Constant ACK_FOR_KEY 

pub const ACK_FOR_KEY: &'static str;

Metadata key for ACK messages indicating which message ID the ACK is for.

Re-exported rather than declared, because a leaf node writes the same key on the acknowledgements it owes and cannot reach this crate. The one home is offline-protocol-sealed; a second spelling here would be a pair that stops answering each other with nothing failing to build. The metadata key naming the message a delivery acknowledgement answers.

An acknowledgement is not a control frame. It carries no prefix and no signature: it is an ordinary message with empty content whose whole meaning is this one metadata entry, so a receiver that does not know this string reads a blank message and a sender that does not write it is never answered. That makes it exactly the kind of number this module holds, and the reason it is not left in the engine, where a leaf node cannot reach it and would have to spell it again.

It is deliberately the only entry a leaf writes. The other two the engine puts on its own acknowledgements say which hop count and which carrier the answer came back over, and both have defaults that are already right for a device: no hops, because a leaf is a direct peer, and BLE, because that is the radio a leaf is paired over. A device does not own its transport, so naming one would be firmware’s guess crossing the wire as fact.