https://arthurchiao.art/blog/how-nat-traversal-works-zh/#231-%E5%8F%8C%E5%90%91%E4%B8%BB%E5%8A%A8%E5%BB%BA%E8%BF%9E%E6%97%81%E8%B7%AF%E4%BF%A1%E9%81%93