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()