Each layer has 2 FIFO queues, one for up Events and one for down Events. When an Event is
received by a layer (calling the internal upcall ReceiveUpEvent), it is placed
in the up-queue where it will be retrieved by the up-handler thread which will invoke method
Up of the layer. The same applies for Events traveling down the stack. Handling
of the up-handler and down-handler threads and the 2 FIFO queues is donw by the Protocol
class, subclasses will almost never have to override this behavior.
The important thing to bear in mind is that Events have to passed on between layers in FIFO
order which is guaranteed by the Protocol implementation and must be guranteed by subclasses
implementing their on Event queuing.
Note that each class implementing interface Protocol MUST provide an empty, public