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Networking - Quick Revision

📌 Last-minute revision before interviews. Scan these points quickly.

OSI Model (7 Layers)

  • 7 Application: HTTP, FTP, DNS, SMTP
  • 6 Presentation: SSL/TLS, encryption, compression
  • 5 Session: Session management, authentication
  • 4 Transport: TCP, UDP, ports, segments
  • 3 Network: IP, ICMP, ARP, routing, packets
  • 2 Data Link: Ethernet, Wi-Fi, MAC, frames
  • 1 Physical: Cables, radio, bits

TCP vs UDP

  • TCP: Connection-oriented, reliable, ordered, slow (HTTP, email, FTP)
  • UDP: Connectionless, unreliable, fast (DNS, gaming, streaming, VoIP)
  • TCP handshake: SYN → SYN-ACK → ACK (3-way)
  • TCP termination: FIN → ACK → FIN → ACK (4-way)
  • Congestion control: Slow start, congestion avoidance, fast retransmit

DNS

  • Resolution: Client → Recursive Resolver → Root → TLD → Authoritative
  • Records: A (IPv4), AAAA (IPv6), CNAME (alias), MX (mail), NS (nameserver)
  • Caching: Browser → OS → Resolver, TTL-based

HTTP

  • Methods: GET (read), POST (create), PUT (update), PATCH (partial), DELETE
  • Status: 2xx (success), 3xx (redirect), 4xx (client error), 5xx (server error)
  • HTTP/1.1: Persistent connections, no multiplexing
  • HTTP/2: Multiplexing, HPACK, server push, single TCP connection
  • HTTP/3: QUIC (UDP), built-in TLS 1.3, 0-RTT, no head-of-line blocking

HTTPS / TLS

  • TLS 1.3: 1-RTT handshake, 0-RTT resumption, mandatory forward secrecy
  • Certificate: CA chain verification, domain validation
  • SSL Pinning: Hardcode expected cert (prevents MITM)

URL Components

https://www.example.com:443/path?key=value#fragment
protocol | subdomain | port | path | query | fragment

WebSocket

  • Full-duplex, persistent connection over TCP
  • Upgrade from HTTP via handshake
  • Use: Real-time chat, gaming, live updates, notifications
  • vs HTTP: HTTP is request-response, WebSocket is bidirectional

REST API

  • Stateless, uniform interface, cacheable
  • CRUD: GET/POST/PUT/DELETE
  • Status codes: 200 OK, 201 Created, 400 Bad Request, 401 Unauthorized, 404 Not Found, 500 Internal Error

Load Balancing

  • L4: Transport layer, routes by IP+port, fast
  • L7: Application layer, routes by HTTP headers/URL, flexible
  • Algorithms: Round Robin, Least Connections, IP Hash, Weighted

Key Protocols

  • NAT: Private → Public IP mapping
  • ARP: IP → MAC address resolution
  • DHCP: Auto IP assignment
  • ICMP: Diagnostics (ping, traceroute)
  • gRPC: HTTP/2 + Protocol Buffers, fast internal APIs
  • GraphQL: Client-specified queries, single endpoint

Key Concepts

  • CDN: Edge servers cache content closer to users
  • VPN: Encrypted tunnel over public internet
  • CORS: Browser cross-origin security
  • Cookie: Client-side, sent with requests
  • Session: Server-side, identified by session ID
  • JWT: Self-contained token, stateless
  • Reverse proxy: Sits in front of servers, handles SSL, load balancing, caching
  • Firewall: Filters traffic based on rules (packet filtering, stateful, WAF)

🔗 Cross-References