[c-nsp] 3560 buffering

[Marian Ďurkovič]

On Wed, 14 Oct 2009 08:44:24 +0200, Gert Doering wrote
> Hi,
> 
> On Tue, Oct 13, 2009 at 07:10:24AM -0500, Jeff Bacon wrote:
> > What's an output drop mean, in a 3560 context?
> 
> Traffic too bursty, output buffers too tiny.
> 
> > Is there a way to tell the 3560 to buffer more aggressively?

There's a fundamental clash between desktop switch design and TCP operation on
recent operating systems. Switches like 3560G by default buffer 100 MTU-sized
packets, i.e. something like 150 kB of data. 3560Es are even worse, they only
buffer 64 MTU-sized packets by default (~100 kB of data).

But in recent Linux kernels (as well as in Windows Vista) TCP buffer autotuning
rises TCP window to megabyte(!) ranges. Hence a *single* TCP connection has no
problem to overrun the buffer space and cause large amount of drops.

  With kind regards,

        M.