ey labels along the top path and darker gray labels along the bottom path, are transparently tunneled across the back- All-Optical Networks Figure 7-16 Label stacks across LSR A the backbone. Host X Backbone Network LSR C LSR D LSR E 21 13 Host Y LSR B bone network in a single outer LSP between LSR C and LSR E. A label stack is arranged with the label for the outer tunnel at the top and the label for the inner LSP at the bottom. On the wire (or fiber), the topmost label is transmitted first and is the only label used for routing the packet until it is popped from the stack and the next highest label becomes the top label. A major asset of MPLS is its traffic engineering. According to draft-ietf- tewg-principles-00.txt, Internet traffic engineering is defined as that aspect of Internet network engineering dealing with the issue of perfor- mance evaluation and performance optimization of operational IP net- works." The objectives are to enhance performance, facilitate reliable network operations, and control and optimize the routing function. In other words, the goal is to forward packets over a path to the destination that offers the maximum efficiency and optimal performance. MPLS traffic engi- neering overrides shortest path routing. It discovers the path that meets requirements and installs the forwarding state along that path. Essentially, it provides connection-oriented service over a connectionless network. MPLS traffic-engineering components include routing protocol extensions, constraint-based path selection, signaling, and label switching. The advan- tages afforded by MPLS traffic engineering are better control over traffic aggregate paths, extensible to circuit-oriented network regimes, and poly- morphic Link State Protocol (LSP) behavior. This is a brief description of how MPLS traffic engineering works:17 1. LSP advertises "unreserved capacity" and administrative attributes per link. 2. Constraints (required bandwidth and policy) are specified for a traffic- engineering trunk at the ingress mode. Chapt