In TCP (Transmission Control Protocol), flags are used to control the state and behavior of a TCP connection. Each flag is a single-bit field in the TCP header that indicates specific actions or statuses. Here are the primary TCP flag types:
1. SYN (Synchronize)
Purpose: Initiates a connection between two TCP endpoints. It is used during the TCP three-way handshake process to synchronize sequence numbers between the sender and receiver.
Bit Position: 2nd bit (from the right) in the TCP header.
2. ACK (Acknowledgment)
Purpose: Acknowledges the receipt of packets. It indicates that the sender of the packet has received all data up to the acknowledgment number.
Bit Position: 4th bit (from the right) in the TCP header.
3. FIN (Finish)
Purpose: Gracefully terminates a connection. It indicates that the sender has finished sending data and wants to close the connection.
Bit Position: 1st bit (from the right) in the TCP header.
4. RST (Reset)
Purpose: Resets a connection. It is used to abort a connection due to errors or unexpected conditions and to inform the other side that the connection is being reset.
Bit Position: 5th bit (from the right) in the TCP header.
5. PSH (Push)
Purpose: Pushes data to the receiving application immediately. It indicates that the data should be delivered to the application as soon as possible, bypassing the normal buffering.
Bit Position: 6th bit (from the right) in the TCP header.
6. URG (Urgent)
Purpose: Indicates that the data contained in the packet is urgent and should be prioritized. The urgent pointer field in the TCP header specifies the end of the urgent data.
Bit Position: 3rd bit (from the right) in the TCP header.
7. ECE (ECN Echo)
Purpose: Echoes back an Explicit Congestion Notification (ECN) to indicate that congestion was encountered. It is used in conjunction with the CWR flag.
Bit Position: 8th bit (from the right) in the TCP header.
8. CWR (Congestion Window Reduced)
Purpose: Indicates that the sender has received a TCP segment with the ECE flag set and has reduced its congestion window accordingly.
Bit Position: 7th bit (from the right) in the TCP header.
9. NS (Nonce Sum)
Purpose: Used in newer ECN (Explicit Congestion Notification) implementations to detect inadvertent ECN support in older systems that might cause protocol issues.
Bit Position: 9th bit (from the right) in the TCP header.
Summary of Flags and Their Positions:
URG: Urgent data (bit 3)
ACK: Acknowledgment (bit 4)
PSH: Push (bit 6)
RST: Reset (bit 5)
SYN: Synchronize (bit 2)
FIN: Finish (bit 1)
ECE: ECN Echo (bit 8)
CWR: Congestion Window Reduced (bit 7)
NS: Nonce Sum (bit 9)
These flags play crucial roles in managing TCP connections, ensuring reliable communication, handling connection establishment and termination, and dealing with congestion and errors.
