通常情况下，当进程使用UDP 和外部主机通信时，NAT 会建立一个Session ，这个Session 能够保留多久并没有标准，或许几秒，几分钟，几个小时。(Typically, when a process uses UDP to communicate with an external host, NAT creates a Session, how long the Session can hold and does not have a standard, maybe a few sec