A distinctive feature of software-defined networking (SDN) is a logically centralized control plane realized using multiple physical controllers. The placement of the controllers, the so-called controller placement problem (CPP), is a crucial design issue. It influences network performance parameters such as latency, flow setup time, network availability, load balance of the controllers, and energy consumption. In this article, we illustrate the formulation of these CPP objectives. We categorize the CPP design solutions as either static or adaptive. In adaptive CPP, the solutions proposed dynamically adapt to the number of controllers required and the switch to controller mapping to varying network traffic. We further differentiate adaptive CPP as wired or wireless. The optimization strategies adopted by the papers are analyzed and grouped into five categories: exact, heuristic, meta-heuristic, clustering, and game theory. The merits and demerits of each approach are discussed. In conclusion, we outline the research challenges worth investigating.

中文翻译：

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软件定义网络中的控制器放置问题：一项调查

软件定义网络 (SDN) 的一个显着特征是使用多个物理控制器实现的逻辑集中控制平面。控制器的放置，即所谓的控制器放置问题 (CPP)，是一个至关重要的设计问题。它会影响网络性能参数，例如延迟、流设置时间、网络可用性、控制器的负载平衡和能耗。在本文中，我们说明了这些 CPP 目标的制定。我们将 CPP 设计解决方案分为静态或自适应。在自适应 CPP 中，所提出的解决方案动态适应所需控制器的数量以及映射到不同网络流量的控制器切换。我们进一步将自适应 CPP 区分为有线或无线。论文采用的优化策略被分析并分为五类：精确、启发式、元启发式、聚类和博弈论。讨论了每种方法的优点和缺点。总之，我们概述了值得研究的研究挑战。