Utilizing Optical Circuits in Hybrid Packet/Circuit Data-Center Networks

Existing Data Center Networks (DCNs) continue to evolve to keep up with application requirements in terms of bandwidth, latency, agility, etc. According to the updated release of the Cisco Global Cloud Index [1], by 2019, more than 86% of traffic workloads will be processed by cloud DCs. Traditional DCNs, which are based on electrical packet switching (EPS) with hierarchical, tree-like topologies can no longer support future cloud traffic requirements in terms of dynamicity, bandwidth and latency. Hence, existing DCNs can be enhanced with OCS (Optical Circuit Switching), which provides high bandwidth, low latency and low power consumption [2], giving rise to hybrid OCS-EPS topologies. In this research, we assess a virtualized, hybrid, flat DCN topology consisting of a single layer of high radix ToR (Top of Rack) switches, interconnected with each other and through an OCS plane. The benefit of such flat topology is twofold: 1) In terms of bandwidth, over-subscription is reduced, and bisection bandwidth is increased; and 2) In terms of latency, the diameter (longest path) of topology is reduced. Moreover, we present new algorithms and orchestration functionality to detect and offload suitable flows (e.g. elephant flows) from the EPS to the OCS plane. Our DC architecture consists of hybrid EPS-OCS DCN, an Openflow(OF) based control plane, and an orchestration layer. Our orchestration layer decouples the elephant flows detection from the rerouting decision logic in the DCN. Specifically, the elephant flows detection is done by flow tagging in the hypervisor, while the flow rerouting is executed at the EPSs, which are connected directly to the OCS. Hence, it provides a more efficient, scalable, and easy to configure architecture as compared to existing hybrid solutions. The orchestrator monitors the ToR switches by sFlow and detects high volume traffic between two ToRs, exceeding a given bandwidth threshold. Such traffic may consist of either few elephant flows or many mice flows. To further increase the optical circuit utilization, we introduce two types of optical circuits: 1) private circuit, presented in existing solutions, is utilized only by flows that originate and end at the ToR switches connected to the circuit endpoints. 2) shared circuit, is part of our novel approach. It can be used also by flows that are transmitted through ToR switches connected to the circuit endpoints, but originate and/or end at other ToRs. Moreover, the orchestrator may dynamically decide to configure private or shared optical circuits, according to various criteria including current network utilization, traffic flows nature, tenants SLAs, etc. Configuring or changing the optical circuit type requires installing a single OpenFlow rule for each ToR connected to the circuit endpoints; hence, enabling low overhead and fast network configuration. To assess the benefit of such optical circuit configurations, we implement the proposed algorithms and test them over an emulated data and control plane environment. We evaluate the network performance for various network traffic scenarios for both private and shared optical circuits, and compare them to an EPS-only baseline topology with the same total link bandwidth. Our preliminary results show that the shared optical circuits introduce an improvement of 5% to 10% as compared to the commonly used private circuits. The research is partially supported by the European Communitys Seventh Framework Programme (FP7/2001-2013) under grant agreement no. 619572 (COSIGN Project).