Deallocating a half-duplex conversation normally

Half-duplex conversations are normally deallocated by the transaction program in SEND or PENDING_SEND state. The deallocation can be unconditional or conditional. It can also include a request to send data or transmit buffered data to the partner LU.

Normal conversation deallocation occurs when the side of the conversation in a sending state issues an APPCCMD CONTROL=DEALLOC macroinstruction. The partner application program is informed of the deallocation request through feedback on an APPCCMD macroinstruction (probably an APPCCMD CONTROL=RECEIVE macroinstruction).

The application program in a receiving state can be informed of the deallocation by way of a bit (DEALLOCATE) in the What-Received field of the RPL extension on an APPCCMD CONTROL=RECEIVE macroinstruction. It also can be informed of the deallocation if it attempts to issue an APPCCMD CONTROL=SEND, QUALIFY=ERROR macroinstruction. If it attempts to issue an APPCCMD CONTROL=SEND, QUALIFY=ERROR macroinstruction, it receives an error return code indicating the partner has deallocated the conversation.

In any of the varieties of the CONTROL=DEALLOC macroinstruction, any buffered data that has not yet been sent to the partner LU is transmitted through the network. The various forms of this macroinstruction involve two major choices:
  • Whether to send more data
  • Whether to send a confirmation request

If the application program will do neither, it should use the QUALIFY=FLUSH form of the macroinstruction. The QUALIFY=DATAFLU form sends additional data and deallocates the conversation. The QUALIFY=CONFIRM form sends a confirmation request and deallocates the conversation. The QUALIFY=DATACON form sends both data and a confirmation request in addition to deallocating the conversation. These choices are shown in Table 1.

Table 1. Macroinstructions for normal deallocation of half-duplex conversations
Sends Data Confirm Macroinstruction
Yes Yes DEALLOC/DATACON
No Yes DEALLOC/CONFIRM
Yes No DEALLOC/DATAFLU
No No DEALLOC/FLUSH
Parent topic: Deallocating a half-duplex conversation