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Network Working Group                                Edwin E. Meyer, Jr.
Request for Comments: 46           Massachusetts Institute of Technology
                                                           17 April 1970

                      ARPA Network Protocol Notes

   The attached document contains comments and suggestions of the
   Network Working Group at Project MAC.  It is based upon the protocol
   outlined in NWG/RFC 33, 36, and later documents.

   This proposal is intended as a contribution to the dialog leading to
   a protocol specification to be accepted by the entire Network Working

   We solicit your comments.


   In this document the Network Working Group at MIT Project MAC suggest
   modifications and extensions to the protocol specified by Carr,
   Crocker, and Cerf in a preprint of their 1970 SJCC paper and extended
   by Crocker in NWG/RFC 36.  This document broadly outlines our
   proposal but does not attempt to be a complete specification.  It is
   intended to be an indication of the type and extent of the protocol
   we think should be initially implemented.

   We agree with the basic concept of simplex communication between
   sockets having unique identifiers.  We propose the implementation of
   a slightly modified subset of the network commands specified in
   NWG/RFC36 plus the ERR command as specified by Harslem and Heafner in
   NWG/RFC 40.

   Given the basic objective of getting all ARPA contractors onto the
   network and talking to each other at the earliest possible date, we
   think that it is important to implement an initial protocol that is
   reasonably simple yet extendable while providing for the major
   initial uses of the network.  It should be a simple protocol so as to
   elicit the broadest possible support and to be easily implementable
   at all installations with a minimum of added software.

   While the protocol will evolve, the fundamentals of a protocol
   accepted and implemented by all installations are likely to prove
   very resistant to change.  Thus it is very important to make the
   initial protocol open-ended and flexible.  A simple basic protocol is
   more likely to succeed in this respect than a complicated one.  This

                                                                [Page 1]

RFC 46                ARPA Network Protocol Notes             April 1970

   does not preclude the existence of additional layers of protocol
   between several installations so long as the basic protocol remains

   We feel that three facilities must be provided for in the initial

   1. Multi-path communication between two existing processes which know
      how to connect to each other.

   2. A standard way for a process to connect to the logger (logging
      process at a HOST) at a foreign HOST and request the creation of a
      user process.  (The login ritual may or may not be standardized.)

   3. A standard way for a newly created process to initiate pseudo-
      typewriter communication with the foreign process which requested
      its creation.

   The major differences between the protocol as proposed by Carr,
   Crocker, and Cerf and this proposal are the following:

   1. The dynamic reconnection strategy specified in Crocker's
      NWG/RFC 36 is reserved for future implementation.  We feel that
      its inclusion would unduly complicate the initial implementation
      of the protocol.  We outline a strategy for foreign process
      creation that does not require dynamic reconnection.  Nothing in
      this proposal precludes the implementation of dynamic reconnection
      at a later date.

   2. We propose that an "instance tag" be added to the socket
      identifier so as to separate sockets belonging to different
      processes of the same user coexisting at one HOST.

   3. The following NCP commands have been added:

      a. The ERR command specified in NWG/RFC 40 is included.

      b. BLK and RSM commands are presented as possible alternatives to
         the "cease on link" IMP command and SPD and RSM commands set
         forth in NWG/RFC 36.  Because these commands operate on socket
         connections rather than link numbers, they do not impede the
         implementation of socket connection multiplexing over a single
         link number, should that later prove desirable.

      c. An INT command that interrupts a process is specified.  We feel
         that it is highly important to be able to interrupt a process
         that may be engaged in unwanted computation or output.  To
         implement the interrupt as a special format within a normal

                                                                [Page 2]

RFC 46                ARPA Network Protocol Notes             April 1970

         message raises severe difficulties: the connection may be
         blocked when the interrupt is needed, and the NCP must scan
         each incoming message for an interrupt signal.

      d. An ECO echoing command to test communications between NCPs is

   4. Sockets are conceptualized as having several states, and these are
      related to conditions under which network requests may be queued.
      This differs from the unlimited queuing feature, which presents
      certain implementation difficulties.

