diff --git a/ws2012/P2P/uebungen/4/src/node/MessageType.java b/ws2012/P2P/uebungen/4/src/node/MessageType.java
index 8637a0bf..ea6fe7e1 100644
--- a/ws2012/P2P/uebungen/4/src/node/MessageType.java
+++ b/ws2012/P2P/uebungen/4/src/node/MessageType.java
@@ -1,11 +1,8 @@
 package node;
 
 public class MessageType {
-	public final static byte INVITE = 7;
+	public final static byte INVITE = 0;
 	public final static byte LEAVE = 1;
-
-	public final static byte NEW_NEIGHBOR = 4;
-
-	public final static byte PING = 2;
-	public final static byte PONG = 3;
+	public final static byte NEW_NEIGHBOR = 2;
+	public final static byte ACK = 3;
 }
diff --git a/ws2012/P2P/uebungen/4/src/node/Node.java b/ws2012/P2P/uebungen/4/src/node/Node.java
index 046b397a..f84f34af 100644
--- a/ws2012/P2P/uebungen/4/src/node/Node.java
+++ b/ws2012/P2P/uebungen/4/src/node/Node.java
@@ -6,9 +6,14 @@ import java.net.SocketAddress;
 import java.nio.ByteBuffer;
 import java.nio.channels.DatagramChannel;
 import java.util.ArrayList;
+import java.util.HashMap;
 import java.util.List;
+import java.util.Map;
+import java.util.Random;
 import java.util.logging.Logger;
 
+import util.BufferUtil;
+
 public class Node {
 
 	private final static Logger LOGGER = Logger.getLogger(Node.class.getName());
@@ -22,11 +27,16 @@ public class Node {
 
 	private List<SocketAddress> neighbors = new ArrayList<SocketAddress>();
 
+	private Map<Integer, Ack> acks = new HashMap<Integer, Ack>();
+
 	private volatile Thread thread;
 	private UDPListen udpListen;
 
+	private Random generator;
+
 	public Node() {
 		System.setProperty("java.net.preferIPv4Stack", "true");
+		generator = new Random(System.currentTimeMillis());
 		try {
 			channel = DatagramChannel.open();
 			channel.socket().bind(new InetSocketAddress("localhost", 0));
@@ -57,8 +67,10 @@ public class Node {
 		// LOGGER.info("Name: " + getName() + ", Spawning new node.");
 		Node newNode = new Node();
 
+		int ack_id = generateAck(MessageType.INVITE, newNode.getAddress());
 		buf.clear();
 		buf.put(MessageType.INVITE);
+		buf.putInt(ack_id);
 		buf.flip();
 
 		try {
@@ -72,6 +84,12 @@ public class Node {
 		return newNode;
 	}
 
+	public int generateAck(byte type, SocketAddress addr) {
+		int ack_id = generator.nextInt();
+		acks.put(ack_id, new Ack(addr));
+		return ack_id;
+	}
+
 	public SocketAddress getAddress() {
 		return channel.socket().getLocalSocketAddress();
 	}
@@ -81,11 +99,16 @@ public class Node {
 		InetSocketAddress a = (InetSocketAddress) addr;
 
 		buf.put(MessageType.NEW_NEIGHBOR);
+		int ack_id = generateAck(MessageType.NEW_NEIGHBOR, addr);
+		buf.putInt(ack_id);
 
 		for (String part : a.getHostString().split("\\.")) {
 			buf.put(Byte.valueOf(part));
 		}
 		buf.putInt(a.getPort());
+
+		// acks.get(ack_id).setBuf(BufferUtil.clone(buf));
+
 		buf.flip();
 	}
 
@@ -116,7 +139,9 @@ public class Node {
 			}
 
 			buf.clear();
+			int ack_id = generateAck(MessageType.LEAVE, neighbors.get(i));
 			buf.put(MessageType.LEAVE);
+			buf.putInt(ack_id);
 			buf.flip();
 
 			try {
@@ -163,6 +188,7 @@ public class Node {
 
 			while (running) {
 				SocketAddress receivedFrom = null;
+				int ack_id;
 
 				try {
 					receivedFrom = channel.receive(buf);
@@ -176,9 +202,40 @@ public class Node {
 						case MessageType.INVITE:
 							LOGGER.info(name + " received invite from "
 									+ receivedFrom.toString());
+
+							ack_id = buf.getInt();
+
+							buf.clear();
+							buf.put(MessageType.ACK);
+							buf.putInt(ack_id);
+							buf.flip();
+							channel.send(buf, receivedFrom);
+
 							neighbors.add(receivedFrom);
 							break;
 
+						case MessageType.ACK:
+							ack_id = buf.getInt();
+
+							LOGGER.info(name + " received ack from "
+									+ receivedFrom.toString());
+
+							if (acks.containsKey(ack_id)) {
+								Ack theAck = acks.get(ack_id);
+								if (theAck.check(receivedFrom)) {
+									acks.remove(theAck);
+
+								} else {
+									LOGGER.info("Received unexpected ack from "
+											+ receivedFrom.toString());
+								}
+							} else {
+								LOGGER.info("Received unexpected ack from "
+										+ receivedFrom.toString());
+							}
+
+							break;
+
 						case MessageType.LEAVE:
 							LOGGER.info(name + ": " + receivedFrom.toString()
 									+ " is leaving. Deleting...");
@@ -191,9 +248,17 @@ public class Node {
 									neighbors.remove(i);
 								}
 							}
+							ack_id = buf.getInt();
+
+							buf.clear();
+							buf.put(MessageType.ACK);
+							buf.putInt(ack_id);
+							buf.flip();
+							channel.send(buf, receivedFrom);
 							break;
 
 						case MessageType.NEW_NEIGHBOR:
+							ack_id = buf.getInt();
 							StringBuilder theAddr = new StringBuilder();
 
 							// Read 4 Bytes and 1 int
@@ -217,13 +282,20 @@ public class Node {
 										+ new_neighbor.toString());
 							}
 
+							buf.clear();
+							buf.put(MessageType.ACK);
+							buf.putInt(ack_id);
+							buf.flip();
+							channel.send(buf, receivedFrom);
+
 							break;
 
 						default:
 							LOGGER.info(name
 									+ " received unknown command from "
-									+ receivedFrom.toString() + ": "
-									+ messageType + new String(buf.array()));
+									+ receivedFrom.toString() + ": ["
+									+ messageType + "]"
+									+ new String(buf.array()));
 						}
 					} else {
 						try {