Wireless Sensor Networks consists of sensor nodes which are placed in harsh and hostile surrounding where an adversary may capture the nodes, replicate them and use it for its own purpose. If the clone nodes remain undetected, it can disrupt the network functions making it vulnerable to attacks. Hence false data can be injected or the legitimate data can be taken out. Thus to detect the node clone attack to preserve security goals is the fundamental problem in WSN.
Centralized methods have been proposed to detect the clones which consist of single point of failure. Hence distributed protocols are more subtle to detect the clone nodes where no central authority is present and each node is responsible for detection. This paper presents various existing witness-based distributed techniques for detecting node clones present in the network.