router.go
func (router *Router) OnGossipUnicast(sender PeerName, msg []byte) error {
	return fmt.Errorf("unexpected topology gossip unicast: %v", msg)
}
func (router *Router) OnGossipUnicast(sender PeerName, msg []byte) error {
	return fmt.Errorf("unexpected topology gossip unicast: %v", msg)
}
// OnGossipUnicast implements Gossiper, but always returns an error, as a
// router should only receive gossip broadcasts of TopologyGossipData.