A service provider runs an MPLS core and offers a DiffServ-based QoS service. A customer complains that when their DSCP-marked traffic transits the provider's MPLS network, any re-marking the provider performs on the outer MPLS EXP bits (for example, during congestion at the P routers) is being copied back onto the customer's IP DSCP field when the packet exits at the egress PE. The customer wants their original IP DSCP values preserved end-to-end regardless of any provider MPLS EXP changes. Which MPLS DiffServ tunneling mode should the provider configure to meet this requirement?