HLD | Network Protocols in Action: How the Internet Communicates

📌 What are Network Protocols?

• Network Protocols are a set of rules that define how two systems communicate over the internet.
• These rules are organized into layers, each with specific responsibilities.

📚 Layers of Network Communication (OSI Model)

1. Application Layer
2. Presentation Layer
3. Session Layer
4. Transport Layer
5. Network Layer
6. Data Link Layer

(Only Application and Transport layers are the main focus here.)

🧠 Application Layer Protocols

1. Client-Server Model Protocols

• These follow a request-response model: the client sends a request, the server responds.
• Examples:
  • HTTP (Hypertext Transfer Protocol)
  • Used for accessing web pages.
  • Stateless, one-time connection per request.
  • Not secure; use HTTPS for encrypted communication.
  • FTP (File Transfer Protocol)
  • Maintains two connections:
    • Control Connection: persistent, used for commands.
    • Data Connection: temporary, used for actual file transfer.
  • Not secure (no encryption on data connection), hence not preferred today.
  • SMTP (Simple Mail Transfer Protocol)
  • Used to send emails.
  • Works with:
    • IMAP (Internet Message Access Protocol) – reads/accesses mail without deleting, maintains mail on server.
    • POP3 (Post Office Protocol v3) – downloads and deletes mail (now outdated).
  • SMTP uses a client-server model:
    • User Agent → SMTP Client → Mail Transfer Agent (MTA) → MTA Server → User Agent

2. Peer-to-Peer Model Protocols

• Communication occurs directly between two peers, without a dedicated server.
• Example:
  • WebRTC (Web Real-Time Communication)
  • Enables real-time communication (video/audio) directly between browsers.
  • Uses UDP for low latency.
  • Example: Live streaming apps (video may blur if packets are lost, but speed is preferred over reliability).

🔁 WebSockets vs HTTP vs WebRTC

🚚 Transport Layer Protocols

1. TCP/IP (Transmission Control Protocol / Internet Protocol)

• Connection-oriented: reliable.
• Data is sent in an ordered fashion.
• Acknowledgments are sent for every packet.
• Slower due to overhead but reliable.
• Used by:
  • HTTP
  • HTTPS
  • FTP
  • SMTP

2. UDP (User Datagram Protocol) / ICMP (Internet Control Message Protocol)

• Connectionless: faster but unreliable.
• No acknowledgments or packet ordering.
• Used in:
  • WebRTC
  • Live video/audio streaming
• Advantage: Low latency.

⚠ Security Note

• FTP is outdated because it lacks encryption on the data connection.
• Prefer HTTPS for secure communication over HTTP.

✅ Summary

🧪 Real-world Examples

• HTTP: Visiting a website like Wikipedia.
• WebSocket: WhatsApp or Slack for chat messaging.
• WebRTC: Zoom, Google Meet, YouTube Live, etc.