The Transmission Control Protocol (TCP) is one of the core protocols of the Internet Protocol (IP) suite and is fundamental to how most communication on the internet and local networks works. It operates at the transport layer (Layer 4) of the OSI model. The primary role of TCP is to provide a reliable, ordered, and error-checked delivery of a stream of data between applications. To achieve this reliability, TCP is a "connection-oriented" protocol. Before any data is sent, the two applications must first establish a connection through a process called a "three-way handshake." Once the connection is established, TCP breaks the application's data stream into smaller packets, adds a sequence number to each one, and sends them. At the receiving end, TCP reassembles the packets in the correct order and checks for errors. If a packet is lost or corrupted, TCP will request a retransmission.
This entire process ensures that the data arrives at its destination complete, uncorrupted, and in the correct sequence. This reliability makes TCP the protocol of choice for applications where data integrity is paramount, such as web Browse (HTTP), file transfer (FTP), and sending commands to a programmable instrument over a LAN. The trade-off for this reliability is a slight increase in latency and overhead compared to its connectionless counterpart, UDP.
The TCP specification has been merged into "RFC 9293 TCP" from "RFC 793 TCP" and others.