One of the possibilities that cancause the controller to be unreachable is a Mirai malware attack. Miraiis a type of malware that automatically finds Internet of Things devices toinfect and conscripts them into a botnet—a group of computing devices that canbe centrally controlled. From there this IoT army can be used to mountdistributed denial of service (DDoS) attacks. In which, a large number of packets are sent to a host or a group ofhosts in a network. If the source addresses of the incoming packets arespoofed, which is usually the case, the switch will not find a match and has toforward the packet to the controller.
The collection of legitimate and DDoSspoofed packets can bind the resources of the controller into continuousprocessing up to the point where they are completely exhausted. This will makethe controller unreachable for the newly arrived legitimate packets and maybring the controller down causing the loss of the SDN architecture. Even ifthere is a backup controller, it has to face the same challenge.