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.