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RFC 55

Network Working Group                                          J. Newkirk
Request for Comments: 55                                        M. Kraley
                                                                J. Postel
                                                               S. Crocker
                                                             19 June 1970

                A Prototypical Implementation of the NCP

   While involved in attempting to specify the formal protocol, we also
   attempted to formulate a prototypical NCP in an Algol-like language.
   After some weeks of concentrated effort, the project was abandoned as
   we realized that the code was becoming unreadable.  We still,
   however, felt the need to demonstrate our conception of how an NCP
   might be implemented; we felt that this would help suggest solutions
   for problems that might arise in trying to mold the formal
   specifications into an existing system.  This document is that
   attempt to specify in a prose format what an NCP could look like.

   There are obvious limitations on a project of this nature.  We do
   not, and cannot, know all of the quirks of the various systems that
   must write an NCP.  We are forced to make some assumptions about the
   environment, system calls, and the like.  We have tried to be as
   general as possible, but no doubt many sites will have completely
   different ways of conceptualizing the NCP.  There is great difficulty
   involved in conveying our concepts and the mechanisms that deal with
   these concepts to people who have wholly different ways of looking at
   things.  We have, however, benefited greatly by trying to actually
   code this program for our fictitious machine.  Many unforeseen
   problems surfaced during the coding, and we hope that by issuing this
   document we can help to alleviate similar problems which may arise in
   individual cases.

   There is, of course, absolutely no requirement to implement anything
   which is contained in this document.  The only rigid rules which an
   NCP _must_ conform to are stated in NWG/RFC#54.  This description is
   intended only as an example, _not_ as a model.

   In the discussion which follows we first describe the environment to
   be assumed and postulate a set of system calls.  We discuss the
   overall architecture of the NCP and the tables that will be used to
   hold relevant information.  Narratives of network operations follow.
   A state diagram is then presented as a convenient method for
   conceptualizing the cause-effect sequencing of events.  The detailed
   processing of each type of network event (system calls or incoming
   network messages) is then discussed.

Newkirk, et al.                                                 [Page 1]

RFC 55             Prototypical Implementation of NCP          June 1970

II. Environment

   We assume that the host will have a time-sharing operating system in
   which the CPU is shared by processes.

   We envision that each process is tagged with a user number.  There
   may be more than one process with the same user number; if so, they
   should all be cooperating with respect to using the network.

   We envision that each process contains a set of ports which are
   unique to the process.  These ports are used for input to or output
   from the process, from or to files, devices, or other processes.

   We also envision that a process is not put to sleep (i.e., blocked or
   dismissed) when it attempts to LISTEN or CONNECT.  Instead it is
   informed when some action is complete.  Of course, a process may
   dismiss itself so that it wakes up only on some external event.

   To engage in network activity, a process attaches a local socket to
   one of its ports.  Sockets are identified by user number, host and
   AEN; a socket is local to a process if the user numbers of the two
   match and they are in the same host.  Thus, a process need only
   specify an AEN when it is referring to a local socket.

   Each port has a status which is modified by system calls and
   concurrent events outside the process (e.g., a 'close connection'
   command from a foreign host).  The process may look at a port's
   status as any time (via the STATUS system call).

   We assume a one-to-one correspondence between ports and sockets.

III. System Calls

   These are typical system calls which a user process might execute.

         We use the notation

                  SYSCALL (ARG1, ARG2....)

                  SYSCALL is the name of the system call
                  ARGk, etc. are the parameters of the system call.

Newkirk, et al.                                                 [Page 2]

RFC 55             Prototypical Implementation of NCP          June 1970


         P        specifies a port of the process
         AEN      specifies a local socket; the user number and host are
         FS       specifies a socket with any user number in any hose,
                  and with any AEN
         CR       the condition code returned

      CONNECT attempts to attach the local socket specified by AEN to
      the port P and to initiate a connection with a specific foreign
      socket, FS.  Possible values of CR are:

         CR=OK          The CONNECT was legal and the socket FS is being
                        contacted.  When the connection is established
                        or refused the status will be updated.

         CR = BUSY      The local socket is in use (illegal command

         CR = BADSKT    The socket specification was illegal.

         CR = NOROOM    Local host's resources are exhausted.

         CR = HOMOSEX   Incorrect send/receive pair

         CR = IMP DEAD  Our imp has died

         CR = LINK DEAD The link to the foreign host is dead because:
                        1. the foreign Imp is dead,
                        2. the foreign host is dead, or
                        3. the foreign NCP does not respond.


