diff --git a/protocol.md b/protocol.md
--- a/protocol.md
+++ b/protocol.md
@@ -1,6 +1,6 @@
 # Protocol #
 
-[Communication protocol proposal. Not yet fully implemented and expected to change.]
+[Communication protocol proposal. Not yet fully implemented and still expected to change.]
 
 Nodes communicate by exchanging messages through network sockets. Basic message format is simple:
 
@@ -19,7 +19,7 @@ The client sends a request (a message) a
 Initiates communication between the client and the server. Checks that the nodes can meaningfully talk to each other.
 
 #### Params ####
-- (str) version: program version. Both the client and the server are to check for compatibility with their partner.
+- [(int) major, (int) minor, (int) tiny] version version: program version. Both the client and the server are to check for compatibility with their partner.
 - {push, pull} action: what the client desires to do
 - (str) filename: name of the file to be synchronized
 - (int) blockSize
@@ -74,7 +74,7 @@ Cease talking and close the connection.
 See [the init command](#init). If the client doesn't like server's protocol version, it is free to [end](#end) the connection.
 
 #### Params ####
-- (str) version
+- [(int) major, (int) minor, (int) tiny] version
 
 
 ### <a name="deny"></a>deny ###
@@ -82,6 +82,7 @@ The server refuses connection. It might 
 
 #### Params ####
 - (int) retry: suggested delay in seconds before another attempt at connection
+- (int) status
 - (str) msg
 
 
diff --git a/src/client.py b/src/client.py
--- a/src/client.py
+++ b/src/client.py
@@ -5,10 +5,11 @@ import logging as log
 from datetime import datetime
 
 import config as conf
+import status
 import stats
 from util import Progress
 from hashtree import HashTree,hashBlock
-from netnode import BaseConnection,NetNode,FailedConnection,LockedException
+from netnode import BaseConnection,NetNode,FailedConnection,LockedException,IncompatibleException
 
 
 class DeniedConnection(Exception): pass
@@ -50,8 +51,12 @@ class Client(NetNode):
 		self._outcoming.writeMsg(jsonData)
 		jsonData,binData=self._incoming.readMsg()
 		if jsonData["command"]=="deny":
+			if jsonData["status"]==status.incompatible.version:
+				raise DeniedConnection("Incompatible client version. Consider upgrading it.")
 			raise DeniedConnection()
 		assert jsonData["command"]=="init"
+		if jsonData["version"]<conf.lowestCompatible:
+			raise IncompatibleException("Incompatible server version. Consider upgrading it.")
 
 	## Asks server for node hashes to determine which are to be transferred.
 	#
diff --git a/src/config.py b/src/config.py
--- a/src/config.py
+++ b/src/config.py
@@ -21,7 +21,8 @@ conf=dict()
 if os.path.isfile(configFile):
 	with open(configFile) as f: conf=json.load(f)
 
-version=(0,0,0)
+version=[0,0,1]
+lowestCompatible=[0,0,0] # tuple is more fitting but json conversion transforms it into a list anyway
 
 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
@@ -50,8 +50,9 @@ def push(args):
 			print(stats.report())
 			print()
 		except FailedConnection: pass
-		except DeniedConnection:
+		except DeniedConnection as e:
 			print("Server {0}:{1} denied connection.".format(*host))
+			print(e)
 
 def pull(args):
 	_checkFile(args.datafile)
@@ -71,8 +72,9 @@ def pull(args):
 		print()
 		print(stats.report())
 	except FailedConnection: pass
-	except DeniedConnection:
+	except DeniedConnection as e:
 		print("Server {0}:{1} denied connection.".format(*host))
+		print(e)
 
 def serve(args):
 	_checkFile(args.datafile)
diff --git a/src/netnode.py b/src/netnode.py
--- a/src/netnode.py
+++ b/src/netnode.py
@@ -12,6 +12,7 @@ lockFile=os.path.join(conf.directory,"di
 
 class FailedConnection(Exception): pass
 class LockedException(Exception): pass
+class IncompatibleException(Exception): pass
 
 
 class BaseConnection: # abstract
diff --git a/src/server.py b/src/server.py
--- a/src/server.py
+++ b/src/server.py
@@ -6,6 +6,7 @@ import logging as log
 from hashtree import hashBlock
 from netnode import BaseConnection,NetNode
 import config as conf
+import status
 
 
 class Connection(BaseConnection):
@@ -40,7 +41,7 @@ class Miniserver:
 				if p and p.is_alive():
 					with connection as c:
 						c[0].readMsg()
-						c[1].writeMsg({"command":"deny"})
+						c[1].writeMsg({"command":"deny","status":status.locked})
 					continue
 				p=multiprocessing.Process(target=Server.run,args=(connection,self._filename,self._treeFile))
 				p.start()
@@ -79,9 +80,11 @@ class Server(NetNode):
 
 		if jsonData["command"]=="init":
 			if jsonData["blockSize"]!=self.BLOCK_SIZE or jsonData["blockCount"]!=self._tree.leafCount:
-				self._outcoming.writeMsg({"command":"deny"})
+				self._outcoming.writeMsg({"command":"deny","status":status.incompatible.parameters})
+			if jsonData["version"]<conf.lowestCompatible:
+				self._outcoming.writeMsg({"command":"deny","status":status.incompatible.version})
 			if jsonData["action"]=="pull" and self.isLocked():
-				self._outcoming.writeMsg({"command":"deny"})
+				self._outcoming.writeMsg({"command":"deny","status":status.locked})
 			if jsonData["action"]=="push" and not self.isLocked():
 				self._lock()
 
diff --git a/src/status.py b/src/status.py
new file mode 100644
--- /dev/null
+++ b/src/status.py
@@ -0,0 +1,5 @@
+class incompatible:
+	version=100
+	parameters=101
+
+locked=102