code: protected boolean isAckReceived(long cSeq) {
  		if(acksReceived == null) {
  			// http://code.google.com/p/sipservlets/issues/detail?id=152
  			// if there is no map, it means that the session was already destroyed and it is a retransmission
  			return true;
  		}
  		Boolean ackReceived = acksReceived.get(cSeq);
  		if(logger.isDebugEnabled()) {
  			logger.debug("isAckReceived for CSeq " + cSeq +" : " + ackReceived);
  		}
  		if(ackReceived == null) {
  			// if there is no value for it it means that it is a retransmission
  			return true;
  		}
  		return ackReceived;
  	}
docs: check if the ack has been received for the cseq in param
  it may happen that the ackReceived has been removed already if that's the case it will return true
  @param cSeq CSeq number to check if the ack has already been received
  @return