[reSIProcate] how to send a CANCEL before connected?

[frank zhao <frankzhaohb@xxxxxxxxx>]

hi,
   In the file of ClientInviteSession.hxx,line 36 .there is some
comments for end() method, as shown in the  bellow code.
-----------------------------------------------------------------------
 /** Makes the specific dialog end. Will send a BYE (not a CANCEL) */
      virtual void end(EndReason reason);
-----------------------------------------------------------------------

   It seems that in the ClientInviteSession, when we want to terminate
a session. The stack can only send BYE. I want to know why not send a
CANCEL message before 2xx received in a clientInviteSession. Is there
some way to send a CANCEL before the call is connected?