private Set<ReceiptReceivedListener> receiptReceivedListeners = Collections
.synchronizedSet(new HashSet<ReceiptReceivedListener>());
private DeliveryReceiptManager(Connection connection) {
ServiceDiscoveryManager sdm = ServiceDiscoveryManager.getInstanceFor(connection);
sdm.addFeature(DeliveryReceipt.NAMESPACE);
this.connection = connection;
instances.put(connection, this);
// register listener for delivery receipts and requests
connection.addPacketListener(this, new PacketExtensionFilter(DeliveryReceipt.NAMESPACE));