TCP (Transmission Control Protocol)

Overview

TCP is a connection-oriented, reliable, and byte-stream based protocol that ensures data transmitted between devices on a network arrives completely and in the correct order.

Key Features

Connection Establishment

TCP uses a three-way handshake process to establish a connection:

1. SYN: Client sends a SYN packet to the server
2. SYN-ACK: Server responds with a SYN-ACK packet
3. ACK: Client acknowledges with an ACK packet

Reliability Mechanisms

• Acknowledgments: Confirmation of received segments
• Sequence Numbers: Track the order of transmitted data
• Checksums: Verify data integrity
• Retransmission: Resend lost or corrupted segments

Flow Control

TCP implements a sliding window protocol that allows the receiver to control how much data the sender can transmit, preventing buffer overflow.

Congestion Control

• Slow Start: Gradually increases transmission rate
• Congestion Avoidance: Adjusts window size based on network conditions
• Fast Retransmit and Recovery: Quickly recovers from packet loss

TCP Segment Structure

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|          Source Port          |       Destination Port        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                        Sequence Number                        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                    Acknowledgment Number                      |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|  Data |           |U|A|P|R|S|F|                               |
| Offset| Reserved  |R|C|S|S|Y|I|            Window             |
|       |           |G|K|H|T|N|N|                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|           Checksum            |         Urgent Pointer        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                    Options                    |    Padding    |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                             Data                              |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

TCP Connection States

• LISTEN: Server waiting for connections
• SYN-SENT: Client has sent SYN, waiting for SYN-ACK
• SYN-RECEIVED: Server received SYN, sent SYN-ACK
• ESTABLISHED: Connection established, data transfer possible
• FIN-WAIT-1: Closing connection, waiting for ACK
• FIN-WAIT-2: Waiting for FIN from remote
• CLOSE-WAIT: Remote closed connection, waiting for application to close
• CLOSING: Both sides closing simultaneously
• LAST-ACK: Waiting for final ACK
• TIME-WAIT: Waiting to ensure remote received final ACK
• CLOSED: Connection fully terminated

TCP Animation

Common Applications

• Web browsing (HTTP/HTTPS)
• Email (SMTP, IMAP, POP3)
• File transfers (FTP, SFTP)
• Remote terminal access (SSH, Telnet)
• Database connections