Prick plums with sterilized needle to prevent bursting. Combine sugar, water, orange peel and cinnamon sticks in a large sauce pot. Bring to a boil. Reduce heat and simmer a few minutes. Add plums, a layer at a time to heat through, about 2 minutes. Remove plums from syrup. Peel skins, if desired. Pack hot plums into hot jars, leaving 1/2 inch head space. Reheat syrup to a boil. Remove from heat; discard cinnamon sticks. Stir in port. Pour over plums, leaving 1/2 inch head space. Remove air bubbles. Adjust caps. Process 20 minutes in boiling water bath. Yield: about 3 quarts.