Wireshark talking about tcp three-way handshake

Wireshark talking about tcp three-way handshake
Capture Data Packets:
Open wireshark --> Capture --> interfaces --> select Nic --> start
Open the browser, enter www.xiyou.edu.cn, and close the browser after the web page is opened successfully.
Click stop in wireshark to stop capturing data)
Analyze Data Packets:
Filter out SYN packets sent from the Local Machine

Wireshark sets a stream index for each tcp connection, so the stream index is 7 here for analysis!
The principle of stream index is not complex. It can be calculated using the tuples of TCP. This is calculated by wireshark in advance.

Three-way handshake packet analysis
First handshake

Seq = 0 no validation number ACK = 0 (notset) SYN = 1
Second handshake

Seq = 0 ack = 1 ACK = 1 SYN = 1
Third handshake

Seq = 1 ack = 1 ACK = 1 SYN = 0
After the three handshakes, a tcp connection is established. The process is simplified as follows:

Let's talk about it again.
The server first creates a transmission control block TCB, prepares to accept client connection requests, and then the server is in the listen status waiting for client connection
The first handshake: the client also needs to create a TCB before connecting and then send a connection request to the server. At this time, the synchronization bit SYN in the header is set to 1, and there is an initial serial number (seq = 0); TCP rules, SYN packets cannot carry data, but also consume a serial number when the message is sent, the client enters the SYS-SENT status
The second handshake: After the server receives a request from the client to connect to the Message segment, if you agree to establish a connection, send a confirmation message to the client. In the confirmation message segment, set both SYN and ACK to 1, the confirmation number is ack = 1 (the seq value in the client is + 1, indicating that the server wants the client to send a message number 1 next time ), at the same time, the server will also select a serial number (seq = 0) for the confirmation message sent this time, and cannot carry the data to consume a serial number, and finally the server enters the SYN-RECD status
The third handshake: after the client is confirmed by the server, it must also send a confirmation to the server. The ACK of this packet segment is set to 1, check no. ack = 1 (seq value in SERVER + 1). The serial number is the seq value of the last sent message + 1, that is, seq = 1. TCP specifies that ACK packets can carry data! However, if no data is carried, no serial number is consumed. Therefore, the sequence number of the next data packet is seq = 1. So far, the TCP connection has been ESTABLISHED and the client enters the ESTABLISHED status, the server enters the ESTABLISHED status after receiving the confirmation message from the client.