Class selector code points provide backward compatibility between IP Precedence and DiffServ. IP Precedence uses the three most significant bits of the former Type of Service field. DiffServ expands QoS marking by using the six-bit DSCP field, but class selector values preserve compatibility by setting the three high-order bits to match the corresponding IP Precedence value while leaving the lower bits as zeros. For example, CS3 corresponds to the old precedence value 3, and CS6 corresponds to precedence 6. This allows devices that still interpret IP Precedence to provide broadly consistent treatment while newer DiffServ-aware devices can use DSCP-based policy. Expedited forwarding is used for low-loss, low-latency traffic such as voice. Assured forwarding provides drop precedence within classes. Default per-hop behavior represents best- effort treatment. None of those are specifically the backward-compatibility mechanism for migration from IP Precedence. Therefore, the design should include class selector code points during the transition to DiffServ.
Reference topics: DiffServ, DSCP, class selector, IP Precedence compatibility, QoS marking migration.