Transmission Control Protocol (TCP) is a transport layer protocol that provides reliable, ordered end-to-end data delivery across networks. Because underlying protocols such as IP only provide best-effort delivery without arrival guarantees, TCP operates on top of IP to ensure that every byte is received accurately and in sequence.

Before data transmission begins, TCP establishes a connection using a three-way handshake: the client sends a SYN (synchronize) message, the server responds with a SYN-ACK acknowledgment, and the client finishes setup with a final ACK. Real data transfer only occurs after this handshake completes.

During transmission, TCP numbers every packet and monitors delivery confirmations. If a packet is lost in transit, the receiver detects a gap in sequence numbers, the sender notices the lack of acknowledgment, and TCP automatically retransmits the missing unit. Additionally, TCP includes congestion control mechanisms, slowing its transmission rate during network congestion and gradually increasing speed as conditions improve.