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 sid