[c-nsp] Needs some help with QOS

[Peter Rathlev]

On Tue, 2009-03-24 at 14:39 -0500, John Lange wrote:
> I followed the examples on that page but I'm not having any luck. As
> far as I can tell the queue is dropping at least some packets that it
> should be prioritizing (look for 582 below).
...
> policy-map parent_shaping
>  class class-default
>   shape average 128000
>   service-policy child_queueing
> 
> policy-map child_queueing
>  class cm-qos1
>   priority percent 70
> 
> interface FastEthernet4
>  service-policy output parent_shaping
...
>      Class-map: cm-qos1 (match-all)
>       3810 packets, 2978841 bytes
>       5 minute offered rate 71000 bps, drop rate 25000 bps
>       Match: access-group name al-qos1
>       Priority: 70% (89 kbps), burst bytes 2200, b/w exceed drops: 582

You only give priority to 89kbps, so if you try to force more traffic
through, the excess will of course be dropped when others use the
remaining bandwidth. The "5 minute offered rate" can never exceed the
configured rate, but it can easily land below. Or did you push traffic
through for more than five minutes?

Maybe the burst allowed in the shaping combined with a harder (less
buffered) limit on the WAN interface drops more packets than necessary?

>                                 As a side question, the file copy now
> seems to work much differently in that it does a big "burst" at the
> start of the copy and then "stalls". Is this a burst while the packet
> queue fills up?

Hmm... TCP should take care of finding the correct level. Is the stall
permanent, i.e. the connection is dropped? Or does it just take a long
break and then resume?

You could try playing with the burst size definitions with "priority
percent 70 <burst-size>". Default is 200 ms of burst, but that may not
suit the purpose. You could try lowering it a little and see if this
works better.

Regards,
Peter