How do I respond to "Hey man, sup?"?

BACK IN MY DAY, you asked that question as you were curious what the person was up to (similarly, “How’s it goin?” was asked to see how the person was).

But these days I see Dood A ask Dood B “Hey man, what’s up?” and Dood B then appropriately responds “Hey man, sup”.

So when somebody asks “What’s up” or some variation, am I supposed to answer the question, or repeat the question back?

I’m mostly unsure because typically, “What’s up” is asked when it’s very obvious what’s up. I’ll log onto Ventrilo (Online chat program, for the uninitiated) and somebody will ask me what’s up. What’s up is me logging into Vent to chat. Or I’ll sit down to eat a sandwich and my roomie will ask what’s goin on. What’s going on is me eating a sandwich.