http://i3xi5qxvbrngh3g6o7czwjfxwjzigook7zxzjmgwg5b7xnjcn5hzciad.onion/proposals/348-udp-app-support.html
Short-lived identifiers are still quite valuable to attackers, because they may reliably track application sessions across changes to the Tor exit. If longer-lived identifiers exist for any reason, that of course provides a powerful tool for call metadata gathering.