TCP Working: 3-Way Handshake & Reliable Communication

1. What is TCP and why it is needed?

   TCP stands for Transmission Control Protocol. It's a set of rules that ensures data sent over the internet arrives:

• In the correct order

• Without errors

• Without loss

• Without duplicates

  Think of it as: Registered mail with tracking and confirmation.

2. Problems TCP is designed to solve

Out of Order Packets: TCP puts them back in sequence.

Lost Packets: TCP resends missing data.

Duplicate Packets: TCP ignores duplicates.

Corrupted Data: TCP detects and rejects bad data.

3. What is the TCP 3-Way Handshake

Before sending any data, TCP establishes a connection. This is called the

3-way handshake:

You: "Can you hear me?" Friend: "Yes, I can hear you. Can you hear me?" You: "Yes, I can hear you too!" Now you both know the connection works.

4. Step-by-step working of SYN, SYN-ACK, and ACK?

Step 1: SYN (Synchronize)

Client sends SYN packet to server.

Message: "Hello Server! Can we talk? My sequence number is 100."

Step 2: SYN-ACK (Synchronize-Acknowledge)

Server responds with SYN-ACK.

Message: "Yes, I got your 100! My sequence number is 200."

Step 3: ACK (Acknowledge)

Client sends ACK.

Message: "Got your 200! Ready to send data."

Connection established! Data transfer begins.

What Are Sequence Numbers?

Sequence numbers track bytes sent.

If you send 100 bytes starting at sequence 1000: - Sequence number = 1000 - Next expected ACK = 1100 This ensures order and completeness.

5. How data transfer works in TCP?

Once connected, data flows with acknowledgements.

The Process:

• Client sends data packet with sequence number

• Server receives and sends ACK with next expected sequence

• Client sends next packet

• Repeat until all data sent

6. How TCP ensures reliability, order, and correctness?

TCP uses multiple mechanisms:

1. Sequence Numbers:

Every byte gets a number. Receiver knows if something's missing.

2. Acknowledgements (ACKs):

Receiver confirms what it received. If sender doesn't get ACK, it resends.

3. Checksums:

Data integrity check. Corrupted data gets rejected and retransmitted.

4. Timeouts & Retransmission:

If ACK doesn't arrive within timeout, sender retransmits.

Ensuring Order

Even if packets arrive out of order, TCP uses sequence numbers to reorder them.

Example: Received: Packet 3, Packet 1, Packet 2 TCP reorders: Packet 1, Packet 2, Packet 3

Ensuring Correctness

Checksum validates data integrity. If checksum fails, packet is discarded and retransmitted.

7. How a TCP connection is closed?

TCP uses a 4-way handshake to close connections gracefully.

4-Way Termination

Step 1: Client sends FIN ("I'm done sending")

Step 2: Server sends ACK ("Got it")

Step 3: Server sends FIN ("I'm also done")

Step 4: Client sends final ACK ("Goodbye")

Connection closed!

TCP Connection Lifecycle

A TCP connection goes through three phases:

• Establish — 3-way handshake

• ]Data Transfer — Send/receive with ACKs

• Terminate — 4-way close

Why TCP Matters

TCP is used everywhere:

• Web browsing (HTTP/HTTPS)

• Email (SMTP, IMAP)

• File transfers (FTP)

• SSH (remote access)

When reliability matters: Use TCP.

When speed matters more: Use UDP (no handshakes, no guarantees).

Quick Summary

TCP = Reliable communication protocol 3-Way Handshake: SYN → SYN-ACK → ACK Data Transfer: Send data → Get ACK → Send more Reliability: Sequence numbers track order - ACKs confirm receipt - Timeouts trigger retransmission - Checksums verify correctness Connection Close: FIN → ACK → FIN → ACK