问题：

[多选] Which two statements about the diffserv model are true？（）

A . a class can be inentified as a single application .multiple applications with similar service nees.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 scalabilty.