[QUIC] h2 mapping - hpack dynamic table by reference

[Patrick McManus <[e-mail]>]

Hi Robbie, Mike, et al. Thanks for getting the ball rolling with
https://tools.ietf.org/html/draft-shade-quic-http2-mapping-00..

I have a thought on the mapping - Putting all of the header frames onto one
in-order stream is a pretty big hammer.

Is there stomach for looking at a strategy where hpack references the
dynamic table state via some kind of instruction counter? That way the
headers could be on the data (>=5) stream and just the dynamic table
manipulation stuff from hpack could be on stream 3.. implementations would
be able to choose whether to generate hpack that referenced the current
counter or an older one (within some window).. an older one is going to be
more robust to reordering happening on stream 3 so the dep between the
streams is loosened. Anecdotally the rate of change of the dynamic table
slows down (though I have no data - could collect it if we're serious)
after an initial burst and at that point not using table updates until
they've been acked could be a reasonable thing to do... and this even
allows pre-populating the dynamic table at setup time with site-local
values rather than in response to a query. Rewinding to any particular
instruction seems to be a big burden, but we could probably do some things
with checkpoints and acks to bound it all.

this is a fairly significant bit of surgery to h2's formats, but given that
one of the real quic opportunities is to perform better under loss it could
be worthwhile to sever ordering dependencies even further. Seems like a
reasonable thing to investigate to me.

Thanks
-Patrick