Computer Networks Essentials / Application Layer

FTP

🤔 What is FTP? (Basic Idea)

Sometimes we need to:

  • Upload files to a server ⬆️

  • Download files from a server ⬇️

  • Manage files remotely 📁

👉 This is done using FTP.

🧠 Simple Definition

📌 FTP (File Transfer Protocol) is an application-layer protocol used to transfer files between a client and a server over a network.

🧠 Why FTP is Needed

FTP is used when:

  • Files are large

  • Regular email attachment is not suitable

  • Files must be stored on a remote server

📌 Examples:

  • Website file upload

  • Software distribution

  • Backup storage

🧠 Real-Life Analogy

🏦 Bank Locker Example

  • You go to the bank (server)

  • You authenticate (login)

  • You deposit or withdraw files (upload/download)

👉 FTP works the same way.

🧭 Where Does FTP Belong in OSI Model?

  • FTP is an Application Layer protocol

  • Uses TCP for reliable transfer

🔌 Ports Used by FTP

FTP uses two different connections:

Connection

Port

Purpose

Control Connection

21

Commands & responses

Data Connection

20 (or dynamic)

Actual file transfer

📌 This makes FTP different from most protocols.

🧩 Two Connections in FTP (Key Concept)

🔹 Control Connection

  • Established first

  • Remains open throughout session

  • Used for:

    • Login

    • Commands

    • Responses

🔹 Data Connection

  • Created only when needed

  • Used for:

    • Upload

    • Download

    • Directory listing

📌 Closed after transfer completes.

🔐 FTP Authentication

FTP supports:

  • 👤 Username

  • 🔑 Password

📌 Two login modes:

🔓 Anonymous FTP

  • Username: anonymous

  • Password: email (optional)

📌 Used for public downloads.

🔒 Authenticated FTP

  • Requires valid username/password

  • Used for secure access

🔁 FTP Working (Step-by-Step)

1️⃣ Client connects to server on port 21

2️⃣ Server asks for username/password

3️⃣ Client authenticated

4️⃣ Control connection stays open

5️⃣ Data connection opened when file transfer requested

6️⃣ File transferred

7️⃣ Data connection closed

8️⃣ Session ends

🔄 Modes of FTP: Active vs Passive

⚡ 1. Active FTP

🧠 How Active FTP Works

1️⃣ Client opens control connection to server

2️⃣ Client tells server its port

3️⃣ Server opens data connection from port 20 to client

📌 Server initiates data connection.

❌ Problems

  • Firewalls block incoming connections

  • Client security issues

🧲 2. Passive FTP

🧠 How Passive FTP Works

1️⃣ Client opens control connection

2️⃣ Client requests passive mode

3️⃣ Server provides a port number

4️⃣ Client opens data connection to server

📌 Client initiates both connections ✔️

✅ Advantages

  • Firewall friendly

  • Most commonly used today

⚖️ Active vs Passive FTP

Feature

Active FTP

Passive FTP

Data connection initiated by

Server

Client

Firewall friendly

❌ No

✅ Yes

Security

Lower

Higher

Usage

Rare today

Common

📜 Common FTP Commands

Command

Purpose

USER

Send username

PASS

Send password

LIST

List directory

RETR

Download file

STOR

Upload file

QUIT

End session

❌ Security Issues with FTP

FTP has major security problems:

  • ❌ Username sent in plain text

  • ❌ Password sent in plain text

  • ❌ Data not encrypted

📌 Anyone can sniff data 🚨

🔐 Secure Alternatives to FTP

Protocol

Description

SFTP

FTP over SSH

FTPS

FTP over SSL/TLS

SCP

Secure copy

📌 Used in modern systems.

🧠 FTP vs HTTP (Quick Comparison)

Feature

FTP

HTTP

Purpose

File transfer

Web data

Connections

Two

One

State

Stateful

Stateless

Security

Low

High (HTTPS)

🧠 Memory Tricks

  • FTP = File Transfer

  • 21 = Control

  • 20 = Data

  • Passive = Client starts everything

⭐ One-Line Summary

🔹 FTP is an application-layer protocol that transfers files using separate control and data connections over TCP.