Only with specialized client software. The connect() system call doesn't return to the caller until the three-way handshake is complete, so you need a fourth p

Only with specialized client software. The connect() system call doesn't return to the caller until the three-way handshake is complete, so you need a fourth packet to send useful data to the server socket. In the other direction, where you have datacenter hardware and custom kernels, it's common to see the stack cheat and start blasting packets back to the client as soon as it gets the initial SYN, just expecting that the ACK will arrive normally.