FIN vs RST in TCP connections

Question

The way I understand this, there are 2 ways to close TCP connection:

• send FIN flag
• send RST flag

RST causes immediate connection termination, while in FIN you get a confirmation.

Do I understand this right, and are there any other distinctions between the two? Can those 2 flags be used together?

Answer 1

• FIN says: "I finished talking to you, but I'll still listen to everything you have to say until you say that you're done."

• RST says: "There is no conversation. I won't say anything and I won't listen to anything you say."

  RST is useful if you have long lasting TCP connection with little traffic. If one of the computers is restarted, it forgets about the connection, and the other computer gets RST, as soon as it sends another packet.

Answer 2

FIN or RST would be sent in the following case

• your process close the socket

• OS is doing the resource cleanup when your process exit without closing socket.

  If your process call close(), FIN would be sent from the closing side by default (note: you can set socket option SO_LINGER to make it send RST instead of FIN)

  If your process exit without closing the socket, kernel would close the tcp connection and do the clean up for your process. FIN or RST can be sent. If there is data in your receive queue, RST would be sent. Otherwise, FIN would be sent.

  You can loop through tcp_close() in tcp.c for more details.(I am using kernel-2.6.32-573.7.1 from redhat branch)