TCP 为什么三次握手而不是两次握手

可以只看这一篇：https://mp.weixin.qq.com/s/NIjxgx4NPn7FC4PfkHBAAQ

参考：TCP 为什么三次握手而不是两次握手

Why do we need a 3-way handshake? Why not just 2-way?

 why not just use a two-way handshake? The short answer is because a two way handshake would only allow one party to establish an ISN, and the other party to acknowledge it. Which means only one party can send data.

But TCP is a bi-directional communication protocol, which means either end ought to be able to send data reliably. Both parties need to establish an ISN, and both parties need to acknowledge the other's ISN.

为什么不只是使用双向握手？ 简短的回答是因为双向握手只允许一方建立ISN，而另一方则承认它。 这意味着只有一方可以发送数据。

但TCP是一种双向通信协议，这意味着任何一端都应该能够可靠地发送数据。 双方都需要建立一个ISN，双方都需要承认对方的ISN。