The normal-flow traffic between an application program and an LU includes requests containing data, requests that contain certain control information (called normal-flow data requests), and the responses to such data requests and control requests. Normal-flow requests are sent sequentially, one after the other, through the network, and a normal-flow request that is sent before another such request arrives sooner. Figure 1 illustrates this principle.

Similarly, responses to normal-flow requests (called normal-flow responses) keep their order as they travel through the network. However, VTAM® does not maintain the exact sequence relationship between requests and responses in relation to each other; that is, a response sent by an LU after a request can be presented to the application program before the request. The only way that an application program can be sure of receiving normal-flow requests and responses in the exact order that they are sent by the LU is by specifying RESPOND=QRESP (and POST=SCHED) on the macroinstruction or in the RPL used to send the request. See Controlling the handling of normal-flow responses for more information about how VTAM handles responses. (Using the authorized path affects the order in which asynchronous operations complete, and because of this, the sequence in which requests are received can be affected. See Additional coding considerations for authorized path for more information about authorized path usage.)

Certain control requests (called expedited-flow data-flow-control-requests) and responses to those requests (called expedited-flow traffic) are sent in a separate flow (called expedited flows) from the normal-flow requests and responses. These, together with all session-control requests and their responses, form the expedited-flow traffic in the network. The expedited-flow requests tell the receiver to do something that has higher priority than receiving normal-flow requests (for example, to stop sending normal-flow requests or to prepare to shut down communication with the other end of the session). Because of this, VTAM sends an expedited-flow request immediately, before sending any normal-flow traffic that might be waiting to be sent. Normal-flow traffic is handled separately from the expedited-flow traffic. Figure 2 illustrates how VTAM gives priority to expedited-flow traffic.

The requests and responses that are sent on the normal flow and the expedited flow are listed in Table 1. Only one expedited-flow data-flow-control request can be sent at a time by each LU in the session; a response must be received to one such request before another can be sent. Similarly, only one session-control request can be sent at a time with the SESSIONC macroinstruction.