http://i3xi5qxvbrngh3g6o7czwjfxwjzigook7zxzjmgwg5b7xnjcn5hzciad.onion/proposals/348-udp-app-support.html
For example, the very permissive endpoint-independent filtering strategy would allow any host on the internet to send datagrams in bulk to all available local ports on a Tor exit in order to map that traffic's effect on any guards they control. Any vehicle for malicious parties to mark traffic can be abused to de-anonymize users. Even if there is a more restrictive filtering policy, UDP's lack of sequence numbers make header spoofing attacks considerably easier than in TCP.