Node Class

The Node class is the core component of the P2P file sharing system. It manages TCP connections, implements the messaging protocol, and coordinates piece downloads and uploads.

Constructor

Creates a new P2P node instance.

const node = new Node({
  port: 6881,
  filePath: './shared/file.txt'
});

options.port

number

required

TCP port for listening to incoming peer connections

options.filePath

string

required

Path to the file. For seeders, this should be an existing file. For leechers, this is the destination path where the file will be downloaded.

Properties

string

Unique peer identifier (16 hexadecimal characters generated from 8 random bytes)

port

number

TCP port for listening to connections

filePath

string

Path to the file being shared or downloaded

fileName

string

Name of the file (extracted from filePath or received from peer metadata)

fileSize

number

Total file size in bytes

fileHash

string

SHA-1 hash of the file content (used as content identifier)

pieceSize

number

default:"65536"

Size of each piece in bytes (64 KiB by default). Automatically adjusted if file is smaller.

numPieces

number

Total number of pieces the file is divided into

isSeed

boolean

Indicates whether this node has the complete file

havePieces

Set<number>

Set of piece indices that this node possesses

missingPieces

Set<number>

Set of piece indices that are still needed for download

pendingPieces

Set<number>

Set of piece indices that have been requested but not yet received

knownPeers

Map<string, PeerInfo>

Map of known peers (key: peer ID, value: peer information object)

fileManager

Manager

File manager instance for reading/writing pieces

Connection Methods

startListening()

Starts the TCP server to accept incoming peer connections.

node.startListening();

This method:

Creates a TCP server on the specified port
Accepts incoming connections from other peers
Sets up socket handlers for each connection
Logs the node ID and listening port

connectToPeer(host, port)

Establishes an outgoing connection to another peer.

node.connectToPeer('192.168.1.100', 6881);

host

string

required

IP address or hostname of the peer

port

number

required

TCP port of the peer

This method:

Creates an outgoing TCP connection
Sends a handshake message upon connection
Sets up socket handlers
Handles connection errors

Protocol Methods

_sendHandshake(socket)

Sends a handshake message containing file metadata.

socket

Socket

required

TCP socket to send the handshake through

// Handshake message structure
{
  type: 'handshake',
  id: 'a1b2c3d4e5f6g7h8',
  fileName: 'example.txt',
  fileSize: 1048576,
  fileHash: 'da39a3ee5e6b4b0d3255bfef95601890afd80709',
  pieceSize: 65536,
  port: 6881
}

_sendBitfield(socket)

Sends a bitfield message indicating which pieces this node possesses.

socket

Socket

required

TCP socket to send the bitfield through

// Bitfield message structure
{
  type: 'bitfield',
  pieces: [0, 1, 2, 5, 8]
}

_sendRequest(socket, index)

Sends a request for a specific piece to a peer.

socket

Socket

required

TCP socket of the peer to request from

index

number

required

Zero-based index of the piece to request

node._sendRequest(socket, 3);

Message Handling Methods

_handleMessage(socket, message)

Routes incoming messages to appropriate handlers based on message type.

socket

Socket

required

TCP socket that received the message

message

object

required

Parsed JSON message object

Supported message types:

handshake - Initial peer identification and file metadata
bitfield - List of pieces a peer possesses
request - Request for a specific piece
piece - Piece data delivery
have - Notification that a peer acquired a new piece
peers - List of additional known peers

_handleHandshake(socket, message)

Processes incoming handshake messages.

// From src/node.js
await this._handleHandshake(socket, message);

This method:

Validates peer ID (rejects if same as own ID)
Verifies file hash matches (disconnects if mismatch)
Adopts file metadata if this is a leecher
Exchanges peer lists for peer discovery
Sends bitfield if this node has pieces

_handleBitfield(socket, message)

Processes bitfield messages containing peer’s available pieces.

// Updates peer's available pieces and schedules requests
const pieces = message.pieces;
peerInfo.availablePieces = new Set(pieces);
this._scheduleRequests();

_handleRequest(socket, message)

Processes piece request messages from peers.

// Reads requested piece and sends it to the peer
const index = message.index;
const buffer = await this.fileManager.readPiece(index);
const pieceMsg = {
  type: 'piece',
  index: index,
  data: buffer.toString('base64')
};
socket.write(JSON.stringify(pieceMsg) + '\n');

_handlePiece(socket, message)

Processes received piece data and writes it to the file.

