diff --git a/ws2012/P2P/uebungen/4/src/RandomGenerator2.java b/ws2012/P2P/uebungen/4/src/RandomGenerator2.java
index 254c6c31..ded8b436 100644
--- a/ws2012/P2P/uebungen/4/src/RandomGenerator2.java
+++ b/ws2012/P2P/uebungen/4/src/RandomGenerator2.java
@@ -5,7 +5,6 @@ import java.util.List;
 import java.util.Random;
 import java.util.logging.LogManager;
 
-
 import node.Node;
 import analysis.NetworkDumper;
 
diff --git a/ws2012/P2P/uebungen/4/src/node/Ack.java b/ws2012/P2P/uebungen/4/src/node/Ack.java
index 3d2a1c61..9583db1e 100644
--- a/ws2012/P2P/uebungen/4/src/node/Ack.java
+++ b/ws2012/P2P/uebungen/4/src/node/Ack.java
@@ -23,10 +23,14 @@ public class Ack {
     // The channel to re-send the message on
     private DatagramChannel channel;
 
-    public Ack(int id, SocketAddress address, DatagramChannel channel) {
+    OnAckReceive callback;
+
+    public Ack(int id, SocketAddress address, DatagramChannel channel,
+            OnAckReceive onReceive) {
         this.id = id;
         this.address = address;
         this.channel = channel;
+        this.callback = onReceive;
         startThread();
     }
 
@@ -54,6 +58,10 @@ public class Ack {
     }
 
     public void setReceived() {
+        if (callback != null) {
+            callback.onReceive();
+        }
+
         // Stop thread
         try {
             if (thread != null) {
@@ -61,7 +69,6 @@ public class Ack {
                 thread.join();
             }
         } catch (InterruptedException e) {
-            e.printStackTrace();
         }
     }
 
diff --git a/ws2012/P2P/uebungen/4/src/node/Node.java b/ws2012/P2P/uebungen/4/src/node/Node.java
index a2275e2a..42090706 100644
--- a/ws2012/P2P/uebungen/4/src/node/Node.java
+++ b/ws2012/P2P/uebungen/4/src/node/Node.java
@@ -40,6 +40,8 @@ public class Node {
 
 	private Random generator;
 
+    protected boolean receivedAckForLastInvite;
+
 	public Node() {
 		System.setProperty("java.net.preferIPv4Stack", "true");
 		generator = new Random(System.currentTimeMillis());
@@ -72,16 +74,23 @@ public class Node {
 	public Node spawn() throws IOException {
         LOGGER.log(Level.FINE, "Name: " + getName() + ", Spawning new node.");
 		Node newNode = new Node();
-
-		sendInvite(newNode);
         addNeighbor(newNode.getAddress());
+        receivedAckForLastInvite = false;
+		sendInvite(newNode);
 
 		return newNode;
 	}
 
-    private boolean sendInvite(Node newNode) {
+    private boolean sendInvite(final Node newNode) {
 		ByteBuffer buffer = ByteBuffer.allocate(BUF_SIZE);
-		Ack ack = generateAck(newNode.getAddress());
+
+        Ack ack = generateAck(newNode.getAddress(), new OnAckReceive() {
+            @Override
+            public void onReceive() {
+                receivedAckForLastInvite = true;
+            }
+        });
+
 		buffer.put(MessageType.INVITE);
 		buffer.putInt(ack.getId());
 		buffer.flip();
@@ -104,12 +113,12 @@ public class Node {
 	 *            the SocketAddress the ack should be received from
 	 * @return the identifier for this ack
 	 */
-	private Ack generateAck(final SocketAddress addr) {
+    private Ack generateAck(final SocketAddress addr, OnAckReceive callback) {
 		int ack_id = generator.nextInt();
         while (acks.containsKey(ack_id)) {
             ack_id = generator.nextInt();
         }
-		Ack newAck = new Ack(ack_id, addr, channel);
+        Ack newAck = new Ack(ack_id, addr, channel, callback);
 		
 		acks.put(ack_id, newAck);
 		return newAck;
@@ -270,13 +279,19 @@ public class Node {
 	 * from the network.
 	 */
 	public void leave() {
+        if (!receivedAckForLastInvite || neighbors.isEmpty()) {
+            // This means the bootstrapping has not been finished. Either I have
+            // not been invited by a node, or the Node I invited didn't ack by
+            // now
+            return;
+        }
 		LOGGER.log(Level.INFO, "Name: {0}, Leaving...", getName());
 
-		for (int i = 0; i < neighbors.size(); i++) {
-		      ByteBuffer buffer = ByteBuffer.allocate(BUF_SIZE);
-            Ack ack = generateAck(neighbors.get(i));
-		        buffer.put(MessageType.LEAVE);
-		        buffer.putInt(ack.getId());
+        for (int i = 0; i < neighbors.size(); i++) {
+            ByteBuffer buffer = ByteBuffer.allocate(BUF_SIZE);
+            Ack ack = generateAck(neighbors.get(i), null);
+            buffer.put(MessageType.LEAVE);
+            buffer.putInt(ack.getId());
 
 			if (neighbors.size() > 2) {
 
diff --git a/ws2012/P2P/uebungen/4/src/node/OnAckReceive.java b/ws2012/P2P/uebungen/4/src/node/OnAckReceive.java
new file mode 100644
index 00000000..3c514902
--- /dev/null
+++ b/ws2012/P2P/uebungen/4/src/node/OnAckReceive.java
@@ -0,0 +1,5 @@
+package node;
+
+public interface OnAckReceive {
+    public void onReceive();
+}
diff --git a/ws2012/P2P/uebungen/4/src/node/UDPHandler.java b/ws2012/P2P/uebungen/4/src/node/UDPHandler.java
index 4aacbebe..5a336741 100644
--- a/ws2012/P2P/uebungen/4/src/node/UDPHandler.java
+++ b/ws2012/P2P/uebungen/4/src/node/UDPHandler.java
@@ -72,9 +72,8 @@ public class UDPHandler implements Runnable {
 
             LOGGER.log(Level.INFO, "{0}: {1} is leaving. Sending ack #{2}",
                     new Object[] { node.getName(), from.toString(), ack_id });
-
-            node.sendAck(from, ack_id);
         }
+        node.sendAck(from, ack_id);
     }
 
     private void receiveBroadcast(SocketAddress from) {