The client will make both connections to the server, but one of them will be to a random high port, which would almost certainly be blocked by a firewall on the server side.

• 因为客户端要和服务器端建立两个连接，其中一个连到一个高位随机端口，而这个端口非常有可能被服务器端的防火墙阻塞掉。