[Bloat] benefits of ack filtering

[Sebastian Moeller]

Well, ACK filtering/thinning is a simple trade-off: redundancy versus bandwidth. Since the RFCs say a receiver should acknoledge every second full MSS I think the decision whether to filter or not should be kept to the enduser and not some misguided middle boxes; if a DOCSIS ISP wants to secure precious upstream bandwidth they should at least re-synthesize the filtered ACKs after passing their upstream bottleneck IMHO. This is not reasonable network management in my irrelevant opinion unless actively opted-in by the user. Or put differently the real fix for DOCSIS ISPs is to simply not sell internet connections with asymmetries that make it impossible to saturate the link with TCP traffic without heroic measures like ack filtering. 
So I am all for cake learning to do that, but I am 100% against recommending using it unless one is "blessed" with a clue-less ISP that has problems calculating the maximal permissible Up/Down asymmetry for TCP...
BTW, I believe older TCPs used the reception of an ACK and not the acknowledged byte increment for widening their send/congestion windows, ack filtering should make slow start behave more sluggish for such hosts. As far as I can tell linux recently learned to deal with this fact as GRO in essence will also make the receiver ACK more rarely (once every 2 super-packets), so linux I think now evaluates the number of acknoledged bytes. But I have no idea about windows or BSD tcp implementations.

Best Regards



> On Nov 29, 2017, at 07:09, Mikael Abrahamsson <swmike at swm.pp.se> wrote:
> 
> On Tue, 28 Nov 2017, Dave Taht wrote:
> 
>> Recently Ryan Mounce added ack filtering cabilities to the cake qdisc.
>> 
>> The benefits were pretty impressive at a 50x1 Down/Up ratio:
>> 
>> http://blog.cerowrt.org/post/ack_filtering/
>> 
>> And quite noticeable at 16x1 ratios as well.
>> 
>> I'd rather like to have a compelling list of reasons why not to do
>> this! And ways to do it better, if not. The relevant code is hovering
>> at:
>> 
>> https://github.com/dtaht/sch_cake/blob/cobalt/sch_cake.c#L902
> 
> Your post is already quite comprehensive when it comes to downsides.
> 
> The better solution would of course be to have the TCP peeps change the way TCP works so that it sends fewer ACKs. I don't want middle boxes making "smart" decisions when the proper solution is for both end TCP speakers to do less work by sending fewer ACKs. In the TCP implementations I tcpdump regularily, it seems they send one ACK per 2 downstream packets.
> 
> At 1 gigabit/s that's in the order of 35k pps of ACKs (100 megabyte/s divided by 1440 divided by 2). That's in my opinion completely ludicrous rate of ACKs for no good reason.
> 
> I don't know what the formula should be, but it sounds like the ACK sending ratio should be influenced by how many in-flight ACKs there might be. Is there any reason to have more than 100 ACKs in flight at any given time? 500? 1000?
> 
> My DOCSIS connection (inferred through observation) seems to run on 1ms upstream time slots, and my modem will delete contigous ACKs at 16 or 32 ACK intervals, ending up running at typically 1-2 ACKs per 1ms time slot. This cuts down the ACK rate when I do 250 megabit/s downloads from 5-8 megabit/s to 400 kilobit/s of used upstream bw.
> 
> Since this ACK reduction is done on probably hundreds of millions of fixed-line subscriber lines today, what arguments do designers of TCP have to keep sending one ACK per 2 received TCP packets?
> 
> -- 
> Mikael Abrahamsson    email: swmike at swm.pp.se
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