IPSec VPN is a widely-acknowledged solution for enterprise network. Which three IPsec VPNstatements are true？（）

A. IKE keepalives are unidirectional and sent every ten seconds
B. IPsec uses the Encapsulating Security Protocol （ESP） or the Authentication Header （AH）protocol for exchanging keys
C. To establish IKE SA, main mode utilizes six packets while aggressive mode utilizes only threepackets
D. IKE uses the Diffie-Hellman algorithm to generate symmetrical keys to be used by IPsec peer