[Bloat] benefits of ack filtering

[Dave Taht]

On Wed, Nov 29, 2017 at 10:21 AM, Juliusz Chroboczek <jch at irif.fr> wrote:
>> The better solution would of course be to have the TCP peeps change the
>> way TCP works so that it sends fewer ACKs.
>
> Which tends to perturb the way the TCP self-clocking feedback loop works,
> and to break Nagle.

Linux TCP is no longer particularly ack-clocked. In the post pacing,
post sch_fq world, packets are released (currently) on a 1ms schedule.
Support was recently released for modifying that schedule on a per
driver basis, which turns out to be helpful for wifi.

see: https://www.spinics.net/lists/netdev/msg466312.html

>
>> In the TCP implementations I tcpdump regularily, it seems they send one
>> ACK per 2 downstream packets.
>
> That's the delack algorithm.  One of the stupidest algorithms I've had the
> displeasure of looking at (a fixed 500ms timeout, sheesh).

Nagle would probably agree. He once told me he wished for 1 ack per
data packet...

We were young then.

>
> And yes, it breaks Nagle.
>
>> I don't want middle boxes making "smart" decisions

Ironically, it was dave reed's (co-author of the end to end argument)
50x1 ratio network connection that was an impetus to look harder at
this, and what I modeled in
http://blog.cerowrt.org/post/ack_filtering/

(I note there is discussion and way more tests landing on the cake mailing list)

The astounding number was that we were able to drop 70% of all packets
(and 90+% of acks) without doing any visible harm on the tests.

>
> I agree, especially if they use transport-layer data to make their
> decisions.

I'm not particularly fond of the idea myself! But I didn't invent
severe network asymmetry, or cpus that can't context switch worth a
damn.

>> Since this ACK reduction is done on probably hundreds of millions of
>> fixed-line subscriber lines today,

What I'd started with was wanting to create impairments for netem that
matched common ack-filtering schemes in the field already.

> what arguments do designers of TCP have
>> to keep sending one ACK per 2 received TCP packets?

this would be a good list to have.

I note osx does stretch acks by default.

>
> I think it's about growing the TCP congestion window fast enough.  Recall
> that that AIMD counts received ACKs, not ACKed bytes.

the cake code has a specific optimization to preserve slow start. It
can be improved.

>
> (And not breaking Nagle.)
>
> -- Juliusz



-- 

Dave Täht
CEO, TekLibre, LLC
http://www.teklibre.com
Tel: 1-669-226-2619