+------------------------------------------------+
        |                                                |
        | Client                      Server             |
        |                                                |
        |               ...                              |
        |   ----------- PING ---------->                 |
        |   <--------- 200 OK ----------                 |
        |   <------- Q4S-ALERT ---------                 |
        |   -------- Q4S-ALERT -------->                 |
        |   <---------- PING -----------                 |
        |   ---------- 200 OK --------->                 |
        |   ----------- PING ---------->                 |
        |   <--------- 200 OK ----------                 |
        |   <---------- PING -----------                 |
        |   ---------- 200 OK --------->                 |
        |        ...                                     |
        |                                                |
        +------------------------------------------------+