         P             specifies a port of the process
         AEN           specifies a local socket
         CR            the condition code returned

      The local socket specified by AEN is attached to port P.  If there
      is a pending call, it is processed; otherwise, no action is taken.
      When a call comes in, the user will be notified.  After examining
      the call, he may either accept or refuse it.  Possible values of
      CR are:

         CR = OK         Connection begun, listening

         CR = BUSY

Newkirk, et al.                                                 [Page 3]

RFC 55             Prototypical Implementation of NCP          June 1970

         CR = NOROOM

         CR = IMP DEAD

         CR = LINK DEAD


         P       specifies a port of the process
         CR      the condition code returned

      Accept implies that the user process has inspected the foreign
      socket to determine who is calling and will accept the call.
      (Note: an interesting alternative defines ACCEPT as the implicit
      default condition.  Thus any incoming RFC automatically satisfies
      a LISTEN.)  Possible values of CR are:

         CR = BADSKT

         CR = NOROOM

         CR = IMP DEAD

         CR = LINK DEAD

         CR = BADCOMM   Illegal command sequence. (E.g., Accept issued
                        before a LISTEN.

         CR = PREMCLS   Foreign user aborted connection after RFC was
                        locally received but before Accept was executed.


         P        specifies a port of the process
         BUFF     specifies the text buffer for transmission
         BITSRQST specifies the length to be transmitted in bits
         BITSACC  returns the number of bits actually transmitted
         CR       the condition code returned

       Transmission takes place.   Possible values for CR are:

         CR = OK

         CR = IMP DEAD

         CR = LINK DEAD

Newkirk, et al.                                                 [Page 4]

RFC 55             Prototypical Implementation of NCP          June 1970

         CR = NOT OPEN  Connection is not open (illegal command

         CR = BAD BOUND BITSRQST out of bounds (e.g., for a receive
                        socket BUFF was shorter than BITSRQST

   INT (P, CR)

         P       specifies the local socket of this process
         CR      the condition code returned

      The process on the other (foreign) side of this port is to be
      interrupted.  Possible values of CR are:

         CR = OK

         CR = BADSKT

         CR = BADCOMM

         CR = IMP DEAD

         CR = LINK DEAD


         P       specifies a port of this process
         RTAB    the returned rendezvous table entry
         CR      the condition code returned

      The relevant fields of the rendezvous table entry associated with
      this port are returned in RTAB.  This is the mechanism a user
      process employs for monitoring the state of a connection.
      Possible values of CR are:

         CR = OK

         CR = BADSKT

Newkirk, et al.                                                 [Page 5]

RFC 55             Prototypical Implementation of NCP          June 1970

   CLOSE (P, CR)

         P       specifies a port of this process
         CR      the condition code returned

      Activity on the connection attached to this port stops, the
      connection is broken and the port becomes free for other use.
      Possible values of CR are:

         CR = OK

         CR = BADSKT

         CR = BADCOMM

         CR = IMP DEAD

         CR = LINK DEAD

IV.  The NCP - Gross Structure

   We view the NCP as having five component programs, several
   associative tables, and some queues and buffers.

      The Component Programs (see Fig. 4.1)

      1. The Input Handler

         This is an interrupt-driven routine.  It initiates Imp-to-Host
         transmission into a resident buffer and wakes up the input
         interpreter when transmission is complete.

      2. The Output Handler

         This is an interrupt-driven output routine.  It initiates Host-
         to-Imp transmission out of a resident buffer and wakes up the
         output scheduler when transmission is complete.

      3. The Input Interpreter

         This program decides whether the input is a regular message
         intended for a user, a network control message, an Imp-to Host
         message, or an error.  For each class of message this program
         invokes a subroutine to take the appropriate action.

Newkirk, et al.                                                 [Page 6]

RFC 55             Prototypical Implementation of NCP          June 1970

      4. The Output Scheduler

         Three classes of messages are sent to the Imp

            (a) Host-to-Imp messages
            (b) Control messages
            (c) Regular messages

         We believe that a priority should be imposed among these
         classes.  The priority we suggest is the ordering above.  The
         output scheduler selects the highest priority message and
         passes it to the output handler.

         Host-to-Imp messages are processed first come first served.
         Control messages are processed individually by host, each host
         being taken in turn.  A control message queue for each foreign
         host is provided.  When any particular host is scheduled for
         output, as many control commands for that host as will fit are
         concatenated into a single message.  Regular messages are
         processed in groups by host and link, each unique combination
         being taken in turn.

