QUIC Protocol Implementation Samples

// HTTP/3 Server Implementation with QUIC Transport
// Complete server setup with TLS 1.3 and HTTP/3 features

const { createServer } = require('http3');
const fs = require('fs');
const path = require('path');

class HTTP3Server {
  constructor(options = {}) {
    this.options = {
      port: options.port || 4433,
      host: options.host || '0.0.0.0',
      key: options.key || fs.readFileSync(path.join(__dirname, 'server.key')),
      cert: options.cert || fs.readFileSync(path.join(__dirname, 'server.crt')),
      ca: options.ca || fs.readFileSync(path.join(__dirname, 'ca.crt')),
      maxStreams: options.maxStreams || 100,
      idleTimeout: options.idleTimeout || 30000,
      ...options
    };

    this.server = null;
    this.streamHandlers = new Map();
    this.middleware = [];
  }

  start() {
    this.server = createServer({
      key: this.options.key,
      cert: this.options.cert,
      ca: this.options.ca,
      alpn: 'h3',
      settings: {
        maxConcurrentStreams: this.options.maxStreams,
        idleTimeout: this.options.idleTimeout,
        maxStreamData: 1024 * 1024, // 1MB
        maxData: 10 * 1024 * 1024, // 10MB
        initialMaxData: 1024 * 1024
      }
    });

    // Setup request handling
    this.server.on('request', (req, res) => {
      this.handleRequest(req, res);
    });

    // Setup stream handling
    this.server.on('stream', (stream, headers) => {
      this.handleStream(stream, headers);
    });

    // Setup connection handling
    this.server.on('connection', (connection) => {
      this.handleConnection(connection);
    });

    // Setup error handling
    this.server.on('error', (error) => {
      console.error('HTTP/3 server error:', error);
    });

    // Start listening
    this.server.listen(this.options.port, this.options.host, () => {
      console.log(`HTTP/3 server listening on https://${this.options.host}:${this.options.port}`);
    });
  }

  async handleRequest(req, res) {
    try {
      // Apply middleware
      for (const middleware of this.middleware) {
        const result = await middleware(req, res);
        if (result === false) return; // Middleware handled the response
      }

      // Route handling
      const route = this.matchRoute(req.method, req.url);

      if (route) {
        await route.handler(req, res);
      } else {
        // Static file serving
        await this.serveStaticFile(req, res);
      }

    } catch (error) {
      console.error('Request handling error:', error);
      res.statusCode = 500;
      res.end('Internal Server Error');
    }
  }

  handleStream(stream, headers) {
    const streamId = headers[':path'] || 'unknown';
    console.log(`New HTTP/3 stream: ${streamId}`);

    stream.on('data', (chunk) => {
      console.log(`Received data on stream ${streamId}:`, chunk.length);
    });

    stream.on('end', () => {
      console.log(`Stream ${streamId} ended`);
    });

    stream.on('error', (error) => {
      console.error(`Stream ${streamId} error:`, error);
    });

    // Send response
    stream.write('HTTP/3 Stream Response\n');
    stream.end();
  }

  handleConnection(connection) {
    console.log('New HTTP/3 connection established');

    connection.on('close', () => {
      console.log('HTTP/3 connection closed');
    });

    connection.on('error', (error) => {
      console.error('HTTP/3 connection error:', error);
    });

    // Connection metrics
    this.updateConnectionMetrics(connection);
  }

  // Route system
  get(path, handler) {
    this.addRoute('GET', path, handler);
  }

  post(path, handler) {
    this.addRoute('POST', path, handler);
  }

  put(path, handler) {
    this.addRoute('PUT', path, handler);
  }

  delete(path, handler) {
    this.addRoute('DELETE', path, handler);
  }

  addRoute(method, path, handler) {
    const key = `${method}:${path}`;
    this.streamHandlers.set(key, { method, path, handler });
  }

  matchRoute(method, url) {
    const path = new URL(url, `http://${this.options.host}`).pathname;
    const key = `${method}:${path}`;
    return this.streamHandlers.get(key);
  }

  // Middleware system
  use(middleware) {
    this.middleware.push(middleware);
  }

  // Static file serving
  async serveStaticFile(req, res) {
    const url = new URL(req.url, `http://${this.options.host}`);
    let filePath = url.pathname;

    // Default to index.html
    if (filePath === '/') {
      filePath = '/index.html';
    }

    // Security: prevent directory traversal
    filePath = path.normalize(filePath).replace(/^\.\./, '');
    const fullPath = path.join(__dirname, 'public', filePath);

    try {
      const stats = await fs.promises.stat(fullPath);

      if (stats.isDirectory()) {
        filePath = path.join(filePath, 'index.html');
      }

      const data = await fs.promises.readFile(fullPath);
      const ext = path.extname(fullPath);

      // Set appropriate content type
      const contentType = this.getContentType(ext);
      res.setHeader('Content-Type', contentType);

      // Enable HTTP/3 specific features
      res.setHeader('Cache-Control', 'public, max-age=3600');
      res.setHeader('Vary', 'Accept-Encoding');

      // HTTP/3 Early Hints
      if (ext === '.html') {
        res.writeEarlyHints({
          link: [
            '</style.css>; rel=preload; as=style',
            '</script.js>; rel=preload; as=script'
          ]
        });
      }

      res.end(data);

    } catch (error) {
      if (error.code === 'ENOENT') {
        res.statusCode = 404;
        res.end('File Not Found');
      } else {
        console.error('Static file error:', error);
        res.statusCode = 500;
        res.end('Internal Server Error');
      }
    }
  }

  getContentType(ext) {
    const contentTypes = {
      '.html': 'text/html',
      '.css': 'text/css',
      '.js': 'application/javascript',
      '.json': 'application/json',
      '.png': 'image/png',
      '.jpg': 'image/jpeg',
      '.gif': 'image/gif',
      '.svg': 'image/svg+xml',
      '.ico': 'image/x-icon',
      '.wasm': 'application/wasm'
    };
    return contentTypes[ext] || 'application/octet-stream';
  }

  // HTTP/3 specific features
  updateConnectionMetrics(connection) {
    const metrics = {
      remoteAddress: connection.remoteAddress,
      state: connection.state,
      version: connection.version,
      alpn: connection.alpn
    };
    console.log('Connection metrics:', metrics);
  }

  // Push resources (HTTP/3 equivalent)
  pushResources(res, resources) {
    for (const resource of resources) {
      res.push(resource);
    }
  }

  stop() {
    if (this.server) {
      this.server.close();
      console.log('HTTP/3 server stopped');
    }
  }
}

// Usage example with HTTP/3 features
const server = new HTTP3Server({
  port: 4433,
  maxStreams: 200,
  idleTimeout: 60000
});

// Add middleware for logging
server.use((req, res) => {
  console.log(`${new Date().toISOString()} - ${req.method} ${req.url}`);
});

// Add routes
server.get('/api/time', (req, res) => {
  res.setHeader('Content-Type', 'application/json');
  res.end(JSON.stringify({
    time: new Date().toISOString(),
    protocol: 'HTTP/3',
    version: '1.0'
  }));
});

server.post('/api/echo', async (req, res) => {
  const chunks = [];
  req.on('data', chunk => chunks.push(chunk));

  req.on('end', () => {
    const body = Buffer.concat(chunks);
    res.setHeader('Content-Type', 'application/json');
    res.end(JSON.stringify({
      echo: body.toString(),
      length: body.length,
      timestamp: Date.now()
    }));
  });
});

// WebSocket alternative with HTTP/3
server.get('/ws', (req, res) => {
  // HTTP/3 doesn't have native WebSocket, but we can simulate
  // with unidirectional streams
  console.log('WebSocket-like connection request');

  res.writeHead(200, {
    'Content-Type': 'text/plain',
    'Protocol': 'websocket-like'
  });

  // Stream-based communication
  setInterval(() => {
    res.write(`data: ${JSON.stringify({ time: Date.now() })}\n\n`);
  }, 1000);
});

// Server-Sent Events with HTTP/3
server.get('/events', (req, res) => {
  res.writeHead(200, {
    'Content-Type': 'text/event-stream',
    'Cache-Control': 'no-cache',
    'Connection': 'keep-alive'
  });

  const sendEvent = (event, data) => {
    res.write(`event: ${event}\ndata: ${JSON.stringify(data)}\n\n`);
  };

  // Send periodic events
  const interval = setInterval(() => {
    sendEvent('ping', { time: Date.now() });
  }, 5000);

  req.on('close', () => {
    clearInterval(interval);
  });
});

// Start server
server.start();

// Graceful shutdown
process.on('SIGINT', () => {
  console.log('Shutting down HTTP/3 server...');
  server.stop();
  process.exit(0);
});

// QUIC Reliable Transport over UDP
// Implementation with congestion control, flow control, and reliability

const dgram = require('dgram');
const crypto = require('crypto');

class QUICConnection {
  constructor(socket, remoteAddress, remotePort, isClient = false) {
    this.socket = socket;
    this.remoteAddress = remoteAddress;
    this.remotePort = remotePort;
    this.isClient = isClient;

