Imagine you send a WhatsApp message, but:

• You don’t know if the message was delivered

• Messages arrive in the wrong order

• Some messages disappear in between

Annoying, right?

If the internet worked like this, then websites, APIs, online banking — everything would fail.

This exact problem is solved by TCP.

What is TCP and Why Is It Needed?

TCP (Transmission Control Protocol) is a transport protocol that makes sure:

• Data reaches the correct destination

• Data arrives in the correct order

• Data arrives completely

Simple goal of TCP:

“Reliable communication, no guessing.”

Problems TCP Is Designed to Solve

Without TCP, these problems would happen:

• Data loss

• Duplicate data

• Wrong order of data

• No delivery confirmation

TCP handles all of this automatically, without user involvement.

What is the TCP 3-Way Handshake?

Before sending data, TCP first creates a connection.
This process is called the 3-Way Handshake.

Meaning:

Both sides confirm they are ready before data transfer starts.

3-Way Handshake Explained

Think of a conversation between a client and a server.

Step 1: SYN

Client says:

“Hello Server, can we talk?” (SYN)

Step 2: SYN-ACK

Server replies:

“Yes, I’m ready. Are you ready too?” (SYN + ACK)

Step 3: ACK

Client replies:

“Yes, I’m ready. Let’s start.” (ACK)

Connection is now established.

How Data Transfer Works in TCP

TCP breaks data into small packets.

Each packet contains:

• Sequence number (to maintain order)

• ACK number (for confirmation)

The receiver says:

“Packet #5 received, send the next one.”

How TCP Ensures Reliability, Order, and Accuracy

TCP guarantees:

• Reliability: Lost packets are resent

• Order: Sequence numbers keep data in order

• Accuracy: Without ACK, data is not considered delivered

That’s why TCP feels a bit slower, but it is very safe.

What Happens If a Packet Is Lost?

If a packet is dropped in between:

• Receiver does not send ACK

• Sender waits

• After timeout, sender resends the packet

The user never notices anything.
TCP works silently in the background.

How a TCP Connection Is Closed

Even while closing, TCP follows proper rules.

Connection closing steps:

1. FIN: “I’m done sending data”

2. ACK: “I understand”

3. Other side also sends FIN

4. Final ACK is sent

After this, the connection is safely closed.