TCP
When a page loads slowly, an API times out, or packet capture shows retransmissions and duplicate acknowledgements, the problem is often not HTTP itself. It is the transport connection beneath it. TCP is usually summarized as a “reliable transport protocol,” but the real engineering judgment comes from how it turns an unreliable network path into a usable byte stream through connection state, acknowledgement, retransmission, windows, and in-order delivery.
TCP’s cost is here too. It solved the reliable transport problem for the open Internet, but it also carried later Web performance constraints, weak-network behavior, and multiplexing limits down below the application layer. The reasons HTTP/2 gets stuck behind packet loss and QUIC wants to redesign transport again are both in TCP’s behavior model.
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