    // QUIC connection state
    this.connectionId = crypto.randomBytes(16);
    this.state = 'idle'; // idle, handshake, connected, closing, closed
    this.version = 1;

    // Streams management
    this.streams = new Map(); // streamId -> QUICStream
    this.nextStreamId = isClient ? 0 : 1;
    this.maxStreamId = 2 ** 60 - 4;

    // Flow control
    this.flowControlWindow = 1024 * 1024; // 1MB
    this.maxStreamData = 256 * 1024; // 256KB per stream
    this.maxData = 10 * 1024 * 1024; // 10MB total

    // Congestion control
    this.congestionControl = new CubicCongestionControl();

    // Buffers
    this.sendBuffer = [];
    this.receiveBuffer = new Map(); // packetNumber -> buffer
    this.ackedPackets = new Set();

    // Encryption
    this.keys = null;
    this.iv = crypto.randomBytes(12);

    // Timers
    this.ackTimer = null;
    this.retransmissionTimer = null;
    this.idleTimer = null;

    this.setupPacketHandling();
  }

  setupPacketHandling() {
    this.packetNumber = 0;
    this.largestAckedPacket = 0;
  }

  async connect() {
    this.state = 'handshake';

    // Send initial packet
    const initialPacket = this.createPacket({
      type: 'initial',
      version: this.version,
      connectionId: this.connectionId,
      packetNumber: ++this.packetNumber,
      payload: this.createCryptoHandshake()
    });

    await this.sendPacket(initialPacket);
    this.startRetransmissionTimer();
  }

  createPacket(options) {
    const packet = {
      header: {
        type: options.type,
        version: options.version,
        connectionId: options.connectionId,
        packetNumber: options.packetNumber
      },
      payload: options.payload || Buffer.alloc(0)
    };

    // Add QUIC frame headers
    if (options.frames) {
      packet.frames = options.frames;
    }

    return packet;
  }

  createCryptoHandshake() {
    // Simplified crypto handshake
    const handshake = {
      crypto: {
        algorithms: ['AES-256-GCM', 'ChaCha20-Poly1305'],
        hash: 'SHA-256',
        keyExchange: 'X25519'
      },
      parameters: {
        maxIdleTimeout: 30000,
        maxPacketSize: 1200,
        initialMaxData: this.maxData,
        initialMaxStreamData: this.maxStreamData
      }
    };

    return Buffer.from(JSON.stringify(handshake));
  }

  async sendPacket(packet) {
    try {
      const encryptedPacket = await this.encryptPacket(packet);
      const packetBuffer = this.serializePacket(encryptedPacket);

      this.socket.send(packetBuffer, this.remotePort, this.remoteAddress);

      console.log(`Sent QUIC packet ${packet.packetNumber} to ${this.remoteAddress}:${this.remotePort}`);

    } catch (error) {
      console.error('Error sending packet:', error);
    }
  }

  async encryptPacket(packet) {
    if (!this.keys) {
      return packet; // Not yet encrypted
    }

    // Implement AEAD encryption (simplified)
    const plaintext = Buffer.from(JSON.stringify(packet));
    const aad = this.createAdditionalAuthenticatedData(packet);

    // Use crypto.createCipheriv for actual implementation
    // This is a simplified version
    return {
      encrypted: plaintext,
      aad: aad
    };
  }

  createAdditionalAuthenticatedData(packet) {
    const header = packet.header;
    return Buffer.from(`${header.type}|${header.version}|${header.packetNumber}`);
  }

  serializePacket(packet) {
    // Simplified packet serialization
    const header = Buffer.alloc(16);
    header.writeUInt8(0x01, 0); // Long header
    header.writeUInt8(this.version, 1);
    header.writeUInt8(0x01, 2); // Initial packet type
    connectionId.copy(header, 4);

    const payload = packet.payload || Buffer.alloc(0);
    return Buffer.concat([header, payload]);
  }

  handleReceivedPacket(packetBuffer) {
    try {
      const packet = this.deserializePacket(packetBuffer);

      if (this.largestAckedPacket < packet.packetNumber) {
        this.largestAckedPacket = packet.packetNumber;
      }

      switch (packet.type) {
        case 'initial':
          this.handleInitialPacket(packet);
          break;
        case 'handshake':
          this.handleHandshakePacket(packet);
          break;
        case '1-RTT':
          this.handleProtectedPacket(packet);
          break;
        default:
          console.log(`Unknown packet type: ${packet.type}`);
      }

      // Send acknowledgment
      this.sendAck();

    } catch (error) {
      console.error('Error handling packet:', error);
    }
  }

  deserializePacket(buffer) {
    const header = {
      type: '1-RTT',
      version: 1,
      packetNumber: 0,
      connectionId: buffer.slice(4, 20)
    };

    return {
      header: header,
      type: header.type,
      version: header.version,
      packetNumber: header.packetNumber,
      connectionId: header.connectionId,
      payload: buffer.slice(20)
    };
  }

  handleInitialPacket(packet) {
    console.log('Handling initial packet');

    if (this.state === 'idle' && this.isClient) {
      // Server responding to client
      this.processHandshake(packet.payload);
    }
  }

  handleHandshakePacket(packet) {
    console.log('Handling handshake packet');
    this.processHandshake(packet.payload);
  }

  handleProtectedPacket(packet) {
    console.log('Handling protected packet');
    this.processFrames(packet.payload);
  }

  processHandshake(payload) {
    try {
      const handshake = JSON.parse(payload.toString());
      console.log('Handshake parameters:', handshake);

      // Setup encryption keys (simplified)
      this.setupEncryption(handshake.crypto);

      // Update flow control parameters
      this.flowControlWindow = handshake.parameters.initialMaxData;
      this.maxStreamData = handshake.parameters.initialMaxStreamData;

      this.state = 'connected';
      console.log('QUIC connection established');

    } catch (error) {
      console.error('Error processing handshake:', error);
    }
  }

  processFrames(payload) {
    // Process QUIC frames (STREAM, ACK, PING, etc.)
    const frames = this.parseFrames(payload);

    for (const frame of frames) {
      switch (frame.type) {
        case 'STREAM':
          this.handleStreamFrame(frame);
          break;
        case 'ACK':
          this.handleAckFrame(frame);
          break;
        case 'PING':
          this.handlePingFrame(frame);
          break;
        case 'CONNECTION_CLOSE':
          this.handleConnectionCloseFrame(frame);
          break;
      }
    }
  }

  parseFrames(payload) {
    const frames = [];
    let offset = 0;

    while (offset < payload.length) {
      const frameType = payload[offset++];

      switch (frameType) {
        case 0x00: // STREAM frame
          const streamFrame = this.parseStreamFrame(payload, offset);
          frames.push(streamFrame);
          offset += streamFrame.length;
          break;
        case 0x02: // ACK frame
          const ackFrame = this.parseAckFrame(payload, offset);
          frames.push(ackFrame);
          offset += ackFrame.length;
          break;
        case 0x01: // PING frame
          frames.push({ type: 'PING', length: 1 });
          break;
        default:
          // Skip unknown frame types
          offset = payload.length;
      }
    }

    return frames;
  }

  parseStreamFrame(buffer, offset) {
    const streamId = this.readVariableLengthInteger(buffer, offset);
    offset += streamId.length;

    const offsetData = this.readVariableLengthInteger(buffer, offset);
    offset += offsetData.length;

    const length = this.readVariableLengthInteger(buffer, offset);
    offset += length.length;

    const data = buffer.slice(offset, offset + length.value);

    return {
      type: 'STREAM',
      streamId: streamId.value,
      offset: offsetData.value,
      length: length.value,
      data: data,
      fin: buffer[offset + length.value] === 0x01
    };
  }

  parseAckFrame(buffer, offset) {
    const largestAcked = this.readVariableLengthInteger(buffer, offset);
    offset += largestAcked.length;

    const ackDelay = this.readVariableLengthInteger(buffer, offset);
    offset += ackDelay.length;

    return {
      type: 'ACK',
      largestAcked: largestAcked.value,
      ackDelay: ackDelay.value,
      length: largestAcked.length + ackDelay.length + 1
    };
  }

  readVariableLengthInteger(buffer, offset) {
    const first = buffer[offset];
    let value = first & 0x3f;
    let length = 1;

    if (first & 0xc0) {
      const additionalBytes = Math.floor((first & 0xc0) >> 6);
      length += additionalBytes;

      for (let i = 1; i <= additionalBytes; i++) {
        value = (value << 8) | buffer[offset + i];
      }
    }

    return { value, length };
  }

  handleStreamFrame(frame) {
    const streamId = frame.streamId;

    if (!this.streams.has(streamId)) {
      this.streams.set(streamId, new QUICStream(streamId, this));
    }

    const stream = this.streams.get(streamId);
    stream.handleData(frame.offset, frame.data, frame.fin);
  }

  handleAckFrame(frame) {
    console.log(`Received ACK for packet ${frame.largestAcked}`);

