The virtual route (VR) pacing window size calculation exit routine allows you 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. The virtual route pacing response indicates that the virtual route receiver is ready to receive more PIUs on the route. The VR pacing window size calculation exit routine is called when a virtual route is activated. It returns the minimum and maximum values for the window of the virtual route.

The IBM-supplied algorithm for window size calculation works 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, subarea nodes along the route request that the end points 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 to set the window sizes to different values than those supplied by IBM®. The VR pacing window size calculation 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.

The logic for calculating the maximum and minimum window sizes for virtual route pacing is contained in the VTAM exit routine named ISTPUCWC. The source code for ISTPUCWC is not provided with the VTAM program. It is available through the View Program Listings (VPL) application from ISM&D (IBM Software Manufacturing and Distribution), Boulder, Colorado. VPL replaces microfiche and provides the required logic online. You can access this information by using ServiceLink or Dial IBM.

For most routes, VTAM sets the minimum window size to the explicit route length and the maximum size to three times the explicit route length. However, if the virtual route ends in a subarea that is adjacent to VTAM, the maximum window size is set to the greater of 15 or 255-16n, where n is the number of explicit routes (defined or operative) originating in the host and passing through and not ending in the adjacent subarea. This increases the maximum window size for a route to a channel-attached NCP that has only a few explicit routes passing through it.

The VR pacing window size calculation exit routine is called when VTAM is preparing to activate a virtual route. VTAM passes the following information to the exit:

• The particular virtual route number and its transmission priority
• The number and length of the explicit route that is associated with the particular virtual route
• The subarea addresses of the destination and adjacent subareas
• The number of defined and operative explicit routes that traverse, but do not end in, the adjacent subarea
• The address of the explicit route characteristics table, which describes the transmission group traversed by the explicit route associated with a virtual route. This table determines the link protocol being used between the host and the adjacent node on the explicit route. (See Table 1 for the format of this table.)

The following topics contain information you need to replace or modify the VR pacing window size calculation exit routine.