cloudy on genius-dev (and follow-ups) found that AlivenessMonitor is currently broken - and likely has been for a while.

He points out that the change 64984 made in AlivenessMonitor by tpantelis likely to clean up generics warnings replaced an alivenessProtocolHandlerRegistry.getOpt(objPayload.getClass()) by .getOpt(Packet.class) but (quote cloudy) :

we don't have any handler registered for abstract base class, Packet is an abstract base class, both ARP and LLDP derives from Packet. AlivenessProtocolHandlerARP and AlivenessProtocolHandlerLLDP both derives from AbstractAlivenessProtocolHandler<P extends Packet> abstract class.

AlivenessProtocolHandlerARP and AlivenessProtocolHandlerLLDP both registers in alivenessProtocolHandlerRegistry MAP with ARP.class and LLDP.class as keys.

Nobody registers a handler with Packet.class as key.