As announced by Ubisoft earlier this week over on their official forum, the company's servers are currently down for maintenance, with the schedule saying this should last from 04:00 to 12:00 UTC (05:00 to 13:00 BST). As expected, this will have an impact on online features throughout their games, including those like Rainbow Six: Siege.
Single-player should still be available, however, and Ubisoft has apologised for any inconvenience, explaining that they are working "to improve our services and ensure the best experience for you this upcoming holiday season."
Does this put a dent in your plans?