In the process of TCP 3-Way Handshake, we can see this diagram often.

We can notice that, client socket state change to [established] after received the 2nd package.

What happens if the 3rd package (client ACK) is missing?

in client:

int client_fd=connect(a,b);

Does connect() return after receive the 2nd package?

What if the 2nd package is missing? Does it keep SYN-SENT?

And why does the server need to receive the 3rd package then change state to [established]? Why not change state to [established] after receiving the 1st package?

I program for a few years now, but I can not understand this.