Transmission Control Protocol

🔧 How TCP Works

1. Connection Establishment: The 3-Way Handshake

To start a TCP session, both sides perform a three-step handshake:

1. SYN – The client sends a synchronize (SYN) message to the server.
2. SYN-ACK – The server responds with a SYN-ACK (synchronize-acknowledge).
3. ACK – The client sends back an ACK, and the connection is established.

This ensures both parties are ready to start data transfer.

2. Data Transmission

• Segmentation: TCP breaks application data into manageable segments.
• Sequence Numbers: Each byte of data is assigned a number. These help with ordering and detecting loss.
• Acknowledgements (ACKs): The receiver sends ACKs back to confirm receipt of data.

3. Reliability Mechanisms

• Retransmission: If an ACK isn't received in time, TCP resends the data.
• Checksums: Each segment includes a checksum to verify data integrity.
• Duplicate Data Handling: Duplicate segments are detected and discarded.

4. Flow Control (Sliding Window)

TCP uses a window size to control how much data can be sent before waiting for an ACK. This:

• Prevents sender from overwhelming the receiver.
• Allows multiple packets to be in transit (pipeline).

5. Congestion Control

To avoid overloading the network:

• Slow Start: TCP starts with a small congestion window (cwnd) and increases it exponentially.
• Congestion Avoidance: Once a threshold is reached, it grows linearly.
• Fast Retransmit & Fast Recovery: Handle loss without waiting for timeout.

6. Connection Termination: 4-Way Teardown

TCP closes a connection using a four-step process:

1. FIN from one side
2. ACK from the other
3. FIN from the second side
4. ACK from the first side

Each side closes independently.

🛠️ TCP Header (Key Fields)

🌐 Typical Applications

TCP is used when data accuracy and order are more important than speed:

• Web browsing (HTTP, HTTPS)
• Email (SMTP, IMAP, POP3)
• File transfers (FTP)
• Remote terminals (SSH, Telnet)