Reliable Data Transfer Protocol

Go-Back-N Implementation over UDP with SSE Web Dashboard

Project Overview

This project implements a Reliable Data Transfer (RDT) Protocol using the Go-Back-N (GBN) algorithm on top of UDP sockets in Java. It features a modern Web Dashboard powered by Server-Sent Events (SSE) for real-time telemetry, allowing you to observe and control the simulation visually.

Since UDP does not provide reliability by default, all reliability mechanisms are built from scratch:

• ✅ Sequence numbering
• ✅ Acknowledgment (ACK) based confirmation
• ✅ Timeout-based retransmission
• ✅ Sliding window (Go-Back-N)
• ✅ Duplicate packet detection and discard
• ✅ Packet loss simulation (random + manual)
• ✅ Data corruption simulation with CRC32 checksum

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File Structure

RDT_Project/
├── src/
│   ├── Config.java           # All tunable parameters (window, timeout, loss %)
│   ├── Packet.java           # Packet structure: seq num, data, CRC32 checksum
│   ├── NetworkSimulator.java # Random drop + manual drop + corruption injection
│   ├── Sender.java           # GBN Sender logic
│   ├── Receiver.java         # GBN Receiver logic
│   ├── ProtocolLogger.java   # Central logger for broadcasting events via SSE
│   ├── RdtWebServer.java     # Zero-dependency HTTP & SSE Server (Entry Point)
│   └── index.html            # Web Dashboard UI
├── compile.sh                # Script to compile all Java files
└── README.md                 # This file

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How to Run

The project is orchestrated via a zero-dependency Web Server.

Method 1: Using the Script (Linux/Mac/Git Bash)

The provided script compiles the project and starts the server automatically.

./compile.sh

Method 2: Manual (Windows/All)

1. Compile the Project:

   cd src
   javac *.java

2. Start the Web Server:

   java RdtWebServer

Access the Dashboard

Open your browser and navigate to http://localhost:8080.

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Web Interface Features

From the web dashboard, you can interact with the protocol simulation in real-time:

• Start Transfer: Type a custom message and click "Start Transfer". The server will instantiate the Sender and begin the GBN protocol.
• Force Drop: Click the "⚡ Force Drop" button to arm a manual packet drop. The next packet (either Data or ACK) will be dropped by the simulator, allowing you to observe retransmission behavior on demand.
• Real-time Logs: Watch the protocol state machine live. Logs are color-coded by source:
  • SND: Sender events (Sent, Retransmit, ACK received)
  • RCV: Receiver events (Incoming, Accepted, Sent ACK/NAK)
  • SIM: Network Simulator events (Random Loss, Corruption, Manual Drop)
  • SYS: System events

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Configuration (Config.java)

You can tune the protocol behavior by editing src/Config.java.

Parameter	Default	Description
WINDOW_SIZE	4	GBN sliding window size (N)
MAX_SEQ_NUM	8	Max sequence number (wraps around)
TIMEOUT_MS	2000	Retransmit after 2 seconds of no ACK
LOSS_PROBABILITY	0.25	25% random packet/ACK loss
CORRUPT_PROBABILITY	0.10	10% random data corruption
SEND_DELAY_MS	300	Delay between packet sends

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Go-Back-N Protocol — How It Works

Sender Window (N=4):
 ┌───────────────────────────────────────────┐
 │  base        nextSeq                      │
 │   ↓            ↓                          │
 │ [0][1][2][3] [4][5][6][7][8]...           │
 │  ←── window ──→  ← not yet sent →         │
 └───────────────────────────────────────────┘

1. Sender sends packets 0,1,2,3 (window full)
2. ACK(0) arrives → window slides: send packet 4
3. Packet 1 lost → TIMEOUT after 2s
4. Sender retransmits 1,2,3,4 (entire window)
5. Receiver discards out-of-order packets
6. Process continues until all packets ACKed

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Requirements Fulfilled

Requirement	Implemented
UDP socket communication	✅
Sequence numbering	✅
ACK-based delivery confirmation	✅
Timeout and retransmission	✅
Packet loss simulation (random)	✅
Packet loss simulation (manual)	✅
Duplicate packet discard	✅
In-order delivery at receiver	✅
Data corruption detection (CRC32)	✅
Sliding window (Go-Back-N)	✅
Transfer summary stats	✅