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Date:      Sat, 3 Mar 2007 02:56:18 -0800
From:      Luigi Rizzo <>
To:        John Mok <>
Subject:   Re: Dummynet cascade of pipes
Message-ID:  <>
In-Reply-To: <>; from on Sat, Mar 03, 2007 at 09:50:43AM +0800
References:  <>

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On Sat, Mar 03, 2007 at 09:50:43AM +0800, John Mok wrote:
> Hi,
> I am new to Dummynet. I would like to setup a FreeBSD QoS box to replace 
> the one using Linux IMQ. However, I have the following questions :-
> 1. Is it possible to cascade pipes, such that the bandwidth management 
> could be easier by hierarchical control rather than flat-out? For instance,

no. hierarchical bandwidth management means inheritance
and this leads to inefficient algorithms.

WFQ does a decent job in most cases where you would want
inheritance, it's just a slightly different way of modeling things.

> pipe 100 is the total bandwidth 2Mbps of the serial link to Internet
> pipe 110 cascade pipe 100 with a weight 8Kbps and ceiling 16 Kbps
> pipe 120 cascade pipe 100 with a weight 32Kbps and ceiling 256 Kbps
> pipe 130 cascade pipe 100 with a weight 384Kbps and ceiling 2Mbps
> pipe 140 cascade pipe 100 with a weight 224Kbps and ceiling 2Mbps
> Pipe 130 and pipe 140 are allowed to consume the total bandwidth if 
> there is no other traffic, but would only get its share 224 / (8 + 32 + 
> 384 + 224) when all pipes are fully loaded.
> 2. Is there any FIFO queue rather WFQ? If there are 3 queues defined top 
> sahre the bandwidth of pipe 110, how to make the traffic to go in FIFO 
> manner?

all queues are FIFO, individually (meaning that packets are served
in FIFO order within each queue).
There is no FIFO servicing of queues, but i am unclear when one would
need this -- it seems _almost_ equivalent to serving individual
packets in FIFO order, except with a bit of uncontrolled reordering
(uncontrolled means that you cannot make estimates on when a
packet will be served, hence cannot compute guarantees etc., which
is the whole point of having WFQ or bandwidth management)

If you can find a convincing motivation for adding this feature,
it can be done - it is not hard or inefficient, just don't see
how it could help.


> Thanks a lot.
> John Mok
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