   5. The protocol regarding creation of a foreign process and
      communication with it is removed to a separate User Control and
      Communication (UCC) protocol level and is more fully specified.


   It seems convenient and useful to view the network as consisting of a
   hierarchy of protocol and implementation levels.  In addition to
   aiding independent software and hardware development, provisions for
   a layered protocol allow additions and substitution of certain levels
   in experimental or special purpose systems.

   We view the initial network communications system as a hierarchy of
   three systems of increasing generality and decreasing privilege
   level.  These are:

   1. IMP Network - The network of IMPs and physical communication lines
      is the basic resource which higher level systems convert into more
      generalized communication facilities.  The IMP network acts as a
      "wholesaler" of message transmission facilities to a highly
      privileged module within each HOST.

   2. Network Control Program - Each HOST contains a module called the
      Network Control Program (NCP) which has sole control over
      communications between its HOST and the IMP network.  It acts as a
      "retailer" of the wholesale communications facilities provided by
      the IMP network.  The network of NCPs can be viewed as a higher
      level communications system surrounding the IMP network which
      factors raw message transmission capabilities between HOSTs into
      communication facilities between ordinary unprivileged processes.

                                                                [Page 3]

RFC 46                ARPA Network Protocol Notes             April 1970

              H O S T  A                      H O S T  C
    ______________________________       ______________________
   |                              |     |                      |
   |  ____   ____   ____   ____   |     |  ____   ____   ____  |
   | |Proc| |Proc| |Proc| |    |  |     | |Proc| |Proc| |    | |
   | | A  | | B  | | C  | |UCC |  |     | | D  | | E  | |UCC | |
   | |____| |____| |____| |____|  |     | |____| |____| |____| |
   |    |     |      |      |     |     |    |     |      |    |
  - - - - - - |- - - |- - - |- - -|- - -|- - |- - -|- - - |- - - - - -
   |    |     |      |      |   NCP NETWORK  |     |      |    |
   |    |     |      |      |     |     |    |     |      |    |
   |   _|_____|______|______|_    |     |   _|_____|______|_   |
   |  |                       |   |     |  |                |  |
   |  |      N C P   A        |   |     |  |   N C P   C    |  |
   |  |_______________________|   |     |  |________________|  |
   |                     ||       |     |       ||             |
   |_____________________||_______|     |_______||_____________|
                         ||                     ||
  - - - - - - - - - - - -|| - - - - - - - - - - ||- - - - - - - - - -
                         ||     IMP NETWORK     ||
                      ___||___              ____||__
                     |        |            |        |
                     |  IMP   |------------|  IMP   |
                     |   A    |            |   C    |
                     |________|            |________|
                         |                     |
                         |       ________      |
                         |      |        |     |
                         +------|  IMP   |-----+
                                |   B    |

                     FIG 1. Modular View Of Network

   3. User Control and Communication Module - The preceding two
      communication systems are sufficient to permit communication
      between unprivileged processes that already exist.  However, one
      of the primary initial uses of the network is thought to involve
      the creation of a foreign user process through interaction with
      the foreign HOST's logger.  The User Control and Communication
      Module (UCC) implements protocol sufficient for a process to
      communicate with a foreign HOST's logger and to make initial
      control communication with a created process.  Such a process is
      to have the same privileges (subject to administrative control) as
      a local (to the foreign HOST) user process.  The UCC module
      communicates through the NCP in a manner similar to an ordinary
      process.  Except for the ability to close connections to a dead

                                                                [Page 4]

RFC 46                ARPA Network Protocol Notes             April 1970

      process, the UCC module has no special network privileges.  The
      UCC protocol is only one of several third-level protocols that
      could be implemented.  For example, a set of batch processing
      systems connected through the NCP system might implement a load-
      sharing protocol, but not a UCC.


