dummynet dropping too many packets

[Eugene Grosbein]

On Mon, Oct 05, 2009 at 03:20:58PM +0500, rihad wrote:

> >>>Taildrop does not really help with this. GRED does much better.
> >>i think the first problem here is figure out _why_ we have
> >>the drops, as the original poster said that queues are configured
> >>with a very large amount of buffer (and i think there is a
> >>misconfiguration somewhere because the mbuf stats do not make
> >>sense)
> >
> >That may be very simple, f.e. wide uplink channel and policy that
> >dictates slower client speeds. Any taildrop queue would drop lots
> >of packets.
> >
> If uplink is e.g. 100 mbit/s, but data is fed to client by dummynet at 1 
> mbit/s, doesn't the _client's_ TCP software know to slow things down to 
> not overwhelm 1 mbit/s?

That's not client's TCP software feeding your router with traffic
but server side.

> Where has TCP slow-start gone? My router box 
> isn't some application proxy that starts downloading at full 100 mbit/s 
> thus quickly filling client's 1 mbit/s link. It's just a router.

While there is no or little competition for bandwidth from the router
to clients, TCP would work just fine. I suspect your shaping policy
makes heavy competition between clients. In this case, TCP behaves
not-so-well without help of router's good shaping algorythms
and taildrop is not good one.

> Although it doesn't yet make sense to me, I'll try going to GRED soon.

"Works for me" :-)

Eugene Grosbein