code: private void ack() {
      // Send a message to the other side of the connection indicating the
      // last message that we received in order. This will allow it to
      // purge messages we've already received from its queue.
      if (open && connected) {
        if (log.isDebugEnabled()) {
          log.debug(String.format("%s - Acking messages up to: %d", this, lastReceived));
        }
        eventBus.send(outAddress, new JsonObject().putString("action", "ack").putNumber("id", lastReceived));
        lastFeedbackTime = System.currentTimeMillis();
      }
    }
docs: Sends an ack message for the current received count.