What Are TCP Communication Flags? A Complete Guide to TCP Header Flags (SYN, ACK, FIN, RST, PSH, URG) With Real-Time Examples and Header Structure

Table of Contents

• Introduction
• What Are TCP Communication Flags?
• The Six Primary TCP Flags and Their Functions
• Structure of TCP Header and Flag Location
• TCP 3-Way Handshake Using Flags
• Real-Time Example of TCP Flags in Action
• Why TCP Flags Matter in Cybersecurity
• TCP Flag-Based Scanning Techniques
• Conclusion
• Frequently Asked Questions (FAQs)

Introduction

In the world of networking, Transmission Control Protocol (TCP) plays a crucial role in enabling reliable, ordered, and error-checked delivery of data between applications running on hosts across a network. One of the key components that make TCP robust is its communication flags, which control every aspect of a TCP session—from connection establishment to termination. In this blog, we’ll explore the structure, purpose, and function of TCP flags, supported by real-time use cases and TCP header diagrams.

What Are TCP Communication Flags?

TCP communication flags are 1-bit field values in the TCP header that manage the state and control of TCP sessions. These flags are responsible for signaling various events, such as connection initiation, acknowledgment of data, and session termination. Out of the 8 bits allocated for flags in the TCP header, 6 are commonly used for core operations.

The Six Primary TCP Flags and Their Functions

Let’s explore each flag in detail with its purpose and real-world relevance.

1. SYN (Synchronize)

The SYN flag is used to initiate a TCP connection. It synchronizes sequence numbers between sender and receiver.

• Function: Begins the TCP 3-way handshake.

• Use Case: A client connects to a server by sending a SYN packet.

2. ACK (Acknowledgment)

The ACK flag is used to acknowledge received data or control messages. It’s used in almost all TCP communications after the initial SYN.

• Function: Confirms receipt of data.

  
  

• Use Case: After receiving a packet, the host sends back an ACK to confirm it.

3. FIN (Finish)

The FIN flag is used to gracefully terminate an active TCP connection between hosts.

• Function: Ends the TCP session cleanly.

• Use Case: When a browser tab is closed, a FIN is sent to the server.

4. RST (Reset)

The RST flag is used to immediately terminate a TCP session due to an error or abrupt end.

• Function: Aborts connection instantly.

• Use Case: If a packet is sent to a closed port, the server responds with an RST.

5. PSH (Push)

The PSH flag instructs the TCP stack to push buffered data to the application immediately instead of waiting for the buffer to fill.

• Function: Forces immediate delivery.

• Use Case: Messaging apps that require real-time delivery use PSH.

6. URG (Urgent)

The URG flag signals that the data in the segment is urgent and should be processed before other queued segments.

• Function: Prioritizes specific data.

• Use Case: Legacy systems like Telnet use URG to send control commands.

Structure of TCP Header and Flag Location

The TCP header is composed of multiple fields, and the TCP Flags field is embedded among them. The key parts of the TCP header include:

• Source Port

• Destination Port

• Sequence Number

• Acknowledgment Number

• Data Offset

• Reserved Bits

• Control Flags (SYN, ACK, FIN, etc.)

• Window Size

• Checksum

• Urgent Pointer

• Options and Padding

Below is the diagram representing the structure:

  0      4       8      12     16     20     24     28     31
+--------+--------+--------+--------+--------+--------+--------+
| Source Port     | Destination Port                          |
+-----------------+-------------------------------------------+
| Sequence Number                                         |
+------------------------------------------------------------+
| Acknowledgment Number                                  |
+-----------------+------+--------+--------+--------+--------+
| Data Offset     | Res  | URG | ACK | PSH | RST | SYN | FIN |
+-----------------+------+--------+--------+--------+--------+
| Window Size                                          |
+------------------------------------------------------------+
| Checksum        | Urgent Pointer                          |
+-----------------+-------------------------------------------+
| Options and Padding                                    |
+------------------------------------------------------------+

TCP 3-Way Handshake Using Flags

To establish a reliable connection, TCP uses a 3-way handshake, leveraging three flags:

1. SYN – Client sends SYN to server.

2. SYN-ACK – Server responds with SYN and ACK.

3. ACK – Client sends ACK to complete the handshake.

This handshake ensures both parties are synchronized and ready to exchange data securely.

Real-Time Example of TCP Flags in Action

Let’s imagine a user opening a banking website:

• SYN: The browser sends a SYN request to the banking server.

• SYN-ACK: The server replies with SYN-ACK, signaling it's ready to connect.

• ACK: The browser confirms with ACK.

• PSH + ACK: The login request is pushed immediately to the server.

• FIN + ACK: After logging out, the session is gracefully closed using FIN.

This example shows how multiple flags work together to maintain a complete session lifecycle.

Why TCP Flags Matter in Cybersecurity

TCP flags are frequently analyzed in network forensics and intrusion detection because they can reveal:

• Unusual scanning patterns (e.g., FIN or NULL scans).

• DDoS attempts (e.g., SYN Floods).

• Session hijacking or resets (via forged RST flags).

• Command and control (C2) channels in malware using abnormal flag combinations.

TCP Flag-Based Scanning Techniques

• SYN Scan: Fast and stealthy; attacker sends SYN and waits for SYN-ACK.

• FIN Scan: Sends only a FIN to discover closed ports (no response means open).

• XMAS Scan: Sends packets with FIN, URG, and PSH to evade detection.

• NULL Scan: Sends a packet with no flags set to bypass firewalls.

