Chapter 5 Introducing SDN Control in MPLS Networks - High ...

195the start of the tunnel, to the same select-group to ensure load-sharing. Importantly we donot need two tunnels for load-sharing, and it does not matter if the two paths have thesame cost or not.DiffServ-Aware Traffic Engineering (DS-TE): DS-TE is a complicatedmarriage of two mechanisms: a) QoS based on the Differentiated Services architecture[115] that controls the per-hop behavior (PHB) of various traffic classes defined by theDiffServ Code Points (DSCP) bits; and b) MPSL-TE that controls the hop-to-hop (orpath-level) behavior for IP traffic.It is worth noting that on-their-own neither can guarantee QoS [92] on an end-to-endbasis (within a network domain). For example, while DiffServ mechanisms can prioritize,shape and police traffic in class-based queues within a switch; it has no control over thepath the traffic takes in the network domain. And so, congestion in a downstream nodecan cause traffic in some classes to suffer even with PHB control, as the congestioninformation is localized and not propagated back to upstream nodes. In this case, TE canhelp by trying to avoid congestion a-priori by steering traffic away via tunnels thatreserve link-bandwidth and are admission-controlled.However tunnel admission control is agnostic to the type (or class) of traffic that isbeing carried in the tunnel. And so traffic for a certain class of traffic cannot be steeredaway with regular TE. Therefore the purpose of DS-TE is to enable tunnel admissioncontrolon a per-QoS class basis.Consider Fig. 5.10a. It shows the queues that feed an outgoing port on which two(regular) MPLS-TE tunnels are routed. The TE tunnels reserve bandwidth on the linkwithout taking into consideration the data-rates supported by the queues. Since thetunnels are unaware of the queues, they carry traffic for all class-types. Note that thehighest-priority traffic in both tunnels comes out of the same queue. During times ofcongestion, it is possible that the min-rate guaranteed by the high-priority queue may notby enough to sustain all the high-priority traffic arriving at the queue.