code: func (bucket *kbucket) Fresh(dht *DHT) {
  	for e := range bucket.nodes.Iter() {
  		no := e.Value.(*node)
  		if time.Since(no.lastActiveTime) > dht.NodeExpriedAfter {
  			dht.transactionManager.ping(no)
  		}
  	}
  }
docs: // Fresh pings the expired nodes in the bucket.