    // Update congestion control
    this.congestionControl.onPacketAcked(frame.largestAcked);

    // Clear retransmission timer
    if (this.retransmissionTimer) {
      clearTimeout(this.retransmissionTimer);
      this.retransmissionTimer = null;
    }
  }

  handlePingFrame(frame) {
    console.log('Received PING frame');
    this.sendPing();
  }

  handleConnectionCloseFrame(frame) {
    console.log('Connection closed by peer');
    this.close();
  }

  // Stream management
  createStream() {
    const streamId = this.nextStreamId;
    this.nextStreamId += 4; // Only client or server initiated streams

    const stream = new QUICStream(streamId, this);
    this.streams.set(streamId, stream);

    return stream;
  }

  getStream(streamId) {
    return this.streams.get(streamId);
  }

  // Congestion control
  async sendData(streamId, data, fin = false) {
    const stream = this.streams.get(streamId);
    if (!stream) {
      throw new Error(`Stream ${streamId} not found`);
    }

    // Check flow control
    if (stream.bytesSent + data.length > this.maxStreamData) {
      throw new Error('Flow control limit exceeded');
    }

    // Check congestion control
    if (!this.congestionControl.canSend(data.length)) {
      // Buffer data until we can send
      stream.bufferData(data);
      return;
    }

    // Create stream frame
    const frame = {
      type: 'STREAM',
      streamId: streamId,
      data: data,
      fin: fin
    };

    // Send packet with stream frame
    const packet = this.createPacket({
      type: '1-RTT',
      packetNumber: ++this.packetNumber,
      frames: [frame]
    });

    await this.sendPacket(packet);

    // Update congestion control
    this.congestionControl.onPacketSent(data.length);

    // Start retransmission timer
    this.startRetransmissionTimer();

    console.log(`Sent ${data.length} bytes on stream ${streamId}`);
  }

  sendAck() {
    const ackFrame = {
      type: 'ACK',
      largestAcked: this.largestAckedPacket,
      ackDelay: 0,
      ackRanges: [[this.largestAckedPacket, this.largestAckedPacket]]
    };

    const packet = this.createPacket({
      type: '1-RTT',
      packetNumber: ++this.packetNumber,
      frames: [ackFrame]
    });

    this.sendPacket(packet);
  }

  sendPing() {
    const pingFrame = { type: 'PING' };

    const packet = this.createPacket({
      type: '1-RTT',
      packetNumber: ++this.packetNumber,
      frames: [pingFrame]
    });

    this.sendPacket(packet);
  }

  startRetransmissionTimer() {
    if (this.retransmissionTimer) {
      clearTimeout(this.retransmissionTimer);
    }

    this.retransmissionTimer = setTimeout(() => {
      this.handleRetransmissionTimeout();
    }, this.congestionControl.getRetransmissionTimeout());
  }

  handleRetransmissionTimeout() {
    console.log('Retransmission timeout');
    this.congestionControl.onRetransmissionTimeout();
    // Retransmit unacknowledged packets
    this.retransmitUnackedPackets();
  }

  retransmitUnackedPackets() {
    // Implement packet retransmission logic
    console.log('Retransmitting unacked packets');
  }

  setupEncryption(cryptoParams) {
    // Simplified encryption setup
    console.log('Setting up encryption with:', cryptoParams);
    // In real implementation, derive keys from TLS handshake
  }

  close() {
    this.state = 'closing';

    // Send connection close frame
    const closeFrame = {
      type: 'CONNECTION_CLOSE',
      errorCode: 0,
      reason: 'Connection closed'
    };

    const packet = this.createPacket({
      type: '1-RTT',
      packetNumber: ++this.packetNumber,
      frames: [closeFrame]
    });

    this.sendPacket(packet);

    // Clean up timers
    if (this.retransmissionTimer) {
      clearTimeout(this.retransmissionTimer);
    }

    this.state = 'closed';
  }
}

class QUICStream {
  constructor(streamId, connection) {
    this.id = streamId;
    this.connection = connection;
    this.state = 'idle'; // idle, open, closed
    this.buffer = [];
    this.bytesReceived = 0;
    this.bytesSent = 0;
    this.flowControlWindow = 64 * 1024; // 64KB
    this.maxData = 256 * 1024; // 256KB
  }

  handleData(offset, data, fin) {
    this.bytesReceived += data.length;

    // Add data to buffer at correct offset
    this.buffer.push({ offset, data });
    this.buffer.sort((a, b) => a.offset - b.offset);

    if (fin) {
      this.state = 'closed';
    }

    // Trigger data available event
    this.onDataAvailable();
  }

  onDataAvailable() {
    // Emit data to application
    const data = this.getBufferedData();
    if (data) {
      console.log(`Stream ${this.id} received ${data.length} bytes`);
      this.emit('data', data);
    }
  }

  getBufferedData() {
    // Return contiguous buffered data
    let data = Buffer.alloc(0);
    let expectedOffset = 0;

    for (const chunk of this.buffer) {
      if (chunk.offset === expectedOffset) {
        data = Buffer.concat([data, chunk.data]);
        expectedOffset += chunk.data.length;
      } else {
        // Gap in data, wait for more
        break;
      }
    }

    return data.length > 0 ? data : null;
  }

  async write(data) {
    if (this.state === 'closed') {
      throw new Error('Stream is closed');
    }

    await this.connection.sendData(this.id, data, false);
    this.bytesSent += data.length;
  }

  async end(data = null) {
    if (data) {
      await this.write(data);
    }

    await this.connection.sendData(this.id, Buffer.alloc(0), true);
    this.state = 'closed';
  }

  emit(event, data) {
    // Simplified event emission
    if (this.listeners && this.listeners[event]) {
      this.listeners[event].forEach(callback => callback(data));
    }
  }

  on(event, callback) {
    if (!this.listeners) {
      this.listeners = {};
    }
    if (!this.listeners[event]) {
      this.listeners[event] = [];
    }
    this.listeners[event].push(callback);
  }

  bufferData(data) {
    this.buffer.push({ offset: this.bytesSent, data });
  }
}

class CubicCongestionControl {
  constructor() {
    this.cwnd = 10 * 1024; // Initial congestion window
    this.ssthresh = Infinity; // Slow start threshold
    this.minCwnd = 2 * 1024; // Minimum congestion window
    this.maxCwnd = 10 * 1024 * 1024; // Maximum congestion window
    this.reno = 0; // Reno recovery
    this.lastMaxCwnd = 0;

    this.rtt = 100; // Initial RTT estimate in ms
    this.rttVar = 50; // RTT variance
    this.rto = 1000; // Retransmission timeout

    this.packetsInFlight = 0;
    this.bytesInFlight = 0;
    this.endOfRecovery = null;
  }

  canSend(bytes) {
    return this.bytesInFlight + bytes <= this.cwnd;
  }

  onPacketSent(bytes) {
    this.packetsInFlight++;
    this.bytesInFlight += bytes;
  }

  onPacketAcked(packetNumber) {
    this.packetsInFlight--;

    if (this.endOfRecovery && packetNumber < this.endOfRecovery) {
      // Still in recovery
      return;
    }

    // Update congestion window based on current phase
    if (this.cwnd <= this.ssthresh) {
      // Slow start
      this.cwnd += Math.min(bytes, 1500); // Acked bytes or 1 MSS
    } else {
      // Congestion avoidance (CUBIC)
      const timeSinceLastMaxCwnd = Date.now() - this.lastMaxCwndTime;
      const k = Math.cbrt(this.lastMaxCwnd / 0.4); // CUBIC constant
      const w = this.lastMaxCwnd + 0.4 * Math.pow(timeSinceLastMaxCwnd / 1000 - k, 3);

      this.cwnd = Math.min(w, this.maxCwnd);
    }

    this.endOfRecovery = null;
    this.bytesInFlight = Math.max(0, this.bytesInFlight - 1500);
  }

  onPacketLost() {
    console.log('Packet lost, entering congestion recovery');

    this.lastMaxCwnd = Math.max(this.cwnd, this.lastMaxCwnd);
    this.lastMaxCwndTime = Date.now();

    // Reduce congestion window
    this.ssthresh = this.cwnd / 2;
    this.cwnd = this.cwnd / 2;
    this.cwnd = Math.max(this.cwnd, this.minCwnd);

    this.endOfRecovery = Date.now() + this.rtt;
  }

  onRetransmissionTimeout() {
    console.log('Retransmission timeout');

    this.ssthresh = this.cwnd / 2;
    this.cwnd = this.minCwnd;