      5. The System Call Interpreter

         This program interprets requests from the user.  Each system
         call has a corresponding routine which takes the appropriate

      The two interesting components are the input interpreter and the
      system call interpreter.  These are similar in that the input
      interpreter services foreign requests and the system call
      interpreter services local requests.

      The diagram in Figure 4.1  is our conception of the Network
      Control Program.  Squishy amoeba-like objects represent component
      programs, cylinders represent queues, and the arrows represent
      data paths.  In this simplified diagram tables are not shown.
      ["Amoeba-like" objects in original hand drawing are now firm
      rectangular boxes: Ed.]

      The abbreviated labels in the figure have the following meanings:

            HIQ       -     Host-to-Imp Queue
            OCCQ      -     Output Control Command Queue
            DQ        -     Data Queue
            IHBUF     -     Input Handler Buffer
            OHBUF     -     Output Handler Buffer

Newkirk, et al.                                                 [Page 7]

RFC 55             Prototypical Implementation of NCP          June 1970

               ^      |                      Fig. 4.1
         |                 |
         |     System      |
         |      Call       |
         |   Interpreter   |
         |_________________|              _____________
            ^  |      |                  |             |
            |  |      |  +---------------|    Input    |
            |  |      |  |         +-----| Interpreter |
            |  |      |  |         |     |             |
            |  V      V  V         V      -------------
          |======| |=========| |=======|     |      ^
          | D Q  | | O C C Q | | H I Q |     |      |
          |======| |=========| |=======|     |      |
            |  ^        |          |         |      |
            |  |        |          |         |      |
            |  +--------)----------)---------+      |
            |           |          |                |
            +-------+   |   +------+                |
                  __V___V___V__                     |
                 |             |                    |
                 |   Output    |                    |
                 |  Scheduler  |                    |
                 |_____________|                    |
                        |                           |
                        V                           |
                  (===========)               (===========)
                  ( O H B U F )               ( I H B U F )
                  (===========)               (===========)
                        |                           ^
                  ______V______               ______|______
                 |             |             |             |
                 |   Output    |             |    Input    |
                 |   Handler   |             |   Handler   |
                 |             |             |             |
                  -------------               -------------
                        |                           ^
                        |                           |
                        +----------+    +-----------+
                                   |    |
                              |              |
                              |     I M P    |

Newkirk, et al.                                                 [Page 8]

RFC 55             Prototypical Implementation of NCP          June 1970

V. Tables in the NCP

   We envision that the bulk of the NCP's data base is in associative
   tables.  By "associative" we mean that there is some lookup routine
   which is presented with a key and either returns successfully with a
   pointer to the corresponding entry, or fails if no entry corresponds
   to the key.  The major tables are as follows:

      1. The Rendezvous Table

         This table holds the attributes of a connection.  The table is
         accessed by the local socket, but other tables may have
         pointers to existing entries.

         The components of an entry are:

            (a) Local Socket
            (b) Foreign Socket
            (c) Link
            (d) Connection State
            (e) Flow State
            (f) Data Queue
            (g) Call Queue
            (h) Port Pointer
            (i) Their Buffer Size (only needed on the send side)
            (j) Error State

         An entry is created when either a CONNECT or a LISTEN system
         call is executed or when a request for connection is received.
         Various fields remain unused until after the connection is

      2. The Input Link Table

         The input interpreter uses the concatenation of the foreign
         host and link as a key into the input table.  The table is used
         in processing a user-destined message on an incoming link by
         providing a pointer into the rendezvous table.

      3. The Output Link Table

         The input interpreter uses the output link table to access the
         flow state as RFNM's return from transmitted messages.  The
         output link table is keyed by host and link and provides a
         pointer into the rendezvous table.

Newkirk, et al.                                                 [Page 9]

RFC 55             Prototypical Implementation of NCP          June 1970

      4. The Port Table

         The system call interpreter uses the concatenation of the
         process identification and the port identification as a key to
         obtain a pointer into the rendezvous table.

      5. The Output Control Command Table

         The system call interpreter and the input interpreter use this
         table to make entries in the appropriate output control command
         queues.  Commands are queued in separate table entries
         corresponding to foreign hosts.  Before output the contents of
         the queue are concatenated into a large control message.  The
         components of an entry are:

            (a)  Host
            (b)  Output Control Command Queue

      6. The Output Request Queue

         This queue contains an entry for each connection which has data
         requiring transmission to the net.  There is only one entry per
         connection, which is deleted when the last packet of data is
         transmitted and is entered whenever a user makes a system
         request for data transmission.

