🧪 EE323 Network Programming Assignments

This repository contains solutions to four major network programming assignments from KAIST's EE323 Computer Networks course. The assignments progressively build an understanding of socket programming, application-layer protocols, network-layer routing, and transport-layer reliability.

🗂 Assignments Overview

📡 Assignment 1: TCP Echo Server and Client

Goal: Implement a simple server that accepts TCP connections and echoes client messages line by line.

• ✅ Each client runs in its own process (via fork()).
• ✅ Message is transmitted upon pressing Enter.
• ✅ Double Enter triggers client exit.
• ✅ Command-line interface using getopt() (-p, -h).
• ✅ Compatible with file redirection and piping (< input.txt, > output.txt).

Files:

• server.c
• client.c
• Makefile

Run:

# Server
$ ./server -p 12345

# Client
$ ./client -p 12345 -h 127.0.0.1

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🌐 Assignment 2: HTTP Proxy Server

Goal: Build a multi-client HTTP/1.0 proxy server supporting basic caching and blacklisting.

• ✅ Accepts requests, forwards them to destination servers, returns responses.
• ✅ Parses HTTP requests (method, URL, host).
• ✅ Filters blacklisted domains and redirects to http://warning.or.kr.
• ✅ Can be tested with browser, telnet, or Python script.
• ✅ Properly handles malformed requests with HTTP 400.

Files:

• proxy.c
• Makefile

Run:

$ ./proxy <PORT> < blacklist.txt

Test (example):

$ telnet localhost <PORT>
GET http://example.com/ HTTP/1.0
Host: example.com

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🚦 Assignment 3: Software Router Implementation (via Mininet + POX)

Goal: Simulate a software router that performs routing, ARP handling, ICMP response, and packet forwarding in a Mininet-emulated network.

• ✅ Responds to ARP requests and maintains ARP cache.
• ✅ Responds to ICMP Echo Requests and Time Exceeded (Traceroute).
• ✅ Filters packets from blacklisted subnets (10.0.2.0/24).
• ✅ Drops packets with unresolved ARP after timeout, replies with ICMP Unreachable.
• ✅ Fully tested using Mininet and POX SDN controller.

Files:

• router/sr_router.c
• router/sr_arpcache.c
• router/Makefile

Run (inside VM):

$ ./run_pox.sh
$ ./run_mininet.sh
$ ./sr

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📦 Assignment 4: Reliable Transport Layer (STCP)

Goal: Implement a simplified TCP-like transport protocol called STCP.

• ✅ Three-way handshake (SYN/SYN-ACK/ACK)
• ✅ Four-way connection teardown (FIN/ACK)
• ✅ In-order, reliable, full-duplex transmission
• ✅ Sliding window with slow start
• ✅ Logging transmission statistics (client_log.txt, server_log.txt)
• ✅ Compatible with mysock interface (provided stub)

Files:

• transport.c (ONLY file modified)
• Compiles with provided Makefile

Run:

# Server
$ ./server

# Client
$ ./client <SERVER_IP>:<PORT> -f testfile.txt

Output:

• File rcvd is saved at the client side.
• Logs are saved in the root directory.

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🛠 Build Instructions

Each assignment contains a Makefile. Run:

$ make

To clean:

$ make clean

⚠ All code is tested and compiled on KAIST lab machines (eelab5, eelab6). Behavior may vary on other environments.

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📚 References

• Beej’s Guide to Network Programming
• RFC 793 – Transmission Control Protocol
• RFC 1945 – HTTP/1.0 Specification
• Mininet & POX documentation

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🚫 Academic Integrity

Do not reuse or redistribute this repository for academic credit. This work is intended for educational reference and self-study only.