In an RPC-like protocol, where numerous requests can be active at the same time and responses can be given in any sequence, the following is possible: Pretend requests are sequentially numbered, and...

In an RPC-like protocol, where numerous requests can be active at the same time and responses can be given in any sequence, the following is possible: Pretend requests are sequentially numbered, and that ACK[N] confirms the receipt of reply[N]. Should the number of ACKs be cumulative? If not, what should happen in the event that an ACK is not received?