I won't judge whether the heartbeat feature itself is reasonable or not. But the inclusion of a payload was unnecessary, and specifying that it has to be copied to the reply message is so useless as to defy description. "Flexibility" my ass. It's downright suspicious.
The payload is meant to let you distinguish responses to different Heartbeats, right? I'm no expert, but sounds reasonable enough. I could see a lower limit on max payload size, but could you please elaborate a more detailed explanation why it's unnecessary and useless?
Okay, that explains things a bit. Could be done with a fixed-size payload though. Also I understand that the flexible size is intended to help with MTU discovery, but I think that would be a separate thing from a heartbeat.
If you are using data-grams (i.e., UDP) you have no guarantees of ordering or delivery, so you have no way to determine which transmission the echo reply you just received corresponds to without a payload that is returned to you.
Now, a 64kbyte payload, that's unnecessary for simply making each packet unique. That size was likely chosen to allow for the path MTU discovery aspects.
One could argue, however, that "keepalive" and "path MTU discovery" should not have been commingled, but they were.
I always assumed ping's payload was for detecting packet fragmentation/size limit issues and as a trivial mechanism for spotting things like packet corruption.
