Pacing (INPACING, OUTPACING, MAXINPACING) parameters

Pacing is required if there is a possibility of overflowing data buffers internal to the controller or the host system. This typically occurs if the controller or host must pass the data to a device that operates at a slow speed. If the host system receives a pacing response, it sends more data frames up to the window size to the controller.

• Pacing determines how many message units (SNA RUs) can be transferred over a session before receiving an acknowledgment from the receiving system. An excessive number of pacing responses may adversely affect network performance. However, the absence of pacing can cause network congestion and unfair utilization of iSeries™ resources (buffers and central processing units). The values that you can use in negotiating the pacing values with the adjacent system are determined from the INPACING and OUTPACING values on the mode description. The iSeries will not allow these values to be negotiated to a higher value. If necessary, the receive pacing value negotiate to a lower value, matching the INPACING value.
• The pacing value is determined at session establishment time and is not changed for the duration of the session for the following reasons:
  • The adjacent system does not support adaptive pacing
  • The transmission priority is low priority
• If the adjacent system does support adaptive pacing, the minimum pacing value is set at session establishment time using the INPACING and OUTPACING values. The location that starts the session establishment (BIND request) is responsible for setting the values. No negotiation of the values is performed. However, support is provided by the system to change or adapt the pacing values based on the system's buffer resources and traffic patterns in the network. The system can now allocate its session buffers automatically to efficiently use its available resources. The MAXINPACING parameter defines the upper limit on the number of session buffers. The default value of *CALC sets an upper limit of 2 for the INPACING value.
• The iSeries system also has the ability to slow down the transfer of data or even stop receiving at any node of any session. This allows for more equity in the network by dynamically turning the flow of messages on to any hop for any session that may be contributing to congestion problems. In general, the value of the INPACING, OUTPACING, and MAXINPACING parameters on the mode description can affect the data rate, network congestion, buffer utilization, and central processing unit (CPU) utilization.