Which two statements about the DiffServ model are true？ （）

• A、 A class can be identified as a single application， multiple applications with similar service needs， or be based on the source or destination IP addresses.
• B、 A drawback of the DiffServ model is that it does not provide backward compatibility with devices that can only use the ToS field.
• C、 DiffServ uses the DiffServ field in the MAC header to mark frames into behavior aggregates （BAs）. 
• D、 The DiffServ field occupies the same eight bits of the MAC header that were previously used for the ToS field.
• E、 The first six high-order bits of the DiffServ field are used to identify the Resource Reservation Protocol （RSVP） value.
• F、 The primary goal of DiffServ is scalability.