Tutorial

Advanced MPLS Signaling
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Traffic Control in MPLS Networks

In networking, MPLS is express traffic that carries four (4) additional bytes of payload.  For taking that effort, it gets to travel the express lanes.  But, as is too often the case in the actual freeway, your nice, smooth-running express lane is subjected to routine traffic being rerouted onto it, causing congestion and slowdowns.

Remember that MPLS is an overlay protocol that overlays MPLS traffic on a routine IP network. The self-healing properties of IP may cause congestion on your express lanes. There is no accounting for the unforeseen traffic accidents and reroutes of routine traffic onto the express lanes. The Internet is self-healing with resource capabilities, but the problem becomes this: how does one ensure that paths and bandwidth that are reserved for their packets do not get overrun by rerouted traffic? (Figures 2 –4)

Figure 2: MPLS with Three Paths

In Figure 2, we see a standard MPLS network with three different paths across the Wide- Area Network.  Path A is engineered to the 90^th percentile of bandwidth of peak busy hour; Path B is engineered to the 100^th percentile bandwidth of peak busy hour; finally, Path C is engineered to the 125^th percentile of peak busy hour.  In theory, Path A will never have to contend with congestion, owing to sound network design (including traffic engineering). In other words, the road is engineered to take more traffic than it will receive during rush hour. The C network, however, will experience traffic jams during rush hour, because it is designed not to handle peak traffic conditions.

The Quality of Service (QoS) in Path C will have some level of unpredictability regarding both jitter and dropped packets, whereas the traffic on Path A should have consistent QoS measurements.

Figure 3: MPLS with a Failed Path C

In Figure 3, we see a network failure in Path C, and the traffic is rerouted (Figure 4) onto an available path – Path A.  Under these conditions, Path A is subjected to a loss of QoS criteria. To attain real QoS, there must be a method for controlling both traffic on the paths and the percentage of traffic that is allowed onto every engineered path.

Figure 4: MPLS with Congestion Caused by a Reroute

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