Shashnet

A custom application layer protocol for reliable, ordered, and lossless data transfer over UDP.

Overview

Shashnet provides a reliable data transfer mechanism over UDP, emulating TCP-like behavior entirely at the application layer. By implementing features such as error detection, packet ordering, retransmission, flow control, and duplicate handling, Shashnet creates a virtual transport layer that enables seamless, dependable communication across unreliable networks.

Features

• Reliability: Ensures all packets are received without loss, even in unreliable network conditions.
• Packet Ordering: Maintains the order of data packets for accurate reassembly at the receiver.
• Error Detection and Handling: Employs checksum verification to detect and handle corrupted packets.
• Flow Control: Balances the sender's rate with the receiver's capacity to prevent overflows.
• Retransmissions: Implements timeouts and retransmissions for unacknowledged packets.
• Duplicate Handling: Detects and discards duplicate packets to maintain data integrity.

Architecture

The system consists of two primary components:

Server

The server handles incoming data packets from the client, validates them, and reassembles them in the correct order. Key tasks include:

• Receiving packets and verifying checksums to detect corruption.
• Maintaining a buffer to handle out-of-order packets and ensuring correct sequencing.
• Sending acknowledgment packets to inform the client about successful receptions.

Client

The client initiates the communication by sending data packets to the server. Core responsibilities include:

• Splitting data into smaller packets and assigning sequence numbers.
• Managing a sliding window to control the flow of data.
• Retransmitting packets when acknowledgments are not received within a timeout period.
• Handling cumulative acknowledgments to optimize performance.

Virtualization on the Application Layer

Shashnet simulates features of a transport layer protocol entirely within the application layer, leveraging UDP as the underlying communication channel. This includes:

1. Custom Packet Format:

   • A structured format containing sequence numbers, acknowledgment numbers, payloads, and checksums.
   • Sequence and acknowledgment numbers are used to track data flow and maintain order.

2. Sliding Window Mechanism:

   • A sender-side buffer manages the number of packets in flight, ensuring that retransmissions and new transmissions occur seamlessly.
   • The receiver-side buffer reorders out-of-sequence packets for proper assembly.

3. Error Handling:

   • Checksum algorithms detect corrupted packets, prompting retransmission requests.
   • Retransmissions are triggered by sender-side timeouts if acknowledgments are delayed.

4. Flow Control:

   • Acknowledgment-based mechanisms ensure the sender adapts to the receiver's processing capacity, preventing data loss due to buffer overflows.

5. Duplicate Detection:

   • Sequence numbers enable the receiver to discard redundant packets, ensuring data integrity.

This virtualization approach allows Shashnet to mimic TCP-like reliability while retaining UDP's flexibility, making it a powerful solution for custom data transfer applications.

Resources

• UDP (RFC 768)
• TCP (RFC 793)
• Principles of Reliable Data Transfer - Keith W. Ross and James F. Kurose

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This project cost me my sanity. Never again.