    // Reset RTT estimates
    this.rto = Math.min(this.rto * 2, 60000); // Exponential backoff
  }

  updateRTT(newRTT) {
    if (this.rtt === 0) {
      this.rtt = newRTT;
      this.rttVar = newRTT / 2;
    } else {
      this.rttVar = (3 * this.rttVar + Math.abs(newRTT - this.rtt)) / 4;
      this.rtt = (7 * this.rtt + newRTT) / 8;
    }

    this.rto = this.rtt + 4 * this.rttVar;
    this.rto = Math.min(Math.max(this.rto, 200), 60000); // Clamp between 200ms and 60s
  }

  getRetransmissionTimeout() {
    return this.rto;
  }
}

// Server implementation
class QUICServer {
  constructor(port = 0) {
    this.port = port;
    this.socket = null;
    this.connections = new Map();
    this.connectionId = 0;
  }

  start() {
    this.socket = dgram.createSocket('udp4');

    this.socket.on('message', (msg, rinfo) => {
      this.handlePacket(msg, rinfo);
    });

    this.socket.on('listening', () => {
      const address = this.socket.address();
      console.log(`QUIC server listening on ${address.address}:${address.port}`);
    });

    this.socket.bind(this.port);
  }

  handlePacket(buffer, rinfo) {
    const connectionKey = `${rinfo.address}:${rinfo.port}`;

    if (!this.connections.has(connectionKey)) {
      console.log(`New connection from ${connectionKey}`);

      const connection = new QUICConnection(
        this.socket,
        rinfo.address,
        rinfo.port,
        false // Server connection
      );

      this.connections.set(connectionKey, connection);

      // Clean up on connection close
      connection.on('close', () => {
        this.connections.delete(connectionKey);
        console.log(`Connection ${connectionKey} closed`);
      });
    }

    const connection = this.connections.get(connectionKey);
    connection.handleReceivedPacket(buffer);
  }

  stop() {
    if (this.socket) {
      this.socket.close();
      console.log('QUIC server stopped');
    }
  }
}

// Usage example
const server = new QUICServer(4433);
server.start();

console.log('QUIC Reliable Transport Server started');

// Client example
const clientSocket = dgram.createSocket('udp4');
const client = new QUICConnection(clientSocket, '127.0.0.1', 4433, true);

client.on('connected', () => {
  console.log('QUIC connection established');

  // Create a stream and send data
  const stream = client.createStream();
  stream.write('Hello, QUIC!');
  stream.end();
});

client.on('data', (data) => {
  console.log('Received data:', data.toString());
});

// Start connection
client.connect();

// Handle incoming packets
clientSocket.on('message', (msg, rinfo) => {
  if (rinfo.address === '127.0.0.1' && rinfo.port === 4433) {
    client.handleReceivedPacket(msg);
  }
});

clientSocket.bind(); // Bind to random port

process.on('SIGINT', () => {
  server.stop();
  clientSocket.close();
  process.exit(0);
});

// QUIC Connection Migration Implementation
// Seamless migration between network interfaces and IP addresses

const dgram = require('dgram');
const os = require('os');
const EventEmitter = require('events');

class QUICConnectionMigrator extends EventEmitter {
  constructor(quicConnection) {
    super();
    this.quicConnection = quicConnection;

    // Migration state
    this.activeConnections = new Map(); // interface -> socket
    this.primaryInterface = null;
    this.connectionId = quicConnection.connectionId;

    // Network interfaces
    this.interfaces = new Map(); // name -> interface info
    this.monitorInterval = null;

    // Path management
    this.paths = new Map(); // pathId -> path info
    this.activePath = null;
    this.pathValidation = new Map(); // pathId -> validation state

    // Migration policies
    this.migrationPolicy = 'automatic'; // automatic, manual, disabled
    this.migrationThreshold = {
      packetLoss: 0.1, // 10% packet loss triggers migration
      latency: 500, // 500ms latency increase triggers migration
      bandwidth: 100 // 100KB/s bandwidth drop triggers migration
    };

    this.initializeInterfaces();
    this.startMonitoring();
  }

  initializeInterfaces() {
    const networkInterfaces = os.networkInterfaces();

    for (const [name, interfaces] of Object.entries(networkInterfaces)) {
      for (const iface of interfaces) {
        if (iface.family === 'IPv4' && !iface.internal) {
          this.interfaces.set(name, {
            name: name,
            address: iface.address,
            netmask: iface.netmask,
            mac: iface.mac,
            family: iface.family,
            internal: iface.internal
          });

          // Create socket for each interface
          this.createSocketForInterface(name, iface.address);
        }
      }
    }

    console.log('Available network interfaces:', Array.from(this.interfaces.keys()));
  }

  createSocketForInterface(interfaceName, address) {
    try {
      const socket = dgram.createSocket('udp4');

      socket.bind({ address: address, port: 0 });

      socket.on('message', (msg, rinfo) => {
        this.handlePacket(msg, rinfo, interfaceName);
      });

      socket.on('error', (error) => {
        console.error(`Socket error for interface ${interfaceName}:`, error);
        this.emit('socketError', { interfaceName, error });
      });

      this.activeConnections.set(interfaceName, socket);

      // Set as primary if this is the first interface
      if (!this.primaryInterface) {
        this.primaryInterface = interfaceName;
        console.log(`Primary interface set to: ${interfaceName} (${address})`);
      }

    } catch (error) {
      console.error(`Failed to create socket for interface ${interfaceName}:`, error);
    }
  }

  startMonitoring() {
    this.monitorInterval = setInterval(() => {
      this.monitorInterfaces();
      this.analyzePerformance();
      this.checkMigrationConditions();
    }, 1000); // Monitor every second
  }

  monitorInterfaces() {
    const currentInterfaces = os.networkInterfaces();
    const activeInterfaceNames = new Set();

    for (const [name, interfaces] of Object.entries(currentInterfaces)) {
      for (const iface of interfaces) {
        if (iface.family === 'IPv4' && !iface.internal) {
          activeInterfaceNames.add(name);

          if (!this.interfaces.has(name)) {
            console.log(`New interface detected: ${name}`);
            this.emit('interfaceAdded', { name, interface: iface });
          }
        }
      }
    }

    // Check for removed interfaces
    for (const [name, iface] of this.interfaces.entries()) {
      if (!activeInterfaceNames.has(name)) {
        console.log(`Interface removed: ${name}`);
        this.interfaces.delete(name);

        const socket = this.activeConnections.get(name);
        if (socket) {
          socket.close();
          this.activeConnections.delete(name);
        }

        this.emit('interfaceRemoved', { name, interface: iface });

        // Migrate if primary interface was removed
        if (this.primaryInterface === name) {
          this.selectNewPrimaryInterface();
        }
      }
    }
  }

  analyzePerformance() {
    for (const [interfaceName, path] of this.paths.entries()) {
      const metrics = this.calculatePathMetrics(path);

      // Update path metrics
      path.metrics = {
        ...path.metrics,
        ...metrics,
        lastUpdate: Date.now()
      };

      // Emit performance events
      this.emit('pathMetrics', {
        interfaceName,
        metrics
      });
    }
  }

  calculatePathMetrics(path) {
    const now = Date.now();
    const recentPackets = path.packets.filter(
      p => now - p.timestamp < 5000 // Last 5 seconds
    );

    const metrics = {
      packetLoss: 0,
      latency: 0,
      bandwidth: 0,
      jitter: 0
    };

    if (recentPackets.length === 0) {
      return metrics;
    }

    // Calculate packet loss
    const lostPackets = recentPackets.filter(p => p.lost).length;
    metrics.packetLoss = lostPackets / recentPackets.length;

    // Calculate latency and jitter
    const rttPackets = recentPackets.filter(p => p.rtt);
    if (rttPackets.length > 0) {
      const rtts = rttPackets.map(p => p.rtt);
      metrics.latency = rtts.reduce((sum, rtt) => sum + rtt, 0) / rtts.length;

      // Calculate jitter (RTT variance)
      const avgRtt = metrics.latency;
      metrics.jitter = Math.sqrt(
        rtts.reduce((sum, rtt) => sum + Math.pow(rtt - avgRtt, 2), 0) / rtts.length
      );
    }

    // Calculate bandwidth
    const timeWindow = 5000; // 5 seconds
    const recentBytes = recentPackets.reduce((sum, p) => sum + (p.size || 0), 0);
    metrics.bandwidth = (recentBytes * 8) / (timeWindow / 1000); // bits per second

    return metrics;
  }

  checkMigrationConditions() {
    if (this.migrationPolicy === 'disabled') {
      return;
    }

    if (this.migrationPolicy === 'manual') {
      this.emit('migrationCheck', this.getMigrationRecommendations());
      return;
    }

    // Automatic migration
    const recommendations = this.getMigrationRecommendations();

    for (const rec of recommendations) {
      if (rec.severity === 'critical' || rec.severity === 'high') {
        console.log(`Triggering automatic migration: ${rec.reason}`);
        this.migrateToInterface(rec.targetInterface);
        break;
      }
    }
  }

  getMigrationRecommendations() {
    const recommendations = [];

    for (const [interfaceName, path] of this.paths.entries()) {
      if (!path.metrics) continue;

      const metrics = path.metrics;

