rfc1647.txt

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                                 contains only the 3270 data stream.

         SCS-DATA         0x01   The data portion of the message
                                 contains SNA Character Stream data.

         RESPONSE         0x02   The data portion of the message
                                 constitutes device-status information
                                 and the RESPONSE-FLAG field indicates
                                 whether this is a positive or negative
                                 response (see below).

         BIND-IMAGE       0x03   The data portion of the message is
                                 the SNA bind image from the session
                                 established between the server and the
                                 host application.

         UNBIND           0x04   The data portion of the message is
                                 an Unbind reason code.

         NVT-DATA         0x05   The data portion of the message is to
                                 be interpreted as NVT data.



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         REQUEST          0x06   There is no data portion present in
                                 the message.  Only the REQUEST-FLAG
                                 field has any meaning.

         SSCP-LU-DATA     0x07   The data portion of the message is
                                 data from the SSCP-LU session.

      8.1.2 REQUEST-FLAG Field

         The REQUEST-FLAG field only has meaning when the DATA-TYPE
         field has a value of REQUEST; otherwise, the REQUEST-FLAG field
         must be ignored by the receiver and should be set to 0x00 by
         the sender.  Possible values for the REQUEST-FLAG field are:

         Request-Flag Name   Code                Meaning
         -----------------   ----   ---------------------------------
         ERR-COND-CLEARED    0x00   The client sends this to the server
                                    when some previously encountered
                                    printer error condition has been
                                    cleared.  (See the section entitled
                                    "The RESPONSES Function" below.)

      8.1.3 RESPONSE-FLAG Field

         The RESPONSE-FLAG field only has meaning for certain values of
         the DATA-TYPE field.  For DATA-TYPE field values of 3270-DATA
         and SCS-DATA, the RESPONSE-FLAG is an indication of whether or
         not the sender of the data expects to receive a response.  In
         this case the possible values of RESPONSE-FLAG are:

         Response-Flag Name  Code                Meaning
         ------------------  ----   ---------------------------------
         NO-RESPONSE         0x00   The sender does not expect the
                                    receiver to respond either
                                    positively or negatively to this
                                    message.  The receiver must
                                    therefore not send any response
                                    to this data-message.

         ERROR-RESPONSE      0x01   The sender only expects the
                                    receiver to respond to this message
                                    if some type of error occurred, in
                                    which case a negative response must
                                    be sent by the receiver.

         ALWAYS-RESPONSE     0x02   The sender expects the receiver to
                                    respond negatively if an error
                                    occurs, or positively if no errors



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                                    occur.  One or the other must
                                    always be sent by the receiver.

         For a DATA-TYPE field value of RESPONSE, the RESPONSE-FLAG is
         an actual response to a previous data message (which must by
         definition have had a DATA-TYPE of either 3270-DATA or SCS-DATA
         and a RESPONSE-FLAG value of either ERROR-RESPONSE or ALWAYS-
         RESPONSE).  In this case the possible values of RESPONSE-FLAG
         are:

         Response-Flag Name  Code                Meaning
         ------------------  ----   ---------------------------------
         POSITIVE-RESPONSE   0x00   The previous message was received
                                    and executed successfully with
                                    no errors.

         NEGATIVE-RESPONSE   0x01   The previous message was received
                                    but an error(s) occurred while
                                    processing it.

         Accompanying status information will be found in the data
         portion of the message.

         For any other values of the DATA-TYPE field, the RESPONSE-FLAG
         field must be ignored by the receiver and should be set to 0x00
         by the sender.

      8.1.4 SEQ-NUMBER Field

         The SEQ-NUMBER field is only used when the RESPONSES function
         has been agreed to.  It contains a 2 byte binary number, and is
         used to correlate positive and negative responses to the data
         messages for which they were intended.  See the section
         entitled "The RESPONSES Function" for further information.
         When the RESPONSES function is not agreed to, this field should
         always be set to 0x0000 by the sender and ignored by the
         receiver.

9.  Basic TN3270E

   As has been stated earlier, whether or not the use of each of the
   TN3270E functions is allowed on a session is negotiated when the
   connection is established.  It is possible that none of the functions
   are agreed to (in this case, the function-list in the FUNCTIONS
   REQUEST and FUNCTIONS IS commands is null).  This mode of operation
   is referred to as "basic TN3270E".  Note that, since neither the
   SCS-CTL-CODES function nor the DATA-STREAM-CTL function is agreed to,
   basic TN3270E refers to terminal sessions only.



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   Basic TN3270E requires the support of only the following TN3270E
   header values:

          Header field         Value
          ------------         -----
           DATA-TYPE          3270-DATA
           DATA-TYPE          NVT-DATA

   The REQUEST-FLAG, RESPONSE-FLAG and SEQ-NUMBER fields are not used in
   basic TN3270E.

