In the past, the NetworkElement would cover almost all the functionality that
e.g. virtual router can cover: firewall, source NAT, static NAT, password,
VPN... So anyone want to implement the NetworkElement would have to implement
these service's specific methods, even it wouldn't support it. Also, if we want
to find a e.g. FirewallServiceProvider, we have to proceed all the current
network service providers, to call a method to know if it support such service.
That's neither elegant nor scaling way to do it.
As the first step, this patch separates each ServiceProvider from NetworkElement
(there are some interface already out of NetworkElement, so this patch slightly
modifies them too), and only the class would implement the correlated interface, would
have the ability to do these services.
Sheng Yang