Hackers usually exploit a protocol$$s certain feature to formulate an attack. Consider

the following example. In TCP$,$ when a computer receives a data packet $($ACK bit $=1)$ for a

closed port, it will respond with a TCP packet with the RST bit set to indicate an error. An

attacker often uses this feature to performance port scanning$$i$.$e$.$ to find out which port is open.

$($a$)$ A simple filter$-$type firewall has been activated. Can this firewall prevent port scanning? $($b$)$

Now suppose the firewall has states and it can remember which TCP channels are currently

active $($remember each TCP channel is defined by $($source$-$IP$,$ source$-$port, destination$-$IP$,$ and

destination port$).$ Can the problem be prevented?