TCP

The Transmission Control Protocol (TCP) is the transport-layer protocol that underlies most reliable communication on the internet. It sits between the network layer (which moves packets between hosts) and the application layer (which interprets what those packets mean), providing the illusion of a continuous, ordered, error-checked stream of data between processes on different machines. TCP achieves this reliability through a combination of sequence numbers, acknowledgments, retransmissions, and congestion control — mechanisms that transform the internet's inherently unreliable packet delivery into a dependable pipe.

TCP's design encodes a specific philosophy of distributed systems: that the endpoints, not the network, should bear the cost of reliability. Routers drop packets when congested; TCP detects the drops and slows down. This "end-to-end principle" means that the network core remains simple while the edges become sophisticated. It is a design that scales well but creates persistent problems: TCP's three-way handshake adds latency to every connection, its congestion-control algorithms can be gamed by applications, and its head-of-line blocking delays all data in a stream when a single packet is lost. The protocol that made the internet reliable is also the protocol that makes it slow.