RUDP - Reliable UDP Implementation

A custom reliable data transfer protocol built on top of UDP with support for file transfer, chat mode, three-way handshake, four-way connection termination, sliding window flow control, and configurable packet loss simulation.

Overview

This project implements a reliable transport protocol called S.H.A.M. (Simple Handshake and Messaging) over UDP, featuring:

Three-way handshake for connection establishment
Four-way handshake for graceful connection termination
Sliding window protocol with Go-Back-N ARQ for reliable data transfer
Flow control with dynamic window advertisement
Timeout and retransmission mechanism
Packet loss simulation for testing reliability
File transfer mode with MD5 checksum verification
Interactive chat mode for real-time messaging
Comprehensive logging for debugging and analysis

Protocol Header (S.H.A.M.)

struct sham_header {
    uint32_t seq_num;       // Sequence Number
    uint32_t ack_num;       // Acknowledgment Number
    uint16_t flags;         // Control flags (SYN, ACK, FIN)
    uint16_t window_size;   // Flow control window size
};

Flags

SYN_FLAG (0x1): Synchronize sequence numbers (connection establishment)
ACK_FLAG (0x2): Acknowledgment
FIN_FLAG (0x4): Finish connection (graceful termination)

Features

Connection Management

Three-Way Handshake: SYN → SYN-ACK → ACK sequence for reliable connection establishment
Four-Way Handshake: FIN → ACK → FIN → ACK sequence for graceful connection termination
Retransmission: Automatic retransmission on timeout with configurable RTO (500ms default)

Reliable Data Transfer

Sliding Window Protocol: Client uses 4-packet window, server buffers up to 20 packets
Selective Acknowledgment: Cumulative ACKs with duplicate detection
Out-of-Order Buffering: Server buffers out-of-order packets for efficient delivery
Flow Control: Dynamic window advertisement prevents receiver buffer overflow

Packet Loss Handling

Configurable Loss Rate: Simulate packet loss with rates between 0.0 and 1.0
Timeout Detection: Detect lost packets using RTO timer
Automatic Retransmission: Retransmit unacknowledged packets on timeout

Logging

Event Logging: Detailed logs for all protocol events (send, receive, drop, timeout)
Timestamped Entries: Microsecond-precision timestamps for debugging
Environment Variable Control: Enable logging with RUDP_LOG=1

Project Structure

.
├── client.c        # Client implementation (sender)
├── server.c        # Server implementation (receiver)
├── sham.c          # Protocol helper functions (byte order conversion)
├── sham.h          # Protocol header definitions
├── Makefile        # Build configuration
├── test.txt        # Sample test file
├── client_log.txt  # Client event log (generated)
└── server_log.txt  # Server event log (generated)

Compilation

Using Makefile (Recommended)

# Build both client and server
make

# Clean build artifacts
make clean

The Makefile automatically:

Detects OpenSSL installation via Homebrew (macOS)
Links with OpenSSL's crypto library for MD5 checksums
Compiles with C11 standard and warning flags

Manual Compilation

# On macOS with Homebrew OpenSSL
gcc -Wall -Wextra -std=c11 -I$(brew --prefix openssl)/include     -L$(brew --prefix openssl)/lib server.c sham.c -o server -lcrypto

gcc -Wall -Wextra -std=c11 -I$(brew --prefix openssl)/include     -L$(brew --prefix openssl)/lib client.c sham.c -o client -lcrypto

# On Linux (standard OpenSSL paths)
gcc -Wall -Wextra -std=c11 server.c sham.c -o server -lcrypto
gcc -Wall -Wextra -std=c11 client.c sham.c -o client -lcrypto

Usage

File Transfer Mode

Server

# Basic usage
./server <port>

# With packet loss simulation
./server <port> <loss_rate>

# Examples
./server 8080
./server 8080 0.1    # 10% packet loss

Client

# Basic usage
./client <server_ip> <port> <input_file> <output_filename>

# With packet loss simulation
./client <server_ip> <port> <input_file> <output_filename> <loss_rate>

# Examples
./client 127.0.0.1 8080 test.txt received.txt
./client 127.0.0.1 8080 test.txt received.txt 0.15    # 15% packet loss

