Which two statements about using the CHAP authentication mechanism in a PPP link are true?()
A、CHAP uses a two-way handshake
B、CHAP authentication periodically occurs after link establishment
C、CHAP has no protection from playback attacks
D、CHAP authentication is performed only upon link establishment
E、CHAP uses a three-way handshake
F、CHAP authentication passwords are sent in plaintext