TCP Flags


When we think about the internet, we often think of websites and social media platforms. However, beneath the surface of these applications lies a complex network of protocols and technologies that communicate data between devices. One such technology is TCP, or Transmission Control Protocol, which is responsible for ensuring reliable data transmission over the internet.

Within TCP are a set of flags that play a critical role in facilitating communication between devices. In this article, we will dive into the world of TCP flags − what they are, why they matter, and how they work.

Definition of TCP Flags

TCP flags are small pieces of data within a TCP packet that provide important information about the state of a network connection. These flags are binary values that can be set to 1 or 0 to indicate whether certain conditions are true or false. The flags serve as control bits that help manage various aspects of network communication.

Explanation of the Different Types of TCP Flags

A Comprehensive Guide to Each Flag's Purpose

The SYN flag is used by devices to initiate a connection and request synchronization with another device. The ACK flag is then sent from the receiving device to acknowledge the receipt of data. The RST flag can be used by either device to terminate a connection abruptly or reset it if there is an issue.

The PSH flag is often used when time−sensitive data needs to be sent through the network stack quickly without waiting for more packets before sending them on to the application layer. The URG flag serves as an indicator that certain data requires immediate attention and should not be delayed in transmission.

The FIN flag marks the end of a connection between two devices. When one device sends this signal, it lets the other know that no more data will be sent and that they can close their end of the connection as well.

SYN Flag

The SYN flag, or "synchronize" flag, is a crucial piece of any TCP connection. It is used to initiate a connection between two devices on a network.

Definition and Purpose

In technical terms, the SYN flag is used to synchronize both communication endpoints before data transmission begins. Its purpose is to establish a starting sequence number for the first packet in the session, which will be used to identify and track packets as they are transmitted between devices.

How it Initiates a Connection

The process of initiating a connection using the SYN flag involves three steps:

  • The sending device sends out a packet with only the SYN (synchronize) bit set in its header. This initiates what's called a "SYN−ACK" exchange.

  • The receiving device responds by sending back an ACK (acknowledgment) message with its own sequence number and an acknowledgment of receipt of the sender's sequence number.

  • The originating device acknowledges receipt of this response by sending another ACK message back to its peer.

ACK Flag

Definition and Purpose

The ACK flag in TCP (Transmission Control Protocol) is used to acknowledge the receipt of data. When a device receives data, it sends an acknowledgment packet back to the sender indicating that it has received the data successfully. The ACK flag is commonly used in network communications protocols where reliable and error−free data transmission is crucial.

How it Acknowledges Receipt of Data

When a device receives a packet from another device, it examines the contents of the packet and checks for any errors or corruption. If there are no errors detected in the packet, then an acknowledgment packet is sent back to the sender with an ACK flag set. The acknowledgment packet contains information such as sequence numbers and window sizes to help ensure reliable transmission of data.

RST Flag

Definition and Purpose

The RST flag is shorthand for "reset" and is used to abruptly terminate a TCP connection. When this flag is sent, it indicates that the recipient should immediately stop any further processing of packets related to the connection being reset.

How it Resets a Connection

When the RST flag is sent, it immediately triggers an abrupt termination of the TCP connection. This means that any remaining data in transit will be lost, and both sides of the connection will need to re−establish communication from scratch if they wish to continue communicating with each other. The use of an RST flag can be seen as a sort of "emergency brake" for TCP connections, allowing one side to quickly shut things down if something has gone wrong.

URG Flag

The URG flag is one of the TCP flags that can be set in a TCP packet header. It stands for "urgent" and is used to indicate that certain data in the packet requires immediate attention. When the URG flag is set, it means that the receiver should prioritize processing this data before any other incoming data.

Definition and purpose

The URG flag has a specific purpose in network communication. It indicates that certain data within a TCP packet must be processed urgently by the receiving device.

This could be because it contains critical information or because it needs immediate action from the receiver. When this flag is set, it instructs the receiving device to process this particular segment as soon as possible, even if it means interrupting currently ongoing communications.

How it indicates that certain data is urgent or requires immediate attention.

When a segment of data with the URG flag set arrives at its destination, the receiver knows that it must give priority to processing this data before any other incoming packets. The receiver will prioritize processing this packet to ensure timely delivery of urgent information. The URG pointer field in a TCP header points to where exactly in the received packet's data stream lies an urgent message, making prioritization possible.

FIN Flag

The FIN flag, which stands for "final," is used to terminate a connection between two devices. When a device sends a FIN packet to another device, it means that it wants to close the connection and stop exchanging data.

Definition and Purpose

The purpose of the FIN flag is to signal the end of a TCP connection. When both devices have sent and received their respective FIN packets, the connection is considered closed. This ensures that there are no open connections left between devices after they have finished communicating.

How it Terminates a Connection Between Two Devices

The process of terminating a TCP connection using the FIN flag involves sending a series of packets back and forth between two devices. Once both devices have sent and received their respective FIN packets, they know that each other wants to end the communication session.

When one device sends a FIN packet, it sets the FIN bit in its TCP header to indicate that it has no more data to send. The other device will respond with an ACK packet as soon as it receives this message.


TCP flags play an essential role in modern network communication. They provide the necessary control mechanisms to ensure that packets are delivered efficiently and reliably across networks.

The different flags serve unique purposes in connection initiation, data acknowledgment, resetting of connections, packet delivery optimization, indication of urgent data, and connection termination. Understanding the functionality of each flag is crucial for network administrators and engineers who work with TCP protocols daily.

Updated on: 11-Jul-2023


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