  +--|------->(    )
         (    )       |      |              |       | +DATA  (    )
         (____)       |______|              |______ |        (____)

         |<---- Host K ----->|<-- Network-->|<----- Host L ----->|

                   A Rendezvous at the Receiver's Host

      ----         ______            _______            ______         ----
     (    )       |      |          |       |          |      |       (    )
     (    ) SEND  |      | OUT+DATA |       |    IN    |      |RECEIVE(    )
     (    )------>|------|--------->|-+  +--|<---------|------|<------(    )
     (    )       |      |          |  \/   |          |      |       (    )
     (    ) (IN)  |      |    IN    |  /\   |OUT+DATA  |      | (OUT) (    )
     (    )<------|------|<---------|-+  +--|--------->|------|------>(    )
     (    )       |      |          |       |          |      | +DATA (    )
     (____)       |______|          |______ |          |______|       (____)

     |<---- Host K ----->|<--Net-->|<-Host->|<--Net-->|<----- Host L ----->|
                                       M

                   A Rendezvous at an Intermediate Host

    ISSUES

    Timeouts.

       The issue of timeouts is a very sticky one.  A coherent system of
       timeouts simplifies everything and does away with races.  However,
       many Hosts are unwilling or unable to use timeouts, especially
       timeouts whose duration is specified.

       Without these timeouts there is probably a need for a negative
       acknowledgment which goes back to the source of an IN or OUT when one
       is timed out.  However, this now leads to races.

       A negative acknowledgment (which we will refer to as a FLUSH message)
       could be employed by a Host to mean:

    Bressler, et al.            Experimentation                     [Page 8]

    RFC 333          MESSAGE SWITCHING PROTOCOL EXPERIMENT          May 1972

          1. I have no room in my table

          2. I have no more available buffer space or

          3. I no longer wish to retain the table entry/buffer.

          In general, we believe that a Host should be allowed to throw away
          an IN or OUT+data whenever it is no longer convenient for the Host
          to hold the messages.  This can be immediately on the arrival of a
          message; for instance, if the Host does not want to buffer traffic
          for which it does not have a user buffer.  In lieu of timeouts,
          any time a process issues a SEND or RECEIVE, it can take it back
          by issuing the matching RECEIVE or SEND.

    Blocking the Process After a Send or Receive.

          This is a question which is left implementation dependent.  In
          general, we do not think it is a good idea to block the process
          after a SEND since it may want to do another to another port or
          even do a RECEIVE.  In fact, we see nothing  inherently wrong with
          a process doing two or more SENDs to the same port as long as the
          communicating processes know what they are doing.  Of course, some
          communicating processes will prohibit several simultaneous
          messages being in transit between the same ports, for instance the
          TELNETs may well prohibit this.  However, for reasons of
          increasing bandwidth, etc., two processes may well want several
          simultaneous messages.  In this case we think it is up to the
          processes to worry about the sequencing of messages; however, we
          refer users desiring their processes to take a care of message
          sequencing to the method used in the IMP/Very Distant Host
          interface which is documented in Appendix F of BBN Report 1822.

    Message Buffering

          A few points are worth mentioning with regard to message
          buffering.  First, most OUTs will probably be accompanied by data.
          Therefore, in general, since the receiver process may be swapped
          out, the receiver Host monitor must be prepared to buffer some
          data somewhere.  To minimize the amount of buffering needed, the
          monitor could refuse further traffic from the IMP until the
          earlier traffic from the IMP has been written on a disk or drum.
          Or the monitor could have a small number of buffers in the monitor
          area of memory which it fills as traffic comes from the IMP, and
          which are swapped with buffers claimed earlier by the receiver
          processes as the receiver processes are swapped in.  Note that the
          buffers may be less than the maximum subnet message size in length
          if the RECEIVEs never specify a longer message length -- of
          course, this can be enforced.  Finally note that the message size,

    Bressler, et al.            Experimentation                     [Page 9]

    RFC 333          MESSAGE SWITCHING PROTOCOL EXPERIMENT          May 1972

          receive-port-id, etc. are available in the first 144 bits which
          come in from the IMP.  It might be useful to read this before
          deciding into which buffer to read the rest of the message.

    Positive Acknowledgments

          Built into the system is a certain form of acknowledgment.  The
          information is always available as to when the receiving process
          has done a RECEIVE.  The sending Host is assured of receiving an
          "IN" when the receive call is issued.

          Further forms of acknowledgment and validation can be implemented
          at the first user level, and advanced protocols will probably
          develop a library of such routines.

