单选题Which attribute is optional for IKE phase 2 negotiations？（）A proxy-IDB phase 2 proposalC Diffie-Hellman group keyD security protocol （ESP or AH）
题目
单选题
Which attribute is optional for IKE phase 2 negotiations？（）
A
proxy-ID
B
phase 2 proposal
C
Diffie-Hellman group key
D
security protocol （ESP or AH）