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RE: TC: Dropped vs Overlimit packets (fwd)

I guess i got 2 copies of this mail one seems to have been private.

---------- Forwarded message ----------
Date: Thu, 31 Aug 2000 21:15:59 -0400 (EDT)
From: jamal <hadi@shell.cyberus.ca>
To: Jonathan Earle <jearle@nortelnetworks.com>
Subject: RE: TC: Dropped vs Overlimit packets

On Thu, 31 Aug 2000, Jonathan Earle wrote:

> > -----Original Message-----
> > From: jamal [mailto:hadi@cyberus.ca]
> > 
> > On Wed, 30 Aug 2000, Jonathan Earle wrote:
> > 
> > Not necessarily, they could be throttled.
> I was referring to my specific case, not the general case, in which you are
> correct.  Since I'm merely specifying a rate and a queue to send packets,
> shouldn't the packets simply be overlimit?  Or dropped?  Why both?

And of course my response to you again:
In your case it is SFQ that is doing the dropping and CBQ doing the
overlimit decisions.

> > In your case it is SFQ that is doing the dropping and CBQ doing the
> > overlimit decisions.
> So, what properties/qualities of SFQ are doing the dropping?  

The SFQ _algorithm_ makes the decision to drop.
It does not necessarily drop the incoming packets because of the queue
being full. It drops a packet from the hash with the longest queue for
example and enqueues this specific one. If you want more details about SFQ
        Paul E. McKenney "Stochastic Fairness Queuing",
        IEEE INFOCOMM'90 Proceedings, San Francisco, 1990.

Essentially, if you use a specific qdisc, you should understand the
repurcassions. Your safest bet is ofcourse to use a FIFO.
Depends on what your requirements are really.

> Shouldn't the
> egress qdisc just obey what the class tells it to do, which in my case is
> "send these packets".  

Not necessarily, refer to above.

> Unless I'm completely confused, and off-track, the
> way I see the flow is something like:
> - Packet enters box.  
> - Filter sees packet, sends it to the proper class.
> - Class looks at the room (its rate specification) and says "Packet, you
> cannot fit into this room, go away".  The packet is dropped on the floor,
> kicked and beaten to death by angry Narns, and counted as overlimit.

No refer to above comments. This is what FIFO would do.

> - Class sends packets which are in the room (within the rate) to the SFQ,
> which just dumbly sends them on their way, like an mall escalator would to
> riders.

If the class has a stashed TBF or CBQ it can do rate control othewise it