# 附錄：TCP連接的狀態 1. LISTEN:首先服務端需要打開一個socket進行監聽，狀態為LISTEN. /* The socket is listening for incoming connections. 偵聽來自遠方TCP端口的連接請求 */ 2. SYN_SENT:客戶端通過應用程序調用connect進行active open.于是客戶端tcp發送一個SYN以請求建立一個連接.之后狀態置為SYN_SENT. /*The socket is actively attempting to establish a connection. 在發送連接請求后等待匹配的連接請求 */ 3. SYN_RECV:服務端應發出ACK確認客戶端的SYN,同時自己向客戶端發送一個SYN.之后狀態置為SYN_RECV /* A connection request has been received from the network. 在收到和發送一個連接請求后等待對連接請求的確認 */ 4. ESTABLISHED: 代表一個打開的連接，雙方可以進行或已經在數據交互了。/* The socket has an established connection. 代表一個打開的連接，數據可以傳送給用戶 */ 5. FIN_WAIT1:主動關閉(active close)端應用程序調用close，于是其TCP發出FIN請求主動關閉連接，之后進入FIN_WAIT1狀態./* The socket is closed, and the connection is shutting down. 等待遠程TCP的連接中斷請求，或先前的連接中斷請求的確認 */ 6. CLOSE_WAIT:被動關閉(passive close)端TCP接到FIN后，就發出ACK以回應FIN請求(它的接收也作為文件結束符傳遞給上層應用程序),并進入CLOSE_WAIT. /* The remote end has shut down, waiting for the socket to close. 等待從本地用戶發來的連接中斷請求 */ 7. FIN_WAIT2:主動關閉端接到ACK后，就進入了FIN-WAIT-2 ./* Connection is closed, and the socket is waiting for a shutdown from the remote end. 從遠程TCP等待連接中斷請求 */ 8. LAST_ACK:被動關閉端一段時間后，接收到文件結束符的應用程序將調用CLOSE關閉連接。這導致它的TCP也發送一個 FIN,等待對方的ACK.就進入了LAST-ACK . /* The remote end has shut down, and the socket is closed. Waiting for acknowledgement. 等待原來發向遠程TCP的連接中斷請求的確認 */ 9. TIME_WAIT:在主動關閉端接收到FIN后，TCP就發送ACK包，并進入TIME-WAIT狀態。/* The socket is waiting after close to handle packets still in the network.等待足夠的時間以確保遠程TCP接收到連接中斷請求的確認 */ 10. CLOSING: 比較少見./* Both sockets are shut down but we still don't have all our data sent. 等待遠程TCP對連接中斷的確認 */ 11. CLOSED: 被動關閉端在接受到ACK包后，就進入了closed的狀態。連接結束./* The socket is not being used. 沒有任何連接狀態 */