Relays are helpful not only when UDP is blocked, but also when both nodes are behind NAT that does random port overloading and port prediction doesn't work.
TCP hole punching uses symmetrical TCP open trick to establish a proper TCP session between nodes each behind its own NAT. Roughly the same idea as with UDP h/p, but requires a coordinating server to make it work. Useful for cases when UDP punching doesn't work or when UDP is blocked completely.
Oh, that kind of TCP hole punching. No, we don't do that, because we run over UDP only. So far, I don't think we've seen network conditions where UDP is blocked but the NAT is friendly enough to permit TCP traversal (same problems as UDP traversal, only with extra state to manage). I suppose we might get to it eventually, but there's a lot of simpler cases we can handle better first :)
TCP hole punching uses symmetrical TCP open trick to establish a proper TCP session between nodes each behind its own NAT. Roughly the same idea as with UDP h/p, but requires a coordinating server to make it work. Useful for cases when UDP punching doesn't work or when UDP is blocked completely.