The Internet has led to the creation of a digital society, where (almost) everything is connected and is accessible from anywhere. However, despite their widespread adoption, traditional IP networks are complex and very hard to manage. It is both difficult to configure the network according to predefined policies, and to reconfigure it to respond to faults, load, and changes. To make matters even more difficult, current networks are also vertically integrated: the control and data planes are bundled together. Software-defined networking (SDN) is an emerging paradigm that promises to change this state of affairs, by breaking vertical integration, separating the network’s control logic from the underlying routers and switches, promoting (logical) centralization of network control, and introducing the ability to program the network. The separation of concerns, introduced between the definition of network policies, their implementation in switching hardware, and the forwarding of traffic, is key to the desired flexibility: by breaking the network control problem into tractable pieces, SDNmakes it easier to create and introduce new abstractions in networking, simplifying network management and facilitating network evolution. In this paper, we present a comprehensive survey on SDN. We start by introducing the motivation for SDN, explain its main concepts and how it differs from traditional networking, its roots, and the standardization activities regarding this novel paradigm. Next, we present the key building blocks of an SDN infrastructure using a bottom-up, layered approach. We provide an in-depth analysis of the hardware infrastructure, southbound and northbound application programming interfaces (APIs), network virtualization layers, network operating systems (SDN controllers), network programming languages, and network applications. We also look at cross-layer problems such as debugging and troubleshooting. In an effort to anticipate the future evolution of this new paradigm, we discuss the main ongoing research efforts and challenges of SDN. In particular, we address the design of switches and control platformsVwith a focus on aspects such as resiliency, scalability, performance, security, and dependabilityVas well as new opportunities for carrier transport networks and cloud providers. Last but not least, we analyze the position of SDN as a key enabler of a software-defined environment.
摘要(abstract)
本文作者在摘要中主要列举了一些传统IP网络的缺点：复杂而且难以管理，最重要的一点就是它的控制面和数据面在同一台设备中，紧密耦合。而SDN就是要改变这种状况，它要求转发面与控制面分离。其次介绍了本文的大体内容：1.什么是SDN？它与传统网络有什么不同. 2. SDN：自下而上. 3. 正在进行的研究工作和挑战.
The distributed control and transport network protocols running inside the routers and switches are the key technologies that allow information, in the form of digital packets, to travel around the world. Despite their widespread adoption, traditional IP networks are complex and hard to manage [1]. To express the desired high-level network policies, network operators need to configure each individual network device separately using low-level and often vendor-specific commands. In addition to the configuration complexity, network environments have to endure the dynamics of faults and adapt to load changes. Automatic reconfiguration and response mechanisms are virtually nonexistent in current IP networks. Enforcing the required policies in such a dynamic environment is therefore highly challenging. To make it even more complicated, current networks are also vertically integrated. The control plane (that decides how to handle network traffic) and the data plane (that forwards traffic according to the decisions made by the control plane) are bundled inside the networking devices, reducing flexibility and hindering innovation and evolution of the networking infrastructure. The transition from IPv4 to IPv6, started more than a decade ago and still largely incomplete, bears witness to this challenge, while in fact IPv6 represented merely a protocol update. Due to the inertia of current IP networks, a new routing protocol can take five to ten years to be fully designed, evaluated, and deployed. Likewise, a clean-slate approach to change the Internet architecture (e.g., replacing IP) is regarded as a daunting taskVsimply not feasible in practice [2], [3]. Ultimately, this situation has inflated the capital and operational expenses of running an IP network. Software-defined networking (SDN) is an emerging networking paradigm that gives hope to change the limitations of current network infrastructures. First, it breaks the vertical integration by separating the network’s control logic (the control plane) from the underlying routers and switches that forward the traffic (the data plane). Second, with the separation of the control and data planes, network switches become simple forwarding devices and the control logic is implemented in a logically centralized controller (or network operating system1), simplifying policy enforcement and network (re)configuration and evolution. A simplified view of this architecture is shown in Fig. 1. It is important to emphasize that a logically centralized programmatic model does not postulate a physically centralized system [7]. In fact, the need to guarantee adequate levels of performance, scalability, and reliability would preclude such a solution. Instead, production-level SDN network designs resort to physically distributed control planes. The separation of the control plane and the data plane can be realized by means of a well-defined programming interface between the switches and the SDN controller. The controller exercises direct control over the state in the data plane elements via this well-defined application programming interface (API), as depicted in Fig. 1. The most notable example of such an API is OpenFlow. An OpenFlow switch has one or more tables of packethandling rules (flow table). Each rule matches a subset of the traffic and performs certain