Re: [Bloat] Questions for Bufferbloat Wikipedia article

[Sebastian Moeller <moeller0@gmx.de>]

Hi Eric,

thanks for your thoughts.

> On Apr 6, 2021, at 02:47, Erik Auerswald <auerswal@unix-ag.uni-kl.de> wrote:
> 
> Hi,
> 
> On Mon, Apr 05, 2021 at 11:49:00PM +0200, Sebastian Moeller wrote:
>> 
>> all good questions, and interesting responses so far.
> 
> I'll add some details below, I mostly concur with your responses.
> 
>>> On Apr 5, 2021, at 14:46, Rich Brown <richb.hanover@gmail.com> wrote:
>>> 
>>> Dave Täht has put me up to revising the current Bufferbloat article
>>> on Wikipedia (https://en.wikipedia.org/wiki/Bufferbloat)
>>> [...]
>> [...] while too large buffers cause undesirable increase in latency
>> under load (but decent throughput), [...]
> 
> With too large buffers, even throughput degrades when TCP considers
> a delayed segment lost (or DNS gives up because the answers arrive
> too late).  I do think there is _too_ large for buffers, period.

	Fair enough, timeouts could be changed though if required ;) but I fully concur that laergeish buffers require management to become useful ;)


> 
>> The solution basically is large buffers with adaptive management that
> 
> I would prefer the word "sufficient" instead of "large."

	If properly managed there is no upper end for the size, it might not be used though, no?


> 
>> works hard to keep latency under load increase and throughput inside
>> an acceptable "corridor".
> 
> I concur that there is quite some usable range of buffer capacity when
> considering the latency/throughput trade-off, and AQM seems like a good
> solution to managing that.

	I fear it is the only network side mitigation technique?


> 
> My preference is to sacrifice throughput for better latency, but then
> I have been bitten by too much latency quite often, but never by too
> little throughput caused by small buffers.  YMMV.

	Yepp, with speedtests being the killer-application for fast end-user links (still, which is sad in itself), manufacturers and ISPs are incentivized to err on the side of too large for buffers, so the default buffering typically will not cause noticeable under-utilisation, as long as nobody wants to run single-flow speedtests over a geostationary satellite link ;). (I note that many/most speedtests silently default to test with multiple flows nowadays, with single stream tests being at least optional in some, which will reduce the expected buffering need).

> 
>> [...]
>> But e.g. for traditional TCPs the amount of expected buffer needs
>> increases with RTT of a flow
> 
> Does it?  Does the propagation delay provide automatic "buffering" in the
> network?  Does the receiver need to advertise sufficient buffer capacity
> (receive window) to allow the sender to fill the pipe?  Does the sender
> need to provide sufficient buffer capacity to retransmit lost segments?
> Where are buffers actually needed?

	At all those places ;) in the extreme a single packet buffer should be sufficient, but that places unrealistic high demands on the processing capabilities at all nodes of a network and does not account for anything unexpected (like another low starting). And in all cases doing things smarter can help, like pacing is better at the sender's side (with better meaning easier in the network), competent AQM is better at the bottleneck link, and at the receiver something like TCP SACK (and the required buffers to make that work) can help; all those cases work better with buffers. The catch is that buffers solve important issues while introducing new issues, that need fixing. I am sure you know all this, but spelling it out helps me to clarify my thoughts on the matter, so please just ignore if boring/old news.


> 
> I am not convinced that large buffers in the network are needed for high
> throughput of high RTT TCP flows.
> 
> See, e.g., https://people.ucsc.edu/~warner/Bufs/buffer-requirements for
> some information and links to a few papers.

	Thanks, I think the bandwidth delay product is still the worst case buffering required to allow 100% utilization with a single flow (a use case that at least for home links seems legit, for a back bone link probably not). But in any case if the buffers are properly managed their maximum size will not really matter, as long as it is larger than the required minimum ;)

Best Regards
	Sebastian


> 
>> [...]
> 
> Thanks,
> Erik
> -- 
> The computing scientist’s main challenge is not to get confused by
> the complexities of his own making.
>                        -- Edsger W. Dijkstra
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