What protocols / ports may be required for IPSec to function?
UDP Port 500 - IKE Phase 1 negotiation
UDP Port 4500 - NAT-T (NAT Transversal) if there is NAT taking place on both endpoints.
Layer 4 Protocol 50 (ESP) - ESP is needed at layer 4 for encapsulation and de-encapsulation of each IPSec packet
Optional (if AH is being used)
Layer 4 Protocol 51 (AH) - AH is not usually used because it does not encrypt packets.