Which three statements are true about the appliance 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 model attempts to deliver a level of service based on the QoS specified by each packet