Chat Mode

Server

# Start server in chat mode
./server <port> --chat [loss_rate]

# Examples
./server 8080 --chat
./server 8080 --chat 0.05    # 5% packet loss

Client

# Connect to server in chat mode
./client <server_ip> <port> --chat [loss_rate]

# Examples
./client 127.0.0.1 8080 --chat
./client 127.0.0.1 8080 --chat 0.1    # 10% packet loss

Chat Commands:

Type message and press Enter to send
Type /quit to exit chat mode and close connection

Enabling Logging

# Set environment variable before running
export RUDP_LOG=1

# Run client/server
./client 127.0.0.1 8080 test.txt output.txt
./server 8080

# Logs will be written to client_log.txt and server_log.txt

Implementation Details

Client (Sender)

Window Management:

Maintains sliding window of 4 packets (configurable with WINDOW_SIZE)
Tracks base sequence number and next sequence to send
Implements dynamic timeout calculation based on oldest unacknowledged packet

Flow Control:

Monitors receiver's advertised window size
Suspends transmission when receiver buffer is full
Resumes transmission when window opens

Retransmission:

500ms RTO (Retransmission Timeout)
Retransmits oldest unacknowledged packet on timeout
Uses non-blocking socket with select() for multiplexing

Server (Receiver)

Buffer Management:

Buffers up to 20 out-of-order packets (2× window size)
Delivers packets to application in order
Dynamically updates advertised window based on buffer occupancy

ACK Generation:

Sends cumulative ACK for next expected sequence number
Sends duplicate ACKs for out-of-order packets
Includes current window size in every ACK

File Reception:

Writes received data to file as it arrives in order
Calculates MD5 checksum after complete file reception
Displays checksum for verification

Chat Mode

Bidirectional Communication:

Uses select() to multiplex between stdin and socket
Sends messages as data packets with S.H.A.M. header
Real-time display of incoming and outgoing messages

Connection Termination:

Client initiates termination with /quit command
Four-way handshake ensures graceful shutdown
Both sides can detect premature disconnection

Protocol Flow

Connection Establishment (Three-Way Handshake)

Client                          Server
   |                               |
   |  SYN (seq=1000)              |
   |----------------------------->|
   |                               |
   |  SYN-ACK (seq=5000, ack=1001)|
   |<-----------------------------|
   |                               |
   |  ACK (ack=5001)              |
   |----------------------------->|
   |                               |
   |  [Connection Established]    |

Data Transfer

Client                          Server
   |                               |
   |  DATA (seq=X, len=1024)      |
   |----------------------------->|
   |                               |
   |  ACK (ack=X+1024)            |
   |<-----------------------------|
   |                               |
   |  DATA (seq=X+1024, len=512)  |
   |----------------------------->|
   |                               |
   |  ACK (ack=X+1536)            |
   |<-----------------------------|

Connection Termination (Four-Way Handshake)

Client                          Server
   |                               |
   |  FIN (seq=Y)                 |
   |----------------------------->|
   |                               |
   |  ACK (ack=Y+1)               |
   |<-----------------------------|
   |                               |
   |  FIN (seq=Z)                 |
   |<-----------------------------|
   |                               |
   |  ACK (ack=Z+1)               |
   |----------------------------->|
   |                               |
   |  [Connection Closed]         |

Configuration Parameters

Client

WINDOW_SIZE: 4 packets
RTO_MS: 500 milliseconds
PAYLOAD_SIZE: 1024 bytes
BUFFER_SIZE: 1040 bytes (header + payload)

Server

WINDOW_SIZE: 10 packets (buffer = 2× window)
PAYLOAD_SIZE: 1024 bytes
BUFFER_SIZE: 1040 bytes

Build Configuration

Makefile Settings

Compiler: GCC with C11 standard
Flags: -Wall -Wextra (strict warnings)
OpenSSL: Automatic detection via Homebrew (macOS)
Library: -lcrypto for MD5 checksums
Targets: server and client executables

Dependencies

GCC compiler with C11 support
OpenSSL library (libssl-dev on Ubuntu/Debian, openssl via Homebrew on macOS)
Standard POSIX headers: sys/socket.h, netinet/in.h, arpa/inet.h