    MESSAGE HEADER

          The following section deals with the specific format of Host to
          Host messages and algorithms describing the proper response to a
          given message.

          Each message begins with a 144 bit header containing the following
          fields:

          1. HOST-TO-IMP leader (32 bits) as specified in BBN Reports 1822

          2. to port ID (i.e., the id of the port receiving the message) (24
             bits)

          3. MSG TYPE (8 bits) IN, OUT, FLUSH, etc.

          4. from port ID (i.e., id or the port sending the message) (24
             bits)

          5. initiating Host's table position (8 bits) see below.

          6. HOST "sourcing" this message (8 bits) see below.

          7. RENDEZVOUS HOST (8 bits)

          8. bit count of data (16 bits)

       The header format has been arranged so that no data item will cross a
       word boundary on machines with 16, 32, and 36-bit words, except where
       the size of the item is greater than the word size.  The actual
       arrangement of bytes within words is shown in the following figures
       for these three word sizes.  For the benefit of 36-bit Hosts, bytes 4
       and 13 (numbering from 0) are unused.  The 2 and 3-byte items do not

    Bressler, et al.            Experimentation                    [Page 10]

    RFC 333          MESSAGE SWITCHING PROTOCOL EXPERIMENT          May 1972

       cross word boundaries except for the port ID's on the 16 bit
       machines.  This attention to packing and unpacking ease was given
       both for general convenience, and in particular because Hosts may
       wish to examine the header at interrupt level to determine where the
       rest of the message should go.

       +-------------+-------------+
    0  |  HOST/IMP   | DESTINATION |
       |   FLAGS     |             |
       +-------------+-------------+
    1  |   LINK      | /////////// |
       |             | /////////// |
       +-------------+-------------+
    2  | /////////// |             |
       | /////////// |             |
       +-------------+             |
    3  |        TO PORT ID         |
       |                           |
       +-------------+-------------+
    4  |  MESSAGE    |             |
       |   TYPE      |             |
       +-------------+             |
    5  |        FROM PORT ID       |
       |                           |
       +-------------+-------------+
    6  |  TABLE      | /////////// |
       |  POSITION   | /////////// |
       +-------------+-------------+
    7  |  SOURCE     | RENDEZVOUS  |
       |   HOST      |   HOST      |
       +-------------+-------------+
    8  |        BIT COUNT          |
       |                           |
       +-------------+-------------+
       |                           |
    9  |           DATA            |
       //                         //
       |                           |
       +-------------+-------------+

             16-bit Host Format

       +-------------+
       |             |            ////////// = unused
       |             |            //////////
       +-------------+
           8 bits

    Bressler, et al.            Experimentation                    [Page 11]

    RFC 333          MESSAGE SWITCHING PROTOCOL EXPERIMENT          May 1972

       0             8            16            24            32     36
       +-------------+-------------+-------------+-------------+------+
    0  | HOST/IMP    |   FOREIGN   |    LINK     | ////////////////// |
       |  FLAGS      |   HOST      |             | ////////////////// |
       +------+------+-------------+-------------+-------+-----+------+
    1  | //// |        TO PORT ID                        |  MESSAGE   |
       | //// |                                          |   TYPE     |
       +------+------+-------------+-------------+-------------+------+
    2  |               FROM PORT ID              |   TABLE     | //// |
       |                                         |   POSITION  | //// |
       +------+-------------+-------------+------+-------------+------+
    3  | //// |   SOURCE    | RENDEZVOUS  |          BIT COUNT        |
       | //// |    HOST     |  HOST       |                           |
       +------+-------------+-------------+---------------------------+
       |                                                              |
    4  |                                                              |
       //                          DATA                              //
       |                                                              |
       |                                                              |
       +-------------+-------------+-------------+-------------+------+

                             36-bit Host Format

       +-------------+-------------+-------------+-------------+
    0  | HOST/IMP    |   FOREIGN   |    LINK     | /////////// |
       |  FLAGS      |   HOST      |             | /////////// |
       +-------------+-------------+-------------+-------------+
    1  | /////////// |             TO PORT ID                  |
       |             |                                         |
       +-------------+-------------+-------------+-------------+
    2  |  MESSAGE    |             FROM PORT ID                |
       |   TYPE      |                                         |
       +-------------+-------------+-------------+-------------+
    3  |  TABLE      | /////////// |  SOURCE     | RENDEZVOUS  |
       |  POSITION   | /////////// |   HOST      |   HOST      |
       +-------------+-------------+-------------+-------------+
       |        BIT COUNT          |                           |
       |                           |                           |
       +-------------+-------------+                           |
       |                                                       |
       //                   DATA                              //
       |                                                       |
       +-------------+-------------+-------------+-------------+

