I am flying United from Washington Dulles to Frankfurt where I will take a connecting flight (Lufthansa) to Lisbon . Both flights were booked in one reservation. What must I do upon arrival at Frankfurt before boarding my flight to Lisbon? Do I (a) retrieve luggage, (b) go through passport control, (c) go through security again? Anything else? And has anybody used the gate to gate service? If so, what was the experience like? Thanks!