Wash and drain the cranberries, and pick them over carefully, removing any spoiled berries or foreign objects. Pour the port into a large nonreactive skillet, and bring to a boil over high heat. Add the cranberries, and cook, stirring constantly, until they begin to pop, about 5 minutes. When the cranberries are popping, pour the sugar, salt, and orange zest over them. Cook, stirring constantly, until the sugar melts and the mixture begins to simmer briskly, 3–5 minutes. Remove from heat and allow to cool before serving.