Software Defined Network (SDN), a very new idea in networking domain, promises to dramatically simplify network management and enable innovation through network programmability. In spite of significant benefits, this emerging architecture suffers from several security challenges which make it vulnerable to different attacks like denial of service attack, man-in-middle attack etc. In this paper, we have discussed about the impact of denial of service attack on SDN environment and several existing solutions to defend it. Finally we have given a new proposal for detecting denial of service attack in SDN environment. © 2016 IEEE.