On @0xB10C‘s approach of pairing clearnet addresses with an externally reachable Tor service endpoint by the same node for establishing clearnet connections: it seems like it would be fine for nodes that do not also run a P2P Tor service. It makes me wonder if it could increase chances of an adversary in correlating addresses when a node which also runs a P2P Tor service shuts down though. Maybe it’s already bad. Having nodes create new Tor services at the same interval as the new ephemeral addr messages would maybe help.
Regarding building a TCP-like protocol over UDP as touched upon by @cedarctic & @sipa I agree it is a major undertaking, but probably a smaller lift than for example multiprocess support being worked on in Core. It still needs champions with a lot of grit though. Maybe the adversarial aspects is what makes people pessimistic? I’ve only seen such protocols used in less adversarial environments, in closed source.
Having the ability to bypass strict ordering / stream semantics on a per packet basis would be at least somewhat beneficial for things like compact block propagation.
Agree on attempting TCP hole punching first though. 