Tcp protocol three-way handshake detailed process

TCP is the host-to-host layer transport control protocol, providing a reliable connection service, using a three-way handshake to establish a connection:

The bit code is the tcp flag. There are 6 kinds of indications: SYN (synchronous connection) ACK (acknowledgement confirmation) PSH (push transmission) FIN (finish end) RST (reset reset) URG (urgent emergency)

Sequence number Acknowledge number

The first handshake: Host A sends a bit code of syn=1, randomly generates a packet of seq number=1234567 to the server, host B is known by SYN=1, and A requests to establish a connection;

Second handshake: After receiving the request, Host B should confirm the online information, send A to ack number= (seq+1 of host A), syn=1, ack=1, and randomly generate the packet with seq=7654321.

The third handshake: After receiving the host A, check whether the ack number is correct, that is, the seq number+1 sent for the first time, and whether the bit code ack is 1. If it is correct, host A will send ack number= again (host B's) Seq+1), ack=1, after the host B receives the seq value and ack=1, the connection is established successfully.

After completing the three-way handshake, Host A and Host B start transmitting data.

In the TCP/IP protocol, the TCP protocol provides a reliable connection service, using a three-way handshake to establish a connection.

First handshake: When establishing a connection, the client sends a syn packet (syn=j) to the server and enters the SYN_SEND state, waiting for the server to confirm;

The second handshake: the server receives the syn packet, must confirm the SYN of the client (ack=j+1), and also sends a SYN packet (syn=k), that is, the SYN+ACK packet, and the server enters the SYN_RECV state; The third handshake: The client receives the SYN+ACK packet from the server and sends an acknowledgement packet ACK (ack=k+1) to the server. After the packet is sent, the client and the server enter the ESTABLISHED state and complete the three-way handshake. After completing the three-way handshake, the client and server start transmitting data.

Example:

IP 192.168.1.116.3337 192.168.1.123.7788: S 3626544836

IP 192.168.1.123.7788 192.168.1.116.3337: S 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 the bit code syn=1, randomly generates the packet of seq number=3626544836 to 192.168.1.123, 192.168.1.123 by SYN=1 to know that 192.168.1.116 requires to establish the connection;

The second handshake: 192.168.1.123 to confirm the online information after receiving the request, send ack number=3626544837, syn=1, ack=1 to 192.168.1.116, randomly generate the package of seq=1739326486;

The third handshake: After receiving 192.168.1.116, check whether the ack number is correct, that is, the seq number+1 sent for the first time, and whether the bit code ack is 1. If it is correct, 192.168.1.116 will send ack number=1739326487 again. Ack=1, 192.168.1.123 is confirmed and seq=seq+1 is confirmed. If ack=1, the connection is successfully established.

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

The first handshake flag (Figure 3)

We can see that there is only one sync bit in the flag, that is, making a request (SYN)

The second handshake flag (Figure 4)

We can see that there is an acknowledgment bit and a sync bit in the flag, that is, it is responding (SYN + ACK).

The third handshake flag (Figure 5)

We can see that there is only one acknowledgment bit in the flag, which is to confirm again (ACK).

A complete three-way handshake is the request --- answer --- reconfirm

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