3次握手，

https://blog.csdn.net/hyg0811/article/details/102366854

https://mp.weixin.qq.com/s?__biz=MzAwMDUwNDgxOA==&mid=2652664500&idx=1&sn=5f4c1e85cf5a4c38d3a1b55ac67fc75e&chksm=810f3429b678bd3f51a2581adb1bb581ad6f62adcbdfb2fa4ed77eaef0154cd34a4da3f87460&scene=21#wechat_redirect

 

https://blog.csdn.net/lee244868149/article/details/80635411