[c-nsp] Flow Control and 10GE interfaces

[Marian Ďurkovič]

On Tue, Nov 24, 2009 at 11:24:26AM -0500, Ross Vandegrift wrote:
> > Yes, what you described is basically a case where the interface runs at faster
> > speed than the data path behind it.
> > 
> > Some examples: oversubcribed 10GE card with only 8 Gbps bandwidth to the switch
> > fabric or system bus, 100 Mpbs ethernet interface in front of 34 Mbps microware
> > link. 
> > 
> > This is exactly the *only* situation, where classic flow control makes sense and
> > does really help, since it properly triggers output queueing at the sending side
> > when the real data-path speed is reached. Any other usage is likely to cause
> > more problems than benefits.
>  
> But in these cases you're saturated!  So why not just drop the frame and
> let the upper-layer figure out that it needs to back off?  You're just
> delaying the inevitable by invoking flow control and hiding the
> information from the upper layer.

Not exactly. By using flow control you're in fact signalling the real 
data-path speed to the sender, i.e. the sender knows it talks to e.g.
"34 Mbps ethernet" interface and not to wirespeed 100 Mbps ethernet.
It can utilize this info to properly apply QOS or to smooth microbursts
using its output buffers - for instance, you'll hardly get IPTV
working over such 34 Mbps microwave links without flowcontrol enabled.

 
   With kind regards,

        M.