diff --git a/ws2012/P2P/uebungen/4/src/node/Node.java b/ws2012/P2P/uebungen/4/src/node/Node.java
index 54952ece..f85fd85a 100644
--- a/ws2012/P2P/uebungen/4/src/node/Node.java
+++ b/ws2012/P2P/uebungen/4/src/node/Node.java
@@ -106,6 +106,9 @@ public class Node {
 	 */
 	private Ack generateAck(final SocketAddress addr) {
 		int ack_id = generator.nextInt();
+        while (acks.containsKey(ack_id)) {
+            ack_id = generator.nextInt();
+        }
 		Ack newAck = new Ack(ack_id, addr, channel);
 		
 		acks.put(ack_id, newAck);