In modern life the internet has become backbone of digital society which is the packet switched distributed network and connected with almost every digital devices, available everywhere in the world. The traditional network carries few challenges like, dependency on vendors, difficulty to manage large network, dynamically changing of forwarding policies and more. To overcome such challenges, today the idea of Software Defined Networking (SDN) came into existence. The basic idea behind SDN is to implement programmable network by separately managing the control plane and data plane to improve the efficiency of network performance. The main problem with SDN is the Controller Placement Problem (CPP), which gives the overview of whole network. Today the main focus of the researchers is to solve the CPP. CPP is a NP-hard problem because the network should consist of minimum controllers and controllers should be placed on appropriate locations. For the large size network, controller deployment is difficult to manage. But the challenge in this area is Quality of Services (QoS) in respect of controller management. This paper investigates the systematic review of QoS based on controller’s problems, analyzed the current research and summarized the findings of the different controller’s performance based on QoS parameters e.g. reliability, scalability, consistency and load balancing. Finally, this paper also highlights the research challenges to improve the QoS in SDN.

在现代生活中，互联网已成为数字社会的骨干，它是分组交换分布式网络，并与世界上几乎所有可用的几乎所有数字设备相连。传统网络几乎没有挑战，例如，对供应商的依赖，难以管理大型网络，动态更改转发策略等等。为了克服这些挑战，如今，软件定义网络（SDN）的思想应运而生。SDN的基本思想是通过分别管理控制平面和数据平面来实现可编程网络，以提高网络性能的效率。SDN的主要问题是控制器放置问题（CPP），它概述了整个网络。今天，研究人员的主要重点是解决CPP。CPP是一个NP难题，因为网络应由最少的控制器组成，并且控制器应放置在适当的位置。对于大型网络，很难部署控制器。但是在该领域的挑战是控制器管理方面的服务质量（QoS）。本文研究了基于控制器问题的QoS的系统综述，分析了当前的研究并总结了基于QoS参数（如可靠性，可伸缩性，一致性和负载平衡）的不同控制器性能的发现。最后，本文还重点介绍了提高SDN QoS的研究挑战。但是在该领域的挑战是控制器管理方面的服务质量（QoS）。本文研究了基于控制器问题的QoS的系统综述，分析了当前的研究并总结了基于QoS参数（如可靠性，可伸缩性，一致性和负载平衡）的不同控制器性能的发现。最后，本文还重点介绍了提高SDN QoS的研究挑战。但是在该领域的挑战是控制器管理方面的服务质量（QoS）。本文研究了基于控制器问题的QoS的系统综述，分析了当前的研究并总结了基于QoS参数（如可靠性，可伸缩性，一致性和负载平衡）的不同控制器性能的发现。最后，本文还重点介绍了提高SDN QoS的研究挑战。