tcp中的状态定义

查看内核源代码中的定义
net/ipv4/tcp.c
/*

Description of States:

 *

 *    TCP_SYN_SENT         

sent a connection request, waiting for ack

 *

 *    TCP_SYN_RECV        

received a connection request, sent ack,

 *                         waiting for final ack in three-way handshake.

 *

 *    TCP_ESTABLISHED          

connection established

 *

 *    TCP_FIN_WAIT1        

our side has shutdown, waiting to complete

 *                         transmission of remaining buffered data

 *

 *    TCP_FIN_WAIT2        

all buffered data sent, waiting for remote

 *                         to shutdown

 *

 *    TCP_CLOSING           

both sides have shutdown but we still have

 *                         data we have to finish sending

 *

 *    TCP_TIME_WAIT       

timeout to catch resent junk before entering

 *                         closed, can only be entered from FIN_WAIT2

 *                         or CLOSING.  Required because the other end

 *                         may not have gotten our last ACK causing it

 *                         to retransmit the data packet (which we ignore)

 *

 *    TCP_CLOSE_WAIT            

remote side has shutdown and is waiting for

 *                         us to finish writing our data and to shutdown

 *                         (we have to close() to move on to LAST_ACK)

 *

 *    TCP_LAST_ACK         

out side has shutdown after remote has

 *                         shutdown.  There may still be data in our

 *                         buffer that we have to finish sending

 *

 *    TCP_CLOSE        

socket is finished

 */

tcp客户端connect发起之后,协议栈就会发送SYN给服务器
                                      协议栈收到来自服务器的响应后,发出Ack后,就进入了established
tcp服务器在收到SYN后,发送SYN+ACK后就进入SYN_RECVD,再次收到客户端的Ack时,也进入established
可见established状态连接的双方进入的时机是有先后的,客户端是先进入的,服务器是后进入的