The VTAM® application program sends and receives most requests and responses on a session by using SEND and RECEIVE and by using certain exit routines. A VTAM application program can receive some requests (expedited-flow data-flow-control requests) and some responses by having VTAM schedule an exit routine designed to handle these requests (a DFASY exit routine) and responses (a RESP exit routine). Alternately, requests and responses can be handled by RECEIVE RTYPE=DFASY or RECEIVE RTYPE=RESP, respectively. See Summary of control block field usage for information on which RPL fields are automatically set by VTAM.

If the Start Data Traffic (SDT) indicator is required by the transmission-services (TS) profile in the session parameters, the sending and receiving of most requests and responses on a session cannot begin until an SDT request has been sent from the primary logical unit (PLU) to the secondary logical unit (SLU). See Specifying a session parameter, for more information on the TS profile. When required, the SDT request must be sent at the beginning of a session, and it must be sent within a session if the request flow is to be restarted after being stopped (with a Clear request). At the beginning of a session, either VTAM or the PLU application program sends the SDT request, depending on how the SDT field of the NIB is set when OPNDST is issued. If the SDT field indicates SYSTEM, the SDT request is sent by VTAM as part of the OPNDST processing. If the SDT field indicated APPL, the SDT request must be sent by the PLU application program, using SESSIONC. SESSIONC CONTROL=CLEAR can also be used to halt the flow of requests and responses. To resume request flow after it has been stopped, the PLU application program issues SESSIONC to send the SDT request.

If the SLU is an application program, VTAM or the SLU application program can send a response to that SDT request, depending upon how the SDT field of the NIB is set when the SLU application program issues the OPNSEC macroinstruction. If the SDT field indicates SYSTEM, VTAM sends the SDT response when the SLU receives the SDT request from the PLU. If the SDT field indicates APPL, the response must be sent by the SLU application program (using a SESSIONC macroinstruction).