code: public void addOngoingTransaction(Transaction transaction) {
  
  		if(transaction != null && ongoingTransactions != null  && !isReadyToInvalidate() ) {
  			boolean added = this.ongoingTransactions.add(transaction);
  			if(added) {
  				if(logger.isDebugEnabled()) {
  					logger.debug("transaction "+ transaction +" has been added to sip session's ongoingTransactions" );
  				}
  				setReadyToInvalidate(false);
  			}
  		}
  	}
docs: Add an ongoing tx to the session.