握手
TCP連接狀態
三次握手
下面是狀態的解釋：
| State | Description |
|---|---|
| LISTEN | The socket is listening for incoming connections. Such sockets are not included in the output unless you specify the --listening (-l) or --all (-a) option. |
| SYN_SENT | The socket is actively attempting to establish a connection. |
| SYN_RECV | A connection request has been received from the network. |
| ESTABLISHED | The socket has an established connection. |
四次釋放
上面的網圖看着還是挺清晰的，但是有個小瑕疵，就是不應該是客戶端對服務端，而是主動關閉端對應被動關閉端。主動關閉端要求進入TIME_WAIT 狀態。
下面是各種狀態的解釋：
| State | Description |
|---|---|
| ESTABLISHED | The socket has an established connection. |
| FIN_WAIT1 | The socket is closed, and the connection is shutting down. |
| FIN_WAIT2 | Connection is closed, and the socket is waiting for a shutdown from the remote end. |
| TIME_WAIT | The socket is waiting after close to handle packets still in the network. |
| CLOSE | The socket is not being used. |
| CLOSE_WAIT | The remote end has shut down, waiting for the socket to close. |
| LAST_ACK | The remote end has shut down, and the socket is closed. Waiting for acknowledgement. |
| CLOSING | Both sockets are shut down but we still don’t have all our data sent. |
TIME_WAIT重置場景
主動端關閉連接之後，被動端發送了FIN報文代表被動端也想要關閉連接。此時，主動端發送的確認報文失效。
被動端沒有收到FIN的確認報文，因此重發FIN報文，主動端收到會重置時鐘，發送ACK確認報文。
