# An SDN Focused Approach for Energy Aware Traffic Engineering in Data Centers
## 4 Emulation Results  
We evaluated the results of our optimization algorithm on various network topologies, sizes (number of nodes) and traffic volumes. We emulated three different network topologies applicable to DC networking (a) classical three tier topology [26], (b) fat tree [27], and (c) leaf and spine [28]. We focused only to inter-switch communications and did not consider the impact of multihomed servers where further benefits can be achieved. Physical topologies and switches were emulated using Mininet [29] that implements OpenvSwitch (OVS) [30] as an Openflow switch, because of its ability to create and maintain large network topologies [31]. Mininet is responsible for instantiating a number of Openflow enabled switches, connecting them to an external SDN-C, creating a number of physical hosts and interconnecting them with virtual links. Mininet allows the development of scripts to deploy large scale topologies based on pre-defined parameters. In this context, we developed a series of scripts that automatically generate the topologies under examination with the number of devices described in Table 2.  
We have chosen Opendaylight (ODL) [32] as the SDN controller and we executed our experiments employing ODL's RESTful API. The experimentation procedure is depicted in Figure 1. Initially we spin up the desired topology on a VM instance executing Mininet software. The OVS instance inside Mininet is connected to an external VM that runs ODL. A python module collects the topology information and the configured link capacities. The CPLEX solver runs on a separate dedicated VM in order not to interfere with ODL. Our software module is fed with the device power consumption model and randomized flows (depending on the scenario) so as to calculate the initial power consumption and link utilization according to current, non-optimized state where all links operate on the highest possible state. The same module includes the routing functionality and generates traffic flows between all devices. Based on the scenario, the appropriate input configuration file for CPLEX is generated and the MIP solver is invoked. According to the results, new optimal consumption and link utilizations are calculated, and the new topology is stored in an external file for review. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Size** & **Access (Number of Switches)** & **Aggregation (Number of Switches)** & **Core (Number of Switches)** \\ \hline \multicolumn{4}{c}{**Classical 3-Layer**} \\ \hline Size 1 (XS) & 8 & 2 & 2 \\ Size 2 (S) & 16 & 4 & 2 \\ Size 3 (M) & 32 & 4 & 2 \\ Size 4 (L) & 64 & 4 & 2 \\ Size 5 (XL) & 64 & 8 & 2 \\ \hline \multicolumn{4}{c}{**Fat Tree**} \\ \hline Size 1 (XS) & 8 & 2 & 2 \\ Size 2 (S) & 16 & 4 & 2 \\ Size 3 (M) & 32 & 4 & 2 \\ Size 4 (L) & 64 & 4 & 2 \\ Size 5 (XL) & 64 & 8 & 2 \\ \hline \multicolumn{4}{c}{**Leaf and Spine**} \\ \hline Size 1 (XS) & 8 & 2 & n/a \\ Size 2 (S) & 16 & 4 & n/a \\ Size 3 (M) & 32 & 4 & n/a \\ Size 4 (L) & 32 & 8 & n/a \\ Size 5 (XL) & 64 & 8 & n/a \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Number of devices on evaluated scenarios. The power consumption model of an interface consists of a baseline value due to the transceiver and a traffic proportional part due to the electronics parts on the Linecards. Typical values for transceiver consumption start from 1.5 W per transceiver for a 10 Gbps multimode optical module till 3.5 W/module for 100 Gbps single mode fiber [33]. On the other hand, the nominal value on the switch side is 3.5 W for a 10 Gbps interface and 14 W for a 40 Gbps one. In our emulation, a non-linear power model is assumed where the initial states tend to consume higher volumes of power. We considered four different states regarding the link power consumption, where the last state corresponds to the maximum power consumption of a link. The link is considered to consume no power when in sleep state. The first step includes the power requirements to maintain the link state (thus consumes relatively more than intermediate steps).