How the tcp three-way handshake works

TCP three-way handshake is a key step in establishing a TCP connection. In this process, through the interaction of three message segments, the client and server confirm each other's identity and communication capabilities, thereby establishing a reliable connection. This mechanism ensures the reliability and integrity of data transmission, but it also brings time overhead, which needs to be weighed and optimized in practical applications.

TCP three-way handshake is the process of establishing a TCP connection. Its working principle is as follows:

1. The first handshake: the client The server sends a connection request segment, which contains the SYN (synchronization) flag bit set to 1 and the client's initial sequence number Seq. This indicates that the client wishes to establish a connection with the server. After the sending is completed, the client enters the SYN_SENT state and waits for confirmation from the server.
2. Second handshake: After the server receives the client's connection request, if it agrees to the connection, it will send a confirmation message segment to the client. In this message segment, the SYN flag is also set to 1, and the ACK (acknowledgement) flag is set to 1. The confirmation number Ack is the client's initial sequence number Seq 1, indicating that the client's connection request has been received. In addition, the server will also generate its own initial sequence number Seq and send it to the client. In this way, the client confirms that the server has received its connection request and that the server is ready to communicate. After the transmission is completed, the server enters the SYN_RECV state.
3. The third handshake: After receiving the confirmation message segment from the server, the client sends a confirmation message segment to the server again, in which the ACK flag bit is set to 1, and the confirmation number Ack is the server's initial sequence number. Seq 1, indicating that the server's confirmation message segment has been received and is ready for data transmission. This way, the server also confirms that the client is ready to communicate. After the transmission is completed, both the client and the server enter the ESTABLISHED state, indicating that the TCP connection has been successfully established and both parties can start transmitting data.

Through these three handshakes, both the client and the server can confirm each other's identity and communication capabilities, thereby establishing a reliable connection. This is also one of the important mechanisms in the TCP protocol to ensure the reliability and integrity of data transmission. At the same time, since each handshake requires sending and receiving message segments, this will also bring a certain amount of time overhead. Therefore, in actual applications, trade-offs and optimizations need to be made based on specific circumstances.

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