RUDP -- Reliable Data Transfer over UDP

A custom application-layer protocol built on top of UDP that provides reliable, ordered file transfer with session management, integrity verification, encryption, and authentication.

Authors: Zach Benedict Hallare & Nathan Laborada

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Overview

RUDP implements reliability features on top of UDP for file transfer operations. The protocol operates in three phases:

1. Session Establishment -- Three-way handshake (SYN, SYN-ACK, ACK)
2. Data Transfer -- Stop-and-Wait ARQ with sequenced DATA/ACK packets
3. Session Termination -- FIN/FIN-ACK exchange

Both file download (server to client) and upload (client to server) are supported.

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Project Structure

UDP-Implementation/
  protocol.py        # Shared protocol definitions, packet format, crypto utilities
  server.py          # RUDP file transfer server
  client.py          # Interactive RUDP file transfer client
  rfc_document.md    # Full RFC-style protocol specification
  server_files/      # Directory the server reads from and writes to

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Requirements

• Python 3.6 or later
• No external dependencies (uses only the standard library)

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Getting Started

1. Start the Server

python server.py

The server binds to 127.0.0.1:12345 by default and serves files from the server_files/ directory.

2. Start the Client

python client.py

The client presents an interactive prompt with the following commands:

Command	Description
download <filename>	Download a file from the server's server_files/ directory
upload <filename>	Upload a local file to the server
exit	Quit the client

Example Session

rudp> download test.txt
rudp> upload myfile.txt
rudp> exit

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Protocol Details

Packet Format

Every packet on the wire follows this structure:

[13-byte Header] [Encrypted Payload (variable)] [32-byte HMAC-SHA256]

Header fields (13 bytes total):

Field	Size	Description
msg_type	1 byte	Packet type (SYN, ACK, DATA, FIN, etc.)
session_id	4 bytes	Unique session identifier
seq_num	4 bytes	Sequence number for ordering
payload_len	2 bytes	Length of the encrypted payload
checksum	2 bytes	CRC-16/CCITT-FALSE integrity check

Reliability

• Stop-and-Wait ARQ -- one packet sent at a time, waiting for ACK before proceeding
• Timeout-based retransmission -- packets are retransmitted after a configurable timeout (default 2 seconds)
• Sequence numbering -- data is reassembled in order regardless of arrival sequence
• Maximum retries -- transfers abort after 5 consecutive failed attempts

Security

• XOR Encryption -- payloads are encrypted with a repeating-key XOR cipher using a shared secret
• HMAC-SHA256 Authentication -- a 32-byte HMAC digest is appended to every packet to prevent tampering and forgery
• CRC-16 Checksum -- header and payload integrity are verified using CRC-16/CCITT-FALSE

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Configuration

All protocol parameters are defined in protocol.py:

Parameter	Default	Description
SERVER_HOST	127.0.0.1	Server listen address
SERVER_PORT	12345	Server listen port
CHUNK_SIZE	1024	Maximum payload bytes per DATA packet
TIMEOUT	2.0	Socket timeout in seconds
MAX_RETRIES	5	Maximum retransmission attempts
SHARED_SECRET	rudp-secret-key-2025	Shared key for encryption and HMAC

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Testing Packet Loss

RUDP includes a built-in simulated packet loss mode for testing retransmission behavior without external tools.

To enable it, edit protocol.py:

SIMULATE_LOSS    = True    # Enable simulated packet loss
LOSS_PROBABILITY = 0.3     # 30% chance of dropping each packet

Dropped packets are logged with a [SIM] prefix. The protocol will retransmit automatically, verifying that file transfers complete correctly even under loss.

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RFC Document

A full RFC-style specification is available in rfc_document.md. It covers the protocol in detail, including state machines, swimlane diagrams, message type definitions, and error handling procedures.