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()