UDP Reliable File Transfer (Java)

A minimal, self-contained project that demonstrates a reliable file transfer protocol over UDP in Java using Stop‑and‑Wait ARQ, a CRC32 checksum, and a gremlin on the client to simulate damage, loss, and delay.

Why Stop‑and‑Wait? It's simple and perfect for showcasing reliability concepts. You can extend it to Go‑Back‑N or Selective Repeat as a follow-up.

Features

• UDP transport with application-level reliability
• Fixed-size data chunks (default 1024 bytes)
• CRC32 checksum per packet
• Client gremlin: random damage, loss, delay
• Clear console logs for learning & debugging
• Works cross‑platform (Java 17+)

Project Layout

udp-reliable-file-transfer-java/
├─ src/
│  ├─ main/java/com/example/udprdt/
│  │  ├─ Client.java
│  │  ├─ Server.java
│  │  └─ Utils.java
│  └─ main/resources/
│     └─ TestFile.html
├─ .gitignore
├─ LICENSE
├─ pom.xml
└─ README.md

Build

mvn -q -e -DskipTests package

Run

1) Start the server (sends file)

By default the server serves src/main/resources/TestFile.html. You can pass a different path.

# Terminal 1
java -cp target/udp-reliable-file-transfer-java-1.0.0.jar com.example.udprdt.Server 50001 src/main/resources/TestFile.html

2) Start the client (receives file)

Arguments: <serverHost> <serverPort> <outFile> <damageProb> <lossProb> <delayProb> <delayMs>

# Terminal 2
java -cp target/udp-reliable-file-transfer-java-1.0.0.jar com.example.udprdt.Client localhost 50001 received_TestFile.html 0.05 0.05 0.05 200

The client will write the received file to received_TestFile.html.

Tip: Try different probabilities (0.0 – 0.9) and delayMs to see retransmissions and checksum NAKs.

Protocol (Stop‑and‑Wait)

Each UDP DATA packet:

[SEQ:1][LEN:2][LAST:1][CRC32:4][DATA:LEN]

• SEQ alternates 0/1
• LEN ≤ 1024
• LAST 1 if final packet, else 0
• CRC32 computed on DATA only

ACK/NAK packet:

[TYPE:1][SEQ:1]

• TYPE = 0x06 for ACK, 0x15 for NAK
• SEQ acknowledges the sequence number

Notes

• The gremlin applies to incoming DATA packets at the client (after receive, before validation).
• Timeouts and retransmissions are handled server‑side (default timeout 1000 ms).
• Tweak chunk size / timeout in Server.java if needed.

Extend

• Add windowing to implement Go‑Back‑N.
• Add per‑packet timestamps and RTT estimation for adaptive timeout.
• Encrypt payloads (e.g., AES/GCM) and authenticate headers.