Connected: An Internet Encyclopedia
3.8 Interfaces

Up: Connected: An Internet Encyclopedia
Up: Requests For Comments
Up: RFC 793
Prev: 3.7 Data Communication
Next: 3.9 Event Processing

3.8 Interfaces

3.8 Interfaces

There are of course two interfaces of concern: the user/TCP interface and the TCP/lower-level interface. We have a fairly elaborate model of the user/TCP interface, but the interface to the lower level protocol module is left unspecified here, since it will be specified in detail by the specification of the lowel level protocol. For the case that the lower level is IP we note some of the parameter values that TCPs might use.

User/TCP Interface

    The following functional description of user commands to the TCP is,
    at best, fictional, since every operating system will have different
    facilities.  Consequently, we must warn readers that different TCP
    implementations may have different user interfaces.  However, all
    TCPs must provide a certain minimum set of services to guarantee
    that all TCP implementations can support the same protocol
    hierarchy.  This section specifies the functional interfaces
    required of all TCP implementations.

    TCP User Commands

      The following sections functionally characterize a USER/TCP
      interface.  The notation used is similar to most procedure or
      function calls in high level languages, but this usage is not
      meant to rule out trap type service calls (e.g., SVCs, UUOs,

      The user commands described below specify the basic functions the
      TCP must perform to support interprocess communication.
      Individual implementations must define their own exact format, and
      may provide combinations or subsets of the basic functions in
      single calls.  In particular, some implementations may wish to
      automatically OPEN a connection on the first SEND or RECEIVE
      issued by the user for a given connection.

      In providing interprocess communication facilities, the TCP must
      not only accept commands, but must also return information to the
      processes it serves.  The latter consists of:

        (a) general information about a connection (e.g., interrupts,
        remote close, binding of unspecified foreign socket).

        (b) replies to specific user commands indicating success or
        various types of failure.


        Format:  OPEN (local port, foreign socket, active/passive
        [, timeout] [, precedence] [, security/compartment] [, options])
        -> local connection name

        We assume that the local TCP is aware of the identity of the
        processes it serves and will check the authority of the process
        to use the connection specified.  Depending upon the
        implementation of the TCP, the local network and TCP identifiers
        for the source address will either be supplied by the TCP or the
        lower level protocol (e.g., IP).  These considerations are the
        result of concern about security, to the extent that no TCP be
        able to masquerade as another one, and so on.  Similarly, no
        process can masquerade as another without the collusion of the

        If the active/passive flag is set to passive, then this is a
        call to LISTEN for an incoming connection.  A passive open may
        have either a fully specified foreign socket to wait for a
        particular connection or an unspecified foreign socket to wait
        for any call.  A fully specified passive call can be made active
        by the subsequent execution of a SEND.

        A transmission control block (TCB) is created and partially
        filled in with data from the OPEN command parameters.

        On an active OPEN command, the TCP will begin the procedure to
        synchronize (i.e., establish) the connection at once.

        The timeout, if present, permits the caller to set up a timeout
        for all data submitted to TCP.  If data is not successfully
        delivered to the destination within the timeout period, the TCP
        will abort the connection.  The present global default is five

        The TCP or some component of the operating system will verify
        the users authority to open a connection with the specified
        precedence or security/compartment.  The absence of precedence
        or security/compartment specification in the OPEN call indicates
        the default values must be used.

        TCP will accept incoming requests as matching only if the
        security/compartment information is exactly the same and only if
        the precedence is equal to or higher than the precedence
        requested in the OPEN call.

        The precedence for the connection is the higher of the values
        requested in the OPEN call and received from the incoming
        request, and fixed at that value for the life of the
        connection.Implementers may want to give the user control of
        this precedence negotiation.  For example, the user might be
        allowed to specify that the precedence must be exactly matched,
        or that any attempt to raise the precedence be confirmed by the

        A local connection name will be returned to the user by the TCP.
        The local connection name can then be used as a short hand term
        for the connection defined by the <local socket, foreign socket>


        Format:  SEND (local connection name, buffer address, byte
        count, PUSH flag, URGENT flag [,timeout])

        This call causes the data contained in the indicated user buffer
        to be sent on the indicated connection.  If the connection has
        not been opened, the SEND is considered an error.  Some
        implementations may allow users to SEND first; in which case, an
        automatic OPEN would be done.  If the calling process is not
        authorized to use this connection, an error is returned.

