Going out of China from Chengdu to Kathmandu via Air China 407

I am an Indian Passport holder, coming in a tour to china, while returning I am planning to go via Kathmandu. But the flight I chose is having a transit stop in Lhasa. So where the immigration/customs will happen? In CD or in Lhasa? If it happens in Lhasa will there be any requirement of having Tibet Permit to borad this flight?