   Each HOST implements a module called the Network Control Program
   (NCP) which controls all network communications involving that HOST.
   The network of NCPs forms a distributed communication system that
   implements communication paths between individual processes.  The NCP
   protocol issues involve:  (i) the definition of these communication
   paths, and (ii) a system for coordinating the distributed NCP system
   in maintaining these communication paths.  These are discussed below.


   Communication between two processes is made through a simplex
   connection between two sockets:  a send socket attached to one
   process and a receive socket attached to another process.  Sockets
   have the following characteristics:

   Socket Identifier - A socket identifier is used throughout the
   network to uniquely identify a socket.  It consists of 48 bits,
   having the following components:

      a. User Number (24 bits) - A socket attached to a process is
         identified as belonging to that process by a user number
         consisting of 8 bits of "home" HOST code plus 16 bits of user
         code assigned by the home HOST.  This user number is the same
         for all sockets attached to any of his processes in any HOST.

      b. Instance Tag (8 bits) - More than one process belonging to a
         user may simultaneously exist within a single HOST.  The
         instance tag identifies the particular process to which a
         socket belongs.  A user's first process at a HOST to use the
         network receives instance tag = 0 by convention.

      c. HOST Number (8 bits) - This is the code of the HOST on which
         the attached process exists.

      d. Socket Code (8 bits) - This code provides for 128 send and 128
         receive sockets in each process.  The low order bit determines
         whether this is a "send" (= 1) or "receive" (= 0) socket.

                                                                [Page 5]

RFC 46                ARPA Network Protocol Notes             April 1970

   States of Sockets - Each socket has an associated state.  The NCP may
   implement more transitory states of a socket, but the three following
   are of conceptual importance.

      a. Inactive - there is no currently existing process which has
         told the NCP that it wishes to listen to this socket.  No other
         process can successfully communicate with an inactive socket.

      b. Open - Some process has agreed to listen to events concerning
         this socket but it is not yet connected.

      c. Connected - This socket is currently connected to another

   Socket Event Queue - A queue of events to be disclosed to the owning
   process is maintained for each open or connected socket.  It consists
   of a chronologically ordered list of certain events generated by the
   action of one or more foreign processes trying to connect or
   disconnect this socket.  An entry in the event queue consists of the
   event type plus the identifier of the foreign socket concerned.  The
   following event types are defined:

      a. "request" - a foreign socket requests connection.  (not queued
         if local socket is already connected)

      b. "accept" - a foreign socket accepts requested connection.

      c. "reject" - a foreign socket rejects requested connection.

      d. "close" - a foreign socket disconnects an existing connection.

   A "request" event is removed from the queue when it is accepted or
   rejected.  The other events are removed from the queue as they are
   disclosed to the owning process.

   Some events are intended to be transparent to the process owning the
   socket, and they do not generate entries in the event queue.

   Although an event queue is conceptually unlimited, it seems necessary
   to place some practical limit on its length.  When an event queue for
   a socket is full, any incoming event that would add to the queue
   should be discarded and the sending NCP notified (via ERR command
   described below).

                                                                [Page 6]

RFC 46                ARPA Network Protocol Notes             April 1970

NCP Control Communications

   The NCP network coordinates its activities through control commands
   passed between its individual components.  These commands generally
   concern the creation and manipulation of socket connections
   controlled by the NCP receiving the command.  A control command is
   directed to a particular NCP by being sent to its HOST as a message
   over link number 1 (designated as the control link), which is
   reserved for that purpose.  The IMP network does not distinguish
   between these messages and regular data messages implementing
   communication through a socket connection.

