Request unit flow

An application program establishes a session with the VTAM® SSCP when it opens its ACB. Communication then proceeds through the exchange of formatted Forward and Deliver request units. Both RUs contain network services request units (NS RUs) and, if appropriate, the name of the resource to receive the NS RU.

Figure 1 shows an example of Forward and Deliver RU flow. The application program (using the SEND macro) sends VTAM a formatted Forward RU. The Forward RU contains an NS RU (indicated in parentheses in Figure 1) and the name of the SSCP or the name of a PU to receive the NS RU. If the Forward RU contains the name of a PU, the VTAM SSCP sends the embedded NS RU to the specified PU over an SSCP-PU session. If the Forward RU contains the name of the SSCP, then the embedded NS RU is routed to the application program specified in the CNM routing table.

If the NS RU is not within a Forward RU, the NS RU is also processed by the SSCP.
Figure 1. Example of Forward and Deliver request unit flow
The diagram shows an example of Forward and Deliver RU flow. See the previous information for detailed explanation.

The application program uses the RECEIVE macroinstruction to receive Forward responses and formatted Deliver request units from the VTAM SSCP. The Deliver RU contains an embedded NS RU (indicated in parentheses in Figure 1). This NS RU might have originated within the SSCP or it might have been sent to the SSCP by the PU over a PU-SSCP session. The PU can send an NS RU to reply to an NS RU (contained in a Forward RU), to transmit unsolicited data, or to request a load.

The application programmer must know the format of the Forward and Deliver RUs, how to specify network names within the network name fields of these RUs, and the types and formats of embedded NS RUs that can be placed within the Forward and Deliver RUs. VTAM internal CNM RUs should only be sent from the CNM application that is currently in session with ISTPDCLU. See Requirements for receiving session-awareness and trace data for more information.