Multi-Protocol Label Switching (MPLS)


Multi-Protocol Label Switching (MPLS) was originally presented as a way of improving the forwarding speed of routers but is now emerging as a crucial standard technology that offers new capabilities for large scale IP networks. Traffic engineering, the ability of network operators to dictate the path that traffic takes through their network, and Virtual Private Network support are examples of two key applications where MPLS is superior to any currently available IP technology.

Although MPLS was conceived as being independent of Layer 2, much of the excitement generated by MPLS revolves around its promise to provide a more effective means of deploying IP networks across ATM-based WAN backbones. The Internet Engineering Task Force is developing MPLS with draft standards expected by the end of 1998. MPLS is viewed by some as one of the most important network developments of the 1990's. This article will explain why MPLS is generating such interest.

The essence of MPLS is the generation of a short fixed-length label that acts as a shorthand representation of an IP packet's header. This is much the same way as a ZIP code is shorthand for the house, street and city in a postal address, and the use of that label to make forwarding decisions about the packet. IP packets have a field in their 'header' that contains the address to which the packet is to be routed. Traditional routed networks process this information at every router in a packet's path through the network (hop by hop routing).

In MPLS, the IP packets are encapsulated with these labels by the first MPLS device they encounter as they enter the network. The MPLS edge router analyses the contents of the IP header and selects an appropriate label with which to encapsulate the packet. Part of the great power of MPLS comes from the fact that, in contrast to conventional IP routing, this analysis can be based on more than just the destination address carried in the IP header. At all the subsequent nodes within the network the MPLS label, and not the IP header, is used to make the forwarding decision for the packet. Finally, as MPLS labeled packets leave the network, another edge router removes the labels.

In MPLS terminology, the packet handling nodes or routers are called Label Switched Routers (LSRs). The derivation of the term should be obvious; MPLS routers forward packets by making switching decisions based on the MPLS label. This illustrates another of the key concepts in MPLS. Conventional IP routers contain routing tables which are looked up using the IP header from a packet to decide how to forward that packet. These tables are built by IP routing protocols (e.g., RIP or OSPF) which carry around IP reachability information in the form of IP addresses. In practice, we find that forwarding (IP header lookup) and control planes (generation of the routing tables) are tightly coupled. Since MPLS forwarding is based on labels it is possible to cleanly separate the (label-based) forwarding plane from the routing protocol control plane. By separating the two, each can be modified independently. With such a separation, we don't need to change the forwarding machinery, for example, to migrate a new routing strategy into the network.

There are two broad categories of LSR. At the edge of the network, we require high performance packet classifiers that can apply (and remove) the requisite labels: we call these MPLS edge routers. Core LSRs need to be capable of processing the labeled packets at extremely high bandwidths.

This is an abstract of the MPLS article contained in The complete article examines MPLS and the opportunities it offers to users and also to the service providers who are designing and engineering the next generation of IP networks. It also describes why new carrier-class edge devices will become a key component in the provisioning of future network services.

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