actions (dropping, forwarding, modifying, etc.) on the traffic. Depending on the rules installed by a controller application, an OpenFlow switch canVinstructed by the controllerVbehave like a router, switch, firewall, or perform other roles (e.g., load balancer, traffic shaper, and in general those of a middlebox). An important consequence of the SDN principles is the separation of concerns introduced between the definition of network policies, their implementation in switching hardware, and the forwarding of traffic. This separation is key to the desired flexibility, breaking the network control problem into tractable pieces, and making it easier to create and introduce new abstractions in networking, simplifying network management and facilitating network evolution and innovation. Although SDN and OpenFlow started as academic experiments [9], they gained significant traction in the industry over the past few years. Most vendors of commercial switches now include support of the OpenFlow API in their equipment. The SDN momentum was strong enough to make Google, Facebook, Yahoo, Microsoft, Verizon, and Deutsche Telekom fund Open Networking Foundation (ONF) [10] with the main goal of promotion and adoption of SDN through open standards development. As the initial concerns with SDN scalability were addressed [11]Vin particular the myth that logical centralization implied a physically centralized controller, an issue we will return to later onVSDN ideas have matured and evolved from an academic exercise to a commercial success. Google, for example, has deployed an SDN to interconnect its data centers across the globe. This production network has been in deployment for three years, helping the company to improve operational efficiency and significantly reduce costs [8]. VMware’s network virtualization platform, NSX [12], is another example. NSX is a commercial solution that delivers a fully functional network in software, provisioned independent of the underlying networking devices, entirely based around SDN principles. As a final example, the world’s largest IT companies (from carriers and equipment manufacturers to cloud providers and financial services companies) have recently joined SDN consortia such as the ONF and the OpenDaylight initiative [13], another indication of the importance of SDN from an industrial perspective. A few recent papers have surveyed specific architectural aspects of SDN [14]–[16]. An overview of OpenFlow and a short literature review can be found in [14] and [15]. These OpenFlow-oriented surveys present a relatively simplified three-layer stack composed of high-level network services, controllers, and the controller/switch interface. In [16], Jarraya et al. go a step further by proposing a taxonomy for SDN. However, similarly to the previous works, the survey is limited in terms of scope, and it does not provide an in-depth treatment of fundamental aspects of SDN. In essence, existing surveys lack a thorough discussion of the essential building blocks of an SDN such as the network operating systems (NOSs), programming languages, and interfaces. They also fall short on the analysis of cross-layer issues such as scalability, security, and dependability. A more complete overview of ongoing research efforts, challenges, and related standardization activities is also missing. In this paper, we present, to the best of our knowledge, the most comprehensive literature survey on SDN to date. We organize this survey as depicted in Fig. 2. We start, in the next two sections, by explaining the context, introducing the motivation for SDN and explaining the main concepts of this new paradigm and how it differs from traditional networking. Our aim in the early part of the survey is also to explain that SDN is not as novel as a technological advance. Indeed, its existence is rooted at the intersection of a series of ‘‘old’’ ideas, technology drivers, and current and future needs. The concepts underlying SDNVthe separation of the control and data planes, the flow abstraction upon which forwarding decisions are made, the (logical) centralization of network control, and the ability to program the networkVare not novel by themselves [17]. However, the integration of already tested concepts with recent trends in networkingVnamely the availability of merchant switch silicon and the huge interest in feasible forms of network virtualizationVare leading to this paradigm shift in networking. As a result of the high industry interest and the potential to change the status quo of networking from multiple perspectives, a number of standardization efforts around SDN are ongoing, as we also discuss in Section III. Section IV is the core of this survey, presenting an extensive and comprehensive analysis of the building blocks of an SDN infrastructure using a bottom-up, layered approach. The option for a layered approach is grounded on the fact that SDN allows thinking of networking along two fundamental concepts, which are common in other disciplines of computer science: separation of concerns (leveraging the concept of abstraction) and recursion. Our layered, bottom-up approach divides the networking problem into eight parts: 1) hardware infrastructure; 2) southbound interfaces; 3) network virtualization (hypervisor layer between the forwarding devices and the NOSs); 4) NOSs (SDN controllers and control platforms); 5) northbound interfaces (to offer a common programming abstraction to the upper layers, mainly the network applications); 6) virtualization using slicing techniques provided by special purpose libraries or programming languages and compilers; 7) network programming languages; and finally 8) network applications. In addition, we also look at cross-layer problems such as debugging and troubleshooting mechanisms. The discussion in Section V on ongoing research efforts, challenges, future work, and opportunities concludes this paper.