      The following NCP control commands are defined:

      a. Request for Connection

         RFC <local socket> <foreign socket> [<link no.>]

      An NCP directs this command to a foreign NCP to attempt to
      initiate a connection between a local socket and a foreign socket.
      If the foreign socket is open, the foreign NCP places a "request"
      event into the socket's event queue for disclosure to the process
      that owns it.  If the foreign process accepts, the foreign NCP
      returns a positive acknowledgement in the form of another RFC.  It
      rejects connection by issuing the CLS command (see below).  An RFC
      is automatically rejected without consulting the owning process if
      the foreign socket is not open (inactive or connected).  Multiple
      RFCs to the same socket are placed into its event queue in order
      of receipt.  Any queued RFCs are automatically rejected by the NCP
      once the owning process decides to accept a connection.  The NCP
      which has control of the "receive" socket of the potentially
      connected pair designates a link number over which messages are to

      b. Close a Connection

         CLS <local socket> <foreign socket>

      An NCP issues this network command to disconnect an existing
      connection or to negatively acknowledge an RFC.  There is a
      potential race problem if an NCP closes a local send socket in
      that the CLS command may reach the foreign NCP prior to the last
      message over that socket connection.  This race is prevented by
      adhering to two standards: (i) A CLS command for a local send
      socket is not transmitted until the RFNM for the last message to
      the foreign socket comes back, and (ii) the foreign NCP processes
      all incoming messages in the order received.

                                                                [Page 7]

RFC 46                ARPA Network Protocol Notes             April 1970

      c. Block Output over a Connection

         BLK <foreign send socket>

      A process may read data through a receive socket slower than
      messages are coming in and thus the NCP's buffers may tend to clog
      up.  The NCP issues this command to a foreign NCP to block further
      transmission over the socket pair until the receiving process
      catches up.

      d. Resume Output over a Blocked Connection

         RSM <foreign send socket>

      An NCP issues this command to unblock a previously blocked

      e. Interrupt the Process Attached to a Connection

         INT <foreign socket>

      Receipt of this message causes the foreign NCP to immediately
      interrupt the foreign process attached to <foreign socket> if it
      is connected to a local socket.  Data already in transit within
      the NCP network over the interrupted connection will be
      transmitted to the destination socket.  The meaning of "interrupt"
      is that the process will immediately break off its current
      execution and execute some standard procedure.  That procedure is
      not defined at this protocol level.

      f. Report an Erroneous Command to a Foreign NCP

         ERR <code> <command length> <command in error>

      This command is used to report spurious network commands or
      messages, or overload conditions that prevent processing of the
      command.  <code> specifies the error type.  If <code> specifies an
      erroneous network command, <command in error> is that command (not
      including IMP header) and <command length> is an integer
      specifying its length in bits.  If <code> specifies an erroneous
      message, <command in error> contains only the link number over
      which the erroneous message was transmitted.  (This is slightly
      modified from the specification in NWG/RFC 40.)

                                                                [Page 8]

RFC 46                ARPA Network Protocol Notes             April 1970

      g. Network Test Command

         ECO <48 bit code> <echo switch>

      An NCP may test the quality of communications between it and a
      foreign NCP by directing to it an ECO command with an arbitrary
      <48 bit code> (of the same length as a socket identifier) and
      <echo switch> 'on'.  An NCP receiving such a ECO command should
      immediately send an acknowledging ECO with the same <48 bit code>
      and <echo switch> 'off' to the originating NCP.  An NCP does not
      acknowledge an ECO with <echo switch> 'off'.  We feel that this
      command will be of considerable aid in the initial shakedown of
      the entire network.

      h. No Operation Command


      An NCP discards this command upon receipt.

User Interface to the NCP

   The NCP at each HOST has an interface through which a local process
   can exercise the network, subject to the control of the NCP.  The
   exact specification of this interface is not a network protocol
   issue, since each installation will have its own interface keyed to
   its particular requirements.  The protocol requirements for the user
   interface to an NCP are that it provide all intended network
   functions and no illegal privileges.  Examples of such illegal
   privileges include the ability to masquerade as another process,
   eavesdrop on communications not intended for it, or to induce the NCP
   to send out spurious network commands or messages.

