 to a wholesale carrier. Service providers could use label stacking to merge hundreds of thousands of LSPs into a relatively small number of backbone tunnels between points of presence. Fewer tunnels mean smaller routing tables, and smaller routing tables make it easier for providers to scale the network core. Why MPLS? MPLS adds two important elements to IP: virtual circuits and capacity reservation. Virtual circuits, in this context, are referred to as LSPs. LSPs provide security simi- lar to that found in Frame Relay: Tunnels are used to isolate customer traffic. (But true security is not simply guaranteed through the use of tunnels; rather, true secu- rity means that customer traffic must be encrypted end to end.) MPLS also allows for capacity reservation, enabling the support of SLAs. Typical parameters might include a packet loss of less than 1% and a round-trip delay of less than 55 to 70 milliseconds. Constraint-based routing is superior to IP because it bases routing decisions on more than just a shortest-path calculation. Hence, using MPLS may be the best way for service providers to provision VPNs that meet customer service quality metrics and permit ISPs to scale their networks and meet traffic-engineering requirements without having to resort to using ATM permanent virtual circuit (PVC) overlay networks. In summary, MPLS adds QoS and virtual tunnels; it provides a common con- trol plane between Layer 2 and Layer 3; it can support multiple Layer 2 protocols such as Frame Relay, ATM, and Ethernet; and it provides Layer 2 performance, which means it has the benefits of connection-oriented networks. MPLS is a com- promise between connectionless Layer 3 and connection-oriented Layer 2, which means it provides deterministic behavior. Using MPLS is the most effective way to integrate IP and ATM in the same backbone network. It reduces the processing overhead in IP routers, improving packet-forwarding performance. MPLS is also another way to provide QoS in net- work backbones, competing with or being complem