## 2 Previous Work  
Early work has studied the consequences on network quality of service, including delay, packet loss and jitter, of putting routers OFF and ON as a way to save energy and adapt energy consumption to actual network load [9; 10]. Recent studies also indicate that energy can be saved by modifying routing policies, for example, by aggregating traffic along a few routes and switching OFF, putting in standby or putting to sleep some network nodes. One such approach is to use dynamic bandwidth allocation, as a way to allow some users to have increased bit rate when others do not require as much [2]. Another approach is dynamic topology optimization where the alterations of the load are exploited in order to select among all topologies that ensure connectivity and satisfy the traffic demands the one that has the lowest overall power consumption [2]. Another technique called proxying transfers the management of traffic to a dedicated proxy entity that stores all packets and responds to requests for connections, enabling other network nodes to sleep for longer periods [11]. In [12], the authors propose an enhancement to the OSPF routing protocol in which a subset of IP routers are selected to be switched OFF during periods of low traffic. In [13] a greener router is proposed by implementing congestion control and capacity scaling. The recent survey [8] describes several techniques for achieving energy efficiency in networks. Early work suggesting energy savings in the Internet [14] proposed routing modifications so as to aggregate traffic along a few routes, leading to a modification of the network topology based on route adaptation and putting nodes and devices to sleep. More recent work illustrated the feasibility of using admission control to restrict the more energy demanding users [15]. The problem of energy aware routing is examined via an analytical approach in [16] where an optimization algorithm is built in order to minimise a composite energy and QoS cost function. In [17] an autonomic algorithm based on the CPN protocol [18] is proposed to minimise power consumption while meeting the requested end-to-end delay bounds. In [19] two widely used routers are measured in terms of system power demand with different configurations of line cards and operating conditions. While traffic was shown to have some impact on the power consumption of a line card, it is measured to be only around 2%. A similar power consumption model [20] includes the effect of individual ports configured on each line card, suggesting that the impact of port utilization on power consumption is under 5% and identifying the challenge for device manufacturers to ensure that networking devices are energy proportional. These studies indicate that the power consumption of current networking equipment is largely independent of its utilization and thus there is significant energy waste when nodes are inactive or lightly loaded. An energy-aware online technique is proposed in [21] to spread the load through multiple paths, based on assuming a step-like model of power consumption as a function of the hardware's processing rate and the ability of nodes to automatically adjust their operating rate to their utilization. Rate-adaptation for individual links is also examined in [22] based on the utilization and the link queuing delay. Moreover, a sleeping approach is proposed, where traffic is sent out in bursts at the edge routers enabling other line cards to sleep between successive bursts of traffic. In [23], the authors select the active links and routers to minimise the power consumption via simple heuristics that approximately solve a corresponding NP-hard problem, while in [24], they discuss a case study based on specific backbone networks, and they propose an estimate of the potential overall energy savings in the Internet ([25]). In [26], the relationship between network robustness, performance and Internet power consumption is investigated based on data collected from Internet sources.