      // Check packet loss
      if (metrics.packetLoss > this.migrationThreshold.packetLoss) {
        recommendations.push({
          sourceInterface: interfaceName,
          targetInterface: this.findBestAlternativeInterface(interfaceName),
          reason: `High packet loss: ${(metrics.packetLoss * 100).toFixed(2)}%`,
          severity: metrics.packetLoss > 0.2 ? 'critical' : 'high',
          metrics
        });
      }

      // Check latency
      if (metrics.latency > this.migrationThreshold.latency) {
        recommendations.push({
          sourceInterface: interfaceName,
          targetInterface: this.findBestAlternativeInterface(interfaceName),
          reason: `High latency: ${metrics.latency.toFixed(2)}ms`,
          severity: metrics.latency > 1000 ? 'critical' : 'medium',
          metrics
        });
      }

      // Check bandwidth
      if (metrics.bandwidth < this.migrationThreshold.bandwidth * 1024) {
        recommendations.push({
          sourceInterface: interfaceName,
          targetInterface: this.findBestAlternativeInterface(interfaceName),
          reason: `Low bandwidth: ${(metrics.bandwidth / 1024).toFixed(2)}KB/s`,
          severity: metrics.bandwidth < 50 * 1024 ? 'high' : 'low',
          metrics
        });
      }
    }

    return recommendations.sort((a, b) => {
      const severityOrder = { critical: 3, high: 2, medium: 1, low: 0 };
      return severityOrder[b.severity] - severityOrder[a.severity];
    });
  }

  findBestAlternativeInterface(currentInterface) {
    const alternatives = Array.from(this.interfaces.keys())
      .filter(name => name !== currentInterface);

    // Score each alternative interface
    const scored = alternatives.map(name => {
      const path = this.paths.get(name);
      let score = 100;

      if (path && path.metrics) {
        // Penalize poor performance
        score -= path.metrics.packetLoss * 100;
        score -= path.metrics.latency / 10;
        score += path.metrics.bandwidth / 1000;
      } else {
        // Unknown interface, give neutral score
        score = 50;
      }

      return { name, score };
    });

    // Return the best scoring interface
    scored.sort((a, b) => b.score - a.score);
    return scored[0]?.name;
  }

  async migrateToInterface(targetInterface) {
    if (!this.interfaces.has(targetInterface)) {
      throw new Error(`Target interface ${targetInterface} not available`);
    }

    if (targetInterface === this.primaryInterface) {
      console.log('Already using target interface');
      return;
    }

    console.log(`Migrating from ${this.primaryInterface} to ${targetInterface}`);

    try {
      // Start migration process
      this.emit('migrationStart', {
        from: this.primaryInterface,
        to: targetInterface
      });

      // Create new path
      const newSocket = this.activeConnections.get(targetInterface);
      const oldSocket = this.activeConnections.get(this.primaryInterface);

      // Send PATH_CHALLENGE to validate new path
      await this.validatePath(targetInterface);

      // Update QUIC connection to use new path
      await this.updateConnectionPath(targetInterface);

      // Gracefully switch to new socket
      const success = await this.switchConnectionSockets(oldSocket, newSocket);

      if (success) {
        // Clean up old socket
        this.primaryInterface = targetInterface;

        console.log(`Successfully migrated to ${targetInterface}`);
        this.emit('migrationComplete', {
          from: oldSocket.address,
          to: newSocket.address,
          success: true
        });

      } else {
        throw new Error('Socket switch failed');
      }

    } catch (error) {
      console.error(`Migration to ${targetInterface} failed:`, error);
      this.emit('migrationFailed', {
        from: this.primaryInterface,
        to: targetInterface,
        error: error.message
      });
    }
  }

  async validatePath(interfaceName) {
    return new Promise((resolve, reject) => {
      const socket = this.activeConnections.get(interfaceName);
      if (!socket) {
        reject(new Error(`Socket not available for interface ${interfaceName}`));
        return;
      }

      const pathId = this.generatePathId();
      const challenge = crypto.randomBytes(8);

      // Send PATH_CHALLENGE
      const packet = this.createPathChallengePacket(pathId, challenge);
      socket.send(packet, this.quicConnection.remotePort, this.quicConnection.remoteAddress);

      // Wait for PATH_RESPONSE
      const timeout = setTimeout(() => {
        reject(new Error('Path validation timeout'));
      }, 5000);

      const onResponse = (msg, rinfo) => {
        // Check if this is the PATH_RESPONSE
        if (this.isPathResponse(msg, pathId, challenge)) {
          clearTimeout(timeout);
          socket.off('message', onResponse);
          resolve(true);
        }
      };

      socket.on('message', onResponse);

      // Store validation state
      this.pathValidation.set(pathId, {
        interfaceName,
        challenge,
        startTime: Date.now()
      });
    });
  }

  generatePathId() {
    return Math.floor(Math.random() * 2 ** 32);
  }

  createPathChallengePacket(pathId, challenge) {
    // Create PATH_CHALLENGE frame (simplified)
    const frame = Buffer.alloc(9);
    frame.writeUInt8(0x1A, 0); // PATH_CHALLENGE frame type
    frame.writeUInt32BE(pathId, 1);
    challenge.copy(frame, 5);

    return frame;
  }

  isPathResponse(packet, pathId, challenge) {
    // Simplified PATH_RESPONSE check
    return packet.length >= 9 &&
           packet.readUInt8(0) === 0x1B && // PATH_RESPONSE frame type
           packet.readUInt32BE(1) === pathId;
  }

  async updateConnectionPath(interfaceName) {
    // Update QUIC connection to use new path
    const interfaceInfo = this.interfaces.get(interfaceName);

    // This would integrate with the actual QUIC implementation
    // to update the connection's local address
    console.log(`Updating connection path to ${interfaceName} (${interfaceInfo.address})`);

    // Record the new path
    const pathId = this.generatePathId();
    this.paths.set(pathId, {
      interfaceName,
      address: interfaceInfo.address,
      packets: [],
      created: Date.now()
    });

    this.activePath = pathId;
  }

  async switchConnectionSockets(oldSocket, newSocket) {
    // Implement seamless socket switching
    return new Promise((resolve) => {
      // Buffer packets on old socket
      const buffer = [];

      const oldPacketHandler = (msg, rinfo) => {
        buffer.push({ msg, rinfo });
      };

      oldSocket.on('message', oldPacketHandler);

      // Small delay to ensure no packets are lost
      setTimeout(() => {
        oldSocket.off('message', oldPacketHandler);

        // Replay buffered packets on new socket
        buffer.forEach(({ msg, rinfo }) => {
          this.quicConnection.handleReceivedPacket(msg, rinfo);
        });

        resolve(true);
      }, 100);
    });
  }

  selectNewPrimaryInterface() {
    const availableInterfaces = Array.from(this.interfaces.keys());

    if (availableInterfaces.length === 0) {
      console.log('No network interfaces available');
      this.emit('noInterfacesAvailable');
      return;
    }

    // Select the first available interface
    const newPrimary = availableInterfaces[0];
    this.primaryInterface = newPrimary;

    console.log(`New primary interface: ${newPrimary}`);
    this.emit('primaryInterfaceChanged', { newInterface: newPrimary });
  }

  handlePacket(packet, rinfo, interfaceName) {
    // Record packet metrics for path analysis
    this.recordPacketMetrics(interfaceName, packet);

    // Check if this packet is for our connection
    if (this.shouldHandlePacket(packet, interfaceName)) {
      this.quicConnection.handleReceivedPacket(packet);
    }
  }

  shouldHandlePacket(packet, interfaceName) {
    // Simplified check - in real implementation, verify connection ID
    return interfaceName === this.primaryInterface || this.paths.size > 0;
  }

  recordPacketMetrics(interfaceName, packet) {
    if (!this.paths.has(interfaceName)) {
      return;
    }

    const path = this.paths.get(interfaceName);
    path.packets.push({
      timestamp: Date.now(),
      size: packet.length,
      lost: false
    });

    // Clean up old packets (older than 1 minute)
    const cutoff = Date.now() - 60000;
    path.packets = path.packets.filter(p => p.timestamp > cutoff);
  }

  // Public API
  setMigrationPolicy(policy) {
    this.migrationPolicy = policy;
    console.log(`Migration policy set to: ${policy}`);
  }

  setMigrationThresholds(thresholds) {
    this.migrationThreshold = { ...this.migrationThreshold, ...thresholds };
    console.log('Migration thresholds updated:', this.migrationThreshold);
  }

  getInterfaceStatus() {
    return Array.from(this.interfaces.entries()).map(([name, info]) => ({
      name,
      address: info.address,
      isPrimary: name === this.primaryInterface,
      hasConnection: this.activeConnections.has(name)
    }));
  }

  getPathMetrics() {
    const metrics = {};

    for (const [pathId, path] of this.paths.entries()) {
      metrics[pathId] = {
        interfaceName: path.interfaceName,
        address: path.address,
        metrics: path.metrics,
        created: path.created
      };
    }

    return metrics;
  }

  stop() {
    if (this.monitorInterval) {
      clearInterval(this.monitorInterval);
      this.monitorInterval = null;
    }

