* Establishes for the client link identified by 'req' a subscription
* to the 'event'.
*
+ * Establishing subscription MUST be called BEFORE replying to the request.
+ *
* @param req the request
* @param event the event to subscribe
*
* link identified by 'req'.
* Returns 0 in case of successful subscription or -1 in case of error.
*
+ * Revoking subscription MUST be called BEFORE replying to the request.
+ *
* @param req the request
* @param event the event to revoke
*