         The entry is re-inserted if transmission is not completed
         (message too long) or is prevented by the flow control
         mechanism.  The only component of an entry is a local socket.

      7. The Host Live Table

         This is a simple table listing the hosts which are alive.  This
         table is checked before establishing a connection and before
         sending any data to ensure that the destination host actually
         exists.  At present the protocol does not define the procedure
         to be followed for the Host up/Host down conditions.  See

      8. The Link Assignment Table

         Link numbers are assigned by the receiver.  This table records
         which links are free and can, therefore, be assigned.

Newkirk, et al.                                                [Page 10]

RFC 55             Prototypical Implementation of NCP          June 1970

VI.  Informal Description of Network Operations

   We present here narratives describing the operation conducted during
   the three major phases of network usage: opening, flow control, and

   A. Opening

      In order to establish a connection for data transmission, a pair
      of RFC's must be exchanged.  An RTS must go from the receive-side
      to the send-side, and an STR must be issued by the send-side to
      the receive-side.  In addition, the receive-side, in its RTS, must
      specify a link number.  These RFC's (RFC is a generic term
      encompassing RTS and STR) may be issued in any time sequence.  A
      provision must also be made for queuing pending calls (i.e., RFC's
      which have not been dealt with by the user program).  Thus, when a
      user is finished with a connection, he may choose to examine the
      next pending call from another process and decide to either accept
      or refuse the request for connection.  A problem develops because
      the user may not choose to examine his pending calls; thus they
      will merely serve to occupy queue space in the NCP.  Several
      alternative solutions to this problem will be mentioned later.

      Utilizing the framework of the prototype system calls described
      above, we envision at least four temporal sequences for obtaining
      a successfully opened connection:

         1. The user may issue a LISTEN, indicating he is willing to
            consider connecting to anyone who sends him an RFC.  When an
            RFC comes in the user is notified.  The user then decides
            whether he wishes to connect to this socket and issues an
            ACCEPT or a CLOSE on the basis of that decision.  A CLOSE '
            refuses' the connection, as discussed under "Closing."  An
            ACCEPT indicates he is willing to connect; an RFC is issued,
            and the connection becomes fully opened.

         2. Upon processing a user request for a LISTEN, the NCP
            discovers that a pending call exists for that local socket.
            The user is immediately notified, and he may ACCEPT or
            CLOSE, as above.

         3. The user issues a CONNECT, specifying a particular foreign
            socket that he would like to connect to.  An RFC is issued.
            If the foreign process accepts the request, it answers by
            returning an RFC.  When this acknowledging RFC is received,
            the connection is opened.

Newkirk, et al.                                                [Page 11]

RFC 55             Prototypical Implementation of NCP          June 1970

         4. When presented with a CONNECT, the NCP may discover that a
            pending call exists from the specified foreign socket to the
            local socket in question.  An acknowledging RFC is issued
            and the connection is opened.

      In all of the above cases the user is notified when the connection
      is opened, but data flow cannot begin until buffer space is
      allocated and an ALL command is transmitted.

      Any of these connection scenarios will be interrupted if a CLS
      comes in, as discussed under "Closing."

         1. Pending Call Queues

            It is essential that some form of queuing for pending RFC's
            be implemented.  A simple way to see this is to examine a
            typical LISTEN-CONNECT sequence.  One side issues a LISTEN,
            the other a CONNECT.  If the LISTEN is issued before the RFC
            coming from the remote CONNECT arrives, all is fine.
            However, due to the asynchronous nature of the net, we can
            never guarantee that this sequence of events will occur.  If
            calls are not queued, and the RFC comes in before the LISTEN
            is issued, it will be refused; if it arrives later, it will
            be accepted.  Thus we have an extremely ambiguous situation.

            Unless one has infinite queue space, it is desirable that
            some mechanism for purging the queues of old RFC's which the
            user never bothered to examine.  An obvious but informal
            method is to note the time when each RFC is entered into the
            queue, and then periodically refuse all RFC's which have
            exceeded some arbitrary time limit.  Another thought, which
            probably should be included within the context of any
            scheme, is for the NCP to send a CLS on all outstanding
            connections or pending calls when a user logs out or blows

            The scheme which is utilized in this description may seem at
            first blush to be non-intuitive; but we feel it is more
            realistic than other proposals.  Basically, when a CONNECT
            is issued, the NCP assumes that this socket wishes to talk
            to the specified foreign socket and to that socket only.  It
            therefore purges from the pending call queue all non-
            matching RFC's by sending back CLS's.  Similarly, when the
            connection is in the RFC-SEND state (a CONNECT has been
            issued), all non-matching RFC's are refused.  If a LISTEN-
            ACCEPT or LISTEN- CLOSE sequence is executed, the remainder

Newkirk, et al.                                                [Page 12]

RFC 55             Prototypical Implementation of NCP          June 1970

            of the pending calls are not removed from the queue, in the
            expectation that the user may wish to accept these requests
            in the future.

