In-Depth Explanation of Each TCP Flag

Purpose: Indicates that the data being sent is urgent and should be processed immediately.

How it Works:

The Urgent Pointer field in the TCP header is used in combination with the URG flag.

This ensures critical data reaches the receiving application without delay.

Use Cases: ✅ Emergency communication (e.g., VoIP, financial transactions)
✅ Interrupt signals (e.g., CTRL+C to terminate a process in SSH)

Purpose: Used to confirm receipt of packets.

How it Works:

The ACK Number field contains the next expected sequence number.

Every packet (except the first SYN) should have this flag set.

Use Cases: ✅ Confirms successful packet delivery
✅ Used in reliable data transfer

Purpose: Forces data to be immediately delivered to the application without buffering.

How it Works:

Normally, data can be buffered before being sent to an application.

The PSH flag bypasses buffering and sends the data instantly.

Use Cases: ✅ Live chat applications
✅ Streaming services
✅ Real-time data transmission

Purpose: Abruptly terminates a connection.

How it Works:

If a device receives an unexpected packet, it may respond with an RST flag.

Prevents unauthorized connections from being established.

Use Cases: ✅ Connection reset when an error occurs
✅ Protection against unauthorized access

🚨 Security Concern:
🔴 Attackers use RST Injection Attacks to forcefully close TCP connections.

Purpose: Used in the TCP 3-way handshake to establish a connection.

How it Works:

The client sends a SYN packet with an Initial Sequence Number (ISN).

The server responds with SYN-ACK.

The client acknowledges with ACK.

Use Cases: ✅ Initiating new connections
✅ TCP handshake

🚨 Security Concern:
🔴 Attackers use SYN Flood Attacks to overwhelm a server by sending multiple SYN requests without completing the handshake.

Purpose: Used to gracefully close a connection.

How it Works:

The sender sends a FIN packet.

The receiver acknowledges with ACK.

The receiver sends its own FIN.

The sender acknowledges with ACK (4-way handshake).

Use Cases: ✅ Graceful termination of TCP sessions
✅ Ending communication after data transfer