实现GBN的功能，利用JAVA语言实现，简单，有利于加深对网络的了解-GBN to achieve the function, the use of JAVA language, simple, and is conducive to deepening understanding of the network

UDP实现可靠数据传输。使用GBN算法，实现了：1.随即丢包(随机数产生器Math.random()) 2.丢包后遇冗余ack重传 3.超时重传 (DatagramSocket和Socket都有成员函数setSoTimeout(delay),通过捕获超时时抛出的IO异常来实现超时重传)-Reliable UDP data transfer. GBN algorithm used to realize: 1. Then packet loss (random number generator Ma