[Bloat] Bloat done correctly?

[Alex Elsayed]

Sebastian Moeller wrote:

> Hi Benjamin,
> 
> To go off onto a tangent:
> 
> On Jun 12, 2015, at 06:45 , Benjamin Cronce
> <bcronce at gmail.com> wrote:
> 
>> [...]
>> Under load while doing P2P(About 80Mb down and 20Mb up just as I started
>> the test) HFSC: P2P in 20% queue and 80/443/8080 in 40% queue with ACKs
>> going to a 20% realtime queue http://www.dslreports.com/speedtest/622452
> 
> I know this is not really your question, but I think the ACKs should go
> into the same queue as the matching data packets. Think about it that way,
> if the data is delayed due to congestion it does not make too much sense
> to tell the sender to send more faster (which essentially is what ACK
> prioritization does) as that will not really reduce the congestion but
> rather increase it. There is one caveat though: when ECN is used it might
> make sense to send out the ACK that will signal the congestion state back
> to the sender faster… So if you prioritize ACKs only select those with an
> ECN-Echo flag ;) @bloat : What do you all think about this refined ACK
> prioritization scheme?

I'd say that this is wrongly attempting to bind upstream congestion to 
downstream congestion.

Let's have two endpoints, A and B. There exists a stream sent from A towards 
B.

If A does not receive an ack from B in a timely manner, it draws inference 
as to the congestion on the path _towards_ B. Prioritizing acks from B to A 
thus makes this _more accurate to reality_ - a lost ack (rather than the 
absence of an ack due to a lost packet) actually behaves as misinformation 
to the sender, causing them to

1.) back off sending when the sending channel is not congested and
2.) resend a packet that _already arrived_.

The latter point is a big one: Prioritized ACKs (may) reduce spurious 
resends, especially on asymmetric connections - and suprious resends are 
pure network inefficiency. Especially since the data packets are likely far 
larger than the ACKs. Which would _also_ get resent.