TCP Unveiled: A Deep Dive into How It Works and All TCP Flags Demystified with Real-Life Examples

TCP (Transmission Control Protocol) uses various control flags within its header to manage communication ....

by Vikash Kumawat
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TCP (Transmission Control Protocol) uses various control flags within its header to manage communication between two devices. These flags indicate different aspects of the TCP connection. Let’s explain each TCP flag with examples:

1. SYN (Synchronize) Flag:

  • Function: It is used to initiate a connection and synchronize sequence numbers.
  • Example: When Computer A wants to establish a connection with Computer B, it sends a TCP packet with the SYN flag set.

Computer A –> SYN –> Computer B

2. ACK (Acknowledge) Flag:

  • Function: It acknowledges the receipt of data or a connection request.
  • Example: After receiving a SYN packet from Computer A, Computer B acknowledges it with a packet that has both the SYN and ACK flags set.

Computer B –> SYN-ACK –> Computer A

3. FIN (Finish) Flag:

  • Function: It is used to release a connection gracefully when data transmission is complete.
  • Example: When Computer A has finished sending data, it sends a FIN packet to Computer B to initiate the connection termination.

Computer A –> FIN –> Computer B

4. RST (Reset) Flag:

  • Function: It resets a connection and is used to indicate an error or problem.
  • Example: If Computer B receives unexpected or malformed data, it may send an RST packet to Computer A to reset the connection.

Computer B –> RST –> Computer A

5. PSH (Push) Flag:

  • Function: It instructs the receiving end to push received data to the application layer immediately.
  • Example: If Computer A wants Computer B to process data without delay, it sets the PSH flag in the TCP packet.

Computer A –> Data with PSH –> Computer B

6. URG (Urgent) Flag:

    • Function: It indicates that the urgent pointer field in the TCP header is valid, and the data should be processed urgently.
    • Example: In some cases, data marked as urgent may be given priority by the receiving application.

7. CWR (Congestion Window Reduced) Flag:

    • Function: It indicates that the sender has reduced its congestion window due to network congestion.
    • Example: If Computer A experiences network congestion, it may set the CWR flag in its TCP packets to inform Computer B about the congestion control measures being taken.

8. ECE (Explicit Congestion Notification Echo) Flag:

    • Function: It is used in conjunction with the CWR flag to signal network congestion and control data flow.
    • Example: When Computer A sets the CWR flag, Computer B may respond with the ECE flag to acknowledge the congestion notification.

These TCP flags help manage the communication between devices and enable various functions like connection establishment, data transmission, error handling, and graceful connection termination. They are essential for ensuring reliable and ordered data transfer in TCP/IP communication.

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