Table of Contents

Understanding TCP PSH Packet Flag

TCP (Transmission Control Protocol) is a crucial part of internet communication, responsible for ensuring the reliable delivery of data packets between devices.

The TCP protocol defines various flags, one of which is the PSH (Push) flag.

In this article, we will delve deeper into the TCP PSH flag, its purpose, and how it affects data transmission.

TCP is a connection-oriented protocol that provides reliable data transfer between devices. It is used in many applications, such as web browsing, file transfer, email, VoIP, and gaming.

TCP packets contain several fields, including the source and destination port numbers, sequence and acknowledgement numbers, and control flags. The PSH flag is one of these control flags and plays a vital role in data transmission.

What is TCP PSH flag?

The TCP PSH (Push) flag is a control flag used to indicate that the receiving device should deliver the data to the receiving application as soon as possible, rather than buffering it.

When the PSH flag is set, it instructs the receiving device to deliver the data immediately to the application layer without waiting for more data to arrive.

3. Why is PSH flag important?

The PSH flag is important in real-time applications that require low latency and high throughput, such as video conferencing, online gaming, and voice over IP (VoIP).

By delivering the data as soon as it arrives, the PSH flag reduces the delay and ensures a smooth and uninterrupted flow of data.

4. How does PSH flag work?

When a sender sets the PSH flag in a TCP packet, it indicates that the data is urgent and should be sent immediately to the receiving device.

Upon receiving the packet, the receiving device delivers the data to the receiving application layer immediately, even if more data is expected. This process reduces the buffering delay, ensuring faster and more efficient data delivery.

5. PSH flag and real-time applications

Real-time applications require low latency and high throughput to provide a seamless experience to the user. The PSH flag is crucial in such applications as it reduces the delay in delivering data, ensuring a smooth and uninterrupted flow of data.

For example, in video conferencing, the PSH flag reduces the delay in delivering audio and video data, ensuring real-time communication.

6. PSH flag vs. URG flag

The URG (Urgent) flag is another control flag in TCP, which is used to indicate that some data in the packet is urgent and requires immediate attention. However, unlike the PSH flag, the URG flag is rarely used in modern applications.

The PSH flag is more commonly used as it reduces the delay in delivering data and improves the overall performance of the application.

7. PSH flag in Wireshark

Wireshark is a popular network protocol analyzer that captures and displays the network traffic in real-time. It provides detailed information about the TCP packets, including the PSH flag. In Wireshark, the PSH flag is represented as a field in the TCP packet header.

To view the PSH flag in Wireshark, select the TCP packet of interest, and look for the "Flags" field in the "Transmission Control Protocol" section. If the PSH flag is set, it will be displayed as "PSH" in the flags field.

Additionally, Wireshark also provides a filter for PSH packets. To filter PSH packets, type "tcp.flags.push == 1" in the filter box, and Wireshark will display only packets with the PSH flag set.

8. PSH flag and HTTP

HTTP (Hypertext Transfer Protocol) is a popular application layer protocol used in web browsing. When a web page is requested by the client, the server sends the page in several packets.

The PSH flag is set in the last packet, indicating that the server has completed sending the page and the client can start rendering it. The PSH flag improves the web browsing experience by reducing the delay in rendering the page.

9. PSH flag and Email

Email is another application that uses TCP for message delivery. When an email is sent, it is divided into several packets, and the PSH flag is set in the last packet, indicating that the email message is complete.

The PSH flag ensures that the message is delivered to the recipient without any delay.

10. PSH flag and VoIP

VoIP (Voice over Internet Protocol) is a real-time application that requires low latency and high throughput.

The PSH flag is crucial in VoIP as it reduces the delay in delivering audio packets, ensuring a smooth and uninterrupted conversation between the parties.

11. PSH flag and Gaming

Online gaming is another real-time application that relies heavily on TCP for data transmission.

The PSH flag is important in gaming as it reduces the delay in delivering game data, ensuring a seamless and lag-free gaming experience.

12. PSH flag and Security

The PSH flag can also be used as a security measure to protect against packet injection attacks. Packet injection attacks involve an attacker injecting malicious packets into the network to disrupt the communication between devices.

By setting the PSH flag in the packets, the receiving device can identify the malicious packets and discard them, ensuring the security of the network.

13. PSH flag and Network Optimization

The PSH flag can also be used to optimize network performance.

By setting the PSH flag, the sender can reduce the number of packets sent, thereby reducing network congestion and improving overall network performance.


In conclusion, network security is a critical aspect of any organization's IT infrastructure, and it's essential to have a comprehensive understanding of the latest security threats and countermeasures. With the rapid evolution of technology, network security threats are becoming increasingly sophisticated, making it imperative for IT professionals to stay up-to-date with the latest security techniques and tools.

Orhan Ergun's network security courses are an excellent resource for IT professionals looking to enhance their knowledge and skills in network security. The courses cover a wide range of topics, including network security fundamentals, threat analysis, network defense, and incident response.

Created by
Stanley Arvey

I am a certified network engineer with over 10 years of experience in the field. I have a deep understanding of networking and IT security, and I am always looking for new challenges.

View profile