"choosing not to forward it" is an interesting phrase. NAT needs to have some expiry for each entry, because we don't have unlimited space for that table. Dropping the mapping entry has the same result as the other side becoming unavailable and is an understood state.
You can't just produce RST out of nowhere on the NAT expiry, because that connection may actually be active somewhere else. Consider a replicating pair of NATs - your connection gets moved from one to the other because (network reasons), but the previous one does not get a message about it because (network reasons). If it sent the RST packets it would actively kill live connections which it should not touch anymore.
You can't just produce RST out of nowhere on the NAT expiry, because that connection may actually be active somewhere else. Consider a replicating pair of NATs - your connection gets moved from one to the other because (network reasons), but the previous one does not get a message about it because (network reasons). If it sent the RST packets it would actively kill live connections which it should not touch anymore.