rfc55.txt

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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.

   NOP

      Discard.

   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).

   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
      ignored.

      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
            alone.




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
            states.

   ERR

      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).

   ECO

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

   ERP

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

   INR, INS

      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
      error.









Newkirk, et al.                                                [Page 21]

RFC 55             Prototypical Implementation of NCP          June 1970


   ALL

      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.

   Data-RFNM

      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
   effect:

   CONNECT

      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.

   LISTEN

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



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.

   ACCEPT

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

      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.

   CLOSE

         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
         sequence.

      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]