Three handshake and four wave in tcp

Three handshake

The three-way handshake is the process of establishing a connection

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Before a connection is established: The server is in the LISTEN state and the client is in the CLOSED state.

First handshake: The client sends a synchronization segment SYN to the server, and specifies the client's initial serial number ISN. At this time, the client is in SYN_SENT status.

The synchronization bit in the header SYN=1 (SYN is just a flag bit, 0 means not SYN, 1 means SYN), initial serial number seq=x .A segment with SYN=1 cannot carry any data, but a sequence number must be specified.

Second handshake: After the server receives the synchronization segment SYN from the client, it will respond with its own SYN message, and also specify its own initial sequence number ISN, and at the same timeThe client's seq+1 will be used as the value of the confirmation sequence number ack, indicating that it has received the client's synchronization segment SYN, and the server is in the state of SYN_RCVD .

In the confirmation segment SYN=1, ACK=1 (ACK is also just a flag, 0 means notACK, 1 means ACK), initial serial number seq=y , confirmation serial number ack=x+1 , p>

Third handshake: After the client receives the synchronization segment SYN from the server, it will send an acknowledgement segment ACK, using the server's seq+1 as the value of ack, indicating thatI have received the synchronization segment SYN from the server.The client enters the ESTABLISHED state, and the server also enters the ESTABLISHED state after confirming the ACK of the segment.

Confirm the message segment ACK=1, the serial number seq=x+1, confirm the serial number ack=y+1,

At this point, both parties have established a connection and can send data normally.

Why is the three-way handshake three times?

Three handshakes to confirm that both parties are ready

The first handshake: the client can't confirm anything, the server confirms: the other party sends normally, and it receives normally

Second handshake: the client confirms: its own sending and receiving are normal, and the other party's sending and receiving are normal; the server confirms: the other party's sending is normal, and its own receiving is normal

The third handshake: The client confirms: its own sending and receiving are normal, and the other party's sending and receiving are normal; the server confirms: the other party's sending and receiving are normal, and its own sending and receiving are normal.

Four waves

Four waving is the disconnection process

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Both the client and the server can actively close the connection, only the first party to request to close will enter the TIME_WAIT state

Before disconnecting: Both client and server are in ESTABLISHED state, both parties can actively disconnect, and the client can actively disconnectIt is better to open the connection.

The first wave: The client intends to disconnect and sends a FIN message to the server. The FIN message will specify a serial number, and then the client enters FIN_WAIT_1 status.

That is, the client sends a connection release segment FIN=1, specifies the serial number seq=u, and actively closesTCP connection, waiting for confirmation from the server.

The second wave: After the server receives the FIN message, it sends an ACK response message to the client, using the client's FIN message sequence number seq+1 as the ACK response messageThe confirmation sequence number ack=u+1 of the segment, and the specified sequence number seq=v.

Then the server enters the CLOSE_WAIT state. At this time, the TCP is in a half-closed state, and the connection between the client and the server is released.After the client receives the ACK response segment from the server, it enters the FIN_WAIT_2 state.

The third wave: The server also intends to disconnect and send a FIN message to the client, after which the server enters the LASK_ACK state, waiting for the clientend confirmation.

In the connection release segment of the server FIN=1, ACK=1, serial number seq=w, confirm the serial number ack=u+1.

Fourth wave: After the client receives the FIN segment from the server, it will send an ACK response segment to the server, and the ACK sequence number ack of the FIN segment is used as the ACKThe sequence number seq of the response segment, and the sequence number seq+1 of the FIN segment is used as the acknowledgment sequence number ack.

After that, the client enters the state of TIME_WAIT. After the server receives the ACK response segment, the server enters the state of CLOSED.At this point, the connection to the server has been closed.

When the client is in the TIME_WAIT state, the TCP has not been released at this time, and it needs to wait for 2MSL before the client enters the CLOSED state.