    // Close all sockets
    for (const socket of this.activeConnections.values()) {
      socket.close();
    }

    this.activeConnections.clear();
    console.log('Connection migration stopped');
  }
}

// Usage example with existing QUIC connection
class MigratableQUICConnection {
  constructor() {
    this.baseConnection = null; // Your existing QUICConnection
    this.migrator = null;
  }

  async connect(serverAddress, serverPort) {
    // Create base QUIC connection
    this.baseConnection = new QUICConnection(/* ... */);

    // Add migration support
    this.migrator = new QUICConnectionMigrator(this.baseConnection);

    // Listen for migration events
    this.migrator.on('migrationStart', (info) => {
      console.log('Connection migration starting:', info);
    });

    this.migrator.on('migrationComplete', (info) => {
      console.log('Connection migration complete:', info);
    });

    this.migrator.on('migrationFailed', (info) => {
      console.error('Connection migration failed:', info);
    });

    // Establish initial connection
    await this.baseConnection.connect();
  }

  // Migration control
  enableAutoMigration() {
    this.migrator.setMigrationPolicy('automatic');
  }

  disableMigration() {
    this.migrator.setMigrationPolicy('disabled');
  }

  setCustomMigrationThresholds(thresholds) {
    this.migrator.setMigrationThresholds(thresholds);
  }

  async manualMigration(targetInterface) {
    return await this.migrator.migrateToInterface(targetInterface);
  }

  getConnectionStatus() {
    return {
      interfaces: this.migrator.getInterfaceStatus(),
      paths: this.migrator.getPathMetrics(),
      primaryInterface: this.migrator.primaryInterface
    };
  }

  close() {
    if (this.migrator) {
      this.migrator.stop();
    }

    if (this.baseConnection) {
      this.baseConnection.close();
    }
  }
}

// Example usage
async function demonstrateMigration() {
  const connection = new MigratableQUICConnection();

  // Configure migration
  connection.enableAutoMigration();
  connection.setCustomMigrationThresholds({
    packetLoss: 0.05, // 5% packet loss
    latency: 300,    // 300ms latency
    bandwidth: 200  // 200KB/s minimum
  });

  // Connect to server
  await connection.connect('example.com', 4433);

  // Monitor connection status
  setInterval(() => {
    const status = connection.getConnectionStatus();
    console.log('Connection status:', status);
  }, 5000);

  // Manual migration example
  // await connection.manualMigration('wlan0');

  // Graceful shutdown
  setTimeout(() => {
    connection.close();
  }, 60000);
}

// Network change detection
class NetworkChangeDetector {
  constructor(migrator) {
    this.migrator = migrator;
    this.currentNetworkState = null;
    this.changeThreshold = 3; // Require 3 consecutive changes
    this.changeCount = 0;
  }

  startDetection() {
    setInterval(() => {
      this.detectNetworkChanges();
    }, 2000);
  }

  detectNetworkChanges() {
    const currentState = this.captureNetworkState();

    if (this.currentNetworkState &&
        !this.compareNetworkStates(this.currentNetworkState, currentState)) {
      this.changeCount++;

      if (this.changeCount >= this.changeThreshold) {
        console.log('Network change detected, triggering migration');
        this.migrator.emit('networkChanged', {
          from: this.currentNetworkState,
          to: currentState
        });
        this.changeCount = 0;
      }
    } else {
      this.changeCount = 0;
    }

    this.currentNetworkState = currentState;
  }

  captureNetworkState() {
    const interfaces = os.networkInterfaces();
    const state = {
      timestamp: Date.now(),
      interfaces: []
    };

    for (const [name, ifaces] of Object.entries(interfaces)) {
      for (const iface of ifaces) {
        if (iface.family === 'IPv4' && !iface.internal) {
          state.interfaces.push({
            name,
            address: iface.address,
            mac: iface.mac
          });
        }
      }
    }

    return state;
  }

  compareNetworkStates(state1, state2) {
    if (state1.interfaces.length !== state2.interfaces.length) {
      return false;
    }

    const sorted1 = state1.interfaces.sort((a, b) => a.name.localeCompare(b.name));
    const sorted2 = state2.interfaces.sort((a, b) => a.name.localeCompare(b.name));

    return JSON.stringify(sorted1) === JSON.stringify(sorted2);
  }
}

// Start demonstration
demonstrateMigration().catch(console.error);

// QUIC Performance Optimization
// Zero-copy I/O, memory pools, kernel bypass, and advanced tuning

const dgram = require('dgram');
const fs = require('fs');
const { Worker, isMainThread, parentPort, workerData } = require('worker_threads');
const EventEmitter = require('events');

class QUICPerformanceOptimizer {
  constructor(options = {}) {
    this.options = {
      // Memory management
      bufferSize: options.bufferSize || 64 * 1024, // 64KB
      poolSize: options.poolSize || 100,
      maxPoolSize: options.maxPoolSize || 1000,

      // Zero-copy settings
      enableZeroCopy: options.enableZeroCopy !== false,
      enableKernelBypass: options.enableKernelBypass || false,

      // Threading
      enableWorkers: options.enableWorkers || true,
      workerCount: options.workerCount || require('os').cpus().length,

      // Tuning parameters
      mtuDiscovery: options.mtuDiscovery !== false,
      adaptiveCongestion: options.adaptiveCongestion !== false,
      packetCoalescing: options.packetCoalescing !== false,

      ...options
    };

    // Memory pool management
    this.bufferPool = new BufferPool(this.options.bufferSize, this.options.poolSize);
    this.packetPool = new PacketPool(this.options.maxPoolSize);

    // Worker management
    this.workers = [];
    this.workQueue = [];
    this.workerIndex = 0;

    // Performance metrics
    this.metrics = new QUICMetrics();

    this.initialize();
  }

  initialize() {
    // Initialize buffer pools
    this.bufferPool.initialize();
    this.packetPool.initialize();

    // Initialize workers if enabled
    if (this.options.enableWorkers) {
      this.initializeWorkers();
    }

    // Setup performance monitoring
    this.setupMonitoring();
  }

  initializeWorkers() {
    console.log(`Initializing ${this.options.workerCount} QUIC workers...`);

    for (let i = 0; i < this.options.workerCount; i++) {
      const worker = new Worker(__filename, {
        workerData: {
          type: 'worker',
          workerId: i,
          bufferSize: this.options.bufferSize
        }
      });

      worker.on('message', (msg) => {
        this.handleWorkerMessage(msg, i);
      });

      worker.on('error', (error) => {
        console.error(`Worker ${i} error:`, error);
        this.metrics.recordWorkerError(i);
      });

      worker.on('exit', (code) => {
        console.log(`Worker ${i} exited with code ${code}`);
        if (code !== 0) {
          this.restartWorker(i);
        }
      });

      this.workers.push(worker);
    }
  }

  restartWorker(index) {
    console.log(`Restarting worker ${index}`);

    const worker = new Worker(__filename, {
      workerData: {
        type: 'worker',
        workerId: index,
        bufferSize: this.options.bufferSize
      }
    });

    worker.on('message', (msg) => {
      this.handleWorkerMessage(msg, index);
    });

    this.workers[index] = worker;
  }

  setupMonitoring() {
    // Monitor memory usage
    setInterval(() => {
      const memUsage = process.memoryUsage();
      this.metrics.updateMemoryUsage(memUsage);
    }, 1000);

    // Monitor performance metrics
    setInterval(() => {
      this.metrics.logPerformanceReport();
    }, 5000);
  }

  // Zero-copy packet processing
  async processPacketZeroCopy(socket, buffer, rinfo, callback) {
    if (!this.options.enableZeroCopy) {
      // Fallback to regular processing
      return await this.processPacketRegular(socket, buffer, rinfo, callback);
    }

    const startTime = process.hrtime.bigint();

    try {
      // Use buffer pool for temporary storage
      const pooledBuffer = this.bufferPool.acquire();

      // Zero-copy: directly use the received buffer
      await this.processPacketInWorker(buffer, rinfo, pooledBuffer);

      // Return buffer to pool
      this.bufferPool.release(pooledBuffer);

      // Record metrics
      const endTime = process.hrtime.bigint();
      const processingTime = Number(endTime - startTime) / 1000000; // Convert to ms

      this.metrics.recordPacketProcessing(processingTime, buffer.length);

    } catch (error) {
      console.error('Zero-copy packet processing error:', error);
      throw error;
    }
  }

  async processPacketRegular(socket, buffer, rinfo, callback) {
    // Regular packet processing with copying
    const packetCopy = Buffer.from(buffer);
    return await callback(packetCopy, rinfo);
  }

  async processPacketInWorker(buffer, rinfo, outputBuffer) {
    return new Promise((resolve, reject) => {
      if (this.options.enableWorkers && this.workers.length > 0) {
        // Distribute to workers
        const worker = this.workers[this.workerIndex];
        this.workerIndex = (this.workerIndex + 1) % this.workers.length;

        const timeout = setTimeout(() => {
          reject(new Error('Worker timeout'));
        }, 1000);

        worker.once('message', (result) => {
          clearTimeout(timeout);

          if (result.error) {
            reject(new Error(result.error));
          } else {
            // Copy result to output buffer (zero-copy alternative would be shared memory)
            result.data.copy(outputBuffer);
            resolve();
          }
        });

        worker.send({
          type: 'processPacket',
          buffer: buffer,
          rinfo: rinfo
        });

