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On 05/21/2011 12:49 PM, Dave Taht wrote:
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cite="mid:BANLkTimFbb4jbv6uooOp+UXmwrguARfkGw@mail.gmail.com"
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<br>
<div class="gmail_quote">On Sat, May 21, 2011 at 10:33 AM,
Srikanth Sundaresan <span dir="ltr"><<a
moz-do-not-send="true" href="mailto:srikanth@gatech.edu">srikanth@gatech.edu</a>></span>
wrote:<br>
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<div class="h5"><br>
On May 21, 2011, at 6:08 PM, Dave Taht wrote:<br>
<br>
><br>
><br>
> On Sat, May 21, 2011 at 9:48 AM, Srikanth Sundaresan
<<a moz-do-not-send="true"
href="mailto:srikanth@gatech.edu">srikanth@gatech.edu</a>>
wrote:<br>
> I do not have a problem with it when we know the
effective bandwidth. My question is, what when do not? We
cannot rely on volunteers to give us reliable information
on that.<br>
><br>
> I say we turn it *on only while testing*. That too,
after we get an idea about each user's bandwidth. THis is
feature that, in its current form needs to be tailored to
each user. It is not a good idea to give everyone a
default setting - as I mentioned in my previous email,
unless we hit bulls eye (unlikely), it is either
crippling, or useless.<br>
><br>
> It could potentially seriously downgrade user
experience.<br>
><br>
><br>
> What part about 800ms latencies under load without
QoS isn't about a degraded user experience?<br>
<br>
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If the cost of reduced upload speed, which could perhaps be
as much as 30%, (if the default is 340kbps - the DSL
connection here in my B&B gets up to 450k), can't be
reduced, I certainly think that it's the higher price to pay
than reduced latency.<br>
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I would urge you strongly to do more realistic testing, with
more real users using the network...<br>
<br>
...before making that call. I am assuming you are unleashing
these devices on real users? with more than one person on the
network?<br>
<br>
RTT times will probably get even worse than 800ms with
multiple streams running. I have not tested that, I'll get to
it.<br>
<br>
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<br>
Take a look at the ISCI data on bufferbloat. The colour versions are
in: <a
href="http://gettys.wordpress.com/2010/12/06/whose-house-is-of-glasse-must-not-throw-stones-at-another/">http://gettys.wordpress.com/2010/12/06/whose-house-is-of-glasse-must-not-throw-stones-at-another/</a>
You can get an explanation for the structure you will see from the
Netalyzr papers. The diagonal lines show the potential latency, and
the "green" line of .5 seconds is already 3x telephony standards.
So a high fraction of the internet has serious bloat problems.<br>
<br>
Several things:<br>
<br>
1) the dataset *underdetected* the problem: there was a bug in
Netalyzr only detected last October (and the dataset covers most of
2010 and back into 2009). So the situation is worse than that
shown. How much, we don't know.<br>
<br>
2) the amount of bloat varies *all* over; some DSL lines are up in
the 6 second range. <br>
<br>
3) the amount of buffering is fixed: so the lower the provisioning,
the longer the latency on the same devices. I was seeing > 1.2
seconds on 20Mbps cable, at which point web surfing is painful
indeed.<br>
<br>
4) If the bottleneck is elsewhere, then the broadband connection
bufferbloat isn't the buffers that will affect the results. So if
the backhaul is lower bandwidth than the broadband/wireless
connections, then the problem will be in the network routers, where
RED (if enabled!) can control it.<br>
<br>
I call the problem the "Daddy, the Internet is slow today" problem:
I was usually the person who was causing saturation on the line, so
when my family was experiencing slowdowns, they would come to me and
I'd try to debug it, and stop whatever it was that was doing them
in. Who may be saturating the link most often varies from household
to household: bittorrent can induce bufferbloat suffering.<br>
<br>
Now, what you do is for you to determine; just understand the
bufferbloat problem is extremely common and extremely variable.
Your mileage *will* vary.<br>
- Jim<br>
<br>
<br>
<br>
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