On Thu, Mar 21, 2013 at 2:14 PM, Oliver Hohlfeld <
oliver@net.t-labs.tu-berlin.de> wrote:

> > I believe you are actually measuring the *fraction of the time* your
> > measurements show bad latency
>
> Yes.
>
> > The best data I've seen on how widespread the problem is is the ICSI
> > Netalyzr scatter plots results
>
> One has to be extremely careful on what to conclude from this data.
> The Netalyzr data shows that bloated buffers _exist_, not that they
> are _used_ in practice. Or as Mark has put it: "[it] shows that large
> delays due to buffering can happen, not that they do happen."
> Empirical evidence cited in my previous mail suggests that buffer
> bloat does not happen often.
>

Heh.

6% of the time isn't "often"?  The measurements is much more a measurement
of how much of the time your competing with someone else (or yourself in
background) for the bottleneck than anything else.

Start up any long lived TCP connection (using a modern TCP; that leaves out
Windows XP).

Stand back a few seconds.

Measure latency.

Think uploading/downloading files/videos, or sending email with images
attached, etc.


More insidious is what visiting a web page with lots of embedded images can
do.

The cross product of:
   1) current web browsers using many more than 2 TCP connections.
   2) web site "sharding".
   3) the initial window of data in those TCP connections.

This causes transients of what is up to hundreds of milliseconds of head of
line blocking even
on very high speed broadband/wifi connections.  Again, I encourage you to
do this measurement on your own broadband connection, by visiting any image
heavy web site and running ping....



>
> > I encourage everyone to run netalyzr and/or the mlabs tests for
> > bufferbloat on your own broadband connections, or do simple copy and
> > ping tests inside your own house over wifi to your local file servers....
>
> While this will identify bloated buffers, it does not help in answering
> what fraction of Internet users actually experience the problem.
>

We've not come across any broadband equipment buffered "correctly".  If
thought is given at all to buffering, it's been sized to the maximum
buffering the device might ever need for a single TCP flow at its maximum
bandwidth.  Example, plug a current DOCSIS 3 modem (capable up to 300Mbps
these days) and use it at 20Mbps; unless the buffersizing amendment has
been turned on (no ISP I am aware of yet does so), you'll be overbuffered
by a factor of 15.

And all the operating systems have problems, to one degree or another (and
as they are all adjacent to wireless links these days, they also contribute
to the problem.

Bufferbloat just waits in hiding to get you when you try to use the network.
                                        - Jim


> Oliver

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