When lwIP was ported to AVR32, it was blended with FreeRTOS, so a lot of
periodicall calling to lwIP is done behind the scene.
In my project, under certain condition, I need to suspend FreeRTOS for
two seconds (interrupts are not disabled), but I want to continue to
receive/send UDP packets during this time, so I think I need to call some
APIs in lwIP so that it will handle the incoming packets.