How Does Transmission Control Protocol Work?

TCP is a transport layer protocol in the TCP/IP protocol suite, used to ensure reliable communication between applications across a network. TCP utilizes its three-way handshake process to operate efficiently in computer networks. 

This article explains the TCP connection establishment process step-by-step. We have explained the TCP protocol, its packet header, and the three-way handshake process to help you understand how the TCP/IP protocol works in computer networks.

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What is TCP?

Transmission Control Protocol is one of the core communication protocols of the Internet Protocol suite, used for reliable data transfer between computers over a network.

TCP (Transmission Control Protocol) ensures reliable data transfer through mechanisms like error checking, flow control, and a connection establishment process known as the three-way handshake. It breaks large chunks of data into smaller packets, which are transmitted across the network and reassembled at the destination.

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What are TCP Packets?

A TCP packet (also called a TCP segment) is a small unit of data that TCP uses to send information over a network. Each packet contains:

1. A header that contains the control information like source/destination ports, sequence numbers, etc.

2. A payload, which is the actual chunk of data being transmitted.

TCP packets are an important component for TCP working, as breaking data into smaller packets enables TCP to:

● Send data efficiently across the network, especially when different routes are used.

● Handle delivery errors by resending only the lost/corrupted packet instead of the entire message.

● Manage flow control, as TCP can manage how much data is sent at a time, preventing the receiver from being overwhelmed.

● Control congestion by adjusting the rate of packet transmission based on network traffic conditions.

Understanding TCP Packet Header

The TCP packet header is the main component responsible for TCP working. The header is a part of a TCP packet, which carries control information like Source and destination port numbers, Sequence and acknowledgment numbers, etc.

The image below shows the TCP packet header format and the different fields in it:

Learn about these fields by reading our detailed article on the TCP Header

How Does TCP Work in Computer Networks?

The TCP working process can be simplified into 3 simple steps:

1. Connection Establishment: TCP establishes a connection between the client and server using a three-way handshake.

2. Data Transfer: Data is broken into smaller chunks called segments and transmitted over the network. During transmission, TCP manages flow control and uses acknowledgment (ACK) flags to confirm successful delivery of data.

3. Connection Termination: After the data transfer is complete, the connection is terminated using a four-way handshake.

The image below shows TCP working:

TCP Connection Establishment with Three-Way Handshake Process

TCP is a connection-oriented protocol, which means a secure connection is established between two hosts before sending and receiving the actual data packets. The Three-Way handshake process for connection establishment is shown step-by-step below:

Step 1. The sender first sends a packet with a SYN flag to the receiver to establish a connection.

Step 2. The receiver responds with a packet having a SYN-ACK flag, acknowledging the request and indicating that it is ready to receive the data.

Step 3. The sender then sends the ACK packet, confirming that the connection has been established.

These 3 steps complete the Three-Way Handshake process and establish a TCP connection between two computers.

How is TCP Connection Terminated?

After the data has been shared between computers, and there is no further need for data transfer, TCP terminates the connection between computers. To terminate the connection between computers, TCP uses the four-way handshake process.

The steps in a four-way handshake are:

Step 1. One device sends a FIN packet to signal the end of the connection.

Step 2. The other device responds with an ACK packet to confirm receipt of the FIN packet.

Step 3. The same device then sends its own FIN packet.

Step 4. The original sender responds with an ACK packet to confirm receipt.

This completes the four-way handshake process, and the connection is closed.

How Does TCP Ensure a Reliable Connection?

TCP ensures reliable communication through several key techniques. It uses acknowledgments to confirm the delivery of data packets, and if an acknowledgment isn't received within a specific time, the sender resends the packet.

Flow control prevents the sender from overwhelming the receiver by using a "window size" to indicate how much data can be handled at a time.

Congestion control avoids network congestion by gradually increasing the data transmission rate until the network stabilizes, allowing for efficient and reliable data transfer.

Summing Up!!

TCP is a complex protocol that is essential for reliable communication between applications across a network. By breaking down data into small packets, providing error detection and correction mechanisms, and using techniques like flow control and congestion control, TCP ensures that data is transmitted efficiently and reliably.

By breaking down data into manageable packets and ensuring their accurate delivery, TCP/IP supports a wide range of applications, from web browsing to email transmission. Its layered architecture allows for flexibility and scalability, making it adaptable to various networking environments.

As technology continues to evolve, the importance of understanding how TCP/IP operates remains paramount for anyone involved in networking, cybersecurity, or IT infrastructure development