[LARTC] Some questions remaining about TC

[LARTC] Some questions remaining about TC

[Emmanuel SIMON]

Hello everybody,

I try to work with TC for a little while in my study and there still are a few 
questions that I can\'t find any answer anywhere.

Firts about qdisc and classes:
 - What is exactly the difference between qdsic and class ? They are 
so \"intimately tied together\" that i\'m not sure not make the difference. Why 
can\'t we have a qdsic HTB with a class CBQ within or reverse.
 - We have classes but is there any heritage (i\'m not sure it is the right 
word) somewhere: between qdiscs, between classes or between qdiscs and classes.

About DSMARK, I don\'t understand the way it works:
 - What is the (mask, Value) ? What is it used for ?
 - Does it mark the packet or the structure in the kernel ?
 - If i can mark the packet, how is it done ? Can i put only \'minimize 
delay\', \'maximize throughput\' and so on, or can i put for exemple 10110111 if i 
want in the TOS field ?
 - Does it exist a howto only about DSMARK ?

Another subject not so far from LARTC:
Chapter 11 of the howto says we can modify the TOS with Iptables. I\'ve tried to 
change it with the table mangle and to set it to a value outside the TOS values 
(again). it failed (of course ?). After having modified the source code of 
iptable and netfilter and removing the check on the TOS values, it stille 
failed ! There is \"something\" in the IP stack that blocks me !! Does anyone 
know how to do that ???

I have read all that i found: howtos, memos of Werner Almersberger, Q/A of the 
list, books but these questions still remain (and maybe any other). So please, 
help.

thanks
Emmanuel

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Re: [LARTC] Some questions remaining about TC

[Stef Coene]

On Monday 09 June 2003 23:18, Emmanuel SIMON wrote:
> Hello everybody,
>
> I try to work with TC for a little while in my study and there still are a
> few questions that I can\'t find any answer anywhere.
>
> Firts about qdisc and classes:
>  - What is exactly the difference between qdsic and class ? They are
> so \"intimately tied together\" that i\'m not sure not make the difference.
> Why can\'t we have a qdsic HTB with a class CBQ within or reverse.
>  - We have classes but is there any heritage (i\'m not sure it is the right
> word) somewhere: between qdiscs, between classes or between qdiscs and
> classes.
A qdisc is a structure where a packet can be enqueued.  The most important is 
the root qdisc.  That's the qdisc that will proces all packets leaving a NIC.  
If you have classfull qdisc you can add classes.  A class will hold some part 
of the traffic.  So all packets leaving a nic are enqueued in the root qdisc 
and will be "forwarded" to a class.  Each class has 1 qdisc attached to it.  
So it's the qdisc attached to a class that will reponsible for sending the 
packet

Sorry, can't help you with your other questions.

Stef

-- 

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

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Re: [LARTC] Some questions remaining about TC

[Lars Landmark]

On Tue, 10 Jun 2003, Stef Coene wrote:

> On Monday 09 June 2003 23:18, Emmanuel SIMON wrote:
> > Hello everybody,
> >
> > I try to work with TC for a little while in my study and there still are a
> > few questions that I can\'t find any answer anywhere.
> >
> > Firts about qdisc and classes:
> >  - What is exactly the difference between qdsic and class ? They are
> > so \"intimately tied together\" that i\'m not sure not make the difference.
> > Why can\'t we have a qdsic HTB with a class CBQ within or reverse.
> >  - We have classes but is there any heritage (i\'m not sure it is the right
> > word) somewhere: between qdiscs, between classes or between qdiscs and
> > classes.
> A qdisc is a structure where a packet can be enqueued.  The most important is 
> the root qdisc.  That's the qdisc that will proces all packets leaving a NIC.  
> If you have classfull qdisc you can add classes.  A class will hold some part 
> of the traffic.  So all packets leaving a nic are enqueued in the root qdisc 
> and will be "forwarded" to a class.  Each class has 1 qdisc attached to it.  
> So it's the qdisc attached to a class that will reponsible for sending the 
> packet
> 
> Sorry, can't help you with your other questions.
> 
> Stef
> 

Hi;

qdisc and classes are as you explained tightly tied together.  
As Stef said, qdisc is a struct containing a pointer to a set of function.  
In addition do HTB and CBQ hold an array in this qdisc struct.  The purpose of 
this array is to hold a pointer to each class configured for either CBQ or 
HTB.  This means; as once you have configured a CBQ qdisc, CBQ allocate this 
array.  When you now configure a class, the pointer to this class struct 
is put into this array.  
But this array can only hold pre defined types of class pointers.  
HTB and CBQ have two different types of class pointers. This is the main  
reason why CBQ and HTB classes can not be allocated to the opposite qdisc.  
If this was not enough; When dequeue functions is invoked, this function 
use different variables in its class struct.  HTB and CBQ do not 
necessarily contains the same variables intended to use for the same purpose. 

