TCP three-way handshake and four-way handshake

TCP three-way handshake and four-way handshake

Three-way handshake:Three handshakes are required to establish TCP. The Client sends the connection request message first. The Server segment receives the connection and replies to the ACK message, and allocates resources for the connection. After the Client receives the ACK packet, it also generates an ACK packet to the Server segment and allocates resources so that the TCP connection is established.

 

Four Waves: the Client initiates a request to interrupt the connection, that is, to send a FIN packet. After the Server receives the FIN message, it means"My Client has no data to send to you.", But if you still have data not sent, you do not have to rush to close the Socket, you can continue to send data. So you should first send ACK ,"Tell the Client that I have received your request, but I am not ready yet. Please wait for your message". At this time, the Client enters the FIN_WAIT status and continues to wait for the Server's FIN message. When the Server determines that the data has been sent, it sends the FIN message to the Client ,"Tell the Client. Okay. I have sent the data and are ready to close the connection.". After the Client receives the FIN message ,"The connection can be closed, but he still does not trust the network, for fear that the Server does not know to close, so the ACK enters the TIME_WAIT status after being sent. If the Server does not receive the ACK, the Server can re-transmit it.. ", After the Server receives ACK ,"The connection can be disconnected.". If the Client still does not receive a reply after waiting for 2MSL, it indicatesThe Server has been shut down normally. Well, my Client can also close the connection.. OK. The TCP connection is closed!

What is TCP three-way handshake and four-way handshake?

1. Establish a connection protocol (three-way handshake)
(1) the client sends a TCP packet with SYN flag to the server. This is packet 1 during the three-way handshake.
(2) The server responds to the client. This is the first packet in the three-way handshake. This packet carries both the ACK mark and SYN sign. Therefore, it indicates the response to the client SYN Packet. It also marks the SYN message to the client and asks whether the client is ready for data communication.
(3) The customer must re-respond to an ACK packet in the service segment, which is packet segment 3. 2. Connection termination agreement (four waves)
Because the TCP connection is full-duplex, each direction must be closed separately. This principle is that when one party completes its data sending task, it can send a FIN to terminate the connection in this direction. Receiving a FIN only means that there is no data flow between the two parties. a tcp connection can still send data after receiving a FIN. First, the party that closes the service will take the initiative to close the service, and the other party will passively close the service.
(1) The TCP client sends a FIN to disable data transmission from the client to the server (packet segment 4 ).
(2) When the server receives the FIN, it sends back an ACK and confirms that the serial number is 1 (packet segment 5 ). Like SYN, a FIN occupies a sequence number.
(3) The server closes the client connection and sends a FIN to the client (packet segment 6 ).
(4) The customer segment sends back the ACK message for confirmation, and sets the confirmation sequence number to receive the serial number plus 1 (packet segment 7 ). CLOSED: this is nothing to say, indicating the initial state. 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. It is very short, 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. 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:
In fact, the true meanings of FIN_WAIT_1 and FIN_WAIT_2 indicate waiting for the FIN message 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 already 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 FIN_WAI... the remaining full text>

Help: What is the entire process of TCP three-way handshake and four-way handshake?

The following is a reference for everyone to learn together ~~

TCP handshake protocol

In TCP/IP, TCP provides reliable connection services and uses three handshakes to establish a connection.

First handshake: when a connection is established, the client sends the syn Packet (syn = j) to the server and enters the SYN_SEND status. Wait for the server to confirm;

The second handshake: when the server receives the syn packet, it must confirm the customer's SYN (ack = j + 1) and send a SYN Packet (syn = k), that is, the SYN + ACK packet, the server enters the SYN_RECV status;

The third handshake: the client receives the server's SYN + ACK package and sends the ACK (ack = k + 1) Confirmation package to the server. After the package is sent, the client and server enter the ESTABLISHED status, complete three handshakes.

After three handshakes, the client and the server start to transmit data. In the above process, there are some important concepts:

Unconnected queue: in the three-way handshake protocol, the server maintains an unconnected queue, which opens an entry for the SYN Packet (syn = j) of each client, this entry indicates that the server has received the SYN Packet and sent a confirmation to the customer, waiting for the customer's confirmation package. The connection identified by these entries is in the Syn_RECV state on the server. When the server receives the customer's confirmation packet, it deletes the entry and the server enters the ESTABLISHED state.
Backlog parameter: Maximum number of unconnected queues.

SYN-ACK retransmission times the server sends the SYN-ACK package, if the customer does not receive the confirmation package, the server for the first retransmission, wait for a period of time has not received the customer confirmation package, for the second retransmission, if the number of retransmission times exceeds the maximum number of retransmission times specified by the system, the system deletes the connection information from the semi-connection queue. Note that the waiting time for each retransmission is not necessarily the same.

Semi-connection survival time: the maximum time for the semi-connection queue to survive, that is, the maximum time for the service from receiving the SYN packet to confirming that the message is invalid, the maximum waiting time of all retransmission request packets. The semi-join survival time is also called Timeout time and SYN_RECV survival time.
Reference: zhidao.baidu.com/question/7331325.html? Si = 2