> So this hypothetical middleman will receive a packet, check that it's not a duplicate, and forward it to the recipient it's proxying for. How will it know that the recipient has actually received it?
It seems like the answer is in the first part, the "check that it's not a duplicate".
Implement at-least-once but with a unique token to identify the request, and the receiver sends back acknowledgement with that token every time it receives the original message, but only hands it off for processing the first time. Stuff this behind library/API so it's hidden to the user and the application code doesn't have to handle it, and... isn't that it?
Yes, but the handoff can fail in the same way (it can't know if the thing it's handing off to actually got it). But the application can also just be resilient to that with idempotent operations and have the handoff be at-least-once.
Everything in a digital system can fail, but by convention (and because it's not so far from the actual truth) some parts are assumed to be 100% reliable when modeling them. If you don't make this assumption, you can't guarantee anything.
It seems like the answer is in the first part, the "check that it's not a duplicate".
Implement at-least-once but with a unique token to identify the request, and the receiver sends back acknowledgement with that token every time it receives the original message, but only hands it off for processing the first time. Stuff this behind library/API so it's hidden to the user and the application code doesn't have to handle it, and... isn't that it?