Error Handling

Socket creation/binding failures with descriptive error messages
File open/read/write errors with errno reporting
Handshake timeout and automatic retransmission
Packet loss detection and recovery via timeout mechanism
Graceful handling of unexpected disconnections
Invalid command-line argument validation

Testing

Basic File Transfer Test

# Terminal 1: Start server
./server 8080

# Terminal 2: Send file
./client 127.0.0.1 8080 test.txt received.txt

# Verify transfer
md5sum test.txt received.txt    # Linux
md5 test.txt received.txt        # macOS

Packet Loss Simulation Test

# Terminal 1: Server with 20% loss
export RUDP_LOG=1
./server 8080 0.2

# Terminal 2: Client with 20% loss
export RUDP_LOG=1
./client 127.0.0.1 8080 test.txt received.txt 0.2

# Check logs
tail -f client_log.txt
tail -f server_log.txt

Chat Mode Test

# Terminal 1: Server in chat mode
./server 8080 --chat

# Terminal 2: Client in chat mode
./client 127.0.0.1 8080 --chat

# Exchange messages, then type /quit on client

Large File Transfer Test

# Create a large test file (10 MB)
dd if=/dev/urandom of=largefile.bin bs=1024 count=10240

# Transfer the file
./server 8080
./client 127.0.0.1 8080 largefile.bin received_large.bin

# Verify integrity
diff largefile.bin received_large.bin

Log File Format

[2025-11-15 01:30:45.123456] [LOG] SND SYN SEQ=1000
[2025-11-15 01:30:45.234567] [LOG] RCV SYN-ACK SEQ=5000 ACK=1001
[2025-11-15 01:30:45.345678] [LOG] SND DATA SEQ=1001 LEN=1024
[2025-11-15 01:30:45.456789] [LOG] RCV ACK=2025
[2025-11-15 01:30:46.567890] [LOG] TIMEOUT SEQ=2025
[2025-11-15 01:30:46.678901] [LOG] RETX DATA SEQ=2025 LEN=512
[2025-11-15 01:30:46.789012] [LOG] DROP DATA SEQ=3000

Troubleshooting

OpenSSL Not Found (macOS)

# Install OpenSSL via Homebrew
brew install openssl

# Update Makefile or set paths manually
export OPENSSL_ROOT=$(brew --prefix openssl)

OpenSSL Not Found (Linux)

# Install OpenSSL development package
sudo apt-get install libssl-dev      # Ubuntu/Debian
sudo yum install openssl-devel       # CentOS/RHEL

Port Already in Use

# Find process using the port
lsof -i :8080           # macOS/Linux
netstat -ano | grep 8080  # Linux

# Kill the process or use a different port
./server 8081

Limitations

Fixed payload size of 1024 bytes per packet
No congestion control (only flow control)
Simple Go-Back-N retransmission (not selective repeat)
Single-threaded server (handles one client at a time)
No encryption or authentication
No adaptive RTO calculation (fixed 500ms timeout)

Author

dawnbliss-coder

Name		Name	Last commit message	Last commit date
Latest commit History 3 Commits
Makefile		Makefile
README.md		README.md
client.c		client.c
server.c		server.c
sham.c		sham.c
sham.h		sham.h

dawnbliss-coder/networking

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RUDP - Reliable UDP Implementation

Overview

Protocol Header (S.H.A.M.)

Flags

Features

Connection Management

Reliable Data Transfer

Packet Loss Handling

Logging

Project Structure

Compilation

Using Makefile (Recommended)

Manual Compilation

Usage

File Transfer Mode

Server

Client

Chat Mode

Server

Client

Enabling Logging

Implementation Details

Client (Sender)

Server (Receiver)

Chat Mode

Protocol Flow

Connection Establishment (Three-Way Handshake)

Data Transfer

Connection Termination (Four-Way Handshake)

Configuration Parameters

Client

Server

Build Configuration

Makefile Settings

Dependencies

Error Handling

Testing

Basic File Transfer Test

Packet Loss Simulation Test

Chat Mode Test

Large File Transfer Test

Log File Format

Troubleshooting

OpenSSL Not Found (macOS)

OpenSSL Not Found (Linux)

Port Already in Use

Limitations

Author

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