                             32-bit Host Format

    Bressler, et al.            Experimentation                    [Page 12]

    RFC 333          MESSAGE SWITCHING PROTOCOL EXPERIMENT          May 1972

       The fields within the Host/IMP leader are already familiar to NCP
       programmers however, two points about these fields are worth
       mentioning.  First, the destination field originally contains the
       number of the rendezvous Host.  After rendezvous at a intermediate
       site, the destination field contains the source of the message
       rendezvous with.  Second, the link field for the MSP experiment can
       only contain link number 192-195.  We have not taken the time to
       figure out a sensible allocation of these four links among all the
       messages which might be sent using the MSP.  One alternative is to
       cycle over the links to increase the bandwidth of the "pipe" between
       any two Hosts. For the time being, until further consideration is
       given to this issue, we suggest each Host at a site using one
       (unique) link for all its communication.

       The message types we have to represent in the message type field are
       few now: we suggest message type 2 for SEND or OUT messages and
       message 3 for RECEIVE or IN messages.  Message type 4 is the FLUSH
       message, if FLUSH is used.

       The rendezvous Host field needs no comment.  Except that the field is
       unnecessary after the rendezvous has taken place and could then be
       used for something else.

       The bit count is a count of data bits in an OUT message or the size
       of the input buffer (not including the header) in an IN message.
       Thus the sender process can tell from the IN message bit count when
       it receives the IN message how much of the data in the OUT message
       was accepted by the receiver process and can use this knowledge to
       retransmit the remainder of the message if so desired.  After the
       rendezvous, we recommend that all of the data in the message be sent
       on the source of the IN message even if the OUT bit count was greater
       than the IN bit count.  Thus, at the receiver Host the monitor has
       the option (if it wants to take it) of discarding the message for
       being too long, sending the number of bits the receiver process has
       done an IN for into the receiver process and discarding the rest, or
       queuing the rest of the bits and somehow notify the receiver process
       that there are more bits which the receiver process can ask for.

       The to- and from-port-id fields are 24-bit numbers.  This size was
       chosen to help the TIPs.  The first eight bits of a port Id should be
       the number of the Host at which this port id was created.  Note well,
       that this is not necessarily the Host at which the port is being
       used.  This is necessary since rendezvous take place at intermediate
       sites and because ports may move from site to site.  We suggest that
       all port ids with the first eight bits all zero be reserved for
       network-wide use.  In particular, a port id with all 24 bits zero
       will be used to mean "ANY".  This gives us the options of:

    Bressler, et al.            Experimentation                    [Page 13]

    RFC 333          MESSAGE SWITCHING PROTOCOL EXPERIMENT          May 1972

                RECEIVE from ANY to SPECIFIC

                RECEIVE from SPECIFIC to SPECIFIC

                SEND from SPECIFIC to ANY

           and  SEND from SPECIFIC to SPECIFIC

       Examples of the use of these options will be given below.

       The other options (RECEIVE to ANY) and (SEND from ANY) we feel are
       kind of useless but would not prohibit them.  We believe that in the
       absence of explicit specification of rendezvous Host, the use of an
       ANY port id in the user's system call should affect the default
       rendezvous site as follows:

          RECEIVE from ANY--rendezvous in receiver

          RECEIVE from SPECIFIC--rendezvous in sender

          SEND to ANY--rendezvous in sender

          SEND to SPECIFIC--rendezvous in sender

       The less significant 16 bits of the id can be used however a Host
       wants to.  For instance, eight bits might be used as a process id and
       eight bits might be used as a channel specification within the
       specified process.  We suggest that each Host reserve the port ids
       with the middle eight bits all zero for special uses as well known
       ports.

       The table position field is included to help prevent costly table
       searches at interrupt level.  Hosts sending INs and OUTs, put in the
       table position field the rendezvous table position of the SEND or
       RECEIVE associated with the IN or OUT.  At an intermediate Host
       rendezvous, the table position fields in the matching IN and OUT are
       swapped so that when the messages arrive at the opposite end, the
       matching SEND and RECEIVE can be found quickly.  The MSP must do the
       swap at the rendezvous, but of course the MSPs need not fill in the
       table position field when first transmitting an IN or OUT in which
       case the information arriving in an IN or OUT will be meaningless.
       The general algorithm, then, is to check the table position as
       specified in this field and if that fails, search the whole table.

