TCP Handshake & Connection

Before two computers can exchange data over TCP, they must agree to talk — that’s the 3-way handshake.

Think of it like calling someone on the phone:

1. You dial the number (SYN) — “Can you hear me?”
2. They answer (SYN-ACK) — “Yes, I hear you. Can you hear me?”
3. You confirm (ACK) — “Yes, I hear you too. Let’s talk.”

After the handshake, the conversation (data transfer) begins.

sequenceDiagram
    participant Client as Client<br/>(Your Browser)
    participant Server as Server<br/>(example.com)

    Note over Client,Server: STEP 1: Establish Connection (3-Way Handshake)
    Client->>Server: SYN<br/>"Hey, can we talk? My sequence # = 100"
    Server-->>Client: SYN-ACK<br/>"Yes! My sequence # = 300, your # = 101"
    Client->>Server: ACK<br/>"Got it! My # = 101, your # = 301"

    Note over Client,Server: ✅ Connection Established — Data Transfer Begins
    Client->>Server: GET /index.html (Data)
    Server-->>Client: HTTP/1.1 200 OK (Data)

    Note over Client,Server: STEP 2: Close Connection
    Client->>Server: FIN<br/>"I'm done talking"
    Server-->>Client: ACK<br/>"Got it, one sec"
    Server-->>Client: FIN<br/>"I'm done too"
    Client->>Server: ACK<br/>"OK, goodbye!"

Sequence numbers are random numbers that track the order of data. They ensure that:

• Data is reassembled in the correct order
• Lost packets can be detected
• Duplicate packets are ignored

When the conversation ends, a 4-step goodbye:

1. Client → Server: FIN (“I’m done sending”)
2. Server → Client: ACK (“Got it, let me finish”)
3. Server → Client: FIN (“OK, I’m done too”)
4. Client → Server: ACK (“Goodbye!”)

• TCP uses a 3-way handshake (SYN → SYN-ACK → ACK) to establish a connection
• Think of it like a phone call — both sides confirm they can hear each other
• Sequence numbers track packet order so data is reassembled correctly
• TCP is reliable — it resends lost packets and checks for errors
• Connection teardown uses 4 steps (FIN → ACK → FIN → ACK)