IIW_final(made with IDLE Clion into a Linux environment-the testing process is made by Ubuntu bash): The main goal of this project is to build an application client-server with program language C. It is able to interchange secure information,more precisely a file, through a reliable transfer which used the transfer protocol UDP. The communication of clients with the server takes place through API Berkeley's socket. Furthermore, as required, the implementation of the reliable service,that is not guaranteed by UDP, is managed by the selective-repeat protocol with the condition probability that we could lose some packet The software should be managed to: -connect the two process client-server without login in, but with request and response messages, -through command: message sends by client to the server-The goal of it is to return the list of all available files from server's directory. -through command: message sends by client to the server-It lets to the client to download the file according condition it is present into server's diretory. -through command: message sends by client to server- It's able to l’upload a file present into the client's directiory to server's directory.