            Although the latter method may seem to be arbitrary and/or
            unnecessarily restrictive, we have not yet concocted a
            scenario which would be prohibited by this method, assuming
            that we are dealing with a competent programmer (i.e., one
            who is wary of race conditions and the asynchronous nature
            of the net).  Of course whatever scheme or schemes a
            particular site chooses is highly implementation dependent;
            we suggest that some provision for the queuing of RFC's be
            provided for a period of time at least of the order of
            magnitude that they are retained in the CONNECT-clear scheme
            mentioned above.

   B. Flow Control

      Meaningful data can only flow on a connection when it is fully
      opened (i.e., two RFC's have been exchanged and closing has not
      begun).  We assume that the NCP's have a buffer for receiving
      incoming data and that there is some meaningful quantity which
      they can advertise (on a per connection basis) indicating the size
      message they can handle.  We further assume that the sending side
      regulates its transmission according to the advertisements of that

      When a connection is opened, a cell (called 'Their Size') is set
      to zero.  The receive-side will decide how much space it can
      allocate and send an ALL message specifying that space.  The
      send-side will increment 'Their Size' by the allocated space and
      will then be able to send messages of length less than or equal to
      'Their Size' When messages are transmitted, the length of the
      message is subtracted from 'Their Size'.  When the receive-side
      allocates more buffer space (e.g. when a message is taken by the
      user, thus freeing some system buffer space), the number of bits
      released is sent to the send-side via an ALL message.

      Thus, 'Their Size' is never allowed to become negative and no
      transmission can take place if 'Their Size' equals zero.

      Notice that the lengths specified in ALL messages are increments
      not the absolute size of the receiving buffer.  This is
      necessitated  by the full duplex nature of the flow control
      protocol.  The length field of the ALL message can be 32 bits long
      (note: this is an unsigned integer), thus providing the facility
      for essentially an infinite "bit sink", if that may ever be

Newkirk, et al.                                                [Page 13]

RFC 55             Prototypical Implementation of NCP          June 1970

   C. Closing

      Just as two RFC's are required to open a connection, two CLS's are
      required to close a connection.  Closing occurs under various
      circumstances and serves several purposes.  To simplify the
      analysis of race conditions, we distinguish four cases: aborting,
      refusing, termination by receiver, termination by sender.

      A user "aborts" a connection when he issues a CONNECT and then a
      CLOSE before the CONNECT is acknowledged.  Typically a user will
      abort following an extended wait for the acknowledgment; his
      system may also abort for him if he blows up.

      A user "refuses" a connection when he issues a LISTEN and, after
      being notified of a prospective caller, issues a CLOSE.  Any
      requests for connection to a socket which is expecting a call from
      a particular socket are also refused.

      After a connection is established, either side may terminate.  The
      required sequence of events suggests that attempts to CLOSE by the
      receive-side should be viewed as "requests" which are always
      honored as soon as possible by the send-side.  Any data which has
      not yet been passed to the user, or which continues over the
      network, is discarded.  Requests to CLOSE by the send-side are
      honored as soon as all data transmission is complete.

         1. Aborting

            We may distinguish three cases:

            a) In the simplest case, we send an RFC followed later by a
               CLS.  The other side responds with a CLS and the attempt
               to connect ends.

            b) The foreign process may accept the connection
               concurrently with the local process aborting it.  In this
               case, the foreign process will believe the local process
               is terminating an open connection.

            c) The foreign process may refuse the connection
               concurrently with the local process aborting it.  In this
               case, the foreign process will believe the local process
               is acknowledging its refusal.

Newkirk, et al.                                                [Page 14]

RFC 55             Prototypical Implementation of NCP          June 1970

         2. Refusing

            After an RFC is received, the local host may respond with an
            RFC or a CLS, or it may fail to respond.  (The local host
            may have already sent its own RFC, etc.)  If the local host
            sends a CLS, the local host is said to be "refusing" the
            request for connection.

            We require that CLS commands be exchanged to close a
            connection, so it is necessary for the local host to
            maintain the rendezvous table entry until an acknowledging
            CLS is returned.

