A wireless sensor network (WSN) consists of spatially distributed autonomous sensors to monitor physical or environmental conditions, such as temperature, sound, pressure, etc. and to cooperatively pass their data through the network to a main location. In WSN the delivery of all sending packet is important so we called it as reliability. In transport layer reliable message delivery and congestion control are important factors. The aptitude of the network to ensure reliable data transmission in a state of continuous change of network structure. This paper focuses on the existing transport protocols provide the entire requirement of transport protocol, the factors affecting the reliability and different ways to achieve reliability. {\textcopyright} 2012 IEEE.