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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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