The IBM-supplied algorithm for window size calculation is designed to work with the route pacing algorithm used in the network. It is appropriate for most installations and configurations. VTAM® calculates the minimum and maximum sizes of virtual route pacing windows based on the link protocol and the explicit route length (that is, the number of transmission groups in the explicit route used by the virtual route). While the virtual route is being used to transmit data, adjacent subarea nodes on the route automatically adjust the window sizes within the minimum and maximum limits according to traffic conditions along the route.

However, after tuning VTAM and analyzing traffic patterns and resource capabilities, you might want to choose your own bounds or code a replacement routine that sets the window sizes to different values than the ones supplied by IBM®. This exit routine is appropriate for systems where the number of resources could vary considerably from one day to the next. For example, you might find one or more resources whose capacities are not consistently used. This might warrant increasing the window sizes. Decreasing the window sizes is less likely to be useful because network flow control protocols are designed to prevent congestion, and setting window sizes too small could reduce traffic flow considerably.

If you use the Information Management System (IMS™), you might want to code a replacement virtual route-pacing window-size calculation exit routine to reduce the number of virtual route pacing responses that VTAM processes for every IMS transaction. For more information about coding the exit routine for IMS, see z/OS Communications Server: SNA Customization.

You can also specify default minimum and maximum window sizes on the VRPWSnn operand of the PATH definition statement when defining a virtual route. This is the simplest way to choose the default and is sufficient for most needs. If you do not specify VRPWSnn, VTAM uses the default algorithm described previously.

The virtual route-pacing window-size calculation exit routine can be used in addition to VRPWSnn operands (coded on the VTAM and the NCP PATH definition statements) to alter the VTAM virtual route pacing window sizes. This exit routine is used to specify the bounds for virtual route pacing windows. A virtual route pacing window represents the quantity of path information units (PIUs) that can be transmitted on a virtual route before a virtual route pacing response is received. This response indicates that the virtual route receiver is ready to receive more PIUs on the route. The exit routine is called when a virtual route is activated. It returns the minimum and maximum values for the window of the virtual route.