Dynamic bandwidth control

I would like to guarantee that one kind of traffic is prioritize when line BW limits are reach. I mean, I want to process traffic to a VS instead of other traffic just when line BW is exhausted.

To do so, I read about using a Rate Shaping profile. But this solution is very stricted, I would rather to process all traffic indistinctly when line limits are not reach.

I also read about apply a rate class using iRules.

when CLIENT_ACCEPTED {
 if { [IP::addr [IP::client_addr] equals xxx.xxx.xxx.xxx] } {
  log local0. "[IP::client_addr] being handled by rateclass class1"
  rateclass class1
 }
}

But in this solution I cannot figure out how much line usage are taken.

Any idea of how to take into account this variable?

KR, Dario.