3.2.1.1.5.1.2 Receiver AvailableWindow
As the receiver queues and releases PDUs in its ReceiveWindow, it MUST locally keep track of how much space it has left in its ReceiveWindow, in bytes. The size of the ReceiveWindow minus the sum of the size of all RPC PDUs that the receiver queued in this ReceiveWindow is called Receiver AvailableWindow. Receiver AvailableWindow can range from 0 to the value of receive window.