回退n协议演示程序 我自己做的,虽然是匆忙做的，但可以运行

Protocol 5 (pipelining) allows multiple outstanding frames. The sender may transmit up to MAX_SEQ frames without waiting for an ack. In addition, unlike the previous protocols, the network layer is not assumed to have a new packet all the time. Instead, the network layer causes a network_layer_ready event when there is a packet to send.