Applies to:

Goal

This document provides information about all TCP socket states

Solution

The possible state values for TCP sockets are as follows:

(Refer to "man netstat".)

BOUND         Bound, ready to connect or listen.

CLOSED        Closed. The socket is not being used.

CLOSING       Closed, then remote shutdown; awaiting acknowledgment.

CLOSE_WAIT    Remote shutdown; waiting for the socket to close.

ESTABLISHED   Connection has been established.

FIN_WAIT_1    Socket closed; shutting down connection.

FIN_WAIT_2    Socket closed; waiting for shutdown from remote.

IDLE          Idle, opened but not bound.

LAST_ACK      Remote shutdown, then closed; awaiting acknowledgment.

LISTEN        Listening for incoming connections.

SYN_RECEIVED  Active/initiate synchronization received and the connection under way

SYN_SENT      Actively trying to establish connection.

TIME_WAIT     Wait after close for remote shutdown retransmission.

The following explains relationship between system calls of socket program and TCP socket states.

Connection Phase in detail

               Server          Client

                 |      Syn       |
    SYN_RECEIVED |
						

							 

       

Closing Phase in detail
						
               Server          Client

                 |      Fin       |
      CLOSE_WAIT |
						

							 



The following are the snoop output for a simple TCP connection.
						
						

							# snoop -i  -r port 1122

client -> server     TCP D=1122 S=33165 Syn Seq=2277924106 Len=0 Win=24820

server -> client     TCP D=33165 S=1122 Syn Ack=2277924107 Seq=484525574 Len=0 Win=8760

client -> server     TCP D=1122 S=33165     Ack=484525575 Seq=2277924107 Len=0 Win=24820

client -> server     TCP D=1122 S=33165     Ack=484525575 Seq=2277924107 Len=256 Win=24820

server -> client     TCP D=33165 S=1122     Ack=2277924363 Seq=484525575 Len=0 Win=8504

server -> client     TCP D=33165 S=1122     Ack=2277924363 Seq=484525575 Len=256 Win=8760

client -> server     TCP D=1122 S=33165     Ack=484525831 Seq=2277924363 Len=0 Win=24820

client -> server     TCP D=1122 S=33165 Fin Ack=484525831 Seq=2277924363 Len=0 Win=24820

server -> client     TCP D=33165 S=1122     Ack=2277924364 Seq=484525831 Len=0 Win=8760

server -> client     TCP D=33165 S=1122 Fin Ack=2277924364 Seq=484525831 Len=0 Win=8760

client -> server     TCP D=1122 S=33165     Ack=484525832 Seq=2277924364 Len=0 Win=24820