umbrella. Because IP is a connectionless protocol, it cannot guarantee that network resources will be available. In addition, IP sends all traffic between the same two points over the same route. During busy periods, therefore, some routes become congested and others remain underutilized. With- out having explicit control over route assignments, the provider has no way to steer excess traffic over less busy routes. One key difference between MPLS and IP is that in MPLS, packets sent between two points can take different paths based on different MPLS labels. How MPLS Works MPLS is connection oriented, like ATM and Frame Relay, and it makes use of label- switched paths (LSPs). MPLS tags, or adds a label to, IPv4 or IPv6 packets SO they can be steered over the Internet along predefined routes. MPLS also adds a label that identifies the type of traffic, the path, and the destination. This allows routers to assign explicit paths to various classes of traffic. Using these explicit routes, ser- vice providers can reserve network resources for high-priority or delay-sensitive flows, distribute traffic to prevent network hotspots, and preprovision backup routes for quick recovery from outages. The Next-Generation Network Infrastructure As shown in Figure 10.15, an MPLS network is composed of a mesh of label- switching routers (LSRs). These LSRs are MPLS-enabled routers and/or MPLS- enabled ATM switches. They are core routers that ignore the IP packet header. LSRs forward packets based on labels, running IP routing protocols and some form of MPLS Label Distribution Protocol (LDP) to allocate and distribute the Forwarding Equivalence Class (FEC) and label bindings. Labels are contained in the packets or cells. They are used by the LSR to index the Label Information Base (LIB) during the label swap. The format is link-layer dependent. As each packet enters the network, an ingress LSR assigns it a label, based on its destination, VPN membership, ToS bits, and other considerations. At each hop, an LSR uses the label to index a