TCP

Socket and connection state

     CLOSED       ---- Closed.  The socket  is  not  being used.
     LISTEN       ---- Listening for incoming connections.
     SYN_SENT     ---- Actively trying to  establish  connection.
     SYN_RECEIVED ---- Initial synchronization of the connection under way.
     ESTABLISHED  ---- Connection has been established.
     CLOSE_WAIT   ---- CloseWait - Remote shut down; waiting  for  the socket to close.
     FIN_WAIT_1   ---- Socket closed; shutting  down  connection.
     CLOSING      ---- Closed,   then   remote   shutdown; awaiting acknowledgement.
     LAST_ACK     ---- Remote  shut  down,  then   closed ;awaiting acknowledgement.
     FIN_WAIT_2   ---- Socket closed; waiting for shutdown from remote.
     TIME_WAIT    ---- TimeWait - Wait after close for  remote  shutdown retransmission.

Flow

• This side closes first:
  • ESTABLISHED -> FIN_WAIT_1-> FIN_WAIT_2 -> TIME_WAIT -> CLOSED.
• The other side closes first:
  • ESTABLISHED -> CLOSE_WAIT -> LAST_ACK -> CLOSED.
• Both sides close at the same time:
  • ESTABLISHED -> FIN_WAIT_1-> CLOSING ->TIME_WAIT -> CLOSED.