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MPLS was designed to bring a more “packet-switching” behavior to IP, similar to ATM or Frame Relay. To investigate this lets take a look at how traditional IP and ATM networks work. In a typical IP network the data is cut up into packets (they can be of varying lengths) and these packets are forwarded based on the destination IP address in the IP header. In these networks routers take independent forwarding decisions at each hop, which can lead to poor service quality as a result of packets re-sequencing.
In the header for IP the most important part is the destination IP address. As we saw different routers in the network can take different routing decisions, so packets from the same source to the same destination can take different paths.
In ATM the header identifies PVC’s – Permanent Virtual Paths. These are paths that the cells must take. So for ATM every cell that has the same source and same destination will travel over the same path – unlike IP. Thus the cells are not re-ordered in transit. The ATM header contains a virtual path indicator and a virtual channel / circuit indicator that define the PVC. These values are locally significant and are switched at each hop along the network.
ATM-based cores offer precise control over traffic flows across the network. ATM allows for traffic engineering as we can manipulate the paths that the cells take. PVC’s are mapped from edge to edge to precisely distribute traffic across all links so that they are evenly utilized. And because ATM is a virtual network, sitting on top of a physical one the virtual network could be reengineered without changing the physical network.
ATM isn’t the only protocol that uses the label concept. Frame relay uses a DLCI – Data Link Connection Identifier. This DLCI has a channel number which is applied to frame relay frames and is similar to a label, it is used by the network to route data. This identifier is locally significant also.
Another technology that can be used is TDM. TDM uses a concept of timeslots. So if we want to bring a more label switching approach to our packet forwarding why don’t we just use ATM? It is quite evident now that future applications will be based on IP. Now MPLS has become more stable and a recognized technology it can combine the benefits of labelswitching over an IP core and control plane.