As wired and wireless networks grow, underlying protocols need to be evaluated so that heterogeneous networks can be linked in an efficient manner. Among the transport layer protocols, user datagram protocol UDP is the only protocol that offers a fast and efficient mechanism to handle voice and video data traffic because it is a message based, connectionless service operating on the IP layer. In addition to the network conditions, it is observed that the performance also depends upon the implementation of the protocol. UNIX is one of the most widely used operating system in the networking world and different UNIX flavors have different UDP stack implementation. In this research work the performance of different UDP implementations were compared under different network conditions. It is observed that no UDP implementation dominates in all the tested network conditions. It is also observed that the selection of an appropriate datagram size had large impact on the overall performance.