are they fighting or playing?

I just got a new kitten and have been doing the whole introduction process with him and my old cat. I've been doing this for about a week now and it's been going ok... except I can't tell if my old cat is really mad or just playing. Whenever I let them out together (for short amounts of time) my old cat hisses and growls A LOT. However, he also does things like rolls over on his back so his tummy is facing up... he'll do this right in front of the new kitten almost like he wants to play... but he is still hissing and growling. Occassionally he'll swat at the new kitten but never with his claws out... he'll even chase the little guy around but he'll never attack the kitten or anything like that... if my old cat wants to play, why is he still hissing and growling so much? Please help.

Sometimes this behavior means the older cat is trying to show who's the boss, but I don't think it wants to hurt the kittne.

Sounds like playing and older kitty is telling new kitty who's boss.