       The source field is filled in INs and OUTs by the MSP which
       originally sends these messages.  At the rendezvous the source of
       each message is preserved in the message being forwarded to the final
       Host.  When an IN or OUT arrives at a process, the process can use

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    RFC 333          MESSAGE SWITCHING PROTOCOL EXPERIMENT          May 1972

       the source information to update its understanding of the rendezvous
       Host (e.g., when the destination Host and rendezvous Host are
       different).

    EXAMPLES

    The typical example.

       We envision communication normally taking place using specifications
       to and from ports and rendezvous at the sender.  For instance, the
       TIP would probably send to other Hosts using this method and would
       certainly receive from other Host until the TIP asks for it.  In this
       "normal" method a monitor could even look at the bit count in the
       arriving IN-message, use that as an allocation and then simulate an
       OUT-message of the exact correct length.

    The logging example

       Consider an example of SEND to SPECIFIC and RECEIVE from ANY with the
       rendezvous at the receiver.  This method might be used by some
       logging receiver process with a well-known to-port.  For instance, a
       measurements program to which statistics are sent from many processes
       throughout the net.

    The program library example

       Suppose within a given time-sharing system there is a particular
       library routine which is available for use by any process in the
       network.  The library process has a RECEIVE from ANY always pending
       at a well-known port.  Eventually, some process sends a message to
       the library process' well-known-port.  This message includes the data
       to be processed, a port to use for sending the answer, and the money.
       The library process takes some of the money and sends it to the
       well-known port of the accounting process which itself has a RECEIVE
       from ANY pending.  The library process then processes the data and
       sends the answer back to the process which requested the service
       using a SEND to SPECIFIC message which rendezvous at the destination
       where there is already a RECEIVE from SPECIFIC pending.  Of course,
       in this message besides the answer, any change the requesting process
       has coming is returned.

    A comment

       As can be seen from our examples, we think rendezvousing at an
       intermediate Host will seldom be done as the chief benefit of this
       comes when it is desirable to move a port (see reference 4 for a
       discussion of this).  We would like to see all Hosts provide some

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       (meager) amount of buffering for this purpose but would not require
       it.  It shouldn't be too painful to provide a little of this kind of
       buffering-especially since a Host can throw away any message it can't
       handle.

       (THIS PAGE WILL BE REPLACED WITH A BETTER DESCRIPTION OF TELNET UNDER
       MSP IN A FEW DAYS--DCW)

    TELNET

       Let us postulate a pair of Telnet programs that maintain two
       bidirectional communication paths, one for data and one for control.
       Let us also assume, for convenience that the port IDs are as follows:

          If the WRITE-CONTROL-ID is N, then --

             READ-CONTROL-ID=N+1,

             WRITE-DATA=N+2,

             READ-DATA=N+3.

       The initial state is the server Telnet sitting with a READ-FROM-ANY
       pending.

       The user Telnet now issues a SEND-TO-SPECIFIC with the data field
       containing the PORT-ID of the SERVER's WRITE-CONTROL-ID. This message
       is sent from the user-Telnet's WRITE-CONTROL-ID.

       Thus all port IDs are specified by the user Telnet, so, if desired,
       he need only remember one number and derive the rest.  Uniqueness is
       preserved since the port IDs supplied by the user Telnet contain his
       Host ID and other information making the ID unique to him.

       Now that these communication paths are established, the two processes
       can exchange data and control information according to established
       Telnet protocols.

    THE INFORMATION OPERATOR

       The Message Switching Protocol itself impose no fixed requirements on
       the use of the port ID's, and the problem of process identification
       is somewhat separated from the means used to effect communication.
       It is, however, very much a part of the overall issue of interprocess
       communication, and so we here specify a facility for handling process
       identification, the information operator.

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       One goal in a process identification scheme is to provide a means by
       which processes can select their own identifiers which can be
       guaranteed unique and can contain information meaningful to the user.
       Problems of efficiency prevent making the port ID's themselves large
       enough to accomplish this aim.  Efficiency questions aside, it would
       appear to be ideal to allow processes to use character strings of
       arbitrary length to identify themselves.  Uniqueness can then be
       easily ensured if, for example, users follow the convention of
       including their names in the process identification string.  Further,
       the remainder of the name can be chosen to have some meaning related
       to its use with obvious advantages and convenience for users.

       One solution is to establish a convention whereby the symbolic
       identifiers are used only during some initial phase of communication
       and not in every message.  That is, processes identify each other
       initially using symbolic identifiers, but exchange local port
       identifiers at the same time which are used for all ensuing messages.

