diff --git a/src/client.py b/src/client.py
--- a/src/client.py
+++ b/src/client.py
@@ -40,11 +40,12 @@ class Client:
 		with Connection() as (incoming,outcoming):
 			jsonData={"command":"init", "blockSize":localTree.BLOCK_SIZE, "blockCount":localTree.leafCount, "version":conf.version}
 			outcoming.writeMsg(jsonData)
+			jsonData,binData=incoming.readMsg()
+			assert jsonData["command"]=="ack"
 
-		# determine which blocks to send
-		print(datetime.now(), "negotiating:")
-		while len(nodeStack)>0:
-			with Connection() as (incoming,outcoming):
+			# determine which blocks to send
+			print(datetime.now(), "negotiating:")
+			while len(nodeStack)>0:
 				i=nodeStack.pop()
 				outcoming.writeMsg({"command":"req", "index":i})
 
@@ -69,8 +70,8 @@ class Client:
 		i1=-1
 
 		print(datetime.now(), "sending data:")
-		for (k,i2) in enumerate(blocksToTransfer):
-			with Connection() as (incoming,outcoming):
+		with Connection() as (incoming,outcoming):
+			for (k,i2) in enumerate(blocksToTransfer):
 				jsonData={"command":"send", "index":i2, "dataType":"data"}
 				if i1+1!=i2:
 					dataFile.seek(i2*HashTree.BLOCK_SIZE)
@@ -79,12 +80,14 @@ class Client:
 				log.info("block #{0}: {1}...{2}".format(i2,binData[:5],binData[-5:]))
 
 				outcoming.writeMsg(jsonData,binData)
-			i1=i2
-			progress(k,len(blocksToTransfer))
+				jsonData,binData=incoming.readMsg()
+				assert jsonData["command"]=="ack" and jsonData["index"]==i2, jsonData
+				i1=i2
+				progress(k,len(blocksToTransfer))
 		print("100%")
 
 		with Connection() as (incoming,outcoming):
 			outcoming.writeMsg({"command":"end"})
 
-		log.info(datetime.now(), "closing session...")
+		log.info("closing session...")
 		dataFile.close()
diff --git a/src/networkers.py b/src/networkers.py
--- a/src/networkers.py
+++ b/src/networkers.py
@@ -7,14 +7,16 @@ class NetworkReader:
 
 	def readMsg(self):
 		data=self.stream.readline()
-		if not data: pass # !! raise something
+		assert data
 		jsonLength=int(data.split(b":")[1].strip()) # "json-length: length" -> length
 		data=self.stream.readline()
-		if not data: pass # !! raise something
+		assert data
 		binLength=int(data.split(b":")[1].strip()) # "bin-length: length" -> length
 		jsonData=self.stream.read(jsonLength)
+		assert len(jsonData)==jsonLength
 		jsonData=json.loads(str(jsonData,encoding="utf-8"))
 		binData=self.stream.read(binLength)
+		assert len(binData)==binLength
 		
 		return (jsonData,binData)
 		
@@ -31,4 +33,5 @@ class NetworkWriter:
 		jsonData=bytes(json.dumps(jsonData)+"\n",encoding="utf-8")
 		jsonLength=bytes("json-length: "+str(len(jsonData))+"\n",encoding="utf-8")
 		binLength=bytes("bin-length: "+str(len(binData))+"\n",encoding="utf-8")
+
 		return b"".join((jsonLength,binLength,jsonData,binData))
diff --git a/src/server.py b/src/server.py
--- a/src/server.py
+++ b/src/server.py
@@ -42,33 +42,38 @@ class Server:
 		self.dataFile=None
 
 	def serve(self):
-		while self._serveOne():
-			pass
-
-	def _serveOne(self):
-		with Connection(self.ss) as (incoming,outcoming):
-			jsonData,binData=incoming.readMsg()
+		while True:
+			with Connection(self.ss) as (incoming,outcoming):
+				try:
+					while True:
+						if not self._serveOne(incoming,outcoming): return
+				except AssertionError:
+					continue
 
-			if jsonData["command"]=="init":
-				assert jsonData["blockSize"]==self.BLOCK_SIZE
-				assert jsonData["blockCount"]==self.tree.leafCount
+	def _serveOne(self,incoming,outcoming):
+		jsonData,binData=incoming.readMsg()
 
-			elif jsonData["command"]=="req":
-				outcoming.writeMsg(*self._requestHash(jsonData))
+		if jsonData["command"]=="init":
+			assert jsonData["blockSize"]==self.BLOCK_SIZE
+			assert jsonData["blockCount"]==self.tree.leafCount
+			outcoming.writeMsg({"command": "ack"})
 
-			elif jsonData["command"]=="send" and jsonData["dataType"]=="data":
-				self._receiveData(jsonData,binData)
+		elif jsonData["command"]=="req":
+			outcoming.writeMsg(*self._requestHash(jsonData))
+
+		elif jsonData["command"]=="send" and jsonData["dataType"]=="data":
+			outcoming.writeMsg(*self._receiveData(jsonData,binData))
 
-			elif jsonData["command"]=="end":
-				log.info("closing session...")
-				if self.dataFile:
-					self.dataFile.close()
-				return False
+		elif jsonData["command"]=="end":
+			log.info("closing session...")
+			if self.dataFile:
+				self.dataFile.close()
+			return False
 
-			else:
-				assert False, jsonData["command"]
+		else:
+			assert False, jsonData["command"]
 
-			return True
+		return True
 
 	def _requestHash(self,jsonData):
 		log.info("received request for node #{0}".format(jsonData["index"]))
@@ -91,4 +96,5 @@ class Server:
 		self.dataFile.write(binData)
 		self._lastWrite=i
 
+		return ({"command": "ack", "index": i},)
 		# never update the hash tree