// Decodes base64 data and writes to file
const dataBuffer = Buffer.from(dataBase64, 'base64');
await this.fileManager.writePiece(index, dataBuffer);

// Updates tracking structures
this.havePieces.add(index);
this.missingPieces.delete(index);
this.pendingPieces.delete(index);

// Notifies other peers with 'have' message
const haveMsg = { type: 'have', index: index };

This method also:

Verifies file integrity when download completes
Transitions node to seed mode after completion
Schedules next piece requests

_handleHave(socket, message)

Processes notifications that a peer acquired a new piece.

peerInfo.availablePieces.add(index);
// Schedules request if we need this piece
if (this.missingPieces.has(index) && !this.pendingPieces.has(index)) {
  this._scheduleRequests();
}

_handlePeers(socket, message)

Processes peer list messages for peer discovery.

// Adds new peers and initiates connections based on ID comparison
for (let peer of peersList) {
  const { id: peerId, host, port } = peer;
  if (this._shouldInitiateConnection(peerId)) {
    this.connectToPeer(host, port);
  }
}

Internal Methods

_setupSocketHandlers(socket)

Configures event handlers for a peer connection socket.

socket

Socket

required

TCP socket to configure

Sets up handlers for:

data - Buffers and parses JSON messages (newline-delimited)
close - Cleans up peer state and reschedules requests
error - Logs connection errors

_scheduleRequests()

Assigns piece download requests to available peers.

// Iterates through peers and assigns missing pieces
for (let [peerId, peerInfo] of this.knownPeers.entries()) {
  if (!peerInfo.busy && peerInfo.availablePieces.size > 0) {
    for (let pieceIndex of peerInfo.availablePieces) {
      if (this.missingPieces.has(pieceIndex) && 
          !this.pendingPieces.has(pieceIndex)) {
        this._sendRequest(peerInfo.socket, pieceIndex);
        break;
      }
    }
  }
}

_shouldInitiateConnection(otherPeerId)

Determines whether this node should initiate a connection to avoid duplicates.

otherPeerId

string

required

ID of the other peer

Returns true if this node’s ID is greater (lexicographically) than the other peer’s ID.

return (otherPeerId && this.id && this.id > otherPeerId);

_startProgressInterval()

Starts a periodic timer to display download progress.

// Displays progress every second
setInterval(() => {
  const bytesDone = this.fileSize - (this.missingPieces.size * this.pieceSize);
  const percent = ((bytesDone / this.fileSize) * 100).toFixed(2);
  const speed = bytesDone / elapsed; // bytes per second
  console.log(`Progreso: ${percent}% (${bytesDone}/${this.fileSize} bytes)`);
}, 1000);

Shows:

Download percentage
Bytes downloaded / total bytes
Download speed in KB/s

Core Classes

Protocol

Constructor

Properties

Connection Methods

startListening()

connectToPeer(host, port)

Protocol Methods

_sendHandshake(socket)

_sendBitfield(socket)

_sendRequest(socket, index)

Message Handling Methods

_handleMessage(socket, message)

_handleHandshake(socket, message)

_handleBitfield(socket, message)

_handleRequest(socket, message)

_handlePiece(socket, message)

_handleHave(socket, message)

_handlePeers(socket, message)

Internal Methods

_setupSocketHandlers(socket)

_scheduleRequests()

_shouldInitiateConnection(otherPeerId)

_startProgressInterval()

Build docs developers (and LLMs) love

Core Classes

Protocol

​Constructor

​Properties

​Connection Methods

​startListening()

​connectToPeer(host, port)

​Protocol Methods

​_sendHandshake(socket)

​_sendBitfield(socket)

​_sendRequest(socket, index)

​Message Handling Methods

​_handleMessage(socket, message)

​_handleHandshake(socket, message)

​_handleBitfield(socket, message)

​_handleRequest(socket, message)

​_handlePiece(socket, message)

​_handleHave(socket, message)

​_handlePeers(socket, message)

​Internal Methods

​_setupSocketHandlers(socket)

​_scheduleRequests()

​_shouldInitiateConnection(otherPeerId)

​_startProgressInterval()

Build docs developers (and LLMs) love

Constructor

Properties

Connection Methods

startListening()

connectToPeer(host, port)

Protocol Methods

_sendHandshake(socket)

_sendBitfield(socket)

_sendRequest(socket, index)

Message Handling Methods

_handleMessage(socket, message)

_handleHandshake(socket, message)

_handleBitfield(socket, message)

_handleRequest(socket, message)

_handlePiece(socket, message)

_handleHave(socket, message)

_handlePeers(socket, message)

Internal Methods

_setupSocketHandlers(socket)

_scheduleRequests()

_shouldInitiateConnection(otherPeerId)

_startProgressInterval()