[Bloat] benefits of ack filtering

[Jonathan Morton]

Taking into account a variety of scenarios, I have difficulty identifying a
case where an ack deleted by a reasonably conservative algorithm would have
given any practical benefit had it remained, *including* considerations of
smoothness of ack-clocking.

If the uplink isn't congested then no deletions occur; if it is congested
then there's a high probability that a flow-isolation scheme would deliver
several acks back to back between larger data packets, so an ack-clocked
sender would still be "lumpy".  That's without even considering aggregation
and discrete MAC-grant links (ie. DOCSIS).

Deleting unnecessary acks from a congested uplink also frees capacity for
competing traffic, which I think we can agree is a good thing when it has
no deleterous side-effects.

I have not yet personally verified that the algorithm now in Cake matches
my assumptions.  If it doesn't, I'll push for modifications.

Incidentally, I am of the opinion that ECE can safely be ignored for
ack-filtering purposes.  Normally ECE remains set in all acks until a CWR
is heard in reply, so it only matters that the ECE signal isn't *delayed* -
which ack-filtering actually helps to achieve.  More sophisticated uses of
ECE should also survive this as long as statistical independence is
maintained.

- Jonathan Morton
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