         3. Terminating by the Sender

            When the user on the send side issues a CLOSE system call,
            his NCP must accept it immediately, but may not send out a
            CLS command until all the data in the local buffers has been
            passed to the foreign host.  It is thus necessary to test
            for both 'buffer-empty' and
            'RFNM-received' before sending the CLS command.  As usual,
            the CLS must be acknowledged before the entry may be

         4. Terminating by the Receiver

            When the user on the receive side issues a CLOSE system
            call, his NCP accepts and sends the CLS command immediately.
            Data may still arrive, however, and this data should be
            discarded.  The send side, upon receiving the CLS, should
            immediately terminate the data flow.

VII. Connection Status

   An excellent mechanism for describing the sequence of events required
   to establish and terminate a connection involves a state diagram.  We
   may assume that each socket can be associated with a state machine,
   and that this state machine may, at any time, be in one of ten
   possible states.  In any state, certain network events cause the
   connection status to enter another state; other events are ignored;
   still others are error.  A transition may also involve the local NCP
   performing some action.  Figure 7.1 depicts the state machine.
   Circles [now boxes: Ed] represent states (described below); arrows
   show legal transitions between states.  The labels on the arrows
   identify the event which caused them (note that CLOSE is a system
   call, CLS is a control command).  Phrases after slashes denote the
   action which should  be performed while traveling over that arrow.
   The arrow labeled '[E]RFC' (found between states 0 and 1) represents

Newkirk, et al.                                                [Page 15]

RFC 55             Prototypical Implementation of NCP          June 1970

   the condition that whenever a connection enters the CLOSED state, the
   pending call queue for that connection is checked [Original was
   backwards "E": Ed.]

   If any pending calls exist in the queue, the connection moves to the
   PENDING state.  If an RFC is received for a socket in the CLOSED
   state, it is also moved along this path to the PENDING state.  Events
   and the actions they cause are described in sections VIII and IX
   below.  Descriptions of the ten states follow:

      (0) CLOSED

          The local socket is not attached to any port and no user has
          requested a connection with it.  (The table entry is non-

      (1) PENDING CALL

          The socket is not attached to any port but one or more
          requests for connection have been received.  A LISTEN system
          call will be satisfied immediately by the first entry in the
          pending call queue for a matching request; all other pending
          calls are deleted.

      (2) LISTENING

          The socket is attached to a port.  We are waiting for a user
          to request connection with this socket.

      (3) RFC-RCVD

          We are listening and an RFC was received.  The local user has
          been informed of the pending call.  He must respond with
          either a CLOSE or an ACCEPT.

      (4) ABORT

          We have notified the user that his LISTEN has been satisfied
          but he has not yet responded; if during this time the foreign
          user aborts the connection by sending a CLS, we send a CLS to
          acknowledge the abort and mark the fact with this state.  When
          the user accepts or refuses the call, we can inform him the
          connection has been prematurely terminated.

Newkirk, et al.                                                [Page 16]

RFC 55             Prototypical Implementation of NCP          June 1970

      (5) RFC-SENT

          This state is entered when:

          a)  The local user has attached this socket to a port by
              issuing a CONNECT.
          b)  An RFC has been sent, and
          c)  No reply has been received.

          When the user issues a CONNECT the pending call queue is

          If a matching RFC is not found, the queue is deleted and this
          state is entered.  As new RFC's arrive they are compared with
          our user's request.  If they do not match, the RFC is
          immediately refused.  If the RFC matches, it completes the
          initialization process and the connection enters the OPEN

      (6) OPEN

          RFC's have been exchanged and the connection is securely
          established.  Transmission may begin following receipt of an
          ALL command from the receive side, and will then proceed
          subject to flow control.

      (7) CLS-WAIT

          After the local user has executed a CLOSE, and we have issued
          a CLS, we must wait for an acknowledging CLS before the
          connection can be completely closed.   If the appropriate CLS
          has not already been received, this state is entered.

      (8) DATA-WAIT

          If we are on the send side and the local user executes a CLOSE
          system call, a CLS cannot be issued if our data buffer is not
          empty or if a RFNM for the last data message is outstanding.
          The connection enters this state to wait for these conditions
          to be fulfilled.  Upon completion and acknowledgement of
          output a CLS may be issued and the connection enters the CLS-
          WAIT state, waiting for the acknowledging CLS.   If a CLS
          arrives while in the DATA-WAIT state we clear our buffer (the
          CLS came from a receive socket, indicating it is no longer
          interested in our data) and enter the RFNM-WAIT state to wait
          for the network to clear.