These scanning methods are part of reconnaissance in ethical hacking and are often executed using tools like Nmap.

Conclusion

TCP communication flags are small, but they play a critical role in the stability, reliability, and security of internet communications. Whether you're debugging a network issue, securing your system, or performing penetration testing, a solid understanding of SYN, ACK, FIN, RST, PSH, and URG is essential.

The TCP flag field enables powerful, flexible, and resilient control over data flow, ensuring that every transmission reaches its destination safely and correctly. With proper usage and monitoring of TCP flags, professionals can maintain robust network health, detect anomalies, and improve cybersecurity posture.

Frequently Asked Questions (FAQs)

What is a TCP flag and why is it used in network communication?

TCP flags are 1-bit control fields within the TCP header that manage the state and flow of a TCP connection. They help initiate, maintain, and terminate sessions between systems.

How many TCP flags are there and what are their names?

There are six main TCP flags: SYN (Synchronize), ACK (Acknowledgment), FIN (Finish), RST (Reset), PSH (Push), and URG (Urgent).

What is the function of the SYN flag in TCP?

The SYN flag is used to initiate a connection between two hosts. It’s the first step in the TCP 3-way handshake process.

Why is the ACK flag important in TCP sessions?

The ACK flag confirms receipt of data. It is used in every step after the initial SYN to acknowledge the previous message.

What happens when the FIN flag is set in a TCP packet?

The FIN flag is used to gracefully terminate a TCP connection, indicating that the sender has finished transmitting data.

When is the RST (reset) flag used in TCP?

The RST flag is used to abruptly terminate a connection when something unexpected occurs or to reject an invalid connection attempt.

What is the use of the PSH (push) flag in TCP?

The PSH flag tells the receiving system to process the data immediately, without waiting for more packets to arrive.

How does the URG (urgent) flag work in TCP?

The URG flag indicates that certain data within the packet should be treated as high-priority and processed immediately.

Are TCP flags always set to 1?

No, each TCP flag is only set to 1 when it is actively being used for that packet’s purpose; otherwise, it remains 0.

Can multiple TCP flags be set at the same time?

Yes, combinations like SYN-ACK or FIN-ACK are common in TCP communication, especially during connection establishment or termination.

What is a TCP 3-way handshake and which flags are used in it?

The TCP 3-way handshake uses SYN, SYN-ACK, and ACK flags to establish a reliable connection between a client and server.

How does the TCP header structure look like?

The TCP header includes fields such as source port, destination port, sequence number, acknowledgment number, and TCP flags, among others.

Where are TCP flags located in the TCP packet?

TCP flags are located in the control bits section of the TCP header, typically in the middle of the structure after sequence and acknowledgment numbers.

What is the size of each TCP flag?

Each TCP flag occupies 1 bit in the header. With six primary flags, the total size of the TCP flags field is 6 bits.

How do TCP flags relate to packet sniffing and analysis?

Security tools like Wireshark use TCP flags to identify the nature of traffic, spot handshake sequences, and detect anomalies or scanning attempts.

What is a SYN flood attack and how does it misuse TCP flags?

A SYN flood attack overwhelms a server by sending numerous SYN requests without completing the handshake, exhausting system resources.

Can TCP flags help detect port scanning techniques?

Yes, Nmap and other scanners use TCP flags in special combinations (like NULL or XMAS scans) to probe ports and bypass firewalls.

What is a FIN scan and how does it exploit TCP flags?

A FIN scan sends packets with only the FIN flag set. If no response is received, the port is considered open, helping evade detection.

How do NULL and XMAS scans use TCP flags to evade firewalls?

NULL scans use packets with no flags set, while XMAS scans set multiple flags (FIN, PSH, URG) to probe systems stealthily.

What is the difference between SYN and ACK flags in TCP?

SYN starts a connection, while ACK confirms the receipt of data or connection steps. SYN is used first, ACK follows in response.

What is the purpose of using the RST flag during invalid sessions?

The RST flag is used when a system receives a packet for a closed port or invalid connection, allowing it to forcibly end the session.

How do TCP flags impact application layer protocols like HTTP?

TCP flags ensure a stable session for protocols like HTTP, which depend on reliable delivery. They help start, manage, and close connections.

What tools can be used to analyze TCP flags in real-time?

Popular tools include Wireshark, TCPDump, Nmap, Zeek, and Snort, all capable of capturing and interpreting TCP flags.

How does Wireshark display TCP flags in packet analysis?

In Wireshark, each TCP packet shows the flags set (e.g., SYN, ACK) in the detailed view of the TCP segment within a packet.

Can improper flag configurations lead to network vulnerabilities?

Yes, incorrect handling of TCP flags may leave systems open to DoS attacks, scanning techniques, or unauthorized access.

What is the significance of the URG pointer in relation to URG flag?

When the URG flag is set, the urgent pointer field points to the location where the urgent data ends in the TCP segment.

Is TCP communication possible without using all flags?

Yes, not all flags are required for every session. Only specific flags are used depending on the communication phase.

How do TCP flags contribute to reliable data delivery?

They coordinate the sending and receiving of data, handle sequencing, retransmission, and ensure error-free, ordered communication.

Can a firewall or IDS block or monitor TCP flags?

Yes, firewalls and intrusion detection systems (IDS) can filter or flag unusual combinations of TCP flags to prevent suspicious traffic.

What are some real-world examples where TCP flags are used maliciously?

Hackers may use SYN floods for DoS attacks, XMAS scans for reconnaissance, or RST flags to disrupt legitimate sessions.