   9.1 3270 Mode and NVT Mode

      At any given time, a TN3270E connection can be considered to be
      operating in either "3270 mode" or "NVT mode".  In 3270 mode, each
      party may send data messages with the DATA-TYPE flag set to 3270-
      DATA; sending a DATA-TYPE flag set to NVT-DATA constitutes a
      request to switch modes.  In NVT mode, each party may send data
      messages with the DATA-TYPE flag set to NVT-DATA; sending 3270-
      DATA is a request to switch modes.  The connection is initially in
      3270 mode when TN3270E operation is successfully negotiated.  When
      a party receives a message with a DATA-TYPE different from the
      mode it is operating in, the mode of operation for the connection
      is switched.  Switching modes results in the client performing the
      equivalent of a 3270 Erase/Reset operation, as described in [5],
      using the default partition (screen) size.  The server cannot
      assume the client preserves any attributes of the previous
      environment across a mode switch.

      Note that even when sending NVT-DATA, each side should buffer data
      until an entire message is built (for the client, this would
      normally mean until the user presses Enter).  At that point, a
      complete TN3270E data message should be built to transmit the NVT
      data.

      Typically, NVT data is used by a server to interact with the user
      of a client.  It allows the server to do this using a simple NVT
      data stream, instead of requiring a 3270 data stream.  An example
      would be a server which displays a list of 3270 applications to
      which it can connect the client.  The server would use NVT data to
      display the list and read the user's choice.  Then the server
      would connect to the application, and begin the exchange of 3270
      data between the application and the client.








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10.  Details of Processing TN3270E Functions

   Agreement by both parties to a specific function in the FUNCTIONS
   REQUEST function-list implies agreement by each party to support a
   related set of values in the TN3270E header.  It also implies a
   willingness to adhere to the rules governing the processing of data
   messages with regard to the agreed upon function.  Either party that
   fails to accept header values associated either with agreed upon
   functions or with basic TN3270E, or attempts to use header values
   associated with a function that is not a part of basic TN3270E and
   was not agreed upon, will be considered non-conforming and in
   violation of the protocol.  The following sections detail for each
   TN3270E function the associated header values and processing rules.

   10.1 The SCS-CTL-CODES Function

      This function can only be supported on a 3270 printer session.

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

          Header field         Value
          ------------         -----
           DATA-TYPE          SCS-DATA

      A client representing a printer device uses this function to
      indicate its willingness to accept a data stream that includes SCS
      control codes.  For the purposes of NVT mode versus 3270 mode,
      SCS-DATA should be treated exactly like 3270-DATA (i.e., it can
      cause a switch from NVT mode to 3270 mode).

      When a printer device-type has been negotiated, either the SCS-
      CTL-CODES function or the DATA-STREAM-CTL function, or both, must
      be negotiated.  This enables the server to know when it should and
      should not accept a session with a host application on behalf of
      the client.  If only the SCS-CTL-CODES function is agreed to, then
      the server will not establish sessions with host applications that
      would send 3270 data stream control.  If both SCS-CTL-CODES and
      DATA-STREAM-CTL are agreed to, then the server will establish
      sessions both with host applications that would send SCS control
      codes and with those that would send 3270 orders.

   10.2 The DATA-STREAM-CTL Function

      This function can only be supported on a 3270 printer session.

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



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          Header field         Value
          ------------         -----
           DATA-TYPE          3270-DATA

      A client representing a printer device uses this function to
      indicate its willingness to accept a data stream that includes
      3270 orders and attributes.

      When a printer device-type has been negotiated, either the SCS-
      CTL-CODES function or the DATA-STREAM-CTL function, or both, must
      be negotiated.  This enables the server to know when it should and
      should not accept a session with a host application on behalf of
      the client.  If only the DATA-STREAM-CTL function is agreed to,
      then the server will not establish sessions with host applications
      that would send SCS control codes in a data stream.  If both SCS-
      CTL-CODES and DATA-STREAM-CTL are agreed to, then the server will
      establish sessions both with host applications that would send SCS
      control codes and with those that would send 3270 orders.

   10.3 The BIND-IMAGE Function

      This function can only be supported when the TN3270E server
      represents SNA terminals and printers.

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

          Header field         Value
          ------------         -----
           DATA-TYPE          BIND-IMAGE
           DATA-TYPE          UNBIND
           DATA-TYPE          SSCP-LU-DATA

      When BIND-IMAGE is in effect, the server must inform the client
      when an SNA session has been established with a host application,
      and when such a session has been terminated.  It uses DATA-TYPE
      values of BIND-IMAGE and UNBIND to convey this information.