        If the PUSH flag is set, the data must be transmitted promptly
        to the receiver, and the PUSH bit will be set in the last TCP
        segment created from the buffer.  If the PUSH flag is not set,
        the data may be combined with data from subsequent SENDs for
        transmission efficiency.

        If the URGENT flag is set, segments sent to the destination TCP
        will have the urgent pointer set.  The receiving TCP will signal
        the urgent condition to the receiving process if the urgent
        pointer indicates that data preceding the urgent pointer has not
        been consumed by the receiving process.  The purpose of urgent
        is to stimulate the receiver to process the urgent data and to
        indicate to the receiver when all the currently known urgent
        data has been received.  The number of times the sending user's
        TCP signals urgent will not necessarily be equal to the number
        of times the receiving user will be notified of the presence of
        urgent data.

        If no foreign socket was specified in the OPEN, but the
        connection is established (e.g., because a LISTENing connection
        has become specific due to a foreign segment arriving for the
        local socket), then the designated buffer is sent to the implied
        foreign socket.  Users who make use of OPEN with an unspecified
        foreign socket can make use of SEND without ever explicitly
        knowing the foreign socket address.

        However, if a SEND is attempted before the foreign socket
        becomes specified, an error will be returned.  Users can use the
        STATUS call to determine the status of the connection.  In some
        implementations the TCP may notify the user when an unspecified
        socket is bound.

        If a timeout is specified, the current user timeout for this
        connection is changed to the new one.

        In the simplest implementation, SEND would not return control to
        the sending process until either the transmission was complete
        or the timeout had been exceeded.  However, this simple method
        is both subject to deadlocks (for example, both sides of the
        connection might try to do SENDs before doing any RECEIVEs) and
        offers poor performance, so it is not recommended.  A more
        sophisticated implementation would return immediately to allow
        the process to run concurrently with network I/O, and,
        furthermore, to allow multiple SENDs to be in progress.
        Multiple SENDs are served in first come, first served order, so
        the TCP will queue those it cannot service immediately.

        We have implicitly assumed an asynchronous user interface in
        which a SEND later elicits some kind of SIGNAL or
        pseudo-interrupt from the serving TCP.  An alternative is to
        return a response immediately.  For instance, SENDs might return
        immediate local acknowledgment, even if the segment sent had not
        been acknowledged by the distant TCP.  We could optimistically
        assume eventual success.  If we are wrong, the connection will
        close anyway due to the timeout.  In implementations of this
        kind (synchronous), there will still be some asynchronous
        signals, but these will deal with the connection itself, and not
        with specific segments or buffers.

        In order for the process to distinguish among error or success
        indications for different SENDs, it might be appropriate for the
        buffer address to be returned along with the coded response to
        the SEND request.  TCP-to-user signals are discussed below,
        indicating the information which should be returned to the
        calling process.


        Format:  RECEIVE (local connection name, buffer address, byte
        count) -> byte count, urgent flag, push flag

        This command allocates a receiving buffer associated with the
        specified connection.  If no OPEN precedes this command or the
        calling process is not authorized to use this connection, an
        error is returned.

        In the simplest implementation, control would not return to the
        calling program until either the buffer was filled, or some
        error occurred, but this scheme is highly subject to deadlocks.
        A more sophisticated implementation would permit several
        RECEIVEs to be outstanding at once.  These would be filled as
        segments arrive.  This strategy permits increased throughput at
        the cost of a more elaborate scheme (possibly asynchronous) to
        notify the calling program that a PUSH has been seen or a buffer

        If enough data arrive to fill the buffer before a PUSH is seen,
        the PUSH flag will not be set in the response to the RECEIVE.
        The buffer will be filled with as much data as it can hold.  If
        a PUSH is seen before the buffer is filled the buffer will be
        returned partially filled and PUSH indicated.

        If there is urgent data the user will have been informed as soon
        as it arrived via a TCP-to-user signal.  The receiving user
        should thus be in "urgent mode".  If the URGENT flag is on,
        additional urgent data remains.  If the URGENT flag is off, this
        call to RECEIVE has returned all the urgent data, and the user
        may now leave "urgent mode".  Note that data following the
        urgent pointer (non-urgent data) cannot be delivered to the user
        in the same buffer with preceeding urgent data unless the
        boundary is clearly marked for the user.

