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 peers