Reliable Message Transfer (Sliding Window) – Client/Server (Python)

Course Project (Networks)
Implements reliable transfer over an unreliable channel: fixed-size encoded packets, sliding windows for time and sequence, retransmissions, ordered ACKs, and end-of-message handshake.

✨ Highlights

• Package format & codec: fixed-size struct.pack/unpack; header, pos, sent_time, payload; matching decoder.
• Windows & retransmissions: adaptive TIME_WINDOW and SEQ_WINDOW with threshold checks and resend logic.
• Server ACK policy: ordered ACKs, can intentionally skip ACKs to simulate loss; handles DONE to rebuild the message.
• Deterministic demos: reproducible runs for lost packets and lost ACKs, with Wireshark traces.

🗂️ Project Layout

• client.py / server.py — runtime logic, windows & resend policy
• package.py — packet class + encode/decode + validations
• functions.py — helpers (params, slicing, utilities)
• get_packages.py — parses Wireshark JSON and decodes frames with our codec

🚀 Quick Start

# 1) install
pip install -r requirements.txt

# 2) run server (terminal A)
python server.py

# 3) run client (terminal B)
python client.py --max-msg-size 4   # minimum accepted

# (Optional) use provided params files in /demo to auto-load scenarios

## 🧪 Repro Demos

Lost packets: PACKAGES_TO_LOSE=[4,9,10] → client resends after time/seq threshold; server resumes ordered ACKs.

Lost ACKs: ACKS_TO_LOSE=[4,9,10] → client infers earlier packets were delivered when it receives later ACKs.

See docs/wireshark.md for annotated traces and decoded frames.

## 🛠️ Implementation Notes

Fixed-size codec via struct to ensure constant framing on the wire.

Sliding window updates on every ACK; resend on threshold pass.

DONE → server acks, reconstructs message in order, then clean shutdown.