        To distinguish among several outstanding RECEIVEs and to take
        care of the case that a buffer is not completely filled, the
        return code is accompanied by both a buffer pointer and a byte
        count indicating the actual length of the data received.

        Alternative implementations of RECEIVE might have the TCP

        allocate buffer storage, or the TCP might share a ring buffer
        with the user.


        Format:  CLOSE (local connection name)

        This command causes the connection specified to be closed.  If
        the connection is not open or the calling process is not
        authorized to use this connection, an error is returned.
        Closing connections is intended to be a graceful operation in
        the sense that outstanding SENDs will be transmitted (and
        retransmitted), as flow control permits, until all have been
        serviced.  Thus, it should be acceptable to make several SEND
        calls, followed by a CLOSE, and expect all the data to be sent
        to the destination.  It should also be clear that users should
        continue to RECEIVE on CLOSING connections, since the other side
        may be trying to transmit the last of its data.  Thus, CLOSE
        means "I have no more to send" but does not mean "I will not
        receive any more."  It may happen (if the user level protocol is
        not well thought out) that the closing side is unable to get rid
        of all its data before timing out.  In this event, CLOSE turns
        into ABORT, and the closing TCP gives up.

        The user may CLOSE the connection at any time on his own
        initiative, or in response to various prompts from the TCP
        (e.g., remote close executed, transmission timeout exceeded,
        destination inaccessible).

        Because closing a connection requires communication with the
        foreign TCP, connections may remain in the closing state for a
        short time.  Attempts to reopen the connection before the TCP
        replies to the CLOSE command will result in error responses.

        Close also implies push function.


        Format:  STATUS (local connection name) -> status data

        This is an implementation dependent user command and could be
        excluded without adverse effect.  Information returned would
        typically come from the TCB associated with the connection.

        This command returns a data block containing the following

          local socket,
          foreign socket,
          local connection name,
          receive window,
          send window,
          connection state,
          number of buffers awaiting acknowledgment,
          number of buffers pending receipt,
          urgent state,
          and transmission timeout.

        Depending on the state of the connection, or on the
        implementation itself, some of this information may not be
        available or meaningful.  If the calling process is not
        authorized to use this connection, an error is returned.  This
        prevents unauthorized processes from gaining information about a


        Format:  ABORT (local connection name)

        This command causes all pending SENDs and RECEIVES to be
        aborted, the TCB to be removed, and a special RESET message to
        be sent to the TCP on the other side of the connection.
        Depending on the implementation, users may receive abort
        indications for each outstanding SEND or RECEIVE, or may simply
        receive an ABORT-acknowledgment.

    TCP-to-User Messages

      It is assumed that the operating system environment provides a
      means for the TCP to asynchronously signal the user program.  When
      the TCP does signal a user program, certain information is passed
      to the user.  Often in the specification the information will be
      an error message.  In other cases there will be information
      relating to the completion of processing a SEND or RECEIVE or
      other user call.

      The following information is provided:

        Local Connection Name                    Always
        Response String                          Always
        Buffer Address                           Send & Receive
        Byte count (counts bytes received)       Receive
        Push flag                                Receive
        Urgent flag                              Receive
  TCP/Lower-Level Interface

    The TCP calls on a lower level protocol module to actually send and
    receive information over a network.  One case is that of the ARPA
    internetwork system where the lower level module is the Internet
    Protocol (IP) [2].

    If the lower level protocol is IP it provides arguments for a type
    of service and for a time to live.  TCP uses the following settings
    for these parameters:

      Type of Service = Precedence: routine, Delay: normal, Throughput:
      normal, Reliability: normal; or 00000000.

      Time to Live    = one minute, or 00111100.

        Note that the assumed maximum segment lifetime is two minutes.
        Here we explicitly ask that a segment be destroyed if it cannot
        be delivered by the internet system within one minute.

    If the lower level is IP (or other protocol that provides this
    feature) and source routing is used, the interface must allow the
    route information to be communicated.  This is especially important
    so that the source and destination addresses used in the TCP
    checksum be the originating source and ultimate destination. It is
    also important to preserve the return route to answer connection

    Any lower level protocol will have to provide the source address,
    destination address, and protocol fields, and some way to determine
    the "TCP length", both to provide the functional equivlent service
    of IP and to be used in the TCP checksum.

Next: 3.9 Event Processing

Connected: An Internet Encyclopedia
3.8 Interfaces