       The means of providing this facility is to establish a process at
       each of a number of Hosts (e.g., all server Hosts) called the
       "information operator".  The function of this process is to associate
       symbolic identification strings and port ID's.  A process can
       identify itself and/or a foreign process to the information operator,
       and may request the port ID of the foreign process.  The symbolic
       identification strings are chosen by the processes and are long
       enough to contain meaningful information, e.g., LOGGER, MURPHY-
       TESTPROG.

       Communication with the information operator, whether by local or
       remote processes, is via the regular MSP functions.  The information
       operator will always have a RECEIVE ANY outstanding on a well-known
       port.  This could in general be the only well-known port in
       existence.  A message received on this port contains the following
       parameters:

          1. String identifying the foreign process with which communication
             is desired.

          2. String identifying the calling process.

          3. Calling process' port number.

          4. A delay specification.

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       The format of these parameters is shown in Fig. 4.  In some cases,
       one or more of the arguments would be null.  Following receipt of a
       message, the information operator will, in some cases, do a SEND
       SPECIFIC to the calling process' port number providing the desired
       information or notice of failure.

       The following two cases would appear to cover all functions of the
       information operator.  They correspond to the SEND/RECEIVE SPECIFIC
       ANY cases of the MSP.

       1. Two processes each knowing the specific identify of the other wish
          to communicate.  Each does a SEND SPECIFIC to the information
          operator, giving parameters 1-2, the default delay spec in this
          case being WAIT.  When the information operator receives the
          second of these and notes that a match exists, it sends to each
          process the port ID of the other process and deletes both strings
          and both port ID's from its tables.  The two processes, which have
          each done a RECEIVE SPECIFIC in anticipation of the foreign port
          number, can then communicate using just the port numbers and basic
          MSP functions.

       2. A process is set up to provide some sort of general service or
          information, and its name and protocol advertised.  This process
          intends to maintain an outstanding SEND or RECEIVE ANY for the
          first (and perhaps only) message transaction, e.g., the library
          process discussed earlier.  Most such processes would be receivers
          initially, but there might be a few cases where a SEND could be
          left outstanding, and a forcing process could come along and pick
          up the information.  In either case, the service process will do
          SEND SPECIFIC to the information operator giving the local
          symbolic ID and local port ID.  The foreign symbolic ID would be
          null, and the default delay spec is NO-WAIT.  That is,

             INFO ( -, local ID, local port)

          The information operator will enter this information in its tables
          but return nothing to the caller.  The caller would proceed to do
          its SEND/RECEIVE ANY to wait for business.  When another process
          wishes to use the advertised service, it asks the logger for the
          port ID of the service process, i.e.,

             INFO (service ID, -, local port)

          The local symbolic ID need not be specified, and the default delay
          spec is NO-WAIT.  The information operator would SEND the port ID
          of the service process to the local port of the caller, and retain
          the table entry for future callers.  Only the service process

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          could request the entry be deleted.  If the service ID was unknown
          to the information operator at the time of this call, it would
          immediately return a failure indication, i.e., zero.

       Communicating processes would normally use the information operator
       local to one or the other, and like the rendezvous Host in the MSP,
       this would be agreed upon in advance.  Service processes would
       normally use the information operator at their local site, and
       correspondingly, user processes would call the information operator
       at the site where the service process was expected to be available.
       There is no restriction on using an information operator at some
       other site of course, and some small and/or lazy servers could use a
       different Host for their service process ID's.  It presents no
       problem for two or more information operators to have entries for the
       same service process, and in fact, this may be very desirable for
       special types of service processes which exist only one place on the
       net and may move around from time to time.

       Processes would specify their own local port numbers, and each system
       would have to provide some way to help user processes do this.  In
       TENEX for example, one would probably use the job number concatenated
       with another number assigned within the job.  The information
       operator cannot supply port numbers because it will be running on a
       different Host than one or both of the communicants and cannot know
       what is a unique number for that Host.  In some cases, processes
       would ask the "unique number process" (described below) for their
       local port ID, and would make it known via the information operator.

       In actual practice, a few exceptions would be made to the rule that
       the only "well-known" port in the world is the information operator.
       Such exceptions would be processes common to many Hosts, e.g.,
       LOGGER, or those in particularly frequent use.  In such cases the
       unique port numbers would be assigned by administrative fiat and
       recorded and published to all users.

       The symbolic identification strings are specified to be from 1 to 39
       (an arbitrary maximum) ASCII characters terminated by a null (byte of
       all zeroes).  The characters will be 7-bit ASCII in 8-bit bytes with
       the high order bit set to zero.  A null string (first byte is null)
       is used where no argument is required.

