What is TCP and why it is needed

TCP (Transmission Control Protocol) is a protocol that allows two devices to exchange data safely and reliably over the internet.

In internet data travels through many routers, cables, and networks. Along the way, problems happen:

• Data can get lost

• Data can arrive in the wrong order

• Data can get damaged

• The receiver may be overloaded

TCP acts like a manager that controls how data is sent and received.

Without TCP: Internet communication would be random and unreliable.

With TCP: Data is delivered correctly, in order, and completely.

That’s why websites, emails, banking apps, and file downloads all use TCP.

Problems TCP is designed to solve

Packet loss — Some data disappears

Out-of-order packets — Message becomes jumbled

Duplicate packets — Same data received twice

Data corruption — Bits change during travel

Receiver overload — System crashes

Network congestion — Internet becomes slow

What is the TCP 3-Way Handshake

Before sending real data, TCP must establish a connection.

It’s like knocking before entering a room.

This process is called the 3-Way Handshake.

Purpose:
Confirm both devices are alive
Synchronize sequence numbers
Prepare resources for communication

Step-by-step working of SYN, SYN-ACK, and ACK

Step 1 — SYN

Client → Server

Means: “I want to connect. My data numbering starts from X.”

Step 2 — SYN-ACK

Server → Client

Means:

“I received your request. I’m ready. My numbering starts from Y.”

Step 3 — ACK

Client → Server

Means:

“I received your response. Connection established.”

Now both sides agree → data transfer begins.

How data transfer works in TCP

TCP breaks big data into smaller pieces called segments.

Each segment has:

Data

Sequence number

Checksum

How TCP ensures reliability, order, and correctness

TCP uses multiple mechanisms together:

Sequence Numbers

Keep data in correct order.

Acknowledgements

Confirm receipt.

Retransmission

If ACK not received → resend data.

Flow Control

Receiver tells sender how much data it can handle (window size).

How a TCP connection is closed

Connection closing also follows rules.

Steps:

1. Client → FIN
   “I’m done sending.”

2. Server → ACK
   “Got your FIN.”

3. Server → FIN
   “I’m done too.”

4. Client → ACK
   Connection fully closed.