Transmission Control Protocol is one of the two core transport protocols of the internet (the other being UDP). TCP provides reliable, ordered delivery of a byte stream between two endpoints, handling retransmission of lost packets, flow control, and congestion management under the hood.
TCP underlies most application protocols — HTTP, HTTPS, SSH, SMTP, and thousands of others — because applications that care about correctness generally cannot handle the packet loss and reordering that raw IP packets exhibit. UDP handles the minority of cases where lower latency is worth giving up delivery guarantees (real- time media, DNS queries, some game traffic).
Modern HTTP variants (HTTP/2, HTTP/3) have complicated the picture. HTTP/3 runs over QUIC, which is built on UDP rather than TCP, to avoid head-of-line blocking issues in TCP-based connection multiplexing. But the vast majority of enterprise networking traffic in 2026 still runs over TCP, and the protocol's role as the internet's default reliable transport is unchanged.