code: AutoBuffer call(AutoBuffer ab) {
      int tnum = ab.getTask();
      RPC<?> t = ab._h2o.taskGet(tnum);
      // Forgotten task, but still must ACKACK
      if( t == null ) return RPC.ackack(ab,tnum);
      return t.response(ab); // Do the 2nd half of this task, includes ACKACK
    }
docs: Received an ACK for a remote Task.  Ping the task.