A sender could always cache the most recently discovered PMTU to a given destination. That would be enough for repeated short UDP flows to the same destination. As for isolated short UDP flows, they just need to use the Internet minimum MTU (576 or 1280) - there's no other possible solution.Minimum MTU for IPv4 is only 68 bytes; minimum EMTU_R is the one that is 576 - right?
I forget the exact terminology, but Brian is talking about the biggest packet one can send and be guaranteed it will not be fragmented. The is 576 bytes for IPv4 and 1280 bytes for IPv6.
I think of it as the biggest smallest packet one can send :-) Bob