TCP Flags (Cont.)

• FIN (F)

• • Initiates a connection teardown.
  • Indicates to the receiver that the sender will not transmit any more data

• RST (R)

• • Tells the recipient that the TCP connection that it is trying to use is no longer or never was valid
  • Response to a connection request to a port with no server
  • • TCP uses this rather than ICMP port unreachable
  • Response to data sent to a valid port but on an invalid socket
  • • E.g., one side reboots and loses connection state
  • Aborts a TCP connection

• URG (U)

• • The urgent pointer field is valid