A beginner-friendly and interview-ready guide to Client-Server Architecture, P2P, Protocols, TCP/UDP, WebSockets, and WebRTC.

Table of Contents

Communication Models

1. Client-Server Architecture

In the Client-Server model, a client sends a request to a central server, and the server processes the request before sending back a response.

           +-----------+
           |  Server   |
           +-----------+
            /    |    \
           /     |     \
          /      |      \
 +---------+ +---------+ +---------+
 | Client1 | | Client2 | | Client3 |
 +---------+ +---------+ +---------+

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Workflow

  1. Client sends Request
  2. Server processes Request
  3. Server returns Response

Examples

  • Google
  • Amazon
  • Facebook
  • Gmail
  • Banking Applications

Key Points

  • Centralized architecture
  • Clients cannot communicate directly
  • Every request passes through the server

2. Peer-to-Peer (P2P)

In Peer-to-Peer architecture, devices communicate directly with one another.

      Peer A
      /    \
     /      \
Peer B ---- Peer C

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Examples

  • WebRTC
  • Torrent

Key Points

  • No central server for media transfer
  • Direct communication between peers
  • Lower latency

Protocols

A protocol is a set of rules that defines how data is transmitted between devices.

HTTP (HyperText Transfer Protocol)

Purpose

Used for loading websites and REST APIs.

Features

  • Request → Response
  • Stateless
  • Runs over TCP
  • Client always starts communication 
Browser --------HTTP Request--------> Server
Browser <-------HTTP Response-------- Server

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Used In

  • Websites
  • REST APIs
  • Login Systems
  • Web Applications

FTP (File Transfer Protocol)

Purpose

Transfers files between client and server.

FTP uses Two Connections

1. Control Connection

  • Login
  • Commands
  • Authentication

2. Data Connection

  • File Upload
  • File Download 
Client

│
├── Control Connection ─────────► Server
│
└── Data Connection ────────────► Server

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Why Two Connections?

Commands and file data are transferred separately, making file transfer easier and more efficient.

SMTP (Simple Mail Transfer Protocol)

Purpose

Used to Send Emails.

Sender

↓

SMTP

↓

Mail Transfer Agent (MTA)

↓

Receiver Mail Server

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Key Points

  • Only sends emails
  • Uses TCP

IMAP (Internet Message Access Protocol)

Purpose

Used to Read, Access and Synchronize Emails.

Mail Server

↓

IMAP

↓

Email Client

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Key Points

  • Reads emails
  • Synchronizes emails across multiple devices
  • Emails remain stored on the server

WebSocket

Purpose

Provides Real-Time Two-Way Communication.

Unlike HTTP, both client and server can send messages whenever they want.

Client 1
    ▲
    │
    ▼
+-----------+
|  Server   |
+-----------+
 ▲    ▲    ▲
 │    │    │
 ▼    ▼    ▼
C2   C3   C4

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Important

  • NOT Peer-to-Peer
  • Client ↔ Server
  • Full Duplex Communication
  • Persistent Connection

Clients cannot communicate directly.

The server forwards messages between clients.

Examples

  • WhatsApp Web
  • Live Chat
  • Multiplayer Games
  • Notifications
  • Stock Market Updates

WebRTC (Web Real-Time Communication)

Purpose

Allows direct Peer-to-Peer communication.

Peer A  <──────────────►  Peer B

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Uses

  • Video Calls
  • Voice Calls
  • Screen Sharing

Important

  • Mostly uses UDP
  • Optimized for low latency
  • Supports audio, video and data channels

Network Layers

Only the important layers are shown.

+-----------------------+
| Application Layer     |
+-----------------------+
| Transport Layer       |
+-----------------------+
| Network Layer         |
+-----------------------+

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Application Layer

Contains protocols like:

  • HTTP
  • FTP
  • SMTP
  • IMAP
  • WebSocket

Transport Layer

Responsible for

  • Reliability
  • Speed
  • Packet Delivery

Protocols

  • TCP
  • UDP

Network Layer

Responsible for

  • Routing
  • IP Addressing

Uses IP Addresses to send packets between devices.

TCP (Transmission Control Protocol)

TCP is a Connection-Oriented protocol.

Before sending data, TCP establishes a connection.

Working 

Connection Established

↓

Data Split into Packets

↓

Sequence Numbers Added

↓

Packets Sent

↓

Receiver Sends ACK

↓

Missing Packets Retransmitted

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Features

  • Reliable
  • Ordered Delivery
  • Acknowledgement (ACK)
  • Retransmission
  • Error Detection
  • Connection-Oriented

Used In

  • HTTP
  • FTP
  • Email
  • Banking
  • Downloads

When to Use TCP

Whenever every packet is important and must arrive in the correct order.

UDP (User Datagram Protocol)

UDP is a Connectionless protocol.

Working 

Packet 1

Packet 3

Packet 2

Packet 5

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Packets may arrive in any order.

Features

  • No Connection Setup
  • No ACK
  • No Retransmission
  • Best Effort Delivery
  • Very Fast

Used In

  • Live Streaming
  • Video Calls
  • Voice Calls
  • Online Games
  • DNS

When to Use UDP

When speed is more important than perfect reliability.

Example:

During a video call, losing one frame is acceptable, but waiting for retransmission would introduce lag.

TCP vs UDP

Feature TCP UDP
Connection ✅ Yes ❌ No
Reliable ✅ Yes ❌ No
Ordering ✅ Maintained ❌ Not Guaranteed
ACK ✅ Yes ❌ No
Retransmission ✅ Yes ❌ No
Speed Slower Faster
Best For Files, Banking, HTTP Streaming, Gaming, Video Calls

HTTP vs WebSocket

HTTP WebSocket
Request → Response Full Duplex
Client starts communication Both client and server can send anytime
Short-lived connection Persistent connection
Not real-time Real-time
Uses TCP Uses TCP

WebSocket vs WebRTC

WebSocket WebRTC
Client-Server Peer-to-Peer
Uses TCP Primarily Uses UDP
Chat Applications Video Calls
Notifications Voice Calls
Multiplayer Synchronization Screen Sharing

Which Protocol Should You Use?

Situation Protocol
Website HTTP
File Transfer FTP
Send Email SMTP
Read Email IMAP
Live Chat WebSocket
Video Call WebRTC
Banking TCP
File Download TCP
Live Streaming UDP
Online Gaming UDP

Interview Questions

Why does FTP use two connections?

One connection is used for commands (Control Connection) and another is used for transferring files (Data Connection).

Why is UDP used in Video Calls?

Because speed is more important than reliability. Missing a few packets is better than introducing delay.

Why is TCP used in Banking?

Every packet must reach the destination correctly and in the correct order.

Is WebSocket Peer-to-Peer?

No.

WebSocket is a Client-Server protocol that supports Full Duplex communication.

Which protocol sends emails?

SMTP

Which protocol reads emails?

IMAP

Quick Revision

Topic Remember
HTTP Request-Response
FTP Two Connections
SMTP Send Email
IMAP Read Email
WebSocket Full Duplex Client-Server
WebRTC Peer-to-Peer
TCP Reliable, Ordered
UDP Fast, Best Effort
Live Streaming UDP
Banking TCP
Video Calls WebRTC + UDP

Summary

  • HTTP → Websites and APIs
  • FTP → File Transfer
  • SMTP → Sending Emails
  • IMAP → Reading Emails
  • WebSocket → Real-time Client-Server Communication
  • WebRTC → Peer-to-Peer Video/Audio Communication
  • TCP → Reliable and Ordered
  • UDP → Fast and Connectionless