What is TCP Header and How it Works?

Transmission Control Protocol (TCP) is a fundamental protocol within the Internet protocol suite that allows for reliable communication between systems on a network.

One key component of TCP's functionality is the TCP header, which contains essential information that allows devices to manage and track data transmission. 

In this article, we'll explore the TCP header in detail, understand its structure, and the different fields in the TCP Header format. We will also look at how a TCP connection is established, its vulnerabilities to Distributed Denial of Service (DDoS) attacks, and how to mitigate such threats. 

Furthermore, if you are interested in learning about the TCP/IP protocol suite, you can check out our Networking courses, where we teach more such protocols in detail.

What is a TCP Header? 

The TCP header is part of every TCP segment sent over the network and contains various fields that manage communication between two devices. These fields facilitate reliable data transmission, connection establishment, error-checking, and flow control.

The size of the TCP header can vary between 20 to 60 bytes depending on the presence of optional fields.

CCNA Training OnlineStart your career in Networking with CCNA Certification training.Explore course

TCP Header Fields

Understanding the individual fields within a TCP header is critical for recognizing how data flows across a network. The following are the key components of the TCP header: 

 1. Source Port Number 

The Source Port Number identifies the port number on the sending device, which is used to distinguish between multiple applications or processes running on that device.

Also Read About Port Numbers

2. Destination Port Number 

The Destination Port Number specifies the receiving port on the destination device. Together with the source port, this helps identify which application should process the incoming data. 

 3. Sequence Number 

The Sequence Number tracks the order of data packets as they are transmitted, ensuring that the recipient can reassemble the data in the correct order. 

 4. Acknowledgment Number 

The Acknowledgment Number is sent by the receiving device to confirm receipt of the data and indicate the next expected byte of data. This mechanism ensures reliability in the communication process. 

 5. Data Offset 

The Data Offset also called the header length, specifies the size of the TCP header. It allows the receiving device to distinguish between header information and the actual payload data. 

 6. Reserved 

The Reserved field is reserved for future use and is set to zero by default. It currently serves no function but ensures compatibility with future protocol updates. 

 7. Flags 

The Flags (also known as control bits) indicate the state or control of the TCP connection. Some key flags include: 

● SYN: Initiates a connection. 

● ACK: Acknowledges received data. 

● FIN: Signals the end of data transmission. 

● RST: Resets the connection. 

● PSH: Pushes data to the receiving application. 

● URG: Marks data as urgent. 

 8. Window Size 

The Window Size field controls the flow of data by indicating how much data the receiving side is willing to accept. This is a key component in managing network congestion. 

 9. Checksum 

The Checksum verifies the integrity of the TCP segment, ensuring that data is not corrupted during transmission. It covers both the header and the data. 

 10. Urgent Pointer 

The Urgent Pointer field indicates that certain data within the segment is urgent and should be processed immediately. This field is only used when the URG flag is set. 

 11. Options 

The Options field is optional and allows for additional functionalities, such as window scaling or timestamping, to improve performance and control of data transmission. 

Check Our Article on IPv4 Header

Why is the TCP Header Important? 

The TCP header is a key component of reliable and efficient communication in network systems. Without the mechanisms provided by the TCP header, data could be lost, corrupted, or delivered out of sequence, leading to poor network performance and degraded user experience.

The points below show why TCP header is an important component of TCP protocol: 

1. Correct Addressing: The TCP header ensures that data packets are directed to the appropriate applications on both the sender’s and receiver’s devices. 

2. Maintaining Proper Order: The sequence and acknowledgment numbers play a crucial role in preserving the order of data packets.

3. Control of Data Transmission: TCP's control flags facilitate important functionalities, such as establishing and closing connections, signaling urgent data, or requesting updates in window size.  

4. Flow Control: The window size field in the TCP header allows for effective flow control, preventing the sender from overwhelming the receiver with too much data at once.

5. Error Detection: The checksum ensures that transmitted data has not been corrupted in transit. It provides a verification mechanism for detecting errors, allowing the receiver to discard damaged packets and request retransmission, thereby preserving the quality of communication. 

6. Enhanced Functionality: The options field enables additional TCP features, such as timestamps, selective acknowledgments, and maximum segment size adjustments. These enhancements improve overall performance, security, and efficiency, ensuring that data transfers are not only fast but also robust against errors and threats. 

How to View and Analyze the TCP Header 

As a network engineer, analyzing the TCP header is essential for tasks like troubleshooting, performance optimization, or learning about network behavior. Several open-source tools provide the ability to view and interpret TCP headers, helping you better understand network traffic. Key tools include: 

1. Wireshark   

Wireshark is a powerful, widely used network protocol analyzer. It allows you to capture both live and offline network traffic, including TCP headers. With Wireshark, you can filter, search, and export specific packets, as well as visualize and decode all fields within the TCP header.

It also offers the ability to generate statistics and graphs, such as throughput, round-trip time, and retransmissions, helping you diagnose and optimize TCP performance. 

2. tcpdump   

For a command-line-based approach, tcpdump is an effective tool that enables you to capture and analyze TCP streams based on specific criteria, like source or destination IP, port, or TCP flags.

tcpdump is excellent for quickly monitoring and troubleshooting TCP traffic in real-time, and it allows you to save and replay captured packets for in-depth post-analysis. 

3. Nmap   

Nmap is a network scanner used to discover and probe the TCP header information of connected devices. It provides insights into operating systems, open ports, and running services.

Nmap is especially useful for mapping your network and evaluating its security posture, while also helping assess TCP performance. 

TCP DDoS Vulnerabilities and Methods of Mitigation 

TCP is vulnerable to Distributed Denial of Service (DDoS) attacks that flood a network, server, or application with excessive TCP requests, rendering it unable to handle legitimate traffic. 

Common TCP-Based DDoS Attacks 

1. SYN Flood: Attackers exploit the SYN phase of the three-way handshake by sending numerous SYN requests without completing the handshake, leading to half-open connections. 

2. ACK Flood: Attackers send a flood of ACK packets, causing the target system to expend resources on processing these bogus acknowledgments. 

3. RST Flood: Attackers send repeated RST packets to reset connections, interrupting legitimate sessions and overwhelming the server. 

How to Handle TCP DDoS Attacks

1. SYN Cookies: Prevents resource exhaustion during a SYN flood by allowing servers to respond to SYN requests without allocating state information until the handshake is completed. 

2. Rate Limiting: Restricts the number of incoming TCP connections or packets, ensuring legitimate traffic can still pass through during an attack. 

3. Traffic Filtering: Configures network devices to filter malicious traffic based on predefined patterns, such as recognizing abnormal rates of SYN packets. 

4. Load Balancing: Distributes incoming traffic across multiple servers to prevent any single system from being overwhelmed, maintaining service availability. 

5. Deep Packet Inspection (DPI): Inspects the content of TCP packets to detect abnormal behavior indicative of DDoS attacks, allowing filtering of malicious traffic before it reaches its target. 

Conclusion 

In summary, understanding TCP headers is crucial for anyone involved in network management and security. The TCP header serves as the backbone of reliable communication, ensuring that data is transmitted accurately and efficiently between devices.

TCP header facilitates correct addressing, maintains the order of packets, controls data flow, detects errors, and enhances functionality using different fields in the TCP header structure.

TCP faces security threats in the form of Distributed Denial of Service (DDoS) attacks. By gaining insights into these vulnerabilities and implementing effective mitigation strategies—such as SYN cookies, rate limiting, and traffic filtering—network administrators can safeguard their systems against potential disruptions.