But is your program actually P2P? (If it were, then: The sender would know when the recipient has completed the download, because the sender was finished sending it and the recipient has stated that they are done receiving it. And there would be no need for gymnastics.)
Exactly like that. My program is half-way there to doing NAT hole-punching, a bunch of other stuff is missing and I'm deep into yak-shaving right now the likes of you wouldn't even believe, but I have had good progress the last few months. (Been tinkering for years, finally decided to do a serious effort. The commit log goes back years but for long periods very silent.)
But is your program actually P2P? (If it were, then: The sender would know when the recipient has completed the download, because the sender was finished sending it and the recipient has stated that they are done receiving it. And there would be no need for gymnastics.)