Three handshakes and four waves

Three handshake

1. The first handshake: The client sends a data packet with the bit code SYN = 1 (the SYN flag bit is set) and randomly generates the initial sequence number Seq = J to the server.The server is informed by SYN = 1 (set) that the client has requested to establish a connection.
2. The second handshake: After the server receives the request, it needs to confirm the connection information, and sends the confirmation number to the client Ack = (client's Seq +1, J+1), SYN = 1, ACK = 1 (SYN,ACK flag bit is set), randomly generated sequence number Seq = K data packet.
3. The third handshake: After the client receives it, check whether the Ack is correct, that is, the Seq +1 (J+1) sent for the first time, and whether the bit code ACK is 1.If it is correct, the client will send Ack = (Seq+1 on the server side, K+1), ACK = 1, and an acknowledgment packet whose sequence number Seq is the server acknowledgment number J.After the server receives it and confirms the previously sent Seq(K+1) value and ACK= 1 (ACK is set), the connection is established successfully.

After these three steps, the client and the server successfully establish a TCP connection.These three steps are collectively referred to as the three-way handshake.
The above Seq represents the sequence number, Ack represents the confirmation number, SYN and ACK and FIN are all flag bits.ACK is set to 1 to indicate that the acknowledgment number field is valid, if ACK is 0, the segment does not contain acknowledgment information.SYN is used in the connection establishment process. In the connection request, SYN = 1 and ACK = 0 indicate that the segment does not have a piggybacked acknowledgment field.The connection reply will piggyback an acknowledgment, so the reply will have SYN= 1 and ACK= 1.Also there is no cost to send an ACK, so we will see that once a connection is established, the ACK flag is always set to 1

Four waves

1. The client sends a FIN Seq = M (FIN set, serial number is M) packet to close the data transfer from the client to the server.
2. When the server receives this FIN, it sends back an ACK, confirming that the sequence number Ack is the received sequence number M+1.
3. The server closes the connection with the client and sends a FIN Seq = N to the client.
4. The client sends back an ACK message for confirmation, and the confirmation sequence number Ack is the received sequence number N+1.
For the four times of wave, in fact, if you look carefully, it is twice, because TCP is full-duplex, and both sides must be closed.At the time of termination, one side is passive, so it seems to be four waves.