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 strongenough to make Google, Facebook, Yahoo, Microsoft,Verizon, and Deutsche Telekom fund Open NetworkingFoundation (ONF) [10] with the main goal of promotionand adoption of SDN through open standards development.As the initial concerns with SDN scalability wereaddressed [11]Vin particular the myth that logical centralizationimplied a physically centralized controller, anissue we will return to later onVSDN ideas have maturedand evolved from an academic exercise to a commercialsuccess. Google, for example, has deployed an SDN tointerconnect its data centers across the globe. This productionnetwork has been in deployment for three years,helping the company to improve operational efficiencyand significantly reduce costs [8]. VMware’s networkvirtualization platform, NSX [12], is another example.NSX is a commercial solution that delivers a fully functionalnetwork in software, provisioned independent of theunderlying networking devices, entirely based aroundSDN principles. As a final example, the world’s largest ITcompanies (from carriers and equipment manufacturers tocloud providers and financial services companies) haverecently joined SDN consortia such as the ONF and theOpenDaylight initiative [13], another indication of theimportance of SDN from an industrial perspective.A few recent papers have surveyed specific architecturalaspects of SDN [14]–[16]. An overview of OpenFlowand a short literature review can be found in [14] and [15].These OpenFlow-oriented surveys present a relativelysimplified three-layer stack composed of high-level networkservices, controllers, and the controller/switch interface.In [16], Jarraya et al. go a step further by proposing ataxonomy for SDN. However, similarly to the previousworks, the survey is limited in terms of scope, and it doesnot provide an in-depth treatment of fundamental aspectsof SDN. In essence, existing surveys lack a thorough discussionof the essential building blocks of an SDN such asthe network operating systems (NOSs), programming languages,and interfaces. They also fall short on the analysisof cross-layer issues such as scalability, security, and dependability.A more complete overview of ongoing researchefforts, challenges, and related standardizationactivities 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, inthe next two sections, by explaining the context, introducingthe motivation for SDN and explaining the mainconcepts of this new paradigm and how it differs fromtraditional networking. Our aim in the early part of thesurvey is also to explain that SDN is not as novel as atechnological advance. Indeed, its existence is rooted atthe intersection of a series of ‘‘old’’ ideas, technology drivers,and current and future needs. The concepts underlyingSDNVthe separation of the control and data planes,the flow abstraction upon which forwarding decisions aremade, the (logical) centralization of network control, andthe ability to program the networkVare not novel bythemselves [17]. However, the integration of already testedconcepts with recent trends in networkingVnamely theavailability of merchant switch silicon and the huge interest in feasible forms of network virtualizationVareleading to this paradigm shift in networking. As a result ofthe high industry interest and the potential to change thestatus quo of networking from multiple perspectives, anumber of standardization efforts around SDN are ongoing,as we also discuss in Section III.Section IV is the core of this survey, presenting anextensive and comprehensive analysis of the buildingblocks of an SDN infrastructure using a bottom-up, layeredapproach. The option for a layered approach is groundedon the fact that SDN allows thinking of networking alongtwo fundamental concepts, which are common in otherdisciplines of computer science: separation of concerns(leveraging the concept of abstraction) and recursion. Ourlayered, bottom-up approach divides the networking probleminto eight parts: 1) hardware infrastructure; 2) southboundinterfaces; 3) network virtualization (hypervisorlayer between the forwarding devices and the NOSs);4) NOSs (SDN controllers and control platforms);5) northbound interfaces (to offer a common programmingabstraction to the upper layers, mainly the network applications);6) virtualization using slicing techniques providedby special purpose libraries or programming languagesand compilers; 7) network programming languages; andfinally 8) network applications. In addition, we also look atcross-layer problems such as debugging and troubleshootingmechanisms. The discussion in Section V on ongoingresearch efforts, challenges, future work, and opportunitiesconcludes 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 (seeFig. 3). The data plane corresponds to the networking devices,which are responsible for (efficiently) forwardingdata. The control plane represents the protocols used topopulate 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 thecontrol functionality. Network policy is defined in the managementplane, the control plane enforces the policy, andthe data plane executes it by forwarding data accordingly.In traditional IP networks, the control and data planesare tightly coupled, embedded in the same networkingdevices, and the whole structure is highly decentralized.This was considered important for the design of the Internetin the early days: it seemed the best way to guaranteenetwork resilience, which was a crucial design goal. Infact, this approach has been quite effective in terms ofnetwork performance, with a rapid increase of line rateand port densities.However, the outcome is a very complex and relativelystatic architecture, as has been often reported in the networkingliterature (e.g., [1]–[3], [6], and [19]). It is alsothe fundamental reason why traditional networks are rigid,and complex to manage and control. These two characteristicsare largely responsible for a vertically integrated industrywhere innovation is difficult.Network misconfigurations and related errors are extremelycommon in today’s networks. For instance, morethan 1000 configuration errors have been observed inborder gateway protocol (BGP) routers [20]. From a singlemisconfigured device, very undesired network behaviormay result (including, among others, packet losses, forwardingloops, setting up of unintended paths, or servicecontract violations). Indeed, while rare, a single misconfiguredrouter is able to compromise the correct operationof the whole Internet for hours [21], [22].To support network management, a small number ofvendors offer proprietary solutions of specialized hardware,operating systems, and control programs (networkapplications). Network operators have to acquire andmaintain different management solutions and the correspondingspecialized teams. The capital and operationalcost of building and maintaining a networking infrastructureis significant, with long return on investment cycles,which hamper innovation and addition of new features andservices (for instance, access control, load balancing,energy efficiency, traffic engineering). To alleviate the lackof in-path functionalities within the network, a myriad ofspecialized components and middleboxes, such as firewalls,intrusion detection systems, and deep packet inspectionengines, proliferate in current networks. A recentsurvey of 57 enterprise networks shows that the number ofmiddleboxes is already on par with the number of routersin current networks [23]. Despite helping in-path functionalities,the net effect of middleboxes has increasedcomplexity 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。