TCP

TCP (Transmission Control Protocol) is the transport-layer protocol that gives applications a reliable, ordered byte stream over an unreliable network. It sets up a connection with a three-way handshake, numbers every byte so lost packets can be retransmitted and out-of-order ones reassembled, and slows itself down when the network is congested.

Worked example: when you load a web page, the browser opens a TCP connection to the server, and TCP guarantees the HTML arrives complete and in order even if individual packets are dropped or reordered on the way. Gotcha: that reliability costs latency — the handshake adds a round trip before any data moves, and head-of-line blocking means one lost packet stalls everything behind it, which is exactly why real-time media and HTTP/3 reach for UDP instead.