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    Format of Information Operator Messages

    To Information Operator: A stream of 8-bit bytes.

    +------+--//---+------+------+--//---+------+------+-------+-------+
    |char 0| 1// n | null |char 0| 1// n | null | port | number| delay |
    |      |  //   |      |      |  //   |      |      |       |spec   |
    +------+--//---+------+------+--//---+------+------+-------+-------+
     \                   /\                     /\             /\      /
      \_________________/  \___________________/  \___________/  \____/
          PARAMETER 1         PARAMETER 2           PARAMETER 3  PARAMETER
                                                                 4
       Parameters given:

          1. String identifying the foreign process with which communication
             is desired. (1 to 39 characters, or null)

          2. String identifying the calling process. (1 to 39 characters, or
             null)

          3. Calling process' port number.

          4. Delay specification:

                0=default
                1=wait for match
                2=don't wait for match

    From Information Operator: 3 8-bit bytes.

       +--------|-------|-------+
       | byte 0 |   1   |   2   |
       +--------|-------|-------+

       Port number (24 bits) of requested foreign port if successful, 0 if
       unsuccessful.

    UNIQUE PORT NUMBERS

       The existence of unique port numbers is essential to the operation of
       the MSP.  For instance, when two communicating processes specify
       message rendezvous at an intermediate site, the processes must be
       able to specify to- and from-ports which are not being used by other
       processes which have specified message rendezvous at the same site or
       else messages may be delivered to incorrect destinations.  We have
       alluded to a method of providing unique port numbers earlier in this
       note.  This method is to partition the 24-bit port number space into
       disjointed segments and give one segment to each Host in the network

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       to distribute when it is called upon to "create" a unique port id.
       Thus each 24-bit Host number will consist of two major parts.  The
       first 8 bits will be the number of the Host "creating" the port id
       and the next 16 bits can be used in any manner the creating Host
       desires.  This gives each Host 2^16 port numbers to distribute, and
       each Host will have the burden of distributing its segment of the
       port number space in a unique manner.  We recommend the convention
       that the port numbers with the middle 8 bits equal to zero be
       reserved for well-known ports in the creating Host's system.  We
       already recommend in an earlier section that port numbers with the
       first 8 bits equal to zero be reserved for network-wide use and in
       particular the port number with all 24 bits equal to zero be used to
       mean ANY.

       Since each Host only has 2-16- port numbers to distribute, in general
       port numbers will not be able to be held and used by processes for
       long periods of time (e.g., weeks and months).  More typically, Hosts
       will probably  implicitly "take back' all port numbers the Host has
       distributed each time the Host's system goes down and will
       redistribute the port numbers as required when the system comes back
       up.  In other words, port numbers will not in general remain unique
       over the going down of the creating Hosts.  Of course, a given Host
       may see to give the same port numbers to a number of standard
       processes (such as the FORTRAN compiler) each time it comes up port
       numbers registered with an information operator will frequently
       remain constant over system ups and downs.

       In spite of the fact that each Host will probably not in general be
       able to distribute port numbers to arbitrary user processes which ca
       be guaranteed to remain unique over a long period of time, there will
       still be demand for provision of long-term unique port numbers.  To
       some, the procedure of going through the information operator smacks
       much too much of making a connection.  These people will insist that
       for a variety of reasons their processes be allowed to communicate
       via ports whose identifiers remain constant for long periods of time.
       Therefore, it would be nice if at one or two places in the network, a
       long-term unique number service was provided.  We'll call a process
       providing this service the Unique Number Process.  The Unique Number
       Process would have assigned to it one segment of the unique port
       number space-all those port numbers, for instance, with the first 8-
       bits equal to 377-8.  This process would have a SEND-to-ANY pending
       from a well-known port with local rendezvous specified.  When any
       process wanted a unique number which it could depend on not to be
       used for all time or until the number is given back, it would send a
       RECEIVE-from-SPECIFIC specifying the well-known port of the Unique
       Number Process and rendezvous at the Unique Number Process' Host.
       The Unique Number Process' pending SEND-to-ANY would contain a unique
       number.  Also, the Unique Number Process would have a RECEIVE-from-

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       ANY always pending at another well-known port with local rendezvous
       specified.  At this port the Unique Number Process would receive
       unique numbers which processes are giving back.  The Unique Number
       Process would maintain a bit table 2-16- bits long indicating the
       state of each of its unique numbers (free or in use) in some long-
       term storage medium such as in the file system.  The Unique Number
       Process might also maintain some information about each process to
       which it gives a unique number so that when the supply of unique
       number gets depleted, processes can be asked to return them.

