How requests and responses are exchanged

Requests and responses are exchanged with an LU on a session by using SEND, RECEIVE, and SESSIONC macroinstructions and by using certain exit routines. Using SEND/RECEIVE communication, requests can be sent simultaneously by the application program and by the LU. Certain control requests can be sent ahead of other control requests and requests that contain data. Requests can be queued and responses correlated by using sequence numbers (a sequence number is automatically assigned to each request). This flow of requests and responses between a VTAM® application program and an LU can be synchronized, if necessary, by stopping the flow, resetting sequence numbers at one or both ends of the session, and then restarting the flow. See SEND, RECEIVE, and SESSIONC macroinstructions for more information about these concepts.

Using the large message performance enhancement outbound (LMPEO) option, an application program can send data that exceeds the maximum RU size (for outbound RUs) with a single SEND operation. VTAM splits the data into multiple request units, which form a chain or partial chain of RUs. Refer to Large message performance enhancement outbound (LMPEO) option for details about LMPEO.