Network Programming Projects in C

This repository contains six assignments developed for the NES 416 – Network Programming course at JUST (Jordan University of Science and Technology).
The assignments cover TCP/UDP socket programming, client-server models, I/O multiplexing, and multithreading in the C programming language.

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🚀 Learning Objectives

• Understand the fundamentals of network programming in C.
• Build TCP and UDP client-server applications.
• Implement iterative and concurrent servers.
• Apply signal handling in UNIX/Linux.
• Use I/O multiplexing for scalable client-server interaction.
• Develop multi-threaded applications for parallel processing.
• Practice file transfer, authentication, and string processing over sockets.

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🛠️ Technologies Used

• Language: C
• Compiler: GCC (GNU Compiler Collection)
• Libraries:
  • <stdio.h>, <stdlib.h>, <string.h>
  • <unistd.h>, <pthread.h>
  • <sys/socket.h>, <netinet/in.h>, <arpa/inet.h>
• Environment: UNIX/Linux

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📂 Assignments Overview

#	Assignment	Key Focus	Description
1	Basic Client Interaction	TCP Basics	Simple client sends name to server → server replies with name + timestamp.
2	Iterative TCP Password Verification	TCP, Iterative Server	Client submits password → server validates against security rules. Connection open until done.
3	Concurrent TCP Server with Signals	Concurrency, Signals	Server forks a child per client, handles multiple requests, client exits on SIGQUIT.
4	UDP File Download Service	UDP, File Transfer	Client authenticates with server, requests file list, downloads files in chunks, adjusts buffer sizes.
5	TCP/UDP with I/O Multiplexing	Hybrid, Multiplexing	Client menu: (1) UDP download, (2) TCP upload, (3) Quit. Uses I/O multiplexing for efficiency.
6	Multithreaded TCP Character Counting	TCP, Threads	Multi-threaded client/server → counts occurrences of specific characters in strings.

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📘 Assignment Details

Assignment 1 – Basic Client Interaction

• Goal: Learn the building blocks of a network application.
• Task: Client sends a name to the server, server replies with name + current time/date.
• Command Example:

  hw1 <server_ip> <name>
  

Assignment 2 – TCP Password Verification

• Goal: Implement TCP client-server with iterative design.
• Client: Sends password to server until user types done.
• Server: Validates password (length 8–16, includes 3 of: uppercase, lowercase, number, special).
• Key Concepts: Iterative server, TCP communication.

Assignment 3 – Concurrent TCP Server with Signals

• Goal: Understand concurrency + signals in client-server systems.
• Server: Forks child per client, prints client IP/port, handles multiple requests.
• Client: Exits on SIGQUIT, not by done.
• Extra Rule: Special characters allowed @#$^&*+= (except %).

Assignment 4 – UDP File Download Service

• Goal:
  Practice using UDP sockets to implement a simple file transfer system.
• Process:
  1. Client authenticates with handshake bytes (0x55, 0xAA).
  2. Server responds with a list of available files.
  3. Client selects a file to download.
  4. File is transferred in 1024-byte chunks.
  5. Client can type done to exit the session.
  6. (Extra Feature): Client adjusts and displays send/receive buffer sizes for performance testing.

Assignment 5 – TCP/UDP with I/O Multiplexing

• Goal: Investigate TCP + UDP with I/O multiplexing.
• Client Menu:
  1. Download file via UDP.
  2. Upload file via TCP (to concurrent server).
  3. Quit.
• Features:
  1. Hostname resolution via /etc/hosts.
  2. Upload/download over different protocols.
  3. Prevents flooding server (UDP call limited to once per 5 sec).

Assignment 6 – Multithreaded TCP Character Counting

• Goal: Combine multithreading with TCP sockets.
• Client:
  1. Input string + characters to count.
  2. Creates one thread per character to send request to server.
• Server:
  1. Multi-threaded, counts occurrences, reports back per client thread.
  2. Tracks number of threads created so far.
• Output Example:

  T2: # of r’s in the string = 1
  T3: # of A’s in the string = 3
  T4: # of 1’s in the string = 2
  Total occurrences = 6