Resumen de Sdn-based solutions for carrier-grade transport networks
According to recent predictions, mobile data traffic will increase 7-fold between 2017 and 2022, generating around 77 exabites of data per month. This data traffic increase will be pushed by innovative services including 8K video, virtual reality, augmented reality or cloud gaming, in addition to the numerous devices that will enable smart cities, smart homes, and Internet of Things (IoT). For that reason, future mobile transport networks (5G and beyond) are expected to offer unprecedented bandwidth with extreme low latency to support all this kind of innovative services, but future networks are also expected to reduce the Operational Expenditure (OPEX) and Capital Expenditure (CAPEX) to a reasonable Return on Investment (ROI) range. These stringent requirements will not only be driven by the introduction of the mmWave radio spectrum, it will require a thorough redesign of the current network architecture, moving towards a new service-based mobile system.
A key architectural component to support 5G’s increased user data rates is the transport network, which is responsible for interconnecting and feeding data to/from the Radio Access Network (RAN). The traditional approach employs fully-fledged radio access points, where all the radio protocol stack, including the baseband processing functionality, is distributed and co-located with the radio frontend, and connected to the Core Network (CN) through a Backhaul (BH) transport network. Early backhaul segment technologies consisted in circuit switching nodes and evolved into a packet-based network, typically consisting in packet-switching nodes interconnected by wires, i.e., optical fiber and/or copper, and in whole or in part, wireless links using high capacity microwave bands. Motivated by the success of mobile technologies, RAN designers have been seeking innovative ways of pushing the limits of RAN performance and reducing costs. As a result, C-RAN has emerged as the technology of choice to realize both performance and cost effectiveness. C-RAN decomposes radio access point functionality into a small footprint, basic radio part called Remote Radio Head (RRH), which includes the lower levels of the protocol stack (amplification, digital-to-analog converters, etc.) and are located at distributed radio sites called Distributed Unit (DU), and a pool-able base band processing part named Base Band Unit (BBU) that centralizes the upper levels of the stack at the Central/Cloud Unit (CU). The bandwidth increase in the upcoming generation of mobile networks poses several issues to the current 4G C-RAN, dramatically increasing the transport capacity and latency requirements, forcing a re-design of the fronthaul segment based on a packet-switched transport network. For that reason, new functional splits are being investigated, using Ethernet as the common factor, blurring the traditional separation between the fronthaul and backhaul network segments, driving into a converged packet-based network, commonly denoted as Crosshaul. The movement towards new packet-based functional splits relax the requirements in the transport network, enabling new opportunities for softwarized solutions, focusing on SDN and NFV technologies for the future network deployments.
The Software Defined Networking (SDN) paradigm is based on decoupling control and data planes while centralizing the network control logic in a node known as controller, leaving behind a static architecture and looking forward to more dynamic and flexible one. In the recent years, SDN has focused the attention of industry and academy. This fact triggered the creation of the Open Networking Foundation (ONF) by Deutsche Telekom, Facebook, Google, Microsoft, Verizon and Yahoo with the objective of promoting SDN and standardize the OpenFlow protocol. Since then, SDN has emerged as a basic toolset for operators to manage their infrastructure, as it opens up the possibility of running a multitude of intelligent and advanced applications for network optimization purposes in a centralized network controller. However, the nature that makes possible this efficient management and operation in a flexible way, the logical centralization, also poses important challenges due to the lack of proper monitoring tools suited for SDN-based architectures. In order to take timely and right decisions while operating a network, centralized intelligence applications need to be fed with a continuous stream of up-to-date network statistics. However, this is not feasible with current SDN solutions due to scalability and accuracy issues derived from the centralized intrinsic nature of the SDN paradigm. Furthermore, the control centralization also poses resiliency, reliability and security problems since a centralized controller is a potential single point of failure and a potential bottleneck, needing special attention in order to support the demanded carrier grade requirements.
