SYN/ACK Port Scan

Attacker => SYN/ACK => Target
case1  (open)
Target =>RST =>Attacker

case2 (close)
 No Response

what happens in case1...?
The attacker does not have connection with target.So there is no need to sent SYN/ACK packet.That is why target sends back a RST packet of port is open.

Advantages:
Very difficult to detect.Requires a manual watch with the help of sniffer and intrusion detection system (IDS).

Disadvantages :
This method is unreliable.This scan can be easily countered by a firewall or route that blocks all incoming SYN/ACK packets from systems with whom no connection has been established.