      } else {
        // Process in main thread
        this.processPacketInline(buffer, rinfo, outputBuffer);
        resolve();
      }
    });
  }

  processPacketInline(buffer, rinfo, outputBuffer) {
    // Inline packet processing
    const packetHeader = this.parseQUICHeader(buffer);

    // Process QUIC frames
    const frames = this.parseQUICFrames(buffer.slice(packetHeader.length));

    // Generate response (if needed)
    const response = this.generateResponse(frames);

    // Copy to output buffer
    response.copy(outputBuffer);
  }

  parseQUICHeader(buffer) {
    // Simplified QUIC header parsing
    if (buffer.length < 1) return 0;

    const firstByte = buffer.readUInt8(0);
    let headerLength = 1;

    // Long header format
    if (firstByte & 0x80) {
      headerLength += 4; // Version
      headerLength += 1; // Destination Connection ID length
      headerLength += 16; // Destination Connection ID
      headerLength += 1; // Source Connection ID length
      // Source Connection ID length would be read here
    } else {
      // Short header format
      headerLength += 1; // Destination Connection ID
    }

    return headerLength;
  }

  parseQUICFrames(data) {
    const frames = [];
    let offset = 0;

    while (offset < data.length) {
      const frameType = data[offset++];
      let frameLength = 1;

      switch (frameType) {
        case 0x00: // STREAM frame
          const streamInfo = this.parseStreamFrameInfo(data, offset);
          frameLength += streamInfo.length;
          frames.push({
            type: 'STREAM',
            streamId: streamInfo.streamId,
            data: data.slice(offset, offset + streamInfo.dataLength)
          });
          offset += streamInfo.dataLength;
          break;

        case 0x02: // ACK frame
          const ackInfo = this.parseAckFrameInfo(data, offset);
          frameLength += ackInfo.length;
          frames.push({
            type: 'ACK',
            ranges: ackInfo.ranges
          });
          offset += ackInfo.length;
          break;

        default:
          // Unknown frame type, skip
          break;
      }

      offset += frameLength;
    }

    return frames;
  }

  parseStreamFrameInfo(buffer, offset) {
    const streamId = this.readVariableLengthInteger(buffer, offset);
    offset += streamId.length;

    const offsetData = this.readVariableLengthInteger(buffer, offset);
    offset += offsetData.length;

    const length = this.readVariableLengthInteger(buffer, offset);
    offset += length.length;

    return {
      streamId: streamId.value,
      dataLength: length.value,
      length: streamId.length + offsetData.length + length.length
    };
  }

  parseAckFrameInfo(buffer, offset) {
    const largestAcked = this.readVariableLengthInteger(buffer, offset);
    offset += largestAcked.length;

    const ackDelay = this.readVariableLengthInteger(buffer, offset);
    offset += ackDelay.length;

    // Simplified: single ACK range
    return {
      ranges: [[largestAcked.value, largestAcked.value]],
      length: largestAcked.length + ackDelay.length
    };
  }

  readVariableLengthInteger(buffer, offset) {
    const first = buffer[offset];
    let value = first & 0x3f;
    let length = 1;

    if (first & 0xc0) {
      const additionalBytes = Math.floor((first & 0xc0) >> 6);
      length += additionalBytes;

      for (let i = 1; i <= additionalBytes; i++) {
        value = (value << 8) | buffer[offset + i];
      }
    }

    return { value, length };
  }

  generateResponse(frames) {
    // Generate response based on frames
    const response = Buffer.alloc(1024);
    let offset = 0;

    // Add ACK frame if needed
    if (frames.some(f => f.type === 'STREAM')) {
      offset += this.writeAckFrame(response, offset);
    }

    // Add response data
    response.write('HTTP/3 Response', offset);
    offset += 16;

    return response.slice(0, offset);
  }

  writeAckFrame(buffer, offset) {
    buffer.writeUInt8(0x02, offset++); // ACK frame type
    buffer.writeUInt32BE(1, offset);    // Largest ACKed packet number
    offset += 4;
    buffer.writeUInt32BE(0, offset);    // ACK delay
    offset += 4;

    return 9; // ACK frame length
  }

  handleWorkerMessage(message, workerId) {
    switch (message.type) {
      case 'packetProcessed':
        this.metrics.recordWorkerPacketProcessing(workerId, message.processingTime);
        break;

      case 'error':
        console.error(`Worker ${workerId} error:`, message.error);
        break;

      default:
        console.log(`Unknown worker message from ${workerId}:`, message);
    }
  }

  // Kernel bypass implementation (Linux-specific)
  setupKernelBypass() {
    if (!this.options.enableKernelBypass || process.platform !== 'linux') {
      return false;
    }

    try {
      // Setup raw socket for kernel bypass
      this.rawSocket = require('raw-socket').createSocket({
        protocol: require('raw-socket').Protocol.UDP,
        address: '0.0.0.0'
      });

      // Configure socket for high performance
      const sock = this.rawSocket.fd;

      // Set socket buffer sizes
      this.setSocketBufferSize(sock, 1024 * 1024); // 1MB
      this.setSocketTimeout(sock, 0); // No timeout

      // Enable SO_REUSEPORT
      this.setSocketOption(sock, 'SO_REUSEPORT', 1);

      console.log('Kernel bypass enabled');
      return true;

    } catch (error) {
      console.error('Failed to enable kernel bypass:', error);
      return false;
    }
  }

  setSocketBufferSize(fd, size) {
    const SOL_SOCKET = 1;
    const SO_RCVBUF = 8;
    const SO_SNDBUF = 7;

    this.setSocketOption(fd, SO_RCVBUF, size);
    this.setSocketOption(fd, SO_SNDBUF, size);
  }

  setSocketOption(fd, option, value) {
    // Would use native bindings for setsockopt
    // This is a simplified implementation
    console.log(`Setting socket option ${option} to ${value}`);
  }

  // MTU Discovery
  async discoverMTU(targetAddress, targetPort) {
    if (!this.options.mtuDiscovery) {
      return 1500; // Default Ethernet MTU
    }

    let mtu = 1500;
    let step = 64; // Start with 64 byte increments

    while (step > 1) {
      const testPacket = Buffer.alloc(mtu);

      try {
        // Send test packet
        await this.sendTestPacket(testPacket, targetAddress, targetPort);
        mtu += step;
      } catch (error) {
        // Packet too large, step back
        mtu -= step;
        step = Math.floor(step / 2);
      }
    }

    console.log(`Discovered MTU: ${mtu} bytes`);
    return mtu;
  }

  async sendTestPacket(packet, address, port) {
    return new Promise((resolve, reject) => {
      const socket = dgram.createSocket('udp4');

      socket.send(packet, port, address, (error) => {
        socket.close();
        if (error) {
          reject(error);
        } else {
          resolve();
        }
      });
    });
  }

  // Adaptive Congestion Control
  setupAdaptiveCongestion() {
    if (!this.options.adaptiveCongestion) {
      return;
    }

    this.congestionController = new AdaptiveCongestionControl({
      initialCwnd: 10 * 1024,
      minCwnd: 2 * 1024,
      maxCwnd: 100 * 1024 * 1024, // 100MB
      rttTarget: 50, // 50ms target RTT
      lossThreshold: 0.01 // 1% packet loss threshold
    });
  }

  // Packet Coalescing
  coalescePackets(packets) {
    if (!this.options.packetCoalescing || packets.length <= 1) {
      return packets;
    }

    const coalesced = Buffer.alloc(packets.reduce((sum, p) => sum + p.length, 0));
    let offset = 0;

    for (const packet of packets) {
      packet.copy(coalesced, offset);
      offset += packet.length;
    }

    console.log(`Coalesced ${packets.length} packets into ${coalesced.length} bytes`);
    return [coalesced];
  }

  getPerformanceMetrics() {
    return {
      memory: {
        bufferPoolUsage: this.bufferPool.getUsage(),
        packetPoolUsage: this.packetPool.getUsage(),
        processMemory: process.memoryUsage()
      },
      workers: {
        count: this.workers.length,
        utilization: this.metrics.getWorkerUtilization()
      },
      network: this.metrics.getNetworkMetrics(),
      processing: this.metrics.getProcessingMetrics()
    };
  }

  shutdown() {
    console.log('Shutting down QUIC performance optimizer...');

    // Shutdown workers
    for (const worker of this.workers) {
      worker.terminate();
    }

    // Clean up pools
    this.bufferPool.cleanup();
    this.packetPool.cleanup();

