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Code

#define TCPS_ESTABLISHED 4 /* established */
(Notice useless comment.)
Documentation

RFC-793
ESTABLISHED - represents an open connection, data received can be delivered to the user. The normal state for the data transfer phase of the connection.
                                   +---------+ ---------\      active OPEN  
                                   |  CLOSED |            \    -----------  
                                   +---------+<---------\   \   create TCB  
                                     |     ^              \   \  snd SYN    
                        passive OPEN |     |   CLOSE        \   \           
                        ------------ |     | ----------       \   \         
                         create TCB  |     | delete TCB         \   \       
                                     V     |                      \   \     
                                   +---------+            CLOSE    |    \   
                                   |  LISTEN |          ---------- |     |  
                                   +---------+          delete TCB |     |  
                        rcv SYN      |     |     SEND              |     |  
                       -----------   |     |    -------            |     V  
      +---------+      snd SYN,ACK  /       \   snd SYN          +---------+
      |         |<-----------------           ------------------>|         |
      |   SYN   |                    rcv SYN                     |   SYN   |
      |   RCVD  |<-----------------------------------------------|   SENT  |
      |         |                    snd ACK                     |         |
      |         |------------------           -------------------|         |
      +---------+   rcv ACK of SYN  \       /  rcv SYN,ACK       +---------+
        |           --------------   |     |   -----------                  
        |                  x         |     |     snd ACK                    
        |                            V     V                                
        |  CLOSE                   +---------+                              
        | -------                  |  ESTAB  |                              
        | snd FIN                  +---------+                              
        |                   CLOSE    |     |    rcv FIN                     
        V                  -------   |     |    -------                     
      +---------+          snd FIN  /       \   snd ACK          +---------+
      |  FIN    |<-----------------           ------------------>|  CLOSE  |
      | WAIT-1  |------------------                              |   WAIT  |
      +---------+          rcv FIN  \                            +---------+
        | rcv ACK of FIN   -------   |                            CLOSE  |  
        | --------------   snd ACK   |                           ------- |  
        V        x                   V                           snd FIN V  
      +---------+                  +---------+                   +---------+
      |FINWAIT-2|                  | CLOSING |                   | LAST-ACK|
      +---------+                  +---------+                   +---------+
        |                rcv ACK of FIN |                 rcv ACK of FIN |  
        |  rcv FIN       -------------- |    Timeout=2MSL -------------- |  
        |  -------              x       V    ------------        x       V  
         \ snd ACK                 +---------+delete TCB         +---------+
          ------------------------>|TIME WAIT|------------------>| CLOSED  |
                                   +---------+                   +---------+