[toast0]

It's not nice that pmtud breaks when ICMP isn't available, but sensible probing can do pretty well. Of course, barely anybody probes and they may not be sensible if they do.

Imagine at the beginning of a connection, sending a burst of packets, you could send (ignoring tcp timestamps because it makes the math hard, PAWS is a waste of bytes for most flows, and etc) [0,1460); [1400, 2800), ...

If you get an ack of the first packet, great. If not, you resend it as a 1400 byte payload and probe again in a future burst. Maybe even premptively resend the first packet as a 1400 byte segment after a short delay. Anyway, have enough failed large packets and probe smaller. Probe bigger again every so often if the connection stays open for a meaningful amount of time.