1. A method comprising: establishing, with a mobile gateway positioned in a mobile access network, a session by which a mobile device is to access a service; determining, by one or more processors of the mobile gateway, that the mobile gateway has incomplete information to activate usage monitoring of the service responsive to receipt of a message to activate a charging rule that includes a request to activate usage monitoring for a service identified by an identifier without having received any message associating the identifier to the service provided via the session; and in response to determining that the mobile gateway has the incomplete information to activate the usage monitoring, rejecting, with the one or more processors of the mobile gateway, the charging rule.