Traditional IP packet forwarding analyzes the destination IP address contained in the network layer header of each packet as the packet travels from its source to its final destination. A router analyzes the destination IP address independently at each hop in the network. Dynamic routing protocols or static configuration builds the database needed to analyze the destination IP address (the routing table). The process of implementing traditional IP routing also is called hop-by-hop destination-based unicast routing.
Although successful, and obviously widely deployed, certain restrictions, which have been realized for some time, exist for this method of packet forwarding that diminish its flexibility. New techniques are therefore required to address and expand the functionality of an IP-based network infrastructure.
This first chapter concentrates on identifying these restrictions and presents a new architecture, known as Multiprotocol Label Switching (MPLS), that provides solutions to some of these restrictions. The following chapters focus first on the details of the MPLS architecture in a pure router environment, and then in a mixed router/ATM switch environment.