code: private void sendAck(String index, String message, GuacamoleStatus status) {
  
          // Error "ack" instructions implicitly close the stream
          if (status != GuacamoleStatus.SUCCESS)
              closeInterceptedStream(index);
  
          sendInstruction(new GuacamoleInstruction("ack", index, message,
                  Integer.toString(status.getGuacamoleStatusCode())));
  
      }
docs: Injects an "ack" instruction into the outbound Guacamole protocol
  stream, as if sent by the connected client. "ack" instructions are used
  to acknowledge the receipt of a stream and its subsequent blobs, and are
  the only means of communicating success/failure status.
  
  @param index
  The index of the stream that this "ack" instruction relates to.
  
  @param message
  An arbitrary human-readable message to include within the "ack"
  instruction.
  
  @param status
  The status of the stream operation being acknowledged via the "ack"
  instruction. Error statuses will implicitly close the stream via
  closeStream().