s described here it is said to work as downstream unsolicited, with liberal label retention and using independent LSP control mode. Each of these modes has an alternative, so, for example, rather than mapping messages being sent unsolicited, the sages can be sent only when an upstream router requests a binding on demand; rather than liberally retaining a label every time a neighbor sends a binding, the router can conserva- tively use labels only for FECs that it knows about; finally, rather than every LSR indepen- dently sending mappings, LSRs can send them only in an ordered manner from egress to ingress. It will be up to the network operator to determine which of these modes is most appropriate. Here the most common configuration is described and the reader should refer to RFC 3031, RFC 5036, RFC 5037, or De Ghein (2006) for a more detailed description and arguments about which modes should be used. Later, when RSVP-TE is considered, it will be seen that it naturally uses downstream on-demand label distribution mode with ordered LSP control. Traffic Engineering The MPLS architecture and its native method for distributing labels (LDP) have been intro- duced. Before considering traffic engineering, it is worthwhile to think about what the technology explained SO far achieves. The architecture defines how unlabeled traffic can be labeled and how labeled traffic can be processed in each router. LDP defines how address prefixes obtained from IP routing tables can be used to set up label bindings in each LSR to create LSPs that follow the same paths that would be followed by conventional IP forwarding. In other words, all of the complex MPLS architecture and signaling described SO far achieve nothing except for the fact that packets are switched using label swapping as opposed to IP forwarding using the longest prefix match. In the 1990s this would have been significant advantage, but modern IP routers can achieve full line rate IP forwarding. Consequently, there has to be an additional need for MPLS for it to be dep