File transfer over UDP

Client/server file transfer application using a connectionless network service (SOCK_DGRAM type of the Berkeley sockets API, i.e. UDP as transport layer protocol).

Term paper of Internet and Web Engineering course.

Overview

The application provides the following services:

• Unauthenticated client-server connection
• Reliable file transfer
• A communication protocol between the client and the server providing the exchange of two types of messages:
  • command messages: sent from the client to the server to request the execution of the various operations
  • response messages: sent from the server to the client in response to a command with the outcome of the operation
• the execution in the user space without requiring root privileges

Server

The server (multithreaded) listening on a default port, provide the following services:

• Send the response message to the list command with the filelist (the list of file names available for sharing)
• Send the response message to the get command with the required file, if any, or an error message
• Sending the response message to the put command with the outcome of the operation
• Configure the listening port

Client

The client provides the user with the following functions:

• Show the files available on the server (via the list command)
• Download a file from the server (via the get command)
• Upload a file on the server (via the put command)

Reliable transmission

The exchange of messages takes place using an unreliable communication service. In order to ensure the correct sending/receiving of messages and files both clients and servers implement at the application level the Go-Back N protocol.

During the tests to simulate the loss of messages over the network (a very unlikely event in a local network let alone when client and server are running on the same host), it is assumed that each message is discarded by the sender with probability $p$.

The size $N$ of the dispatch window, the probability $p$ of loss of messages, and the duration $T$ of the timeout, are three configurable constants and the same for all processes. In addition to the use of a fixed timeout, it is possible to choose the use of a value for the adaptive timeout calculated dynamically based on the evolution of the observed network delays.