[Cake] [Bloat] benefits of ack filtering
dave.taht at gmail.com
Wed Nov 29 13:41:41 EST 2017
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.
>> 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
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
(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
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
>> 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
CEO, TekLibre, LLC
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