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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


 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


 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


 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
    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.
  1. ————
  • 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.


 Familiarly with the following references will be helpful to the
 reading of the rest of this note.
    2) NIC document 9348 on the Telnet 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


 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

+——————–+ +———-+ +——————–+

(PROCESS) +———-+
( S ) HOST
[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 now sends an "OUT" message with S's data to Host RNDZ.

HOST SND                               HOST RNDZ

+————+ +—————————+

——————––> _ _ _ 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
   HOST RNDZ                          HOST RCV

+————————+ +———————–+

+-_ _ _ "IN" _ _ _
_ _ _———-_ _ _←\
_ _ _ _ _ _ \ ←-
_V_ \
_| | | ( 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

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.
  1. —– _ ——

( ) | | ( )

   (      ) SEND  |         | RECEIVE (      )
   (      )------>|--+  +---|<--------(      )
   (      )       |   \/    |         (      )
   (      ) (IN)  |   /\    |  (OUT)  (      )
   (      )<------|--+   +--|-------->(      )
   (______)       |_________| +DATA   (______)
   |<------------- Host K ------------------>|
             A Rendezvous at the Sender's Host
  1. _ —-

( ) | | | | ( )

   (    ) 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

  1. _ —-

( ) | | | | ( )

   (    ) SEND  |      |   OUT+DATA   |       | RECEIVE(    )
   (    )------>|------|------------->|-+  +--|<-------(    )
   (    )       |      |              |  \/   |        (    )
   (    ) (IN)  |      |      IN      |  /\   | (OUT)  (    )
   (    )<------|------|<-------------|-+  +--|------->(    )
   (    )       |      |              |       | +DATA  (    )
   (____)       |______|              |______ |        (____)
   |<---- Host K ----->|<-- Network-->|<----- Host L ----->|
             A Rendezvous at the Receiver's Host
  1. _ —- ( ) | | | | | | ( ) ( ) SEND | | OUT+DATA | | IN | |RECEIVE( ) ( )——>|——|———>|-+ +–|←——–|——|←—–( ) ( ) | | | \/ | | | ( ) ( ) (IN) | | IN | /\ |OUT+DATA | | (OUT) ( ) ( )←—–|——|←——–|-+ +–|———>|——|——>( ) ( ) | | | | | | +DATA ( ) () || | | || ()

|←— Host K —–>|←-Net–>|←Host→|←-Net–>|←—- Host L —–>|

             A Rendezvous at an Intermediate Host



 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.


    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
    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
    3. MSG TYPE (8 bits) IN, OUT, FLUSH, etc.
    4. from port ID (i.e., id or the port sending the message) (24
    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.


 |   FLAGS     |             |

1 | LINK | / | | | / |


2 | / | | | / | |

 +-------------+             |

3 | TO PORT ID |

 |                           |

4 | MESSAGE | |

 |   TYPE      |             |
 +-------------+             |


 |                           |

6 | TABLE | / | | POSITION | / |



 |   HOST      |   HOST      |


 |                           |
 |                           |

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 | | |



 | //// |                                          |   TYPE     |


 |                                         |   POSITION  | //// |


 | //// |    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
          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
 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

Bressler, et al. Experimentation [Page 14] 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


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

Bressler, et al. Experimentation [Page 15] RFC 333 MESSAGE SWITCHING PROTOCOL EXPERIMENT May 1972

 (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


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

Bressler, et al. Experimentation [Page 16] RFC 333 MESSAGE SWITCHING PROTOCOL EXPERIMENT May 1972

 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-
 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
    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.

Bressler, et al. Experimentation [Page 17] RFC 333 MESSAGE SWITCHING PROTOCOL EXPERIMENT May 1972

 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

Bressler, et al. Experimentation [Page 18] RFC 333 MESSAGE SWITCHING PROTOCOL EXPERIMENT May 1972

    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.

Bressler, et al. Experimentation [Page 19] RFC 333 MESSAGE SWITCHING PROTOCOL EXPERIMENT May 1972

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

+——+–—+——+——+–—+——+——+——-+——-+ \ /\ /\ /\ /

\_________________/  \___________________/  \___________/  \____/
 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
    3. Calling process' port number.
    4. Delay specification:
          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


 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

Bressler, et al. Experimentation [Page 20] RFC 333 MESSAGE SWITCHING PROTOCOL EXPERIMENT May 1972

 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-

Bressler, et al. Experimentation [Page 21] RFC 333 MESSAGE SWITCHING PROTOCOL EXPERIMENT May 1972

 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
 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

Bressler, et al. Experimentation [Page 22] RFC 333 MESSAGE SWITCHING PROTOCOL EXPERIMENT May 1972

 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.


 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
    source of message
    rendezvous Host
    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

Bressler, et al. Experimentation [Page 23] RFC 333 MESSAGE SWITCHING PROTOCOL EXPERIMENT May 1972

    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
       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
 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


 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] RFC 333 MESSAGE SWITCHING PROTOCOL EXPERIMENT May 1972

 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.


 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  ]

Bressler, et al. Experimentation [Page 26]

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