Understanding of TCP status transition

LISTEN: this is also an easy-to-understand state, indicating that a SOCKET on the server is in the listening state and can accept connections. SYN_RCVD: this status indicates that the SYN packet is received. Normally, this status is an intermediate status of the server SOCKET during the three-way handshake session when a TCP connection is established, basically, you can hardly see this status with netstat unless you write a client test program and deliberately disable the last ACK packet from the three TCP handshakes. Therefore, when an ACK packet is received from the client, it enters the ESTABLISHED status. If an RST signal is received, it is returned to the LISTEN state. The following is an arrangement of this scenario: telnet thread 1 registers socket resources and establishes a connection, telnet thread 2 also registers the socket resource and enters the link state. Then, telnet thread 2 ends the connection and releases the socket resource. Then, telnet thread 1 will receive an RST signal and terminate the connection. SYN_SENT: this status echo SYN_RCVD. When the client SOCKET executes the CONNECT connection, it first sends the SYN packet, and then it enters the SYN_SENT status, and wait for the server to send 2nd messages in the three-way handshake. SYN_SENT status indicates that the client has sent SYN packets. ESTABLISHED: This is easy to understand, indicating that the connection has been ESTABLISHED. FIN_WAIT_1: The FIN_WAIT_1 and FIN_WAIT_2 statuses both indicate waiting for the FIN packets of the other party. The difference between the two States is that the FIN_WAIT_1 state is actually when the SOCKET is in the ESTABLISHED State, it wants to actively close the connection and send a FIN packet to the other party, the SOCKET enters the FIN_WAIT_1 state. When the other Party responds to the ACK packet, it enters the FIN_WAIT_2 status. Of course, under normal circumstances, the other party should immediately respond to the ACK packet, regardless of the situation, therefore, the FIN_WAIT_1 status is generally difficult to see, while the FIN_WAIT_2 status is often seen using netstat. FIN_WAIT_2: The above has explained in detail this status. In fact, the SOCKET in the FIN_WAIT_2 status indicates a semi-connection, that is, either party requires a close connection, but also tells the other party, I have some data to send to you, and close the connection later. TIME_WAIT: indicates that the FIN packet of the other party is received, and the ACK packet is sent concurrently, so that 2MSL can return to the CLOSED available status. If FIN_WAIT_1 receives a message with both the FIN mark and ACK mark, it can directly enter the TIME_WAIT status without passing through the FIN_WAIT_2 status. CLOSING: this is a special exception that is rare in actual situations. Normally, when you send a FIN packet, you should first (or simultaneously) receive the ACK packet from the other party and then receive the FIN packet from the other party. However, the CLOSING status indicates that after you send the FIN packet, you have not received the ACK packet from the other party, but have also received the FIN packet from the other party. Under what circumstances will this happen? In fact, it is not difficult to come to the conclusion that if both parties close a SOCKET at the same time, both parties may send FIN packets at the same time, that is, the CLOSING status may occur, both parties are closing the SOCKET connection. Another case is that ACK is lost. CLOSE_WAIT: the meaning of this state is actually waiting to be closed. How can this problem be solved? When the other party closes a SOCKET and sends a FIN packet to itself, your system will undoubtedly respond to an ACK packet to the other party, and then enters the CLOSE_WAIT status. Next, in fact, what you really need to consider is to check if you still have data to send to the other party. If not, you can close the SOCKET and send the FIN packet to the other party, that is, close the connection. So what you need to do in CLOSE_WAIT is to wait for you to close the connection. LAST_ACK: this status is easy to understand. It passively closes a side and waits for the other side's ACK packet after sending the FIN message. After receiving the ACK message, you can enter the CLOSED available status. 1. Why is the three-way handshake while the four-way handshake when the connection is closed? This is because the SOCKET in the LISTEN status of the server can respond to ACK and SYN after receiving the SYN Packet connection request) it is sent in a message. However, when the connection is closed, when the other party receives the FIN Message notification, it only indicates that the other party has no data to send to you; but not all your data may have been sent to the other party, therefore, you may not close the SOCKET immediately, that is, you may need to send some data to the other party, and then send the FIN message to the other party to indicate that you agree to close the connection now, therefore, the ACK messages and FIN messages are sent separately in most cases. 2. Why does the TIME_WAIT status still need to be 2 MSL before it can be returned to the CLOSED status? A: reliable termination of TCP full-duplex connections. B: allow the passage of late packets in the network. Author: china_hearts