The first three interface functions are called at node startup
(init) as well as whenever the module is switched on (start)
or off (stop) at run time. This enables the preservation of
the internal state of the module at all times. The four gates
enable a module to interface with other modules. A module
receives packets through its fromUp, fromDown functions
and sends packets to other modules via toUp, toDown. The
connections among modules are not static, but are managed
at runtime by the module manager (see below).