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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.

cheers,
jamal
---------- 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
read:
        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
cant.

cheers,
jamal