diff --git a/src/server.py b/src/server.py
--- a/src/server.py
+++ b/src/server.py
@@ -71,9 +71,12 @@ class Server(NetNode):
 		jsonData,binData=self._incoming.readMsg()
 
 		if jsonData["command"]=="init":
-			assert jsonData["blockSize"]==self.BLOCK_SIZE
-			assert jsonData["blockCount"]==self._tree.leafCount
-			self._outcoming.writeMsg({"command": "ack"})
+			if jsonData["blockSize"]!=self.BLOCK_SIZE or jsonData["blockCount"]!=self._tree.leafCount:
+				self._outcoming.writeMsg({"command":"deny"})
+			if jsonData["action"]=="pull" and self.isLocked():
+				self._outcoming.writeMsg({"command":"deny"})
+
+			self._outcoming.writeMsg({"command":"init", "version":conf.version})
 
 		elif jsonData["command"]=="req":
 			if jsonData["dataType"]=="data":
@@ -86,7 +89,7 @@ class Server(NetNode):
 
 		elif jsonData["command"]=="end":
 			self._finalize()
-			self._locked=False
+			if jsonData.get("action")=="push": self._unlock()
 			return False
 
 		else:
@@ -115,6 +118,7 @@ class Server(NetNode):
 		return (jsonResponse,binResponse)
 
 	def _receiveData(self,jsonData,binData):
+		if not self.isLocked(): self._lock()
 		log.info("received data block #{0}: {1}...{2}".format(jsonData["index"],binData[:5],binData[-5:]))
 
 		i=jsonData["index"]