Re: cannot fill 100Mbps pipe (over 20ms rtt via netem)?

[slashdev]

On Wed, Apr 30, 2008 at 5:27 PM, John Heffner <johnwheffner@xxxxxxxxx> wrote:
>
> On Wed, Apr 30, 2008 at 5:13 PM, slashdev <slashdev@xxxxxxxxx> wrote:
>  >
>  > On Wed, Apr 30, 2008 at 4:34 PM, John Heffner <johnwheffner@xxxxxxxxx> wrote:
>  >
>  >  > On Wed, Apr 30, 2008 at 2:45 PM, slashdev <slashdev@xxxxxxxxx> wrote:
>
>  >  >  >  i can provide the tcpdump trace. but that too looks ok. i.e. window
>  >  >  >  scale is set to
>  >  >  >  healthy "10" and the receiver does endup opening large window. but i
>  >  >  >  may be missing
>  >  >  >  something.
>  >  >  >
>  >  >  >  if you need any other information please do ask.
>  >  >  >
>  >  >  >  i am bit puzzled at the moment :-)
>  >  >
>  >  >  Linux should easily be able to support this "out of the box" these
>  >  >  days.  Can you post the binary tcpdump somewhere?
>  >
>  >  http://slashdev.googlepages.com/traces/
>  >
>  >  i've added traces for 20ms rtt and normal lan rtt (aka 0ms rtt :-)
>  >  let me know if you have trouble accessing them or want any more
>  >  information.
>  >
>  >  thanks for your help
>
>  Both traces show lots of packet loss -- enough to cause an equilibrium
>  cwnd of less than 100 KB.  With a short RTT, this window is adequate
>  to fill a 100 Mbps pipe, but with even a 20 ms RTT, it's hurting you.
>  I'm not sure where the packet loss is coming from.  Possibly an
>  overloaded switch or bad cable?

thanks for the insight. will take a look at my setup and see why that
should happen. will report back once i am done fixing things at my
end.

btw (maybe a stupid question): how did you deduce the cwnd from
the traces? i can see large windows 300K+ being advertised by the
receiver and the sender transmitting 100K+ of data many a times...
(or is that because of TSO, is see those large values in tcpdump) ?

thanks again!
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