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# TCP Protocol TCP (Transmission Control Protocol) is one of the core protocols in the Internet protocol suite, located at the transport layer. It provides a reliable, connection-oriented, byte-stream-based data transmission service. The main characteristics of TCP are ensuring that data is not lost, not duplicated, and arrives in order during transmission. Below is a detailed explanation of how TCP works and its key features. * * * ## How TCP Works TCP establishes connections through "three-way handshake", terminates connections through "four-way wave", and uses acknowledgment mechanisms, retransmission mechanisms, and flow control during data transmission to ensure reliability. ### 1. **Three-Way Handshake to Establish Connection** !(#) * **SYN**: The client sends a SYN packet (synchronization request) to the server, requesting to establish a connection. * **SYN-ACK**: After receiving the SYN packet, the server replies with a SYN-ACK packet (synchronization acknowledgment), indicating agreement to establish the connection. * **ACK**: After receiving the SYN-ACK packet, the client sends an ACK packet (acknowledgment), indicating the connection has been established. ### 2. **Data Transmission** After the connection is established, TCP ensures reliable data transmission through the following mechanisms: * **Sequence Number and Acknowledgment Number**: Each data packet has a sequence number, and the receiver uses the acknowledgment number to inform the sender which data has been successfully received. * **Retransmission Mechanism**: If the sender does not receive an acknowledgment, it will resend the data packet. * **Flow Control**: Through the sliding window mechanism, the sending rate is dynamically adjusted to prevent the receiver's buffer from overflowing. * **Congestion Control**: Through algorithms such as slow start and congestion avoidance, the sending rate is dynamically adjusted to prevent network congestion. ### 3. **Four-Wave Handshake to Terminate Connection** !(