How to get a really unique transaction id in SQL Server?

For auditing reasons I have to determine the current transaction's unique id. 

Using sys.dm_exec_requests.transaction_id (or sys.dm_tran_current_transaction.transaction_id) doesn't work for me because transaction_id restarts at 0 after a server restart.

Is there another transaction id, that is really unique (DB scope)? Or is there another reliable value that I could use to combine the transaction_id with, like for instance the time when the transaction_id counter was reset?

Thanks!