rfc1647.txt

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      When establishing an SNA session on behalf of a client, the server
      will receive a Bind RU from the host application.  It will also
      receive a Start Data Traffic RU.  Once both of these have been
      responded to positively by the server, it must then inform the
      client of the presence of this session by sending it a data
      message with the DATA-TYPE flag set to BIND-IMAGE.  The data
      portion of this message must contain the bind image exactly as it
      was received in the Bind RU that the server accepted on behalf of
      the client.




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      When an SNA session between the server and a host application is
      terminated, the server should send a data message to the client
      with the DATA-TYPE flag set to UNBIND.  If the server was notified
      of the session termination via an SNA Unbind RU, it should include
      the Unbind reason code in the data portion of the message it sends
      to the client.  If the server itself requested the SNA session
      termination (for example, as part of SYSREQ key processing), it
      should set the data portion of the UNBIND message to 0x01,
      indicating "normal end of session".

      Another aspect of the BIND-IMAGE function alters the allowable
      DATA-TYPE flag values slightly from the behavior described in the
      section entitled "Basic TN3270E".  When BIND-IMAGE is in effect,
      data messages with DATA-TYPE set to 3270-DATA or SCS-DATA are not
      allowed before the first BIND-IMAGE is received by the client;
      only SSCP-LU-DATA or NVT-DATA can be used to transmit user-
      oriented data.  The same applies to data messages exchanged after
      an UNBIND is sent and before another BIND-IMAGE is received by the
      client.  Once the client receives a BIND-IMAGE data message, the
      allowable DATA-TYPE values include 3270-DATA and/or SCS-DATA,
      depending on whether a terminal or printer device-type was
      negotiated, and whether a printer client agreed to DATA-STREAM-CTL
      or SCS-CTL-CODES, or both.  (See the section entitled "The SYSREQ
      Function" for further discussion of the SSCP-LU session in an SNA
      environment.)

   10.4 The RESPONSES Function

      This function can be supported for both terminal and printer
      sessions connected to both SNA and non-SNA servers.

      Agreement to support this function requires that the party support
      the following TN3270E header values:

          Header field         Value
          ------------         -----
           DATA-TYPE          RESPONSE
           DATA-TYPE          REQUEST
           RESPONSE-FLAG      -all values-
           REQUEST-FLAG       ERR-COND-CLEARED
           SEQ-NUMBER         binary values from 0-32767

      Whenever a data message is sent with a DATA-TYPE of either SCS-
      DATA or 3270-DATA, the sender must set the RESPONSE-FLAG field to
      either NO-RESPONSE, ERROR-RESPONSE, or ALWAYS-RESPONSE.  It is
      anticipated that the client side will normally set RESPONSE-FLAG
      to NO-RESPONSE.  The server, if it represents an SNA device,
      should set RESPONSE-FLAG to reflect the response value set in the



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      RH of the RU that generated this data message - Definite Response
      resulting in a RESPONSE-FLAG value of ALWAYS-RESPONSE, Exception
      Response resulting in ERROR-RESPONSE being set, and No Response
      causing a setting of NO-RESPONSE.  A non-SNA server should set
      RESPONSE-FLAG to ERROR-RESPONSE.

      In addition, the sender must keep a count of the messages with a
      DATA-TYPE of 3270-DATA or SCS-DATA that it sends on a given
      session.  This counter should start at zero for the first such
      message, and be incremented by one for each subsequent message.
      If the counter reaches the maximum of 32767, it should be
      restarted at zero.  The sender should place this value in the
      SEQ-NUMBER field of the TN3270E header before it sends the
      message.  Note that the SEQ-NUMBER field must be set regardless of
      the value of the RESPONSE-FLAG field.

      10.4.1 Response Messages

         Whenever a data message with a DATA-TYPE of either SCS-DATA or
         3270-DATA is received, the receiver must attempt to process the
         data in the data portion of the message, then determine whether
         or not it should send a data message with a DATA-TYPE of
         RESPONSE.  If the data message it has just processed had a
         RESPONSE-FLAG value of NO-RESPONSE, or if it had a value of
         ERROR-RESPONSE and there were no errors encountered while
         processing the data, then no RESPONSE type message should be
         sent.  Otherwise, a data message should be sent in which the
         header DATA-TYPE field is set to RESPONSE, and in which the
         SEQ-NUMBER field is a copy of the SEQ-NUMBER field from the
         message to which this response corresponds.  The RESPONSE-FLAG
         field in this header must have a value of either POSITIVE-
         RESPONSE or NEGATIVE-RESPONSE.  A POSITIVE-RESPONSE should be
         sent if the previously processed message's header specified
         ALWAYS-RESPONSE and no errors were encountered in processing
         the data.  A NEGATIVE-RESPONSE should be sent when

          1) the previously processed message specified ERROR-RESPONSE
             or ALWAYS-RESPONSE and

          2) some kind of error occurred while processing the data.

