180 Copyright © 2016, IGI Global. Copying or distributing in print or electronic forms without written permission of IGI Global is prohibited. Chapter 8 DOI: 10.4018/978-1-5225-0105-3.ch008 ABSTRACT Increasingly system software and user applications are becoming automated and thus many of inter machine communications are not user action driven. Some of these automated communications like OS updates, database synchronization will not pose security threats, while others can have malicious behavior. Automated communications pose a threat to the security of systems if initiated by unwanted programs like keyloggers and Botnets. As these applications are programmed to contact a peer host regularly, most of these communications are periodic in nature. In this chapter we describe a method for detecting periodic communications by analyzing network fows for security monitoring. In particular we use a clustering technique to identify periodic communications between hosts. We experiment with both simulated and real world data to evaluate the efcacy of method. 1. INTRODUCTION The global Internet landscape is changing due to growing number users, increasing network bandwidth, intelligent applications and increasing number of mobile user base. Each of these dimensions bring a kind of challenge to the networking community. Accommodating and serving diverse user base with varied demands is a challenge in itself. While the network bandwidth is increasing there are always bandwidth hungry applications which are ready to consume it. For example peer to peer applications may consume lot of bandwidth. Providing a fair access share to all the applications is a problem many ISPs are striving to address. To address and understand all these issues network traffic is monitored and modeled. In this chapter we study a problem which is an implication of intelligent applications in the network. Discovering Periodicity in Network Flows for Security Monitoring Neminath Hubballi Indian Institute of Technology Indore, India Deepanshu Goyal Indian Institute of Technology Guwahati, India