questions about USB enumeration

Hi, 

I'm developing a USB system on TMS320C6747 recently. I have some questions about the USB enumeration process.

In my opinion, the enumeration process should be: get device descriptor, set address, get device descriptor again, get configuration descriptor and so on.

But in my program, the enumeration process is: get device descriptor, set address, get device descriptor, set address, get device descriptor, set address, it seems to be a circle.

I'm confused. Does anyone know anything about the reason?

Thanks very much!