udp 穿透， 传透NAT，打洞。不仅有文档的介绍，将原理介绍清楚，而且有可以直接运行的源码。学习udp打洞的好资料。-udp penetrate, pass through the NAT, holes. Not only describes the document will introduce a clear principle, but also can run the source code. Good information to learn udp hole punching.