• Why TCP is needed

• What is 3-Way Handshake

• How TCP sends data safely

• How TCP closes connection

Why TCP is Needed?

Imagine you want to send a file to your friend.

The file is broken into many small packets:

Packet 1, Packet 2, Packet 3, ...

If there are no rules:

• Packet 5 may get lost

• Packet 10 may come before packet 1

• Packet 20 may get damaged

Final result
File becomes useless garbage

So internet needed a protocol that can say:

• Send in order

• Check if packet reached

• Resend if lost

• Make sure data is correct

That protocol is TCP

What is TCP

TCP = Transmission Control Protocol

TCP is a transport protocol which ensures:

• Reliability

• Order

• Correctness

TCP says:

“Don’t worry, I will deliver your data properly.”

TCP Connection

TCP does not send data directly.

First it says:

“Let us confirm both sides are ready”

This process is called
3-Way Handshake

What is 3-Way Handshake?

3-Way Handshake is connection setup process of TCP.

Why 3 steps?
Because both sides must agree before data transfer.

Analogy

Think of two friends:

• a → Client

• b → Server

They are using walkie-talkie

Step-by-Step 3-Way Handshake

Step 1: SYN

a says:

“Hello b! Can you hear me?”

Technical meaning:

• Client sends SYN packet

• SYN = Synchronize

• Means: “I want to start connection”

Step 2: SYN-ACK

b replies:

“Yes a, I can hear you. Can you hear me?”

Technical meaning:

• Server sends SYN + ACK

• ACK = Acknowledge

• Means: “I heard you, and I am ready too”

Step 3: ACK

Rahul replies:

“Yes b, I can hear you too. Let’s talk.”

Technical meaning:

• Client sends ACK

• Now connection is ESTABLISHED

After Handshake: Data Transfer Starts

Now both client and server trust each other.

Data starts flowing in segments.

But TCP still does checking.

How TCP Sends Data Safely

TCP adds extra information with every packet:

• Sequence Number

• Acknowledgement Number

Sequence Number

Tells:

“This is packet number X”

Acknowledgement Number

Tells:

“I received till packet number X”

What if Packet is Lost?

Suppose:

• Packet 5 is lost

Receiver says:

“I received till packet 4 only”

Sender understands:

“Packet 5 missing”

TCP will:

• Resend packet 5

• Not whole file, only missing part

This is called retransmission

How TCP Connection is Closed

Connection also needs proper ending.

TCP does not just disappear.

FIN-ACK Process

1. Client says:

   “I am done” (FIN)

2. Server replies:

   “Okay” (ACK)

3. Server says:

   “I am also done” (FIN)

4. Client replies:

   “Okay” (ACK)

Connection closed safely.

TCP Connection Life Cycle

1. Establish (3-Way Handshake)

2. Transfer Data

3. Close Connection