The current window size that this DLSw is using in its role
as a data receiver. This is the number of additional paced
SSP messages that this DLSw would be authorizing its DLSw
partner to send, if this DLSw were to send a flow control
indication with the bits specifying `repeat window'.
The value zero should be returned if this circuit is not
running the DLSw pacing protocol.