Analysis of TCP three-way handshake process

Analysis of TCP three-way handshake process

TCP (Transmission Control Protocol) Transmission Control Protocol
TCP is the transmission control protocol used by the host to control the host layer. It provides reliable connection services and uses three handshakes to establish a connection:
The location code is the tcp flag. There are 6 types of tags: SYN (synchronous established online) ACK (acknowledgement confirmation) PSH (push transfer) FIN (finish ended) RST (reset) URG (urgent)
Sequence number Acknowledge number (confirmation number)

The first handshake: host A sends A packet with A syn = 1 and generates A random seq number = 1234567 packet to the server. Host B is known as SYN = 1, and host A requires online connection;
The second handshake: after receiving the request, host B needs to confirm the online information and send ack number = (seq + 1 of host A), syn = 1, ack = 1, generate a random seq = 7654321 Packet
The third handshake: After receiving the handshake, host A checks whether the ack number is correct, that is, the seq number + 1 sent for the first time, and whether the ack is 1. If yes, host A will send ack number = (seq + 1 of host B), ack = 1 again. After host B receives the message, confirm that the seq value is set to ack = 1, and the connection is established successfully.
After three handshakes are completed, data is transmitted between host A and host B.

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 SYN + ACK packet from the server and sends the ACK (ack = k + 1) Confirmation packet to the server, the client and server enter the ESTABLISHED status and complete the three-way handshake. After three handshakes are completed, the client and the server start to transmit data.
Instance:
IP 192.168.1.116.3337> 192.168.1.123.7788: S 3626544836: 3626544836
IP 192.168.1.123.7788> 192.168.1.116.3337: S 1739326486: 1739326486 ack 3626544837
IP 192.168.1.116.3337> 192.168.1.123.7788: ack 1739326487, ack 1
The first handshake: 192.168.1.116 sends a location code syn = 1, and generates a random seq number = 3626544836 packet to 192.168.1.123, 192.168.1.123 by SYN = 1 knowing 192.168.1.116 requires online connection;
The second handshake: after receiving the request, 192.168.1.123 needs to confirm the online information and send ack number = 3626544837, syn = 1, ack = 1 to 192.168.1.116. packets with seq = 1739326486 are randomly generated;
The third handshake: After 192.168.1.116 is received, check whether the ack number is correct, that is, the seq number + 1 sent for the first time, and whether the ack is 1. If yes, 192.168.1.116 then sends ack number = 1739326487, ack = 1, 192.168.1.123 receives the message and confirms that seq = seq + 1, ack = 1, and the connection is established successfully.

Illustration:
A three-way handshake process (Figure 1, Figure 2)


(Figure 1)

(Figure 2)


The flag of the first handshake (figure 3)
We can see that there is only one synchronization bit in the flag, that is, the request (SYN)
 
(Figure 3)

The flag of the second handshake (Figure 4)
We can see that there is a confirmation and synchronization bit in the flag, that is, the response (SYN + ACK)
 
(Figure 4)

The flag of the third handshake (figure 5)
We can see that there is only one confirmation bit in the flag, that is, re-confirmation (ACK)
 
 
(Figure 5)

A complete three-way handshake, that is, request-response-confirm again