DFSYN, DFASY, and RESP types of RUs

VTAM® classifies the data and data-flow-control request and response units that can be received by an application program into three types:

DFSYN RUs, the normal-flow request units for both data and data-flow-control requests
DFASY RUs, the expedited-flow data-flow-control requests
RESP RUs, normal-flow response units for both data and data-flow-control requests.

Table 1 describes these three types in more detail. Optionally, as described in Normal operating system environment for a VTAM application program, certain normal-flow response units can be treated as DFSYN RUs instead of as RESP RUs. These special responses are called DFSYN responses. The application program cannot receive expedited data-flow-control responses. These are intercepted by VTAM.

Table 1. Summary of requests and responses transmitted on normal flow and expedited flow
Normal-flow	Expedited-flow
Data	Session Control: BIND CLEAR Request Recovery (RQR) Set and Test Sequence Numbers (STSN) Start Data Traffic (SDT) UNBIND SWITCH
Data Flow Control: BID Bracket Initiation Stopped (BIS) CANCEL CHASE Logical Unit Status (LUSTAT) Quiesce Complete (QC) Ready to Receive (RTR)	Data Flow Control: Quiesce at End of Chain (QEC) Release Quiesce (RELQ) Request Shutdown (RSHUTD) Shutdown (SHUTD) Shutdown Complete (SHUTC) Stop Bracket Initiation (SBI)

This classification of RUs is useful because it allows the application program to have separate routines to handle RUs of each type. On each RECEIVE, the application program can specify which RU type will satisfy the RECEIVE. More than one type can be specified as eligible. Also, exit routines instead of RECEIVE macroinstructions can be used to receive DFASY RUs and to receive RESP RUs. Finally, the three types are handled independently for eligibility to satisfy a RECEIVE OPTCD=ANY macroinstruction, as described in Receiving input from any session versus from a specific session.

For more information about RECEIVE and about the DFASY and RESP exits, see Explicit RECEIVEs and EXLST exit routines.