So we need to give an abstract model about the forwarding
network status, which abstract the information for the control
network and management network to form necessary and
dynamic views of the forwarding network. Since there are
differences between the information needed by the control
network and that needed by the management work, the
forwarding network needs to provide two kinds of interfaces
for status perception: one for the control network; the other for
the management network. And that means we may need to
design or use two protocols for status perception: one for the
communication between a controller and a forwarder; the other
for the communication between a manager and a forwarder.