Hopefully this will explain why a CBQ class can not be attached to a HTB 
qdisc and vice versa.


Regard 

Lars
Student 
 

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RE: [LARTC] Some questions remaining about TC

[Emmanuel SIMON]

First, thank you Stef for answering.
That's what I understood. I think my question was not enough precise (??)

My reel question/problem is why are parameters of the bandwidth (such as
rate, burts ...) set on classes rather than on qdiscs ?
I guess a class should only map a flow and should not know how this flow
will be sent. Especially that classes don't send packets. What is the reason
why it is done so ? And what if i don't specify parameters fir a leaf class
? Does it herit from the parent ?

Thanks once more
Manu

> -----Message d'origine-----
> De : lartc-admin@mailman.ds9a.nl
> [mailto:lartc-admin@mailman.ds9a.nl]De
> la part de Stef Coene
> Envoyé : mardi 10 juin 2003 20:55
> À : Emmanuel SIMON; lartc@mailman.ds9a.nl
> Objet : Re: [LARTC] Some questions remaining about TC
>
>
> On Monday 09 June 2003 23:18, Emmanuel SIMON wrote:
> > Hello everybody,
> >
> > I try to work with TC for a little while in my study and
> there still are a
> > few questions that I can\'t find any answer anywhere.
> >
> > Firts about qdisc and classes:
> >  - What is exactly the difference between qdsic and class ? They are
> > so \"intimately tied together\" that i\'m not sure not make
> the difference.
> > Why can\'t we have a qdsic HTB with a class CBQ within or reverse.
> >  - We have classes but is there any heritage (i\'m not sure
> it is the right
> > word) somewhere: between qdiscs, between classes or between
> qdiscs and
> > classes.
> A qdisc is a structure where a packet can be enqueued.  The
> most important is
> the root qdisc.  That's the qdisc that will proces all
> packets leaving a NIC.
> If you have classfull qdisc you can add classes.  A class
> will hold some part
> of the traffic.  So all packets leaving a nic are enqueued in
> the root qdisc
> and will be "forwarded" to a class.  Each class has 1 qdisc
> attached to it.
> So it's the qdisc attached to a class that will reponsible
> for sending the
> packet
>
> Sorry, can't help you with your other questions.
>
> Stef
>
> --
>
> stef.coene@docum.org
>  "Using Linux as bandwidth manager"
>      http://www.docum.org/
>      #lartc @ irc.oftc.net
>
> _______________________________________________
> LARTC mailing list / LARTC@mailman.ds9a.nl
> http://mailman.ds9a.nl/mailman/listinfo/lartc HOWTO: http://lartc.org/
>

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Re: [LARTC] Some questions remaining about TC

[Stef Coene]

On Wednesday 11 June 2003 12:26, Emmanuel SIMON wrote:
> First, thank you Stef for answering.
> That's what I understood. I think my question was not enough precise (??)
>
> My reel question/problem is why are parameters of the bandwidth (such as
> rate, burts ...) set on classes rather than on qdiscs ?
You have to see a class as a virtual channel.  Each class is responsible for 
the packets that it holds.  So the rate / burst / ceil and so one are only 
valid for that class.  So you have to put packets in the same class that 
somehow are the same.  Like all telnet/ssh packets.  So you can give that 
class a small burst and a low latency.  Or you put the packets from 1 client 
in 1 class so all the traffic to the client is one 1 big channel and he is 
reponsible for this.

> I guess a class should only map a flow and should not know how this flow
> will be sent. Especially that classes don't send packets. What is the
> reason why it is done so ? And what if i don't specify parameters fir a
> leaf class ? Does it herit from the parent ?
A class is not reponsible for sending the packets because the class is only 
used to create the virtual channels (and so to limit the packets in that 
class).  Once a packet is dequeued from a virtual channel (and so is dequeued 
form a class) it's give to the qdisc attached to that class.  So you can add 
a small fifo qdisc to that class so packets are send fast.  Or you can add a 
sfq qdisc so each flow has his own little queue.  So you have a lot of 
flexibility.

You can even add a classfull qdisc to a class, but that's just a waste of CPU 
cycles and will only add extra delays.

Each class needs it's own parameters when you create it.  It will not inheret 
something from its parent.

> Thanks once more
This is even for more confusing when I have to think about ut.  This is in no 
docs explained, but I don't think that I'm telling big lies.

Stef

-- 

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

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