code: public void queryUnflushedStreams()
      throws SIResourceException
    {
      synchronized (streamSets) {
        for(Iterator i=streamSets.iterator(); i.hasNext();)
        {
          StreamSet next = (StreamSet) i.next();
  
          // Note the use of -1 for the request ID.  This guarantees
          // that we won't accidentally overlap with a request
          // in the local request map.
          upControl.sendAreYouFlushedMessage(next.getRemoteMEUuid(),
                                             next.getDestUuid(),
                                             next.getBusUuid(),
                                             -1,
                                             next.getStreamID());
        }
      }
    }
docs: Sends an "are you flushed" query to the source of any unflushed streams.
  We use this to determine when it's safe to delete a destination with
  possibly indoubt messages.