   We outline here an interface based on the Carr, Crocker, and Cerf
   proposal that is sufficient to fully utilize the network.  While this
   particular set of calls is intended mainly for illustrative purposes,
   it indicates the types of functions necessary.

      The following calls to the NCP are available:

      a. LISTEN <my 8 bit socket code>

      A user opens this socket, creating an empty event queue for it.
      This LISTEN call may block waiting for the first "request" event,
      or it may return immediately.

                                                                [Page 9]

RFC 46                ARPA Network Protocol Notes             April 1970

      b. INIT <my socket code> <foreign socket>

      A user attempts to connect <my socket> to <foreign socket>.  The
      local NCP sends an RFC to the foreign NCP requesting that the
      connection be created.  The returned acknowledgemnet is either an
      RFC (request accepted) or CLS (request rejected).  At the caller's
      option, the INIT call blocks on the expected "accept" or "reject"
      event, or it can return immediately without waiting.  In this case
      the user must call STATUS (see below) at a later time to determine
      the action by the foreign NCP.  When a blocked INIT call returns,
      the "accept" or "reject" event is removed from the event queue.

      c. STATUS <my socket code>

      This call reports out the earliest previously unreported event in
      the queue of <my socket>.  The STATUS call deletes the event from
      the queue if that type of event is deleteable by disclosure.

      d. ACCEPT <my socket code>

      The user accepts connection with the foreign socket whose
      "request" event is earliest in the event queue for <my socket>.
      An acknowledging RFC is sent to the accepted foreign socket, and
      the "request" event is deleted from the event queue.  Should any
      other "request" event exist in the queue, the NCP automatically
      denies connection by sending out a CLS command and deleting the

      e. REJECT <my socket code>

      The user rejects connection with the foreign socket whose
      "request" event is earliest in the event queue for <my socket>.
      The NCP sends out a CLS command and deletes the "request" event
      from the queue.

      f. CLOSE <my socket code>

      The user directs the NCP to disconnect any active connection to
      this socket and to deactivate the socket.  The NCP sends out a CLS
      command to the foreign socket if a connection has existed.  The
      status of the foreign socket also becomes closed once the "close"
      event is disclosed to the foreign process.

      g. INTERRUPT <my socket code>

      The user directs the NCP to send out an INT command to the foreign
      socket connected to <my socket>.

                                                               [Page 10]

RFC 46                ARPA Network Protocol Notes             April 1970

      h. TRANSMIT <my socket code> <pointer> <nbits>

      The user wishes to read (<my socket> is receive) or write (<my
      socket> is send) <nbits> of data into or out of an area pointed to
      by <pointer>.  A call to write returns immediately after the NCP
      has queued the data to send a message over the connection.  The
      call to write blocks only if the connection is blocked or if the
      local NCP is too loaded to process the request immediately.  Data
      to be transmitted over a connection is formatted into one or more
      IMP messages of maximum length 8095 bits and transmitted to the
      foreign HOST over the link number specified in the RFC sent by the
      NCP controlling the receiving connection.  A "close" event in the
      event queue for <my socket> is disclosed through the action of
      TRANSMIT.  A call to write discloses the "close" event
      immediately.  A read call discloses it when all data has been

The History of a Connection From a User View

An Illustrative Example

   Assume that process 'a' on HOST A wishes to establish connection with
   process 'b' on HOST B.  Before communication can take place, two
   conditions must be fulfilled:

      a. process 'a' must be able to specify to its NCP a socket in 'b's
      socket space to which it wants to connect.

      b. process 'b' must already be LISTENing to this socket.

   1. Establishing the Connection

      a. process 'b' LISTENs to socket 'Bb9'.

      b. process 'a' INITs 'Bb9' to its 'Aa12'.  The NCP at A generates
      an RFC specifying link number = 47, which it chooses from its
      available set of links.  This is the link over which it will
      receive messages if the connection is ACCEPTed by process 'b'.

      c. process 'b' is informed of A's INIT request.  He may REJECT
      connection (NCP B sends back a CLS) or ACCEPT (NCP B sends back an

      d. If process 'b' ACCEPTs, the confirming RFC establishes the
      connection, and messages can now flow.

