An extensive study on application level gateways (ALGs)

Application level gateways (ALGs) are physical network entities that are used to provide application specific proxies and deep packet inspection. ALG typically evaluates and examines all the incoming traffic to its interface and after applying security policies it allows traffic to enter into the internal network. In this paper we explain ALG network architecture, ALG working principle and its impact on different applications. We also identify various applications that require ALG while operate behind NATs or firewalls. This paper also discusses why different application layer protocols require ALG functionality to successfully operate with other applications in different address realm.