Peer-to-Peer File Sharing System

A simplified BitTorrent-like peer-to-peer file sharing system implemented in Java.

Peers connect to each other using TCP sockets and collaboratively download pieces of a file until every peer obtains the complete file.

Demo

Peers exchanging pieces of a file across the network.

The system implements core BitTorrent mechanisms such as:

• Piece-based file distribution
• Bitfield exchange
• Interested / Not Interested messages
• Request / Piece transfer
• Choking / Unchoking mechanism
• Preferred neighbor selection
• Optimistic unchoking

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System Overview

Each peer acts as both a client and a server:

• Server role: Accept incoming connections from other peers.
• Client role: Initiate connections to peers that started earlier.

Once connected, peers exchange information about the pieces they possess and request missing pieces from neighbors. The file is split into fixed-size pieces, which are downloaded and assembled locally by each peer.

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Project Structure

project_root/
├── src/ # Java source files
├── build/ # Compiled .class files (generated after build)
├── Makefile # Build and run configuration
├── Common.cfg # Global configuration parameters
└── PeerInfo.cfg # Peer information configuration

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Building and Running the Project

Build Without Make

javac -d ../build *.java  
  
cd ../build  
java PeerProcess 1001

Build Using Makefile

make run PEER=1001

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Running Multiple Peers

Currently, peers must be started manually.

Example:

make run PEER=1001  
make run PEER=1002  
make run PEER=1003

Each peer reads configuration files and connects to other peers based on the order defined in PeerInfo.cfg.

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Configuration Files

PeerInfo.cfg

Defines peers participating in the network. 1001 localhost 6001 0
1002 localhost 6002 1
1003 localhost 6003 0

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Common.cfg

Defines global parameters.

NumberOfPreferredNeighbors 3
UnchokingInterval 5
OptimisticUnchokingInterval 10
FileName thefile
FileSize 2167705
PieceSize 16384

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Protocol Workflow

The system follows a message-based protocol similar to BitTorrent.

• Handshake Peers establish TCP connections and exchange a handshake message containing peer identifiers.
• Bitfield Exchange Peers share a bitfield indicating which pieces they possess.
• Interest Determination Peers determine whether they are interested in pieces from another peer. Possible messages: - interested - not interested
• Choking / Unchoking Peers regulate bandwidth by controlling which neighbors may request pieces. Two mechanisms are used:
  • Preferred neighbors
  • Optimistic unchoking
• Piece Requesting Interested peers send request messages for missing pieces.
• Piece Transfer Peers send the requested data using piece messages.
• File Reconstruction Once all pieces are received, the peer reconstructs the complete file.

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Choking / Unchoking Mechanism

Every UnchokingInterval seconds:

1. The peer calculates download rates from neighbors
2. The top K peers become preferred neighbors
3. Preferred neighbors are unchoked
4. Others are choked.

Every OptimisticUnchokingInterval seconds:

• One randomly selected peer is optimistically unchoked.

This ensures fairness and prevents peers from being permanently choked.

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File Handling

The file is split into pieces of size PieceSize.

Each peer:

1. Downloads pieces independently
2. Stores pieces locally
3. Updates its bitfield
4. Broadcasts have messages
5. Reconstructs the file after receiving all pieces

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Environment

Testing was performed in a local environment using:

• localhost for peer communication
• Multiple peers running on different ports
• Java TCP socket communication The system can also run across multiple machines if hostnames and ports are configured accordingly.