Re: [LARTC] Explanation reqd. for few (more than few!) concepts in TC <long>

[Stef Coene <stef.coene@docum.org>]

On Wednesday 29 October 2003 16:02, Suraj Shankar wrote:
> > But the leaf class only contains the traffic and can
> > not send them.  To do so,
> > you need to add a qdisc to that class.  It's the
> > qdisc that will send the
> > packets.  This can be a class-full qdisc so you can
>
> now, if you are saying a class differs from a qdisc in
> that it cannot send out packets and qdisc can ... i
> think that is not perfectly true, in that, if a fifo
> qdisc was attached as a child to a classful qdisc (say
> cbq) the fifo qdisc will not be able to send packets
> out directly, it has to dequeue to the parent and only
> the root can dequeue-out.
That's even confusing for me.  I just ignore that part :(

> which (size of the packet) could vary, right?
> this would then lead to a dynamically sized queue, is
> this how it works?
Yes, you can stor a a number of packets in a pfifo.  So the total queue length 
(in bytes) can vary.

> > That's a difficult one, because YOU have to be the
> > bottleneck to get some good
> > shaping results.  You can try the htb or the cbq
> > qdisc and take a higher ceil
> > then your link.  You will not be the bottleneck and
> > loose some control.
>
> okey, so can you please tell me, if I fail to be the
> bottleneck, what maximum would happen ... IMO, I could
> carry on shaping but not for that extra few bytes, now
> available, right?
Most modems have a hugh queue to speed up downloads.  But that queue can kill 
the shaping you did.  So you have to be sure the queue is never full.  And 
the only thing to do so is to never send more data then the modem can handle.  
So you have to make sure YOU are the bottleneck.

> > If you have a bounded class, the ceil is the rate.
> > If it's not bounded, the
> > ceil will be the rate of a bounded parent class.
> 'ceil' IMHO, is a little different from 'cell', that
> the lartc howto talks about.
What part of the lartc howto ?

> > Just take some example tc commands and copy the
> > parameters you don't know.
> > The only parameters you have to change is rate and
> > weight.
> yeah, but the lartc howto talks about more ... so was
> wondering!
One of the reasons why I like htb is you don't need these confusing options.  
And htb don't rely on the nic hardware to calculate the actual rate.

> > Do you mean the wrr qdisc?  See the homepage of the
> > wrr qdisc.
>
> oops, I am sorry I meant according to the lartc howto,
> there are few WRR parameters that need to be set for
> CBQ!?
Ah the WRR schedule.  I think one of these parameters is the weight.  Just 
take weight = rate / 10.

> IMHO, the root qdisc (by default) is a pfifo_fast
> (classless) qdisc ... which means if want a classful
> qdisc you would need to replace the default qdisc.
> By "replace", I mean attach it to root ... which then
> follows all packets 'enter here' ... which then gets
> me back to my original Q!
> have been to your site ... went back today ... but
> couldn't find pointers to my confusion.
If you add a class qdisc as the root qdisc, all packets are queued in the 
class less qdisc and processed.  So you don't need a filter.

Stef
-- 
stef.coene@docum.org
 "Using Linux as bandwidth manager"
     http://www.docum.org/
     #lartc @ irc.openprojects.net

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