[altq 287] Re: Ingress traffic shaping?

[Kenjiro Cho <kjc@csl.sony.co.jp>]

Karl Pielorz wrote:
> We're running AltQ-2.1 on FreeBSD 3.3, and we have a config which sets up fxp0
> to be an interface with a forwarding rate of 512kbs, i.e.
> 
> interface fxp0 bandwidth 512k cbq
> 
> Outoing traffic seems to be pretty much kept to this overall 512k limit...
> However, incoming traffic (fxp0 connects via a cisco to a higher speed link,
> i.e. 2mb) doesn't seem to be limited... I remember reading somewhere that
> whilst you can schedule / shape outgoing traffic you can't do the same to
> incoming traffic? 

That's right.

> - If this is true, surely the traffic coming 'in' from the
> router is going to 'out' of the FreeBSD AltQ machine, and thus could be
> shaped?

Managing incoming traffic is tricky:
 - when congestion occurs, packets are dropped at the other side of
   your bottleneck link.  you can't do much about it.
 - if incoming traffic is all adaptive (e.g., tcp), there is a chance
   to control incoming traffic to some extent.
   but, for packet scheduling to work, packets need to be backlogged
   in the queue.  
   if the link is 512Kbps and you shape it to 512Kbps, packets are
   never backlogged.
   so, you have to shape it at a slightly lower rate in order to
   create a bottleneck for the packet scheduler.

> ps. We do actually have valid reasons for wanting to shape 'incoming' requests
> :-)

So does everyone :)

-Kenjiro