Starting from what is SDN, the image vividly illustrates the development of SDN. Then introduce the status quo of SDN, presented in the form of a table, the idea is clear and clear. Then introduce the characteristics of SDN and the SDN technology based on OpenFlow. Finally, the application scenario analysis of SDN and security issues are closed. The whole article is from the shallower to the deeper, and it is worthy of recollection. SDN (Software Defined Networking) is a software-defined network that has revolutionized the network technology and will have a profound impact on the future of the network world. So what exactly is SDN, why is this technology present, and what are the problems with the scalability and security of the SDN network architecture, which is worthy of in-depth analysis and thinking. 1.1 What is SDN? If all the network devices in the network are regarded as managed resources, then referring to the principle of the operating system, the concept of a network operating system can be abstracted. The network operating system (ie, the control layer) abstracts the underlying network devices on the one hand (ie, The specific details of the data layer, while also providing a unified management view and programming interface for the upper application (application layer), as shown in Figure 1. In this way, based on the network operating system platform, users can develop various applications to define logical network topologies through software to meet the different needs of network resources without concern for the physical topology of the underlying network. Figure 1 SDN three-tier architecture 1.2 The origin of SDN In 2006, Stanford's MarTIn Casado led a project on network security and management, Ethane, which attempted to enable network administrators to easily define network flow-based security control policies through a centralized controller and to enforce these security policies. It is applied to various network devices to achieve security control of the entire network communication. Inspired by this project, MarTIn and Nick McKeown proposed to separate the data module (Data Plane) and the route control (Control Plane) function modules of the traditional network device, and use a centralized controller (Controller) to standardize the interfaces. The management and configuration of network devices will provide more possibilities for the design, management and use of network resources, which will make it easier to promote network innovation and development, so the concept of OpenFlow came into being. The first paper in 2008, "OpenFlow: Enabling InnovaTIon in Campus Networks", which was introduced in detail in the OpenFlow concept, was published in ACM SIGCOMM, marking the opening of the SDN era. Figure 2 shows the development of SDN/OpenFlow. Figure 2 Development of SDN/OpenFlow Regarding the concept and implementation of SDN, there is a big controversy in the industry. The mainstream is the OpenFlow-based SDN implementation. The SDN that we usually say refers to SDN based on OpenFlow. 1.3 Status of SDN In academia, GENI, Internet2, Europe's OFELIA, and Japan's JGN2plus have launched research and deployment of SDN. Domestically led by Tsinghua University, the Chinese Academy of Sciences Institute of Computing, Beiyou, Southeast University, Peking University and other units participated in the 863 project "Future Network Architecture and Innovation Environment" 863 project research. In the industry, SDN startups represented by Nicira (which was acquired by VMware in July 2012) and Big Switch continue to emerge. In order to seize the opportunities in the new technology and its own marketing considerations, Juniper, NEC, HP, IBM, Cisco and other vendors have released SDN hardware products supporting OpenFlow; NEC, BigSwitch, IBM, Cisco or autonomous or joint development SDN controller VMware, Juniper, Cisco and other vendors have released SDN-based virtual network solutions; some of these products have been deployed in the SDN research field. Domestic equipment manufacturers such as Huawei, ZTE, and Shengke, mobile, China Unicom, and telecommunications operators are also making rapid progress in the development of SDN products and solutions. In terms of standardization, in addition to the standardization work of ONF, IETF (Internet Engineering Task Force), IRTF (Internet Research Task Force), ITU-T (International Telecommunication Union Telecommunication Standardization Sector), ETSI (European Telecommunications Standardization Association), etc. have also been established. The working group, which conducts standard research and development work around SDN applications and architectures in their respective fields. Since 2011, SDN has gradually become one of the hottest focuses in the emerging technology market. Among the 10 new technologies that InfoWorld announced in November 2011 that will affect the next 10 years, SDN ranked second. After 4 years of accumulation and precipitation, it can be seen that SDN has stepped out of the initial concept hype stage. With continuous improvement and development, the trend of commercialization of SDN in carrier networks and data centers is becoming more and more obvious. According to IDC, in 2018, the market size of SDN in enterprises and cloud service providers will increase from 960 million US dollars in 2014 to 8 billion US dollars, with a compound annual growth rate of 89.4%. The core idea of ​​SDN is mainly reflected in three aspects: 1) separation of control plane and forwarding plane; 2) concentration of control plane; 3) behavior management of data plane by programming in control plane. Therefore, SDN mainly presents the following characteristics: Centralized fine control Compared with traditional networks, SDN controllers can achieve more centralized network management, and the defined management policies and behaviors are more granular. Open unified interface SDN breaks the closedness of traditional network equipment, enabling devices that support unified standards to complete network forwarding functions, and upper-layer applications can be more open and diverse. Flexible dynamic expansion Because behaviors and policies can be customized at the application layer, users can dynamically adjust and flexibly expand based on usage requirements. The hydrogel Screen Protector has super ductility and shrinkage, has a strong and effective self-repair function, impact resistance, durability, better toughness, and has a certain buffering effect on sharp objects. The use of hydrogel film can adapt to the contours of any device, so it can be attached to curved screens and rounded edges. The full-coverage screen protector can perfectly fit your screen and provide maximum protection. Hd Clear Hydrogel Protector Sheet,Anti Blue-Ray Hydrogel Protector,Matte Hydrogel Film,Privacy Hydrogel Film Shenzhen TUOLI Electronic Technology Co., Ltd. , https://www.szhydrogelprotector.com
SDN is a new type of network architecture. Its core idea is to make the network more open, flexible and efficient through software control and centralization. The specific performance is to separate the control plane of the network from the forwarding plane (ie, the data plane): the control plane provides the user with a standard programming interface to facilitate centralized deployment of the network management application; the forwarding plane remains in the hardware through the standard protocol interface ( Such as OpenFlow) receives and executes the forwarding policy. 
The predecessor of SDN can trace the concept of "programmable network" at the earliest. Since then, some scholars have proposed corresponding programmable network ideas and methods. In 2004, the 4D Project proposed a Clean-Slate network design solution that highlighted the separation of interaction protocols and routing decision logic between network elements. 
In 2011, researchers such as McKeown formed the Open Networking FoundaTIon (ONF), which is responsible for the development and promotion of relevant standards, including OpenFlow standards, OpenFlow configuration protocols and SDN white papers, which greatly promoted OpenFlow and The standardization work of SDN has also made it a research hotspot in the field of open network architecture and network virtualization. 
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