Back in the OSI post I mentioned that Layer 4's whole job is service-to-service delivery — making sure data reaches the right application on a host, not just the right machine — and that TCP and UDP are the two strategies for doing it. This post is that comparison properly unpacked, because "TCP is reliable, UDP is fast" is the kind of half-explanation that sounds fine until someone asks you what "reliable" actually means at the protocol level, and you realize you don't quite know. ...