[Bloat] Goodput fraction w/ AQM vs bufferbloat

[Stephen Hemminger]

On Fri, 6 May 2011 14:56:01 -0700
Fred Baker <fred at cisco.com> wrote:

> 
> On May 6, 2011, at 8:14 AM, richard wrote:
> > If every packet takes two attempts then the ratio will be 1/2 - 1 unit
> > of googput for two units of throughput (at least up to the choke-point).
> > This is worst-case, so the ratio is likely to be something better than
> > that 3/4, 5/6, 99/100 ??? 
> 
> I have a suggestion. turn on tcpdump on your laptop. Download a web page with lots of imagines, such as a google images web page, and then download a humongous file. Scan through the output file for SACK messages; that will give you the places where the receiver (you) saw losses and tried to recover from them.
> 
> > Putting a number to this will also help those of us trying to get ISPs
> > to understand that their Usage Based Bilking (UBB) won't address the
> > real problem which is hidden in this ratio. The fact is, the choke point
> > for much of this is the home router/firewall - and so that 1/2 ratio
> > tells me the consumer is getting hosed for a technical problem.
> 
> I think you need to do some research there. A TCP session with 1% loss (your ratio being 1/100) has difficulty maintaining throughput; usual TCP loss rates are on the order of tenths to hundredths of a percent.

There is some good theoretical work which shows relationship
between throughput and loss.
  http://www.slac.stanford.edu/comp/net/wan-mon/thru-vs-loss.html

Rate <= (MSS/RTT)*(1 / sqrt{p})

where:
Rate: is the TCP transfer rate or throughputd
MSS: is the maximum segment size (fixed for each Internet path, typically 1460 bytes)
RTT: is the round trip time (as measured by TCP)
p: is the packet loss rate. 

It is interesting that longer RTT which can be an artifact of
bloat in the queues, will hurt throughput in this case.