Spitoon
You can ask whatever you want, we don't mind
VdeV
It's speed through the air that pilots fly their machines at, not speed over the ground. So if high up we would be on a Mach number and when lower down an Indicated Air Speed. We are often asked to slow down whilst descending and can do it, it's just that big jets generally are quite slippery beasts and find it hard to slow down whilst maintaining anything much more than 500fpm rate of descent. We can trade off one for the other and use speedbrake to increase either. It depends on the 'urgency' that the controller makes the request with, as generally if descending we will use quite a few miles to slow significantly.
PP