TCP Connect Port Scan

In this kind of port scan the port scanner establishes a full 3 way TCP/IP handshake connection with all ports on the remote system.
Attacker =>SYN => Target

case 1:
 Target  => SYN/ACK => Attacker

case 2:
 Target => RST/ACK => Attacker

Attacker => ACK => Target

TCP port scan is very difficult to determine.This is because these type of scanning never establishes a connection with the target.

Countermeasures:

Create a router or firewall rule that creates a buffer and limits the number of connections that can be established from the same IP