       It has already been mentioned that some of the process ID's
       registered along with their symbolic names at the information
       operator might be long-term unique numbers gotten from the Unique
       Number Process.  It should also be mentioned that there would seem to
       be no reason, other than scarcity of storage space, that in addition
       to the port number through which primary access is gained to a
       process and which was called the process ID in the previous section,
       arbitrary port numbers along with their symbolic identified could not
       be registered with an information operator.  For instance, rather
       than registering the name BBN-FORTRAN and a single port number, one
       could perhaps register the port numbers whose symbolic identifiers
       were BBN-FORTRAN-CONTROL-TELETYPE, BBN-FORTRAN-INPUT-FILE, BBN-
       FORTRAN-LISTING-FILE, and BBN-FORTRAN-BINARY-OUTPUT-FILE.  This is
       perhaps at odds with standard practice within operating systems, but
       is consistent with the philosophy of reference 4 that communication
       is done with ports and not processes.

       Let us now address an issue which has been ignored up to now and
       which was only alluded to in reference 4, the issue of port
       protection.  We have not given this matter a great deal of thought;
       however, one mechanism for port protection seems quite
       straightforward.  The heart of this mechanism is a process at each
       Host which we shall call (alliteratively) the Port Protection Process
       (PPP).  The PPP maintains a list of all processes which exist at the
       Host and for each process the numbers of all ports which the process
       has "legally" obtained.  Every time a process does a SEND or RECEIVE,
       the monitor checks with the PPP to see if the process has specified
       port numbers it has the right to use; i.e., those legally obtained.
       The PPP has some RECEIVEs always pending at well-known ports.  When
       one process wants to pass a port to some other process, the first
       process sends a message to the PPP specifying the number of the port
       to be sent, the Host number at which the second process resides, a
       port at which the second process is expecting to receive the port,
       etc.  The PPP looks up in its tables whether the first process has
       the port it wants to send.  If it does, it sends a message to the PPP
       at the destination site.  The message contains the number of the port
       to be transferred and the RECEIVE port for the destination process.
       The destination PPP checks in its table whether the process has the

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       RECEIVE port, and if so, passes the new port to the process and
       updates its tables to indicate the process now possesses the new
       port.  The messages to a PPP will optionally be able to specify that
       a copy of a port be sent, a port be deleted, etc.  The PPPs would
       probably have some built-in legal ports for each process,
       particularly the port's processes used to communicate with the PPP.
       The exact specification requires development but that should not be
       hard (see (3),(6), and (7) in reference 4).  The main difficulty we
       see is efficient checking of the PPP's tables by the monitor for
       every RECEIVE or SEND without entirely supplanting the monitor's
       current protection system.

    FLOW CHART

       The following section describes a flow chart for most of the MSP.  A
       distinction is made between calls made by local processes called SEND
       and RECEIVE, and messages coming in over the NET called IN and OUT.
       An additional distinction is made between calls (or messages) with a
       local rendezvous and those with a foreign rendezvous Host.

       Since the code is quite similar, the distinction need not be made,
       but will be included for the sake of clarity.

       It is assumed that the MSP has table provisions for the following
       items:

          source of message
          rendezvous Host
          FROM-PORT-ID
          TO-PORT-ID
          table position
          type of message
          data size and location
          data about the user process

       User does a SEND or RECEIVE

       A. Rendezvous is at a foreign host

          1. Store the appropriate table data

          2. Send a message to the rendezvous host

             a. SEND: OUT + DATA

             b. RECEIVE: IN

       B. Rendezvous is local - look for entry in table

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          1. Entry NOT found: create entry with appropriate data

          2. A matching entry exists in table:

             a. RECEIVE: give user the data

             b. Send a message to the other host (as specified by the source
                field of the original msg)

                1)SEND: OUT+DATA
                2)RECEIVE: IN

             c. Alert user to the fact that transaction is complete

             d. Clear table entry

       An IN is received over the NET-search table for matching entry.

