1. Introduction
The concepts of network security and users privacy expanded to denote issues pertaining to when and how network resources are used. Network security attacks are classified into two classes: active and passive. In active attacks, an attacker modifies traffic, and hence causing direct damage. In passive attacks, an attacker simply listens or eavesdrop packets without actively changing anything, so it is more difficult to detect than active ones. To deal with these attacks, a variety of countermeasures have been developed, and they can be classified as either offensive or defensive.