On 12/06/15 15:35, Daniel Havey wrote:
On Fri, Jun 12, 2015 at 6:00 AM, Alan Jenkins
<alan.christopher.jenk...@gmail.com> wrote:
On 12/06/15 02:44, David Lang wrote:
On Thu, 11 Jun 2015, Sebastian Moeller wrote:
On Jun 11, 2015, at 03:05 , Alan Jenkins
<alan.christopher.jenk...@gmail.com> wrote:
On 10/06/15 21:54, Sebastian Moeller wrote:
One solution would be if ISPs made sure upload is 100% provisioned.
Could be cheaper than for (the higher rate) download.
Not going to happen, in my opinion, as economically unfeasible for a
publicly traded ISP. I would settle for that approach as long as the ISP is
willing to fix its provisioning so that oversubscription episodes are
reasonable rare, though.
not going to happen on any network, publicly traded or not.
Sure, I'm flailing. Note this was in the context of AQSM as Daniel
describes it. (Possibly misnamed given it only drops. All the queuing is
"underneath" AQSM, "in the MAC layer" as the paper says :).
Noooooooooo! I am a huge supporter of ECN. ECE Everywhere! I'm sure
I wrote "mark/drop" in the paper. I might have dyslexically written
"drop/mark", but, if I ever gave the impression then I categorically
deny that right now and till forever. ECN everywhere :^)
My bad! Imagine I wrote mark/drop. It was just a definitional wibble.
Is it AQM if it's not your Q, is it policing (and does AQM include
policing)?
- AQSM isn't distinguishing up/down bloat. When it detects bloat it has to
limit both directions in equal proportion.
=> if there is upload contention (and your user is uploading), you may hurt
apps sensitive to download bandwidth (streaming video), when you don't need
to.
What would the solutions look like?
i) If contention in one direction was negligible, you could limit the other
direction only. Consumer connections are highly asymmetric, and AQSM is
only measuring the first IP hop. So it's more feasible than 100% in both
directions. And this isn't about core networks (with larger statistical
universes... whether that helps or not).
I'm sure you're right and they're not asymmetric _enough_.
ii) Sebastian points out if you implement AQSM in the modem (as the paper
claims :p), you may as well BQL the modem drivers and run AQM. *But that
doesn't work on ingress* - ingress requires tbf/htb with a set rate - but
the achievable rate is lower in peak hours. So run AQSM on ingress only!
Point being that download bloat could be improved without changing the other
end (CMTS).
This is pretty cool. I had not considered BQL (though Dave and Jim
were evangelizing about it at the time :). This solves the
upload/download problem which I was not able to get past in the paper.
BQL on the egress and ASQM for the ingress. BQL will make sure that
the upload is under control so that ASQM can get a good measurement on
the download side. Woot! Woot! Uncooperative ISP problem solved!
BTW...Why doesn't BQL work on the ingress?
Both AQM & BQL manage transmit queues. It's the natural way to work.
If you don't control the transmit queue(s) at the bottleneck, you have
to insert an artificial bottleneck that you _do_ control.
The problem is we don't have the source to hack any modems, only Linux
routers like in your paper :). It's why, to set up a router with our
SQM, you have to know the line rate (both directions). And tell SQM to
use maybe 95% of it. And assume the full rate is available even during
peak hours :(.
So we still want co-operative ISPs to solve this, because that's who
procures the modems. That said, once they've been _developed_ it's
going to be easier to buy a good modem regardless of ISP.
We rate-limit with htb, so we build a queue for fq_codel to work on.
(Then Johnathan smushed the two qdiscs together and called the resulting
code "cake").
Regarding ingress rate-limiting, note that
- it involves creating a virtual network interface (IFB), to get an
artificial transmit queue to apply AQM to. (Something like that, I
don't know the exact syntax).
- bursts can still pile up at the upstream bottleneck, so you can't
eliminate latency increases altogether. (E.g. caused by tcp's initial
multiplicative increase). It can still be a big win though, because isp
queues would often be allowed to grow many times larger than needed
(expected rtt / ~100ms).
The question is not "can the theoretical max of all downstream devices
exceed the upstream bandwidth" because that answer is going to be "yes" for
every network built, LAN or WAN, but rather "does the demand in practice of
the combined downstream devices exceed the upstream bandwidth for long
enough to be a problem"
it's not even a matter of what percentage are they oversubscribed.
someone with 100 1.5Mb DSL lines downstream and a 50Mb upstream (30% of
theoretical requirements) is probably a lot worse than someone with 100 1G
lines downstream and a 10G upstream (10% of theoretical requirements)
because it's far less likely that the users of the 1G lines are actually
going to saturate them (let alone simultaniously for a noticable timeframe),
while it's very likely that the users of the 1.5M DSL lines are going to
saturate their lines for extended timeframes.
The problem shows up when either usage changes rapidly, or the network
operator is not keeping up with required upgrades as gradual usage changes
happen (including when they are prevented from upgrading because a peer
won't cooperate)
As for the "100% provisioning" ideal, think through the theoretical
aggregate and realize that before you get past very many layers, you get to
a bandwidh requirement that it's not technically possible to provide.
David Lang
Yuppers! Dave is right. The FCC studies (especially the 80/80 study
out of UNC) from 2010 - 2014 (footnoted in the paper) indicate that
during peak hours it is quite common for an ISP not to provide 100% of
the rated throughput.
Note the same technical problem applies if they provide 100%-150%.[1]
If bloat is a problem, you can probably still hack around it as a user,
but then you lose the _benefits_ of the oversubscription.
[1] Comcast "Powerboost" http://www.dslreports.com/faq/14520
In fact in 2014 it indicates that a full on 50%
of the ISPs measured provided less than 100%. The 100% all the time
goal is unreasonable because it implies too much waste. Many ISPs get
to 90% or above even during peak hours. This is good! We could live
with that :) Providing that last 10% would mean that they would have
to provide for a lot of excess capacity that goes unused during
non-peak hours. Wasteful. That money should be allocated for more
important things like providing AQM for all or saving the planet or
something else. :^)
:)
I grant Dave's caveats. Couldn't resist looking for the graphs. (Around
page 40
http://data.fcc.gov/download/measuring-broadband-america/2014/2014-Fixed-Measuring-Broadband-America-Report.pdf).
It looks like cable ISPs happen to hit an average of 100% during peak
hours (as you might hope from figure of 50% going below, apparently 50%
are going above).
Alan
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