Adding how websocket works with AppGW

Author: abshamsft

articles/application-gateway/application-gateway-websocket.md

@@ -13,7 +13,7 @@ ms.devlang: na
 ms.topic: article
 ms.tgt_pltfrm: na
 ms.workload: infrastructure-services
-ms.date: 05/08/2017
+ms.date: 03/11/2019
 ms.author: amsriva
 
 ---
@@ -25,7 +25,15 @@ WebSocket protocol standardized in [RFC6455](https://tools.ietf.org/html/rfc6455
 
 You can continue using a standard HTTP listener on port 80 or 443 to receive WebSocket traffic. WebSocket traffic is then directed to the WebSocket enabled backend server using the appropriate backend pool as specified in application gateway rules. The backend server must respond to the application gateway probes, which are described in the [health probe overview](application-gateway-probe-overview.md) section. Application gateway health probes are HTTP/HTTPS only. Each backend server must respond to HTTP probes for application gateway to route WebSocket traffic to the server.
 
-## Listener configuration element
+It's used in apps that benefit from fast, real-time communication, such as chat, dashboard, and game apps.
+
+## How does WebSocket work
+
+To establish a WebSocket connection, a specific HTTP-based handshake is exchanged between the client and the server. If successful, the application-layer protocol is "upgraded" from HTTP to WebSockets, using the previously established TCP connection. Once this occurs, HTTP is completely out of the picture; data can be sent or received using the WebSocket protocol by both endpoints, until the WebSocket connection is closed. 
+
+![addcert](.\media\application-gateway-websocket\websocket.png)
+
+### Listener configuration element
 
 An existing HTTP listener can be used to support WebSocket traffic. The following is a snippet of an httpListeners element from a sample template file. You would need both HTTP and HTTPS listeners to support WebSocket and secure WebSocket traffic. Similarly you can use the [portal](application-gateway-create-gateway-portal.md) or [PowerShell](application-gateway-create-gateway-arm.md) to create an application gateway with listeners on port 80/443 to support WebSocket traffic.