Computer Networks

TCP/IP Model (practical):

TCP vs UDP
• TCP connection-oriented, reliable, ordered delivery. Slower. Used by: HTTP, SSH, databases.
• UDP connectionless, no guarantee. Faster. Used by: DNS, video streaming, gaming, VoIP.

TCP Handshake (3-way) 1. Client → Server: SYN 2. Server → Client: SYN-ACK 3. Client → Server: ACK → connection established

IP Addressing
• IPv4: 32-bit (192.168.1.1), ~4.3B addresses
• IPv6: 128-bit (2001:db8::1), 340 undecillion addresses
• CIDR: 192.168.0.0/24 = first 24 bits are network, last 8 are host (256 addresses)

DNS Resolution `` Browser cache → OS cache → Recursive resolver (ISP) → Root nameserver → TLD (.com) → Authoritative nameserver → IP address ``

HTTP/2 vs HTTP/3
• HTTP/1.1: one request per TCP connection, head-of-line blocking
• HTTP/2: multiplexing (multiple requests over one connection), header compression
• HTTP/3: runs on QUIC (UDP-based), eliminates TCP head-of-line blocking, faster mobile

TLS Handshake (HTTPS) 1. Client Hello supported TLS versions, cipher suites 2. Server Hello chosen cipher, certificate 3. Client verifies certificate against trusted CA 4. Key exchange → shared session key 5. Encrypted communication begins

Common Ports | Port | Protocol | |---|---| | 80 | HTTP | | 443 | HTTPS | | 22 | SSH | | 5432 | PostgreSQL | | 3306 | MySQL | | 6379 | Redis | | 27017 | MongoDB |

HTTP Version Comparison | Feature | HTTP/1.1 | HTTP/2 | HTTP/3 | |---|---|---|---| | Protocol | TCP | TCP | QUIC (UDP) | | Multiplexing | ❌ | ✅ | ✅ | | Header compression | ❌ | ✅ HPACK | ✅ QPACK | | Head-of-line blocking | TCP + app | TCP only | ❌ Eliminated | | Server push | ❌ | ✅ | ✅ | | Connection setup | 1.5 RTT | 1 RTT | 0-1 RTT | | Best for | Legacy | Most apps | Mobile, high loss |

WebSocket Protocol ```javascript // Server (Node.js) const { WebSocketServer } = require('ws'); const wss = new WebSocketServer({ port: 8080 });

wss.on('connection', (ws, req) => { const userId = authenticate(req); ws.on('message', (data) => { const msg = JSON.parse(data); // Broadcast to all connected clients wss.clients.forEach(client => { if (client.readyState === WebSocket.OPEN) { client.send(JSON.stringify({ from: userId, text: msg.text, timestamp: Date.now() })); } }); }); ws.on('close', () => removeUser(userId)); ws.on('error', (err) => console.error(err)); // Heartbeat to detect stale connections const heartbeat = setInterval(() => { if (ws.readyState === WebSocket.OPEN) ws.ping(); }, 30_000); ws.on('close', () => clearInterval(heartbeat)); });

// Client const ws = new WebSocket('wss://example.com/ws'); ws.onopen = () => console.log('connected'); ws.onmessage = (e) => console.log(JSON.parse(e.data)); ws.onerror = (e) => console.error(e); ws.onclose = () => setTimeout(reconnect, 3000);

function send(text) { if (ws.readyState === WebSocket.OPEN) { ws.send(JSON.stringify({ text })); } } ```