       A. No matching entry create an entry with appropriate data.

       B. A match exists

          1. Entry was cause by a local SEND

             a. Send "OUT _ DATA" to source of IN

             b. Inform user of transaction

             c. Clear table entry

          2. Entry was caused by an OUT received over net-acting as third
             host.

             a. Send IN to site that created table entry

             b. Send OUT + DATA (previously buffered) to site sending the IN

             c. Clear table entry

       An OUT + DATA is received over the NET -search table for matching
       entry

       A. No match is found

          1. buffer data

          2. create appropriate table information

    Bressler, et al.            Experimentation                    [Page 24]

    RFC 333          MESSAGE SWITCHING PROTOCOL EXPERIMENT          May 1972

       B. A match is found

          1. Table entry was caused by locally executed RECEIVE

             a. give data to the user and alert him to its existence.

             b. send a matching "IN" to the source of the "OUT"

             c. remove entry from table

          2. Table entry was caused by the receipt of an "IN" over the NET,
             thus we are acting as a third party host

             a. send the "OUT + DATA" to the host stored in the table

             b. send an "IN" to the host from which the "OUT" had just
             arrived.

    MSP VARIATIONS

       It may of interest to the reader to know of some of the other MSPs we
       have considered while arriving at the present one.

       The simplest we considered is an MSP based on all rendezvous being
       done at the destination Host.  The sender process sends an OUT-
       message plus the data to the destination Host.  The receiver process
       does an IN which stays at the receivers Host.  The OUT and RECEIVE
       rendezvous and the data is passed to the receiver process.  The
       transmission is now complete, except in some variations of this MSP
       an acknowledgement is sent to the sender process.  This MSP has
       couple of disadvantages: In the simplest formulation, the RECEIVE had
       to be waiting when the OUT+data arrived, otherwise the out data were
       thrown away.  This puts too tight a constraint on the timing of the
       SEND and RECEIVE, especially since the sender and receiver processes
       can be a continent apart.  However, if the IN is allowed to arrive
       first and must be held until matched by a RECEIVE, the monitor must
       buffer an indeterminate amount of data in all cases including the
       normal one.  Further, basing everything on rendezvous at the
       destination makes the process of moving a port difficult.

       The next simplest MSP we considered was the IPC of reference 4.  This
       works just the opposite of the above described MSP in that it is
       based on almost all rendezvous being done at the source Host with two
       special messages to handle the relatively uncommon cases when a
       rendezvous must be done at the destination or an intermediate Host.
       This system, its advantages, and disadvantages is discussed at very
       great length in the reference.

    Bressler, et al.            Experimentation                    [Page 25]

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       A third variation on the MSP, suggested by Crowther, is the same as
       the present MSP in that the OUT and IN rendezvous at a process
       specified rendezvous Host and the OUT is sent to the source of the IN
       and the IN to the source of the OUT, but the data is not sent along
       with the OUT.  Instead, when the OUT finally reaches the source of
       the IN, another message is sent from the receiver Host to the source
       Host requesting the data to be sent.  The data finally is transmitted
       to the destination in response to this data request message.  Our
       main objection to this system is its lack of symmetry, but we do
       recognize that it does not require any Host to buffer data for which
       a process has not set up an input buffer and perhaps for that reason
       it is a better system than the MSP we are presenting.

       In the last MSP variation we considered, the difference between SEND
       or RECEIVE and OUT or IN was discarded.  In this case only one
       message is used which we will call TRANSFER.  When a process executes
       a TRANSFER it can specify an input buffer, an output buffer, both, or
       neither.  Two processes wishing to communicate both execute TRANSFERs
       specifying the same to and from port ids and the same rendezvous
       Host.  The TRANSFERs result in TRANSFER-messages plus data in the
       case that an output buffer was specified which rendezvous at the
       rendezvous Host.  When the rendezvous occurs, the TRANSFER-messages
       plus their data cross and each is sent to the source of the other.
       The system allows processes not to know whether they must do a SEND,
       or RECEIVE and is (perhaps) a nice generalization of the MSP
       presented in this note.  For instance, two processes can exchange
       data using this system, or two processes can kind of interrupt each
       other by sending dataless TRANSFERs.  This variation of the MSP is a
       development of a suggestion of Steve Crocker.  Its disadvantages are:
       (1) unintentional matches are more likely to occur, (2) rendezvous
       selection site is more complex, and (3) it's hard to think about.

    APPENDIX

       A system for Interprocess Communication in a Resource Sharing
       Computer Network.  Communications of the ACM, April, 1972.
       Permission to reprint this paper was granted by permission of the
       Association for Computing Machinery. [Omitted in republished version
       of RFC 333.]

       N.B. The ideas of section 4 of the following paper are in no way
       critical to the ideas developed in section 3--DCW.

             [ This RFC was put into machine readable form for entry ]
                [ into the online RFC archives by Via Genie 3/00  ]

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