Roast Duck with Orange Sauce Recipe

4 brightly colored navel oranges
A 5½-lb ready-to-cook duckling
½ tsp salt
Pinch of pepper
3 Tb granulated sugar
¼ cup red wine vinegar
2 cups strong, brown duck stock (follow directions for brown chicken stock using duck giblets instead of chicken giblets)
2 Tb arrowroot blended with 3 Tb port or Madeira
The rest of the blanched orange peel
The 4 oranges, skinned
½ cup port or Madeira
The prepared sauce base
2 or 3 Tb good orange liqueur
Drops of orange bitters or lemon juice
2 Tb softened butter
A 4-cup saucepan