traffic shaping with dummynet and priorize questions

[vladone]

Hi!
I want to build some traffic shaping. I want to have clients with
128kbs/s and 256kbits/s. So i make two pipe:
ipfw pipe 1 config bw 128kbits/s mask dst-ip 0xffffff
ipfw pipe 1 config bw 256kbits/s mask dst-ip 0xffffff
With this any host in my network receive the maximum bandwith for that
pipe according with their account.

My question:
If network is very busy, total bandwith is not sufficient for all
and obviously not reach 128kbits/s, i want to priorize traffic. users
with hight bandwith are priorized against users with low bandwith.

How i can make this?