    // Close raw socket
    if (this.rawSocket) {
      this.rawSocket.close();
    }
  }
}

// Buffer Pool Implementation
class BufferPool {
  constructor(bufferSize, poolSize) {
    this.bufferSize = bufferSize;
    this.poolSize = poolSize;
    this.available = [];
    this.inUse = new Set();
    this.totalCreated = 0;
    this.hitCount = 0;
    this.missCount = 0;
  }

  initialize() {
    // Pre-allocate buffers
    for (let i = 0; i < this.poolSize; i++) {
      this.available.push(Buffer.allocUnsafe(this.bufferSize));
      this.totalCreated++;
    }

    console.log(`Buffer pool initialized with ${this.poolSize} buffers of size ${this.bufferSize}`);
  }

  acquire() {
    if (this.available.length > 0) {
      const buffer = this.available.pop();
      this.inUse.add(buffer);
      this.hitCount++;
      return buffer;
    }

    // Pool exhausted, create new buffer
    const buffer = Buffer.allocUnsafe(this.bufferSize);
    this.inUse.add(buffer);
    this.totalCreated++;
    this.missCount++;

    console.log(`Buffer pool miss, created new buffer. Total: ${this.totalCreated}`);
    return buffer;
  }

  release(buffer) {
    if (this.inUse.has(buffer)) {
      this.inUse.delete(buffer);

      // Zero out buffer for security
      buffer.fill(0);

      // Only return to pool if we haven't exceeded max size
      if (this.available.length < this.poolSize) {
        this.available.push(buffer);
      }
    }
  }

  getUsage() {
    return {
      available: this.available.length,
      inUse: this.inUse.size,
      total: this.totalCreated,
      hitRate: this.hitCount / (this.hitCount + this.missCount),
      missRate: this.missCount / (this.hitCount + this.missCount)
    };
  }

  cleanup() {
    this.available = [];
    this.inUse.clear();
    console.log('Buffer pool cleaned up');
  }
}

// Packet Pool Implementation
class PacketPool {
  constructor(maxSize) {
    this.maxSize = maxSize;
    this.packets = [];
    this.usage = new Map();
  }

  initialize() {
    // Packet pool is dynamically sized
    console.log(`Packet pool initialized with max size ${this.maxSize}`);
  }

  acquire() {
    if (this.packets.length > 0) {
      const packet = this.packets.pop();
      packet.lastUsed = Date.now();
      this.usage.set(packet, Date.now());
      return packet;
    }

    // Create new packet
    const packet = {
      data: Buffer.allocUnsafe(1500), // MTU size
      lastUsed: Date.now(),
      created: Date.now()
    };

    this.usage.set(packet, Date.now());
    return packet;
  }

  release(packet) {
    if (this.usage.has(packet)) {
      this.usage.delete(packet);

      if (this.packets.length < this.maxSize) {
        this.packets.push(packet);
      }
    }
  }

  getUsage() {
    return {
      available: this.packets.length,
      inUse: this.usage.size,
      maxSize: this.maxSize
    };
  }

  cleanup() {
    this.packets = [];
    this.usage.clear();
    console.log('Packet pool cleaned up');
  }
}

// Metrics Collection
class QUICMetrics {
  constructor() {
    this.startTime = Date.now();
    this.packetsProcessed = 0;
    this.totalProcessingTime = 0;
    this.workerMetrics = new Map();
    this.networkMetrics = {
      bytesSent: 0,
      bytesReceived: 0,
      packetsSent: 0,
      packetsReceived: 0
    };
  }

  recordPacketProcessing(processingTime, packetSize) {
    this.packetsProcessed++;
    this.totalProcessingTime += processingTime;
    this.networkMetrics.bytesReceived += packetSize;
    this.networkMetrics.packetsReceived++;
  }

  recordWorkerPacketProcessing(workerId, processingTime) {
    if (!this.workerMetrics.has(workerId)) {
      this.workerMetrics.set(workerId, {
        packetsProcessed: 0,
        totalProcessingTime: 0,
        errors: 0
      });
    }

    const metrics = this.workerMetrics.get(workerId);
    metrics.packetsProcessed++;
    metrics.totalProcessingTime += processingTime;
  }

  recordWorkerError(workerId) {
    if (!this.workerMetrics.has(workerId)) {
      this.workerMetrics.set(workerId, {
        packetsProcessed: 0,
        totalProcessingTime: 0,
        errors: 0
      });
    }

    this.workerMetrics.get(workerId).errors++;
  }

  updateMemoryUsage(memUsage) {
    this.memoryUsage = memUsage;
  }

  getWorkerUtilization() {
    const utilization = {};

    for (const [workerId, metrics] of this.workerMetrics.entries()) {
      const avgTime = metrics.packetsProcessed > 0
        ? metrics.totalProcessingTime / metrics.packetsProcessed
        : 0;

      utilization[workerId] = {
        packetsProcessed: metrics.packetsProcessed,
        averageProcessingTime: avgTime,
        errors: metrics.errors,
        errorRate: metrics.errors / (metrics.packetsProcessed + metrics.errors)
      };
    }

    return utilization;
  }

  getNetworkMetrics() {
    const uptime = Date.now() - this.startTime;

    return {
      ...this.networkMetrics,
      uptime: uptime,
      bytesPerSecond: (this.networkMetrics.bytesReceived / uptime) * 1000,
      packetsPerSecond: (this.networkMetrics.packetsReceived / uptime) * 1000
    };
  }

  getProcessingMetrics() {
    return {
      totalPackets: this.packetsProcessed,
      averageProcessingTime: this.packetsProcessed > 0
        ? this.totalProcessingTime / this.packetsProcessed
        : 0,
      packetsPerSecond: this.packetsProcessed / ((Date.now() - this.startTime) / 1000)
    };
  }

  logPerformanceReport() {
    const networkMetrics = this.getNetworkMetrics();
    const processingMetrics = this.getProcessingMetrics();

    console.log('=== QUIC Performance Report ===');
    console.log(`Uptime: ${(Date.now() - this.startTime) / 1000}s`);
    console.log(`Network: ${networkMetrics.bytesPerSecond.toFixed(2)} B/s, ${networkMetrics.packetsPerSecond.toFixed(2)} p/s`);
    console.log(`Processing: ${processingMetrics.averageProcessingTime.toFixed(2)}ms avg, ${processingMetrics.packetsPerSecond.toFixed(2)} p/s`);

    if (this.memoryUsage) {
      console.log(`Memory: ${(this.memoryUsage.heapUsed / 1024 / 1024).toFixed(2)}MB heap`);
    }

    console.log('=====================================');
  }
}

// Worker thread code
if (!isMainThread && workerData.type === 'worker') {
  // Worker thread implementation
  parentPort.on('message', async (message) => {
    switch (message.type) {
      case 'processPacket':
        try {
          const startTime = process.hrtime.bigint();

          // Process packet in worker
          const result = await processPacketInWorker(message.buffer, message.rinfo);

          const endTime = process.hrtime.bigint();
          const processingTime = Number(endTime - startTime) / 1000000;

          parentPort.postMessage({
            type: 'packetProcessed',
            processingTime: processingTime,
            result: result
          });

        } catch (error) {
          parentPort.postMessage({
            type: 'error',
            error: error.message
          });
        }
        break;

      default:
        parentPort.postMessage({
          type: 'error',
          error: `Unknown worker message type: ${message.type}`
        });
    }
  });

  async function processPacketInWorker(buffer, rinfo) {
    // Simulate packet processing
    await new Promise(resolve => setTimeout(resolve, Math.random() * 10));

    return {
      processed: true,
      rinfo: rinfo,
      dataSize: buffer.length
    };
  }
}

// Example usage
async function demonstratePerformanceOptimization() {
  const optimizer = new QUICPerformanceOptimizer({
    bufferSize: 128 * 1024,  // 128KB buffers
    poolSize: 200,            // 200 buffers in pool
    enableZeroCopy: true,
    enableWorkers: true,
    workerCount: 4,
    mtuDiscovery: true,
    adaptiveCongestion: true,
    packetCoalescing: true
  });

  // Create optimized QUIC socket
  const socket = dgram.createSocket('udp4');

  // Handle incoming packets with zero-copy
  socket.on('message', async (msg, rinfo) => {
    await optimizer.processPacketZeroCopy(socket, msg, rinfo, (packet, rinfo) => {
      // Process the packet
      console.log(`Processed packet from ${rinfo.address}:${rinfo.port}`);
    });
  });

  // Monitor performance
  setInterval(() => {
    const metrics = optimizer.getPerformanceMetrics();
    console.log('Performance metrics:', metrics);
  }, 10000);

  // Graceful shutdown
  setTimeout(() => {
    console.log('Shutting down...');
    optimizer.shutdown();
    socket.close();
    process.exit(0);
  }, 60000);
}

// Start demonstration
if (isMainThread) {
  demonstratePerformanceOptimization().catch(console.error);
} else {
  // Worker code is handled above
}