说明：  实现go back n算法，模仿数据链路层的gobackn协议
(To implement the Go-Back-N (GBN) protocol on top of UDP on both nodes to guarantee that all packets can be successfully delivered to the higher layers in the correct order. To emulate an unreliable channel, the receiver or the sender needs to drop an incoming data packet or an ACK, respectively, with a certain probability. 2.2)