Session-level pacing
Session-level pacing is the mechanism by which the receiver of data (Db2, in this case) can control the pace at which the sender sends data (in the form of RUs).
The pacing size is the number of RUs that VTAM® sends across the line at one time, and you can set that value by using the VPACING option of the VTAM APPL definition statement. You set the RU size in the MODEENT macro. The receiving VTAM stores these RUs in its IOBUF pool; it uses pacing so that its buffers do not become flooded with data.
The pacing process works as shown in the following figure. The system at the sending side (assume it is USIBMSTODB22) passes data to its VTAM system. VTAM formats the data into RUs and sends those RUs across the network. If, for example, the pacing size is 2, then it sends two RUs. A 29-byte network header is sent with each RU.
After the USIBMSTODB22 VTAM system sends the specified number of RUs, it does not send any more data on this session until it receives a pacing response from the VTAM system at USIBMSTODB21. The USIBMSTODB21 VTAM system does not send a response until VTAM transfers the data into the Db2 buffers.
Although it is generally true that the receiving system can control inbound pacing, both communicating systems negotiate final pacing values.