当建立了一个 [私有ip:端口]-[公网ip:端口] 端口绑定之后，对于来自同一个[私有ip:端口]会话，锥形nat服务器允许发起会话的应用程序 重复使用这个端口绑定，一直到这个会话结束才解除（端口绑定）。
for example, suppose client a in the diagram below initiates two simultaneous outgoing sessions through a cone nat, from the same internal endpoint (10.0.0.1:1234) to two different external servers, s1 and s2. the cone nat assigns just one public endpoint tuple（元组）, 155.99.25.11:62000, to both of these sessions, ensuring that the "identity" of the client's port is maintained across address translation. since basic nats and firewalls do not modify port numbers as packets flow across the middlebox, these types of middleboxes can be viewed as a degenerate form of cone nat.
