Working of Transmission Control Protocol (TCP) Explained

TCP is a transport layer protocol in the TCP/IP protocol suite, used to ensure reliable communication between applications across a network. To understand the TCP working process, you should be familiar with how TCP establishes a connection, transfers data, and then terminates the connection. 

In this article, we have explained how TCP works in detail. From three-way handshake for connection establishment to showing TCP header for data transfer, this guide explains the working of TCP protocol in simple words..

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What is TCP?

TCP stands for Transmission Control Protocol, and it is a fundamental communication protocol used in computer networking. It is part of the Internet Protocol suite, and is used in various internet applications, including web browsing, email, and file transfers.

TCP works by breaking down data into smaller packets, addressing them, sending them through the network, and ensuring they are reassembled in the correct order at the destination. This process also includes error checking and retransmission of lost packets.

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TCP Header Structure

At the packet level, a TCP packet consists of many different fields, and each field has its own functions. The TCP header diagram is shown below:

When a TCP packet is sent from one host to another. It is first encapsulated with an IP header, which has the source and destination IP addresses. Using this, a packet can be routed across the networks. In the transmitted network, a packet can be delayed.

Read our detailed article on TCP Header

How TCP Works?

TCP connection and data transfer can be understood in 5 easy steps:

Step 1: Connection Establishment: TCP establishes a connection between the client and server using a three-way handshake (SYN, SYN-ACK, ACK).

Step 2: Data Transfer: Data is broken into smaller chunks called segments and transmitted over the network.

Step 3: Acknowledgment: The receiving end acknowledges the received segments, ensuring reliable delivery.

Step 4: Flow Control: TCP regulates the flow of data to prevent the sender from overwhelming the receiver.

Step 5: Connection Termination: The connection is terminated using a four-way handshake (FIN, ACK, FIN, ACK).

How TCP Establishes a Connection

TCP is a connection-oriented protocol, which means a secure connection is established between two hosts before sending and receiving the actual data packets between these two hosts. This is achieved by establishing a three-way handshake connection between the sender and the receiver. The following are the steps:

Step 1. The host who wants to send data (client) first sends a packet with a SYN flag to the receiving host to establish a connection.

Step 2. The receiving host responds (Server) with a packet having a SYN-ACK flag and also acknowledging the request and indicating that it is ready to receive the data.

Step 3. The sending host responds (Client) with an ACK packet, confirming that the connection has been established and data can now be sent.

The data can be sent and received between the two hosts only when connections have been established using TCP packets.

How is TCP Connection Terminated

Similarly, whenever a host wants to terminate its connection with the other host, TCP uses a four-way handshake process to terminate connections.

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

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

Step 3. The other end 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 receipt of data packets; 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