Newkirk, et al.                                                [Page 17]

RFC 55             Prototypical Implementation of NCP          June 1970

      (9) RFNM-WAIT

          If we are on the send side and a CLS command arrives, we
          cannot issue an acknowledging CLS if we have not received the
          RFNM for our last data message.  We enter this state to await
          the RFNM, and cease all further data transmission.  When the
          RFNM comes in, a CLS may then be issued, and the connection
          will be closed.

Newkirk, et al.                                                [Page 18]

RFC 55             Prototypical Implementation of NCP          June 1970

                     |              |       CLOSE
      CONN/          |    CLOSED    |<---------------------------+
      send RFC       |     (0)      |       LISTEN               |
    +----------------|              |-----------------------+    |
    |                |______________|                       |    |
    |                     |    ^                            |    |
    |              [E]RFC |    |  CLS/send CLS              |    |
    |                  ___V____|____                     ___V____|____
    |  non-matching   |             |                   |             |
    |  CONN/send RFC  |   PENDING   | LISTEN        RFC |  LISTENING  |
    |   +-------------|    (1)      |----------+   +----|     (2)     |
    |   |             |_____________|          |   |    |_____________|
    |   |       matching     |                 |   |
 ___V___V_____  CONN/send RFC|               __V___V______
|             |              |     ACCEPT/  |             | CLS/
|   RFC-SENT  | RFC          |     send RFC |   RFC-RECD  | send CLS
|     (5)     |----------+   |   +----------|     (3)     |---------+
|_____________|          |   |   |          |_____________|         |
   |   |                 |   |   |               |                  |
   |   |              ___V___V___V___  SND&CLOSE |   ____________   |
   |   |    RCV&CLS/ |               |-----------)->|            |  |
   |   |    send CLS |      OPEN     | SND&CLS   |  |  DATA-WAIT |  |
   |   |   +---------|      (6)      |--------+  |  |    (8)     |  |
   |   |   |         |_______________|        |  |  |____________|  |
   |   |   |      RCV&CLOSE/ |                |  |   |              |
   |   |   |       send CLS  |                |  |   |              |
   |   |   |                 |                |  |   | CLS          |
   |   |   |           ______V______          |  |   |              |
   |   |   |   CLOSE/ |             |CLOSE/   |  |   |              |
   |   |   |  send CLS|   CLS-WAIT  |send CLS |  |   |              |
   |   +---)--------->|     (8)     |<--------)--+   |              |
   |       |          |_____________|         |      |              |
   |       |                 |             ___V______V_       ______V___
   |       |                 |            |            |     |          |
   |       |                 |            |  RFNM-WAIT |     |   ABORT  |
   |       |             CLS |            |     (9)    |     |    (4)   |
   |       |                 |            |____________|     |__________|
   |       |                 |                   |                 |
   |       |           ______V_______  RFNM/     |                 |
   |       |          |              | send CLS  |                 |
   |  CLS/ +--------->|    CLOSED    |<----------+                 |
   | send CLS         |     (0)      |                ACCEPT|CLOSE |
   +----------------->|              |<----------------------------+

                         Figure 7.1
                  Connection State Diagram

Newkirk, et al.                                                [Page 19]

RFC 55             Prototypical Implementation of NCP          June 1970

VIII.  Algorithms for the Input Interpreter

   The following is a concise description of the NCP's responses to
   incoming network commands.  CS always indicates Connection State.
   Note, CLOSE is a system call executed by the local user process, and
   CLS is a network command.



   RFC (RTS or STR)

      If no entry exists, create one with status = PENDING CALL, and
      queue the message.

      If CS = LISTENING, then queue the entry, enter the RFC-RCVD state,
      and inform the user of the request.

      If CS = RFC-SENT but the new RFC does not match the request,
      refuse the RFC.

      In all other cases, check the RFC for a match.  If none exists,
      queue the RFC.  If the RFC matches, then if:

         CS = RFC-SENT, we enter the OPEN state.

         CS = CLOSE-WAIT, the RFC is ignored.

         otherwise, the request is illegal in all states which indicate
         it has already been received (these states are 1,3,4,6,8,9).

      In any case, if processing the RFC causes an overflow condition
      (resources are exhausted), refuse the connection (send a CLS).


      The pending call queue is searched.  If the CLS doesn't match the
      current request, but does match some other request, then delete
      that request and issue a CLS.  If there is no match, the CLS is

      If the CLS matches the current request, and CS =

         PENDING, then delete the current request.  If the request queue
            is empty, delete the entry; otherwise, leave the entry

Newkirk, et al.                                                [Page 20]

RFC 55             Prototypical Implementation of NCP          June 1970

         RFC-RCVD, Issue a CLS and enter the ABORT state.
         ABORT, ignore.

