[c-nsp] DTS traffic shaping & voice queuing

[Rodney Dunn]

On Mon, Dec 20, 2004 at 09:51:56AM +0100, Djerk Geurts wrote:
> Who can tell me how voice is processed when doing DTS (QoS)

Depends to what level you are asking.
The most basic is:

shaping creates backpressure to the queueing engine by
saying the interface can transmit at X rate.
Once the queueing kicks in when the shaper is allowed
to send more data the queueing code determines which
packets (in what order) are given to the shaper to send.

No congestion, it's FIFO for all traffic.

> 
> The principle is to have a hyrarchical policy-map with the parent shaping to
> CIR and the child doing the actual QoS queuing and bandwidth statements
> according to the set classes. 

Yep.

The purpose is to apply QoS to a Fast-Ethernet
> sub-interface where the sub-interface should be rate limited.

That's fine.  It should work on an IOS router.  There are some gotchas
where the driver code didn't work just right for the shaping (ie:
ethernet interfaces).  I don't remember the details of which but
I know I've seen it before.  As for a L3 switch like the 65xx
I'm not sure about this.  I don't think it will do shaping.

> 
> As far as I know now:
> Shape average = Commits to CIR (best as I need to commit to CIR)
> Shape peak    = Sends Bc & Be (not good when doing voice? If Be=0 then the
> same as average?)
>

It all depends on how the downstream handles the traffic.  Most people
just do "shape average <CIR>" and leave it as that.

 
> - Shaping delays traffic upon congestion as it's queued in the shaping
> process. Which is not good for voice traffic.

Not true.  Shaping simply acts as the gateway to the wire on how
fast it lets data go out.  You will introduce queueing delay in the
QOS queues but that's what you get when you have congestion.
You put your VOICE in a priority (LLQ) so the queueing code makes
sure those packets get out first so there is little to no delay
in that class.

The other classes will see some small amount of delay but in a BW class
that just means that the queueing code will guarantee that the
class gets the configured amount of BW over time.  It does NOT mean
it will go out first.  That's what LLQ is for.  Send the voice first.

> 
> - Policing no queueing happens but packets are just dropped. This is not
> good for voice either.

Not a good idea.

> 
> Regards,
> 
> --
> Djerk Geurts
> 
> 
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