Reliable Transport

Build reliable delivery on top of an unreliable network — the problem TCP solves. Packets get lost, duplicated, and reordered; the transport layer hides all of it. Implement sequence numbers, a receiver that reassembles in order with cumulative acknowledgements, a sender that tracks what's been acked, and a sliding window with retransmission. The machinery that turns a lossy channel into an ordered, gap-free byte stream.