TCP

TCP is the protocol that turns the internet’s unreliable packet delivery into a dependable two-way stream of bytes between two programs. The two sides first agree to talk with a short handshake, then the sender numbers every byte, the receiver acknowledges what arrived, and anything not acknowledged in time gets sent again, so data comes out complete and in order. It also paces itself: flow control stops a fast sender from swamping a slow receiver, and congestion control speeds up until packets start getting lost, then backs off sharply to avoid clogging the network. Web pages, email, SSH and most database connections all ride on TCP.