The softwarization trend does not stop with the incorporation of SDN solutions, it goes hand-in-hand with the virtualization of network functions. Actual networks are composed of several network functions, chained and connected in order to conform a network service functionality. Actual network services are rigid and static and performed by specific network hardware, usually provided by closed vendors that do not allow the interoperability with others vendors’ hardware. The Network Function Virtualization (NFV) paradigm is based on decoupling these network functions from hardware to software, allowing the execution on Commercial Off-the-Shelf (COTS) hardware, reducing the CAPEX and OPEX and increasing the flexibility and programmability of the network. This network functions run in virtualized environments being agnostic from the underlying hardware layers and are commonly known as Virtual Network Functions (VNFs). For that reason, future mobile networks will make extensive use of Data Center Networks (DCNs), running most virtualized network services and data processing. Not only big cloud data centers will store, compute and analyze all the data, also smaller edge and fog data centers will be used, aiming to reduce the overall network latency required in the new generation of mobile networks. This increasing demand is pushing the limits of current DCNs, requiring, like mobile transport networks, higher scalability, resiliency and performance while reducing the network deployment cost. Traditional approaches for layer-2 switching in DCNs rely on table lookups, where a flat 48-bit MAC address space is searched in order to find the next hop to forward the packet to. Very recently, the IEEE 802 has defined new mechanisms for the structured use of the local MAC address space. This new flexibility enables the embedding of semantics in the MAC addresses that can be used for new applications, such as improving the standard layer-2 forwarding process.
From this perspective, this thesis proposes a set of SDN-based solutions and architectures for next generation networks, including both mobile transport and DCN. Departing from the mobile transport, this thesis theoretically analyzes the centralization degree of eNB functions needed in a crosshaul configuration, motivating the need of a flexible design to accommodate the different functional splits that will coexist in future mobile networks. This thesis presents a novel design of a fully integrated Crosshaul solution, implemented and deployed using cost-efficient SDN-based forwarding elements. The pipeline design of the Crosshaul forwarding elements presented in this thesis has been carefully optimized, eliminating the latency-impacting operations, making it suitable for lower layer functional splits. The Crosshaul design introduced in this thesis has been extensively evaluated, being the first empirical validation of a fully integrated Crosshaul network, transporting flows from various functional split options over the same switching infrastructure.
The presented SDN-based Crosshaul solution is accompanied by a set of tools devoted to enrich the control plane functions. This thesis firstly analyzes the challenges and limitations of SDN transport networks in terms of network operation, telemetry and monitoring. The stateless nature of OpenFlow collides with the statefulness managing and monitoring necessities in carrier grade networks, precluding OpenFlow to keep information on the forwarded traffic, and making impossible the generation and injection of packets. This thesis proposes an Adaptative Telemetry System (ATS), formed by an ATS application, ATS plugin and ATS agent that enables the local execution of OAM procedures directly on the switches, enabling active measurements (e.g., delay, bandwidth, etc.), their reporting (e.g., alarms) to the network controller and the remote configuration using Finite State Machine (FSM) modelling, being fully compatible with the OpenFlow standard. This monitoring solution was experimentally evaluated in a virtualized tree topology formed by 43 nodes and 84 ports, each of these nodes was implemented as an LXD container running the ATS agent and a virtual switch. The obtained results demonstrated that ATS brings significant benefits in terms of control plane offloading and higher accuracy in the telemetry measurements, complying with the performance requirements defined by 3GPP for 5G networks. This thesis also makes an extensive analysis of the reliability and scalability reliability issues present in the SDN centralized nature, focusing on the connection between the controller and the data plane elements, the so called control channel. Future mobile networks will offer unprecedented amount of bandwidth and extremely low latency, but will also face stringent availability requirements for lot of innovative services that will take place in the next generation of networks. This thesis presents the design of a multipath-based solution that aims to secure and robustize the control channel connection, creating multiple disjoint redundant paths through the out-of-band and in-band management networks. The proposed solution is able to seamlessly migrate the OpenFlow connection from the failing path to an available one without traffic interruption. This solution was evaluated in a real topology formed by Alix devices, showing an increased resiliency against path failure, boosting both the robustness and the scalability of the OpenFlow channel, aiming to achieve the required 99.99999% network availability required in the next generation of mobile networks.
Finally, this thesis also investigates the opportunities present in the aforementioned IEEE 802 mechanisms for the structured use of the local MAC address space, and its applicability to current DCNs L2 switching schemas. This thesis includes a novel SDA solution that allows to embed semantics into the MAC addresses, giving meaning to the address structure, eliminating the need of look-up tables. This thesis incorporates a unit testing validation of the proposed SDA solution, comparing it with a baseline switching method. Due to the positive results obtained in the unit testing validation, the presented contribution also evaluates the flexibility of SDA illustrating a potential application for segregating the latency-sensitive small-frame flows from the large data-intensive large flows, evaluating it in an emulated DCN, demonstrating the potential benefits of this traffic segregation in terms of network congestion.
As a conclusion, it can be said that this thesis analyzes some technical gaps of the future generation of mobile networks, ranging from mobile transport to data center networks, proposing novel SDN-based solutions aiming to increase the overall flexibility, performance and robustness while reducing the cost in order to match the stringent requirements of future network services.