                                                               [Page 11]

RFC 46                ARPA Network Protocol Notes             April 1970

          HOST  A               |          HOST B
          INITIATOR             |          ACCEPTOR
          PROCESS 'a'           |          PROCESS 'b'
                                |  a. LISTEN 'socket code 9'
 b. INIT 'socket code 12' 'Bb9' |
      RFC 'AA12' 'Bb9' 'link 47' ==========>
                                | c. ACCEPT 'socket code 9'
                                |        RFC 'Bb9' 'Aa12'
                                | d. TRANSMIT 'send buffer' 'len'
                                |                        'socket 9'
                     <============== IMP message 'link 47' 'send buffer'
 e. TRANSMIT 'rec buffer' 'length'
                    'socket 12' ============>
                                | f. CLOSE 'socket code 9'
                             last RFNM ===>
                      <============== CLS 'Bb9' 'Aa12'
     closes socket 'Aa12'       |

     FIG 2.  Establishing and Communicating over a Socket Connection

   2. Sending Messages over a Connection.

      a. Process 'b' issues a TRANSMIT call to send data through the
      connection.  NCP B formats this into an IMP message and sends it
      to NCP A with link number = 47 as specified by A's RFC.

      b. NCP A receives the raw message from NCP B with link number =
      47.  NCP A uses this link number in deciding who the intended
      recipient is, and stores the message in a buffer for the recipient

      c. Process 'a' may issue a read (TRANSMIT) call for socket code 12
      at any arbitrary time.  The read call blocks if there is no data
      pending for the socket.  The read call picks up the specified
      number of bits transmitted over socket code 12, perhaps across an
      IMP message boundary.  The boundaries of the IMP messages are
      invisible to the read call.

                                                               [Page 12]

RFC 46                ARPA Network Protocol Notes             April 1970

      d. Should process 'b' send data over the connection at a faster
      rate than process 'a' picks it up, NCP A can issue a BLK command
      to NCP B if A's buffers start filling.  Later, when process 'a'
      catches up NCP A can tell B to resume transmission via an RSM

   3. Process 'b' Closes the Connection

      a. Process 'b' decides to close the connection, and it issues the
      CLOSE call to NCP B.  To avoid race problems B waits for the RFNM
      from the previous message over this connection, then sends the CLS
      command to NCP A.  When the RFNM from the CLS command message
      returns, NCP B flushes socket 'Bb9' from its tables, effecting the
      close at its end and deactivating 'Bb9'.

      b. Because of sequential processing within NCP A, the last message
      to socket 'Aa12' is guaranteed to have been directed to a process
      before the CLS from NCP B comes through.  Upon receipt of the CLS
      from B, NCP A marks socket 'Aa12' as "close pending" and places a
      "close" event into the event queue of 'Aa12'.

      c. Process 'a' can still issue read calls for socket 'Aa12' while
      there is buffered data pending.  When 'a' issues a read call after
      the buffer has been emptied, the "close" event is disclosed to
      inform 'a' of the closure, and socket 'Aa12' is flushed from the
      tables of NCP A.

   4. Process 'a' Closes the Connection

      a. Let us return to step 2. and assume that process 'a' wants to
      close the connection from its end.  There is no race problem
      because we assume that once 'a' issues a CLOSE call, it no longer
      wants to read messages over that socket.

      b. Assume that process 'a' issues a CLOSE call on socket 'Aa12'.
      NCP A immediately sends out a CLS command to NCP B and marks
      socket 'Aa12' as "close pending".  Any data buffered for read on
      'Aa12' is discarded.  To allow remaining messages already in
      transit from process 'b' to percolate through the IMP network to
      NCP A and be discarded without error comments, NCP A retains
      'Aa12' in its tables for a suitable period of time after receiving
      the RFNM from the CLS command.  During this period NCP A discards
      all messages received over the closing connection.  After allowing
      a reasonable amount of time for these dead messages to come in,
      NCP A flushes 'Aa12' from its tables, effectively closing the
      connection and deactivating 'Aa12'.  Further messages to socket
      'Aa12' result in NCP A sending an ERR "erroneous command" to the
      originating NCP.

