CS425 – A1 to A4 (Ultra‑Explained Edition)

This repository contains four networking assignments rewritten with friendly, step‑by‑step explanations so that you can skim comments and understand the “why” as well as the “what”.

A1 — Epoll‑based Group Chat (TCP)

• Folder: CS425/A1
• Files: server_grp.cpp, client_grp.cpp, server_grp.h, Makefile, users.txt
• What you’ll learn:
  • How a TCP server handles many clients using epoll (no threads required).
  • How to authenticate clients and keep tiny session state.
  • Private messages, broadcast, groups (create/join/leave), group messages.
• Start here: read the big header at the top of server_grp.cpp. It explains the approach and data structures.

A2 — DNS Resolver (Iterative vs Recursive)

• Folder: CS425/A2_220285_220276_220638
• Files: dnsresolver.py, README.md
• What you’ll learn:
  • How real DNS resolution walks ROOT → TLD → AUTHORITATIVE servers.
  • How to use the system’s recursive resolver.
  • How to send queries, parse answers, and handle timeouts.
• Start here: the docstring at the top of dnsresolver.py explains the flow.

A3 — Raw TCP Handshake (Client + Server)

• Folder: CS425/A3220285220276220638
• Files: client.cpp, server.cpp, Makefile, README
• What you’ll learn:
  • What happens under the hood of a TCP 3‑way handshake.
  • How to craft IP/TCP headers and checksums manually (for learning only!).
• Start here: read the headers at the top of client.cpp and server.cpp.

A4 — Routing Simulator (Distance Vector + Link State)

• Folder: CS425/A4220638220276220285
• Files: routing_sim.cpp, input*.txt, README.md, Makefile
• What you’ll learn:
  • How routing tables are computed in Distance Vector and Link State.
  • How next hops emerge from Bellman‑Ford style and Dijkstra updates.
• Start here: the header at the top of routing_sim.cpp sets the mental model.

Philosophy of this rewrite

• Explain Like I’m New: each main file begins with a plain‑language overview.
• Name things after what they do: data structures are described with intent.
• No magic leaps: tricky spots (epoll, raw sockets) are annotated.
• Preserve functionality: code still builds and runs as before; it’s just more readable.

Build & Run Quick Notes

• A1: cd CS425/A1 && make (then run server, then client on another terminal).
• A2: cd CS425/A2_220285_220276_220638 && python3 dnsresolver.py iterative example.com
• A3: cd CS425/A3220285220276220638 && make (root permissions might be needed for raw sockets).
• A4: cd CS425/A4220638220276220285 && make then ./routing_sim < input1.txt

If you want even more guidance (e.g., diagrams / flowcharts in the READMEs), say the word and I’ll add them.