Which three statements are true about the application of QoS models？ （）

• A、 The DiffServ model requires applications to signal the network with QoS requirements.
• B、 The DiffServ model can be used to deliver QoS based upon IP precedence， or source and destination addresses.
• C、 The DiffServ model requires RSVP.
• D、 The best effort model is suitable for applications such as file transfer and e-mail
• E、 The IntServ model requires applications to signal the network with QoS requirements.
• F、 The IntServ model attempts to deliver a level of service based on the QoS specified by each packet