# An SDN Focused Approach for Energy Aware Traffic Engineering in Data Centers
## 2 Related Work  
For the ICT domain, the main method to limit \(\text{CO}_{2}\) emissions is by decreasing the power consumption of network topologies. A lot of work is performed in this area on various layers of the network architecture. Existing methods can be classified into two main categories (a) evolving hardware to support power consumption proportional to traffic served [11] and (b) putting unused links and devices into sleep mode after applying traffic engineering techniques to reroute traffic [12]. Techniques to allow unused links to get into sleep states can be applied to 'access' part of the network (both wired and wireless), to 'transport' (covering wired aggregation and mobile backhauling based on optical links) and to 'core' [13]. Based on work performed in core networks, a number of mechanisms have been proposed and that can be separated into four main categories (a) new routing protocols [14], (b) traffic engineering techniques [15], (c) new network architectures [16], and (d) modifications to existing routing protocols [17; 18] so that routing decisions will be based on energy consumption criteria. The main drawback of existing approaches is the requirement for full knowledge of network topology and link utilizations. Even though getting a full layer-3 topology is rather easy even on large scales networks, it is not feasible to gather per flow statistics even when utilizing modern monitoring systems with telemetry [19] and big data analytics [20]. On the DC domain, a more holistic approach is taken into consideration that addresses the power consumption not only on networking and computing devices but on supporting equipment such as cooling devices. On the networking layer, similar approaches have been proposed that try to put unused links in sleep state. Furthermore, they only focus on legacy devices with sleep states,neglecting other possible power states [21]. In contrast, modern switches and especially fiber optics links in DCs support a certain degree of energy proportionality with a step-wise approach [22]. A typical modular network device consumes power only in two concrete states (a) maximum power when operating in normal mode and (b) minimum power when operating in sleep state. This mode of operation is applicable on chassis layer, line cards and routing processors. There is no direct correlation between traffic volumes and power consumption, but only on environmental aspects like DC room temperature and the number of routing table entries. Interfaces on the other hand operate on energy aware states according to the amount of traffic they serve. The number of states and the maximum power consumption of an interface varies among manufacturers and depends on the link type (multimode optics, single mode optics, DAC cables, CAT-6 copper cables) [23]. Until recently, all proposed mechanisms have been applied only to experimental topologies and lack adoption from major networking equipment manufacturers. In addition, there is a lack in real life deployments of such solutions due to a number of reasons: (a) protocol expansions are not eagerly adopted from manufactures, (b) network statistics (on per flow basis) and topology monitoring cannot scale well, and (c) advances in the area of SDN are not taken into account. The SDN architecture [24] of separated data and control plane has been successfully used to build networks inside a DC regardless of the purpose served and the workloads hosted, from 5G and NFV services to generic IT applications and data storage. Furthermore, SDN already provides the mechanisms to efficiently collect measurements and apply forwarding rules to existing equipment without modifications to devices or routing protocols. Finally, SDN design allows to easily integrate external applications via well-defined software APIs in SDN controllers (SDN-C). Therefore, the use of an SND-C application seems to be the only viable method for network policies in any aspect of DC networking.