Can you explain why it is neccesary or functional that "one side of the protocol can send large messages, and the counterparty can reply with smaller ones, rely

Can you explain why it is neccesary or functional that "one side of the protocol can send large messages, and the counterparty can reply with smaller ones, relying on the fact that the message includes discardable padding." ?? If the payload has variable size to facilitate MTU discovery, then there is some logic to sending messages of varying sizes. An MTU can never exceed the 1500 of ethernet though, so this is weird stuff. What good can a payload of 64k really do? And MTU discovery doesn't involve any echoing by the receiving party. Either packets get blocked (at the IP layer) and error messages get returned (by the router blocking them) OR the packages arrive fully (or get reassembled) so this Heartbeat code won't know about any issues anyway. And the "echo" proves nothing. I really don't get this, what is the point...?