I believe it is by your feet - unless they have a spot some where else within the cabin. I would ask they airline first. I know my friend brought her cats on when she was moving and they were by her feet. I have also read where they have a spot on one airline where they have a spot within the cabin, they can put them.
It would be too dangerous to have them in above compartent with luggage. My dog is easy going in that way, in that if he were elsewhere in the cabin, he'd just either sleep or chew his bone.