I wonder if DummyDTLSSession is the correct name for this class.  If it has the word "Dummy" in it I expect it to be a derived class that stubs out the base class required API or something.  In this case it appears to be a base class API specification for a real DTLSSession.

DummyDTLSSession is used in only one place.  Perhaps you could merge this class with DTLSSession and make it possible via the API to create a "dummy" instance that does what you want.