引言(introduction)
传统网络的缺点：(1) 复杂且难以管理；
(2) 控制面和数据面在同一台设备中，紧密耦合
SDN优点：(1) 控制面与转发面分离
(2) 简化了策略的执行和网络(Re)的配置和演化。
最著名的南向接口标准：openflow
介绍接下来的内容.
Computer networks can be divided in three planes of functionality: the data, control, and management planes (see Fig. 3). The data plane corresponds to the networking devices, which are responsible for (efficiently) forwarding data. The control plane represents the protocols used to populate the forwarding tables of the data plane elements. The management plane includes the software services, such as simple network management protocol (SNMP)- based tools [18], used to remotely monitor and configure the control functionality. Network policy is defined in the management plane, the control plane enforces the policy, and the data plane executes it by forwarding data accordingly. In traditional IP networks, the control and data planes are tightly coupled, embedded in the same networking devices, and the whole structure is highly decentralized. This was considered important for the design of the Internet in the early days: it seemed the best way to guarantee network resilience, which was a crucial design goal. In fact, this approach has been quite effective in terms of network performance, with a rapid increase of line rate and port densities. However, the outcome is a very complex and relatively static architecture, as has been often reported in the networking literature (e.g., [1]–[3], [6], and [19]). It is also the fundamental reason why traditional networks are rigid, and complex to manage and control. These two characteristics are largely responsible for a vertically integrated industry where innovation is difficult. Network misconfigurations and related errors are extremely common in today’s networks. For instance, more than 1000 configuration errors have been observed in border gateway protocol (BGP) routers [20]. From a single misconfigured device, very undesired network behavior may result (including, among others, packet losses, forwarding loops, setting up of unintended paths, or service contract violations). Indeed, while rare, a single misconfigured router is able to compromise the correct operation of the whole Internet for hours [21], [22]. To support network management, a small number of vendors offer proprietary solutions of specialized hardware, operating systems, and control programs (network applications). Network operators have to acquire and maintain different management solutions and the corresponding specialized teams. The capital and operational cost of building and maintaining a networking infrastructure is significant, with long return on investment cycles, which hamper innovation and addition of new features and services (for instance, access control, load balancing, energy efficiency, traffic engineering). To alleviate the lack of in-path functionalities within the network, a myriad of specialized components and middleboxes, such as firewalls, intrusion detection systems, and deep packet inspection engines, proliferate in current networks. A recent survey of 57 enterprise networks shows that the number of middleboxes is already on par with the number of routers in current networks [23]. Despite helping in-path functionalities, the net effect of middleboxes has increased complexity of network design and its operation.
网络化现状(STATUS QUO in networking)
计算机网络可以分为三个功能层面：数据面、控制面和管理面。
在传统的IP网络中，控制面和数据面是紧密耦合的，嵌入在同一个网络设备中，这在早期的互联网设计中被认为是重要的。然而，其结果是一个非常复杂和相对静态的体系结构，这也是传统网络僵化、管理和控制复杂的根本原因。