         RFC-SENT, issue a CLS.  If the pending call queue is empty
            delete the entry, else enter the PENDING state.

         OPEN, If we are on the receive side, response is identical to
            the response for RFC-SENT.  If we are on the send side,
            clear the data queue, and if a RFNM is still pending enter
            the RFNM-WAIT state.  Otherwise response is identical to the
            response for RFC-SENT.

         CLS-WAIT, Issue a CLS and if the pending call queue is empty,
            delete the entry, otherwise CS = PENDING.

         DATA-WAIT, clear the data queue and enter the RFNM-WAIT state.
            A matching CLS cannot occur in the CLOSED or LISTENING


      Errors are queued for later attention by system programmers, and
      are considered to be a system error in the host that originated
      the exchange.  (Not associated with any state).


      The op code is changed to ERP and retransmitted (Not associated
      with any state).


      Upon receipt of an ERP, the system passes the text of the command
      back to the process which issued the ECO.


      These commands are enabled only in the OPEN state.  Upon receiving
      an INTERRUPT, the system causes an event to be sent to the
      associated process.  An INTERRUPT is ignored in the CLS-WAIT,
      DATA-WAIT, and RFNM-WAIT states.  In any other state it is an

Newkirk, et al.                                                [Page 21]

RFC 55             Prototypical Implementation of NCP          June 1970


      ALLOCATE is valid only in the OPEN state, and may be sent only to
      a send socket.  The NCP increments the 'Their Size' field in the
      associated rendezvous table entry by the size specified in the
      ALLOCATE command.

      In the CLS-WAIT and DATA-WAIT states this command is ignored; in
      any other state it is an error.


      If in the OPEN state, mark the Flow Control Status field in the
         appropriate rendezvous table entry as RFNM-RECVD, and send more
         data if required.

      If in the DATA-WAIT state, maintenance the Flow Control Status.
         If the data queue is empty issue a CLS and enter the CLS-WAIT
         state; otherwise, transmit the next message.

      If in the RFNM-WAIT state, maintenance the Flow Control Status and
         issue a CLS.  If the Pending Call queue is empty delete the
         rendezvous table entry, otherwise CS = PENDING.

      A Data-RFNM is an error in all other states.

IX.  Algorithms for the System Call Interpreter

   Each System Call is discussed, giving the state changes it may


      If there is no entry, create one, issue an RFC, and enter the
         RFC-SENT state.

      If CS = PENDING, search the queue and reject all non-matching
         requests.  If no match is found issue an RFC and enter the
         RFC-SENT state.  If a match is found, issue an RFC and enter
         the OPEN state.  Transmission can commence as soon as buffer
         space has been allocated.

      In any other state this command is illegal.


      If an entry doesn't exist, create one, and enter the LISTENING

Newkirk, et al.                                                [Page 22]

RFC 55             Prototypical Implementation of NCP          June 1970

      If CS = PENDING, inform the user and enter the RFC-RCVD state.

      In any other state this command is illegal.


      If CS = RFC-RCVD, then issue an RFC and enter the OPEN state.
         Data transmission can occur as soon as buffer space is

      If CS = ABORT, inform the user of the premature termination of the
         connection.  If the pending call queue is empty, delete the
         entry; otherwise, enter the PENDING state.

      This command cannot be legally executed in any other state.


         If CS =

      LISTENING, then delete the entry.

      RFC-RCVD, then issue a CLS and enter the CLS-WAIT state.

      ABORT, inform the user of the premature termination of the
         connection.  If the pending call queue is empty, delete the
         entry; otherwise, enter the PENDING state.

      RFC-SENT, then issue a CLS and enter the CLS-WAIT state.

      OPEN, if we are on the send side, and the data queue is not empty,
         or if a Data-RFNM is still outstanding, enter the DATA-WAIT
         state; otherwise, issue a CLS and enter the CLS-WAIT state.

      CLS-WAIT, issuing a CLOSE in this state is a USER ERROR.

      DATA-WAIT, issuing a CLOSE in this state is also an illegal

      RFNM-WAIT, ignore the CLOSE.

      A valid CLOSE cannot be issued if an entry does not exist, or if a
         socket is in the PENDING state.

           [ This RFC was put into machine readable form for entry   ]
           [ into the online RFC archives by Anthony Anderberg 5/00 ]

Newkirk, et al.                                                [Page 23]