                                                               [Page 13]

RFC 46                ARPA Network Protocol Notes             April 1970

      c. When NCP B receives the CLS command, socket 'Bb9' is marked as
      "close pending", and the CLS event is placed into the event queue
      of 'Bb9'.  The next time process 'b' wishes to write over that
      socket, the CLS event is disclosed to inform him of the closure,
      and socket 'Bb9' is removed from NCP B's tables.


   Some process must exist which agrees to listen to anybody in the
   network and create a process for him upon proper identification.
   This process is called the logger and interacts through the NCP via
   the network-related User Control and Communication (UCC) module,
   which implements the necessary protocol.  Except for one instance
   (CLOSEing connections of dead processes), the process operating the
   UCC module does not have special network privileges.

   Under the UCC protocol a "requestor" process which has directed the
   creation of a "foreign" process maintains two full-duplex pseudo-
   typewriter connections:  one to the foreign logger, and one to the
   created process.  The duplex connection to the foreign logger is used
   to identify the requestor process to the logger, and after login to
   return to the requestor process basic information concerning the
   health of the created process.  The duplex connection to the created
   process is used for control communication to it.

   Maintaining two full-duplex connections avoids reconnection problems
   both when the logger transfers communication to the created process
   and when it needs to regain control.  This is at the modest expense
   of requiring the requestor process to switch typewriter
   communications between two sets of connections.

   The way that communication is established is essentially as follows:
   the requestor process first reserves four of its sockets having
   contiguous socket codes.  Then it "signals" the UCC, specifying one
   of these sockets.  From the "signal" the UCC knows which process is
   calling, and by protocol, on which requestor socket pair the UCC is
   to communicate with the requestor process, and which requestor socket
   pair the created process is to use for its communications.  This is
   specified below in more detail.

Establishing and Operating a Remote Process

   The UCC at each HOST always keeps a send socket with user number = 0,
   instance tag = 0 open (active and unconnected) as a "signal" socket,
   and periodically checks for INITs to this socket.  Processes wishing
   to create a process at this HOST must first signal the UCC by issuing
   an INIT to this socket.

                                                               [Page 14]

RFC 46                ARPA Network Protocol Notes             April 1970

   The requesting process must have four free sockets with contiguous
   socket codes:  <base_socket> (receive) through <base_socket+3>
   (send).  The high numbered send/receive set of sockets is used for
   typewriter communication with the foreign UCC, the low numbered set
   for typewriter communication with the created process.

   1. The "requestor" process calls LISTEN twice to open the
   <base_socket+2> and <base_socket+3> receive/send pair over which it
   will talk to the foreign UCC.  Then it sends out a "signalling" INIT
   call on <base_socket> to the UCC "signal" socket.  The only thing
   that the UCC does with this "signalling" INIT call is to note down
   the socket number <base_socket> from which it originated.  The UCC
   immediately rejects this request so as to keep its "signal" socket
   open for other signals.

   2. After receiving the expected REJECT on its initial INIT call to
   the UCC's signal socket, the requestor process issues LISTENs for
   <base_socket> and <base_socket+1>.  (The created process will INIT
   these sockets to establish control communication with the requestor
   process.)  The requestor process then blocks by calling STATUS
   <base_socket+2> .

