now suppose that client a wants to establish a udp communication session directly with client b.? if a simply starts sending udp messages to b's public address, 138.76.29.7:31000, then nat b will typically discard these incoming messages (unless it is a full cone nat), because the source address and port number does not match those of s, with which the original outgoing session was established. similarly, if b simply starts sending udp messages to a's public address, then nat a will typically discard these messages.
假如这个时候 client a 想与 client b建立一条udp通信直连，如果 client a只是简单的发送一个udp信息到client b的公网地址138.76.29.7:31000的话，nat b会不加考虑的将这个信息丢弃（除非nat b是一个 full cone nat），因为 这个udp信息中所包含的地址信息，与client b和服务器s