         Normally only the client will be constructing and sending these
         RESPONSE messages.  A negative response sent by the client to
         the server is the equivalent of a Unit Check Status [7].  All
         references to device status and sense codes in this section
         rely on [7].





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         The data portion of a RESPONSE message must consist of one byte
         of binary data.  The value of this byte gives a more detailed
         account of the results of having processed the previously
         received data message.  The possible values for this byte are:

           For a RESPONSE-FLAG value of POSITIVE-RESPONSE -

             Value            Meaning
             -----            -------
             0x00      Successful completion (when sent by the client,
                       this is equivalent to "Device End").

           For a RESPONSE-FLAG value of NEGATIVE-RESPONSE -

             Value            Meaning
             -----            -------
             0x00      An invalid 3270 command was received
                       (equivalent to "Command Reject").

             0x01      Printer is not ready (equivalent to
                       "Intervention Required").

             0x02      An illegal 3270 buffer address or order
                       sequence was received (equivalent to
                       "Operation Check").

             0x03      Printer is powered off or not connected
                       (equivalent to "Component Disconnected").

         When the server receives any of the above responses, it should
         pass along the appropriate information to the host application.
         The appropriate information is determined by whether the server
         represents an SNA or a non-SNA device.

         An SNA server should pass along a POSITIVE-RESPONSE from the
         client as an SNA positive Response Unit to the host
         application.  It should translate a NEGATIVE-RESPONSE from the
         client into an SNA negative Response Unit in which the Sense
         Data Indicator bit is on and which contains one of the
         following sense codes:











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             RESPONSE-FLAG        Equivalent        SNA Sense Code
             -------------        ----------        --------------
                 0x00           Command Reject        0x10030000

                 0x01        Intervention Required    0x08020000

                 0x02           Operation Check       0x10050000

                 0x03        Component Disconnected   0x08310000

         A non-SNA server should pass along a POSITIVE-RESPONSE from the
         client by setting the Device End Status bit on.  It should
         reflect a NEGATIVE-RESPONSE from the client by setting the Unit
         Check Status Bit on, and setting either the Command Reject,
         Intervention Required, or Operation Check Sense bit on when
         responding to the Sense command.

         In the case of Intervention Required or Component Disconnected
         being passed by the server to the host application, the host
         would normally refrain from sending any further data to the
         printer.  If and when the error condition at the client has
         been resolved, the client must send to the server a data
         message whose header DATA-TYPE field is set to REQUEST, and
         whose REQUEST-FLAG is set to ERR-COND-CLEARED.  Note that this
         message has no data portion.  Upon receipt of this message, the
         server should pass along the appropriate information to the
         host application so that it may resume sending printer output.
         Again, the form of this information depends on whether the
         server represents an SNA or a non-SNA device.

         An SNA server should reflect an ERR-COND-CLEARED to the host
         application by sending an SNA LUSTAT RU with one of the
         following sense codes:

          - if the previous error condition was an Intervention
            Required, the server should send sense code 0x00010000

          - if the previous error condition was Component
            Disconnected, the server should send sense code 0x082B0000

         A non-SNA server should set the corresponding bits in the
         Ending Status and Sense Condition bytes.









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   10.5 The SYSREQ Function

      This function can only be supported when the TN3270E server
      represents SNA devices.

      Agreement to support this function requires that the party support
      the following TN3270E header values:

          Header field         Value
          ------------         -----
           DATA-TYPE          SSCP-LU-DATA

      The 3270 SYSREQ key can be useful in an SNA environment when the
      ATTN key is not sufficient to terminate a process.  (See the
      section entitled "The 3270 ATTN Key" for more information.)

      10.5.1 Background

         In SNA, there is a session between the host application (the
         PLU, or Primary Logical Unit) and the TN3270E server
         representing the client (the SLU, or Secondary Logical Unit).
         This is referred to as the PLU-SLU session, and it is the one
         on which normal communications flow.  There is also a session
         between the host telecommunications access method (the SSCP, or
         System Services Control Point) and the SLU, and it is referred
         to as the SSCP-LU session.  This session is used to carry
         various control information and is normally transparent to the
         user; normal 3270 data stream orders are not allowed in this
         data.  For more information, refer to [7].

         The terminal display and keyboard are usually "owned" by the
         PLU-SLU session, meaning any data the user types is sent to the
         host application.  The SYSREQ key is used to toggle ownership
         of the keyboard and display between the PLU-SLU session and the
         SSCP-LU session.  In other words, the user is able to press
         SYSREQ and then communicate directly with the host SSCP.  The
         user may then enter any valid Unformatted Systems Services
         commands, which are defined in the USS table associated with
         the SLU.  The most common USS command users employ is "LOGOFF,"
         which requests that the SSCP immediately terminate the PLU-SLU
         session.  The usual reason for requesting such an action is
         that the host application (the PLU) has stopped responding
         altogether.

         Whenever the keyboard and display are owned by the SSCP-LU
         session, no data is allowed to flow in e