router.go
func (router *Router) OnGossipBroadcast(_ PeerName, update []byte) (GossipData, error) {
	origUpdate, _, err := router.applyTopologyUpdate(update)
	if err != nil || len(origUpdate) == 0 {
		return nil, err
	}
	return &topologyGossipData{peers: router.Peers, update: origUpdate}, nil
}
func (router *Router) OnGossipBroadcast(_ PeerName, update []byte) (GossipData, error) {
	origUpdate, _, err := router.applyTopologyUpdate(update)
	if err != nil || len(origUpdate) == 0 {
		return nil, err
	}
	return &topologyGossipData{peers: router.Peers, update: origUpdate}, nil
}
// OnGossipBroadcast receives broadcasts of TopologyGossipData.
// It returns the received update unchanged.