In order to allow Ipsec & NAT-T traffic incoming :

# ESP TRAFFIC
-A INPUT -p 50 -j ACCEPT
# IPSEC TRAFFIC
-A INPUT -p udp -d 12.34.56.78 –dport 500 -j ACCEPT
# NAT TRAFFIC
-A INPUT -p udp -d 12.34.56.78 –dport 4500 -j ACCEPT