Two laptops attach to a wi-fi hub connected to a DSL line provided by a major telco. One starts a large download. Why doesn't it completely monopolize the skinny straw that connects the 10+mb/s network it actually sees to the gigabit network beyond the DSL? Why can the second laptop start another large download and both see reasonable download speeds? At a more basic level, how do these TCP's even know how fast to drive the transfer? All they see is a multimegabit wireless network. They're not psychic, and there's no configuration box or DHCP setting for "256 kb/s dsl uplink".

That's what TCP actually buys you.

You can make a reliable UDP protocol with a few tens of lines of code. It's called "TFTP", and it's god-awful slow.