[Babel-users] RTT-aware branch of Babel

[Dave Taht]

On Fri, Jun 21, 2013 at 10:04 AM, Henning Rogge <hrogge at googlemail.com> wrote:
> The main problem with RTT and buffers is that you will not get "really
> bad RTTs" until the link is saturated.

Sigh. Humans have a tendency to think of bandwidth as some number
taken from samples over a second, or many seconds. At the timescales
that networks operate at, saturation can occur on timescales much less
than that. At the lowest timescale possible on a given link you are
either at capacity 1 or 0.

There are multiple examples of this in my talks of late. For example,
DASH traffic (what netflix basically does) starts a new transaction
every 10 seconds, ramps up rapidly (because they are co-located
usually in the same data center as the ISP) until a 6mbit stream is
delivered, then it goes quiet until the next 10 seconds of video need
to be delivered. The last few moments of this induce dozens of ms of
latency on the cable modem link I looked at when I had time.

A simpler example of this fallacy is to flood ping a network for 1
second every 10 seconds. If you take an average, you might see 30%
utilization and think everything is fine, but for 3 seconds it's
unusable unless "something is done" to fq other packets inbetween.

One place where this really gets to me is in looking at mrtg
statistics, which use a 5 minute average. I know full well that the
network is periodically saturated based on the drop statistics, but
the averages are consistently well below the provided rate due to the
bursty nature of traffic in general.

I need to get around to monitoring packet drops better via mrtg in
particular and add some more instrumentation to the kernel as to when
and why they happen...

And I keep wondering about what traffic on wifi "really" looks like in
relation to this wonderful, old, paper, that has embedded itself in
many a conciousness...

https://plus.google.com/u/0/107942175615993706558/posts/BPd4TpA2Ssg
-- 
Dave Täht

Fixing bufferbloat with cerowrt: http://www.teklibre.com/cerowrt/subscribe.html