On Fri, 25 Jul 2014, Neil Davies wrote:
Sebastian
On 25 Jul 2014, at 15:17, Sebastian Moeller <moell...@gmx.de> wrote:
But how do you propose to measure the (bottleneck) link capacity then?
It turns out for current CPE and CMTS/DSLAM equipment one typically can not
relay on good QoE out of the box, since typically these devices do not use
their (largish) buffers wisely. Instead the current remedy is to take back
control over the bottleneck link by shaping the actually sent traffic to stay
below the hardware link capacity thereby avoiding feeling the consequences of
the over-buffering. But to do this is is quite helpful to get an educated
guess what the bottleneck links capacity actually is. And for that purpose a
speediest seems useful.
I totally agree that what you are trying to do is to take control "back" for
the upstream delay and loss (which is the network level activity that directly
influences QoE). Observationally the "constraining link" is the point at which
the delay and loss start to grow as the the offered load is increased (there
are interesting interactions with the scheduling in the CMTS/3GPP node B - but
they are tractable) if we don't have direct access to the constraint (which in
the CPE, for ADSL you have) we track that "quality attenuation" inflection
point. Saturating the path is a bit of a sledgehammer (and has nasty
cost/scaling implications).
The thing is that there is little effect on latency until the congestion starts,
so we can only measure the problem when there is congestion.
Saturating the link is a bit of a sledgehammer, but there really isn't any other
way to get to the worst case situation.
In terms of scaling, have the server detect that all the requests have combined
to saturate it's link, and have it tell the clients that it's overloaded, wait a
random amount of time and retry (or try another location)
cost of bandwidth for this is just something to get someone to pay for (ideally
someone with tons of bandwidth already who won't notice this sort of test, even
if there are a few going on at once.)
David Lang
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