> And if the program can't discard messages, then it's a DoS. If the program can instead rely on the receiver to keep up so it doesn't need to make this choice,

> And if the program can't discard messages, then it's a DoS. If the program can instead rely on the receiver to keep up so it doesn't need to make this choice, then there's a trust assumption between these processes. It doesn't work like that and is getting into hypothetical non real world systems again. Trust is especially interesting in this context, because if you don't trust other processes, you absolutely have to be able to discard their messages. They can misbehave, crash, stop responding at any time. But say somehow you can't discard messages and don't trust them. It's still only about backpressure handling. For example, kernel can just refuse to send messages to a particular recipient it knows is not consuming its incoming messages and instead can return messages back to senders into their incoming buffers or rejected buffers or whatever. Senders can decide what to do with that information, wait for a particular recipient to become ready again, waiting is not DoS, stop generating messages for that recipient or accumulate them or just drop them, minimal cooperation is required of course, but not trust, if they don't cooperate they don't hurt anyone but themselves. It's all still pure asynchronous stuff. And in fact, all high performance real world asynchronous communications deal with backpressure all without DoS and trust, they all also can discard messages though.