   3.  The UCC INITs a free send/receive socket pair to the requestor's
   <base_socket+2> and <base_socket+3> on which the requestor process is
   presumably LISTENing.  The requestor process has called STATUS
   <base_socket+2> with block option after LISTENing for the two
   sockets, so that when the INIT from the foreign UCC reaches the
   requestor process, STATUS returns with the INIT indication.  The
   requestor process verifies that the UCC is the process that is
   calling, then it ACCEPTs the call.  The requestor process then calls
   STATUS <base_socket+3> and returns when the INIT for that socket
   reaches it.  It does a similar verify and ACCEPT.  (There is an
   arbitrary choice as for which socket the requestor process first
   calls STATUS.)  Two way communication is established when the
   requestor process has ACCEPTed both INITs from the UCC.  This
   connection is maintained during the login ritual and throughout the
   life of the created process.  Should the requestor process fail to
   respond properly within a limited amount of time to the INITs of the
   UCC, the UCC abandons the connection attempt.

   4. The requestor process must then perform the login ritual with the
   UCC.  (The initial protocol might standardize the login ritual.)  If
   the logger is not satisfied and wishes to cut off the requestor, the
   UCC module CLOSEs both <base_socket+2> and <base_socket+3>, perhaps
   after the logger has sent a suitable message.

                                                               [Page 15]

RFC 46                ARPA Network Protocol Notes             April 1970

   5.  If satisfied, the logger creates a process for the user.  The UCC
   maintains direct communication with the requestor, but this
   connection is now used only to report basic information concerning
   the created process.

   6. The first task of a created process is to establish a dual
   pseudo-typewriter control connection with its requestor process.  The
   created process INITs one of its send/receive socket pairs to the
   requestor's <base_socket> and <base_socket+1>.  If both requests are
   ACCEPTed, the created process sends an initial message over this
   connection.  Then it goes to command level, in which it awaits a
   typewriter command message over the connection.  If the created
   process is unable to establish duplex communication with the
   requestor process, it should destroy itself.  The UCC will either
   CLOSE its own connections with the requestor or make arrangements for
   another process to be created.

   7. When a created process is logged-out, the UCC uses a privileged
   entry to the NCP to CLOSE all connections between the dead process
   and other processes, and to deactivate all open sockets of the dead
   process.  The UCC transmits a message back to the requestor process,
   then CLOSEs the dual connections between it and the requestor

   8. The INTERRUPT call has a standard "quit" meaning when sent from a
   requestor process to a created process over the requestor's receive
   socket <base_socket>.  All pending output from the created process is
   aborted, and the it enters "command level" where it awaits a command
   over the typewriter connection to the requestor process.  The
   interrupted processing is resumable by issuing a "start" command to
   the created process.  (Note that the rule about pending output is
   more restrictive than that implemented by the INT NCP command.)

      This document was prepared through the use of the MULTICS "runoff"
      command.  A source file consisting of intermixed text and "runoff"
      requests was created using the "qed" text editor.  This file was
      then compiled by the "runoff" command to produce a finished copy.
      The latest version of this document exists online in MULTICS in
      the segment



                                                               [Page 16]

RFC 46                ARPA Network Protocol Notes             April 1970

      REQUESTOR                                  FOREIGN
      PROCESS                                    LOGGER
      --------------                             -------------
      a. LISTEN to sockets
      <base_socket+2> and
      <base_socket+3> to be
      connected to foreign logger.

      b. INIT <base_socket>
      to "signal" socket of
      foreign logger.

                                                c. remember <base_socket>
                                                   and REJECT connection
                                                   to signal socket.

      d. LISTEN to sockets                      e. INIT a logger socket
      <base_socket> and                            pair to the requestor's
      <base_socket_1> to be                       <base_socket+2> and
      connected to the created  process.          <base_socket+3>.

      f. ACCEPT connection
      with sockets from
      foreign logger.

                             PERFORM LOGIN RITUAL
                                                g. INIT any socket pair
                                                   to requestor's
                                                   <base_socket> and
      h. ACCEPT connection
      with sockets from created

               FIG. 4 Establishing a Process at a Foreign HOST

          [ This RFC was put into machine readable form for entry ]
          [ into the online RFC archives by Miles McCredie 11/99  ]

                                                               [Page 17]