How TCP works | TCP Header | Source Port | Destination Port | Window | SYN | FIN | PSH | URG | RST

TCP works by establishing a reliable and ordered communication channel between two devices, typically a client and a server, over an IP network. Here's a step-by-step overview of how TCP works:

Connection Establishment: Before data transfer can occur, a TCP connection must be established. The client initiates the connection by sending a SYN (synchronize) packet to the server. The SYN packet contains an initial sequence number that helps identify the data segments being sent.

Connection Acknowledgment: Upon receiving the SYN packet, the server sends a SYN-ACK (synchronize-acknowledge) packet back to the client. The SYN-ACK packet acknowledges the client's request and contains the server's own initial sequence number.

Connection Confirmation: The client responds to the server's SYN-ACK packet with an ACK (acknowledge) packet. This acknowledges the server's acknowledgment and confirms the connection establishment. At this point, the connection is considered open, and both devices can start exchanging data.

Data Transfer: Once the connection is established, the client and server can exchange data. The data to be transmitted is divided into smaller units called segments. Each segment includes a sequence number to ensure ordered delivery and a checksum for error detection.

Segment Transmission and Acknowledgment: The sender (client or server) transmits data segments to the receiver, which buffers the incoming segments. The receiver sends acknowledgment packets (ACK) back to the sender to confirm the successful receipt of segments. If the sender does not receive an ACK within a specified timeout period, it assumes that the segment was lost or corrupted and retransmits it.

Flow Control: TCP implements flow control mechanisms to regulate the rate at which data is transmitted. The receiver advertises its buffer size to the sender using a window size value. The sender adjusts its transmission rate based on the window size to avoid overwhelming the receiver with more data than it can handle.

Congestion Control: TCP also incorporates congestion control mechanisms to prevent network congestion and ensure fair sharing of network resources. It monitors the network conditions, such as packet loss and delay, and dynamically adjusts the sending rate to avoid overloading the network.

Connection Termination: When the data transfer is complete, the connection is terminated. The client or server sends a FIN (finish) packet to initiate the connection termination process. The other device responds with an ACK packet, and both devices gradually close the connection by exchanging FIN and ACK packets in a similar manner to the connection establishment process.

TCP provides a reliable and robust communication channel by ensuring the ordered delivery, error detection, and congestion control of data packets. It forms the foundation for many internet-based applications and protocols, allowing for efficient and secure data transmission.