# 附录：TCP连接的状态 1. LISTEN:首先服务端需要打开一个socket进行监听，状态为LISTEN. /\* The socket is listening for incoming connections. 侦听来自远方TCP端口的连接请求 \*/ 2. SYN\_SENT:客户端通过应用程序调用connect进行active open.于是客户端tcp发送一个SYN以请求建立一个连接.之后状态置为SYN\_SENT. /\*The socket is actively attempting to establish a connection. 在发送连接请求后等待匹配的连接请求 \*/ 3. SYN\_RECV:服务端应发出ACK确认客户端的SYN,同时自己向客户端发送一个SYN.之后状态置为SYN\_RECV /\* A connection request has been received from the network. 在收到和发送一个连接请求后等待对连接请求的确认 \*/ 4. ESTABLISHED: 代表一个打开的连接，双方可以进行或已经在数据交互了。/\* The socket has an established connection. 代表一个打开的连接，数据可以传送给用户 \*/ 5. FIN\_WAIT1:主动关闭(active close)端应用程序调用close，于是其TCP发出FIN请求主动关闭连接，之后进入FIN\_WAIT1状态./\* The socket is closed, and the connection is shutting down. 等待远程TCP的连接中断请求，或先前的连接中断请求的确认 \*/ 6. CLOSE\_WAIT:被动关闭(passive close)端TCP接到FIN后，就发出ACK以回应FIN请求(它的接收也作为文件结束符传递给上层应用程序),并进入CLOSE\_WAIT. /\* The remote end has shut down, waiting for the socket to close. 等待从本地用户发来的连接中断请求 \*/ 7. FIN\_WAIT2:主动关闭端接到ACK后，就进入了FIN-WAIT-2 ./\* Connection is closed, and the socket is waiting for a shutdown from the remote end. 从远程TCP等待连接中断请求 \*/ 8. LAST\_ACK:被动关闭端一段时间后，接收到文件结束符的应用程序将调用CLOSE关闭连接。这导致它的TCP也发送一个 FIN,等待对方的ACK.就进入了LAST-ACK . /\* The remote end has shut down, and the socket is closed. Waiting for acknowledgement. 等待原来发向远程TCP的连接中断请求的确认 \*/ 9. TIME\_WAIT:在主动关闭端接收到FIN后，TCP就发送ACK包，并进入TIME-WAIT状态。/\* The socket is waiting after close to handle packets still in the network.等待足够的时间以确保远程TCP接收到连接中断请求的确认 \*/ 10. CLOSING: 比较少见./\* Both sockets are shut down but we still don't have all our data sent. 等待远程TCP对连接中断的确认 \*/ 11. CLOSED: 被动关闭端在接受到ACK包后，就进入了closed的状态。连接结束./\* The socket is not being used. 没有任何连接状态 \*/