Different between TCP RST and TCP FIN?

 TCP (Transmission Control Protocol) RST (Reset) and FIN (Finish) are two different TCP flags used in the TCP header during the process of establishing and terminating a connection. Here's a brief explanation of each:

 

### TCP RST (Reset):

 

- **Purpose:**

  - Sent by a TCP entity to reset a connection.

  - Typically used to indicate an error or an unexpected condition, such as an attempt to establish a connection to a closed port or an attempt to send data on a connection that has already been closed.

 

- **Connection Termination:**

  - A TCP RST is not part of the normal connection termination process.

  - It is more abrupt and does not go through the usual graceful closure process.

 

- **Example Scenarios:**

  - If a client attempts to connect to a port on a server that is not listening, the server may respond with a TCP RST to indicate that the connection request is not accepted.

  - If a sender attempts to send data on a connection that has already been closed by the receiver, the receiver may respond with a TCP RST.

 

- **Header Flag:**

  - The RST flag is set in the TCP header to indicate the RST condition.

 

### TCP FIN (Finish):

 

- **Purpose:**

  - Sent to initiate a graceful connection termination.

  - Indicates that the sender has finished sending data and wants to close the connection.

 

- **Connection Termination:**

  - The FIN flag is part of the standard connection termination process.

  - Initiates a three-way handshake for connection closure: FIN, ACK, FIN.

 

- **Example Scenarios:**

  - When a client or server has no more data to send, it sends a TCP FIN to signal the end of its data transmission and initiate the connection termination process.

 

- **Header Flag:**

  - The FIN flag is set in the TCP header to indicate the end of the sender's data.

 

### Summary:

 

- **TCP RST:** Abruptly terminates a connection and is often used to indicate errors or unexpected conditions. It is not part of the graceful connection termination process.

 

- **TCP FIN:** Initiates a graceful connection termination, indicating that the sender has finished sending data and wishes to close the connection. It is part of the standard connection closure process.

 

In summary, RST is used for abrupt connection termination and error signaling, while FIN is used to gracefully initiate the process of closing a TCP connection.