Not sure I can agree with the statement about TCP stacks. Yes, kernel TCP stack aren't built with TDD. But it's not a fundamental property of the problem space,

Not sure I can agree with the statement about TCP stacks. Yes, kernel TCP stack aren't built with TDD. But it's not a fundamental property of the problem space, it's just another instance of kernel programming sucking in general. A TCP stack can be written in a way that makes it simple to test, and getting full branch coverage isn't that onerous a task. Does that catch all bugs? Of course not. But it'll still catch many simple logic errors like this one. Such logic errors do creep into code quite often, can be hard to trigger or detect during manual testing, and will be hellish to debug if they manage to get all the way through to a live system. (This isn't idle speculation. I'm the lead engineer for a high performance userspace TCP stack that's handled petabytes of data, probably for millions of endpoints. We have deterministic unit tests for a large proportion of TCP behavior. Getting into a position where we could do proper testing took a while since the initial version was not written with testability in mind, but that work was definitely worth it. But I'll take your word on TLS.)