Ask HN: Learning Network Programming

I haven't had exposure working with lower-level network programming and have been trying to find a learning path to do so. Books on network programming discuss entry-level concepts for networking protocols but aren't instructive about writing networked applications. For instance, I don't understand flow control mechanisms and how to use them effectively and I haven't written my own custom binary protocols nor a message parser. I think I'm looking for a syllabus for a network programming course that doesn't exist, or I'm just not aware of. Suggestions are welcome!