How Does TCP Work: TCP IP Protocol Suite

TCP is a transport layer protocol in the TCP/IP protocol suite which is used in computer networking. Its purpose is to ensure reliable communication between applications across a network.

It breaks down data chunks into small packets. It also has some rules and procedures for the packets to transmit and receive from one end to another. This protocol suite encompasses several layers, each responsible for different aspects of data handling, including packetization, addressing, routing, and error checking.

Understanding how TCP/IP works is essential for grasping the fundamentals of internet connectivity and data exchange in today's digital landscape. In this article, we will learn how does TCP/IP protocol works with packet-level details.

Additionally, obtaining Cisco enterprise certifications can further enhance your understanding of TCP/IP and networking concepts. Certifications such as Cisco Certified Network Associate (CCNA) and Cisco Certified Network Professional (CCNP) provide valuable knowledge and skills that are highly regarded in the IT industry.

What is TCP/IP?

TCP/IP stands for Transmission Control Protocol/Internet Protocol, which is a suite of communication protocols that form the foundation of the Internet and facilitate communication between devices across networks.

TCP/IP breaks down data into smaller packets, addresses them, sends them through the network, and ensures they are reassembled in the correct order at the destination. This process includes error checking and retransmission of lost packets.

TCP/IP was developed in the 1970s by the U.S. Department of Defense for ARPANET, the precursor to the internet. It was adopted as a standard protocol suite in 1983 and has since become the backbone of modern networking.

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Working of TCP

Look at the diagram below, where a client is initiating a connection with the server. So client is an active opener and the server is a passive opener.

How is a TCP Connection Established

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 sender and receiver. The following are the steps:

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.

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.

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.

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

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

3. The other end sends its own FIN packet.

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 Work- Step by Step

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).

TCP Header Structure

At the packet level a TCP packet consists of many different fields and each field has its function as follows, here TCP packet header diagram for your reference

● Source port: This is a 16-bit field that specifies the port number of the sending application. If the application is HTTP and it is sending data then its port number is 80.

● Destination port: This is also a 16-bit field, but it specifies the port number of the receiving application. If the receiving application is SSH then its port number is 22.

● Sequence number: This is a 32-bit field that identifies the number of bytes in the data. The data packet follows some order while sending and receiving so the sequence number is used to keep track of the order of packets. The initial data packet has a unique sequence number called ISN (Initial Sequence Number).

The sending host used this number to send data while the receiving host reassembled it and brought the packets in the correct order.

● Acknowledgment number: This is also a 32-bit field, but it is only used by the receiver. It contains the sequence number of the next expected packet, and it is used to acknowledge receipt of packets that have been successfully received.

● Header length: This is a 4-bit field which specifies the TCP header length. The TCP header contains several different fields, and its length can vary depending on which fields are included.

● Reserved: This is a 6-bit field that is reserved for future use and is currently set to zero.

● Control bits: This is a 6-bit field that contains several control flags that control packet processing. Flags like SYN, ACK, FIN, RST, PSH, and URG are used to establish and terminate connections and control how data is processed.

Here is a brief explanation of TCP flags.

1. SYN (Synchronize) - Used to initiate a connection between two hosts.

2. ACK (Acknowledge) - Used to confirm receipt of a packet.

3. FIN (Finish) - Used to terminate a connection.

4. RST (Reset) - Used to reset a connection or signal an error.

5. PSH (Push) - Used to signal the receiving host to pass data directly to the application without buffering.

6. URG (Urgent) - Used to signal that a packet contains urgent data.

● Window size: This is a 16-bit field and specifies the amount of data the receiving host can accept.

● Checksum: This is a field 16-bit field used to verify packet integrity. Checksum uses various factors such as IP addresses and packet contents etc. to calculate its values.

● Urgent pointer: This is a 16-bit field that is used in conjunction with the URG flag to indicate that certain data within the packet is urgent and should be processed immediately.

● Options: This is an optional field that is mostly used to carry information like MSS i.e. Maximum Segment Size. It is the maximum-sized segment that the sender wants to receive.

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.

Features of TCP

The TCP provides reliable communication by using a variety of techniques to detect and correct errors that may occur during transmission.

● Acknowledgments: When a sender sends data packets it expects to receive an acknowledgment from the receiver if the sender receives the acknowledgment within a specific time which ensures successful packet delivery and if acknowledgment is not received by the sender from the receiver that means that packet is lost in the transmission and the sender will send that data packet again.

● TCP flow control is another important feature that prevents the sender from sending too much data that the receiver can not accept. The receiver uses its window size field in the TCP header which specifies how much data it can accept from the receiver within a specific time. The receiver reduces its window size if the sender is sending data too fast which forces the sender to slow down.

● TCP congestion control mechanisms to prevent network congestion and ensure efficient data transmission. One of the primary methods used by TCP for congestion control is called slow start. Slow start gradually increases the amount of data sent until the network reaches an equilibrium state. Once this happens, data transmission can occur more quickly.

How does TCP Work?- 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

In the past, I took a detailed live session on the TCP (in regards to the Load Balancer virtual servers) it should be very useful to understand how virtual servers establish TCP connections with LTM, here is the video recording for the same. I hope this will help many of you who want to understand TCP in the easiest possible way.