🌐 MiniTCP Download Manager

A two-part networking project featuring a custom, reliable transport protocol (MiniTCP) built entirely on top of UDP, and a Download Manager that utilizes this protocol to fetch files concurrently.

Overview

1. MiniTCP Protocol: A reliable, connection-oriented transport layer implemented over standard UDP sockets. It ensures data integrity, sequencing, and delivery guarantees across lossy networks.
2. Download Manager: A client/server application that splits a requested file into 9 distinct chunks. The client downloads all 9 chunks simultaneously over 9 independent MiniTCP connections, reassembling the file upon completion.

Core Protocol Features

MiniTCP implements the fundamental mechanics of standard TCP:

• Connection Management: 3-way handshake (SYN, SYN-ACK, ACK) for connection setup and a symmetric 4-way close (FIN, ACK, FIN, ACK) for teardown.
• Reliability & Sequencing: 15-byte custom packet headers containing Sequence (SEQ) and Acknowledgement (ACK) numbers, alongside CRC32 checksums for data integrity.
• Loss Detection & Recovery: Implements Stop-and-Wait ARQ with timeout-based loss detection and automatic retransmission with backoff.
• Concurrency: Each file chunk connection operates on its own ephemeral UDP port via independent Python threads.

Run Guide

Requirements: Python 3.8+, standard library only — nothing to pip install.

1. Start the server (in one terminal). This serves the sample file, split into 9 chunks, on UDP port 9000, with verbose packet-level logging:

python3 server.py --file testfiles/sample.bin --port 9000 --chunks 9 -v

You should see it confirm the file/chunk setup and then sit listening for connections.

2. Run the client (in a second terminal), also verbose:

python3 client.py --host 127.0.0.1 --port 9000 --out downloaded.bin -v

This fetches file info, opens 9 parallel MiniTCP connections, downloads all 9 chunks concurrently, and reassembles them into downloaded.bin. With -v on, you'll see every SYN/ACK/DATA/FIN packet exchanged on both ends — useful for actually watching the handshake and retransmission logic work in real time.

3. Verify the download is byte-for-byte correct:

sha256sum testfiles/sample.bin downloaded.bin

Both hashes should match exactly.

Example run:

Optional — test reliability under packet loss: add --loss 0.1 to either command (drops ~10% of outgoing packets) to watch MiniTCP's timeout/retransmission recover automatically:

python3 server.py --file testfiles/sample.bin --port 9000 --chunks 9 --loss 0.1 -v
python3 client.py --host 127.0.0.1 --port 9000 --out downloaded.bin --loss 0.1 -v

📂 Repository Structure

├── client.py               # Download Manager client script
├── server.py                # Download Manager server script
├── minitcp/                 # The custom protocol package
│   ├── protocol.py          # Packet formatting, encoding, and checksums
│   └── connection.py        # Handshake, send/recv, retransmission logic
├── docs/                     # System architecture and protocol design PDFs
└── testfiles/                # Sample binary files for transfer testing