http://i3xi5qxvbrngh3g6o7czwjfxwjzigook7zxzjmgwg5b7xnjcn5hzciad.onion/proposals/348-udp-app-support.html
It also makes possible clear-net hosting of UDP servers using an exit node's IP, which may have undesirable abuse properties. This permissive filter is also incompatible with our proposed mitigation to local port exhaustion on exit relays. Even with per-circuit rate limiting, an attacker could trivially overwhelm the local port capacity of all combined UDP-capable Tor exits.