The ClientInviteSession and ServerInviteSession classes define behaviors which are used on "early" invite sessions. Once the sessions are no longer "early", the behaviors on the base class InviteSession are used.
The dispatch methods on the InviteSession classes handle most of the behavior. The InviteSessionHandler class provides the means by which the application is included in processing that is above SIP protocol/dialog level.
The state machines for the InviteSessions are described by documentation in
ClientInviteSession.dot dum-serverinvite-state.dot dum-clientinvite-state.dot dum-dialogset-state.dot dum-invite-connected-state.dot ServerInviteSession.dot