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With differentiated services (Diffserv), packets are classified at the edge of the network. The differentiated service-fields (DS-fields) of the packets are set accordingly. In the middle of the network, packets are buffered and scheduled in accordance to their DS-fields by weighted random early detection (WRED) and weighted round robin (WRR). Important traffic such as network control traffic and traffic from premium customers will be forwarded preferentially.

In terms of support for QoS, MPLS provides the CoS field which enables different service classes to be offered for individual labels. For more fine-grained QoS provisioning, the CoS field could be ignored, using a separate label for each class. In this instance, the label would represent both the forwarding and service classes. As noted earlier, MPLS is able to provide QoS support on a per-flow basis using either flow detection or request-based control traffic from protocols such as RSVP to trigger label assignment. More general QoS differentiation can be achieved by such means as label assignment on a per-user basis, and using more general traffic engineering techniques.

A typical example for QoS application is that tunnels (from ingress to egress) can be preset across the MPLS network and QoS can be provisioned to each such tunnel. This concept has existed for quite some time in Layer 2 protocols such as ATM and frame relay. Preset tunnels are simple and efficient, but pre-provisioning them in interconnected networks makes relatively inefficient circuit-like use of resources that must be constantly tuned.





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