Abstract: A packet transmission protocol, which operates in a full-duplex mode in a system, which includes a transmitter, a receiver, and a communications network having a channel that logically ties the transmitter and the receiver together through the network, is disclosed. The receiver regularly sends a control packet to the transmitter. The control packet includes a plurality of data fields, which are useful in describing the state of the receiver to the transmitter. The transmitter receives the receiver's control packet and analyzes the data. If it finds that a particular block of packets had been received with an error (or not received at all), then the transmitter retransmits the block. The protocol includes two windows for controlling the volume of information, e.g. number of blocks, in the network.