[Dovecot] Dovecot communication with sasl in dovecot source code

[Meisam Navaki]

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.