The Impact of Software-Defined Optical Networks on Data Centre Optimization (Invited) Siamak Azodolmolky, Philipp Wieder, Ramin Yahyapour Gesellschaft für Wissenschaftliche Datenverarbeitung mbH Göttingen (GWDG), Am Faßberg 11, 37077 Göttigen, Germany, e-mail: {siamak.azodolmolky, Philipp.Wieder, Ramin.Yahyapour}@gwdg.de. ABSTRACT Delivering scalable packet-switched interconnects that can support the bursty Ethernet traffic, which is common in many data centre applications is a challenging problem that is only getting harder. Implementing the control logic capable of deciding how to forward each packet individually in current packet-switched Ethernet interconnects is not optimized and researchers are proposing hybrid optical/electronic Top of Rack (ToR) architectures to improve the performance towards an optimized operation. Emerging applications (e.g., Big Data) and potential cross-layer performance improvement is another interesting topic that should be properly addressed in future data centres. In this paper a summary of recent developments of optical networking in data centres will be presented. Our software-defined optical network emulation platform is introduced and initial WDM link modelling as a tool for cross layer investigation and optimization are presented. A nonlinear optical signal propagation tool (as a network application) and related results are presented. Keywords: Software-defined optical network, network emulation, hybrid optical/packet switching, hybrid top of rack, cross-layer optimization, application driven networking 1. INTRODUCTION Ranging from small enterprises to large scale compute cloud providers, most of the existing IT systems and services are strongly depending on highly scalable and efficient data centres. These infrastructures still pose significant challenges on providers regarding computing, storage and networking [1]. Concerning the networking issues, data centres should be designed and deployed in such a way as to offer flexible and high cross-section bandwidth and low latency, QoS based on the application requirements, high levels of resilience, intelligent resource utilization to reduce energy consumption and improve overall efficiency, agility in provisioning network resources[2]. Existing data centres are typically designed with a fat-tree or oversubscribed fat-tree interconnection topology. The number of racks per physical point of delivery (POD) is typically between 40 and 60, but even denser values can be found. Each rack is typically equipped with a Top-of-Rack switch (ToR) that provides southbound interfaces to the end hosts and northbound interfaces to the higher levels of the tree. Two approaches are followed in fat-tree networks: 1) the traditional fat-tree approach using higher rate ports towards the root of the tree, or 2) utilising low-port-rate commodity switches in a folded Clos topology in order to provide low-rate multiple paths between the physical servers. Although the cost of Clos is lower than the traditional fat-tree approach, it still scales super-linearly. Furthermore, the cabling of the huge number of switches becomes quite challenging and is error prone during setup and maintenance. The emergence of software-defined networking (SDN) has been expected to change the current state of quo in networking. Early research efforts have indeed showed that data centre networking can significantly benefit from SDN in solving different problems such as live network migration, improved network management, eminent failure avoidance, rapid deployment from development to production networks, troubleshooting, and optimization of network utilization. SDN can also offer networking primitives for cloud applications, solutions to predict network transfers of applications, mechanisms for fast reaction to operation failures, network-aware VM placement, QoS support, real-time network monitoring and fault detection, security policy enforcement services and mechanisms, and enable programmatic adaptation of transport protocols. SDN enables providers to expose more networking primitives to their customers, by allowing virtual network isolation, custom addressing, and the placement of middle boxes and virtual desktop cloud applications [3]. In this work we provide a condense survey on hybrid data centre networking and then introduce our software- defined optical network emulation platform as a tool for cross-layer optimization and application driven networking. 2. HYBRID DATA CENTRE NETWORKING The main limitations of current data centre architectures can be summarized as: i) super linear scaling, ii) High load independent energy consumption, iii) Cumbersome and complex cabling, iv) High (on average) CAPEX and OPEX due to underutilized network (oversubscribed fat-tree can exhibit congestion problems while is still