anchor.diff:  
   + Response 101
   
  +## Group Ping
  +
  +In order to warranty the WebSocket connectivity between the client and the Kurento Media Server, a keep-alive method is implemented. This method is based on a `ping` method sent by the client, which must be replied with a `pong` message from the server. If no response is obtained in a time interval, the client is aware that the connectivity with the media server has been lost.
  +
  +### Request
  +
  +A `ping` request contains the following parameters:
  +
  +* `method` (required, string). Value: `ping`.
  +* `params` (required, object). Parameters for the invocation of the ping message, containing these member:
  +    * interval (required, number). Time out to receive the `pong` message from the server, in milliseconds. By default this value is `240000` (i.e. 40 seconds).
  +
  +This is an example of `ping`:
  +
  ++ Body (application/json)
  +
  +        {
  +            "id": 1,
  +            "method": "ping",
  +            "params": {
  +                "interval": 240000
  +            },
  +            "jsonrpc": "2.0"
  +        }
  +
  +### Response
  +
  +The response to a `ping` request must contain a `result` object with a `value` parameter with a fixed name: `pong`. The following snippet shows the `pong` response to the previous `ping` request:
  +
  ++ Body (application/json)
  +
  +        {
  +            "id": 1,
  +            "result": {
  +                "value": "pong"
  +            },
  +            "jsonrpc": "2.0"
  +        }
  +
   ## Group Create
   
  -Create message requests the creation of an element of the toolbox. The parameter type specifies the type of the object to be created. The parameter `constructorParams` contains all the information needed to create the object. Each message needs different constructorParams to create the object.
  -
  -A `sessionId` parameter is included with the identifier of the current session. The value of this parameter is sent by Kurento Media Server to the client in each response. Only the first request from client to server is allowed to not include the `sessionId` (because at this point is unknown for the client).
  +Create message requests the creation of an Media Pipelines and Media Elements in the Media Server. The parameter type specifies the type of the object to be created. The parameter `params` contains all the information needed to create the object. Each message needs different parameters to create the object.
  +
  +A `sessionId` parameter is included with the identifier of the current session. The value of this parameter is sent by Kurento Media Server to the client in each response. Only the first requests from client to server are allowed to not include the `sessionId` (because at this point is unknown for the client).
   
   ### Request
   
  
positive.code: * `method` (required, string). Value is `invoke`.
  * `params` (required, object)
      * `object` (required, number). Identifier of the source media element.
          * `operation` (required, string). Operation invoked. Allowed Values:
              * `connect`. Connect two media elements.
              * `play`. Start the play of a media (PlayerEndPoint).
              * `record`. Start the record of a media (RecorderEndPoint).
      * `operationParams` (optional, object). 
          * `sink` (required, number). Identifier of the sink media element.
          * `sessionId` (required, string). Session identifier.
  
  The following example shows a Request message requesting the invocation of the operation connect on the object `442352747` with parameter sink `6829986`. The `sessionId` is also included as is mandatory for all requests in the session (except the first one):
  
  + Body (application/json)
  
          {
            "jsonrpc": "2.0",
            "id": 2,
            "method": "invoke",
            "params": {
              "object": "442352747", "operation": "connect",
              "operationParams": {
                "sink": "6829986"
              },
              "sessionId": "c93e5bf0-4fd0-4888-9411-765ff5d89b93"
            }
          }
  
  ### Response
  
negative.code: 
  * `result` (required, object)
      * `sessionId` (required, string). Session identifier.
      * `value` (optional, object). Additional object which describes the result of the `Invoke` operation. For example, in a `WebRtcEndpoint` this field is the SDP response (WebRTC SDP negotiation).
  
  The following example shows a typical response while invoking the operation connect:
  
  + Body (application/json)
  
          {
              "id": 5,
              "result": {
                  "sessionId": "bd4d6227-0463-4d52-b1c3-c71f0be68466"
              },
              "jsonrpc": "2.0"
          }
  
  ## Group Release
  