Abstract: Covert channels leaks information where information travels overlooked. Encryption used to protect the communication from being deciphered by unlawful users. But covert channels hide the presence of communication. Covert channels are serious security intimidation. There are many existing techniques available for development of covert channels by influencing certain fields in the network protocols such as HTTP, IP, TCP, etc. The available packet length based covert channels are having tamper resistance capability but due to abnormal traffic distribution results in recognition possibility. In this paper we present overview of different protocol as well as some packet length based covert channels.