PING frame behavior

Hi all,

The current PING frame language is a bit vague. I'd appreciate
clarification on the following issues (and I'd be happy to open a pull
request incorporating the answers into the spec).

1. Are PING frame receivers obligated to respond with PING+ACK?
2. Must the PING+ACK payload be identical to that of the original PING?
3. May the sender generate additional PING frames before the first is ACK'd?
4. May intermediaries forward PING frames?

I have my own opinions on the best answers to the above, of course, but I'm
new around here, and I'd like to know more about the original intention of
the PING frame first, in case I've misunderstood something.

Thanks,
Alek