[Dovecot] Dovecot communication with sasl in dovecot source code

Hi all, I'm reading dovecot source codes(2.0.13),and I need to know how dovecot communicate with sasl? I traced the source code from cmd_login function,and found out something about sasl_callback. but i never found where this function called and also I couldn't understand how sasl inform the dovecot from authentication result. I'll be thankful if someone could help me out.