> The reliability signals this app needs are built into both TCP and the socket programming interface, from what I can tell. The application-layer acknowledgement it wants appears duplicative.

An ACK only suggests that a segment has reached the recieve buffer successfully. If the receiving application crashes between buffering incoming data and successfully processing it, that's expressly not TCP's problem. The RST will signal that the receiver has died, but does not imply whether or not the application has successfully processed any buffered data up to that point.

AFAIK, an application-layer acknowledgement is the only way to solve this problem.