说明：  大部分的通信都不是一方只是发送，而另一方只是接收，而是双向的。因此在这种情况下返回的Ack帧通常可由反向发送的数据帧一起捎带回来。这只要在数据帧中再加一个确认帧号就可以了。这种技术称作捎带确认。
(Most of the communication is not a party to only send, while the other is only receiving, but the two-way street. Therefore, in this case to return to the Ack frame is usually sent by the reverse data frames back together incidentally. This is as long as the frame in the data frame by adding a confirmation number on it. This technology, known as piggybacking confirmed.)