Transmission Control Protocol (TCP) is a fundamental communication protocol in computer networks, responsible for facilitating reliable and ordered data transmission between devices over IP networks. Here’s a more detailed breakdown of how TCP works:
- Segmentation: Before data transmission, TCP divides the data into smaller units known as segments. Each segment contains a header and a chunk of data. The header includes control information such as source and destination port numbers, sequence numbers, acknowledgment numbers, and flags.
- Connection Establishment: TCP uses a three-way handshake to establish a connection between two devices:
- SYN: The initiating device sends a SYN (synchronize) segment to the receiving device, indicating its desire to establish a connection and specifying an initial sequence number.
- SYN-ACK: The receiving device responds with a SYN-ACK (synchronize-acknowledgment) segment, acknowledging the SYN segment and also sending its own SYN segment with an incremented sequence number.
- ACK: Finally, the initiating device sends an ACK (acknowledgment) segment, acknowledging the SYN-ACK segment and completing the connection establishment process.
- Data Transfer: Once the connection is established, data transfer begins. TCP ensures that data is delivered reliably and in order by using sequence numbers and acknowledgments:
- Sequence Numbers: Each segment contains a sequence number, which helps in ordering segments at the receiving end.
- Acknowledgments (ACKs): The receiving device sends ACK segments to inform the sender about the successful receipt of segments. If a segment is lost or corrupted, the sender retransmits it until the receiver acknowledges its receipt.
- Flow Control: TCP employs flow control mechanisms to manage the rate of data transmission between sender and receiver:
- Window Size: TCP uses a sliding window mechanism to control the amount of data sent before receiving an acknowledgment. The receiver advertises its receive window size, indicating how much data it can accept. The sender adjusts its transmission rate based on this window size to avoid overwhelming the receiver.
- Connection Termination: When data exchange is complete, TCP terminates the connection using a four-way handshake:
- FIN: One party sends a FIN (finish) segment to initiate the connection termination process.
- ACK: The other party sends an ACK segment in response to the FIN segment.
- FIN: The responding party also sends a FIN segment to indicate its agreement to terminate the connection.
- ACK: Finally, the initiating party acknowledges the FIN segment, completing the connection termination process.
By following these steps and mechanisms, TCP ensures reliable, ordered, and error-checked data transmission between devices over IP networks.
- Time-Wait State: After the connection is terminated, each side enters a TIME_WAIT state to ensure that any delayed packets related to the closed connection are properly handled. This prevents confusion with new connections using the same port numbers.
