diff --git a/lib/tfw/networking/server/zmq_websocket_proxy.py b/lib/tfw/networking/server/zmq_websocket_proxy.py
index 6df7c9a..5ab0b10 100644
--- a/lib/tfw/networking/server/zmq_websocket_proxy.py
+++ b/lib/tfw/networking/server/zmq_websocket_proxy.py
@@ -14,6 +14,7 @@ LOG = logging.getLogger(__name__)
 class ZMQWebSocketProxy(WebSocketHandler):
     # pylint: disable=abstract-method
     instances = set()
+    sequence_number = 0
 
     def initialize(self, **kwargs): # pylint: disable=arguments-differ
         self._event_handler_connector = kwargs['event_handler_connector']
@@ -59,14 +60,20 @@ class ZMQWebSocketProxy(WebSocketHandler):
         """
         Invoked on ZMQ messages from event handlers.
         """
+        self.sequence_message(message)
         LOG.debug('Received on pull socket: %s', message)
         self.proxy_eventhandler_to_websocket(message)
 
+    def sequence_message(self, message):
+        self.sequence_number += 1
+        message['seq'] = self.sequence_number
+
     def on_message(self, message):
         """
         Invoked on WS messages from frontend.
         """
         message = json.loads(message)
+        self.sequence_message(message)
         LOG.debug('Received on WebSocket: %s', message)
         self.proxy_websocket_to_eventhandler(message)