What does the word "retransmit" actually mean? I thought I knew. I'd taken the networking class. I'd read the chapter. I could draw the slow-start curve on a whiteboard. Then I tried to write it and discovered I had no idea.

This is a series about rebuilding TCP from scratch in Java, and the parts of it I only understood after the code actually ran.

What I built

LEAP (Loss-aware End-to-end Acknowledged Protocol) is a TCP-style reliable transport I built from scratch on top of UDP. About 1,900 lines of Java. It has sequence numbers, cumulative ACKs, a sliding window, fast retransmit on three duplicate ACKs, adaptive RTO in the style of RFC 6298, slow start and AIMD congestion control with an ssthresh floor, and end-to-end SHA-256 integrity verification on every transfer.

There's a CLI:

./bin/leap receive --port 4040 --output /tmp/in ./bin/leap send myfile.bin --to localhost:4040

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And a benchmark harness that sweeps file size × loss rate × trial across LEAP and…