As you can read also here (
http://msdn.microsoft.com/en-us/library/cc645026(v=vs.95).aspx[
^]), this functionality depend on the transport protocol you use. Since http uses a tipical request-replay scenario and in most cases the port opened for listening on client side would be inaccessible to the server, you have only this polling mechanism. But if your environment enables you to use WCF over TCP, than you will have a real duplex communication.