3次握手完成两个重要的功能，既要双方做好发送数据的准备工作（双方都知道彼此已准备好），也要允许双方就初始序列号进行协商，这个序列号在握手过程中被发送和确认。-3 handshake to complete two important functions, it is necessary to make the two sides prepare to send data (both sides know each other is ready), but also allows both sides to negotiate the initial sequence number, the serial number was in the handshake sending and confirmation.