> can you somehow make the system wait for the buffer to empty? If sending a packet syncronously, the system call would just hang until the buffers have drained enough.

It sounds like you're trying to make UDP lossless. Use TCP instead.

What if I'm running VOIP? Or a real-time video game? I don't want to wait for the dropped datagram to be received before processing the next one. It's too late! It should have been received already. Skip it and use the next one.

> Does the outside network just bang bits through the cable like a radio station?

Basically, yes. Ethernet has no flow control. IP has no flow control. UDP has no flow control. TCP does.

> is there, deep down on the lowest levels of the stack, actually some kind of request/response going on, even for UDP?

No. Ethernet, as commonly deployed, has no need for this, but CSMA/CD is the retransmit part of the specification in case you are using coax tap or a hub. Each Ethernet frame also has a Frame Check Sequence, and the receiving node will discard the frame if it doesn't match up. It's not up to Ethernet to retransmit. Use TCP for that instead.