Fix rtt, including bug in time compute

hbd/client/main.py

@@ -115,13 +115,14 @@ class AsyncConnection:
         self.logger.debug(f"Sent {msg_id} message ({len(data)} bytes)")
     
     def handle_ack(self, msg: dict, now: float):
-        """Handle ACK message from server."""
-        try:
-            self.lastack = msg.get("time", now)
-            rtt = (self.lastack - self.lastsend) * 2000.0  # Convert to ms
-        except Exception:
-            self.lastack = now
-            rtt = (self.lastack - self.lastsend) * 1000.0
+        """Handle ACK message from server.
+        
+        RTT is calculated as: (time ACK received) - (time HTB sent)
+        """
+        self.lastack = now
+        
+        # Calculate RTT: time ACK received minus time HTB sent
+        rtt = (now - self.lastsend) * 1000.0  # Convert to ms
         
         self.rtts.append(rtt)
         if len(self.rtts) > 10: