reasons. The first reason is that distributed pro- tocols are fast and easy to program, but as their action is local, they are not generally able to perform a real optimization of the network, as we will see when describing the Transmission Control Protocol/Internet Protocol (TCP*/IP) control plane. In order not to allow a progressive sliding of the network state from the optimum, for example, in terms of routing or backup capacity assignment, centralized traffic engineer- ing algorithms are needed that allows routing and capacity assignment to be enforced on the network. Even more important is the need for hardware maintenance and field intervention to substitute malfunctioning hardware parts. As a matter of fact, survivability protocols are able to reroute a signal that is not transmitted in the correct way and they can even identify * It is to be noted that the acronym TCP in this context is used as "Transmission Control Protocol." is also used with the meaning of "Terminal Connection Point" in the general layering theory. The two meanings indicate very different concepts SO that no difficulty should arise to identify the correct meaning from the context. Networks Fundamentals and Present Architectures Connection Control controller plane plane Client Routing element Management plane Management controller FIGURE 3.16 Network decomposition in data plane, control plane, and management plane. EMS, element management sys- tem; NMS, network management system. the problem underlying the performance degradation, but, in case of out-of-order network elements, for sure, protocols cannot change the failed hardware with a working one. Another important reason justifying the presence of a centralized network management is the need of a carrier to certify the network performances in front of the service level agreement (SLA) that guarantee to customers a certain performance level. Finally, when a new service is enabled in the network or a new software release is down- loaded over some network elements, all the setup procedu