diff --git a/src/client.py b/src/client.py
--- a/src/client.py
+++ b/src/client.py
@@ -11,6 +11,9 @@ from hashtree import HashTree,hashBlock
 from netnode import BaseConnection,NetNode,FailedConnection
 
 
+class DeniedConnection(Exception): pass
+
+
 class Connection(BaseConnection):
 	def __init__(self,host,port):
 		super().__init__()
@@ -42,6 +45,14 @@ class Client(NetNode):
 		print(datetime.now(), "initializing...")
 		super().__init__(filename,treeFile)
 
+	def init(self,action):
+		jsonData={"command":"init", "blockSize":self._tree.BLOCK_SIZE, "blockCount":self._tree.leafCount, "version":conf.version, "action":action}
+		self._outcoming.writeMsg(jsonData)
+		jsonData,binData=self._incoming.readMsg()
+		if jsonData["command"]=="deny":
+			raise DeniedConnection()
+		assert jsonData["command"]=="init"
+
 	## Asks server for node hashes to determine which are to be transferred.
 	#
 	# Uses a binary HashTree, where item at k is hash of items at 2k+1, 2k+2.
@@ -52,12 +63,6 @@ class Client(NetNode):
 		blocksToTransfer=[]
 		nodeStack=collections.deque([0]) # root
 
-		# initialize session
-		jsonData={"command":"init", "blockSize":localTree.BLOCK_SIZE, "blockCount":localTree.leafCount, "version":conf.version}
-		self._outcoming.writeMsg(jsonData)
-		jsonData,binData=self._incoming.readMsg()
-		assert jsonData["command"]=="ack"
-
 		# determine which blocks to send
 		print(datetime.now(), "negotiating:")
 		progress=Progress(localTree.leafCount)
@@ -115,12 +120,13 @@ class Client(NetNode):
 			progress.p(k)
 		progress.done()
 
-		self._outcoming.writeMsg({"command":"end"})
+		self._outcoming.writeMsg({"command":"end","action":"push"})
 
 		log.info("closing session...")
 		dataFile.close()
 
 	def pullData(self,blocksToTransfer):
+		self._lock()
 		log.info(blocksToTransfer)
 		dataFile=open(self._filename, mode="rb+")
 		i1=-1
@@ -150,6 +156,7 @@ class Client(NetNode):
 
 		log.info("closing session...")
 		dataFile.close()
+		self._unlock()
 
 		if self._treeFile:
 			self._updateTree()
diff --git a/src/config.py b/src/config.py
--- a/src/config.py
+++ b/src/config.py
@@ -21,7 +21,7 @@ conf=dict()
 if os.path.isfile(configFile):
 	with open(configFile) as f: conf=json.load(f)
 
-version=0
+version=(0,0,0)
 
 hosts=conf.get("hosts",["127.0.0.1"])
 port=conf.get("port",9901)
diff --git a/src/morevna.py b/src/morevna.py
--- a/src/morevna.py
+++ b/src/morevna.py
@@ -7,7 +7,7 @@ from util import spawnDaemon, splitHost
 import config as conf
 import stats
 from hashtree import HashTree
-from client import Client, Connection as ClientConnection, FailedConnection
+from client import Client, Connection as ClientConnection, FailedConnection, DeniedConnection
 from server import Miniserver
 
 
@@ -43,12 +43,15 @@ def push(args):
 		try:
 			with ClientConnection(*host) as con:
 				c.setConnection(con)
+				c.init("push")
 				blocksToTransfer=c.negotiate()
 				c.sendData(blocksToTransfer)
 			print()
 			print(stats.report())
 			print()
-		except FailedConnection: continue
+		except FailedConnection: pass
+		except DeniedConnection:
+			print("Server {0}:{1} denied connection.".format(*host))
 
 def pull(args):
 	_checkFile(args.datafile)
@@ -62,11 +65,14 @@ def pull(args):
 	try:
 		with ClientConnection(*host) as con:
 			c.setConnection(con)
+			c.init("pull")
 			blocksToTransfer=c.negotiate()
 			c.pullData(blocksToTransfer)
 		print()
 		print(stats.report())
 	except FailedConnection: pass
+	except DeniedConnection:
+		print("Server {0}:{1} denied connection.".format(*host))
 
 def serve(args):
 	_checkFile(args.datafile)
diff --git a/src/netnode.py b/src/netnode.py
--- a/src/netnode.py
+++ b/src/netnode.py
@@ -1,11 +1,17 @@
+import os
 import socket
 import logging as log
 
+import config as conf
 from networkers import NetworkReader,NetworkWriter
 from hashtree import HashTree
 
 
+lockFile=os.path.join(conf.directory,"dirty.lock")
+
+
 class FailedConnection(Exception): pass
+class LockedException(Exception): pass
 
 
 class BaseConnection: # abstract
@@ -47,6 +53,19 @@ class NetNode:
 
 		self._newLeaves=dict()
 
+	def isLocked(self):
+		return os.path.isfile(lockFile)
+
+	def _lock(self):
+		try:
+			f=open(lockFile,"x")
+			f.close()
+		except FileExistsError:
+			raise LockedException()
+
+	def _unlock(self):
+		os.remove(lockFile)
+
 	def _updateTree(self):
 		log.info("updating hash tree...")
 		self._tree.batchUpdate(self._newLeaves.items())
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"]