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Positive Features of MPLS

Efficient Packet Forwarding

MPLS and multilayer routing techniques in general allow efficient packet forwarding to enable high-speed data transfer. Although in the case of MPLS the link layer is not specified, the approaches all provide a scenario where it is possible to fully integrate and couple traditional datagram routing concepts with link-layer switching devices supported within the telecommunications industry. MPLS functionality is now being supported directly within hardware, with routing and switching mechanisms combined at the chip level in order to provide integration at high speeds, thus increasing its viability.


MPLS-capable devices are able to provide additional functionality beyond the best-effort packet forwarding found within a gigabit router. This flexibility means that in principle it is possible to support ideas such as QoS differentiation. The fundamental separation between forwarding class and label assignment provides a great deal of flexibility. While packets within a class are to be processed in the same way, this approach means that traffic can be engineered to varying extents.

Traffic Engineering

Alone, IP does not lend itself to the idea of traffic engineering, that is, the ability to manage bandwidth and routes in order to provide equal loading of resources within the network. Until now, it has been reliant on other technologies (e.g., ATM) and associated encapsulation techniques in order to offer this functionality. MPLS provides support for traffic engineering through the deployment of constraint-based routing. Stemming from the idea of QoS routing, constraint-based routing not only provides routes that are able to meet the QoS requirements of a flow, but also considers other constraints including network policy and usage. Label distribution protocols supporting label switching for end-to-end constraint-based paths allow traffic characteristics to be described in terms of peak rate and committed rate bandwidth constraints, along with a specified service granularity (which can be used to define the delay variation constraint).

Explicit routing (a subset of constraint-based routing) allows the specification of the route to be taken across the network. This is enabled within MPLS by allowing a label to represent a route, without the overhead of source routing found within normal IP forwarding (making it too resource-intensive for use in most circumstances). Different paths can be selected in order to allow traffic engineering to be carried out effectively, allowing network load to be balanced in a far more flexible manner than manually configuring virtual circuits (as with other primitive approaches to engineering IP traffic). The engineering of paths in such a way implies a simple mechanism for measuring traffic between edge network devices making use of an LSP.

In Internet service provider (ISP) environments where service differentiation is likely to mean users will be charged in terms of the network QoS exploited, the ability within the MPLS architecture to specify per-host and per-user label assignment is likely to be very useful for billing purposes.

Figure 14 - The traffic engineering required to override the shortest path route.

Figure 15 - Explicitly routed LSPs as tunnels enable traffic engineering.

Virtual Private Networks

One service currently delivered using a connection-oriented network is a virtual private network (VPN). Such networks are useful in providing the internal network to a distributed organization. A typical example is the interconnection of several remote field offices with a corporate headquarters. Such a network may not have Internet access and has stringent privacy requirements on its traffic. This application is frequently addressed today using frame relay/ATM.

In an MPLS network, a VPN service could be delivered in a variety of ways. One way would be direct emulation of frame relay, ATM. Another approach would be to deliver the service using MPLS-aware subscriber equipment. Either approach allows a service provider to deliver this popular service in an integrated manner on the same infrastructure they use to provide Internet services.

Figure 16 - An IP VPN ingress LER.




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